M.Vicomte (1),C. Claud (1), M. Rojo (1), P.-E. Mallet (1), T. Laffineur (2) (1)CNRS/IPSL/LMD, Palaiseau, France (UPMC) (2) ENM, Météo France, Toulouse, France

o Very intense mesoscale storms < km diameter, generally < 24 h lifetime --> difficult to forecast. o A maritime phenomenon, declining rapidly over land (because of the larger fluxes of heat and moisture from the ocean). o Usually occur during winter season, between October and March o Formation requires a cold air outbreak over relatively warm water o Mean winds in excess of 25m/s, strong precipitation. Polar Low (PL) 28 Oct :08 UTC Image AVHRR NOAA 18 (

Within WP1 core theme, our goal is : TO BETTER UNDERSTAND THE ROLE OF THE SST AND SEA ICE CONDITIONS ON THE SPATIAL AND TEMPORAL DISTRIBUTION OF POLAR L OWS

Large interannual variability : Few PL in October ≤ 2 per year Much more in March with a maximum of 13 PLs in 2013 Formation areas of polar lows Focus on Norwegian and Barents Seas (Noer et al., 2011) OctoberNovemberDecember JanuaryFebruaryMarch

(Laffineur et al., Monthly Weather Review, 2014) Representation of PLs in reanalyses compared to observations between 1999 and 2001 : ERA-40 (1.125° resolution) : 24% ERA-I (0.75° resolution) : 45 % Atmospheric reanalyses can however be used for studying the large scale circulation associated with development of PLs

Composite standardized anomalies of Z500, SST-T500, PV300 and winds (ERAI) and significance during cold season for PL days PL formation when : Negative geopotential height (Z500) Large instability (SST- T500 positive) Stratospheric intrusion at lower tropopause (PV 300 positive) Northerly winds at 925 hPa Anomalies last 8-10 days (Mallet et al, JGR Atmosphere, 2013) Z 500 hPa SST-T500 PV 300 hPa Winds 925 hPa

(Rojo et al, Tellus A, 2014 in revision) Most of Pls form along the Norwegian Atlantic Current and other warm oceanic currents ~25% of PLs form at the edge of sea ice during period

Example of polar low (21/02/2012) which formed at the edge of sea ice and lasted 2 days Start in the Norwegian Sea and end in the Barents Sea Formation in high SST gradients. Trajectory followed wind direction at 925 hPa. ERA-I winds at 925 hPa at 22/02/2012 SST and SIC on 21/02/2012 from OSTIA (Operational SST and SIC Analysis) 06:12 21/02 02:32 23/02

A single case of Polar Low observed north of Svalbard, 8 January 2010 Sea Ice Courtesy of Gunnar Noer, Norv Met Institute (NOAA-17 MOZAIKK :40, polarlow.met.no/stars/ ‎ ) A single case of Polar Low observed 16 January 2009 over Kara Sea during period

Observations : 0 PL in March PL in March 2013 Why this difference?

Differences between composites of SST, SST-T500, Z500 and winds in March for the years with negative anomalies of SIC (SI-) in January/February and years with positive anomalies (SI+) (lag of about 2 months) For negative anomalies of SIC in January /February, atmospheric conditions are favorable to PL developments in March over the Barents Sea. (Mallet, 2013) SI- - SI+ (Mallet, 2013 PHD) SI - - SI +

SIC january 2007 SIC january 2013 PLs march 2007PLs march 2013 Analysis of SIC in January Strong negative anomalies of SIC in January 2013 compared to 2007 over Barents Sea Favorable conditions for formation of PLs SIC maps from OISST data sets (NOAA Optimum Interpolation Sea Surface Temperature V2)

Decrease of Polar Lows occurrences in NDJ What is the impact of the Arctic summer sea ice extent on the early/mid- winter occurrence of polar lows? (Rojo, M.)

ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/Sep/N_09_area.txt Arctic sea ice extent anomalies in September for

SST-T 500 hPa In December Z 500 hPa PV 300 hPa Winds 925 hPa Correlation between SIC extent index in September and Z500, PV300, winds and SST- T500 in December For positive anomalies of September SI extent, atmospheric conditions allow formation of PLs over the nordic seas

We have observed that a large part of polar lows develop along relatively warm oceanic currents. Nevertheless 25% of them develop at the edge of sea ice where SST gradient is high. Polar Lows develop over open water areas. The retreat of sea ice the last years opens new areas of polar low formation. Reduced sea ice over the Barents Sea in mid-winter creates more favorable conditions for PL development at the end of the season. Arctic sea ice decline at the end of summer may have impact on the lower activity observed in early/mid winter during the last years.

The associations are investigated in climate simulations : o ECHAM5/MPI-OM (1.875°-1.5° resolution) SIC A1B-20C SST-T500 A1B-20C Similar features, more or less marked, for the 3 scenarios (B1, A1B, A2) : Large retreat of SIC to the north Increased stability over the north Atlantic Ocean Northward shift of favorable regions for polar low formation Comparison of SIC and SST-T500 in NDJFM between 2 scenarios of ECHAM5/MPI-OM: A1B ( ) and 20C ( )

SST SIC Standardized anomalies of SST/SIC using ERAI ERAI database, resolution 0.75°x0.75° Anomaly = (SST (/SIC) March 2007(/2013) - mean SST (/SIC) March 79-13)/ standard deviation February 2007 February 2013 Observations : 0 PL in March PL in March 2013 Why this difference? Analysis of SST/SIC in February Strong negative anomaly of SIC in February 2013 Positive SST anomalies predominant

March 2007March 2013 SST : Strong positive anomalies in March 2007 Colder SST in March 2013 than in February due to several cold air outbreaks SIC : Negative SIC anomaly stronger in March 2007 than in February Formation of sea ice between February and March 2013 over the Barents Sea likely due to several cold air outbreaks Standard anomalies of SST/SIC using ERAI ERAI database, resolution 0.75°x0.75° Anomaly = (SST (/SIC) March 2007(/2013) - mean SST (/SIC) March 79-13)/ standard deviation SST SIC

SST Jan SST Jan 2007 SST Feb SST Feb 2007 SST Mar SST Mar 2007 SST Apr SST Apr 2007 Use of OISST database, monthly data, resolution 1°x1°, since 1982 January and February 2013 : SST warmer March and April 2013 : SST colder

SIC Jan SIC Jan 2007 SIC Feb SIC Feb 2007 SIC Mar SIC Mar 2007 SIC Apr SIC Apr 2007 January and February 2013 : less SIC around Svalbard and at West of Novaya Zemlya March and April 2013 : more SIC at North and East of Barents Sea

PV 300hPa ERAI SST-T500 ERAI LHF NCEP SHF NCEP Wind 925 hPa ERAI Warm winds ~ 270 K from SW prevent the development of PL No stratospheric intrusion that could trigger PL development Low SST-T500 < 37 K (indicator of convective tropospheric heating driven by surface fluxes/static stability) Weak turbulent flux : LHF < 90 W.m-2, SHF ~ 50 W.m-2 T (K)

PV 300hPa ERAI SST-T500 ERAI LHF NCEP SHF NCEP Cold winds ~ 255 K from NE allow formation of sea ice at North and East of Barents Strong signature of PV anomaly > 50 pvu at SE of Barents Strong SST-T500 > 47 K at North of Norway, where PL form Weak turbulent flux : LHF > 115 W.m-2, SHF ~ 250 W.m-2 Wind 925 hPa ERAI T (K)

Analysis of several cases of long duration Polar Lows : , , Study of SST, SIC and atmospheric parameters Exceptional case in Kara Sea in

Large Interannual variability

In December SST-T 500 hPa Z 500 hPa PV 300 hPa Winds 925 hPa

In January SST-T 500 hPa Z 500 hPa PV 300 hPa Winds 925 hPa

The research leading to these results has received funding from the European Union 7th Framework Programme (FP ), under grant agreement n NACLIM