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IVANA HERCEG BULIĆ

DELAYED ENSO IMPACT ON SPRING PRECPITATION OVER NORTH ATLANTIC/EUROPEAN REGION. IVANA HERCEG BULIĆ ANDRIJA MOHOROVIČIĆ GEOPHYSICAL INSTITUTE, DEPARTMENT OF GEOPHYSICS, FACULTY OF SCIENCE, UNIVERSITY OF ZAGREB, ZAGREB, CROATIA FRED KUCHARSKI

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IVANA HERCEG BULIĆ

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  1. DELAYED ENSO IMPACT ON SPRING PRECPITATION OVER NORTH ATLANTIC/EUROPEAN REGION 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 EMS&ECAM 2011 12 – 16 September 2011; Berlin, Germany

  2. Introduction 2 • 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 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region

  3. Introduction 3 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 ENSO signal in warmer climate conditions: PNA – weakening NAE – strengthening (e.g. Müller & Roeckner, ClimDyn 2008; Herceg Bulić et al., ClimDyn 2011) Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region

  4. Motivation 4 • 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 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region

  5. Experimental design 5 • 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) Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region

  6. Experimental design 6 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 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region

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

  8. Results - correlation PC1(AMJ PREC) → JFM SSTA (tropical Pacific) 8 • 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 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region

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

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

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

  12. Results – Physical mechanism of time delayed ENSO impact on precipitation in NAE region 12 • 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 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region

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

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

  15. Conclusions – Winter ENSO impact on spring precipitation in the NAE region 15 • 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 Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region

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