Ocean mass changes from the lower degree harmonics of the GRACE/GFO monthly gravity fields.

Abstract

The study is based on the low degree harmonics to focus on the accurate long wavelength parts of the GRACE/GFO gravity fields. To reduce the contamination of mass changes on land a set of point masses are estimated and analyzed to verify estimates of the melting of the major ice sheets. Subsequently, residual gravity fields are used to recover characteristics and regional patterns of changes in the ocean mass.Initially, ocean mass changes are computed using the direct approach using coefficients filtered at harmonic degree and order 20, 40 and 60. The results show that smoothed GSM fields may be used as well, though a wider buffer zone is required. In all cases, values for the global mean ocean mass trends of 2.13-2.47 mm/y were found which are pretty consistent with what others had found already.The second part focus on using the lower degree and more accurate parts of the GRACE/GFO gravity fields. A method for removing land signals contaminating the ocean signal, is tested. It is based on point masses. Though the locations of the point masses at fixed grid nodes may not be optimal, the method seems to work well. The total mass loss on land is about 660 Gt corresponding to 1.83 mm/y ocean sea level gain which is on the low side compared to the values from the initial study. However, using the residual fields from where the continental signals have been removed and no buffer zone, a trend of 1.82 mm/y is obtained. Hence, there is a very good agreement with the corresponding land value. The results show that the method is very successful in removing land signal from the GRACE/GFO monthly fields and facilitates further studies of the changes in the ocean masses.