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A collective list of historical El Niño and La Niña events has been developed, based on an examination of different indices describing the El Niño‐Southern Oscillation (ENSO) phenomenon. Based on this list, tropical cyclone (TC) data from a newly created TC archive for the Southern Hemisphere (SH) have been stratified accordingly and significant changes in TC occurrences depending on warm or cold phases of ENSO have been identified. TC trends in the SH (area south of the equator, 30°E to 120°W) have been examined. For the 1981/82 to 2005/06 TC seasons, there are no apparent trends in the total numbers and cyclone days of TCs, nor in numbers and cyclone days of severe TCs with minimum central pressure of 970 hPa or lower. However, significant positive trends in occurrences and cyclone days of severe TCs with minimum central pressure of 945 hPa or lower have been identified.

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SUMMARYMajor oods occurred in the United Kingdom during autumn 2000. These were caused by a rapid sequence of heavy rainfall events that occurred over a period of many weeks leading to record-breaking monthly-to-seasonal rainfall totals. The question was raised as to whether such rainfall events may be related to human-induced climate change.Climate-model predictions of future changes in mean precipitation behaviour are well established. However, to understand ooding requires an examination of predictions of extreme rainfall behaviour at a relatively small spatial scale. For three areas within the United Kingdom, output from a Hadley Centre regional climate model, 'nested' within one of its general-circulation models, is compared with raingauge data averaged over these areas for the period . This shows that the modelling system is good at predicting the statistical likelihood of extreme rainfall events seen in historical data. This result holds for extreme rainfall totals over daily to monthly timescales.When the modelling system is used to predict changes in these extreme events resulting from atmospheric CO 2 concentrations that may be representative of the period 2080-2100, signi cant reductions in the return periods of such events are seen. For example, 30-day rainfall totals, which happened in the recent past on average once in 20 years, are predicted to happen once in 3-5 years. An interpolation method based upon climate-model output and incorporating raingauge data is used to estimate how rainfall extremes may have changed between the middle of the 19th century, and for a period centred on the year 2000. This also predicts that increased greenhouse gases have led to reduced return periods of extreme rainfall events for three sites of interest, though in this case the changes are not statistically signi cant.

Estuaries around Great Britain may be at heightened risk of flooding because of the simultaneous occurrence of extreme sea surge and river flow, both of which may be caused by mid-latitude cyclones. A measure especially suited for extremes was employed to estimate dependence between river flow and sea surge. To assist in the interpretation of why flow-surge dependence occurs in some areas and not in others, the dependence between precipitation and surge and between precipitation and river flow was also studied. Case studies of the meteorological situations leading to high surges and/or river flows were also carried out. The present study concerns catchments draining to the south and west coasts of Great Britain. Statistically significant dependence between river flow and daily maximum sea surge may be found at catchments spread along most of this coastline. However, higher dependence is generally found in catchments in hilly areas with a southerly to westerly aspect. Here, precipitation in south-westerly airflow, which is generally the quadrant of prevailing winds, will be enhanced orographically as the first higher ground is encountered. The sloping catchments may respond quickly to the abundant rainfall and the flow peak may arrive in the estuary on the same day as a large sea surge is produced by the winds and low atmospheric pressure associated with the cyclone. There are three regions where flow-surge dependence is strong: the western part of the English south coast, southern Wales and around the Solway Firth. To reduce the influence of tide-surge interaction on the dependence analysis, the dependence between river flow and daily maximum surge occurring at high tide was estimated. The general pattern of areas with higher dependence is similar to that using the daily maximum surge. The dependence between river flow and daily maximum sea surge is often strongest when surge and flow occur on the same day. The west coast from Wales and northwards has slightly stronger flow-surge dependence in summer than in winter, whereas dependence is stronger in winter than in summer for the southern part of the study area.

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