Effects of Arctic sea ice loss on Northern Hemisphere blocking highs

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1 Effects of Arctic sea ice loss on Northern Hemisphere blocking highs B. Ayarzagüena 1, J. Screen 1, E. Barnes 2 1 University of Exeter, UK 2 Colorado State University, USA

2 Motivation Arctic amplification (AA) is tightly connected with sea ice loss. AA has been linked to changes in the frequency of blockings (BHs) and extreme weather events by some authors [e.g. Francis & Vavrus, 2012, 2015] but some others did not find any significant change [Barnes, 2013; Screen and Simmonds, 2013]. Aim of our study: Analyze the changes in BH due to future sea ice loss by applying different algorithms to experiments of different models.

3 BH algorithms (1D) Doblas-Reyes et al. (2002) (D-R) Tibaldi & Molteni (1990) 1. Instantaneously blocked longitudes (IBL) defined by: - Reversal of mean flow + splitting of jet stream around the block (Z at 500 hpa) - At a constant central latitude (50 N) (a latitudinal shift of ± 5 N is allowed) (not very convenient for a model changing climate analysis) 2. Persistence of IBL of at least 5 days (with some exceptions). Barnes et al. (2012) (B) 1. IBL defined as DB, but the central latitude is not constant (depends on the maximum climatological TEKE). 2. IBL should belong to a group of neighbor blocked latitudes (GIBL). 3. A persistence of 5 days is also imposed to the GIBL (a slight spatial shift of the group is allowed).

4 Comparison of BHs results of both algorithms in ERA-40 ( ) Doblas-Reyes et al. Barnes et al. From Barnes et al. 2012] Main difference is found over the Pacific in DJF and MAM D-R identifies BHs that are low pressure minima on the northern flank of the Pacific jet [Pelly and Hoskins, 2003].

5 Data: sea ice experiments of HadGAM2 & CAM4 Hadley Centre Global Atmospheric Model version 2 (HadGAM2) Resolution: N96L38 Top of the model: 10hPa Prescribed (SST and) SICs: from CMIP5 historical/rcp8.5 runs of HadGEM2-ES Community Atmosphere Model version 4 (CAM4) Resolution: (0.9 lat. x 1.25 lon.) L26 Top of the model: 10hPa Prescribed SST and SICs: from CMIP5 historical/rcp8.5 runs of CCSM4 Sea ice experiments (260 years): - Control (L20) run: SSTs and SICs averaged over from CMIP5 historical run. - Sensitivity runs: SICs from RCP8.5 run: - M21 run: averaged over L21 run: averaged over

6 Performance of the models (D-R algorithm in control simulations) Bias of HadGAM2 High BH frequency in AMJ over the Atlantic. The BH frequency in JFM over the Pacific is much reduced. Related to mean climate bias Bias of CAM4 Very low frequency over the Atlantic. Masato et al. (2013)

7 Future sea ice loss effects on BHs: HadGAM2 (Barnes et al.) JFM MAM JAS OND Only statistical changes are detected in winter and summer. Thick line: stat. signif. M21-L20 Thick line: stat. signif. L21-M21 (Monte-Carlo test: 100 permutations)

8 Future sea ice loss effects on BHs: HadGAM2 (JFM) Doblas-Reyes et al. Barnes et al. Thick line: stat. signif. M21-L20 Thick line: stat. signif. L21-M21 (Monte-Carlo test: 100 permutations) Common features:- Increase of BH frequency over eastern Atlantic in L21 respect to M21. - Decrease in frequency over 180 E-240 E. Differences: No change in the western edge of the European basin in D-R.

9 Future sea ice loss effects on BHs: HadGAM2 (JFM) 300hPa L21 M21 Contours: M21 Shading: stat. 95% confidence level (t-test) 300hPa L21 M21 Possible explanation of changes in BHs over the Atlantic: Southward shift of the storm tracks related to a pseudo-negative phase of NAO increase of BHs over the Atlantic.

10 Future sea ice loss effects on BH (JFM): HadGAM2 vs CAM4 (D-R) HadGAM2 CAM4 Thick line: stat. signif. M21-L20 Thick line: stat. signif. L21-M21 (95% conf. level; Monte-Carlo test: 100 permut.) As expected, there is no agreement in the results over the European sector between both models. Main possible reason is the differences between both models in that area already in L20 (DR not appropriate).

11 BH-related extreme weather events: CAOs in winter (DJF) No changes in the duration or frequency of CAOs in general. Only changes in intensity in high and mid-latitudes regions. Focusing on CAOs in Central Europe: CAOs become warmer in the future only in M21 due to sea ice loss due to From Ayarzagüena & Screen (2016) Advection of warmer polar air into central Europe

12 BH-related extreme weather events: CAOs in winter (DJF) Clim. TAS No changes in the duration or frequency of CAOs in general. Focusing on CAOs in Central Europe: CAOs become warmer in the future only in M21 due to sea ice loss Anom. SLP due to Advection of warmer polar air into central Europe From Ayarzagüena & Screen (2016)

13 Conclusions Model biases in representing BHs should be taken into account in the analysis of future projections. In our preliminary analysis, we have not found robust significant changes in the frequency of BH due to sea ice loss among algorithms in most of the seasons. A lack of changes in BH-induced extreme weather events (CAOs) is found in terms of frequency or duration. Only the intensity seems to change in most of high and mid-latitudes regions because of thermodynamics reasons. These are only preliminary results and more is coming

14 Thank you very much!

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16 Barnes et al. Doblas-Reyes et al. Future sea ice loss effects on BHs HadGAM2 (AMJ, JAS & OND) AMJ JAS OND No robust changes in the other seasons And same conclusion for CAM4 Thick line: stat. signif. M21-L20 Thick line: stat. signif. L21-M21(95% conf. level; Monte-Carlo test:100 permut.)

17 Performance of the models (D-R algorithm in control simulations) BH computed by replacing the climatological mean of Z in models by that of ERA-Interim as in Scaife et al. (2010) Most of the biases are related to model climatological bias

18 Future sea ice loss effects on BHs: HadGAM2 (JFM) 300hPa L21 M21 Central blocking latitude M21 L21 Possible explanation of the discrepancy in the results in Europe: Southward shift of the central blocking latitude due to change in the maximum of EKE and position of the jet. CBL differs from the fixed position (50 N) of D-B.