ESSO - Indian National Centre for Ocean Information Services

(An Autonomous Body under the Ministry of Earth Sciences, Govt. of India)


Importance of air-sea fluxes in the tropics

The tropics play an important role in the earth’s climate and energy cycle. The tropical oceans gain heat from downward shortwave radiation, allowing for very high surface temperature (above 28°C) in the Indo-Pacific and Atlantic warm pools. This high sea surface temperature can cause the atmospheric boundary layer to destabilize through upward latent and sensible fluxes and by providing moisture through evaporation, leading to the development of deep atmospheric convection. These air-sea interactions are crucial in setting up the Walker circulation, an essential component of the earth climate. These interactions are also active through a continuum of time scales, from diurnal to interannual, and are involved in many tropical climate modes. At the intraseasonal timescale, for example, air-sea heat fluxes play a central role in theories of the Madden and Julian Oscillation, and in the monsoon active and break phases (Sobel et al. 2008). At the interannual timescale, air-sea momentum fluxes play a key role in the Bjerknes feedback, a positive feedback between the surface wind signal related to deep atmospheric convection and its dynamical oceanic response, which is critical to phenomena like El Niño (e.g. McPhaden et al. 2006) and the Indian Ocean Dipole (Webster et al. 1999; Saji et al. 1999). Those examples illustrate how an accurate knowledge of air-sea heat and momentum fluxes is required to understand the main modes of tropical climate variability. The TropFlux project aims at providing state of the art heat and momentum fluxes that resolve the main phenomena of the tropical climate, at intraseasonal (Madden-Julian Oscillation, monsoon active and break phases), seasonal and interannual (El Niño, Indian Ocean Dipole ...) time scales.

Why a new air-sea flux product ?

  • Air-sea fluxes derived from in situ observations are extremely useful for establishing reference climatologies, but do not allow resolving intraseasonal variations due to limited data coverage
  • Air-sea fluxes obtained from re-analyses generally suffer from systematic biases, and do not perform extremely well for surface shortwave fluxes (Praveen Kumar et al. 2012)
  • The OAFLUX project is a very useful initiative that delivers high-quality surface heat fluxes from a synthesis of various re-analyses, satellite observations and in situ datasets. Unfortunately, OAFlux does not provide momentum fluxes so-far, and the availability of its net heat fluxes is conditioned by the availability of high quality ISCCP shortwave surface fluxes, which are only updated once every year or so.

The TropFlux project aims at providing timely (i.e. a few months behind present) estimates of both momentum and net heat flux (and their components). These fluxes should compare well to the largest observational database of estimates of air-sea fluxes in the tropics: Global Tropical Moored Buoy Array (McPhaden et al. 2010).


McPhaden, M.J.,S. E. Zebiak, and, M.H. Glantz, 2006: ENSO as an integrating concept in Earth science, Science, doi:10.1126/science.1132588

McPhaden, M.J., K. Ando, B. Bourles H.P. Freitag, R. Lumpkin, Y. Masumoto, V.S.N. Murty, P. Nobre, M. Ravichandran, J. Vialard, D. Vousden, and W. Yu, 2010: The global tropical moored buoy array, In Proceedings of the “OceanObs‘09: Sustained Ocean Observations and Information for Society” Conference (Vol. 2), Venice, Italy, 21-25 September 2009, Hall, J., D.E. Harrison, and D. Stammer, Eds., ESA Publication WPP-306, doi:10.5270/OceanObs09.cwp.61.

Saji NH, Goswami BN, Vinayachandran PN, Yamagata T, 1999: A dipole mode in the tropical Indian Ocean, Nature, doi:10.1038/43855

Sobel, A.H., E.D. Maloney, G. Bellon and D.M. Frierson, 2008: The role of surface fluxes in tropical intraseasonal oscillations, Nature Geo., doi:10.1038/ngeo312

Webster, P. J., Moore, A. M, Loschnigg, J. P, and Leben, R. R, 1999: Coupled oceanic-atmospheric dynamics in the Indian Ocean during 1997-98, Nature, doi:10.1038/43848