2 An assessment of the seasonal mixed layer salinity budget 3 in the Southern Ocean 4 Shenfu Dong, 1 Silvia L. Garzoli, 2 and Molly Baringer 2 5 Received 23 December 2008; revised 10 August 2009; accepted 1 September 2009; published XX Month 2009. 6 [1] The seasonal cycle of mixed layer salinity and its causes in the Southern Ocean are 7 examined by combining remotely sensed and in situ observations. The domain-averaged 8 terms of oceanic advection, diffusion, entrainment, and air-sea freshwater flux 9 (evaporation minus precipitation) are largely consistent with the seasonal evolution of 10 mixed layer salinity, which increases from March to October and decreases from 11 November to February. This seasonal cycle is largely attributed to oceanic advection 12 and entrainment; air-sea freshwater flux plays only a minimal role. Both oceanic 13 advection-diffusion and the freshwater flux are negative throughout the year, i.e., reduce 14 mixed layer salinity, while entrainment is positive year-round, reaching its maximum in 15 May. The advection-diffusion term is dominated by Ekman advection. Although the 16 spatial structure of the air-sea freshwater flux and oceanic processes are similar for the 17 steady state, the magnitude of the freshwater flux is relatively small when compared to that 18 of the oceanic processes. The spatial structure of the salinity tendency for each month is 19 also well captured by the sum of the contributions from the air-sea freshwater flux, 20 advection-diffusion, and entrainment processes. However, substantial imbalances in the 21 salinity budget exist locally, particularly for regions with strong eddy kinetic energy and 22 sparse in situ measurements. Sensitivity tests suggest that a proper representation of 23 the mixed layer depth, a better freshwater flux product, and an improved surface salinity 24 field are all important for closing the mixed layer salinity budget in the Southern Ocean. 25 Citation: Dong, S., S. L. Garzoli, and M. Baringer (2009), An assessment of the seasonal mixed layer salinity budget in the Southern 26 Ocean, J. Geophys. Res., 114, XXXXXX, doi:10.1029/2008JC005258. 28 1. Introduction 29 [2] Temperature inversions in the upper water column 30 where the surface layer is colder than the subsurface layer 31 are common in the Southern Ocean [e.g., de Boyer 32 Montegut et al., 2007; Dong et al., 2007], suggesting that 33 salinity plays an important role in stabilizing the water 34 column. To illustrate the importance of salinity in the 35 Southern Ocean, we examined the contributions of temper- 36 ature and salinity to the density seasonal cycle in the mixed 37 layer. A monthly mixed layer temperature/salinity climatology 38 was constructed from Argo float profiles (described in section 39 2) to compute density, and only regions with data for all 40 months were included. The salinity contributions (r s ) were 41 calculated using the time-mean temperature and monthly 42 salinity fields, whereas the temperature contributions (r t ) 43 were calculated using the monthly temperature and time- 44 mean salinity fields. 45 [3] Figure 1 shows the ratio between the amplitudes of 46 the seasonal variations in r s and r t , A(r s )/A(r t ), where A(r s ) 47 and A(r t ) are the amplitudes of the seasonal harmonic of r s 48 and r t , respectively, which suggests that the seasonal 49 variations in the mixed layer density are generally domi- 50 nated by temperature changes (ratios less than 1). However, 51 salinity plays an increasingly important role farther south 52 (particularly near the sea ice edge where seasonal migra- 53 tions of the ice edge play a large role in determining the 54 seasonal mixed layer density) and at other specific 55 geographical regions. The ratio (Figure 1) exceeds 0.5 in 56 most regions south of 40°S, particularly for regions in close 57 proximity to sea ice and within a 5° latitude band north of 58 the Subantarctic Front (SAF) where Subantarctic Mode 59 Water (SAMW) is formed. This suggests that salinity 60 contributions to the seasonal variations in the mixed layer 61 density are about half of or nearly equal to the contributions 62 from temperature. Therefore, the role of salinity cannot be 63 neglected, and understanding salinity variability and what 64 controls it are important to understanding SAMW 65 formation, which has been linked to the upper limb of 66 the meridional overturning circulation [e.g., Sloyan and 67 Rintoul, 2001; Rintoul and England, 2002]. 68 [4] Other efforts that have examined the role of salinity in 69 the ocean have shown that salinity plays an important role 70 in the dynamic height variability of the tropics [Maes, 1998; 71 Maes et al., 2002]. Antonov et al. [2002] examined the 72 steric sea level variations for 1957 – 1994 and suggested that 73 in the subpolar North Atlantic the contributions of temper- 74 ature and salinity to the total steric sea level were nearly 75 equal but of opposite sign. The sparseness of data in the JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, XXXXXX, doi:10.1029/2008JC005258, 2009 Click Here for Full Article 1 CIMAS, RSMAS, University of Miami, Miami, Florida, USA. 2 NOAA, AOML, Miami, Florida, USA. Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JC005258$09.00 XXXXXX 1 of 16