Roaring forties / Furious fifties
Monitoring the Southern Ocean (SO) salinity is important due to its strong influence on density stratification, ocean circulation, water cycle, and biological productivity [Gille (2002); Dong et al. 2008; Liu et al., 2010,McNeil and Matear(2008); Montes-Hugo et al., 2009, Sarmiento et al., 1998, Toggweiler and Samuels, 1995]. Monitoring large- and small-scale changes in sea surface salinity (SSS) in the SO is important because it offers an unprecedented opportunity to investigate the relationship of salinity changes with sea ice melting-freezing cycle, buoyancy, precipitation, and response to surface forcing. The Southern Ocean is governed primarily by the seasonal advance and retreat of the sea ice cover and the Antarctic circumpolar current, which is a strong eastward flowing current that connects the major world oceans and redistributes oceanic properties such as heat, salt and nutrients. It is also the location of the Antarctic polar fronts that separate distinct surface water masses and the scene of strong lateral gradients in temperature, salinity and biological productivity (Moore and Abbott, 2002). The movement of salt between sea ice and the ocean influences ocean density (Dietrich, 1986) that in turn affects polar ocean circulation and deep-water formation. The presence of a surface lens of meltwater can also shape the marine ecosystem by providing a stable water column and a shallow mixed layer, which are necessary for phytoplankton bloom development (Dierssen et al., 2002; Smith and Comiso, 2008). Recent studies have shown widespread freshening in higher latitudes (Hellmer et al., 2011; Durack et al., 2012; Jacobs et al., 2002; Jacobs and Guilivi, 2010), which can point to possible connections to global warming.
The Antarctic region has been an area of intense interest recently because of the reported positive trend in the sea ice extent since 1978 and an unpredictable interannual variability of the ice cover in recent years (Comiso et al., 2017; Turner and Comiso, 2017). The northward transport of freshwater associated with the increasing extent of the sea ice cover has been observed to cause significant changes in salinity distribution in the Southern Ocean.
Retrieving SSS from L-band radiometers in the polar regions is especially challenging because of significantly lower sensitivity of changes in brightness temperature to changes in SSS at relatively low sea surface temperature (SST) (Lagerloef et al., 2015; Fig. 1 of Garcia-Eidell et al., 2017). Satellite-derived salinity estimations in the SO are also heavily influenced by the strong winds of the high-latitude Southern Hemisphere. Despite the potential inaccuracies of satellites and differences within the satellite salinity products, the increased temporal and spatial scales and measuring the top few centimeters of the ocean surface have made satellite-derived salinity a useful quantity for air–sea interaction studies.
Validation studies in the Southern Ocean were made to complement the validation of the standard SSS products that have been performed primarily in warm waters and in the Arctic region (Garcia-Eidell et al., 2017). In particular, comparative studies of satellite SSS were performed with available quality-controlled Polarstern thermosalinograph (TSG) measurements and aggregated in-situ measurements called CORA (Coriolis Ocean database for ReAnalysis) (Szekely et al, 2015; Cabanes et al, 2013). Large-scale biases in the polar region have indeed been observed (Melnichenko et al., 2014) but some studies indicate that the biases were in part due to land and sea ice contaminations (Kohler et al., 2014; Brucker et al., 2014). Despite these difficulties, as reviewed here below several recent studies demonstrated the interest of satellite SSS for the southern ocean studies.
A Comparison of Satellite-Derived Sea Surface Salinity and Salt Fluxes in the Southern Ocean
SSS derived from Aquarius/SAC–D, SMAP and SMOS were compared by Ferster and Subrahmanyam (2018) and used to estimate horizontal advective salt fluxes in the Southern Ocean (SO). In comparison with an Argo product, all three satellites estimate similar SSS in the Southern Hemisphere mid-latitudes (30°S–45°S) with low variability among the products. At high latitudes, there are temporal patterns of bias (relative to Argo) in Aquarius during Austral summer and in SMOS during Austral winter. Differences in the satellite products and Argo exist along coastal boundaries, low temperatures, and strong currents. Satellite-derived salinity indicates low temporal–mean standard deviations with Aquarius (0.215) and moderate standard deviations with SMOS (0.294) and SMAP (0.325) against Argo in the SO. Differences in satellite-derived zonal and meridional SSS gradients are large; standard deviation values are 2.52 and 1.49 × 10−6 psu m−1, respectively, and similarly located within the sub-tropical salinity maxima, Antarctic Circumpolar Current, and coastal zones. Differences in the horizontal advective fluxes are on average small, but large variability greater than 275 mm.month−1 indicates errors of similar magnitude to the estimated Argo flux. Based on these results, the authors concluded that the use of satellite-derived salinity may prove to be a useful resource for observing salinity and horizontal salt fluxes, outside the inaccuracies associated with the high latitudes and coastal currents between the various remotely sensed products, and could significantly influence the results depending on the product.
Sea Surface Salinity Distribution in the Southern Ocean as Observed from Space
ALarge scale spatial and temporal variabilities of SSS in the SO from 2011 to 2017 were studied using products derived from Aquarius, SMOS and SMAP in Garcia Eidell et al., (2019). Four products, three from Aquarius and one from SMOS, were evaluated and shown to be generally consistent within 0.3 to 0.6 psu and agree favorably with in situ measurements. However, although the Aquarius products show consistent seasonality of SSS with high values of 34.45 in October and low values of 33.40 in May, the SMOS and SMAP products lack such seasonal variations. This may be caused by larger uncertainties in the SMOS and SMAP data due in part to the lack of concurrent scatterometer measurements that is used to correct for roughness effects. The four products provide similar spatial distributions of SSS with root-mean-square-difference from 0.25 to 0.58 psu. Differences among Aquarius products are mainly due to varying salinity retrieval algorithms, smoothing, and masking of sea ice while the SMOS product showed the highest SSS deviation that is likely due to the bias-adjustment done on the dataset. The analyses of Garcia Eidell et al., (2019) show that SSS in the Southern Ocean region has significant meridional variations with the lowest SSS near the ice edge and highest at lower latitudes. The SSS is also lowest in summer indicating the predominant influence of sea ice and glacial melt but it stays low near ice edges even during the growth season.