meteorologisk institutt met.no ocean surface warming by polar lows - observational evidence from...

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Ocean Surface Warming by Polar lows- Observational Evidence from Microwave DataØyvind Saetra, Torsten Linders and Steinar EastwoodNorwegian Meteorological Institute

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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?

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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

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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?

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Hydrographic data from IMR

Highest frequency of polar lows

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Temperature inversion (Gimsøy section)

CoastContinental shelfDeep Sea

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Vertical temperature profile

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SST response

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Heat and moist fluxes

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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

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Satellite data from REMSS

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SST 17/12 2004

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SST 18/12 2004

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Difference in SST between 17 and 18 December 2004 (interval is 0.25 deg)

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17/12

18/12

19/12 20/12

21/1228/12

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Daily averages based on the satellite swathsOnly highest confidence level data

16/12 2004 18/12 2004

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19/12 2004

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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 15/3-15/11 2007

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Buoy locations

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Error statistics

St1

St2

RMS (K)

0.57 0.49

BIAS (K)

-0.29 -0.17

STD (K)

0.49 0.46

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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