Interannual and decadal variability in the tropical and midlatitude Pacific Ocean

Forty-four years of mechanical and expendable bathythermograph observations are assimilated into a general circulation model of the Pacific Ocean. The model is run from 1950 through 1993 with forcing at the surface from obser ved monthly mean wind stress and temperature. The resulting analysis is use d to describe the spatial and temporal patterns of variability at interannu al and decadal periods. Interannual variability has its largest surface tem perature expression in the Tropics and decadal variability has its largest surface temperature expression in the midlatitude Pacific. However, there a re important interannual surface temperature changes that occur in the midl atitude Pacific and there are important decadal surface temperature changes in the Tropics. An empirical orthogonal function (EOF) analysis of model data that has been bandpass filtered to retain energy at periods of 1-5 yr and at periods gre ater than 5 yr is presented. The results suggest that interannual variabili ty is dominated by a positive feedback mechanism in the Tropics and a negat ive feedback mechanism in the midlatitude ocean, resulting in larger anomal ies in the Tropics. A second EOF analysis of model data that has been low-p ass filtered to retain periods greater than 5 yr reveals patterns of wind a nd surface temperature anomalies that have strikingly similar structure to the interannual patterns; however, the sequencing between the first and sec ond EOFs is different. Even though there are large decadal anomalies of win d stress in the Tropics, the largest anomalies of surface temperature and o cean heat content occur at mid- and high latitudes. The EOF analysis indica tes that decadal variability has a positive feedback mechanism that operate s in the midlatitude ocean and a negative feedback mechanism that operates in the Tropics, so that the largest temperature anomalies are in midlatitud es. Previous studies have cited the contribution of heat flux anomalies as the primary cause of decadal surface temperature anomalies. These model stu dies indicate that meridional advection of heat is at least as important. T he timing of interannual and decadal changes in the atmosphere and in the o cean suggests that the atmosphere plays an important role connecting these phenomena. One interpretation of the results is that interannual and decada l variability are manifestations of the same climate phenomena but have cru cially different feedback mechanisms.