Abstract.A new method is introduced for determining ocean isothermal layer depth (ILD) from temperature profiles and ocean mixed layer depth (MLD) from density profiles that can be applied in all regions of the world's oceans. This method can accommodate not only in situ data but also climatological data sets that typically have much lower vertical resolution. The sensitivity of the ILD and MLD to the temperature difference criteria used in the surface layer depth definition is discussed by using temperature and density data, respectively: (1) from 11 ocean weather stations in the northeast Pacific and (2) The MLD definition presented in this investigation accurately represents the depth to which turbulent mixing has penetrated and would be a useful aid for validation of one-dimensional bulk mixed layer models and ocean general circulation models with an embedded mixed layer.

[1] The spatial and monthly variability of the climatological mixed layer depth (MLD) for the global ocean is examined using the recently developed Naval Research Laboratory (NRL) Ocean Mixed Layer Depth (NMLD) climatologies. The MLD fields are constructed using the subsurface temperature and salinity data from the World Ocean Atlas 1994 . To minimize the limitations of these global data in the MLD determination, a simple mixing scheme is introduced to form a stable water column. Using these new data sets, global MLD characteristics are produced on the basis of an optimal definition that employs a densitybased criterion having a fixed temperature difference of ÁT = 0.8°C and variable salinity. Strong seasonality of MLD is found in the subtropical Pacific Ocean and at high latitudes, as well as a very deep mixed layer in the North Atlantic Ocean in winter and a very shallow mixed layer in the Antarctic in all months. Using the climatological monthly MLD and isothermal layer depth (ILD) fields from the NMLD climatologies, an annual mean ÁT field is presented, providing criteria for determining an ILD that is approximately equivalent to the optimal MLD. This enables MLD to be determined in cases where salinity data are not available. The validity of the correspondence between ILD and MLD is demonstrated using daily averaged subsurface temperature and salinity from two moorings: a Tropical Atmosphere Ocean array mooring in the western equatorial Pacific warm pool, where salinity stratification is important, and a Woods Hole Oceanographic Institute (WHOI) mooring in the Arabian Sea, where strongly reversing seasonal monsoon winds prevail. In the western equatorial Pacific warm pool the use of ILD criterion with an annual mean ÁT value of 0.3°C yields comparable results with the optimal MLD, while large ÁT values yield an overestimated MLD. An analysis of ILD and MLD in the WHOI mooring show that use of an incorrect ÁT criterion for the ILD may underestimate or overestimate the optimal MLD. Finally, use of the spatial annual mean ÁT values constructed from the NMLD climatologies can be used to estimate the optimal MLD from only subsurface temperature data via an equivalent ILD for any location over the global ocean.INDEX TERMS: 4227 Oceanography: General: Diurnal, seasonal, and annual cycles; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4572 Oceanography: Physical: Upper ocean processes; 4599 Oceanography: Physical: General or miscellaneous; KEYWORDS: mixed layer, isothermal layer, seasonal cycle, temperature, salinity, verification Citation: Kara, A. B., P. A. Rochford, and H. E. Hurlburt, Mixed layer depth variability over the global ocean,

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