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.

Are models under-forcing the ocean?

A B

Current Forcing

Actual forcing

site of deep convection

Key region for ocean circulation = High mesoscale cyclone density

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

Bogus in missing vortices?

• Approximate each polar mesoscale vortex by a Rankine vortex

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.