Thermosteric Effects on Long-Term Global Sea Level Change Jianli Chen Center for Space Research, University of Texas at Austin, USA WPGM, July , Beijing, China OS35A-01 Wed. 16:45 PM I sincerely apologize for being absent due to an unexpected urgency, and am grateful to Richard for his kind help. - Jianli Chen

What may cause sea level change?  Warming of the ocean (thermal expansion)  Salinity change (seawater density change)  Ocean current (dynamic sea surface height)  Polar ice sheet & glacial melting (water mass exchange)  Ocean tide (solar & lunar gravitational force)  Pole tide (rotational deformation)  Terrestrial & atmospheric water storage change  Atmospheric pressure loading (inverted barometer)  Solid Earth deformation (uplift & subsidence)  … Steric Change

Global Long-Term Sea Level Change Rates AVISO Merged Satellite Altimeters World Ocean Atlas 2001 (WOD01) Observed Sea Level Rates (cm/year) Steric Sea Level Rates (cm/year)

 Global Mean Sea Level Change Using Merged Satellite Altimeters Measurements.  Thermal Steric Sea Level Changes Using Temperature & Salinity Data From the World Ocean Database 2001 & World Ocean Atlas  Non-steric Sea Level Changes From Terrestrial Water Storage Change, Atmospheric Water Vapor Variation, & Polar Ice Sheet Melting. Objectives

About Sea Level Change  Long-Term Sea level Change  Thermal expansion & salinity change  Polar ice sheet & glacial melting  Glacial isostatic adjustment (GIA)  Terrestrial water storage change  Seasonal Sea Level Change  Thermal expansion & salinity change  Terrestrial water storage  Atmospheric water vapor  Others

 Satellite Radar Altimeter Sea Level Measurements  AVISO merged mean sea level anomaly  TOPEX/Poseidon, Jason-1, ERS-1/2, Envisat  October 1992 to August 2004  7-day intervals  1/3° x 1/3° Mercator grids  Data from 65° S to 65° N are included. Data and Processing

 World Ocean Database 2001 (WOD01)  Yearly temperature anomaly  , 1° x 1° grids  16 layers ( m depth)  Pentadal (5-year running) temperature anomaly  , 1° x 1° grids  28 layers ( m depth)  World Ocean Atlas 2001 (WOA01)  Climatologies of temperature and salinity fields  Jan, Feb, … Dec, 1° x 1° grids  24 layers ( m depth)  Data from 65° S to 65° N are included (for both WOD01 and WOA01) Data and Processing (cont.)

 Terrestrial Water Storage Model 1:  US Climate Prediction Center (CPC) Land Data Assimilation System  Soil moisture and snow, monthly, Jan present, 1  x 1  grids Model 2:  NASA Global Land Data Assimilation System (GLDAS)  Soil moisture and snow, 3-hourly, Jan Dec. 2004, 1° x 1° grids  Atmospheric Water Vapor  NCEP Reanalysis Surface Pressure data  Daily, Jan Aug (same as altimeter data)  Gaussian grids (~ 1.904° x 1.875° ) Data and Processing (cont.)

Steric Sea Level Change  =  (T, S, P) -  0 in which  0 is the mean density of sea water (1028 kg/m 3 ), and  is the density change as a function of temperature (T), salinity (S), and pressure (P). The integral is from the ocean bottom to the sea surface (h=0). T is from either yearly or pentadal temperature fields, S from the mean salinity of the WOD01 climatology, and P is computed from the mean depth of each layer.  is computed using the UNESCO (United Nations Educational, Scientific and Cultural Organization) standard equations (Fofonoff and Millard 1983)  =  (T, S, P) -  0 in which  0 is the mean density of sea water (1028 kg/m 3 ), and  is the density change as a function of temperature (T), salinity (S), and pressure (P). The integral is from the ocean bottom to the sea surface (h=0). T is from either yearly or pentadal temperature fields, S from the mean salinity of the WOD01 climatology, and P is computed from the mean depth of each layer.  is computed using the UNESCO (United Nations Educational, Scientific and Cultural Organization) standard equations (Fofonoff and Millard 1983) Steric sea level change can be computed from seawater density change as,

Global Water Mass Balance & Sea Level Change Assuming the total water mass on the Earth surface is conserved, so  M ocean +  M land_water +  M vapor = 0 or  M ocean = - (  M land_water +  M vapor ) So, non-steric global mean sea level (GMSL) change due to water mass exchange between ocean and land/atmosphere can be computed as,  GMSL non-steric =  M ocean /Ocean_Area  GMSL non-steric = - (  M land_water +  M vapor )/ /Ocean_Area

Data and Processing (cont.) Assuming the total water mass on the Earth surface is conserved, so  GMSL altimeter =  GMSL steric +  GMSL TWS +  GMSL ice_melting + … The big challenge is how to close the budget.   GMSL altimeter = AVISO Mean Sea Level Anomaly   GMSL steric = WOD01/WOA01   GMSL TWS = CPC/NCEP   GMSL ice_melting = GRACE ( Gravity Recovery and Climate Experiment )

 Long-Term Sea Level Change  AVISO merged altimeter estimate, 2.6 mm/year during  WOD01 steric effect, 1.2 mm/year during , and mm/year during last 50 years.  GRACE observed polar ice sheet & mountain glacial melting.  Interannual Sea Level Change  AVISO merged altimeter estimate  GLDAS terrestrial water storage  NCEP water vapor Results

Global Mean Sea Level (GMSL) Change From Satellite Altimeters

Long-Term Global Mean Sea Level Change From Altimeters & Steric Effects Long-Term Global Mean Sea Level Change From Altimeters & Steric Effects

Steric Effects on Global Mean Sea Level Change From WOD01/WOA01

The large bias of WOD98 estimated steric effects on global mean sea level change in 1997 & 1998 is apparently caused by poor data quality and incomplete data collection in WOD98. An algorithm error was later discovered and fixed in WOD01. Data Quality Data Collection

Interannual Global Mean Sea Level Change From CPC Terrestrial Water Storage & NCEP Water Vapor Variations

GRACE observed long-term ice melting:  Antarctic ice sheet ~ – 152 ± 80 km 3 /year [Velicogna & Wahr 2006, Science]  Greenland ice sheet ~ – 239 ± 23 km 3 /year [Chen et al. 2006, Science]  Alaskan mountain glaciers ~ – 101 ± 22 km 3 /year [Chen et al. 2006, EPSL] Polar Ice Sheet & Mountain Glacial Melting Effects So, the total contribution of ice melting to the global ocean from Antarctica, Greenland, and Alaskan mountain glaciers is ~ – 492 km 3 /year, equivalent to global sea level rise of ~ 1.33 mm/year.

Long-Term and Interannual Sea Level Change About Long-Term and Interannual Sea Level Change  Steric effect accounts for about half of the observed long-term sea level rise during the period (1.2 vs. 2.6 mm/year).  Polar ice sheets & mountain glacial melting may cause sea level rise of ~ 1.3 mm/year (during the period ).  Combined steric and ice melting effect may account for ~ 2.5 mm/year, compared with 2.6 mm/year from satellite altimeters.  Steric and non-steric effects appear equally important in driving the global sea level rise. Conclusions

Long-Term and Interannual Sea Level Change About Long-Term and Interannual Sea Level Change  There appears notable acceleration of steric global mean sea level rise in the last 10 years as compared with the entire 50 years period, which can be associated with strong decadal variations.  WOD01 estimated steric effects fail to show corresponding interannual variability during the 1997/1998 El Nino event.  Terrestrial water storage and atmospheric water vapor do show strong interannual variability well correlated with altimeter observations. Conclusions (cont.)

Thanks! Results presented here are being published in, Chen, J.L., C.R. Wilson, B.D. Tapley, X.G. Hu, Thermosteric Effects on Interannual and Long-Term Global Mean Sea Level Change, J. Geodesy, DOI /s , 2006 (in press). Preprints are available