Origin of the Recent Tropical Atlantic SST warming:

the role of ocean dynamics?

J. Servain1,3, G. Caniaux2, G.A. Hounsou-Gbo3,

Y.K. Kouadio4, M.J. McPhaden5, and M. Araújo3

1. IRD/LOCEAN, 4 Place Jussieu, Paris, FRANCE

2. CNRM/GAME (Météo-France/CNRS), Toulouse, FRANCE

3. Universidade Federal de Pernambouco, Recife-PE, BRAZIL

4. LPA, Université de Cocody, Abidjan, IVORY COAST

5. PMEL/NOAA, Seattle-WA, U.S.A.

Introduction

•Observed oceanic trends in part reflect our changing climate •The intensity of the long-term SST warming is different depending on ocean bassin and the period of reference: -between 1850 and 2004: NH +0.59°C and SH +0.46°C [Rayner et al., 2006] -between 1956 and 2004, the Atlantic ocean gained +0.059°C/decade; +0.043°C/decade elsewhere [Palmer et al., 2007] •Here we focus mainly on the tropical Atlantic ocean and recent long-term climatic trends, by using a coherent data set (SST and winds) , mainly from VOS since 1964 [Servain et al., 1985]

SST anomalies +0.7°C over 48 years

Pseudo wind stress (PWS) anomalies

+12 m²/s² over 48 years

Temperature and wind tendencies

SERV Mean SST and PWS

1964-2012

•Data base SERV:~6000 observations since January 1964 •Monthly grided SST and pseudo-wind stress (2°x2°), objectively analyzed on 2°x2° boxes with a method based on

Cressman [1959] •Available at: http://www.funceme.br/index.php/areas/oceanografia/campos-numericos-do-atlantico

SST

SST and PWS tendency analyses [Liebmann et al., 2010]

PWS

SST

+0.71°C over 36 years

SST

+0.71°C

SST/PWS SERV

1964-1975

SST/PWS SERV

1976-2012

SST and PWS: regional analysis

•Decrease of the total change (linear regression trends x length of the series) down to -1.2°C in the NH, in agreement with the wind increase (~+10m²/s²) •Slow warming (+0.5°C but locally up to +0.8°C) in the SH

Significant SST change (max: +1.6°C) and PWS

strengthening (up to 20 m²/s² along the African coast) over

the whole basin

AT OAFlux

1976-2012

Comparison with other data sets: AT and SST

AT anomalies I-COADS [Woodruff et al., 2011] OAFlux [Yu and Weller, 2007] TropFlux [Kumar et al., 2012] SST anomalies SERV

Similar to the regional pattern

of SSTs

Significant warming since the 1970’s and similar interannual variability

Surface wind tendencies I-COADS [Woodruff et al., 2011]

OAFlux [Yu and Weller, 2007] WASWind [Tokinaga and Xie, 2011]

TropFlux [Kumar et al., 2012]

Normalized Serv PWS and TropFlux wind-stress

tendencies

Comparison of surface winds and PWS

Long-term increase in wind magnitude (resp. +0.17 m/s, +0.06 m/s, +0.06 m/s, +0.02 m/s per decade),

wind-stress (+6.5x10-4 N/m²) and PWS (+2.7 m²/s²)

PWS SERV

1964-1975

WASWinds 1964-1975

OAFlux 1964-1975

PWS and wind tendencies : regional analysis

•Intensification of the winds and PWS in the NH in all the three data sets •The change in the SH is less consistent, but relatively similar between SERV and OAFlux

NHF TropFlux

1976-2012

NHF I-COADS

1976-2012

Origin of SST tendencies: surface heat fluxes

Surface net heat fluxes I-COADS TropFlux OAFlux

Decomposition of the NHF

TropFlux Latent heat fluxes

PIRATA buoys network

x x x x x x x x

x x

Origin of SST tendencies: in-situ data

Section at 38°W

Section at 0°N

Section at 10°W

Tendencies 1998-2012

S N

N S

W E

Origin of SST tendencies: mean sea level

SLA AVISO

1993-2012

4-6 cm over the whole area max values (10 cm) in the NH

around the ITCZ

1993

Normalized 12-month moving averages of SERV SST and SLA anomalies

•Positive MSL long-term trend clearly related to the long- term trend of SST •Good correspondance between MSL and SST variability (Corr = 0.5 for the detrended non-smoothed series)

EN3 HC 0-700m 1964-2012

Origin of SST tendencies: heat content 0-700 m

0

700m

dzHC in °C.m

Normalized 12-month moving averages of SERV SST and EN3 heat content anomalies EN3 (Ingleby and Huddleston, 2007): ECMWF analysis since 1950

•Marked warming in the HC similar to Palmer et al. (2007) i.e. +30 °Cm/decade •Stronger trend in the N and E of the area where SST changes are the strongest

Conclusions

•SST increase of 0.7°C between 1976-2012 and simultaneous PWS increase of +12 m²/s² over the last five decades •The tendencies are confirmed over other independent parameters (AT, SLA) and other data sets •(1) The negative tendency of surface net heat flux, (2) the positive tendencies of the upper thermocline temperatures (PIRATA buoys), (3) of SLA, (4) of the 0-700 m heat content, tend to prove that the oceanic dynamics is the main actor of the equatorial SST warming •Role of the AMOC (suggested by Rahmstorf et al. [2013], Grist et al. [2010], Wen et al. [2010, 2011], Desbruyères et al. [2013]), role of STCs [Kröger et al., 2005], role of the Agulhas leakage and the Indian Ocean [Lee et al., 2011; Rühs et al., 2013; Shen and Tung, 2014]?