Separating & Quantifying the Distinct Impacts of El Ni no and - PowerPoint PPT Presentation

Separating & Quantifying the Distinct Impacts of El Ni˜ no and Stratospheric Sudden Warmings on North Atlantic and Eurasian Wintertime Climate Jessica Oehrlein 1 , Gabriel Chiodo 1 , Lorenzo Polvani 1,2 1 Columbia University, 2 Lamont-Doherty Earth Observatory January 8, 2019 Jessica Oehrlein SSWs and El Ni˜ no

Stratospheric Sudden Warming & El Ni˜ no Surface Impacts The mean surface signatures of stratospheric sudden warmings (SSWs) and El Ni˜ no are a negative Northern Annular Mode. Figure: Sea level pressure Figure: Baldwin & Dunkerton 2001. NAM composite for El Ni˜ no events index (blue/purple positive, yellow/red 1880-1988. From Fraedrich negative) following weak vortex events. and M¨ uller 1992. Jessica Oehrlein SSWs and El Ni˜ no

ENSO and SSW Frequency El Ni˜ no increases SSW frequency by a factor of 1.3. Figure: SSW frequency across ENSO phases. From Polvani et al 2017. Jessica Oehrlein SSWs and El Ni˜ no

Relative Impacts of El Ni˜ no and SSWs Observational (Butler et al. 2014) and modeling (Polvani et al. 2017) studies: comparable differences due to SSWs, ENSO Modeling studies (Ineson and Scaife 2009, Sigmund et al. 2013, Domeisen et al. 2015, Scaife et al. 2016): anomalous mean European El Ni˜ no winter conditions only when SSWs occur . Other modeling work (Bell et al. 2009, Li and Lau 2012): negative NAO shift in El Ni˜ no winters in absence of active stratosphere . Jessica Oehrlein SSWs and El Ni˜ no

Approach Large ensembles of one-year model integrations Prescribed neutral-ENSO and strong El Ni˜ no SSTs Compare effects of El Ni˜ no and SSWs to basic state of neutral-ENSO without SSW Jessica Oehrlein SSWs and El Ni˜ no

Summary of Runs Whole Atmosphere Community Climate Model (WACCM) 200 one-year integrations for ENSO-neutral and El Ni˜ no Neutral-ENSO SSTS: 1950-2014 observed climatology El Ni˜ no SSTS: composite of warm ENSO events over 1950-2014 Warm ENSO winters defined by N3.4 index with anomaly above 1 K for three consecutive three-month seasons Jessica Oehrlein SSWs and El Ni˜ no

SSW Frequency Table: Summary of stratospheric sudden warming (SSW) events in EN and Neutral phases. EN Neutral Total Winters 200 200 SSW events 174 85 SSW frequency/decade 8.7 4.3 Winters with SSWs 140 72 Winters with 2 SSWs 30 11 Winters with 3 SSWs 2 1 Jessica Oehrlein SSWs and El Ni˜ no

Effect on NAO Index JFM NAO Index Distributions 0.5 Neutral without SSW Neutral with SSW 0.45 EN without SSW EN with SSW 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 -8 -6 -4 -2 0 2 4 NAO Index (standardized units) Figure: Fitted Gaussians of NAO indices for each state normalized with respect to the neutral-ENSO without SSW base state. Jessica Oehrlein SSWs and El Ni˜ no

Composite Methodology Composites with SSWs: 60 days after first SSW of the winter Composites without SSWs: Same sets of 60 days as in Neutral-ENSO with SSW, with years chosen randomly subject to constraint that no single day appears in the composite twice. All composites shown are anomalies from Neutral-ENSO without SSWs. Jessica Oehrlein SSWs and El Ni˜ no

60 Days After Event: Neutral-ENSO with SSW Figure: Surface climate response in the 60 days following an SSW in Neutral-ENSO conditions. Jessica Oehrlein SSWs and El Ni˜ no

60 Days After Event: El Ni˜ no without SSW Figure: Surface climate response in 60-day periods during El Ni˜ no winters with a quiescent stratosphere. Jessica Oehrlein SSWs and El Ni˜ no

60 Days After Event: El Ni˜ no with SSW Figure: Surface climate response in the 60 days following an SSW in El Ni˜ no conditions. Jessica Oehrlein SSWs and El Ni˜ no

60 Days After Event: Linear Additivity Figure: Surface climate response to SSWs in Neutral-ENSO and El Ni˜ no conditions. Jessica Oehrlein SSWs and El Ni˜ no

Regional Impacts Table: Eurasian (60 ◦ -75 ◦ N, 30 ◦ -120 ◦ E) surface temperature and Mediterranean (35 ◦ − 45 ◦ N, 10 ◦ − 25 ◦ E) precipitation anomalies. Neutral w/ SSW EN no SSW EN w/ SSW Eurasian T (K) -2.51 -0.59 -2.21 Med. precip (mm/mo) +6.18 +6.91 +11.06 Jessica Oehrlein SSWs and El Ni˜ no

Local Impacts Table: Surface temperature anomalies (K) and precipitation rate anomalies (mm/month) for Paris, Stockholm, and Madrid. Neutral w/ SSW EN w/o SSW EN w/ SSW Stockholm Surface T -1.49 -1.34 -2.34 Precipitation -25.58 -28.09 -44.24 Paris Surface T -0.45 -0.57 -1.06 Precipitation +7.81 -4.75 -1.44 Madrid Surface T -0.20 -0.32 -0.42 Precipitation +6.28 +11.83 +14.04 Jessica Oehrlein SSWs and El Ni˜ no

Conclusions SSWs and strong El Ni˜ nos have comparable effects on the NAO, precipitation. SSWs contribute much more to Eurasian cooling. SSW effect is similar regardless of EN/Neutral-ENSO conditions. Stratosphere must be well-resolved for accurate European wintertime forecasts. Jessica Oehrlein SSWs and El Ni˜ no