Ocean Heat Fluxes
Accurate estimates of the ocean surface turbulent and radiative fluxes are of great interest for a variety of air-sea interaction and climate variability interaction issues. Being the language of communication of the ocean and atmosphere, surface fluxes play a key role in the coupling of the Earth climate system, controlling most important feedbacks between the ocean and atmosphere. Furthermore, accurate turbulent flux estimates are essential to assess the Earth’s global energy budget. Changes in the Ocean Heat Content (OHC) of the upper ocean layers can be quantified through the estimation of imbalance of surface flux components. The main source of the long-term time series of such fluxes over the global ocean are reanalyzes based on numerical weather prediction (NWP) models and data assimilation, voluntary observing ship measurements (VOS), and remotely sensed data.
Ocean Heat Fluxes at Ifremer / LOPS
Measuring ocean heat fluxes from space is challenging. Radars and radiometers onboard satellites provide valuable global measurements used to estimate the turbulent fluxes. The methodology for obtaining the surface turbulent fluxes uses physical properties of active and passive satellite instrument measurements, empirical and inverse models relating satellite observations and surface parameters, and objective analysis merging various satellite estimates.
A number of studies aiming at assessing the quality of turbulent fluxes have been published in recent years. They outlined that although the available satellite products exhibit quite similar spatial and temporal variability patterns, regional differences between flux estimates can be significant. It has been also shown that the uncertainties in the net radiative heat flux at the sea surface can be as large as the variations in the turbulent heat fluxes. The reasons for the differences observed relate to differences in input data as well as to differences in inverse and direct methods used to retrieve geophysical parameters from remotely sensed measurements. The main studies relied on the investigations of ocean heat flux estimate quality concluded that the improvements of satellite latent heat flux (LHF) and sensible heat flux (SHF) estimation requires improvements of the remotely sensed surface wind (W), specific air humidity (Qa), sea surface temperature (SST), and air temperature (Ta) at global and regional scales. There are difficulties in comparing surface flux estimates (both turbulent and radiative) due to inconsistencies in methodology and data input. For instance, the studies mentioned above emphasized that the improvement of satellite fluxes should include the improvement of the interpolation method used to calculate gridded fields over the global ocean to better reflect conditions during synoptic-scale storms and fronts.
Our activities aim at providing the user community with long series of improved estimation of fluxes, in particular with the support of international collaboration projects such as ESA OceanHeatFlux project.
CERSAT provides, using an objective method, long time series of global multi-satellite wind vector, wind stress, latent and sensible heat fluxes, with a spatial resolution of 1 degree, and weekly and monthly temporal resolutions, excluding sea ice areas. The used input satellite data come from available scatterometers (ERS-1, ERS-2, NSCAT, Seawinds, QuikScat, ASCAT-A, ASCAT-B,...), and several defense Meteorological Satellite Program (DMSP) radiometers (Special Sensor Microwave/Imager [SSM/I] F10 - F18).