IVANA HERCEG BULIĆ ANDRIJA MOHOROVIČIĆ GEOPHYSICAL INSTITUTE, DEPARTMENT OF GEOPHYSICS, FACULTY OF SCIENCE, UNIVERSITY OF ZAGREB, ZAGREB, CROATIA FRED KUCHARSKI THE ABDUS SALAM INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS, EARTH SYSTEM PHYSICS SECTION, TRIESTE, ITALY DELAYED ENSO IMPACT ON SPRING PRECPITATION OVER NORTH ATLANTIC/EUROPEAN REGION EMS&ECAM – 16 September 2011; Berlin, Germany

Introduction SST anomalies in tropical oceans – strong generator of climate variability on Earth SST anomalies in tropical Pacific associated with ENSO – affect both tropical and extratropical atmosphere Impact on midlatitudes – tropical extratropical-teleconnections – Rossby wave propagation ENSO impact on Pacific-North American (PNA) region – widely investigated and well documented ENSO impact on North Atlantic/European region (NAE) – controversial 2 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region

Introduction Winter ENSO forcing: Herceg Bulić and Branković (ClimDyn 2007); Herceg Bulić, Branković and Kucharski (ClimDyn 2011):  Symmetrical ENSO signal in the winter climate anomalies over the PNA region (PNA pattern) in Z200 anomalies as well as in other atmospheric variables (precipitation, temperature, mslp …)  Weak but detectable winter ENSO signal over the NAE region (in line with some previous observational as well modelling studies; e.g. Fraedrich, Tellus 1994; Brönnimann, RevGeophys 2007) Generally, El Niño ⇒ cyclonic type of weather La Niña ⇒ anticyclonic type of weather 3 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region ENSO signal in warmer climate conditions: PNA – weakening NAE – strengthening (e.g. Müller & Roeckner, ClimDyn 2008; Herceg Bulić et al., ClimDyn 2011)

Motivation 4 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region  Besides concurrent (JFM → JFM) atmospheric response to the ENSO forcing, time- delayed ENSO effect on NAE region has been also reported by some authors (e.g. Rodó et.al. ClimDyn 1997; Kiladis and Dias JClim 1989; van Oldenborg et.al. IntJClimatol 2000)  According to some findings, spring precipitation anomaly in the NAE region may be even stronger than that in winter – possible time-lagged teleconnection between winter tropical Pacific SST conditions and NAE region  Delayed ENSO impact on seasonal precipitation anomalies cannot be explained solely by Rossby wave propagation theory → a lack of dynamical interpretation underpinning the time delayed tropical – extratropical teleconnections

Experimental design 5 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region  Time period: 1901 – 2002  Observational data: Precipitation: Climatic Research Unit (CRU) gridded monthly dataset (0.5°×0.5°) Sea-level pressure: HadSLP re-analysis (5°×5°); provided by Hadley Centre, UK Sea-surface temperature: NOAA_ERSST_V2 data (provided by NOAA/OAR/ESRL PSD, USA) –Sea-ice climatology (provided by Hadley Centre, UK)

Experimental design 6 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region Modeled data: ensembles of numerical simulations performed by ICTP AGCM (SPEEDY; T30L8) 1. CTRL experiment: ensemble of 20 ICTP AGCM numerical simulations forced with observed global monthly SST anomalies 2. MIX experiment: ensemble of 10 integrations performed with ICTP AGCM coupled with a passive slab ocean layer in the North Atlantic and SST forcing acting in the Indo- Pacific tropical basin 3. MIX_winter_ENSO experiment: same as the MIX experiment, but with SST forcing in the Indo-Pacific basin acting from October until March

Results – correlation PC1(CRU PREC) NAE → global JFM SSTA 7 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region Focus on AMJ seasonal precipitation

Results - correlation PC1(AMJ PREC) → JFM SSTA (tropical Pacific) 8 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region  Time-lagged correlations: suggestion of time delayed (JFM → AMJ) impact of tropical Pacific  But, they may be associated also with contemporaneous (AMJ → AMJ) influence as a consequence of seasonal persistence of SST conditions in tropical Pacific

Results - correlation PC1(AMJ PREC) → AMJ SSTA (tropical Pacific) 9 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region  Concurrent (AMJ-AMJ) correlations considerably weaker than the time- lagged (JFM-AMJ)

Results - correlation PC1(AMJ PREC) → JFM SSTA (tropical Pacific) – MIX_winter_ENSO experiment 10 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region MIX_winter_ENSO experiment: ENSO forcing restricted to the cold part of year (Oct – Mar)

11 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region Results – precipitation warm and cold ENSO composites

Results – Physical mechanism of time delayed ENSO impact on precipitation in NAE region 12 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region  CRU and SPEEDY precipitation data have demonstrated time delayed ENSO impact on precipitation in the NAE region  Results obtained by MIX_winter_ENSO experiment exclude seasonal persistence of ENSO as a potential explanation  Rossby wave propagation mechanism can not explain winter-spring teleconnection  Some intermediary physical mechanism is expected to be involved

Results – a tentative diagram of physical mechanism explaining time delayed ENSO impact 13 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region Atmospheric bridge (Lau and Nath, JClim 1996) Trop Pac SST JFM Trop Atmosphere JFM Midlat atmosphere JFM Midlat Atl SST AMJ Midlat atmosphere AMJ

Results – physical mechanism 14 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region

Conclusions – Winter ENSO impact on spring precipitation in the NAE region 15 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region Observed as well as modelled data showed significant time-lagged NAE precipitation response to the winter ENSO forcing JFM warm (cold) ENSO events are associated with wetter (drier) AMJ precipitation conditions in the central part of the NAE region Results obtained by SPEEDY simulations with the ENSO forcing restricted to the cold part of year underlie those findings and exclude ENSO seasonal persistence as a major mechanism causing delayed atmospheric response Physical mechanism undrepinning time-delayed atmospheric response to the ENSO forcing includes Rossby wave-train which constitutes atmospheric-bridge between the tropical Pacific and North Atlantic. Induced mid–latitude Atlantic SST anomalies interact with the overlaying atmosphere altering westerly winds and onshore moisture advection. As a result, downstream sea-level pressure pattern is modulated likewise the precipitation