A COMPARISON BETWEEN GENERAL-CIRCULATION MODEL SIMULATIONS USING 2 SEA-SURFACE TEMPERATURE DATASETS FOR JANUARY 1979

Simulations with the UCLA atmospheric general circulation model (AGCM) using two different global sea surface temperature (SST) datasets for January 1979 are compared. One of these datasets is based on COADS (S STs) at locations where there are ship reports, and climatology elsewh ere; the other is derived from measurements by instruments onboard NOA A satellites. In the former dataset (COADS SST), data are concentrated along shipping routes in the Northern Hemisphere; in the latter datas et (HIRS SST), data cover the global domain. Ensembles of five 30-day mean fields are obtained from integrations performed in the perpetual- January mode. The results are presented as anomalies, that is, departu res of each ensemble mean from that produced in a control simulation w ith climatological SSTs. Large differences are found between the anoma lies obtained using COADS and HIRS SSTs, even in the Northern Hemisphe re where the datasets are most similar to each other. The internal var iability of the circulation in the control simulation and the simulate d atmospheric response to anomalous forcings appear to be linked in th at the pattern of geopotential height anomalies obtained using COADS S STs resembles the first empirical orthogonal function (EOF 1) in the c ontrol simulation. The corresponding pattern obtained using HIRS SSTs is substantially different and somewhat resembles EOF 2 in the sector from central North America to central Asia. To gain insight into the r easons for these results, three additional simulations are carried out with SST anomalies confined to regions where COADS SSTs are substanti ally warmer than HIRS SSTs. The regions correspond to warm pools in th e northwest and northeast Pacific, and the northwest Atlantic. These w arm pools tend to produce positive geopotential height anomalies in th e northeastern part of the corresponding oceans. Both warm pools in th e Pacific produce large-scale circulation anomalies with a pattern tha t resembles that obtained using COADS SSTs as well as EOF 1 of the con trol simulation; the warm pool in the Atlantic does not. These results suggest that the differences obtained with COADS SSTs and HIRS SSTs a re mostly due to the differences in the datasets over the northern Pac ific. There was a blocking episode near Greenland in late January 1979 . Both simulations with warm SST anomalies over the northwest and nort heast Pacific show a tendency toward increased incidence of North Atla ntic blocking; the simulation with warm SST anomalies over the northwe st Atlantic shows a tendency toward decreased incidence. These results suggest that features in both SST datasets that do not have a counter part in the other dataset contribute significantly to the differences between the simulated and observed fields. The results of this study i mply that uncertainties in current SST distributions for the world oce ans can be as important as the SST anomalies themselves in terms of th eir impact on the atmospheric circulation. Caution should be exercised , therefore, when linking anomalous circulation and SST patterns, espe cially in long-range prediction.