Exchanges of heat, water and momentum (wind stress) at the sea surface are important factors for driving the ocean circulation. Changes in the air–sea fluxes may result from variations in the driving surface meteorological state variables (air temperature and humidity, SST, wind speed, cloud cover, precipitation) and can impact both watermass formation rates and ocean circulation. Air–sea fluxes also influence temperature and humidity in the atmosphere and, therefore, the hydrological cycle and atmospheric circulation. AR4 concluded that, at the global scale, the accuracy of the observations is insufficient to permit a direct assessment of changes in heat flux (AR4 Section 5.2.4). As described in Section 3.4.2, although substantial progress has been made since AR4, that conclusion still holds for this assessment.
The net air–sea heat flux is the sum of two turbulent (latent and sensible) and two radiative (shortwave and longwave) components. Ocean heat gain from the atmosphere is defined to be positive according to the sign convention employed here. The latent and sensible heat fluxes are computed from the state variables using bulk parameterizations; they depend primarily on the products of wind speed and the vertical near-sea-surface gradients of humidity and temperature respectively. The air–sea freshwater flux is the difference of precipitation (P) and evaporation (E). It is linked to heat flux through the relationship between evaporation and latent heat flux. Thus, when considering potential trends in the global hydrological cycle, consistency between observed heat budget and evaporation changes is required in areas where evaporation is the dominant term in hydrological cycle changes. Ocean surface shortwave and longwave radiative fluxes can be inferred from satellite measurements using radiative transfer models, or computed using empirical formulae, involving astronomical parameters, atmospheric humidity, cloud cover and SST. The wind stress is given by the product of the wind speed squared, and the drag coefficient. For detailed discussion of all terms see, for example, Gulev et al. (2010).
Atmospheric reanalyses, discussed in Box 2.3, are referred to frequently in the following sections and for clarity the products cited are summarised here: ECMWF 40-year Reanalysis (referred to as ERA40 hereafter, Uppala et al., 2005), ECMWF Interim Reanalysis (ERAI, Dee et al., 2011), NCEP/NCAR Reanalysis 1 (NCEP1, Kalnay et al., 1996), NCEP/DOE Reanalysis 2 (NCEP2, Kanamitsu et al., 2002), NCEP Climate Forecast System Reanalysis (CFSR, Saha et al., 2010), NASA Modern Era Reanalysis for Research and Applications (MERRA, Rienecker et al., 2011) and NOAA-CIRES 20th Century Reanalysis, version 2 (20CRv2, Compo et al., 2011).