50 Year Analysis Global Ocean Surface Heat Flux

Lisan Yu, Woods Hole Oceanographic Institution

Our present knowledge of the global air-sea heat flux distribution stems primarily from the bulk
parameterizations of air-sea fluxes as functions of surface meteorological variables (e.g., wind
speed, temperature, humidity, cloud cover, etc). The source of observations for those flux-related
variables includes marine surface weather reports from Voluntary Observing Ships (VOS)
collected by Comprehensive Ocean-Atmosphere Data Set (COADS) and satellite remote sensing
from various platforms. Atmospheric reanalyses from numerical weather prediction (NWP)
centers such as National Centers for Environmental Prediction (NCEP) and the European Centre
for Medium-Range Weather Forecasts (ECMWF) provide additional model-based database.
Nonetheless, none of the three data sources are perfect as each suffers from at least one of the
four deficiencies: (1) incomplete global coverage, (2) relatively short time series, (3) systematic
bias, and (4) random error.
While improving the quality of each data source is a necessary step toward improving the
estimates of surface heat fluxes, this project takes an alternative approach, i.e., to improve the
quality of the flux estimates through objectively synthesizing the advantages of the three data
sources. The synthesis approach has been applied successfully to generate gridded products of
surface vector wind, SST, and precipitation. This project, which is termed “Objectively Analyzed
air-sea heat Fluxes (OAFlux),” develops an equivalent global synthesis product for surface heat
fluxes by utilizing the methodology developed and experience learnt from a previous pilot study
for the Atlantic Ocean.
The project has two main objectives. The first objective is to produce a 50-year (from the mid
1950s onward) analysis of surface latent, sensible, net shortwave and net longwave radiation
fluxes over the global oceans with improved accuracy. This is to be achieved by an appropriate
combination of COADS data, NWP reanalysis outputs, and satellite retrievals using advanced
objective analysis. The target resolution is 1° longitude by 1° latitude and monthly. Daily flux
fields are produced when satellite data are available. The second objective is to use the data to
study the heat flux variability on seasonal, annual, interannual, decadal and longer timescales and
their relation to global climate change. The scientific investigation helps to assess the quality and
reliability of the dataset in depicting the multi-decade climate record since 1950s and to provide
physical insights into the dataset.
Users of the OA Flux products include: (1) those analyzing the spatial and temporal patterns of
the surface exchanges of latent and sensible heat flux, using the 50-year, 1° resolution flux fields
as the basis for these analyses; (2) those working to quantify the role of the ocean in climate
variability and change by examining, using the OA Flux fields, the ocean’s role as a source or
sink of heat and freshwater; (3) those examining the realism of coupled ocean-atmosphere
models by comparison of the models surface fluxes with the OA Flux product; (4) those working
with ocean models, who use the OA Flux product as the forcing fields for model runs; and
(5) those developing alternate air-sea flux fields from remote sensing and/or in situ data, who
compare their products to the OA Flux fields.