On the recent Atlantic Niño influence on Pacific ENSO events Irene Polo Javier García-Serrano Teresa Losada Elsa Mohino Roberto Mechoso Fred Kucharski Belén Rodríguez de Fonseca GCL2006:04471

Several presentations of these days have shown: -Changes in the ocean (Canary upwelling..Vigo) from the 70’s -Changes in the precipitation from the 70’s (Cantabria, UCM…) So… What has happened from the 70’s? Have the teleconnections changed? We will try to address this in this talk

Atlantic and Pacific host their own El Niño events: Atlantic Niño (Merle, 1980; Zebiak, 1993): peaking in boreal spring-summer Pacific Niño (Philander, 1990): peaking in boreal winter Reynolds SST anomaly MJJA

Do Pacific and Atlantic Niños correlate to each other? …. “ The Pacific El Niño can affect the Tropical North Atlantic through the Walker and Hadley circulations, favoring the TNA warming in the subsequent spring of the Pacific El Niño year” …. “The Atlantic Niño is mostly independent of the Pacific ENSO variability; it has a shorter characteristic time scale and is not to be confused with the tropical Atlantic response to the Pacific ENSO” …. Both the tropical Pacific and Atlantic host an equatorial mode of interannual variability called the Pacific El Niño and the Atlantic Niño, respectively. Although the Pacific El Niño does not correlate with the Atlantic Niño, anomalous warming or cooling of the two equatorial oceans can form an inter-Pacific-Atlantic sea surface temperature (SST) gradient variability that induces surface zonal wind anomalies over equatorial South America and over some regions of both ocean basins …the Nino3 does not contemporaneously correlate with the Atl3 (r~0.04)… Links between those events have been sought with modest success (Enfield & Mayer,1997; Latif & Grötzner, 2000; Wang,2001,2005,2006; Chang, 2006…) pointing to a Pacific lead by about six months

Climate shift -Change in the Pacific SSTs (Miller et al 1994) - Change in the Indian Monsoon ENSO relationship and Atlantic influence of the Indian Monsoon (Kucharski et al,2007,2008) -Change in the backgroud state of El Niño( Fedorov and Philander, 2000) -Widening of the tropical belt (Seidel et al., 2008) -Change in the Sahelian-ENSO relationship (Janicot et al., 1996) etc…. - Statistical Atlantic-Pacific connection ???

Summer West African rainfall and Tropical Atlantic SST variability AMMA-Project: Period of Study Polo I., B. Rodríguez-Fonseca, T. Losada, J. García-Serrano, Journal of Climate, 2008 scf=31% ruv=0.45 Statistically significant regression maps of precipitation, SST, from the EMCA (summer pt, SST evolution) JJAS prp

Simultaneously the Equatorial Mode appears associated with anomalies of different sign over the equatorial Pacific. As the Atlantic Niño damps, Pacific La Niña develops. Statistically significant regression global SST maps onto Atlantic Niño-index (lag0=JJAS) Lead-lag correlation Niño3-index and Equatorial Mode-index Niño3 leadingNiño3 lagging Polo I., B. Rodríguez-Fonseca, T. Losada, J. García-Serrano, Journal of Climate, 2008 García-Serrano, J.; T. Losada, B. Rodríguez-Fonseca, I. Polo, J., Journal of Climate, 2008 AMMA-Project

Keenslyde and Latif, 2007 This statistical relationship has been suggested in several recent papers Jury et al., 2002: “upper zonal winds in the central Atlantic lead the Niño-3 SST index, particularly in the 1980s and 1990s” Melice and Servain, 2003: Tropical South Atlantic leads the SOI by 4 months after In early decades essentially no relationship appears to exist. In the last three decades, Atlantic SSTAs precede anomalies in the Pacific by 6 months

Observed Relationship between Atl3-Niño 3 ATL3 (Zebiak,1993)

Correlation maps between ATL3 jjas and lagged SSTs

Correlation maps between ATL3 jjas and lagged SSTs

Lag -1 Regression maps of the Atlantic WAM-SST exp. Coeff. index onto the anomalous SST, precipitation,200 hPa velocity potential, surface divergence, surface winds, 200 hPa stream function SST, precipitation 200 hPa Velocity potential surface divergence, surface winds 200 hPa stream function

Lag 0 SST, precipitation 200 hPa Velocity potential surface divergence, surface winds 200 hPa stream function

Lag +1 SST, precipitation 200 hPa Velocity potential surface divergence, surface winds 200 hPa stream function

Lag +2 SST, precipitation 200 hPa Velocity potential surface divergence, surface winds 200 hPa stream function

Lag +3 SST, precipitation 200 hPa Velocity potential surface divergence, surface winds 200 hPa stream function

Lag +4 SST, precipitation 200 hPa Velocity potential surface divergence, surface winds 200 hPa stream function

Thermocline depth

Hypothesis: During recent Atlantic Niños, SST anomalies in the eastern equatorial Atlantic are superimposed on a basic state that has been warming up. The increased deep-convection associated with the thermal forcing impacts the Walker circulation, whereby in the central Pacific surface divergence is enhanced, SST decreases, and the thermocline becomes steeper The hypothesis is tested using a coupled model

Sensitivity Experiments The atmospheric model: ICTP AGCM19,20, version 40. Ocean model: extended 1.5 layer reduced-gravity model Experiment: The ocean model is coupled to the ICTP AGCM in the tropical Indo- Pacific region (between 30ºS and 30ºN), outside which the latter uses climatological SSTs, from the Atlantic sector, where observed, monthly varying SSTs are used. This ocean model has been used extensively in the study of the Pacific El Niño (Chang,1994). Ocean model coupled to the AGCM Prescribed observed SSTs

Simulated relationship between Atl3 and Niño 3

Correlation maps between ATL3 jjas and lagged SSTs Simulations

Correlation maps between ATL3 jjas and lagged SSTs Simulations

Conclusions -The observations show how, from the last three decades, Atlantic Niños (or Atlantic Equatorial mode events) tend to lead Pacific Niñas. -From these decades, there has been a change in the basic state with respect to early ones. -In this way, for this time period, during Atlantic Niño events, and from the early spring, the development of the Equatorial Mode goes together with an enhancement in the ascending branch of the Walker circulation over the equatorial Atlantic and, subsequent displacement of anomalous convection towards the Amazon basin. -This anomalous convection induces anomalous surface divergence over the eastern equatorial Pacific, thus enhancing a Bjerknes feedback mechanism and favouring La Niña development. -Coupled AGCM experiments with prescribed SSTs over the Atlantic reproduce this change and the associated mechanism.