Trends and interannual variability of mass and steric sea level in the Tropical Asian Seas

News - 17 July 2017

In the study we have separated, for the first time, the contributions of mass and steric sea level to trends and interannual variability in the Tropical Asian Seas (TAS) over eight years by a statistically weighting the measurements of satellite altimetry, satellite gravimetry and ocean reanalysis estimates. 

Observations of mass in this area are particularly difficult due to possible signal leakage from land hydrology and a the large Sumatra-Andaman earthquake in the vicinity of the target area, while estimating the steric contribution is difficult due to the absence of Argo temperature and salinity estimates and the complex ocean dynamics in the region. It is however an interesting area for a variety of reasons, for example the large interannual fluctuations due to Pacific and Indian ocean dynamics and due to one of the largest sea level rise signals in the world.

We find that the interannual steric signal is small in the shallow regions and that it is driven in the deep regions by the Indian Ocean Dipole (a fluctuations in temperature between the western and eastern parts of the Indian Ocean) and the wind stress in the equatorial Pacific. The interannual mass signal is largest in the shallow regions and it is driven by the Pacific equatorial wind stress, which is related to the ENSO. We argue that during La Nina events, water enters the TAS as a response to larger steric expansion in the western Pacific.

The trends of mass and steric sea level are respectively 4.5 and 6.4 mm/yr. The total sea level trend in the TAS is therefore 3-4x larger than the global average. This affects the global sea level trend, by decreasing it by 0.3 mm/yr in case the area is omitted, which is usually done. Finally we removed the contributions of the interannual variability to the trends, which reduced especially the mass trend. On top of that we removed the effect of the (18.6 year) nodal cycle to the mass trend. The residual 2 mm/yr is in line with the trends we find from mass redistribution fingerprinting the contributions of the mass changes in Greenland, Antartica and the other hydrological components.