meteorologisk institutt met.no ocean surface warming by polar lows - observational evidence from...
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Meteorologisk Institutt met.no
Ocean Surface Warming by Polar lows- Observational Evidence from Microwave DataØyvind Saetra, Torsten Linders and Steinar EastwoodNorwegian 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?
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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