The Effect of Ocean Currents on Sea Surface Temperature Anomalies

Abstract

Global and regional correlations between observed anomalies in sea surface temperature and height are investigated. A strong agreement between the two fields is found over a broad range of latitudes for different ocean basins. From a global frequency spectrum, a mean SST damping timescale of 2?3 months was found with westward and eastward propagating anomalies contributing more or less equally to the total variance. Time?longitude plots and wavenumber?frequency spectra show a significant advective forcing of SST anomalies by a first-mode baroclinic eddy or wave field on spatial scales down to 400 km and timescales as short as 1 month. A scaling analysis of forcing terms in a mixed layer temperature model suggests that Ekman terms are expected to be smaller in magnitude. Even though the magnitude of the mean background temperature gradient determines the effectiveness of the forcing, there is no obvious seasonality that can be detected in the amplitudes of SST anomalies. Instead, individual wavelike signatures in SST can be followed for several months, in some cases even up to two years. The phase lag between SST and SSH anomalies is dependent upon frequency and wavenumber and displays a general decrease toward higher latitudes where the two fields come into phase at low frequencies. Linear feedback coefficients, estimated for an intermediate-scale advective-forcing model, generally increase with latitude. They are much higher than previous estimates for atmospheric damping processes and suggest the importance of ocean mixing processes for the evolution of SST. Estimates of atmospheric feedback on small scales were obtained by matching simultaneous transient features in both SST and surface heat flux fields, yielding values of 30?40 W m?2 for several zonal sections in the North Pacific.