Satellite and In Situ Salinity: Understanding Near-Surface Stratification and Subfootprint Variability

Abstract

International audienceRemote sensing of salinity using satellite-mounted microwave radiometers provides new perspectives for studying ocean dynamics and the global hydrological cycle. Calibration and validation of these measurements is challenging because satellite and in situ methods measure salinity differently. Microwave radiometers measure the salinity in the top few centimeters of the ocean, whereas most in situ observations are reported below a depth of a few meters. Additionally, satellites measure salinity as a spatial average over an area of about 100 × 100 km². In contrast, in situ sensors provide pointwise measurements at the location of the sensor. Thus, the presence of vertical gradients in, and horizontal variability of, sea surface salinity complicates comparison of satellite and in situ measurements. This paper synthesizes present knowledge of the magnitude and the processes that contribute to the formation and evolution of vertical and horizontal variability in near-surface salinity. Rainfall, freshwater plumes, and evaporation can generate vertical gradients of salinity, and in some cases these gradients can be large enough to affect validation of satellite measurements. Similarly, mesoscale to submesoscale processes can lead to horizontal variability that can also affect comparisons of satellite data to in situ data. Comparisons between satellite and in situ salinity measurements must take into account both vertical stratification and horizontal variability

Assessment of Aquarius Sea Surface Salinity

“…Figure 3b also demonstrates that the largest RMSD, exceeding 0.2 psu, are found in the regions of strong variability in SSS, such as along the North Pacific and North Atlantic ITCZ, the North Pacific sub-polar front, the Gulfstream, and near outflows of major rivers such as the Amazon in the tropical North Atlantic. In this regard, the observed relatively large RMSD between the Aquarius and Argo float data in some areas are not necessarily due to errors in Aquarius measurements only, but RMSD may include the disparity between time and space scales captured by two different observational platforms [17][18][19] and the difference in measurement depth between Aquarius (ocean surface) and Argo (~5 m depth) [17]. Larger bias and RMSD is also observed in the high latitudes.…”

“…All three beams show small differences with little variations for the global median. The remaining differences may be related to the uncertainties of salinity observations, such as near-surface stratification or the sub-footprint variations [17]. The important conclusion from Figure 2 is that Aquarius measurements exhibit no spurious trends and drifts.…”

“…It is important to note that Aquarius measurements represent salinity in the top centimeter of the ocean. Near-surface salinity stratification in the upper few meters can cause differences between Aquarius and Argo SSS, especially under rain bands [17].…”

“…This is because Argo float distributions are too sparse to produce robust monthly mean values at 1 • × 1 • scale in regions with strong spatiotemporal variability such as tropical rain bands (e.g., under the ITCZ), near river plumes, western boundary currents, and the Antarctic Circumpolar Current. High-resolution in-situ thermosalinograph (TSG) data indeed show significant variations of SSS within the scales of Aquarius footprint [17]. Therefore, the Argo gridded data in these regions can be significantly affected by the sampling errors of the Argo array.…”

The Salinity Retrieval Algorithms for the NASA Aquarius Version 5 and SMAP Version 3 Releases

“…Aquarius measures ocean salinity within a few centimeters of the surface, whereas ARGO measures salinity at 5 m depth. In order to avoid mismatches between the two measurements resulting from salinity stratification within the upper ocean layer under precipitation [14], it is necessary to have information on rain rate, which is used to flag observations with rain. Our IRR, provided by the Aquarius Rain Accumulation (RA) product [15], is used as an Remote Sens.…”