Closing the Global Sea Level Budget

Global mean sea level (Delta SL) can change due to ocean water mass increase/decrease and ocean volume expand/shrink. This simple concept can be described by the following equation which is also called the sea level budget

Delta SL = Delta SL_{mass}+Delta SL_{steric}

Delta SL_{mass} (or called barystatic sea level in the IPCC reports) includes the influences from ice sheets, glacier and ice caps, land hydrology, and atmospheric water vapor. Notice that water mass moving around within the ocean will not affect on the Delta SL_{mass}.

Delta SL_{steric}, on the other hand, is the density changing over the ocean due to thermal expansion and salinity changes.

The figure below shows how the two components contribute on the total sea level variations through time. Total sea level is derived from the satellite altimetry provided by CMEMS (Copernicus Marine Environment Monitoring Service). Steric sea level is derived from the gridded Argo float data from Scripps institute at UCSD. Barystatic sea level is derived from the GRACE satellite data provided by CSR (Center of Space Research) at UT Austin.

From three independent measurements, the sum of the two dash line (click on the legend of the sum will show the sum of the two in red solid line) would match perfectly with the green solid line if everything is observed flawlessly. This is certainly not the case in the real world. Much observational error and missing components will cause the differences between the sum and the direct observation of total sea level. However, we can say the sea level budget is closed if the range of the standard error based on the present day knowledge (includes both observational and missing components) of the two estimates overlapped. Currently, the trend estimate (3.0+-0.7 mm/yr V.S. 2.9+-0.6 mm/yr) and annual amplitude estimate is closed based on our study [Hsu and Velicogna, 2017].

Different from other studies which is using GRACE observation over the ocean to measure the mass changes, we calculate how each components contribute on the barystatic sea level from land and atmosphere. Figure below is showing the water mass changes in each component observed by GRACE.

If we focus on the total mass change over the ocean (Sum in the above figure), we can find some interesting features in the figure below.

1. There is a large seasonal fluctuation. The components contributing to this significant variation is mainly land water mass and atmospheric water vapor which can be proved by fig 1.

2. The orange line which represents the same signal with seasonal signal removed also shows interesting fluctuation at the interannual timescale. The interannual feature is clearer if one click off the blue line which represents the total barystatic sea level signal. One may notice the significant sea level drop after 2010 and acceleration start from mid-2011. This global scale fluctuation is initiated by the famous ENSO and other atmospheric condition [Boening et al., 2012, Fasullo et al., 2013].

3. The linear trend showed by the green line indicates a 1.7 mm/yr increase of Delta SL_{mass} based on the past 14 years of GRACE observation.

This project focuses not just on closing the current global sea level budget, but also helps us to better understand and predict the future sea level changes through studying how each component contributes to the total sea level.