Oral

IAPSO

JMP05 - Variability and change in Pacific Ocean-Atmosphere system

Importance of the meridional heat transport through the western boundary pathway of the Subtropical Cells in Tropical Pacific Decadal Variability

The decadal to interdecadal variability in the tropical Pacific region, so called Tropical Pacific Decadal Variability (TPDV), is known to have profound influence on global climate, making it essential to improve our understanding of its mechanisms. The shallow meridional overturning circulations, called the Subtropical Cells (STCs), are recognized as important elements of the TPDV, because they regulate the decadal variability of the heat content in the tropical Pacific by the changes of meridional heat transport through the tropics-subtropics boundaries. The variation of the STCs is suggested to include the horizontal circulation, consisting of the transport within the interior ocean (IN) far from the boundaries and the western boundary currents (BC), as well as the meridional overturning component consisting of the surface Ekman Transport (EKM) and the IN. Although most of the studies focus on the IN just as a part of overturning circulation, the contribution of the BC in the horizontal component of the STCs could have significant impacts on the heat transport across the subtropics-tropics boundaries. In this study, the role of the STCs in the TPDV is revisited in detail using outputs from an eddy resolving OGCM, with an emphasis on the contribution of the BC with respect to the meridional heat transport across 10N/S. The decadal variations of the meridional heat transport convergence by the STCs largely explain those of the ocean heat content in the tropical Pacific. Comparison of the heat transport variations among branches of the STCs, i.e. the EKM, IN and BC shows that a significant part of the IN is compensated by the BC in the Northern Hemisphere and about a half of the IN in the Southern Hemisphere. It turns out that the rate of compensation greatly varies over the study period depending on the compensation among the overturning components, particularly in the Northern Hemisphere. These results clearly demonstrate the importance of the BC for the quantitative assessment of the role of STCs in the TPDV.