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Recent intensification of wind-driven circulation in the Pacific and the
ongoing warming hiatus
England, M. H., S. McGregor, P. Spence, G. A. Meehl, A. Timmermann, W. Cai, A. Sen Gupta, M. J McPhaden, A.
Purich, A. SantosoNature Clim. Change 4, 222–227 (2014)
Yu Kosaka, Journal Club (Apr 24, 2014)
The global warming hiatus
• The surface global warming appears to have stopped for this century• CMIP5 historical simulation (until 2005) + RCP scenario (after 2006)
does not reproduce thus hiatus as ensemble mean
Hiatus is attracting wide interests
• Mar 2014 issue of Nature Clim Change was a special issue on hiatuso Editorialo 1 of Correspondenceo 5 Commentarieso News Featureo 1 Article (England et al.)
• Mar 2014 issue of Nature Geoscio Editorialo 2 Commentarieso 1 of Letters
http://www.nature.com/nclimate/focus/slowdown-global-warm/index.html
• News Feature on Jan 15 issue of Nature
What is causing this hiatus?
Changes in radiative forcing• Solar activity minimum ~2009 (Kaufmann et al. 2011 PNAS)
• Anthropogenic aerosol increase (Kaufmann et al. 2011 PNAS)
• Small volcanic eruptions (Solomon et al. 2011 Science, Santer et al. 2014 Nat Geosci)
• Decline in methane emissions (Estrada et al. 2013 Nat Geosci)
• Stratospheric water vapor increase (Solomon et al. 2010 Science)
(Probably) internal variability • La Niña-like decadal trend (or PDO, IPO) (Meehl et al. 2011 Nat CC, 2013 J Clim, Kosaka & Xie 2013 Nature)
Energy budget• Deeper ocean heat uptake (Watanabe et al. 2013 GRL,
Levitus et al. 2012 GRL, Balmaseda et al. 2013 GRL)
– But is this internal or forced?
Obs. global-mean temperature and
trade winds• Observed hiatus and
accelerated GW coincide with IPO+ and IPO–, respectively
• Recent IPO– consistent with negative wind stress in the Eq Pacific
• Note large uncertainty in reanalysis before 1980s (Tokinaga et al. 2013 Nature)
o The recent trend is not necessarily “unprecedented” in WASWind data (Tokinaga and Xie 2011)
ANN GISTEMP anom and 5-yr running mean
Wind stress anom and 20-yr trends[6ºS-6ºN, 180º-150ºW]
Observed (& CMIP5) trends for 1992-
2011
• Acceleration of Walker circulation (against slowdown due to GW)
• Negative phase of IPO (La Niña-like + extratropical anom)
SLP & sfc winds (ERAi)
SSH (AVISO)
SST (HadISST)
SAT (GISTEMP)
SAT (CMIP5 historical + RCP4.5)
Emphasis on the tropical Pacific
• IPO is defined as EOF3 of 13-yr low-pass filtered global SST
• Observed recent sfc wind trend is stronger than regressed trend onto the IPO index
• Suggesting that tropical processes induced the recent IPO trend
Ocean circulation trend (1992-2008)
• Surface Ekman divergence• Thermocline convergence advecting warm
subtropical water to the equatorial subsurface• Acceleration of Equatorial undercurrent
Surface u Surface v
100-200m u 100-200m v
Based on SODA
Trends in ocean-atmos circulation &
ocean temp
Stronger trade winds
Shallower thermocline in east Pacific
Surface cooling in central & east Pacific
Ocean temperature based on reanalyses
Deeper thermocline in west Pacific
Subsurface warming in west Pacific
Global SAT change
x-z perspective
Trends in ocean-atmos circulation &
ocean temp
Stronger trade winds
Equatorial upwelling
Surface cooling
Ocean temperature based on reanalyses
East-west SSH gradient
Subsurface warming
Equatorward subsurface current
(subtropical cell)
Global SAT change
y-z perspective
Model experiment I: OGCM• OGCM + atmos EBM + sea ice model + land model
o Atmos. heat and moisture are coupled with ocean/ice/lando Pacific surface wind (momentum + scalar wind) is prescribed
Global SAT anom Global SAT change(?)
Radiative forcing
historical (no volcano) RCP6
Sfc wind
control climatology –
Pacific wind-forced
climatology
climatology + obs linear
trend(45ºS-45ºN
Pac)
(1)Trend reverses at 2012(2)Stabilizes at 2012 level(3)Trend continues until
20202011/20121991/1992 time
OGCM trend for 1992-2011
• Negative IPO trend• Intensification of
subtropical cell (sfc div, thermocline conv)
• Sfc cooling, subsfc warming
• Eq Pacific cooling is far stronger (> 2x) than obs
SST & ocean circulation Pacific zonal-mean temp & circulation
• Thermocline warming is consistent with obs• Net ocean heat gain of 1.2 x 1022 J at 2012 due to sfc wind• Explains ~50% of slowdown in global SAT rise until 2012
Ocean reanalysis trend
OGCM trend for 1992-2011
• Accelerated subtropical cell circulation
• Intensified subtropical gyre
v and (u,v) trends
SST and column integrated circulation trends
Model experiment II: CGCM• CGCM (CSIRO-Mk3L)• Pacific surface wind (momentum flux only) to ocean
is prescribed
Radiative forcing 20C3M SRES A1B
Sfc wind
control climatology
Pacific wind-forced
climatology
climatology + obs linear trend(45ºS-45ºN Pac)
(1)Trend reverses at 2012
(2)Stabilizes at 2012 level
(3)Trend continues until 2020
2000/2001
2011/20121991/1992 time
• 12-member ensemble each
• Controls model IPO to follow observed trend while ocean heat budget is kept closed
(cf. Kosaka and Xie (2013) restored tropical eastern Pacific SST → Ocean heat budget was not closed)
Global SAT
• CMIP3 (20C3M + ?) + CMIP5 (historical + RCP4.5 or RCP8.5)
• 5-yr running mean, changes from 2000 is added to obs
• Adjusted CMIP result with (Pacific wind-forced – control) of O/CGCM
• CGCM (w/ atmos circulation) explains the current hiatus
Spatial structure of SAT trends for
1992-2011
• Negative IPO pattern• Pacific cooling is too strong, cooling expands to the entire tropics
CGCM control
CGCM Pacific wind trend-forced
Tropical Pacific wind stress vs global
SAT
• 20-yr tropical Pacific τ trend vs. 10-yr global SAT trend
• Positive correlation (r = 0.3) b/w tropical Pacific τ and global SAT trends in CMIP5
• Obs: some outside the CMIP5 cluster; global SAT less sensitive to τ
c.f. 20-yr tropical Pacific SST trend vs. 20-yr global SAT trend by Fyfe & Gillett (2014)
from Fyfe & Gillett (2014 Nature CC)1993-2012 trend
r = 0.63
CMIP5 captures the current hiatus?
• The recent τ trends is far outside the CMIP5 ensemble spread
Very low probability of the current hiatus in CMIP5 (Fyfe & Gillett 2014)
• Amplitude (or phase transition speed) of IPO is underestimated
or • The forced equatorial Pacific τ
trend (Walker circulation slowdown) is overestimated
Ob
s 1992
-2011 20-yr Pacific trade wind trends
(48 members)
Summary & Discussion• Prescribing Pacific sfc wind trends to OGCM/CGCM reproduces the current
hiatus in global SAT changes (50% by ocean processes, 50% by atmos. feedback)
• Intensification of Pacific trade winds due to negative IPO redistribute heat vertically in the ocean, through accelerating the subtropical cell circulationo Cooling at the surface → lowers global SATo Warming in the subsurface (below ~125m)
• The recent intensification of Pacific trade winds is “unprecedented” in 20CR (but considering uncertainty in reanalysis, this is not necessary; Tokinaga et al. 2013)
• The recent trade wind acceleration is outside the CMIP5 ensemble spread
CMIP5 predicts very low chance of the current hiatus in global SAT (Fyfe & Gillett 2014)
• What is inducing this acceleration?o CMIP models project deceleration associated with the global warming
o Rapid Indian Ocean warming (Luo et al. 2012 PNAS)
o Phase transition of AMO in the mid-1990s (Chikamoto et al. 2012 GRL)
o Pacific internal process (after 1997/98 El Niño) (Trenberth & Fasullo 2013 Earth’s Future)