Sea level extremes in the Caribbean Sea

Sea level extremes in the Caribbean Sea are analyzed on the basis of hourly records from 13 tide gauges. The largest sea level extreme observed is 83 cm at Port Spain. The largest nontidal residual in the records is 76 cm, forced by a category 5 hurricane. Storm surges in the Caribbean are primarily caused by tropical storms and stationary cold fronts intruding the basin. However, the seasonal signal and mesoscale eddies also contribute to the creation of extremes. The five stations that have more than 20 years of data show significant trends in the extremes suggesting that flooding events are expected to become more frequent in the future. The observed trends in extremes are caused by mean sea level rise. There is no evidence of secular changes in the storm activity. Sea level return periods have also been estimated. In the south Colombian Basin, where large hurricane-induced surges are rare, stable estimates can be obtained with 30 years of data or more. For the north of the basin, where large hurricane-induced surges are more frequent, at least 40 years of data are required. This suggests that the present data set is not sufficiently long for robust estimates of return periods. ENSO variability correlates with the nontidal extremes, indicating a reduction of the storm activity during positive ENSO events. The period with the highest extremes is around October, when the various sea level contributors' maxima coincide

Decadal variability of European sea level extremes in relation to the solar activity

This study investigates the relationship between decadal changes in solar activity and sea level extremes along the European coasts and derived from tide gauge data. Autumn sea level extremes vary with the 11 year solar cycle at Venice as suggested by previous studies, but a similar link is also found at Trieste. In addition, a solar signal in winter sea level extremes is also found at Venice, Trieste, Marseille, Ceuta, Brest, and Newlyn. The influence of the solar cycle is also evident in the sea level extremes derived from a barotropic model with spatial patterns that are consistent with the correlations obtained at the tide gauges. This agreement indicates that the link to the solar cycle is through modulation of the atmospheric forcing. The only atmospheric regional pattern that showed variability at the 11 year period was the East Atlantic pattern

Interannual variations in precipitation: the effect of the North Atlantic and Southern oscillations as seen in a satellite precipitation data set and in models

Precipitation is a parameter that varies on many different spatial and temporal scales. Here we look at interannual variations associated with the North Atlantic Oscillation (NAO) and the Southern Oscillation (SO), comparing the spatial and temporal changes as shown by three data sets. The Global Precipitation Climatology Project (GPCP) product is based upon satellite data, whereas both the National Centers for Environmental Prediction (NCEP) and European Centre for Medium-Range Weather Forecasts (ECMWF) climatologies are produced through reanalysis of atmospheric circulation models. All three products show a consistent response to the NAO in the North Atlantic region, with negative states of the NAO corresponding to increases in precipitation over Greenland and southern Europe, but to a decrease over northern Europe. None of the climatologies display any net change in total rainfall as a result of the NAO, but rather a redistribution of precipitation patterns. However, this redistribution of rain is important because of its potential effect on oceanic overturning circulation. Similarly, all three data sets concur that the SO has a major effect on precipitation in certain tropical regions; however, there is some disagreement amongst the data sets as to the regional sensitivity, with NCEP showing a much weaker response than GPCP and ECMWF over Indonesia. The GPCP and NCEP climatologies show that the various phases of El Niño and La Niña act to redistribute, rather than enhance, the freshwater cycle. Given that the models incorporate no actual observations of rain, and are known to be imperfect, it is surprising how well they represent these interannual phenomena

Sea level extremes at the coasts of China

Hourly sea level records from 1954 to 2012 at twenty tide gauges at and adjacent to the Chinese coasts are used to analyse extremes in sea level and in tidal residual. Tides and tropical cyclones determine the spatial distribution of sea level maxima. Tidal residual maxima are predominantly determined by tropical cyclones. The 50-year return level is found to be sensitive to the number of extreme events used in the estimation. This is caused by the small number of tropical cyclone events happening each year which lead to other local storm events included thus significantly affecting the estimates. Significant increase in sea level extremes is found with trends in the range between 2.0-14.1 mm yr-1. The trends are primarily driven by changes in median sea level but also linked with increases in tidal amplitudes at three stations. Tropical cyclones cause significant interannual variations in the extremes. The interannual variability in the sea level extremes is also influenced by the changes in median sea level at the north and by the 18.6-year nodal cycle at the South China Sea. Neither of PDO and ENSO is found to be an indicator of changes in the size of extremes, but ENSO appears to regulate the number of tropical cyclones that reach the Chinese coasts. Global mean atmospheric temperature appears to be a good descriptor of the interannual variability of tidal residual extremes induced by tropical cyclones but the trend in global temperature is inconsistent with the lack of trend in the residuals

An intercomparison of global oceanic precipitation climatologies

Large-scale patterns of precipitation are important for the changes they may effect upon the circulation of the ocean. However, marine precipitation is very hard to quantify accurately. Four independent climatologies are examined to compare their estimates of the annual mean precipitation, and the seasonal and interannual variations. One data set, Global Precipitation Climatology Project (GPCP), is based upon satellite data, the other three on output of weather forecast reanalyses from the National Centers for Environmental Prediction (NCEP) and the European Centre for Medium-range Weather Forecasts (ECMWF). Although all datasets have their errors, there is general agreement on the geographical patterns of precipitation. All the models had higher rain rates in the tropics than shown by the satellite data, and also greater seasonal ranges. However, GPCP has 10-25% more precipitation than NCEP and ECMWF in most of the southern regions, because of their weak representation of convergence zones; NCEP2, a more recent version of the NCEP reanalysis, shows a marked improvement in this area. However, in most regions NCEP2 exhibits a larger seasonal range than shown by other datasets, particularly for the tropical Pacific. Both NCEP and NCEP2 often show a seasonal cycle lagging two months or more behind GPCP. Of the three reanalysis climatologies, ECMWF appears best at realising the position and migration of rain features. The interannual variations are correlated between all four datasets, however the correlation coefficient is only large for regions that have a strong response to El Niño and La Niña event, or for comparisons of the two NCEP reanalyses. Of the datasets evaluated, GPCP has the most internal consistency, with no long-term trend in its regional averages, and it alone shows the deficit in Mediterranean precipitation coincident with the Eastern Mediterranean Transient

Impact of the atmospheric climate modes on Mediterranean sea level variability

The relationships of Mediterranean sea level, its atmospherically driven and thermosteric components with the large scale atmospheric modes over the North Atlantic and Europe are explored and quantified. The modes considered are the North Atlantic Oscillation (NAO), the East Atlantic pattern (EA), the Scandinavian pattern (SCAN) and the East Atlantic/Western Russian (EA/WR). The influence of each mode changes between winter and summer. During winter the NAO is the major mode impacting winter Mediterranean sea level (accounting for 83% of the variance) with SCAN being the second (56%) mode in importance. Both NAO and SCAN effects are partly due to direct atmospheric forcing of sea level through wind and pressure changes. However NAO and SCAN are correlated with each other during winter and they explain the same part of variability. The EA/WR also affects the atmospheric sea level component in winter (13%), acting through atmospheric pressure patterns. In winter, the thermosteric contribution is correlated with the SCAN in parts of the Eastern Mediterranean (9%). The rate of change of the thermosteric component in winter is correlated with the EA (24%). During the summer season, the sea level variance is much reduced and the impact of the large scale modes is in most parts of the Mediterranean Sea non significant