1. 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

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 Current Forcing Actual 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. 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? Key Questions to Answer

7. 2 1 = ERA-40 lows = Satellite mesocyclones 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 Compare mesocyclone location in ERA-40 with satellite imagery

8. Diameter (km) 01002003004005006007008009001000 Number of polar mesocyclones 0 50 100 150 200 250 300 350 400 450 500  ERA-40 is deficient at resolving mesocyclones below 500km in size <  See Condron et al. 2006, Mon. Wea. Rev. ERA-40 consistently detects 75% of mesocyclones > 500 km diameter Cyclones captured in ERA40 (%) 0 10 20 30 40 50 60 70 80 90 100 ERA-40

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

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

13. Impacts on the Ocean? 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 4x10 10 J heat extracted.

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.