Ocean warming and sea level rise

1. Ocean warming and sea level rise D. Roemmich1, J. Willis2, J. Gilson1 1Scripps Institution of Oceanography, UCSD 2NASA Jet Propulsion Laboratory Understanding Sea-level Rise and Variability WCRP/COPES Workshop UNESCO, Paris, June 2006

2. Outline • Consider estimates of steric sea level from 3 time periods, distinguished by major evolutions of the ocean observing system. • 50-year record: Sparse hydrography, especially in the Southern Hemisphere. • The last 12 years: Altimetry, quasi-global XBT networks, WOCE hydrography. • The present day: Altimetry, Argo, GRACE. • Conclusions and recommendations.

3. Ocean warming and sea level rise Questions: • What is the long-term rate of global mean steric sea level rise and how does it vary on seasonal, interannual, and decadal time-scales? • How does steric sea level compare to total sea level on each time-scale? • What are the spatial patterns of variability of steric height on each time-scale? • Does knowledge of the spatial pattern and the separation into steric and eustatic components help in understanding the processes that cause sea level variability and change? • Has the steric expansion accelerated in the past decade compared to previous decades?

4. Evolution of the ocean observing system: 1950s Sea level at coastal stations and some island gages. Most of the ocean was not observed. Can we estimate the global spatial average? Hydrographic station data: depth > 500 m, 1950-1959. Coverage was very sparse, especially in the Southern Hemisphere. Is it reasonable to estimate multi-decadal changes with this starting point?

5. Warming of the world ocean, 1955–2003 Levitus et al. (2005) Estimates of global ocean heat content for the 50-year period have been made by Levitus and colleagues and by Ishii and colleagues, using objective interpolation to map anomalies from the long-term mean. OI estimates of anomaly tend toward zero where there is no data, as in the Southern Hemisphere. (Above) Heat gain in the air/sea/land climate system is dominated by oceans. A curious result is that the hemispheres gained similar amounts of heat, despite the Southern Hemisphere oceans having twice the area of the Northern Hemisphere oceans. Is this an artifact of sampling deficiencies? n.b. 5.1 x 1022 J ~ 1 cm

6. Gregory et al., 2004 – (Global) • Representative averages: assume that the average temperature anomaly of sampled points in the layer is representative of the average over the whole volume of the layer. • Zero anomalies: where temperature is not sampled assume that the temperature anomaly is zero and hence does not contribute to the heat content anomaly of the layer. • Gille, 2006 – (Southern Hemisphere) • Assume that regions with no data have the same long-term trend as regions with data: • bin by 0.2 dynamic meter intervals (red line), • by 10o latitude bins (orange line), • or by 5o by 5o bins (green line).  • Assume a zero trend in all 5o by 5o bins with missing data.  • Lyman et al., 2006 – (Global) • This error estimate is formed by comparing altimetric height, globally averaged in each of the 13 years (1993-2003), subsampled at stations from the given (earlier) year, with that from the full altimetric height dataset in each of the 13 years. • Because altimetric height only represents variability over a 13 year period, variance from multi-decadal timescales is not represented. n.b. 5.1 x 1022 J ~ 1 cm

7. 50-year estimates of globally-averagel total sea level from Church et al. (2004, black line) and the thermosteric (0 – 700 m) component from Antonov et al. (2005, red line) and Ishii et al. (2006, blue line). Inclusion of a greater depth range (0 - 3000 m) and the halosteric component would add about 6 mm to the red and blue lines by the end of the time-series. Part of the difference between total and steric sea level estimates may be due to the use of different analysis techniques. Total sea level Church et al., 2004 Thermosteric sea level Antonov et al., 2005 Ishii et al., 2006 Either ocean warming was small component of the 50-year sea level rise (about 25 mm out of the 100 mm total), or it was substantially underestimated through sparse Southern Hemisphere sampling (Gregory et al., 2004, Gille, 2006). The plausible range in 50-year steric sea level rise needs further study.

8. Evolution of the ocean observing system: 1990s The sea level network included more coastal and island stations. Satellite altimetry, using the sea level network for calibration, provided global views of sea level. How accurate are the annual global mean values? + floats XBTs provided extensive coverage of 0-750 m temperature profiles. Some gaps remain. Hydrographic stations, 1990-1999, included the WOCE global survey

9. Thermosteric sea level in the recent period (since 1993) Right: Total sea level (red, source:AVISO) and thermosteric (0 – 750 m) sea level estimated from in situ data only (green) and using the altimeter/ thermosteric height correlation as a first guess (blue) for objective mapping of the in situ data. (From Willis et al., 2004 and Lyman et al., 2006). There appears to be substantial cooling in the period 2003-2005 (Lyman et al., 2006). Left: Temperature change, 1993 – 2003, 60oS to 60oN

10. 5 estimates of upper ocean thermosteric sea level rise, 1993 - 2003 Domingues et al., 2006 Willis et al., 2004, Lyman et al. 2006 Estimates of thermosteric sea level rise from 1993 – 2003 using a variety of analysis techniques (Willis et al., 2004, Lombard et al., 2005, Domingues et al., 2006) are similar, about 1.6 mm/yr (about 1.3 mm/yr for OI estimates), but with interannual differences. All of these are strongly dependent on XBT data. Does the 1998 ENSO signal seen in sea level have a thermosteric component? Lombard et al., 2005 Calculations with a common dataset are needed to reconcile these differences.

11. Consistency of ocean heat storage and satellite radiation budget Interannual comparison of global ocean heat storage (blue, from Willis et al., 2004) against global net flux anomalies from ERBE/ERBS Nonscanner WFOV Edition3_Rev1 (red) and CERES/Terra FM1 Scanner ES4 Edition2_Rev1 (green) for a 10-year period from 1993 to 2003 (from Wong et al., 2006). In a gross sense, the interannual variability in ocean heat storage described by Willis et al. (2004) is supported by the ERB satellite data.

12. Pattern of global steric height increase for the period 1993 – 2003, from Domingues et al. (2006), above, and Willis et al. (2004), below. Panels on the right show globally averaged and globally integrated values. The patterns give clues of the underlying causes. Dynamic height 0/2000 dbar For example, the increase of sea level and heat content in the southwest Pacific was shown to be caused by increased wind stress curl (Southern Annular Mode) spinning up the gyre circulation, Roemmich et al. (2006, right) and Qiu and Chen (2006).

13. Evolution of the ocean observing system: present day New additions to the observing system since 2000: Argo: ~90,000 CTD profiles in 2006.

14. Annual mean steric height for 2005 (Top) Distribution of 62,000 Argo T/S profiles used for calculation of 2005 steric height. (Bottom) Mean 0/2000 dbar steric height (2005) from Argo, calculated in 5o x 3o boxes

15. The 2005 annual cycle in sea surface height and its components 3-month running mean values of 2005 global mean Argo steric height, 0/2000 dbar (black line). Also shown are the 2005 global mean eustatic height (blue line, coutesy D. Chambers) and the sum of the two (magenta line). This plot is meant as a demonstration of the system and to encourage more careful examination of the complete observing system.

16. The 2005 annual cycle in sea surface height and its components (IB) 3-month running mean values of 2005 global mean Argo steric height, 0/2000 dbar (black line). Also shown are the 2005 global mean eustatic height (blue line,courtesy D.Chambers) and the sum of the two (magenta line), and 2005 altimetric height (green line, source AVISO, includes IB correction). This plot is meant as a demonstration of the system and to encourage careful examination of the complete observing system.

17. The global pattern of monthly anomalies Argo steric height March anomaly from 12-month 2005 mean. • The hemispheric patterns due to seasonal warming/cooling are similar. • Many regional features (tropical signals, enhanced magnitude in wbc) are also similar in the two datasets, though not well-resolve by Argo sampling. • Altimetric height contains barotropic variability, though smaller in annual cycle than steric height in most regions. AVISO altimetric height smoothed March anomaly from 12-month 2005 mean.

18. Conclusions • Estimates of ~0.4 mm/yr in steric sea level rise over the 50-year period based on objective interpolation provide a lower bound by representing sampled regions in a global area average. • For the recent period, 1993 – 2003, thermosteric sea level rise is about 1.6 mm/yr and roughly half of the total sea level rise. • Estimates in this period are dependent on XBT data and differ from one another in interannual variability. • Patterns of steric sea level rise provide important clues to the causes of regional change. • The present observing system is characterized by critical improvements with respect to earlier eras: • Global (+/- 60o, 0 - 2000 m) coverage by Argo with much better temperature data quality than XBT, plus salinity and velocity. • Independent measurement of eustatic sea level (GRACE). • High value in the Argo + Altimetry + GRACE combination.

19. Recommendations • Further attention to the 50-year record is needed to estimate the trend and uncertainty in global ocean heat content and steric sea level. • Studies of thermosteric sea level for 1993-2003 using different techniques should be repeated with a common dataset, in order to understand differences in interannual variability and in 10-year changes. • Continued measurement is essential of sea surface height (altimetry + SL gages) and its steric (Argo) and eustatic (GRACE) components long enough to understand the error budgets of the individual elements and the capabilities of the combined system. • Investigation is needed of the other synergies between Argo, altimetry, and GRACE, related to sea level. • Freshwater budget: Ocean salinity variability (Argo) in relation to sea-ice volume and eustatic sea level rise (GRACE). • Large-scale geostrophic dynamics: 1000 m velocity and 0/1000 m steric height (Argo) in relation to mean surface slopes (GRACE), and time-varying surface slopes (altimetry, GRACE).