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

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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

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