A quantitative assessment of changes in seasonal potential predictability for the twentieth century

Abstract

Changes over the twentieth century in seasonal mean potential predictability (PP) of global precipitation, 200 hPa height and land surface temperature are examined by using 100-member ensemble. The ensemble simulations have been conducted by using an intermediate complexity atmospheric general circulation model of the International Center for Theoretical Physics, Italy. Using the Hadley Centre sea surface temperature (SST) dataset on a 1 grid, two 31 year periods of 1920-1950 and 1970-2000 are separated to distinguish the periods of low and high SST variability, respectively. The standard deviation values averaged for the (''Nino-3.4''; 5S-5N, 170W-120W) region are 0.71 and 1.15 C, for the periods of low and high SST variability, respectively, with a percentage change of 62 % during December-January-February (DJF). The leading eigenvector and the associated principal compo- nent time series, also indicate that the amplitude of SST variations have positive trend since 1920s to recent years, particularly over the El Nino Southern Oscillation (ENSO) region. Our hypothesis states that the increase in SST variability has increased the PP for precipitation, 200 hPa height and land surface temperature during the DJF. The analysis of signal and noise shows that the signal-to-noise (S/N) ratio is much increased over most of the globe, particularly over the tropics and subtropics for DJF pre- cipitation. This occurs because of a larger increase in the signal and at the same time a reduction in the noise, over most of the tropical areas. For 200 hPa height, the S/N ratio over the Pacific North American (PNA) region is increas- ing more than that for the other extratropical regions, because of a larger percentage increase in the signal and only a small increase in noise. It is also found that the increase in seasonal mean transient signal over the PNA region is 50 %, while increase in the noise is only 12 %, during the high SST variability period, which indicates that the increase in signal is more than the noise. For DJF land surface temperature, the perfect model notion is utilized to confirm the changes in PP during the low and high SST variability periods. The correlation between the perfect model and the other members clearly reveal that the sea- sonal mean PP changed. In particular, the PP for the 31 years period of 1970-2000 is higher than that for the 31 years period of 1920-1950. The land surface tempera- ture PP is increased in northern and southern Africa, central Europe, southern South America, eastern United States and over Canada. The increase of the signal and hence the seasonal mean PP is coincides with an increase in tropical Pacific SST variability, particularly in the ENSO region.