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An Overview and Brief History of Southern Hemisphere Tropical Cyclones
(Released June 2012)

 
  by Adam Arnold  

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  1. A Global View of Equatorial Waves and Tropical Cyclogenesis

    Carl J. Schreck III, John Molinari and Anantha Aiyyer.

    Monthly Weather Review, Vol. 140, No. 3, Mar 2012, pp. 774-788.

    This study investigates the number of tropical cyclone formations that can be attributed to the enhanced convection from equatorial waves within each basin. Tropical depression (TD)-type disturbances (i.e., easterly waves) were the primary tropical cyclone precursors over the Northern Hemisphere basins, particularly the eastern North Pacific and the Atlantic. In the Southern Hemisphere, however, the number of storms attributed to TD-type disturbances and equatorial Rossby waves were roughly equivalent. Equatorward of 20 degree N, tropical cyclones formed without any equatorial wave precursor most often over the eastern North Pacific and least often over the western North Pacific. The Madden-Julian oscillation (MJO) was an important tropical cyclone precursor over the north Indian, south Indian, and western North Pacific basins. The MJO also affected tropical cyclogenesis by modulating the amplitudes of higher-frequency waves. Each wave type reached the attribution threshold 1.5 times more often, and tropical cyclogenesis was 3 times more likely, within positive MJO-filtered rainfall anomalies than within negative anomalies. The greatest MJO modulation was observed for storms attributed to Kelvin waves over the north Indian Ocean. The large rainfall rates associated with tropical cyclones can alter equatorial wave-filtered anomalies. This study quantifies the contamination over each basin. Tropical cyclones contributed more than 20% of the filtered variance for each wave type over large potions of every basin except the South Pacific. The largest contamination, exceeding 60%, occurred for the TD band near the Philippines. To mitigate the contamination, the tropical cyclone-related anomalies were removed before filtering in this study.

  2. How Does Shifting Pacific Ocean Warming Modulate on Tropical Cyclone Frequency over the South China Sea?

    Guanghua Chen.

    Journal of Climate, Vol. 24, No. 17, Sep 1, 2011, pp. 4695-4700.

    The different modulation of El Niño Modoki and canonical El Niño events on tropical cyclone (TC) frequency over the South China Sea (SCS) during boreal summer and fall for 1960-2009 is investigated. The bootstrap resampling method and two-sample permutation procedure are applied to simulate sampling distributions and conduct statistical tests, respectively. Results from the hypothesis testing indicate that the above-normal TC frequency over the SCS occurs during June-August (JJA) for the El Niño Modoki years, whereas the below-normal TC frequency is significant during September-November (SON) for the canonical El Niño years. The remarkably opposite modulations can be attributed to the different large-scale circulation anomalies, which are consistent with Matsuno-Gill-type responses to the tropical heating source/sink over the western North Pacific (WNP) and Maritime Continent for two kinds of Pacific Ocean warming events. In response to a broad-scale convection anomaly over the WNP during JJA for El Niño Modoki, a zonally elongated cyclonic anomaly dominates the WNP and SCS, leading to enhanced TC activity. In contrast, during SON for the canonical El Niño, a markedly strengthened cooling source centered in the Maritime Continent induces an anticyclonic anomaly over the SCS, resulting in suppressed TC activity. [PUBLICATION ABSTRACT]

  3. Variability and decline in the number of severe tropical cyclones making land-fall over eastern Australia since the late nineteenth century

    Jeff Callaghan and Scott B. Power.

    Climate Dynamics, Vol. 37, No. 3-4, Aug 2011, pp. 647-662.

    Recent studies have raised concerns that tropical cyclones (TCs), particularly severe TCs, have become more frequent in many places in response to global warming. Other studies discuss errors in TC data that can cause large inaccuracies in some of the observed trends. Additional studies conclude that TCs are likely to become more intense in the future in response to global warming, while regional modelling studies for the south-west Pacific near north-eastern Australia project an intensification of TCs and either a decrease or no change in TC numbers. Here we describe and use a new data base of severe land-falling TCs for eastern Australia derived from numerous historical sources, that has taken over a decade to develop. It provides one of the world's longest reliable records of tropical cyclone activity, and allows us to document changes over much longer periods than has been done previously for the Southern Hemisphere. Land-fall numbers are shown to vary a great deal on interannual, decadal and longer time-scales. The interannual variability is consistent with previous studies using much shorter data sets: land-fall numbers are well-simulated as a Poisson process and are modulated by the El Nino-Southern Oscillation (ENSO). Land-falls occurred almost twice as often in La Nina years as they did in El Nino years, and multiple land-falls only occurred during La Nina years. The statistical link between land-falls and pre-season values of the Southern Oscillation Index provides a modest predictive capability. Decadal variability in ENSO drives some of the decadal variability in land-fall numbers. The sign and magnitude of trends calculated over 30 years periods vary substantially, highlighting that caution needs to be taken in making inferences about trends based on e.g. satellite era data only. The linear trend in the number of severe TCs making land-fall over eastern Australia declined from about 0.45 TCs/year in the early 1870s to about 0.17 TCs/year in recent times-a 62% decline. This decline can be partially explained by a weakening of the Walker Circulation, and a natural shift towards a more El Nino-dominated era. The extent to which global warming might be also be partially responsible for the decline in land-falls-if it is at all-is unknown.

  4. Climatology of extratropical cyclones over the South American-southern oceans sector

    David Mendes, Enio P. Souza, José A. Marengo, Monica C. Mendes and D.

    Theoretical and Applied Climatology, Vol. 100, No. 3-4, May 2010, pp. 239-250.

    A climatology of extratropical cyclones is presented. Extratropical cyclones, their main characteristics and their predominant tracks, as well as their interannual variability, affect weather in South America. For that purpose, a storm track database has been compiled by applying a cyclone tracking scheme to six-hourly sea level pressure fields, available from the National Center for Environmental Prediction-National Center for Atmospheric Research reanalyses II for the 1979-2003 period. The spatial distribution of the cyclogenesis frequency shows two main centers: one around Northern Argentina, Uruguay, and Southern Brazil in all seasons and the other near to the North Antarctic Peninsula. The lifetime of extratropical cyclones in the South American sector exhibits small seasonality, being typically of the order of 3.0 days during most of the year and slightly higher (3.5 days) in austral summer. The distance travelled by the cyclones formed in the South American sector tends to be smaller than the total paths found in other areas of the Southern Hemisphere. A k-mean clustering technique is used to summarize the analysis of the 25-year climatology of cyclone tracks. Three clusters were found: one storm-track cluster in Northeast Argentina; a second one west of the Andes Cordillera; and a third cluster located to the north of the Antarctic Peninsula (around the Weddell Sea). The influence of the Antarctic Oscillation (AAO) in the variability of extratropical cyclones is explored, and some signals of the impacts of the variability of the AAO can be observed in the position of the extratropical cyclones around 40°S, while the impacts on the intensity is detected around 55°S.[PUBLICATION ABSTRACT]

  5. Pacific Climate Change Science Program (PCCSP): Improving the Understanding of Tropical Cyclone Climatology

    Y. Kuleshov, D. Jones, M. Doyle, J. Phan and M. Schweitzer.

    Proceedings from the 2010 AGU Western Pacific Geophysics Meeting, Jun 2010

    The PCCSP is dedicated to help Pacific Island countries to gain a better understanding of how climate change will impact their regions [1]. One of the key projects for PCCSP is Improving the Understanding of Tropical Cyclone Climatology. In this project, we are further developing the tropical cyclone (TC) climatology through a detailed study of the large-scale environmental factors responsible for the Pacific TC variability and change. In this paper, we describe a TC data portal which is an integral part of the project. A TC archive for the Southern Hemisphere (SH) has been prepared at the National Climate Centre in collaboration with Meteo-France and the Meteorological Services of Fiji and New Zealand [2] and it now consists of best track data for the 1969/70 to 2006/07 TC seasons [3]. To provide a user-friendly means for accessing historical TC data, a specialised website Tropical Cyclones in the Southern Hemisphere (www.bom.gov.au/climate) was developed in 2007. Currently, we are developing a new design for the website using OpenLayers platform. This allows dynamic map navigation, presenting detailed information for a selected region in the Pacific and the display of changes in TC intensity over the lifetime of a cyclone. TC tracks can be displayed over the base layer where orange dots represent best track 6-hourly positions of the TC when its central pressure was estimated above 970 hPa and red dots represent the severe TC (970 hPa or below [3]). The tracks can be also displayed over the Elevation and bathymetry layer where dashed line connects points of the TC best track when the system was at a stage of Tropical Depression (estimated pressure is above 995 hPa) and solid line connects the points when the storm attained Tropical Cyclone intensity (995 hPa or below). Multiple TC tracks can be displayed to satisfy users requirements to present tracks over the whole TC season, or a number of seasons. Additionally, Tropical Cyclone Track Details feature allows users to display TC data for a selected storm (date and time, latitude and longitude, and intensity in terms of central pressure) in a separate window. The Report on Specific Location feature allows one to display tracks for TCs crossing within specified distance (within a radius of 50, 100, 200 and 400 km) from a specified position (a point with specific geographical coordinates or as a location selected from a list). The project will benefit the Pacific Island Countries through improved TC data availability and hence support further climate change research and vulnerability assessments. Acknowledgement The Australian Government Department of Climate Change and the Australian Government Overseas Aid Program (AusAID) provided support for this project through the PCCSP. References [1] PCCSP web site http://www.csiro.au/partnerships/Pacific-Climate-Change-Science-Pr ogram.htm l [2] Kuleshov et al, 2009: Improving preparedness to natural hazards: Tropical cyclone prediction for the Southern Hemisphere. Advances in Geosciences, 12 Ocean Science, (ed. Gan, J.), World Sci. PuB. Singapore, 127-143 [3] Kuleshov et al 2010: Trends in tropical cyclones in the South Indian Ocean and the South Pacific Ocean, J. Geophys. Res. 115, doi:10.1029/2009JD012372

  6. Seasonal prediction of tropical cyclone genesis frequency over the western North Pacific using teleconnection patterns

    Ki-seon Choi, Ja-yeon Moon, Do-woo Kim and Pao-shin Chu.

    Theoretical and Applied Climatology, Vol. 100, No. 1-2, Mar 2010, pp. 191-206.

    This study has developed a multiple linear regression model for the seasonal prediction of the summer tropical cyclone genesis frequency (TCGF) in the western North Pacific using the three teleconnection patterns. These patterns are representative of the Siberian High Oscillation (SHO) in the East Asian continent, the North Pacific Oscillation in the North Pacific, and Antarctic Oscillation (AAO) near the Australia during the boreal spring (April-May). This statistical model is verified through the two analyses: (a) statistical method of cross validation and (b) differences between the high TCGF years and low TCGF years that is hindcasted by the statistical model. The high TCGF years are characterized by the following anomalous features: Three anomalous teleconnection patterns such as anticyclonic circulation (positive SHO phase) in the East Asian continent, pressure pattern like "north-high and south-low" in the North Pacific, and cyclonic circulation (negative AAO phase) near the Australia were strengthened during the period from boreal spring to boreal summer. Thus, anomalous trade winds in the tropical western Pacific (TWP) were weakened by anomalous cyclonic circulations that located in the subtropical western Pacific (SWP) in both hemispheres. In consequence, this spatial distribution of anomalous pressure pattern suppressed convection in the TWP but strengthened convection in the SWP.[PUBLICATION ABSTRACT]

  7. Summertime moisture transport over Southeastern South America and extratropical cyclones behavior during inter-El Nino events

    Gyrlene Aparecida Silva, Mendes Da and T. Ambrizzi.

    Theoretical and Applied Climatology, Vol. 101, No. 3-4, Aug 2010, pp. 303-310.

    The impact of the inter-El Nino (EN) variability on the moisture availability over Southeastern South America (SESA) is investigated. Also, an automatic tracking scheme was used to analyze the extratropical cyclones properties (system density—SD and central pressure—CP) in this region. During the austral summer period from 1977-2000, the differences for the upper-level wave train anomaly composites seem to determine the rainfall composite differences. In fact, the positive rainfall anomalies over most of the SESA domain during the strong EN events are explained by an upper-level cyclonic center over the tropics and an anticyclonic center over the eastern subtropical area. This pattern seems to contribute to upward vertical motion at 500 hPa and reinforcement of the meridional moisture transport from the equatorial Atlantic Ocean and western Amazon basin to the SESA region. These features may contribute to the positive SD and negative CP anomalies explaining part of the positive rainfall anomalies found there. On the other hand, negative rainfall anomalies are located in the northern part of SESA for the weak EN years when compared to those for the strong events. Also, positive anomalies are found in the southern part, albeit less intense. It was associated with the weakening of the meridional moisture transport from the tropics to the SESA that seems have to contributed with smaller SD and CP anomalies over the most part of subtropics, when compared to the strong EN years. [PUBLICATION ABSTRACT]

  8. Tropical cyclones in the SW Indian Ocean. Part 1: inter-annual variability and statistical prediction

    Denis S. Chang-seng and Mark R. Jury.

    Meteorology and Atmospheric Physics, Vol. 106, No. 3-4, Mar 2010, pp. 149-162.

    The southwestern Indian Ocean (SWIO) is characterized by significant climate variability and frequent tropical cyclones (TC). Year-to-year fluctuations of TC and associated oceanic and atmospheric fields in the period 1961-2002 are studied with reanalysis data as composites and cross-correlations, with wavelet filtering and cross-modulus analysis, and by hovmoller analysis and multi-variate statistical modeling. Observational limitations in the early part of the record are recognized. An intense TC-days index is formed and is characterized by quasi-biennial to decadal cycles that relate to ocean Rossby waves and high latitude atmospheric circulations, respectively. New variables are uncovered that significantly improve the seasonal prediction of SWIO TC. One predictor is the geopotential height in the SE Pacific, which explains 31% of SWIO TC variability. It foretells of downstream oscillations in the sub-tropical jet stream, which govern wind shear, an equatorial duct and attendant circulation anomalies over the SWIO. An anti-phase association between Amazon convection and intense TCs is found to be related to the Atlantic Zonal Circulation. Drought across the Amazon is related to an increase in TC activity in the SWIO, when zonal wind anomalies over the Atlantic become upper easterly/lower westerly. This feature is related to Pacific Ocean El Niño Southern Oscillation phase. A La Niña signal favors TC development through a westward propagating cyclonic circulation and downweling Rossby wave in the South Indian Ocean that enhances thermodynamic energy. It is recommended to repeat this analysis every few years to determine whether teleconnections evolve due to climate drift or improving observations.[PUBLICATION ABSTRACT]

  9. Cold Events over Southern Australia: Synoptic Climatology and Hemispheric Structure

    Linden Claire Ashcroft, Alexandre Bernardes Pezza and Ian Simmonds.

    Journal of Climate, Vol. 22, No. 24, Dec 15, 2009, pp. 6679-6682,6684,6688-6695,6697-6698.

    Cold events (CEs) are an important feature of southern Australian weather. Unseasonably cold conditions can have a significant impact on Australia's agricultural industry and other aspects of society. In this study the bottom 0.4% of maximum temperatures in Melbourne and Perth from the 1958-2006 period are defined as CEs, representing the large-scale patterns affecting most of extratropical Australia. Compiling 6-hourly progressions of the tracks of the cyclones and anticyclones that are geostrophically associated with CEs gives for the first time a detailed synoptic climatology over the area. The anticyclone tracks display a "cloud" of high density across the Indian Ocean, which is linked, in the mean, to weak but significant negative SST anomalies in the region. The cyclone tracks display much variability, with system origins ranging from subpolar to tropical. Several CEs are found to involve tropical and extratropical interaction or extratropical transition of originally tropical cyclones (hurricanes). CE-associated systems travel farther and exhibit longer life spans than similar, non-CE systems. Upper-level analyses indicate the presence of a wave train originating more than 120° west of the CE. This pattern greatly intensifies over the affected area in conjunction with a merging of the subpolar and subtropical jets. The upper-level wave train is present up to five days before the CE. The absence of large orographic features in Australia highlights the importance of wave amplification in CE occurrence. No consistent trend in CE intensity over the period is found, but a significant negative trend in event frequency is identified for both Melbourne and Perth. [PUBLICATION ABSTRACT]

  10. Southern hemisphere tropical cyclone intensity forecast methods used at the Joint Typhoon Warning Center, Part I: control forecasts based on climatology and persistence

    JA Knaff and C. R. Sampson.

    Australian Meteorological and Oceanographic Journal, Vol. 58, No. 1, Mar 2009, pp. 1-7.

    The development of a simple statistical tropical cyclone (TC) intensity forecast model is described. The primary purpose of this model, called southern hemisphere five-day statistical typhoon intensity forecast scheme (SH ST5D), is to provide a skill/no-skill control forecast for verifying other TC intensity forecasts. However, it also provides useful and always-available forecasts of TC intensity in the southern hemisphere. The model is created by fitting an optimal combination of factors related to climatology and persistence (or CLIPER) using multiple linear regression. These CLIPER factors are determined from the best track tropical cyclone dataset produced by the United States of America's Joint Typhoon Warning Center (JTWC) in the years 1980-2002. In 2004 the SH ST5D model became part of the operational suite of tropical cyclone intensity guidance run at JTWC. The forecasts from the model since that time have outperformed both climatology (i.e. a constant 65 knots or 33 ms-1 forecast) and the persistence of initial conditions in a statistically significant manner in independent testing during 2004-2007. This documentation is provided to promote the use of this model's output and provide adequate background for the development of similar models.

  11. Southern hemisphere tropical cyclone intensity forecast methods used at the Joint Typhoon Warning Center, Part II: statistical – dynamical forecasts

    JA Knaff and C. R. Sampson.

    Australian Meteorological and Oceanographic Journal, Vol. 58, No. 1, Mar 2009, pp. 9-18.

    The development and performance of a statistical – dynamical tropical cyclone intensity forecast model, which was developed for the United States of America's Joint Typhoon Warning Center (JTWC), is described. This model, called the Southern Hemisphere Statistical Typhoon Intensity Prediction Scheme (SH STIPS), mirrors similar capabilities created for use in the western North Pacific and North Indian Ocean tropical cyclone basins. The model is created by fitting an optimal combination of factors related to climatology and persistence, intensification potential, vertical wind shear, dynamic size/intensity forecasts and atmospheric stability. All of these factors except the climatology and persistence information are derived from global forecast model analyses and forecasts. In July 2005 the SH STIPS model began a real-time evaluation period. The forecasts from the SH STIPS model have outperformed the combined climatology and persistence based forecast and thus are skillful in independent testing since that time. Since October 2006, SH STIPS has been the primary member in an operational consensus forecast of tropical cyclone intensity change provided to the JTWC. Documentation is provided for potential users of forecasts based on this methodology and for researchers interested in developing similar capabilities in the future.

  12. Southern hemisphere tropical cyclone intensity forecast methods used at the Joint Typhoon Warning Center, Part III: forecasts based on a multi-model consensus approach

    C. R. Sampson and JA Knaff.

    Australian Meteorological and Oceanographic Journal, Vol. 58, No. 1, Mar 2009, pp. 19-27.

    Consensus forecasts aids, those derived from forecasts from several models, and ensemble forecasts aids, those derived from several forecast created by a single model, have become commonplace. Consensus forecast aids are now an integral part of operational tropical cyclone forecasting at the Joint Typhoon Warning Center (JTWC). These consensus aids generally have lower average errors than individual forecast aids and benefit from the skill and independence of their members. This paper discusses the performance of one ensemble forecast aid and one consensus forecast aid run in real-time and made available to JTWC during the 2007 and 2008 southern hemisphere tropical cyclone seasons. The ensemble forecast aid is shown to be as skillful as the top performing ensemble member at each forecast time. The consensus forecast aid is shown to be the most skillful aid available to JTWC during the 2007 and 2008 seasons. Further experiments indicate that adding more forecast aids to the intensity consensus may marginally improve both skill and forecast availability of the consensus forecast aid.

  13. Changes in Tropical Cyclone Activity due to Global Warming: Results from a High-Resolution Coupled General Circulation Model

    S. Gualdi, E. Scoccimarro and A. Navarra.

    Journal of Climate, Vol. 21, No. 20, Oct 2008, pp. 5204-5228.

    This study investigates the possible changes that greenhouse global warming might generate in the characteristics of tropical cyclones (TCs). The analysis has been performed using scenario climate simulations carried out with a fully coupled high-resolution global general circulation model. The capability of the model to reproduce a reasonably realistic TC climatology has been assessed by comparing the model results from a simulation of the twentieth century with observations. The model appears to be able to simulate tropical cyclone-like vortices with many features similar to the observed TCs. The simulated TC activity exhibits realistic geographical distribution, seasonal modulation, and interannual variability, suggesting that the model is able to reproduce the major basic mechanisms that link TC occurrence with large-scale circulation. The results from the climate scenarios reveal a substantial general reduction of TC frequency when the atmospheric CO2 concentration is doubled and quadrupled. The reduction appears particularly evident for the tropical western North Pacific (WNP) and North Atlantic (ATL). In the NWP the weaker TC activity seems to be associated with reduced convective instabilities. In the ATL region the weaker TC activity seems to be due to both the increased stability of the atmosphere and a stronger vertical wind shear. Despite the generally reduced TC activity, there is evidence of increased rainfall associated with the simulated cyclones. Finally, the action of the TCs remains well confined to the tropical region and the peak of TC number remains equatorward of 20 degree latitude in both hemispheres, notwithstanding the overall warming of the tropical upper ocean and the expansion poleward of warm SSTs.

  14. Large-scale Factors in Tropical and Extratropical Cyclone Transition and Extreme Weather Events

    Alexandre Bernardes Pezza and Ian Simmonds.

    Annals of the New York Academy of Sciences, Vol. 1146, Dec 2008, pp. 189-211.

    Transition mechanisms characterizing changes from hurricanes to midlatitude cyclones and vice-versa (extratropical and tropical transition) have become a topic of increasing interest, partially because of their association with recent unusual storms that have developed in different ocean basins of both hemispheres. The aim of this work is to discuss some recent cases of transition and highly unusual hurricane developments and to address some of their wider implications for climate science. Frequently those dramatic cyclones are responsible for severe weather, potentially causing significant damage to property and infrastructure. An additional manifestation discussed here is their association with cold surges, a topic that has been very little explored in the literature. In the Southern Hemisphere, the first South Atlantic hurricane, Catarina, developed in March 2004 under very unusual large-scale conditions. That exceptional cyclone is viewed as a case of tropical transition facilitated by a well-developed blocking structure. A new index for monitoring tropical transition in the subtropical South Atlantic is discussed. This 'South Atlantic index' is used to show that the unusual flow during and prior to Catarina's genesis can be attributed to tropical/extratropical interaction mechanisms. The 'Donald Duck' case in Australia and Vince in the North Atlantic have also been examined and shown to belong to a category of hybrid-transitioning systems that will achieve at least partial tropical transition. While clearly more research is needed on the topic of transition, as we gain further insight, it is becoming increasingly apparent that features of large-scale circulation do play a fundamental role. A complex interaction between an extratropical transition case and an extreme summer cold surge affecting southeastern Australia is discussed as an example of wider climate implications.

  15. Seasonal climate summary southern hemisphere (summer 2007-08): mature La Nina, an active MJO, strongly positive SAM, and highly anomalous sea-ice

    M. C. Wheeler.

    Australian Meteorological Magazine, Vol. 57, No. 4, Dec 2008, pp. 379-393.

    Atmospheric and oceanic conditions in the southern hemisphere are reviewed for the austral summer (December-February) of 2007-08, with particular emphasis given to the drivers of climate variability in the Australian region and their linkages and mechanisms. In the tropical Pacific and nearby, a La Nina (cold) event dominated, peaking in about February 2008. It was the strongest La Nina since 1988 by most measures. The global tropics were also highly influenced by several strong Madden-Julian Oscillation (MJO) events, the strongest activity seen since autumn 2005. In the extratropics, the positive phase of the Southern Annular Mode (SAM) dominated many aspects of the large-scale atmospheric flow, and was the most positive occurrence of the SAM since July 1993. Each of these three large-scale drivers had a discernible influence on some aspects of the summer rainfall and/or temperature seen in regional Australia, but other aspects were apparently unrelated. For example, while the wet conditions over much of eastern and northern Australia were favoured by the La Nina, the drier than normal conditions in the centre of the continent were unlike what is usually expected during a La Nina or a highly positive SAM. For the MJO, its impacts were a strong intraseasonal modulation of rainfall in the far north of Australia and an implied role in the genesis of each of the season's four Australian-region tropical cyclones. The wet conditions in south-central Queensland in January were, however, more a consequence of the lingering effects of an ex-tropical cyclone (Helen). Besides these known large-scale drivers of southern hemisphere climate, also of note for the summer of 2007-08 was a highly anomalous sea-ice extent around Antarctica, being the largest positive sea-ice extent anomaly since satellite records began in 1979. In December 2007 sea ice extent was 15 per cent greater than the 1979-2006 average, and in January 2008 was 33 per cent greater than average, being in stark contrast to the much-reduced sea-ice seen in the northern hemisphere in its summer of 2007.

  16. The South Pacific and southeast Indian Ocean tropical cyclone season 2005-06

    J. B. Courtney and B. J. Santos.

    Australian Meteorological Magazine, Vol. 57, No. 1, Mar 2008, pp. 53-64.

    Fifteen tropical cyclones (TCs) formed in the South Pacific and southeast Indian Ocean during the 2005-06 TC season. This total was below the long-term average of 19 and is the seventh consecutive year of below average TC occurrence, although numbers were about average in the Australian region. Significantly there were five intense TCs, all in the Australian region. TC Monica reached a peak intensity of 69 m s-1 (135 kn) mean winds, the equal highest on record in the Australian region, as it crossed the Northern Territory coast. Most notably, TC Larry peaked in intensity at 56 m s-1 (110 kn) prior to crossing the Queensland coast, devastating the area around Innisfail. There were another four TCs that crossed the Australian coast. All of these crossed the Pilbara coast of northwest Australia, having a significant economic impact on the industry-rich region. The season occurred during a neutral El-Nino Southern Oscillation (ENSO) phase, whilst sea-surface temperatures (SSTs) remained warmer than normal over much of the region. Active phases of the intraseasonal Madden-Julian Oscillation (MJO) coincided with the development of eleven of the fifteen TC events.

  17. Mechanisms for the Interannual Variability in the Tropical Indian Ocean. Part II: Regional Processes

    Bohua Huang and J. Shukla.

    Journal of Climate, Vol. 20, No. 13, Jul 1, 2007, pp. 2937-2941,2944-2949,2951-2955,2957-2960.

    To understand the mechanisms of the interannual variability in the tropical Indian Ocean, two long-term simulations are conducted using a coupled ocean-atmosphere GCM-one with active air-sea coupling over the global ocean and the other with regional coupling restricted within the Indian Ocean to the north of 30°S while the climatological monthly sea surface temperatures (SSTs) are prescribed in the uncoupled oceans to drive the atmospheric circulation. The major spatial patterns of the observed upper-ocean heat content and SST anomalies can be reproduced realistically by both simulations, suggesting that they are determined by intrinsic coupled processes within the Indian Ocean. In both simulations, the interannual variability in the Indian Ocean is dominated by a tropical mode and a subtropical mode. The tropical mode is characterized by a coupled feedback among thermocline depth, zonal SST gradient, and wind anomalies over the equatorial and southern tropical Indian Ocean, which is strongest in boreal fall and winter. The tropical mode simulated by the global coupled model reproduces the main observational features, including a seasonal connection to the model El Niño-Southern Oscillation (ENSO). The ENSO influence, however, is weaker than that in a set of ensemble simulations described in Part I of this study, where the observed SST anomalies for 1950-98 are prescribed outside the Indian Ocean. Combining with the results from Part I of this study, it is concluded that ENSO can modulate the temporal variability of the tropical mode through atmospheric teleconnection. Its influence depends on the ENSO strength and duration. The stronger and more persistent El Niño events in the observations extend the life span of the anomalous events in the tropical Indian Ocean significantly. In the regional coupled simulation, the tropical mode is still active, but its dominant period is shifted away from that of ENSO. In the absence of ENSO forcing, the tropical mode is mainly stimulated by an anomalous atmospheric direct thermal cell forced by the fluctuations of the northwestern Pacific monsoon. The subtropical mode is characterized by an east-west dipole pattern of the SST anomalies in the southern subtropical Indian Ocean, which is strongest in austral fall. The SST anomalies are initially forced by surface heat flux anomalies caused by the anomalous southeast trade wind in the subtropical ocean during austral summer. The trade wind anomalies are in turn associated with extratropical variations from the southern annular mode. A thermodynamic air-sea feedback strengthens these subtropical anomalies quickly in austral fall and extends their remnants into the tropical ocean in austral winter. In the simulations, this subtropical variability is independent of ENSO. [PUBLICATION ABSTRACT]

  18. Use of a Genesis Potential Index to Diagnose ENSO Effects on Tropical Cyclone Genesis

    S. J. Camargo, KA Emanuel and AH Sobel.

    Journal of Climate, Vol. 20, No. 19, Oct 2007, pp. 4819-4834.

    ENSO (El Nino-Southern Oscillation) has a large influence on tropical cyclone activity. The authors examine how different environmental factors contribute to this influence, using a genesis potential index developed by Emanuel and Nolan. Four factors contribute to the genesis potential index: low-level vorticity (850 hPa), relative humidity at 600 hPa, the magnitude of vertical wind shear from 850 to 200 hPa, and potential intensity (PI). Using monthly NCEP Reanalysis data in the period of 1950-2005, the genesis potential index is calculated on a latitude strip from 60 degree S to 60 degree N. Composite anomalies of the genesis potential index are produced for El Nino and La Nina years separately. These composites qualitatively replicate the observed interannual variations of the observed frequency and location of genesis in several different basins. This justifies producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology, to determine which among the factors are most important in causing interannual variations in genesis frequency. Specific factors that have more influence than others in different regions can be identified. For example, in El Nino years, relative humidity and vertical shear are important for the reduction in genesis seen in the Atlantic basin, and relative humidity and vorticity are important for the eastward shift in the mean genesis location in the western North Pacific.

  19. Oceanic mixed layer variations during the movement of cyclone along symmetric tracks in the Indian Ocean

    A. A. Deo, D. W. Ganer and P. S. Salvekar.

    Indian Journal of Marine Sciences, Vol. 35, No. 2, Mar 2006, pp. 111-122.

    The understanding of significant changes in the oceanic mixed layer is important for dynamical prediction of tropical cyclone. Present study aims at examining the variations in the upper ocean parameters during the movement of tropical cyclone in northern and southern Indian Ocean, by applying 1 one half layer wind driven reduced gravity ocean model. Different idealized tracks in the Bay of Bengal and their mirror images in southern hemisphere are considered. The model produced mixed layer depth, temperature and currents are compared for northern and southern hemispheric cyclone cases. Also the effect of latitude for westward moving cyclones in both the hemisphere is investigated. For this, tracks in the Arabian Sea and their counter parts in the southern hemisphere are considered. The maximum cooling found in the wake of cyclone is of the order of 3- 4 degree C, which is comparable with earlier studies and observations. This significant cooling can cause weakening of the storm. This can be useful for prediction especially in the event of a cyclonic storm heading towards land and likely to make land fall. Moreover it is also seen that the mixed layer is cooled more on the right (left) of the storm track in northern (southern) hemisphere. This cooling decreases for the storms translating along higher latitudes implying the sensitivity of the latitudinal location of the storm.

  20. The South Pacific and southeast Indian Ocean tropical cyclone season 2003-04

    R. McInerney, J. Farlow, E. Henry, et al.

    Australian Meteorological Magazine, Vol. 55, No. 4, Dec 2006, pp. 313-323.

    Twelve tropical cyclones (TCs) formed in the combined southeast Indian Ocean and South Pacific Ocean basins, all of which formed between December and March. This was well below the long-term average of 18.9 and becomes the fifth consecutive year of below average occurrences of TCs over this region. However, of the 20 TCs to occur in the southern hemisphere as a whole, eight of those became intense tropical cyclones (ITC), resulting in an above average number of ITCs and ITC days in the southern hemisphere. The El Nino Southern – Oscillation (ENSO) should not have been a modulating influence, in this ENSO-neutral season. The majority of TCs coincided with active phases of the Madden-Julian Oscillation (MJO).

  21. The Effect of Regional Climate Model Domain Choice on the Simulation of Tropical Cyclone-Like Vortices in the Southwestern Indian Ocean

    Willem A. Landman, Anji Seth and Suzana J. Camargo.

    Journal of Climate, Vol. 18, No. 8, Apr 15, 2005, pp. 1263-1274.

    A regional climate model is tested for several domain configurations over the southwestern Indian Ocean to examine the ability of the model to reproduce observed cyclones and their landfalling tracks. The interaction between large-scale and local terrain forcing of tropical storms approaching and transiting the island landmass of Madagascar makes the southwestern Indian Ocean a unique and interesting study area. In addition, tropical cyclones across the southern Indian Ocean are likely to be significantly affected by the large-scale zonal flow. Therefore, the effects of model domain size and the positioning of its lateral boundaries on the simulation of tropical cyclone-like vortices and their tracks on a seasonal time scale are investigated. Four tropical cyclones, which occurred over the southwestern Indian Ocean in January of the years 1995-97, are studied, and four domains are tested. The regional climate model is driven by atmospheric lateral boundary conditions that are derived from large-scale meteorological analyses. The use of analyzed boundary forcing enables comparison with observed cyclones in these tests. Simulations are performed using a 60-km horizontal resolution and for an extended time integration of about 6 weeks. Results show that the positioning of the eastern boundary of the regional model domain is of major importance in the life cycle of simulated tropical cyclone-like vortices: a vortex entering through the eastern boundary of the regional model is generally well simulated. The size of the domain also has a bearing on the ability of the regional model to simulate vortices in the Mozambique Channel, and the island landmass of Madagascar additionally influences storm tracks. These results show that the regional model can produce cyclonelike vortices and their tracks (with some deficiencies) given analyzed lateral boundary forcing. Statistical analyses of GCM-driven nested model ensemble integrations are now required to further address predictive skill of cyclones in the southwestern Indian Ocean and to test if the model can realistically simulate tropical storm genesis as opposed to advecting existing tropical disturbances entering through the model boundaries. [PUBLICATION ABSTRACT]

  22. Southern Hemisphere Blocking Onsets Associated with Upper-Tropospheric Divergence Anomalies

    F Javier Sáez de Adana and Stephen J. Colucci.

    Journal of the Atmospheric Sciences, Vol. 62, No. 5, May 2005, pp. 1614-1625.

    Adana and Colucci investigate the upper-tropospheric divergence anomalies and divergence tendencies prior to and during the onset of blocking over the Southern Hemisphere in search of links between the upper-tropospheric response to tropical convective activity and the onset of blocking in midlatitudes. Results indicate that divergence tendencies over midlatitudes in the block-onset region were forced primarily by horizontal advection, ageostropic relative vorticity, and a nonlinear effect.

  23. Characteristics of Tropical Cyclones in the Australian Region

    Richard A. Dare and Noel E. Davidson.

    Monthly Weather Review, Vol. 132, No. 12, Dec 2004, pp. 3049-3065.

    Dare and Davidson examine the characteristics of 500 tropical cyclones in the Australian region and its three individual basins are examined. Among other things, recurvature generally follows attainment of maximum intensity, suggesting the importance of trough interactions on this behavior.

  24. Differences in El Nino Response over the Southern Hemisphere

    Carolina Vera, Gabriel Silvestri, Vicente Barros and Andrea Carril.

    Journal of Climate, Vol. 17, No. 9, May 1, 2004, pp. 1741-1753.

    Southern Hemisphere (SH) circulation conditions during austral springs of El Nino Southern Oscillation (ENSO) events are examined. Based on previous knowledge that SST variations over the subtropical south-central Pacific (SSCP) region are linked to differences among El Nino (EN) events, a stratification of the springs associated with EN events was performed according to SST conditions over the El Nino-3.4 sector and over the SSCP region. Results showed that the interdecadal variability observed in the Pacific by many previous studies influences the circulation response to ENSO over the SH, mainly through changes in the activity of the SH annular mode.

  25. Tropical cyclone environments over the northeastern Pacific, including mid-level dry intrusion cases

    A. H. Fink and D. G. Vincent.

    Meteorology and Atmospheric Physics, Vol. 84, No. 3-4, Oct 2003, pp. 293-315.

    This study uses a 1 degree x 1 degree lat/long dataset, extracted from ECMWF re-analyses for the 15-year period 1979-1993 (ERA-15), to diagnose the synoptic-scale kinematic, thermodynamic and moisture environments in the vicinity of named tropical cyclones (TCs) in the eastern North Pacific. Based on the NCDC best track dataset, TCs are partitioned into one of three categories: weak (W), strong (S) or intensifying (I). In total, 63TCs are examined: 8Ws and 20Is at point A (maximum intensification) and 11Ws, 13Ss and 11Is at point B (maximum frequency). Composite maps are compiled for all five groups, and six individual case studies are examined, four for extreme TC cases and two for cases involving dry air intrusions. For the most part, peak values and patterns of composited ERA-15 variables display circulation, thermodynamic and moisture characteristics that are compatible with the strength represented by a group's classification. Intercomparison between Ws and Is at points A and B yielded larger conditional instability of low-level air parcels and upper-level outflow within the region of maximum intensification (point A). The intrusions of dry versus moist mid-level air are addressed for each storm with the assistance of 72-hour backward trajectories. Trajectory density maps indicate two preferred paths of air parcels that reach the environment of W storms at point A on the 700 and 500hPa levels. The first one crossed Central America in the region of the Isthmus of Tehuantepec and the second one south of the Central American mountains. Several storms revealed that these trajectories were associated with dry air intrusions into the larger storm area, and this might be one reason for their weak status at point A. One documented example is Kevin (1985). By the time it reached point B, the dry air was replaced by air that was moist and Kevin intensified, although it remained a W system. In contrast, Narda (1989) received a dry air intrusion from Central Mexico at 500hPa as a weak storm at point B and did not intensify. Despite possible analyses problems, the documentation in this study of mid-level dry air intrusions into eastern Pacific TCs from the Mexican-Central American region suggests a hitherto unexploited forecast potential. [PUBLICATION ABSTRACT]

  26. Large-scale circulation features associated with decadal variations of tropical cyclone activity over the Central North Pacific

    Chu Pao-Shin.

    Journal of Climate, Vol. 15, No. 18, Sep 15, 2002, pp. 2678-2689.

    Tropical cyclone frequency in the central North Pacific (CNP) from 1966 to 2000 has exhibited decadal-scale variability. A statistical changepoint analysis reveals objectively that the shifts occur in 1982 and 1995, with fewer cyclones during the 1966 and 1995 epochs and more during the 1982 epoch. A bootstrap resampling method is then applied to determine the frequency distribution of the mean annual cyclones for the 1966, 1981 and 1982 epochs, as well as to infer the confidence intervals of the observed mean and variance of cyclones for each epoch.

  27. The Madden-Julian oscillation, barotropic dynamics, and North Pacific tropical cyclone formation. Part I: Observations

    Eric D. Maloney and Dennis L. Hartmann.

    Journal of the Atmospheric Sciences, Vol. 58, No. 17, Sep 1, 2001, pp. 2545-2558.

    Low-level barotropic dynamics may help to explain the modulation of eastern and western North Pacific tropical cyclones by the Madden-Julian oscillation (MJO) during Northern Hemisphere summer. The MJO is characterized by alternating periods of westerly and easterly 850-mb zonal wind anomalies across the tropical Pacific Ocean.

  28. The Madden-Julian oscillation, barotropic dynamics, and North Pacific tropical cyclone formation. Part II: Stochastic barotropic modeling

    Dennis L. Hartmann and Eric D. Maloney.

    Journal of the Atmospheric Sciences, Vol. 58, No. 17, Sep 1, 2001, pp. 2559-2570.

    A stochastic barotropic model linerarized about the 850-mb flow is used to investigate the relationship between wind variations associated with the Madden-Julian oscillation (MJO) and eddy kinetic energy variations in the Tropics. Such a model is successful in predicting the observed location of eddy kinetic energy maxima during the westerly phase of the MJO and the suppression of eddy activity during the easterly phase of the MJO.

  29. Modelling low frequency variability in Southern Hemisphere extra-tropical cyclone characteristics and its sensitivity to sea-surface temperature

    CJC Reason and R. J. Murray.

    International Journal of Climatology, Chichester, UK, Vol. 21, No. 2, 2001, pp. 249-267.

    The outflow regions of the western boundary currents of the Southern Hemisphere and their extensions into the South Indian Ocean, South Pacific and South Atlantic Currents are prominent areas of low frequency variability in sea-surface temperature (SST). One mechanism of decadal/multidecadal variability involves modulation of surface heat flux exchange and the track and intensity of extra-tropical cyclones and associated frontal systems in response to a zonal mid-latitude SST anomaly. Here, the influence of this type of SST pattern on cyclone properties is investigated by applying a vortex tracking scheme to atmospheric general circulation model integrations. In order to assess the subdecadal to multidecadal scale response, these integrations were conducted for a 21-year period with forcing consisting of a warm SST anomaly stretching across the Southern Hemisphere mid-latitudes. In response to this forcing, a large-scale trough at mid-latitudes and ridging at higher latitudes are generated in the model. Associated with this change in Southern Hemisphere baroclinicity are increases in the density and flux of cyclonic systems in the 40 degrees -55 degrees S zone but decreases poleward of this latitude belt. It is found that the model response weakens throughout the Southern Hemisphere mid-to high-latitudes in the second decade compared to the first but that there is less evidence of a hemispheric weakening in the second quinquennium relative to the first. Implications of these results for mechanisms of decadal /multidecadal climate variability are discussed.

  30. The wintertime southern hemisphere split jet: Structure, variability, and evolution

    Teresa M. Bals-Elsholz, Eyad H. Atallah, Lance F. Bosart, Thomas A. Wasula and et al.

    Journal of Climate, Vol. 14, No. 21, Nov 1, 2001, pp. 4191-4215.

    A persistent feature of the Southern Hemisphere upper-level time-mean flow is the presence of a split jet across the South Pacific east of Australia during the austral winter. The split jet is composed of the subtropical jet (STJ) on its equatorward branch and the polar front jet (PFJ) on its poleward branch.

  31. Interannual variations in the southern hemisphere winter circumpolar vortex: Relationship with the semiannual oscillation

    Adam W. Burnett and Alexander R. McNicoll.

    Journal of Climate, Vol. 13, No. 5, Mar 1, 2000, pp. 991-999.

    Several studies of Southern Hemisphere circulation report changes in the late 1970s and 1980s through the mid- and high latitudes that are associated with a weakening in the semiannual oscillation (SAO) of sea level pressure and geopotential height. This weakening is thought to reflect changes in the amplitude of zonal wave-number 3, a deepening of the subantarctic trough, intensification of subtropical ridging, and an increase in the strength of the westerlies.

  32. Mean southern hemisphere extratropical cyclone behavior in the 40-year NCEP-NCAR reanalysis

    Ian Simmonds and Kevin Keay.

    Journal of Climate, Vol. 13, No. 5, Mar 1, 2000, pp. 873-885.

    This paper presents a new climatology of Southern Hemisphere (SH) extratropical cyclones. This has been compiled by applying a state-of-the-art cyclone tracking scheme to the 6-hourly National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) global reanalyses spanning the period 1958-97.

  33. Climatic determinants and statistical prediction of tropical cyclone days in the Southwest Indian Ocean

    Mark R. Jury, Beenay Pathack and Bhawoodien Parker.

    Journal of Climate, Vol. 12, No. 6, Jun 1999, pp. 1738-1746.

    Climatic determinants of tropical cyclone (TC) days in the southwest Indian Ocean area (10deg-25degS, 50deg-70degE) are analyzed using statistical techniques.

  34. The South Pacific and southeast Indian Ocean tropical cyclone season 1996-97

    B. N. Hanstrum, G. Reader and P. W. Bate.

    Australian Meteorological Magazine, Vol. 48, No. 3, Sep 1999, pp. 197-210.

    Tropical cyclone occurrences were above the long-term averages during the 1996-97 season. The weak La Nina phase which characterised 1996 declined and some early El Nino indicators emerged by the end of the season. These included the appearance of strong westerly equatorial wind anomalies near the date-line, increasing sea-surface temperature in the near-equatorial central and eastern Pacific, a fall of the Southern Oscillation Index to negative values and increased convective activity in the South Pacific convergence zone. The monsoon in the summer hemisphere was of average development or better, with strongest anomalies in the southwest Pacific. Three major cycles of the 30 to 60-day intraseasonal oscillation were diagnosed. The 26 cyclones that formed spanned every month (July 96-June 97) except August and September.

  35. Tropical cyclone motion forecasting using satellite water vapor imagery

    V. F. Dvorak and H. M. Mogil.

    NOAA Technical Report NESDIS, Vol. , No. 83, Dec 1994, pp. ii.

    This paper describes the development and applications of a technique for using water vapor imagery to assess how a tropical cyclone will respond as it interacts with nearby systems. The technique is based on a limited sample of cases from both the Atlantic and Pacific basins. Due to the unavailability of water vapor imagery from the western Pacific region, the technique has not been evaluated on typhoons and Southern Hemisphere tropical cyclones. This report has been written to supplement training courses conducted by NOAA and the Department of Defense. We also anticipate much interest by meteorological services in tropical and middle latitude regions affected by tropical cyclones. For this reason, we have structured the report as a "training" document. After reading about the technique and how to apply it, forecasters can try out their understanding by completing two case studies.

  36. Southern Hemisphere climate scenarios

    AB Pittock and MJ Salinger.

    Climatic Change, Dordrecht, The Netherlands, Vol. 18, No. 2-3, 1991, pp. 205-222.

    There is little doubt that between now and 2050 Earth faces global warming and other changes in climate unprecedented in magnitude since the end of the last glaciation some 10,000 years ago. Predicting the exact nature of that change is, however, difficult. Arguments from palaeoclimatic analogues, comparisons of recent warm versus cool years, physical reasoning and computer simulations are all subject to error and uncertainty. This is more so in the relatively less well understood climate system of the Southern Hemisphere, and at the local and regional scale, than in the Northern Hemisphere and at a zonally averaged scale. Nevertheless some broad features can be described with some confidence, and we can at least identify some of the major uncertainties and processes which we need to understand better. Increased poleward penetration of the subtropical monsoonal regimes is likely, and tropical cyclones may also occur at higher latitudes than at present. The role of the oceans, especially at high southern latitudes and in the tropics, and effects which may change with time as greenhouse gas concentrations gradually increase (`transient' effects) are particularly important and uncertain in the Southern Hemisphere. We know enough to declare the urgency of slowing down and eventually limiting the greenhouse effect. However, more research is needed to guide decision makers and planners at the local and regional level as they try to cope with those climatic changes which are unavoidable. Regional cooperation is essential to make the best use of the research and planning facilities available.

  37. Downstream development in the Southern Hemisphere monsoon during FGGE/WMONEX

    N. E. Davidson and H. H. Hendon.

    Monthly Weather Review, Vol. 117, No. 7, 1989, pp. 1458-1470.

    Evidence is presented of a downstream development mechanism operating across the entire longitudinal span of the 1978/79 Southern Hemisphere monsoon. Observationally it is seen as progressive cyclonic and anticyclonic vorticity increases that develop eastward in the monsoon trough at a speed of approximately 5 m/s. The process results in many tropical cyclone and tropical depression formations over northern Australia and the South Pacific. It is shown that the downstream development process is generally consistent with linearized barotropic dynamics, and that the Southern Hemisphere monsoon, because of an intrinsic westerly basic state, is a particularly suitable region for downstream events. It is also shown that some apparent contradictions in previous observational studies can be rationalized by the theory. The interactions between the regional components of the monsoon (Indonesian, Australian and South Pacific sectors) can also be better understood. We further suggest that the process has implications for other features of the monsoon circulation, namely onset and 40-50 day events. (DBO)

  38. Forecast verification and reconnaissance data for Southern Hemisphere tropical cyclones (July 1982 through June 1984)

    W. P. Wirfel and SA Sandgathe.

    United States.Naval Oceanography Command Center, Joint Typhoon Warning Center, COMNAVMARIANAS, San Francisco, CA., NOCC/JTWC Tech Note 86-1, Feb., 1986., pp. 102.

    The Joint Typhoon Warning Center (JTWC) area of responsibility includes the Southern Hemisphere, from 180 degrees long. westward to the east coast of Africa. This report documents tropical cyclones that developed in the southern Indian Ocean and southern Pacific Ocean from July 1, 1982, to June 30, 1984. It provides composite storm best tracks, a brief summary of each year's tropical cyclone season, and detailed position and reconnaissance fix data. Also included are forecast accuracy statistics for Southern Hemisphere tropical cyclones.

  39. Role of the general circulation in western Pacific tropical cyclone genesis

    Geoff Love.

    Colorado.State Univ., Ft.Collins, Dept.of Atmospheric Science, Atmospheric Science Paper March, 1982., No. 340, Mar 1982, pp. 215.

    This study investigates the role that large-scale circulation patterns play in tropical cyclone genesis. The data available for this study were largely from stations within the eastern hemisphere; accordingly, this paper addresses the problem of defining the important flow features observed prior to cyclone genesis in the northwest Pacific and in the Australian region. A combination of case study and compositing was used to identify those weather systems that interact with a pregenesis tropical cloud cluster, enhancing its development potential. A comparison of precyclone genesis conditions with those observed during noncyclone genesis periods reveals more vigorous Hadley circulations between the subtropical ridges in both hemispheres and the ITCZ. The acceleration of the winter hemisphere Hadley cell in response to cold surge forcing appears to be very dominant. To better understand the planetary-scale adjustment processes resulting from these winter hemisphere surge situations, a series of simple numerical model experiments were conducted. The shallow water equations were solved for an arbitrary time-dependent forcing in either wind or height field. Spherical geometry and a realistic pole-to-pole zonal wind profile were used. These experiments indicate that energy propagation from the midlatitudes to the deep Tropics is dependent upon the scale of the forcing and the background wind field. For Northern Hemisphere midlatitude forcings that are equivalent barotropic in nature, it is possible to generate large-amplitude responses in the Southern Hemisphere, regardless of the intervening wind fields. An important role of the Tropics in all simulations appears to be to provide a region where down-the-pressure-gradient flow may readily occur, assisting in the adjustment processes to planetary-scale forcings.

  40. The Synoptic Meteorological Analysis on the 7619 Tropical Storm in the Southern Hemisphere

    Y. Huang.

    ACTA OCEANOL.SIN., Vol. 3, No. 3, 1981, pp. 390-399.

    According to the analysis of the pressure field, the constant level chart, the temperature-humidity field (Ose,) the cloud chart and other materials, this paper discusses the development of Storm 7619 in detail. The emphasis is especially on the features of development in the streamline field. This paper states that the features of the horizontal shear and the vertical shear of streamline field near the ITCZ are the most important factors for the generating and the dying of a storm. Storm 7619 formed in the deep Easterlies near the southern part of the ITCZ boundary in the area of the smallest vertical shear (2.0-5.0m/s), and in the ensuing 24-48 hours period the lower level shear value increased rapidly. The above discussion will provide information for weather prediction.