Linkages between Arctic sea ice loss and midlatitude

Transcription

1 Linkages between Arctic sea ice loss and midlatitude weather patterns Response of the wintertime atmospheric circulation to current and projected Arctic sea ice decline Gudrun Magnusdottir and Yannick Peings Earth System Science, UC Irvine

2 Background Several studies suggest that the recent AA can increase the likelihood of cold spells over mid-latitudes Supported by recent cold winters Effect of AA on recent more negative NAO/NAM winters and more numerous cold extreme events difficult to discern over the short obs record, also high natural variability Turn to an atmospheric global climate model (AGCM), NCAR s CAM5 What is the effect of the recent extensive sea-ice loss ( ) on the atmospheric circulation in winter? -- compare to response to projected sea-ice change at the end of century ( ) Follows other studies using AGCMs with sea-ice forcing (e.g., recent study by Screen et al)

3 Motivated the following experiments 50-yr control (CTL) simulation, annually repeating sea ice concentration (SIC) & sea surface temperature (SST) that represent climatology (from the monthly HadISST) Two perturbation experiments, each is a 50 member ensemble. Each ensemble member is run for 13 months, started from 1. April of each yr of CTL: 2010C is forced with the annual cycle of the mean SIC for (from HadISST). 2090C is forced with the annual cycle of the mean SIC corresponding to climate model projections for

4 Model experiments CTL: 2010C: 50 yr CAM5 simulation forced with average annual cycle of observed SST and SIC over Ensemble of fifty 13-month simulations forced with average annual cycle of SIC over C: Ensemble of fifty 13- month simulations forced with average annual cycle of SIC projected for Annual cycle of SIC averaged north of 60N

5 SIC averaged over DJF for CTL and two experiments 2010C-CTL 2090C-CTL Also changed SST where SIC was changed by more than 10%

6 Results For pixels with SIC>10% in CTL 2010C Focus on the black curve, the seasonal evolution of net surface flux response 2090C Scale 3 times above SST anom (K) -0.1 SIC anom

7 DJF-mean response in 2m T

8 DJF mean response 2010C 2090C SLP (hpa) Z500 (m)

9 Daily polar cap (north of 65N) Z response Note Feb Oct Mar

10 Summary so far Both experiments (from xsection): negative NAM response in late winter. It is especially impressive for 2010C despite the weaker forcing Late winter response in 2010C has a stratospheric signature whereas 2090C does not show clear stratosphere-troposphere coupling. 2090C has stronger tropospheric response than 2010C. 2090C also has negative NAM response in Dec.

11 Focusing on Feb response in Z500 and U at 10hPa

12 February EP flux and divergence (red) Zonal mean zonal wind (black)

13 February, vertical component of Plumb flux 850hPa

14 Strong seasonality of the response, linear interference Recent studies have emphasized the importance of linear interference between the forced and climatological waves The phase of the anomalous waves resulting from the external forcing is critical for getting a significant impact Constructive interference when in phase Destructive interference when out of phase For 2010C, the forced wave is in phase with the climatological wave in Feb, but not earlier in the season. The spatial distribution of forcing is important since greater decline of sea ice in 2090C does not lead to a stratospheric response

15 Tropospheric winter response in the two experiments, current sea-ice forcing (2010C) and projected end of century sea-ice forcing (2090C). What about mid-latitude extreme events?

16 DJF mean Zonal mean T response Thickness response 1000 to 500hPa Response in transient activity (variance of Z500) 2010C 2090C

17 Is there an increase in extremes over mid-latitudes? Range of the 5400m isoline of height on the 500hPa surface for each day DJF Distribution of daily ranges CTL: white 2010C: light grey 2090C: dark grey

18 Response of 10 th percentile of the 1000hPa daily T

19 Summary current Arctic sea-ice conditions (2010C) favor more intense cold extremes over mid-latitudes (here mostly confined to Asian sector) With stronger sea-ice forcing (2090C), the intensity of cold extremes decreases everywhere north of 45N due to the extension of the Arctic warm anomaly over northern continents. In 2090C (as in 2010C) cold extremes are more intense south of 45N despite far stronger forcing in 2090C the intensity of cold extremes does not change much. Nonlinear relationship between sea-ice retreat and mid-latitude temperature.

20 Net surface flux response for Dec, Jan and Feb

21 SIC (DJF) Net surface flux