Temperature Versus Salinity Gradients Below the Ocean Mixed Layer
NAVAL RESEARCH LAB STENNIS DETACHMENT STENNIS SPACE CENTER MS OCEANOGRAPHY DIV
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We characterize the global ocean seasonal variability of the temperature versus salinity gradients in the transition layer just below the mixed layer using observations of conductivity temperature and depth and profiling float data from the National Ocean Data Center s World Ocean Data set. The balance of these gradients determines the temperature versus salinity control at the mixed layer depth MLD. We define the MLD as the shallowest of the isothermal, isohaline, and isopycnal layer depths ITLD, IHLD, and IPLD, each with a shared dependence on a 0.2 deg C temperature offset. Data are gridded monthly using a variational technique that minimizes the squared analysis slope and data misfit. Surface layers of vertically uniform temperature, salinity, and density have substantially different characteristics. By examining differences between IPLD, ITLD, and IHLD, we determine the annual evolution of temperature or salinity or both temperature and salinity vertical gradients responsible for the observed MLD. We find ITLD determines MLD for 63 and IHLD for 14 of the global ocean. The remaining 23 of the ocean has both ITLD and IHLD nearly identical. It is found that temperature tends to control MLD where surface heat fluxes are large and precipitation is small. Conversely, salinity controls MLD where precipitation is large and surface heat fluxes are small. In the tropical ocean, salinity controls MLD where surface heat fluxes can be moderate but precipitation is very large and dominant.
- Physical and Dynamic Oceanography