Closing the Time-Varying Mass and Heat Budgets for Large Ocean Areas: The Tasman Box
Abstract The role of oceanic advection in seasonal-to-interannual balances of mass and heat is studied using a 12-yr time series of quarterly eddy-resolving expendable bathythermograph (XBT) surveys around the perimeter of a region the authors call the Tasman Box in the southwestern Pacific. The region contains the South Pacific’s subtropical western boundary current system and associated strong mesoscale variability. Mean geostrophic transport in the warm upper ocean (temperature greater than 12°C) is about 3.8 Sv (1 Sv ≡ 106 m3 s−1) southward into the box across the Brisbane, Australia–Fiji northern edge. Net outflows are 3.3 Sv eastward across the Auckland, New Zealand–Fiji edge, and 2.7 Sv southward across Sydney, Australia–Wellington, New Zealand. Mean Ekman convergence of 2.2 Sv closes the mass budget. Net water mass conversions in the upper ocean consist of inflow of waters averaging about 26°C and 35.4 psu balanced by outflow at about 18°C and 35.7 psu, and reflect the net evaporation and heat loss in the formation of South Pacific Subtropical Mode Water. The mean heat balance shows good agreement between ocean heat flux convergence (42.3 W m−2), heat loss to the atmosphere from the NCEP–NCAR reanalysis (39.2 W m−2), and heat storage calculated from data in the box interior (1.3 W m−2). On interannual time scales, volume transport through the box ranges from about 1 to 9 Sv, with heat flux convergence ranging from about 20 to 60 W m−2. An interannual balance in the heat budget of the warm layer is achieved to within about 10 W m−2 (or 6 W m−2 for the upper 100 m alone). Maxima in the advective heat flux convergence occurred in 1993, 1997, and 1999–2000, and corresponded to maxima in air–sea heat loss. The evolution of surface-layer temperature in the region is the residual of nearly equal and opposing effects of ocean heat flux convergence and air–sea exchange. Hence, ocean circulation is a key element in the interannual heat budget of the air–sea climate system in the western boundary current region.
