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Variability of the Oceanic Mixed Layer 1960-2004 James A. Carton, Semyon A. Grodsky, and Hailong Liu November 30, 2006 Submitted to the Journal of Climate Department of Atmospheric and Oceanic Science University of Maryland, College Park, MD 20742 Corresponding author: [email protected] 0
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ABSTRACT A new monthly uniformly gridded analysis of mixed layer properties based on the World Ocean Atlas 2005 global ocean data set is used to examine subseasonal changes in mixed layer properties during the 45-year period 1960-2004. The analysis 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. Years with deep winter-spring mixed layers coincide with years in which winter- spring SST is low, etc. 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. Interestingly, at decadal periods the winter-spring mixed layers of both basins show trends, deepening by 10-40m over the 45-year period of this analysis. At tropical latitudes the boreal winter mixed layer varies in phase with the Southern Oscillation Index, deepening in the eastern Pacific and shallowing in the western Pacific and eastern Indian Oceans during El Niños. In contrast, in boreal summer the mixed layer in the Arabian Sea region of the western Indian Ocean varies in response to changes in the strength of the Southwest Monsoon. 1
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1. Introduction The oceanic mixed layer provides a connection between atmosphere and ocean and thus plays a central role in climate variability. For example, recent studies suggest that changes in the maximum depth of the mixed layer from one winter to the next may explain the reemergence of sea surface temperature (SST) anomalies and thus persistence of wintertime SST patterns ( Timlin et al., 2002; Deser et al., 2003) . Here we exploit the availability of a newly expanded archive of profile observations to determine the spatial and temporal structure of mixed layer depth variability during the 45-year period 1960-2004. Our goal is to document these changes to the extent possible given the limitations of the historical observational record. In the extratropics mixed layers undergo large seasonal depth variations as a result of seasonally varying balances in the mixed layer heat and salt budgets. Summer conditions of high sunlight and mild winds produce shallow, strongly stratified mixed layers. The maximum mixed layer depths (MLDs), in excess of 100m at 40 o N, occur in winter and early spring as the result of reductions in surface buoyancy flux and increases in turbulent mixing (e.g. Monterey and Levitus , 1997; Kara et al ., 2002; and
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  • Spring '17
  • william james
  • The Land, Atlantic Ocean, Pacific Ocean, North Pacific Ocean, North Pacific, mixed layer

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