1. Background and MotivationIt is well established that robust dynamical coupling exists between the extra-tropical troposphere and stratospheric polar vortex (e.g., Baldwin et al. 2010, Gerber et al. 2010).

Annual cycle of the stratospheric polar vortex includes an abrupt termination (stratospheric final warming or SFW) with sizable interannual variability (Waugh et al. 1999).

SFWs help organize seasonal march of the tropospheric circulation (Black et al. 2006); SFW knowledge enhances tropospheric predictability (Hardiman et al. 2011).

Implication: Accurately representing the seasonal cycle & interannual variability of the stratospheric polar vortex is important to simulating regional tropospheric climate.

Motivating question: How well are the spatial structure, climatological seasonal cycle and interannual variability of the boreal polar vortex represented in CMIP5 models?

2. Data and MethodsObservational data: NCEP R1, ERA Interim & JRA 25

Pilot study of several CMIP5 simulations

HadCM3: 20th Century Historical (2 members)HadGEM2-A: 20th Century AMIP Simulation HadGEM2-CC: 20th Century Historical SimulationIPSL-CM5A-LR: 20th Century Historical Simulation

(HadGEM2-CC & IPSL-CM5A-LR -> high top models)

NCEP R1 analysis: 1958-2005Historical simulation analyses: 1950-2005AMIP simulation analysis: 1979-2008

3. Polar vortex structure (zonal wind field)

7. Summary

Acknowledgements: This research is jointly supported by the National Science Foundation under Grant ARC-1107384 and the Department of Energy under Grant DE-SC0004942.The NCEP-NCAR reanalyses come from the NOAA Climate Diagnostics Center from their web site at http://www.cdc.noaa.gov/

5. Stratospheric Final Warming EventsDefined as the last time the 50 hPa zonal-mean zonal wind at 70oN drops below zero without returning to the specified positive threshold value (5 ms-1) until the following autumn (following the methods of Black et al. 2006). The criterion is applied to running 5 day averages of zonal-mean wind.

4. Seasonal Evolution of the Polar Vortex

6. Organizing the Tropospheric Circulation

Figure 1Figure 1. Top : Daily values of the 10 hPa zonal mean zonal wind averaged from 60N to 80N for 40years. Pink curve: Long-term seasonal trend. Blue curve: Composite of SFW events (centered onmean SFW date). Black curve: Analogous composite of SFC events (centered on mean SFC date) .Bottom : A composite of the daily AO and NAO indices with respect to SFW dates. The analysisis performed for the 54 years (1950-2003) after firs t removing a remnant annua l cycle (taken as thelong-term daily average seasonal cycle smoothed with a 31 day running average operator).

Figure 2Figure 2. The daily time evolution of zonal-mean zonal wind (m s -1) averaged from 60N to 80N.Top Left: The climatological-mean time evolution centered on April 14 (denoted Lag 0), TopRight : The parallel time evolution for a composite constructed with respect to the annual timing ofSFW events at 50 hPa (Lag 0). Bottom Left (Right) : Composite of 22 earliest (latest) SFW events.

Stratosphere-Troposphere Coupling: The Stratospheric Seasonal Cycle in CMIP5 Models

Robert X. Black1 , Brent McDaniel2 and Yun-Young Lee1

1Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta; 2Biology and Physics, Kennesaw State University, Kennesaw, GA

Left: Zonal-mean zonal wind (m s-1) time averaged over the boreal cool season (Black & McDaniel 2009).

Right: Daily values of 10 hPa zonal mean zonal wind averaged from 60- 80oN. Climatological seasonal cycle (purple) and SFW composite (blue) (Black et al. 2006).

Daily evolution of zonal-mean zonal wind (m s-1) averaged from 60-80oN

Left: Climatological-mean evolution centered on April 14 (denoted lag 0) (Black et al. 2006).

Right: Composite time evolution of SFW events (Black et al. 2006).

NCEP/NCAR HadCM3 (hist-r1) HadCM3 (hist-r3)

HadGEM2-A (amip) HadGEM2-CC (hist) IPSL-CM5A-LR (hist)

Daily average zonal-mean zonal winds are used to assess:

a) Structure and seasonal evolution of polar vortex (PV) b) Interannual variability in polar vortex demise (SFW)

c) Organization of tropospheric winds by SFW events

NCEP/NCAR HadCM3 (hist-r1) HadCM3 (hist-r3)

HadGEM2-A (amip) HadGEM2-CC (hist) IPSL-CM5A-LR (hist)

Boreal polar vortex too weak in HadCM3 simulationsPolar vortex too strong in HadGEM2-A (amip) & IPSLHadGEM2-CC provides best structural representation

Boreal structural evolution poor in HadCM3 simulationsPolar vortex persists too long in HadGEM2-A & IPSLSeasonal evolution well represented in HadGEM2-CC

Dataset Mean Date σ (days)

NCEP APR 14 20

HadCM3 r1 MAY 01 17

HadCM3 r2 APR 29 17

HadGEM2-A APR 24 16

HadGEM2-CC APR 14 22.5

IPSL-CM5A-LR MAY 10 21

HadCM3 HadGEM2-A (amip) HadGEM2-CC IPSL-CM5A-LR

SFW onset determination insensitive to observational datasetWeak trend toward later boreal SFW events between 1958-2011SFW events are delayed in all simulations except HadGEM2-CCInterannual variability well represented in both high-top models

Observed Boreal SFW events Simulated SFW statistics

Daily evolution of zonal-mean zonal wind averaged from 60-80oN

Top Row: Climatological cycle centered on Mean Date (lag 0) Bottom Row: Composite SFW time evolution (lag 0 = SFW onset)

PV breakdown more abrupt in HadGEM2-A & IPSL-CM5A-LRPV breakdown considerably more abrupt in SFW composites Tropospheric tendencies focused at Day 0 in SFW compositesAll models exhibit qualitative resemblance with observations

SFW events provide an organizing influence on the troposphereCMIP5 models are capable of representing this influence, but…PV often too strong (cold pole) and SFW delayed vs observationsGreater interannual variability observed in high top modelsHadGEM2-CC closely resembles observed PV/SFW behaviorAdditional work required for comprehensive model assessment