On the Observed Trends and Changes in Global Sea Surface Temperature and Air–Sea Heat Fluxes (1984–2006)
Abstract Global satellite observations show the sea surface temperature (SST) increasing since the 1970s in all ocean basins, while the net air?sea heat flux Q decreases. Over the period 1984?2006 the global changes are 0.28°C in SST and ?9.1 W m?2 in Q, giving an effective air?sea coupling coefficient of ?32 W m?2 °C?1. The global response in Q expected from SST alone is determined to be ?12.9 W m?2, and the global distribution of the associated coupling coefficient is shown. Typically, about one-half (6.8 W m?2) of this SST effect on heat flux is compensated by changes in the overlying near-surface atmosphere. Slab Ocean Models (SOMs) assume that ocean heating processes do not change from year to year so that a constant annual heat flux would maintain a linear trend in annual SST. However, the necessary 6.1 W m?2 increase is not found in the downwelling longwave and shortwave fluxes, which combined show a ?3 W m?2 decrease. The SOM assumptions are revisited to determine the most likely source of the inconsistency with observations of (?12.9 + 6.8 ? 3) = ?9.1 W m?2. The indirect inference is that diminished ocean cooling due to vertical ocean processes played an important role in sustaining the observed positive trend in global SST from 1984 through 2006, despite the decrease in global surface heat flux. A similar situation is found in the individual basins, though magnitudes differ. A conclusion is that natural variability, rather than long-term climate change, dominates the SST and heat flux changes over this 23-yr period. On shorter time scales the relationship between SST and heat flux exhibits a variety of behaviors.