Eun-Pa Lim

Synoptic Activity in the Seas around Antarctica

Abstract The recent NCEP–Department of Energy (DOE) Reanalysis-2 update of the original NCEP–NCAR dataset provides what is arguably the highest quality analyses spanning two decades available for the high southern latitudes. It therefore offers an excellent starting point from which to assemble a modern, comprehensive, and reliable picture of synoptic activity in the subantarctic region. This set, covering the “modern satellite” era from January 1979 to February 2000, is used herein. In addition, the exploration in this study has been conducted with sophisticated feature-tracking and trajectory analysis software. It is shown that the high southern latitude cyclone system density is greatest in the Indian Ocean and to the south of Australia near, or to the south of, 60°S. The numbers in winter exceed those in summer, except over a few, but important, regions such as the Bellingshausen Sea. The Antarctic coastal region is confirmed as one of high cyclonicity, as is that in the northern part of the Antarctic...

Explosive Cyclone Development in the Southern Hemisphere and a Comparison with Northern Hemisphere Events

Abstract A compilation of Southern Hemisphere (SH) explosively developing cyclones (or “bombs”) has been assembled based on the National Centers for Environmental Prediction–Department of Energy reanalysis-2 data over the 21-yr period from 1979 to 1999. The identification of these features was undertaken with an objective automated cyclone finding and tracking scheme. The procedure allows for the confounding influence of spatial variations of climatological mean pressure on the pressure deepening of explosive cyclones, a perspective of particular importance in the SH. On average, 26 explosive cyclones occur per year in the SH. They are more prevalent in winter although their seasonality is more modest than that seen in the Northern Hemisphere (NH). The distribution of SH explosive cyclones has a close association with that of strong baroclinicity, although the relationship is not one to one. It is found that many of these cyclones occurring south of 50°S show equatorward movement, in contrast with the pol...

Southern Hemisphere Winter Extratropical Cyclone Characteristics and Vertical Organization Observed with the ERA-40 Data in 1979–2001

Abstract The mean characteristics and trends of Southern Hemisphere (SH) winter extratropical cyclones occurring at six levels of the troposphere over the period 1979–2001 have been investigated using the 40-yr ECMWF Re-Analysis (ERA-40) data. Cyclonic systems were identified with the Melbourne University cyclone finding and tracking scheme. This study shows that mean sea level pressure (MSLP) cyclones are more numerous, more intense, smaller, deeper, and slower moving than higher-level cyclones. The novel vertical tracing scheme devised for this research revealed that about 52% of SH winter MSLP cyclones have a vertically well organized structure, extending through to the 500-hPa level. About 80% of these vertically coherent SH cyclones keep their westward tilt until the surface cyclones reach their maximum depths, and the mean distance is 300 km between the surface and the 500-hPa level cyclone centers when the surface cyclones obtain their maturity. According to the authors’ definition of vertical orga...

Dynamical Forecast of Inter–El Niño Variations of Tropical SST and Australian Spring Rainfall

Abstract The relationship between variations of Indo-Pacific sea surface temperatures (SSTs) and Australian springtime rainfall over the last 30 years is investigated with a focus on predictability of inter–El Nino variations of SST and associated rainfall anomalies. Based on observed data, the leading empirical orthogonal function (EOF) of Indo-Pacific SST represents mature El Nino conditions, while the second and fourth modes depict major east–west shifts of individual El Nino events. These higher-order EOFs of SST explain more rainfall variance in Australia, especially in the southeast, than does the El Nino mode. Furthermore, intense springtime droughts tend to be associated with peak warming in the central Pacific, as captured by EOFs 2 and 4, together with warming in the eastern Pacific as depicted by EOF1. The ability to predict these inter–El Nino variations of SST and Australian rainfall is assessed with the Australian Bureau of Meteorology dynamical coupled model seasonal forecast system, the Pr...

Seasonal Predictability of the Southern Annular Mode due to Its Association with ENSO

AbstractPredictability of the southern annular mode (SAM) for lead times beyond 1–2 weeks has traditionally been considered to be low because the SAM is regarded as an internal mode of variability with a typical decorrelation time of about 10 days. However, the association of the SAM with El Nino–Southern Oscillation (ENSO) suggests the potential for making seasonal predictions of the SAM. In this study the authors explore seasonal predictability and the predictive skill of SAM using observations and retrospective forecasts (hindcasts) from the Australian Bureau of Meteorology dynamical seasonal forecast system [the Predictive Ocean and Atmosphere Model for Australia, version 2 (POAMA2)].Based on the observed seasonal relationships of the SAM with tropical sea surface temperatures, two distinctive periods of high seasonal predictability are suggested: austral late autumn to winter and late spring to early summer. Predictability of the SAM in the austral cold seasons stems from the association of the SAM w...

Dynamical, Statistical-Dynamical, and Multimodel Ensemble Forecasts of Australian Spring Season Rainfall

Abstract The prediction skill of the Australian Bureau of Meteorology dynamical seasonal forecast model Predictive Ocean Atmosphere Model for Australia (POAMA) is assessed for probabilistic forecasts of spring season rainfall in Australia and the feasibility of increasing forecast skill through statistical postprocessing is examined. Two statistical postprocessing techniques are explored: calibrating POAMA prediction of rainfall anomaly against observations and using dynamically predicted mean sea level pressure to infer regional rainfall anomaly over Australia (referred to as “bridging”). A “homogeneous” multimodel ensemble prediction method (HMME) is also introduced that consists of the combination of POAMA’s direct prediction of rainfall anomaly together with the two statistically postprocessed predictions. Using hindcasts for the period 1981–2006, the direct forecasts from POAMA exhibit skill relative to a climatological forecast over broad areas of eastern and southern Australia, where El Nino and th...

Seasonal Variations of Subtropical Precipitation Associated with the Southern Annular Mode

AbstractSeasonal variations of subtropical precipitation anomalies associated with the southern annular mode (SAM) are explored for the period 1979–2011. In all seasons, high-polarity SAM, which refers to a poleward-shifted eddy-driven westerly jet, results in increased precipitation in high latitudes and decreased precipitation in midlatitudes as a result of the concomitant poleward shift of the midlatitude storm track. In addition, during spring–autumn, high SAM also results in increased rainfall in the subtropics. This subtropical precipitation anomaly is absent during winter. This seasonal variation of the response of subtropical precipitation to the SAM is shown to be consistent with the seasonal variation of the eddy-induced divergent meridional circulation in the subtropics (strong in summer and weak in winter). The lack of an induced divergent meridional circulation in the subtropics during winter is attributed to the presence of the wintertime subtropical jet, which causes a broad latitudinal spa...

Biases in the calculation of Southern Hemisphere mean baroclinic eddy growth rate

[1] The maximum Eady growth rate measure of baroclinic instability is very commonly used in the literature. Its average is usually calculated directly from the time-mean flow. It is suggested here that this approach is not entirely suitable, but rather one should obtain the Eady growth rates at all relevant synoptic times and average these. It is found at the 850 hPa level in the Southern Hemisphere that the time-mean of the instantaneous rates exceed those calculated from the time-mean field over much of the mid and high latitudes, and the difference is even more marked at 500 hPa. At both levels the axes of the maxima Eady growth rates are displaced to the south. Some implications are discussed, including the need for caution when diagnosing changes in cyclone properties from changes in Eady growth rate calculated directly from the time-mean flow in climate change model simulations.

The Role of Air–Sea Interaction for Prediction of Australian Summer Monsoon Rainfall

AbstractForecast skill for seasonal mean rainfall across northern Australia is lower during the summer monsoon than in the premonsoon transition season based on 25 years of hindcasts using the Predictive Ocean Atmosphere Model for Australia (POAMA) coupled model seasonal forecast system. The authors argue that this partly reflects an intrinsic property of the monsoonal system, whereby seasonally varying air–sea interaction in the seas around northern Australia promotes predictability in the premonsoon season and demotes predictability after monsoon onset. Trade easterlies during the premonsoon season support a positive feedback between surface winds, SST, and rainfall, which results in stronger and more persistent SST anomalies to the north of Australia that compliment the remote forcing of Australian rainfall from El Nino in the Pacific. After onset of the Australian summer monsoon, this local feedback is not supported in the monsoonal westerly regime, resulting in weaker SST anomalies to the north of Au...

Seasonal Forecasting in the Pacific Using the Coupled Model POAMA-2

AbstractThe development of a dynamical model seasonal prediction service for island nations in the tropical South Pacific is described. The forecast model is the Australian Bureau of Meteorologys Predictive Ocean–Atmosphere Model for Australia (POAMA), a dynamical seasonal forecast system. Using a hindcast set for the period 1982–2006, POAMA is shown to provide skillful forecasts of El Nino and La Nina many months in advance and, because the model faithfully simulates the spatial and temporal variability of rainfall associated with displacements of the southern Pacific convergence zone (SPCZ) and ITCZ during La Nina and El Nino, it also provides good predictions of rainfall throughout the tropical Pacific region. The availability of seasonal forecasts from POAMA should be beneficial to Pacific island countries for the production of regional climate outlooks across the region.