Elements of tropical Pacific decadal variability

Abstract

The evolution of decadal variability in the tropical Pacific is investigated using a global assimilation reanalysis. At the nexus of this study are monthly means of an ocean general circulation model coupled with a data assimilation routine of the period from 1965 to 1999. Model results show Pacific basin decadal changes, including the prominent 1976-77 climate shift, in agreement with previous research. The reanalysis indicates an evolution intrinsic to the tropical Pacific which propagates the decadal signal from the Southern Hemisphere onto the equator, reversing the phase in the eastern tropical Pacific, and subsequently moves into the Northern Hemisphere primarily west of the dateline. Oceanic pathways, most significantly in the Southern Hemisphere, advect spiciness anomalies of the opposite sign to the cold tongue sea surface temperature anomalies away from the outcropping area. After reaching the equator spiciness anomalies are transferred eastward by the equatorial undercurrent and have a potential to change the background state of the eastern tropical Pacific. Simultaneously, a redistribution of atmospheric vorticity in the western tropical Pacific affects isopycnal depth and therefore ocean thermal structure progressing the decadal change from the Southern to the Northern Hemisphere. In this scenario both elements of spiciness and isopycnal surface depth anomalies are important for the evolution and maintaining of decadal variability.