The influence of temperature and salinity variability on the upper ocean density and mixed layer
R. W. Helber, J. G. Richman, and C. N. Barron
Naval Research Laboratory, Ocean Dynamics and Prediction Branch – Code 7321, Stennis Space Center, MS 39529, USA
Received: 04 Aug 2010 – Accepted for review: 06 Aug 2010 – Discussion started: 17 Aug 2010
Abstract. The relative influence of both temperature and salinity on the mixed layer depth (MLD) is evaluated using a relationship of binned regressions of MLD on vertical density compensation and isothermal layer depth (ILD) from a global set of in situ profile observations. Our approach is inspired by the observations of the difference between the MLD and the sonic layer depth (SLD) that evolve seasonally around the global ocean. In this article, we hypothesize that vertical density compensation governs SLD-MLD differences and can be used for mapping the relative influence of temperature and salinity on upper ocean structure. The Turner angle, computed between the surface and 200 m (bulk Turner angle, BTA), serves as a measure of vertical density compensation that quantifies times and areas where either temperature or salinity is destabilizing. For temperature destabilization the ocean exhibits cool/fresh overlying hot/salty water. For salinity destabilization the ocean exhibits hot/salty overlying cool/fresh water. These two classes of density compensation have seasonal variability with different geographical characteristics. Profiles with salinity controlled stable density and destabilizing temperature gradient are found most often at high latitudes. Profiles with temperature controlled stable density and destabilizing salinity gradient are found in the tropics and subtropics of all oceans. Results indicate that about half of the ocean has vertical density compensation that is a necessary condition for SLD-MLD differences. While density compensation is necessary, it is not a sufficient condition for predicting the dependence of MLD on BTA. Density compensation is the dominant factor in MLD variability in heavy river input and subduction regions that cover only ~14% of the ocean.
Helber, R. W., Richman, J. G., and Barron, C. N.: The influence of temperature and salinity variability on the upper ocean density and mixed layer, Ocean Sci. Discuss., 7, 1469-1495, doi:10.5194/osd-7-1469-2010, 2010.