These weekly gridded maps are made using the Aquarius sea surface salinity retrievals. They were obtained with algorithms developed by the Calibration/Validation working group, and applied to the microwave (L-band, about 1.4 GHz) brightness temperature measurements along the spacecraft swath. These maps show sea surface salinity expressed in practical salinity unit (psu) in the northern and southern hemispheres at latitudes higher than 50°. Blue colors represent lower salinity, whereas red colors represent higher salinity. The date shown on the images (below) is the first day of that weekly measurement.
Salinity can only be retrieved from space when the ocean is not covered by sea ice. Sea ice is frozen ocean water. It forms, grows, and melts according to the seasons, but some sea ice remains year round in certain regions of both the Arctic and Antarctic. When a significant fraction of sea ice is present in the Aquarius measurements, salinity cannot be retrieved accurately. These areas were masked on the maps. Other areas without data correspond to either a lack of observations due to orbit geometry and spacecraft maneuvers, or are coastal regions where salinity is inaccurate.
The data presented are a weekly gridded average of the swath data distributed by NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC) and provided by Ludovic Brucker (1,2) and Emmanuel Dinnat (1,3).
Aquarius Level-3 weekly polar-gridded data sets can be downloaded from the National Snow and Ice Data Center (NSIDC).
When ocean surface salinity retrievals are possible in the Arctic Ocean, values are smaller than for other regions. One can also see that blue colors are usually present in the vicinity of the sea ice margin. This may result from sea ice contamination of the Aquarius measurements, and do not necessarily reflect a low salinity.
These maps also show the significant difference between the saltier Atlantic Ocean and the fresher Pacific Ocean. Ocean surface salinity in polar regions varies seasonally as ice freezes (i.e., leaving saltier water in the ocean) and melts (i.e., adding fresh water to the ocean).
In the southern hemisphere, the month of minimum sea ice extent is February, whereas the month of maximum extent is October. In general, the Antarctic sea-ice cover exhibits faster seasonal ice extent variations than that monitored in the northern hemisphere.