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 2009Session 3: Changes in Ocean Circulation Related to Regional Climate
Lynne D. TalleyScripps Institution of Oceanography, UCSD, La Jolla, CA, USA
IPCC AR4 Towards IPCC AR5
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
• 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
• 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)
Salinity
Salinity balances and changes: example of importance of zonal redistribution pathways in addition to meridional
Saltier Atlantic and IndianFresher Pacific
1. Freshwater convergences required to maintain mean salinity distribution
1500 m
surface
2. Meridional FW transports from evaporative tropics/subtropics to high latitudes
3. Zonal FW transports from Atlantic/Indian to Pacific
(Talley, PiO 2008)
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
Salinity trends and relation to changes in freshwater forcing
Salty oceans becoming saltierFresher oceans becoming fresher
Atlantic saltier
Pacific fresher
Indian saltier
Global: neutral
Boyer et al. (2005)
Linear trend 1955-1998, zonally-averaged salinity
Salinity variation: new data set to observe global pattern
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.
Saltier N. AtlanticFresher N. Pacific
Hosoda et al., 2008
Surface salinity 2005 (Argo) minus WOA01 climatology
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
1950 2000
2007 temp. anomalies
IPCC AR4 Towards IPCC AR5
Salinity changes from quasi-synoptic data: trends or variability?
Freshening of AAIW, LSW, NADW,NPIWSalinification of subtropics
Atlantic (Curry et al., 2003)
Pacific (after Wong et al., 2001) Pacific (2006 minus 1991)
Atlantic (2003 minus 1989)
1990s minus 1950s-1960s 2000s minus 1990s
Freshening of SAMWSaltier northern N. AtlanticMixed results in subtropics
Indian (2007 minus 1995)
Salinity changes in the last decade?Southern hemisphere subtropical gyre pycnoclines
1500 1500
Talley et al. (in prep)
Atlantic 2003 minus 1992 Indian: 2002 minus 1987 Pacific: 2003 minus 1991
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
Salinity trends and relation to changes in freshwater forcing: global patterns (zonally averaged)
Bindoff et al. (2009)
Salinity trend 1970-2005Top: shallow salinity maximumBottom: NPIW-SAMW-AAIW
Bottom: zonally-averaged salinity changeMiddle: implied change in P-E
Top: IPCC model results
Shallower (lower latitude outcropping) isopycnals are saltierSlightly deeper (higher lat. outcropping) isopycnals are fresher
Consistent with higher precipitation at higher latitudes, higher evaporation in the subtropics
Oxygen
Oxygen changes/variability: N. Pacific subpolar and subtropical decline
Similar result for 30°N thermocline - oxygen decrease: Mecking et al. (2006, 2008)
N. Pacific oxygen decreases at base of pycnocline
Reduced ventilation at these densities (warmer or slower)
Deutsch et al., 2005
Oxygen changes/variability: N. Atlantic subpolar and subtropical decline
Johnson and Gruber, 2005
Data from I. Yashayaev
Labrador Sea
(1999 minus 1990)Eastern central (2003 minus 1988)
SPMW layer (27.1-27.6) (O2 decrease)
•Oxygen decline pronounced at base of pycnocline (mode water)
NADW layer (28.0) (O2 increase)
LSW/NADW layer (27.9) (O2 decrease)
•Decline in LSW•Increase in shallower part of NADW
Oxygen decline in the tropical O2 minima: a global patternStramma et al (2008)Oxygen minimum zones expanding, oxygen content decreasing
Consistent with climate change response (Bopp et al., 2002).
Time series 1960-present
Oxygen in the oxygen minima
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)
Pacific ST O2 increase, also consistent with gyre spinup (Talley, in prep)
Atlantic ST O2 increase, also consistent with gyre spinup (Talley, in prep)
Global oxygen changes: overall decline
Bindoff et al. (2009)
Decline in oxygen throughout the upper ocean, stronger at higher latitudes (poleward of 40°).Net decrease in oxygen inventory, reduction in water mass formation
Oxygen trend 1970-2005
Top: change on density surfacesBottom: change due to heaving of density surfaces
O2, pressure and pot. Temp. changes as a function of density
What are the implications of ocean O2 decline?
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?
1990 2007
O2/N2 ratio (per meg)
Atmospheric oxygen decline (R. Keeling and group)
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
DIC change due to circulation change
Anthropogenic DIC change
Pacific 150W (central meridian)
Circulation
Southern ocean circulation: intensification and shift of ACC
Circumpolar warming at mid-depthConsistent with slight southward shift of ACC systemConsistent with southward shift and strengthening
of westerlies, which would also strengthen the SH subtropical gyres
Gille (2002)Cai (2006)
Temperature trend at 900 m from 1930s to 2000
Southern ocean circulation: intensification of Indian ST gyre
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
McDonagh et al. (2005)
Southern ocean circulation: intensification of S. Pacific ST gyre
Increase in Southern Annular ModeStrengthening of SH westerlies in the 1990s20% intensification of S. Pacific ST gyre, based
on SSH, subsurface floats
Change in SLP and Ekman pumping
Southern Annular Mode 5-year running mean (CPC)
Dyn.ht. changes Argo-WOCE
1000
/180
020
0/18
00
Change in SST
II
Roemmich et al. (2007)
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
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?
IPCC AR4 Towards IPCC AR5
Ocean Observations: schematics of climate change
• Global view• Ocean circulation changes• Ocean oxygen changes• Changes in natural climate modes
• Global view• Ocean circulation changes• Ocean oxygen changes• Changes in natural climate modes
?