An assessment of the seasonal mixed layer salinity budget in the Southern Ocean

Abstract

[1] The seasonal cycle of mixed layer salinity and its causes in the Southern Ocean are examined by combining remotely sensed and in situ observations. The domain-averaged terms of oceanic advection, diffusion, entrainment, and air-sea freshwater flux (evaporation minus precipitation) are largely consistent with the seasonal evolution of mixed layer salinity, which increases from March to October and decreases from November to February. This seasonal cycle is largely attributed to oceanic advection and entrainment; air-sea freshwater flux plays only a minimal role. Both oceanic advection-diffusion and the freshwater flux are negative throughout the year, i.e., reduce mixed layer salinity, while entrainment is positive year-round, reaching its maximum in May. The advection-diffusion term is dominated by Ekman advection. Although the spatial structure of the air-sea freshwater flux and oceanic processes are similar for the steady state, the magnitude of the freshwater flux is relatively small when compared to that of the oceanic processes. The spatial structure of the salinity tendency for each month is also well captured by the sum of the contributions from the air-sea freshwater flux, advection-diffusion, and entrainment processes. However, substantial imbalances in the salinity budget exist locally, particularly for regions with strong eddy kinetic energy and sparse in situ measurements. Sensitivity tests suggest that a proper representation of the mixed layer depth, a better freshwater flux product, and an improved surface salinity field are all important for closing the mixed layer salinity budget in the Southern Ocean.

Mixed-layer salinity budget in the tropical Indian Ocean: seasonal cycle based only on observations

“…where S m is the salinity average in the mixed layer, t is the time, E is evaporation, P is precipitation, h m is the mixed-layer depth (MLD), u m is the horizontal surface velocity vector averaged over the mixed layer (having u and v components defined positive eastward and northward, respectively), S h m is the salinity at the base of the mixed layer, w e ¼ w þ ∂h m ∂t is the entrainment velocity (at depth z=−h m ) which corresponds to the difference between the vertical velocity w (positive when upwards and estimated from the horizontal currents through the continuity equation) at the mixed-layer base and the mixed-layer deepening rate, H (w e ) is the Heaviside step function (H (w e )=w e if w e >0 and H (w e )=0 if w e <0), and K (set to 500 m 2 s −1 as in Yu 2011 andin Dong et al 2009) is the horizontal diffusivity. Note that the river runoff is not quantified in this study because the studied areas are far from coastal regions.…”

“…We tested two density criteria (0.03 and 0.125 kg m −3 ) of the MLD which are used in several studies (e.g., de Boyer Montégut et al 2004;Dong et al 2009;Yu 2011), and we also used the MLD product of de Boyer Montégut et al (2004) based on density criteria (0.03 kg m −3 ). The reference depth for the vertical density gradient is set to 10 m because of better data sampling than 0 or 5 m. For the E-P, we tested the product described in the data section with two reanalysis products: the monthly evaporation and precipitation dataset from the ERA-Interim reanalysis (Dee et al 2011) of the European Centre for Medium-Range Weather Forecasts (ECMWF) available at 0.5°resolution and the monthly evaporation and precipitation dataset from the National Center for Environmental Prediction (NCEP) reanalysis 1 which are available at 2°resolution (Kalnay et al 1996).…”

“…The MLD is derived from the monthly temperature and salinity fields of this climatology using density criteria which have been used by several authors (e.g., de Boyer Montégut et al 2004;Dong et al 2009;Yu 2011). Based on the sensitivity tests discussed in the Appendix, we find that the criterion of a 0.125-kg-m −3 density change relative to the near surface value (selected at 10 m because of better data sampling than 0 or 5 m) is more appropriated to close the MLS budget in our studied area; thus, it is used for the reference experiment.…”

Observations of watermass modification by mesoscale eddies in the subtropical frontal region of the Indian ocean sector of southern ocean

“…XCTD probes were launched at every 30 nautical mile intervals and the quality of the profiles was controlled by following the guidelines in the CSIRO Cookbook (1993). The mixed layer depth was calculated based on a density difference criterion, Δρ¼0.03 kg m À 3 , where Δρ is the density difference from the topmost near-surface value (Dong et al, 2009).…”

Evaluation of the tropical variability from the Beijing Climate Center’s real-time operational global Ocean Data Assimilation System

“…The mixed layer depth (MLD) is one of the most important quantities of the upper ocean, and its variability strongly influences the physics of the upper ocean and marine biological processes (e.g., de Boyer Montégut et al, 2004;D'Ortenzio et al, 2005;Dong et al, 2007Dong et al, , 2009Ren et al, 2011;Sallée et al, 2008;Moore et al, 2013). We estimate the monthly mean MLD from the temperature and salinity based on the density criterion of 0.125 (kg m −3 ), in which the density is supposed to change from that of the ocean surface of 0.125 kg m −3 (Levitus, 1982).…”