Reduced ascending/descending pass bias in SMOS salinity data demonstrated by observing westward-propagating features in the South Indian Ocean

Abstract

a b s t r a c tThe European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) satellite has been providing data, including sea surface salinity (SSS) measurements, for more than five years. However, the operational ESA Level 2 SSS data are known to have significant spatially and temporally varying biases between measurements from ascending passes (SSS A ) and measurements from descending passes (SSS D ). This paper demonstrates how these biases are reduced through the use of SSS anomalies. Climatology products are constructed using SMOS Level 2 data to provide daily, one-degree by one-degree climatologies separately for ascending and descending passes using a moving window approach (in time and space). The daily, one-degree products can then be averaged to provide values of climatological SSS at different spatial and/or temporal resolutions.The averaged values of the SMOS climatology products are in good general agreement with data from the World Ocean Atlas 2013. However, there are significant differences at high latitudes, as well as in coastal and dynamic regions, as found by previous studies. Both the mean and standard deviation of the differences between data from ascending passes and data from descending passes for the anomalies are reduced compared with those obtained using the original salinity values. Geophysical signals are clearly visible in the anomaly products and an example is shown in the Southern Indian Ocean of westward-propagating signals that we conclude represent the surface expression of Rossby waves or large-scale non-linear eddies. The signals seen in salinity data agree (in speed) with those from sea surface temperature and sea surface height and are consistent with previous studies.

Mechanisms of Mixed‐Layer Salinity Seasonal Variability in the Indian Ocean

“…Various studies (e.g., Banks et al, ; Köhler et al, ; Tong et al, ) showed that satellite‐retrieved SSS is corrupted by systematic errors coming from ice and land contamination (Font et al, ; Gabarro et al, ; Oliva et al, ), radio frequency interference (RFI; Oliva et al, ), seasonal biases, or latitudinal biases mainly due to thermal drift of the instrument (e.g., Boutin et al, ; Gourrion et al, ; Reul et al, ). Those biases are difficult to characterize and several institutions provide satellite‐retrieved SSS products based on different correction methods (e.g., Kolodziejczyk et al, ).…”

Salinity from space

“…All these factors lead to errors making the measurement accuracy aim of the mission of 0.1 in salinity over 10–30 days and 100–200km very challenging. A recent review of SMOS performance concluded that the accuracy aim has not been met globally but mostly achieved in regions within 45°S–45°N with SSTs higher than ~10°C (Mecklenburg et al ., ; for higher‐latitude colder waters see Köhler et al ., ; and for approaches to dealing with biases see Banks et al ., ; Kolodziejczyk et al ., ). Despite these challenges good SSS data are being obtained from SMOS, giving a global picture of the changing ocean salinity over the last 7 years.…”

“…These have been previously identified as Rossby waves (or possibly ocean eddies) based on SST and sea surface height observations from space and, as expected, they are also visible in the SSS anomalies. The propagation speeds derived from these data are consistent with theoretical estimates of Rossby wave speeds, typically ~5–10cms −1 depending on latitude and Rossby wave mode (see figure 6 of Banks et al ., ). Detecting these signals in SSS would not have been possible using in situ measurements.…”

“…Clearly visible is the propagation of Rossby waves from east to west (diagonal striations in the plots). Dashed lines represent estimates of the Rossby wave propagation speeds from each of the observables (after Banks et al, ).…”

“…As temperature and salinity together determine the density of the sea water, this provides new opportunities for understanding the ocean circulation, from the mesoscale through to the gyre, basin and global scales. Some issues remain with regard to improving the quality, and utility, of SSS observations from space, including: RFI; biases in SMOS data (Banks et al ., ; Kolodziejczyk et al ., ); and difficulties in accurate retrievals close to land and at high latitudes (Köhler et al ., ). However, ongoing research is making improvements in all of these areas, and future salinity missions are under consideration; the prospects are therefore good for continued measurements of SSS from space.…”