Global Oceanic Mixed Layer Properties
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In this dissertation, the global oceanic mixed layer properties are explored in three aspects: variability of the oceanic mixed layer, subseasonal variability of the barrier layer/ compensated layer and comparison of bulk sea surface and mixed layer temperatures. (1) The analysis of variability of the oceanic mixed layer from 1960-2007 reveals substantial variability in the winter-spring depth of the mixed layer in the subtropics and midlatitudes. In the North Pacific an Empirical Orthogonal Eigenfunction analysis shows a pattern of mixed layer depth variability peaking in the central subtropics. This pattern occurs coincident with intensification of local surface winds and may be responsible for the SST changes associated with the Pacific Decadal Oscillation. In the North Atlantic a pattern of winter-spring mixed layer depth variability occurs that is not so obviously connected to local changes in winds or SST, suggesting that other processes such as advection are more important. Over the 48-year period the winter-spring mixed layers of both basins show deepening trends by 10-100m. (2) The strongest variability of barrier layer/ compensated layer from monthly climatology is found over 100m in the subpolar latitudes of the North Atlantic in winter. Compensated layers in the eastern North Atlantic vary interannually associated with a North Atlantic Oscillation-like pattern of anomalous sea level pressure. In the winter a barrier layer exists in the subpolar North Pacific, while further south along the Kuroshio extension a compensated layer exists, both of which have variability of up to 60m and a significant long-term trend (shrinkage of the barrier layer in the subpolar gyre and growth of the compensated layer to the south) . These changes are also associated with meteorological shifts. (3) Mixed layer temperature (MLTT) and sea surface temperature (SST) are frequently used interchangeably or assumed to be proportional in climate studies. Historical analyses of bulk SST and MLTT from contemporaneous ocean profile observations show that globally and time averaged MLTT is lower than SST by approximately 0.1 oC. In the upwelling zone of the Equatorial East Pacific this negative MLTT-SST difference varies out of phase with seasonal SST, but on interannual timescales MLTT-SST varies in phase with SST with small differences during El Niños as a result of low solar heating and enhanced rainfall. On shorter diurnal timescales, during El Niños, MLTT-SST differences associated with temperature inversions occur in response to nocturnal cooling in presence of nearsurface freshening. Near surface freshening produces persistent shallow (a few meters depth) warm layers in the northwestern Pacific during boreal summer when solar heating is strong. In contrast, shallow cool layers occur in the Gulf Stream area of the Northwest Atlantic in boreal winter when fresh surface layers developed due to lateral interactions are cooled down by abundant turbulent heat loss.