The Relationship of Western Boundary Current Heat Transport and Storage to Midlatitude Ocean-Atmosphere Interaction

Abstract

Much of the heat transported poleward by the oceans is carried in the midlatitude western boundary currents in the northern hemisphere. As these currents separate from the coastal boundaries and extend eastward into the ocean interior, they ∞ux some of their heat to the atmosphere and store some of their heat in the recirculation gyres south of the current core; the heat content anomalies are negatively correlated with changes in the volume of an isothermal layer known as the \subtropical mode water. An analysis of upper ocean heat content observations (1955{2001) shows that there are substantial interannual variations in the amount of heat stored in the upper 400 m of the water column. About 26% of the variations in heat content in the North Atlantic and North Paciflc Oceans (corresponding to the flrst principal component and with maxima in the western boundary current extension regions) are in phase and slightly lag the atmospheric Northern hemisphere Annular Mode (NAM or Arctic Oscillation). The simplest explanation, that changes in the westerlies cause corresponding changes in the air-sea ∞uxes and therefore in heat content, can be ruled out by by the sign of the correlation: strong westerlies (strong AO) are correlated with positive heat content anomalies. This conclusion is supported by previous analyses of the upper ocean heat budget, which show that the heat content anomalies are primarily caused by variations in ocean advection. The heat content anomalies, rather than being caused by changes in air-sea ∞uxes, instead appear to be the source of interannual variations in those ∞uxes. The magnitude of the ∞ux anomalies, their association with advection and heat storage in the mode water, and the coherence between the two oceans suggest a role for ocean circulation in interannual to decadal variations in climate variability through local air-sea interaction.