Transcript Copenhagen, 2009 changes in ocean`s T, S, oxygen

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
Scripps Institution of Oceanography, UCSD, La Jolla, CA, USA
Ocean Observations: schematics of climate change
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Thermal expansion 1995-2003 (Ishii et al.,
2006, IPCC AR4)
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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
IPCC AR4
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Global view
Including zonal redistributions
Ocean circulation changes
Ocean oxygen changes
Changes in natural climate modes
Towards IPCC AR5
Salinity
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
1500 m
Saltier Atlantic and Indian
Fresher Pacific
2. Meridional FW transports from
evaporative tropics/subtropics to high
latitudes
3. Zonal FW transports from
Atlantic/Indian to Pacific
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
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
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
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.
2007 salinity anomalies
1950
2000
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 temp. anomalies
Salinity changes from quasi-synoptic data: trends or variability?
1990s minus 1950s-1960s
Atlantic (Curry et al., 2003)
Pacific (after Wong et al., 2001)
Freshening of AAIW, LSW, NADW,NPIW
Salinification of subtropics
IPCC AR4
2000s minus 1990s
Atlantic (2003 minus 1989)
Pacific (2006 minus 1991)
Freshening of SAMW
Saltier northern N. Atlantic
MixedTowards
results in
subtropics
IPCC
AR5
Indian (2007 minus 1995)
Salinity changes in the last decade?
Southern hemisphere subtropical gyre pycnoclines
Fresher South Pacific, saltier Indian and eastern Atlantic
Talley et al. (in prep)
Salinity changes are within the subducted thermocline,
SAMW, shallower than the AAIW
Consistent with stronger ST gyre circulation
1500
Atlantic 2003 minus 1992
1500
Indian: 2002 minus 1987
Pacific: 2003 minus 1991
Salinity trends and relation to changes in freshwater forcing:
global patterns (zonally averaged)
Shallower (lower latitude outcropping) isopycnals are saltier
Bindoff et al. (2009)
Slightly deeper (higher lat. outcropping) isopycnals are fresher
Consistent with higher precipitation at higher latitudes, higher
evaporation in the subtropics
Salinity trend 1970-2005
Top: shallow salinity maximum
Bottom: NPIW-SAMW-AAIW
Bottom: zonally-averaged salinity change
Middle: implied change in P-E
Top: IPCC model results
Oxygen
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)
Oxygen changes/variability: N. Atlantic subpolar and subtropical decline
Johnson and Gruber, 2005
•Oxygen decline pronounced at base of
pycnocline (mode water)
•Decline in LSW
•Increase in shallower part of NADW
Labrador Sea
Eastern central
(1999 minus 1990)
SPMW layer (27.1-27.6)
(O2 decrease)
LSW/NADW layer (27.9)
(O2 decrease)
NADW layer (28.0)
(O2 increase)
Data from I. Yashayaev
(2003 minus 1988)
Oxygen decline in the tropical O2 minima: a global pattern
Oxygen minimum zones expanding, oxygen content decreasing
Stramma et al (2008)
Consistent with climate change response (Bopp et al., 2002).
Oxygen in the oxygen minima
Time series 1960-present
Southern hemisphere subtropical gyre (30°S) oxygen increase in
thermocline
Atlantic ST O2
increase, also
consistent with gyre
spinup (Talley, in prep)
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)
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
Oxygen trend 1970-2005
Top: change on density surfaces
Bottom: change due to heaving of density surfaces
Bindoff et al.
(2009)
O2, pressure and pot. Temp.
changes as a function of density
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
Pacific 150W (central meridian)
DIC change due to
circulation change
Anthropogenic DIC change
Atmospheric oxygen decline
(R. Keeling and group)
O2/N2 ratio (per meg)
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
Circulation
Southern ocean circulation: intensification and shift of ACC
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
Gille (2002)
Cai (2006)
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 Mode
Strengthening of SH westerlies in the 1990s
20% intensification of S. Pacific ST gyre, based
on SSH, subsurface floats
Roemmich et al. (2007)
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Southern Annular Mode 5-year running mean (CPC)
Change in SLP and Ekman pumping
200/1800
1000/1800
Dyn.ht. changes Argo-WOCE
Change in SST
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?
Ocean Observations: schematics of climate change
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IPCC AR4
Global view
Ocean circulation changes
Ocean oxygen changes
Changes in natural climate modes
Towards IPCC AR5