Download - IVANA HERCEG BULIĆ
IVANA HERCEG BULIĆANDRIJA MOHOROVIČIĆ GEOPHYSICAL INSTITUTE, DEPARTMENT OF
GEOPHYSICS, FACULTY OF SCIENCE, UNIVERSITY OF ZAGREB, ZAGREB, CROATIA
FRED KUCHARSKITHE 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 2011 12 – 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
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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
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Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region
ENSO signal in warmer climate conditions: PNA – weakeningNAE – strengthening
(e.g. Müller & Roeckner, ClimDyn 2008; Herceg Bulić et al., ClimDyn 2011)
Motivation4
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 design5
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 design6
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
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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)
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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)
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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
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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)
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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
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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
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Herceg Bulić and Kucharski: Delayed ENSO impact on spring precipitation over North Atlantic/European region
Atmospheric bridge (Lau and Nath, JClim 1996)
Trop PacSSTJFM
TropAtmosphereJFM
MidlatatmosphereJFM
Midlat AtlSSTAMJ
MidlatatmosphereAMJ
Results – physical mechanism14
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
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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