Mesoscale Air-Sea Interactions Atmosphere ➜ wind & heat flux ➜ Ocean Atmosphere ⬅ ︎ SST ⬅ ︎ Ocean Hyodae Seo Physical Oceanography Department Woods Hole Oceanographic Institution WHOI Summer Lecture Series July 28, 2014 hseo@whoi.edu
World's Oceans are full of mesoscale North Atlantic eddies and fronts! Current Oyashio Gulf Stream Kuroshio Coastal Upwelling Coastal Upwelling Tropical Instability Waves Coastal Coastal Upwelling Upwelling Antarctic Circumpolar Current AMSR-E SST, June 1 2003 http:/ /aqua.nasa.gov/highlight.php
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) North Atlantic Oscillation SST and wind anomaly patten related to NAO Mean wind is westerly ➜ Stronger wind speed ➔ lower SST via mixing and Mean wind is easterly ← turbulent flux Negative correlation: Atmosphere drives the Kushnir et al. 2002 ocean. rtime (Dec–Mar), anomalous SST, ocean–atmosphere turbulent heat flux
Air-sea interaction at oceanic mesoscale (fast and short scales) TRMM SST and QuikSCAT wind stress on 3 September 1999 TRMM SST QSCAT WIND STRESS Enhanced wind speed over higher SST! Seo et al. 2007
Air-sea interaction at oceanic mesoscales Correlation coefficients between high-pass filtered wind speed and SST Xie et al. 2004 Enhanced wind speed over warm SST Reduced wind speed over cold SST Positive correlation: Ocean drives the atmosphere. How does the mesoscale SST influence the surface wind?
Eddies alter the stability of the lower atmosphere Radiosonde observations in the E. Pacific Free troposphere U free Planetary Boundary Layer ABL (1-2 km) Height 2 = τ u w = u * − " " ρ o Cold Warm Increased wind Decreased Max. wind Min. wind sea surface wind temperature Warm Warm SST Cold Cold SST Hashizume et al. 2002 F IG . 9. (top) Longitude–height section of zonal wind velocity (vectors) and virtual potential temperature (K)(contours 10-100km Unstable boundary layer and Decoupled stable increased mixing boundary layer T � ➜ PBL stability ➜ WS Wallace et al. 1989
How do this coupling affect the ocean and atmosphere? WARM curl curl div div div COLD Chelton et al. 2004 ! • Wind curl ➔ Ekman pumping ➔ Ocean circulation τ W ek = ∇× ! ρ ( f + ζ ) • Wind convergence and divergence ➔ Atmospheric vertical motion and planetary-scale circulation ∇⋅ ! " % w ≈ 1 ε z '∇ 2 SST u ≈ −∇ 2 SST $ ε 2 + f 2 ρ o # &
Wind stress curl and divergence from satellites Tropical Instability Waves SST • Wind stress curl and convergence co-propagate with the front. Curl • Large-amplitude and persistence of the anomalies ➜ • Could be an important factor for Divergence dynamics of the large-scale ocean and atmosphere? Animation from D. Chelton OSU
Impact on the ocean via Ekman pumping: western Arabian sea upwelling case from a coupled model 6 / 1 / 2 0 0 2 - 8 / 3 1 / 2 0 0 2 2 5 k m S C O A R W . A r a b i a n S e a S S T , S S H , W I N D W e k & S S T , R A I N m/day m/day • Large Ekman pumping velocity induced by wind stress curls ! • |Wek/W|~O(1) τ W ek = ∇× ! • A significant factor for evolution of eddies. ρ ( f + ζ ) Seo et al. 2008
Impact on the atmosphere via vertical motion: Gulf Stream case from the observations rain rate: ERA-I � ∙ u Satellites Wind convergence, upward wind a satellite (10 − 6 s − 1 ) a Observed rain rate, satellite a Upward wind (10 − 2 Pa s − 1 ) 50° N 5 50° N 5 200 50° N 300 45° N 4 45° N 4 westerly tropopause! 45° N Pressure (hPa) 400 40° N 4 500 40° N 4 40° N 600 35° N 3 35° N 35° N divergence 3 700 800 30° N 3 30° N convergence 30° N 3 900 25° N 2 1,000 25° N 80° W 70° W 60° W 50° W 40° W 32° N 34° N 36° N 38° N 40° N 42° N 25° N 2 40° W 80° W 70° W 60° W 50° W 40° W 80° W 80° W 70° W 60° W 50° W 40° W –2 –1.5 –1 –0.5 0.5 1 1.5 2 2.5 3 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 –8 –6 –4 –2 2 4 6 8 mm d –1 Minobe et al. 2008 • 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.
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 observations 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 the troposphere . Nature, 452, 206-209. • Seo, H., A. J. Miller and J. O. Roads, 2007: The Scripps Coupled Ocean-Atmosphere Regional (SCOAR) model, with applications in the eastern Pacific sector . J. Climate, 20, 381-402. • Seo, H., R. Murtugudde, M. Jochum, and A. J. Miller, 2008: Modeling of Mesoscale Coupled Ocean- 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 in the eastern equatorial Pacific: Seasonal and interannual variability . J. Climate, 2, 1492–1499. • Xie, S.-P. 2004: Satellite observations of cool ocean–atmosphere interaction . Bull. Amer. Meteor. Soc., 85, 195–209.
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