Ocean heat transport in Simple Ocean Data Assimilation: Structure and mechanisms

摘要

The trend and variability of global ocean heat transport for the period 1958-2004 are investigated using the Simple Ocean Data Assimilation (SODA) analysis. The ocean model is forced with the European Center for Medium Range Weather Forecast (ECMWF) ERA-40 atmospheric reanalysis winds from 1958 to 2001 and with QuikSCAT winds from 2002 to 2004. The assimilation is based on a sequential estimation algorithm, with observations from the historical archive of hydrographic profiles supplemented by ship intake measurements, moored hydrographic observations and remotely sensed sea surface temperature. Heat transport is calculated using temperature and velocity from the ocean analysis. Mean heat transport from the analysis generally agrees with previously published estimates from observational and modeling studies. Trends of heat transport show a range of behaviors. In the Atlantic and Pacific Oceans there is mostly increasing poleward heat transport with two important exceptions. In the Atlantic Ocean there is decreasing heat transport around 50°N and 60°N, and in both the Atlantic and Pacific Oceans there is decreasing heat transport near 10°S. There is also prominent interannual and decadal variability in all of the ocean basins. The results suggest that ocean heat transport variability is primarily determined by the strength of the meridional overturning circulation (MOC), which is controlled by complex processes governing fresh water flux in the northern North Atlantic and surface wind stress. However, the role of temperature variability increases at high latitude, particularly in the northern North Atlantic Ocean. Eddies play an important role in heat transport in the Gulf Stream and its extension in the Atlantic Ocean, and the Kuroshio and its extension in the Pacific Ocean and enhanced Subtropical cells (STCs) affect heat transport estimates in the tropics. In the northern North Atlantic Ocean, a small increase in meridional heat transport and a slight weakening of MOC are detected. Weakening in the northern North Atlantic MOC mainly arises from a freshening in the Labrador Sea and slowdown of the overflows from the Nordic Seas into the northern North Atlantic Ocean. Trends in North Atlantic surface momentum forcing are uniform across several atmospheric reanalyses, however there is less agreement in the role of precipitation in forcing trends of MOC and this exists as a primary source of uncertainty in our analysis.