Estimation of heat and freshwater transports in the North Pacific using high-resolution expendable bathythermograph data

The mean heat and freshwater transports in the North Pacific subtropical gyre during 1998-2002 are estimated. High-resolution expendable bathythermograph/expendable conductivity-temperature-depth (XBT/XCTD) transects (PX-40, Honolulu to Japan; PX-37, San Francisco to Honolulu; PX-10, Honolulu to Guam; PX-44, Guam to Taiwan/Hong Kong) are used to calculate geostrophic transport across each of the ship tracks. Ekman transport is estimated from satellite-scatterometer wind stress. The mean heat and freshwater transport convergences into the northern box bounded by the PX-40/37 transects and the Tsushima and Bering Straits are 0.26 ± 0.16 pW (pW = 10¹⁵ W) and -0.26 ± 0.11 Sv (Sv = 10⁶ m³/s), respectively. Heat and freshwater transport convergences into the western box bounded by the PX-40/10/44 transects and the Tsushima Strait are estimated to be 0.32 ± 0.17 pW and 0.08 ± 0.07 Sv, respectively. In both boxes, warmer waters transported inward by the Ekman flow and by the Kuroshio are compensated by the export of waters at cooler temperatures, whose peaks are found in the temperatures of the mode waters formed in the North Pacific. The salt budget is also described to consider the mechanisms of freshwater transport. Since the western box includes the region with the strongest heat loss to the atmosphere and is possibly a key region for climatic decadal variation, it is necessary to continue the high-resolution XBT/XCTD measurement and to make an effort at improving the estimation of heat and freshwater transports in order to contribute to advancing climate studies.