CLIMATE PREDICTABILITY EXPERIMENTS WITH A GENERAL-CIRCULATION MODEL

The atmospheric response to the evolution of the global sea surface te mperatures from 1979 to 1992 is studied using the Max-Planck-Institut 19 level atmospheric general circulation model, ECHAM3 at T 42 resolut ion. Five separate 14-year integrations are performed and results are presented for each individual realization and for the ensemble-average d response. The results are compared to a 30-year control integra tion using a climate monthly mean state of the sea surface temperatures an d to analysis data. It is found that the ECHAM3 model, by and large, d oes reproduce the observed response pattern to El Nino and La Nina. Du ring the El Nino events, the subtropical jet streams in both hemispher es are intensified and displaced equatorward, and there is a tendency towards weak upper easterlies over the equator. The Southern Oscillati on is a very stable feature of the integrations and is accurately repr oduced in all experiments. The inter-annual variability at middle- and high-latitudes, on the other hand. is strongly dominated by chaotic d ynamics, and the tropical SST forcing only modulates the atmospheric c irculation. The potential predictability of the model is investigated for six different regions. Signal to noise ratio is large in most part s of the tropical belt, of medium strength in the western hemisphere a nd generally small over the European area. The ENSO signal is most pro nounced during the boreal spring. A particularly strong signal in the precipitation field in the extra tropics during spring can be found ov er the southern United States. Western Canada is normally warmer durin g the warm ENSO phase, while northern Europe is warmer than normal dur ing the ENSO cold phase. The reason is advection of warm air due to a more intense Pacific low than normal during the warm ENSO phase and a more intense Icelandic low than normal during the cold ENSO phase, res pectively.