Satellite Observations of Cool Ocean – Atmosphere Interaction

| H igh sea surface temperature (SST) is generally required for deep convection that reaches the tropopause. In the current climate, the SST threshold for deep convection is somewhere around 26°–27°C, depending upon region and season (Gra-ham and Barnett 1987; Waliser et al. 1993). Over such warm oceans, SST changes cause deep convective adjustment , which in turn excites the dynamic response of predominantly first baroclinic mode structure with strong surface wind signals. This robust relation between SST, deep convection, and wind has led to the rapid advance in understanding and modeling the tropical ocean–atmosphere interaction that gives rise to phenomena ranging from the El Niño–Southern Oscillation (ENSO; Neelin et al. 1998) to the northward displacement of the intertropical convergence zone Atmospheric general circulation models (GCMs) show considerable skills in simulating this deep adjustment to SST anomalies associated with ENSO (e.g., Alexander et al. 2002). Over cool oceans where deep convection does not occur, 1 the atmospheric adjustment to changing SST differs markedly from that over warm oceans. Cool ocean–atmosphere interaction is poorly understood, and this lack of understanding is a stumbling block in the current effort to study non-ENSO climate variability. Unlike their success in simulating the Southern Oscillation, atmospheric GCMs disagree among themselves in their atmospheric response to SST anomalies in the extratropics (see Kushnir et al. 2002 for a review). New observations from space reveal surprisingly robust patterns of air–sea coupling over cool oceans where such coupling has been thought to be weak. 1 While the SST threshold is a convenient way to divide the warm and cool regimes for air–sea interaction, local SST is not the only factor for deep convection, which is also influenced by other factors such as large-scale subsidence. It may be more physically appropriate to divide the warm and cold regimes according to whether there is significant deep convection or not.