Modelling the impact of polar mesoscale cyclones on ocean circulation Are we under-forcing our ocean...
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Transcript of Modelling the impact of polar mesoscale cyclones on ocean circulation Are we under-forcing our ocean...
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/MIT2University of Sheffield
3University of East Anglia, Norwich
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.
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
• 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
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
Diameter (km)
0 100 200 300 400 500 600 700 800 900 1000
Num
ber
of p
olar
mes
ocyc
lone
s
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 E
RA
40 (%)0
10
20
30
40
50
60
70
80
90
100
ERA-40
What about the missing vortices?
Diameter (km)
0 100 200 300 400 500 600 700 800 900 1000
Num
ber
of p
olar
mes
ocyc
lone
s
0
50
100
150
200
250
300
350
400
450
500
Cyc
lone
s ca
ptur
ed
in E
RA
40 (
%)
0
10
20
30
40
50
60
70
80
90
100
75% of mesocyclones
x
Above: 13:41 GMT 27 February 1984
Above: airborne wind speed observations
Above: ERA-40 12 UTC 27 February 1984
• 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
x
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 4x1010 J heat extracted.
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
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
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.