Mesoscale Air-Sea Interactions Atmosphere wind & heat flux - - PowerPoint PPT Presentation

Mesoscale Air-Sea Interactions

Hyodae Seo Physical Oceanography Department Woods Hole Oceanographic Institution WHOI Summer Lecture Series July 28, 2014

hseo@whoi.edu

Atmosphere ➜ wind & heat flux ➜ Ocean Atmosphere ⬅︎ SST ⬅︎ Ocean

AMSR-E SST, June 1 2003 http:/ /aqua.nasa.gov/highlight.php

World's Oceans are full of mesoscale eddies and fronts! Tropical Instability Waves Coastal Upwelling Coastal Upwelling Coastal Upwelling Coastal Upwelling Kuroshio Gulf Stream Antarctic Circumpolar Current Oyashio North Atlantic Current

Outline

• 1. Air-sea interaction on mesoscale vs large-scale?
• 2. Mechanism for mesoscale air-sea interaction?
• 3. Impact on the ocean and atmosphere?
• 4. Summary

Air-sea interaction at basin-scale

(slow and large scales)

Stronger wind speed ➔ lower SST via mixing and turbulent flux Negative correlation: Atmosphere drives the

• cean.

Kushnir et al. 2002

SST and wind anomaly patten related to NAO

rtime (Dec–Mar), anomalous SST, ocean–atmosphere turbulent heat flux

Mean wind is westerly ➜ Mean wind is easterly ←

North Atlantic Oscillation

Air-sea interaction at oceanic mesoscale

(fast and short scales)

QSCAT WIND STRESS TRMM SST

Enhanced wind speed over higher SST!

TRMM SST and QuikSCAT wind stress on 3 September 1999

Seo et al. 2007

Enhanced wind speed over warm SST Reduced wind speed over cold SST Positive correlation: Ocean drives the atmosphere.

Correlation coefficients between high-pass filtered wind speed and SST

Xie et al. 2004

Air-sea interaction at oceanic mesoscales

How does the mesoscale SST influence the surface wind?

Eddies alter the stability of the lower atmosphere

Increased wind

Warm SST Cold SST

• Min. wind
• Max. wind

sea surface temperature

Decoupled stable boundary layer Unstable boundary layer and increased mixing T➜ PBL stability ➜ WS Cold

− " u " w = u*

2 = τ

ρo

Decreased wind

Wallace et al. 1989

Warm

Planetary Boundary Layer (1-2 km)

Ufree

Free troposphere 10-100km

• FIG. 9. (top) Longitude–height section of zonal wind velocity (vectors) and virtual potential temperature (K)(contours

Cold Warm ABL Height

Hashizume et al. 2002

Radiosonde observations in the E. Pacific

How do this coupling affect the ocean and atmosphere?

Chelton et al. 2004

COLD WARM

• Wind curl ➔ Ekman pumping ➔ Ocean circulation
• Wind convergence and divergence ➔ Atmospheric vertical motion and

planetary-scale circulation

div div div curl curl

! Wek = ∇× ! τ ρ( f +ζ)

w ≈ 1 ρo εz ε 2 + f 2 " # $ % & '∇2SST

∇⋅ ! u ≈ −∇2SST

• Wind stress curl and convergence

co-propagate with the front.

• Large-amplitude and persistence
• f the anomalies ➜
• Could be an important factor for

dynamics of the large-scale ocean and atmosphere?

Wind stress curl and divergence from satellites

SST Divergence Curl

Animation from D. Chelton OSU

Tropical Instability Waves

Impact on the ocean via Ekman pumping:

western Arabian sea upwelling case from a coupled model

• Large Ekman pumping velocity induced by wind stress curls
• |Wek/W|~O(1)
• A significant factor for evolution of eddies.

! Wek = ∇× ! τ ρ( f +ζ)

S S T , S S H , W I N D , R A I N W e k & S S T 6 / 1 / 2 2

• 8

/ 3 1 / 2 2 2 5 k m S C O A R W . A r a b i a n S e a m/day

Seo et al. 2008

m/day

Impact on the atmosphere via vertical motion: Gulf Stream

case from the observations

1 1.5 2 2.5 3 3.5

mm d–1

4 4.5 5 5.5 6 40° W 80° W 70° W 60° W 50° W 40° W 80° W

a

Wind convergence, satellite (10−6 s−1)

50° N 45° N 40° N 35° N 30° N 25° N 80° W –8 –6 –4 –2 2 4 6 8 70° W 60° W 50° W 40° W 5 4 4 3 3 2

a

Observed rain rate, satellite

50° N 45° N 40° N 35° N 30° N 25° N 5 4 4 3 3 2 80° W 70° W 60° W 50° W 40° W

∙u Satellites rain rate: ERA-I

Upward wind (10−2 Pa s−1) Pressure (hPa)

200 300 400 500 600 700 800 900 1,000 32° N 34° N 36° N –2 –1.5 –1 –0.5 0.5 1 1.5 2 2.5 3 38° N 40° N 42° N

a

50° N 45° N 40° N 35° N 30° N 25° N

upward wind convergence

• Wind convergence (divergence) over warmer (colder) flank of the GS.
• Intense precipitation where wind converges.
• Vertical motion reaching all the way up to the tropopause!
• This will excite the planetary-scale Rossby waves and influence the

atmospheric general circulation. divergence

Minobe et al. 2008

tropopause! westerly

Summary

SST variations associated with mesoscale eddies and fronts cause coherent perturbations in the atmosphere. a ubiquitous feature observed throughout the World Oceans, potentially important for mesoscale ocean dynamics and atmospheric circulation, net effect on large-scale climate dynamics remains uncertain but is an active area of research. In situ data, satellite observations and and high-resolution climate models are all important tools to examine the dynamics of coupling and the effect on large-scale flows.

Thanks! hseo@whoi.edu

References

• Chelton, D.B., M. Schlax, M.H. Freilich and R.F

. Milliff, 2004: Satellite measurements reveal persistent small-scale features in ocean winds. Science, 303, 978–983.

• Hashizume, H., M. Fujiwara, M. Shiotani, T. Watanabe, Y

. Tanimoto, W . T. Liu and K. Takeuchi, 2002: Direct

• bservations of atmospheric boundary layer response to SST variations associated with tropical

instability waves over the eastern equatorial Pacific. J. Climate, 15, 3379–3393.

• Lindzen, R. S., and S. Nigam, 1987: On the role of sea surface temperature gradients in forcing low-level

winds and convergence in the tropics. J. Atmos. Sci., 44, 2418–2436.

• Minobe, S., A. Kuwano-Yoshida, N. Komori, S.-P. Xie, and R.J. Small, 2008: Influence of the Gulf Stream on

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Atmosphere Interaction and its Feedback to Ocean in the Western Arabian Sea. Ocean Modell., 25, 120-131.

• Wallace, J.M., T.P. Mitchell and C. Deser, 1989: The influence of sea surface temperature on surface wind

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