Meteorologisk Institutt met.no Ocean Surface Warming by Polar lows - Observational Evidence from Microwave Data Øyvind Saetra, Torsten Linders and Steinar Eastwood Norwegian Meteorological Institute

Meteorologisk Institutt met.no Background Tropical hurricanes induces intense vertical mixing in the ocean that leads to entrainment of cold subsurface water to the surface and hamper hurricane intensification Over loop-currents in the Gulf of Mexico, deep war cores of warm waters prevent this surface cooling and hurricane intensification is often observed Is there any corresponding effects for polar lows?

Meteorologisk Institutt met.no The Nordic Seas Hydrography of the Nordic Seas is very different from the Tropical oceans –Density is mainly determined by salinity –Temperatures may increase with depth without compromising stability Warm saline water transport by The North-Atlantic Current (NAC) At high latitudes, the NAC sub-ducts under colder and less saline waters and results in the frequently observed temperature inversions

Meteorologisk Institutt met.no How may this interact with polar lows? Can surface entrainment of this water mass be induced by strong winds from polar lows? If so, is this a possible positive feedback mechanism for polar lows? And also, can this be a mechanism for ocean cooling subsequently strengthening of the thermohaline circulation?

Meteorologisk Institutt met.no Hydrographic data from IMR Highest frequency of polar lows

Meteorologisk Institutt met.no Temperature inversion (Gimsøy section) Coast Continental shelf Deep Sea

Meteorologisk Institutt met.no Vertical temperature profile

Meteorologisk Institutt met.no SST response

Meteorologisk Institutt met.no Heat and moist fluxes

Meteorologisk Institutt met.no Observational evidence of surface warming Polar low observation in the Norwegian Sea 18 December 2004 from NOAA satellite images Strong signal of surface warming observed in microwave satellite data in areas affected by the polar low

Meteorologisk Institutt met.no Satellite data from REMSS

Meteorologisk Institutt met.no SST 17/

Meteorologisk Institutt met.no SST 18/

Meteorologisk Institutt met.no Difference in SST between 17 and 18 December 2004 (interval is 0.25 deg)

Meteorologisk Institutt met.no 17/12 18/12 19/12 20/12 21/12 28/12

Meteorologisk Institutt met.no Daily averages based on the satellite swaths Only highest confidence level data 16/ /

Meteorologisk Institutt met.no 19/

Meteorologisk Institutt met.no Microwave data compared with buoy observations Two buoys deployed in the Barents Sea in March 2007 The buoys record the water temperature at 3 meter depth Daily averages from the buoys have been compared with the SST from optimally SST product from Remote Sensing Systems over the period 15/3-15/

Meteorologisk Institutt met.no Buoy locations

Meteorologisk Institutt met.no

Error statistics St1St2 RMS (K) BIAS (K) STD (K)

Meteorologisk Institutt met.no Summary surface warming observed by satellite is consistent with the hypothesis: vertical mixing induced by strong winds –No warming observed in area outside the North Atlantic current –Strongest signal on the maximum wind side Signal is present the satellite data for almost a week Satellite data show more small scale temporal variations than the buoy data – this may however, be caused by the fact that the buoys observe the 3 metre temperature