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Alan Condron 1 , Grant Bigg 2 and Ian Renfrew 3 1 Woods Hole/MIT 2 University of Sheffield

Modelling the impact of polar mesoscale cyclones on ocean circulation Are we under-forcing our ocean models?. Alan Condron 1 , Grant Bigg 2 and Ian Renfrew 3 1 Woods Hole/MIT 2 University of Sheffield 3 University of East Anglia, Norwich. Background.

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Alan Condron 1 , Grant Bigg 2 and Ian Renfrew 3 1 Woods Hole/MIT 2 University of Sheffield

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  1. Modelling the impact of polar mesoscale cyclones on ocean circulationAre we under-forcing our ocean models? Alan Condron1, Grant Bigg2 and Ian Renfrew3 1Woods Hole/MIT 2University of Sheffield 3University of East Anglia, Norwich

  2. Background • Ocean models are commonly forced with atmospheric reanalysis data. • Intense mesoscale storms occur in the polar regions where air-sea heat exchanges initiate open ocean convection. • Consequently, a failure to represent small-scale vortices in these datasets will lead to the under-forcing of the ocean.

  3. Are models under-forcing the ocean? A B Actual forcing Current Forcing

  4. site of deep convection Key region for ocean circulation = High mesoscale cyclone density

  5. Mesoscale Vortices:Polar mesocyclones Mesoscale (<1000 km dia.) Longevity: 3-48 hr (Short-lived) minor vortices  intense “Polar Lows” (wind >15 ms-1) Most vigorous: hurricane force winds (>32ms-1). Frequently occur in cold arctic air outbreaks. Polar mesocyclones have all the necessary ‘factors’ to influence open ocean deepwater convection

  6. Key Questions to Answer • What percentage of mesoscale atmospheric vortices are missing from reanalysis data (ERA-40)? • What impact on the ocean is there in ‘bogusing’ these missing mesoscale cyclones into the atmospheric forcing fields?

  7. Compare mesocyclone location in ERA-40 with satellite imagery = ERA-40 lows = Satellite mesocyclones 2 1 • Match a period in ERA-40 to a satellite image in Harold et al. (1999) database. • 2. Count number of cyclones in ERA-40 that are present on satellite imagery

  8. ERA-40 consistently detects 75% of mesocyclones >500 km diameter 100 90 80 70 60 50 Cyclones captured in ERA40 (%) 40 30 20 ERA-40 10 0 500 450 400 350 300 Number of polar mesocyclones 250 200 150 100 50 0 0 100 200 300 400 500 600 700 800 900 1000 Diameter (km) • ERA-40 is deficient at resolving mesocyclones below 500km in size < • See Condron et al. 2006, Mon. Wea. Rev.

  9. What about the missing vortices? 75% of mesocyclones

  10. Bogus in missing vortices? • Approximate each polar mesoscale vortex by a Rankine vortex

  11. x • 26-27th February, 1984. • ~400 km diameter • Max wind speed: 35 m/s (hurricane force) in main cloud band Above: 13:41 GMT 27 February 1984 Above: airborne wind speed observations Above: ERA-40 12 UTC 27 February 1984

  12. x Above: 13:41 GMT 27 February 1984 Above: airborne wind speed observations Above: ERA-40 12 UTC 27 February 1984

  13. Bogus in two years of polar mesoscale vortices into ERA-40 (2500 vortices) Run control and perturbed forcing ocean modelling experiments using stretched- grid OGCM (FRUGAL, based on MOM) Mean heat fluxes differences are small 158 & 172 W m-2 versus 160 & 173 W m-2 But larges differences over 200 W m-2 Additional 4x1010 J heat extracted. Impacts on the Ocean?

  14. Impacts on the Ocean? • Buoyancy forcing results in the Nordic Gyre ‘spinning up’ • by four times interannual variability • Greenland Sea Deep Water formation generally increases • By 20% in one month, but large variability

  15. Impacts on the Ocean? • Increase in deep water overflow through Denmark Strait of -3.4x10-2 Sv (+2.4%) • This is significant compared to interannual variability

  16. Conclusions • Polar mesoscale cyclones are under-represented in global meteorological analyses (and climate models) • Therefore, where they are common, atmospheric forcing will be too weak • ‘Bogusing’ in 2 years of polar mesoscale cyclones and running control and perturbed ocean modelling runs: • Enhanced heat fluxes • Nordic gyre spin up • Generally increased GSDW formation • Increased deep water overflows through Denmark Strait Condron, A., G. R. Bigg, and I. A. Renfrew (2008), Modeling the impact of polar mesocyclones on ocean circulation, J. Geophys. Res., 113, C10005, doi:10.1029/2007JC004599.

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