Ramasamy Suppiah

Changes in the Probability of Heavy Precipitation: Important Indicators of Climatic Change

A simple statistical model of daily precipitation based on the gamma distribution is applied to summer (JJA in Northern Hemisphere, DJF in Southern Hemisphere) data from eight countries: Canada, the United States, Mexico, the former Soviet Union, China, Australia, Norway, and Poland. These constitute more than 40% of the global land mass, and more than 80% of the extratropical land area. It is shown that the shape parameter of this distribution remains relatively stable, while the scale parameter is most variable spatially and temporally. This implies that the changes in mean monthly precipitation totals tend to have the most influence on the heavy precipitation rates in these countries. Observations show that in each country under consideration (except China), mean summer precipitation has increased by at least 5% in the past century. In the USA, Norway, and Australia the frequency of summer precipitation events has also increased, but there is little evidence of such increases in any of the countries considered during the past fifty years. A scenario is considered, whereby mean summer precipitation increases by 5% with no change in the number of days with precipitation or the shape parameter. When applied in the statistical model, the probability of daily precipitation exceeding 25.4 mm (1 inch) in northern countries (Canada, Norway, Russia, and Poland) or 50.8 mm (2 inches) in mid-latitude countries (the USA, Mexico, China, and Australia) increases by about 20% (nearly four times the increase in mean). The contribution of heavy rains (above these thresholds) to the total 5% increase of precipitation is disproportionally high (up to 50%), while heavy rain usually constitutes a significantly smaller fraction of the precipitation events and totals in extratropical regions (but up to 40% in the tropics, e.g., in southern Mexico). Scenarios with moderate changes in the number of days with precipitation coupled with changes in the scale parameter were also investigated and found to produce smaller increases in heavy rainfall but still support the above conclusions. These scenarios give changes in heavy rainfall which are comparable to those observed and are consistent with the greenhouse-gas-induced increases in heavy precipitation simulated by some climate models for the next century. In regions with adequate data coverage such as the eastern two-thirds of contiguous United States, Norway, eastern Australia, and the European part of the former USSR, the statistical model helps to explain the disproportionate high changes in heavy precipitation which have been observed.

Trends in total rainfall, heavy rain events and number of dry days in Australia, 1910–1990

Trends in heavy rainfall, total rainfall and number of dry days in Australia have been analysed using daily rainfall records at 125 stations. Summer and winter halves of the year were considered separately for the period 1910–1990. The summer half-year is defined as November–April, while the winter-half is May–October. Heavy rainfall is defined as the 90th and 95th percentiles of daily rainfall in each half-year. The magnitude of trends was derived from linear regression while statistical significance was determined by Kendall-Tau and field significance tests. Increasing trends in heavy rainfall and total rainfall have occurred during the summer half-year, but only 10–20% of stations have statistically significant trends. During the winter half-year, heavy rainfall and total rainfall have also increased, except in far southwest Western Australia and inland Queensland. There has been a reduction in the number of dry days in both halves of the year, except in far southwest Western Australia and at a few stations in eastern Australia where there has been an increase in the number of dry days in the winter half-year. Changes in the number of dry days were statistically significant at over 50% of stations. Hence there are regions showing coherent increases and decreases in rainfall which may be due to systematic changes in climate during the last century. Trends were averaged over three broad regions with adequate station coverage. There has been a general decrease in dry days with an increase in total and heavy rainfall intensity in the northeast and southeast, and a decrease in total and heavy rainfall in the southwest. These rainfall changes are related to changes in other climate variables such as temperature and cloud cover in Australia.

Changes in Climate Extremes Over the Australian Region and New Zealand During the Twentieth Century

Analyses of high quality data show that there have been some interesting recent changes in the incidence of some climate extremes in the Australian region and New Zealand.

Spatial and temporal variations in the relationships between the Southern oscillation phenomenon and the rainfall of Sri Lanka

Changes in temporal and spatial relationships between rainfall of Sri Lanka and the Southern Oscillation Index (SOI) during the 110 year period (1881–1990) are presented. The relationships during the first intermonsoon (FIM) and south-west monsoon (SWM) seasons indicate periods of weak positive and negative correlations. On the basis of major turning points in the summer rainfall over India and Sri Lanka, four distinct epochs were identified which cover the years 1881–1900, 1901–1930, 1931–1960 and 1961–1990. Indian and Sri Lankan summer rainfall shows a weak in-phase relationship from 1881 to 1900, a strong in-phase relationship from 1961 to 1990, and an out-of-phase relationship from 1901 to 1960. The correlation between the SOI and SWM seasons rainfall is primarily negative from 1931 to 1960 and positive during other epochs. Strong negative correlations are generally present for the SIM season and strengthening and weakening of negative correlations during this season coincide with changes in the SWM season rainfall and its relationships with the SOI. The north-east monsoon (NEM) season correlations are negative and insignificant. Major changes in spatial patterns of correlations between seasonal rainfall and the SOI have occurred in Sri Lanka during SWM and SIM seasons. The periods of strong positive (negative) correlations during the SWM season coincide with weak (strong) negative correlations during the SIM season. This contrasting pattern is clear when the Indian and Sri Lankan summer monsoon rainfalls were out of phase between 1900 and 1960, but not before 1900, or after 1960. The sudden change in correlations around 1960 suggests a change in the coupled ocean–atmosphere system that dominates the climate of these regions. Changes in temporal and spatial patterns of correlations between the SOI and rainfall have been linked to changes in the location of the active intertropical convergence zone.

EXTREMES OF THE SOUTHERN OSCILLATION PHENOMENON AND THE RAINFALL OF SRI LANKA

Influences of extreme phases of the Southern Oscillation (SO) phenomenon, El Nino and La Nina events, on the seasonal rainfall of Sri Lanka are examined by using composite maps of seasonal rainfall and sea-surface temperature (SST) anomalies. There were 27 El Nino and 22 La Nina events, during the period from 1881 to 1990. Positive and negative rainfall anomalies during the south-west monsoon (SWM) season are associated with La Nina and El Nino events, but negative and positive rainfall anomalies are linked to La Nina and El Nino events during the second intermonsoon (SIM) season. These contrasting patterns are dominant in the dry zone of Sri Lanka. Rainfall anomalies during first intermonsoon (FIM) and north-east monsoon (NEM) seasons do not show clear contrasting patterns as in other seasons and show positive and negative values. On the basis of wettest 20 per cent, mid-20 per cent and driest 20 per cent of years of seasonal total rainfall, composite maps of SST anomalies over the Pacific and Indian Oceans were made. As in rainfall patterns, SST anomalies during FIM and NEM seasons do not show clear contrasts between El Nino and La Nina events. During the SWM season, wet (dry) years are associated with negative (positive) SST anomalies over central and eastern Pacific and west Indian Oceans, but opposite SST anomalies are found over the ‘maritime continent’. During the SIM season, wet (dry) years are associated with positive (negative) SST anomalies over central Pacific and west Indian Oceans and opposite SST anomalies over the ‘maritime continent’. Based on the results of this study and previous studies on synoptic circulation patterns, and the dominance of the intraseasonal oscillation, a plausible explanation is given for larger anomalies during the SWM and SIM seasons in Sri Lanka.

Climatic Background to Past and Future Floods in Australia

Publisher Summary This chapter discusses climatic background and future floods in Australia. Floods in the Murray–Darling Basin (MDB) can be due to local severe storms leading to flash flooding, or to heavy rains from much larger scale systems leading to basis-wide great floods that can take months to travel downstream. The Basin is subject to large scale rain events from diverse synoptic origins, mainly tropical lows in the summer half year in the northern region and fronts and cutoff lows in the southern sector during the winter half year. Great year-to-year variability is associated with fluctuations in the El Nino–Southern Oscillation (ENSO) regime, and there have been marked interdecadal variations in rainfall in the past, some of which have occurred abruptly. Changes due to the enhanced greenhouse effect will include higher temperatures and increased potential evaporation, and likely increases in rainfall intensity associated with major storm systems, including cutoff lows of both tropical and midlatitude origin. Annual mean rainfall is generally expected to increase in northern regions of the MDB, especially inland, but to decrease in the southernmost areas south of the Murray River. Changes in rainfall intensity in models are scale-dependent, with finer resolution models generally predicting larger increases. More work is needed to resolve these uncertainties, and to apply flood routing to rainfall scenarios.

The Australian summer monsoon: CSIRO9 GCM simulations for 1 × CO2 and 2 × CO2 conditions

In this study, the CSIRO9 general circulation model (GCM) simulated temperature, mean sea level pressure (MSLP), circulation features at lower and upper levels of the atmosphere, and interannual variations of monsoon trough and precipitation during the months from December to February are compared with observed data. The study domain includes tropical Australia and the Indonesian region. Changes in monsoon circulation features and in precipitation under 2 × CO2 conditions are also discussed based on the differences between 2 × CO2 and 1 × CO2 experiments. The CSIRO9 GCM simulates many of the observed large-scale and synoptic-scale circulation features of the Australian summer monsoon. It also captures the seasonality of rainfall. However, the model fails to simulate the correct magnitude of the observed wind and the precipitation. It also underestimates the interannual variability of precipitation during the monsoon season. Under a 2 × CO2 scenario, the simulated Australian monsoon circulation is strengthened and precipitation is increased by about 20%, but the interannual variability of precipitation remains unchanged in north Australia. Similarly in central Australia, the model simulates a greater increase in rainfall under 2 × CO2 conditions, but does not show a significant change in variability. Results of the CSIRO9 simulations revealed that on average the monsoon shear line moves toward central Australia under 2 × CO2 conditions, but such a movement is not statistically significant and also shows little variation on interannual time scales. The monsoon shear line does not show any changes under 2 × CO2 conditions in its location over oceanic regions off the northwest and northeast coasts of Australia.