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: Baldwin & Dunkerton 2001. NAM index (blue/purple positive, yellow/red negative) following weak vortex events. Figure: Sea level pressure composite for El Ni˜ no events 1880-1988. From Fraedrich 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

• 8
• 6
• 4
• 2

2 4 NAO Index (standardized units) 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 JFM NAO Index Distributions

Neutral without SSW Neutral with SSW EN without SSW EN with SSW

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