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Role of the ocean in variability of seasonal evaporation and long-term changes in global water cycle

Role of the ocean in variability of seasonal evaporation and long-term changes in global water cycle. Lisan Yu and Bob Weller Woods Hole Oceanographic Institution. NOAA Annual System Review Silver Spring, Maryland Sept 3-5, 2008.

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Role of the ocean in variability of seasonal evaporation and long-term changes in global water cycle

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  1. Role of the ocean in variability of seasonal evaporation and long-term changes in global water cycle Lisan Yu and Bob Weller Woods Hole Oceanographic Institution NOAA Annual System Review Silver Spring, Maryland Sept 3-5, 2008

  2. Ocean Evaporation:a key component of global water cycle • How has the ocean evaporation changed in past decades? • What are the implications for global hydrological cycle?

  3. WHOI OAFlux project produces global ocean evaporation (1958-present) Annual Mean • OAFlux: • Objectively Analyzed Air-Sea Fluxes • Project funded by NOAA to PIs Yu & Weller • Website: http://oaflux.whoi.edu/ • Data products: an objective blending of satellite observations and atmospheric reanalyses outputs. • Products online (1958-2006): • Evaporation • Latent and Sensible heat fluxes • Air-sea variables (wind speed, humidity, temperature) • References: • Yu, L., X. Jin, and R. A. Weller, 2008:Multidecade Global Flux Datasets from the Objectively Analyzed Air-sea Fluxes (OAFlux) Project: Latent and sensible heat fluxes, ocean evaporation, and related surface meteorological variables. WHOI, OAFlux Technical Report. OA-2008-01, 64pp. • Yu, L., X. Jin, and R. A. Weller, 2008:Air-sea latent and sensible heat flux estimates at the global buoy sites. J. Clim. sub judice. • Yu, L., and R. A. Weller, 2007:Objectively Analyzed air-sea heat Fluxes (OAFlux) for the global oceans. Bull. Ameri. Meteor. Soc., 88, 527-539. • Yu, L., 2007:Global variations of oceanic evaporation: The role of the changing wind speed. J. Clim., 20, 5376-5390.

  4. Assessing the change in ocean evaporation Annual-mean time series of E and SST • The Clausius–Clapeyron equation supports that increase of temperature increases the evaporation rate. • How has the pattern of evaporation changed? • how has SST impacted evaporation?

  5. (1) Spatial pattern How has ocean evaporation changed? Annual mean time series 1980 - 1985 2000 - 2005

  6. (2) Seasonal cycle How has ocean evaporation changed?

  7. (3) Linear trends How has ocean evaporation changed? E Annual-mean Trends in E and SST are large-scale and coordinated

  8. How has SST impacted E? U E qs– qa The upward trend in E is driven by both qs – qa and U. Winds carry water vapor away from the evaporating surface so that a new sea–air humidity gradient can be established and the surface continues to evaporate.

  9. Implications to hydrological cycle ?? (1) Is the atmosphere getting wetter? The upward tendency in EOAFlux is consistent with the tendency in column-integrated water vapor of SSM/I observations. (2) Is the global hydrological cycle changing?

  10. Global E – P The study of global water cycle is partially supported by NSF physical oceanography program

  11. Are GPCP and CMAP different?

  12. What does the difference suggest?

  13. Linear trends in E – P • The two sets of combination produce opposite E – P trends in the tropical oceans. • Which one is sensible?

  14. Can Salinity Observations be used to constrain the water cycle? The large-scale consistency suggests that SSS is influenced by time-integrated effects of E – P

  15. Salinity as an indicator of the change in E–P Sea Surface Salinity Observation provided by T. Delcroix (http://www.legos.obs-mip.fr/en/observations/sss/datadelivery/products/)

  16. SSS: Model predicted versus observations Salinity observations are a useful indicator of global water cycle and its changes

  17. Has the global water cycle been accelerating? • Accurate quantification of the rate of the change in global water cycle requires accurate estimates of evaporation, precipitation, and runoffs. Yet, uncertainties in these estimates are large. • Salinity is an independent source of verification of estimated changes in global water cycle. • The upcoming Aquarius salinity mission will help not only to better understand the global freshwater fluxes but also to improve the estimates of these fluxes.

  18. Summary • Evaporation has been rising steadily since early 1980s in concert with sea surface warming. – There is an amplification of the magnitude of evaporation during wintertime, particularly in regions between 40S and 40N. • Salinity observations are an important indicator of global water cycle and its change. – The E-P fluxes from the OAFlux evaporation and GPCP precipitation suggest an acceleration of global water cycle • The ocean is an important component of global water cycle. – Global salinity observations from remote sensing will help to improve the understanding of the changing water cycle and improve the global estimates of freshwater fluxes.

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