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. Santoso Nature 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 hiatus o Editorial o 1 of Correspondence o 5 Commentaries o News Feature o 1 Article (England et al.) Mar 2014 issue of Nature Geosci o Editorial o 2 Commentaries o 1 of Letters News Feature on Jan 15 issue of Nature

What is causing this hiatus? Changes in radiative forcing Solar activity minimum ~2009 (Kaufmann et al PNAS) Anthropogenic aerosol increase (Kaufmann et al PNAS) Small volcanic eruptions (Solomon et al Science, Santer et al Nat Geosci) Decline in methane emissions (Estrada et al Nat Geosci) Stratospheric water vapor increase (Solomon et al Science) (Probably) internal variability La Niña-like decadal trend (or PDO, IPO) (Meehl et al Nat CC, 2013 J Clim, Kosaka & Xie 2013 Nature) Energy budget Deeper ocean heat uptake (Watanabe et al GRL, Levitus et al GRL, Balmaseda et al 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 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 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 ( ) Surface Ekman divergence Thermocline convergence advecting warm subtropical water to the equatorial subsurface Acceleration of Equatorial undercurrent Surface uSurface v m u m 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/land o Pacific surface wind (momentum + scalar wind) is prescribed Global SAT anom Global SAT change(?) Radiative forcing historical (no volcano)RCP6 Sfc wind controlclimatology– 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 / /1992 time

OGCM trend for 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 circulationPacific zonal-mean temp & circulation Thermocline warming is consistent with obs Net ocean heat gain of 1.2 x J at 2012 due to sfc wind Explains ~50% of slowdown in global SAT rise until 2012 Ocean reanalysis trend

OGCM trend for 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 20C3MSRES A1B Sfc wind controlclimatology 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 / / /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 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) 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 Obs 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 circulation o Cooling at the surface → lowers global SAT o 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 ) 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 PNAS) o Phase transition of AMO in the mid-1990s (Chikamoto et al GRL) o Pacific internal process (after 1997/98 El Niño) (Trenberth & Fasullo 2013 Earth’s Future)