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Climate Change Copenhagen 2009 Session 3: Changes in Ocean Circulation Related to Regional Climate

Review of ocean temperature, salinity and oxygen changes in the Pacific and subtropical southern hemisphere Red = In IPCC AR4 Green = after IPCC AR4. Climate Change Copenhagen 2009 Session 3: Changes in Ocean Circulation Related to Regional Climate Lynne D. Talley

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Climate Change Copenhagen 2009 Session 3: Changes in Ocean Circulation Related to Regional Climate

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  1. Review of ocean temperature, salinity and oxygen changes in the Pacific and subtropical southern hemisphereRed = In IPCC AR4 Green = after IPCC AR4 Climate Change Copenhagen 2009 Session 3: Changes in Ocean Circulation Related to Regional Climate Lynne D. Talley Scripps Institution of Oceanography, UCSD, La Jolla, CA, USA

  2. Ocean Observations: schematics of climate change • Sea level, heating, P-E, ice sheets, sea ice, ocean T, salinity, CaCO3, pH • Zonally-averaged • Focus on meridional redistributions • Focus on Atlantic view • Almost no circulation changes incorporated • Global view • Including zonal redistributions • Ocean circulation changes • Ocean oxygen changes • Changes in natural climate modes ? Thermal expansion 1995-2003 (Ishii et al., 2006, IPCC AR4)

  3. Salinity

  4. Salinity balances and changes: example of importance of zonal redistribution pathways in addition to meridional (Talley, PiO 2008) surface 1. Freshwater convergences required to maintain mean salinity distribution 2. Meridional FW transports from evaporative tropics/subtropics to high latitudes 1500 m Saltier Atlantic and Indian Fresher Pacific 3. Zonal FW transports from Atlantic/Indian to Pacific

  5. Climate change and precipitation/evaporation: IPCC AR4 A warmer world pumps more water vapor into the atmosphere (with the ocean an enormous holding tank for the water): increased hydrological cycle Impacts of change are recorded in ocean salinity Potential for (indirect) feedbacks on climate through changed ocean stratification Predicted precipitation change Dry areas become drier Wet areas become wetter

  6. Salinity trends and relation to changes in freshwater forcing Boyer et al. (2005) Salty oceans becoming saltier Fresher oceans becoming fresher Linear trend 1955-1998, zonally-averaged salinity Atlantic saltier Pacific fresher Indian saltier Global: neutral

  7. Salinity variation: new data set to observe global pattern Hosoda et al., 2008 Example of what will be possible with many years of Argo. Just one year, but the pattern much cleaner than historical trend because sampling is so much better. Fresher N. Pacific Saltier N. Atlantic Surface salinity 2005 (Argo) minus WOA01 climatology

  8. Salinity variations: northern North Atlantic salinification Hughes et al., 2008 (ICES Report 291) Surface salinity increasing since about 1996, following fresh period commencing with the Great Salinity Anomaly in the 1970s. Surface temperatures also increasing, so salinity change is somewhat compensating in density. Water mass salinities are a different matter – see other talks in session. 2007 salinity anomalies 2007 temp. anomalies 1950 2000

  9. Salinity changes from quasi-synoptic data: trends or variability? 1990s minus 1950s-1960s 2000s minus 1990s Atlantic (2003 minus 1989) Atlantic (Curry et al., 2003) Pacific (after Wong et al., 2001) Pacific (2006 minus 1991) Freshening of SAMW Saltier northern N. Atlantic Mixed results in subtropics Freshening of AAIW, LSW, NADW,NPIW Salinification of subtropics Indian (2007 minus 1995)

  10. Salinity changes in the last decade?Southern hemisphere subtropical gyre pycnoclines Fresher South Pacific, saltier Indian and eastern Atlantic Salinity changes are within the subducted thermocline, SAMW, shallower than the AAIW Consistent with stronger ST gyre circulation Talley et al. (in prep) 1500 1500 Atlantic 2003 minus 1992 Indian: 2002 minus 1987 Pacific: 2003 minus 1991

  11. Salinity trends and relation to changes in freshwater forcing: global patterns (zonally averaged) Shallower (lower latitude outcropping) isopycnals are saltier Slightly deeper (higher lat. outcropping) isopycnals are fresher Consistent with higher precipitation at higher latitudes, higher evaporation in the subtropics Bindoff et al. (2009) Bottom: zonally-averaged salinity change Middle: implied change in P-E Top: IPCC model results Salinity trend 1970-2005 Top: shallow salinity maximum Bottom: NPIW-SAMW-AAIW

  12. Oxygen

  13. Oxygen changes/variability: N. Pacific subpolar and subtropical decline Deutsch et al., 2005 N. Pacific oxygen decreases at base of pycnocline Reduced ventilation at these densities (warmer or slower) Similar result for 30°N thermocline - oxygen decrease: Mecking et al. (2006, 2008)

  14. Oxygen changes/variability: N. Atlantic subpolar and subtropical decline Johnson and Gruber, 2005 • Oxygen decline pronounced at base of pycnocline (mode water) SPMW layer (27.1-27.6) (O2 decrease) • Decline in LSW • Increase in shallower part of NADW Labrador Sea (1999 minus 1990) LSW/NADW layer (27.9) (O2 decrease) Eastern central (2003 minus 1988) NADW layer (28.0) (O2 increase) Data from I. Yashayaev

  15. Oxygen decline in the tropical O2 minima: a global pattern Oxygen minimum zones expanding, oxygen content decreasing Consistent with climate change response (Bopp et al., 2002). Stramma et al (2008) Oxygen in the oxygen minima Time series 1960-present

  16. Southern hemisphere subtropical gyre (30°S) oxygen increase in thermocline Indian Ocean ST gyre O2 increase. Consistent with gyre spinup of 10-20%. McDonagh et al. (2005) Atlantic ST O2 increase, also consistent with gyre spinup (Talley, in prep) Pacific ST O2 increase, also consistent with gyre spinup (Talley, in prep)

  17. Global oxygen changes: overall decline Decline in oxygen throughout the upper ocean, stronger at higher latitudes (poleward of 40°). Net decrease in oxygen inventory, reduction in water mass formation Bindoff et al. (2009) Oxygen trend 1970-2005 Top: change on density surfaces Bottom: change due to heaving of density surfaces O2, pressure and pot. Temp. changes as a function of density

  18. What are the implications of ocean O2 decline? Calculating anthropogenic carbon contributions to DIC increase (Sabine et al., 2008): O2 decline helps quantify ocean ventilation change, so that natural changes in ocean carbon budget can be removed from observed Atmospheric oxygen decline (R. Keeling and group) 400 ppm decline in atmospheric oxygen, not as much as expected given the observed CO2 increase. Is the ocean a part of the decreasin budget? O2/N2 ratio (per meg) Pacific 150W (central meridian) 1990 2007 DIC change due to circulation change Anthropogenic DIC change

  19. Circulation

  20. Southern ocean circulation: intensification and shift of ACC Gille (2002) Cai (2006) Circumpolar warming at mid-depth Consistent with slight southward shift of ACC system Consistent with southward shift and strengthening of westerlies, which would also strengthen the SH subtropical gyres Temperature trend at 900 m from 1930s to 2000

  21. Southern ocean circulation: intensification of Indian ST gyre McDonagh et al. (2005) Indian Ocean subtropical oxygen increase Helium/tritium and oxygen OUR used to date water. Conclusion: 15-20% increased circulation, hence increased subduction advecting near-surface waters northward faster Presumed due to increased wind forcing

  22. I I Southern ocean circulation: intensification of S. Pacific ST gyre Roemmich et al. (2007) Increase in Southern Annular Mode Strengthening of SH westerlies in the 1990s 20% intensification of S. Pacific ST gyre, based on SSH, subsurface floats Dyn.ht. changes Argo-WOCE Southern Annular Mode 5-year running mean (CPC) 1000/1800 Change in SLP and Ekman pumping 200/1800 Change in SST

  23. Large-scale decadal climate patterns that strongly affect the Pacific and S.O. What is the projection of climate change on these and other natural modes? El Nino Southern Oscillation (Southern Oscillation Index) Pacific Decadal Oscillation (could also show the PNA) Southern Annular Mode

  24. Moving towards the IPCC AR5 Equal attention should be given to global-reaching changes originating in the Antarctic/Southern Ocean (e.g. deep heat content increases – Johnson et al., Kawano et al.) Analysis in terms of global redistributions, not just meridional changes Apparent trends based on decadal differences must be treated with caution; changes in integrating properties such as salinity, temperature, oxygen can be better interpreted in terms of trends than can synoptic changes in circulation Can ocean heat, chemistry and circulation changes be partially understood in terms of the natural modes of variability?

  25. Ocean Observations: schematics of climate change • Global view • Ocean circulation changes • Ocean oxygen changes • Changes in natural climate modes ?

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