Estimation Of Global Ocean Mass Change Rate And Analysis Of Seismic Effect Influence From Satellite Gravity

The ocean mass change is caused by the exchange of water between ocean and the other components of the climate system,such as melting of the polar ice-sheet and mountain glaciers,surface run-off,precipitation and water evaporation between ocean and atmosphere,etc.Quantitative study and monitoring of the ocean mass change can provide useful information for understanding the characteristics of current ecological environment change and global climate system change.GRACE(Gravity Recovery and Climate Experiment,launched in 2002)and GRACE Follow-On(GFO,launched in 2018)satellite gravimetry missions have provided important opportunities for monitoring global mass change.Due to the advantages of wide space coverage and high sensitivity to large-scale mass migration,satellite gravimetry provides an effective means to quantify the ocean mass change.In this study we estimate the global mean ocean mass(GMOM)change rate using the GRACE time-variable gravity data.As the technical notes associated with the GRACE Release-06(RL06)data(released in May 2018)indicate that the new version has incorporated improvement in atmosphere and ocean background models as well as data processing methods(compared to the RL05 version),we first compare the RL06 and RL05 data and analyze their differences in the inversion of global surface mass change,which may benefit the estimation of ocean mass change rate.We compare the noise level and signal level between the RL06 and RL05 GSM data from January 2003 to December 2016.The results show that the average noise level of RL06 data is ?40% lower than that of RL05 data.In addition,the signal amplitude in both seasonal variations and long-term trends,as reflected by RL06 and RL05 data,are comparable at the time series of selected feature points.We further analyze the differences in the estimation of regional total mass change and global mean ocean mass change rates between RL06 and RL05 data.We find that the RL05 mass loss rate over the Antarctica region is 40.22 Gt/year larger than that from RL06 data,while the mass loss rate from RL05 data in the Greenland region is 7.18 Gt/year smaller than that from RL06 data.In both the Antarctica and Greenland regions,the contributions from low-degree spherical harmonic(SH)terms of GRACE data(mainly including the geo-center motion terms)play a leading role in the differences of total mass change rate between RL06 and RL05.Furthermore,we compare the GMOM change rates from RL06 and RL05 data during the period from January 2005 to December 2016.The results demonstrate that the RL06 GMOM change rate is 2.25 mm/year,while the RL05 GMOM change rate is 2.06 mm/year,which indicates the RL06 GMOM change rate is 0.19 mm/year larger than that from RL05 data.Through the analysis of the contributions from various components to the results,we find the main reasons for the 0.19 mm/year difference included the following aspects:(1)There is a significant difference in the geo-center motion(i.e.degree-1)terms between the RL06 and RL05 data.(2)The background models(e.g.atmosphere and ocean models)used in the RL06 and RL05 data processing are quite different and lead to notably different mass change rates for a global scale,and(3)The noise levels of RL06 and RL05 data are different and contribute non-negligibly to global ocean mass change estimation.In addition,we combine the GRACE and GFO RL06 data to estimate the GMOM change rate over a longer time period.The GMOM change rate comes out as 2.25 mm/year during the 16 years from the January 2003 to the December 2019(with a data gap between GRACE and GFO from June 2017 to June 2018).The accurate estimation of global ocean mass change using time-variable satellite gravimetry data still has challenges,such as the larger uncertainty of low-degree SH terms in the data,the significant errors of current GIA models necessary for ocean mass change estimation.Among the challenges(or uncertainties)the effect of large earthquake is a factor that has not been well studied.The mass change effect of large earthquakes occurring in land-ocean boundary areas during the GRACE observation mission period contaminates the extraction of ocean mass change signal.As the large earthquakes inducing significant mass change signals during the GRACE mission mainly include the December 2004 Mw9.1 Sumatra earthquake,April 2012 Mw8.6 Sumatra earthquake,March 2011 Mw9.0 Tohoku-Oki earthquake and February 2010 Mw8.8 Chile earthquake,we take these four earthquake regions as case studies in the correction of seismic impact.The results show that the impact of the above four earthquakes on the GMOM change rate estimation during the period January 2003 to December 2015 is-0.07±0.02 mm/year(with the GMOM change rates 2.12±0.30 mm/year and 2.05 ± 0.30 mm/year before and after the seismic effect correction,respectively).Although the influence of seismic effect is under the uncertainty level of the current GRACE GMOM change rate estimation(-0.07 vs.±0.30 mm/year),it is a systematic bias and should be taken into account for better understanding of the global sea level change.In particular,along with the upcoming improvement of satellite gravimetry in precision as well as temporal and spatial resolutions,the seismic effect correction will become more necessary for more refined GMOM change study.