Naval Research Laboratory Mixed Layer Depth (NMLD) Climatologies

Abstract : Monthly isothermal layer depth (ILD) and mixed layer depth (MLD) fields for the global ocean are presented from the Naval Research Laboratory (NRL) Mixed Layer Depth (NMLD) climatologies. The ILD is defined using only temperature while the MLD is defined using a density-based criterion. These fields are constructed from the I -degree monthly mean climatologies of the World Ocean Atlas 1994 (WOA94) using a method for determining layer depth that can accommodate the wide variety of temperature and density profiles that occur within the global ocean. The MLD, constructed using a density criterion based on a 0.8 C temperature difference (AT) that also accounts for variable salinity, provides an optimal representation of the depth of the mixed layer. This optimal MLD is recommended as the most appropriate depth to use for purposes of model validation, mixed layer heat budgets, and ocean biology studies. The NMLD climatologies are used to examine the spatial and seasonal variability of the ILD and MLD for the latitudes 65N to 72S. Strong seasonality in MLD is found in the subtropical Pacific Ocean at high latitudes. The very deep mixed layer in the North Atlantic Ocean in winter is reproduced as well as the very shallow mixed layer in the Antarctic throughout the year. The correspondence between ILD and the optimal definition of MLD is also investigated, and maps of AT values are provided to enable the best ILD to be determined for any month and location in the global ocean. Large variations in the NMLD climatologies constructed using different criteria highlight the importance of using an optimally defined MLD climatology.

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