Mixed layer depth variability over the global ocean

Abstract

[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,

Multidecadal-scale adjustment of the ocean mixed layer heat budget in the tropics: examining ocean reanalyses

“…Toyoda et al (2015) show that monthly mean ocean mixed layer depths from de Boyer Montegut et al (2004) are more shallow than individual observed profiles due to rapid restratification in spring and vertical resolution issues. The Naval Research Lab (Kara et al 2003) annual mean mixed layer depth climatology is shown in Fig. 1f.…”

“…, and e density with a variable threshold criterion equivalent to a 0.2 °C decrease (DReqDTm02). f Naval Research Lab (Kara et al 2003) annual mean mixed layer depth climatology constructed using monthly mean temperature and salinity climatologies from the World Ocean Atlas (Levitus et al 1994) and a density criterion Fig. 2 Climatologies F NET , the net heat flux from the atmosphere to the ocean, from the a GODAS, b GECCO2, c ORA-S3, and d ORA-S4 ocean reanalyses, and the e NCEP2, f NCEP1, g ERAI, and h JRA-55 atmospheric reanalyses.…”

Comparison of mixed layer depth and barrier layer thickness for the Indian Ocean using two different climatologies

“…Hence, to improve our understanding of the climate system in a better perspective, MLD climatology is an essential pre-requisite to the ocean modelling community. This information also supplements to improve existing parameterization schemes in ocean general circulation models (Kara, 2003;de Boyer et al, 2004). Several researchers (Monterey and Levitus, 1997;Kara et al, 2003) have attempted to study MLD climatology of the global oceans using the world ocean atlas (WOA).…”

“…This information also supplements to improve existing parameterization schemes in ocean general circulation models (Kara, 2003;de Boyer et al, 2004). Several researchers (Monterey and Levitus, 1997;Kara et al, 2003) have attempted to study MLD climatology of the global oceans using the world ocean atlas (WOA). Studies utilizing Array for Real-time Geostrophic Oceanography (ARGO) data was made by de Boyer et al (2004) and as such, studies for north Indian Ocean utilizing WOA and ARGO was attempted by Thadathil et al, (2007Thadathil et al, ( , 2008.…”

Eos, Transactions, American Geophysical Union Volume 88, Number 26, 26 June 2007

“…Using equation (1) [Kara et al, 2003]. My calculations show that this small change in aerosol optical thickness produces a cooling of the ocean of only 0.1 K, more than a factor of 10 smaller than the values reported by Lau and Kim.…”