Ocean mixed layer depth: A subsurface proxy of ocean‐atmosphere variability

Abstract

[1] A new criterion, based on the shallowest extreme curvature of near surface layer density or temperature profiles, is established for demarking the mixed layer depth, h mix . Using historical global hydrographic profile data, including conductivitytemperature-depth and expendable bathythermograph data obtained during World Ocean Circulation Experiment, its seasonal variability and monthly to interannual anomalies are computed. Unlike the more commonly used D criterion, the new criterion is able to deal with both different vertical resolutions of the data set and a large variety of observed stratification profiles. For about two thirds of the profiles our algorithm produces an h mix/c that is more reliable than the one of the D criterion. The uncertainty for h mix/c is ±5 m for high-(<5 m) and ±8 m for low-(<20 m) resolution profiles. A quality index, QI mix , which compares the variance of a profile above h mix to the variance to a depth of 1.5 Â h mix , shows that for the 70% of the profile data for which a clearly recognizable well-mixed zone exists near the surface, our criterion identifies the depth of the wellmixed zone in all cases.

In situ transcriptomic analysis of the globally important keystone N2-fixing taxon Crocosphaera watsonii

“…Characteristics of C. watsonii population The mixed layer depth, as calculated per Lorbacher et al (2006) was 25.9 m, where surface salinity was depressed by B0.75 relative to waters immediately below. Crocosphaera-like cell abundances at station KM0703.025 reached 3.84-4.70 Â 10 3 cells per ml between 37 and 45 m depth (the C. watsoniienriched horizon), but were much less abundant at the surface, at 75 m, and were not detected by microscopy at the depth of the deep chlorophyll maximum (106 m, see Figure 1).…”

OMIP contribution to CMIP6: experimental and diagnostic protocol for the physical component of the Ocean Model Intercomparison Project

“…In contrast, the prognostic sea surface height in Boussinesq models does not incorporate global steric effects (Greatbatch, 1994). When removing the global mean, sea level patterns from Boussinesq and nonBoussinesq models are directly comparable (Losch et al, 2004;Griffies and Greatbatch, 2012).…”

“…So most techniques to estimate the MLD rely on either a threshold gradient or a threshold change in one of these quantities, normally in potential temperature θ or density (see for example Lorbacher et al, 2006;de Boyer Montégut et al, 2004;Monterey and Levitus, 1997). Relying solely on θ has the advantage of good observational data coverage, but this approach neglects salinity stratification associated with barrier layers (see e.g.…”

Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue

“…In the study of Foltz et al (2003) MLDs are calculated with the temperature threshold criterion of 0.5 °C, but using only the profiles between 5.00 and 7.00 local time as a reference for the threshold to avoid the influence of shallow diurnal mixed layers. Different temperature thresholds (0.2 and 0.5 °C) referenced against SSTs between 5.00 and 7.00 local time as well as the temperature gradient criterion described in Lorbacher et al (2006) for PIRATA subsurface temperatures have been evaluated and compared to the MLDs of the climatology of de Boyer Montégut et al (2004) at the PIRATA locations. Best agreement was found for the temperature threshold of 0.5 °C, which is therefore further used here to estimate MLDs from PIRATA subsurface temperatures.…”