50 Year Analysis Global Ocean Surface Heat Flux

Lisan Yu, Woods Hole Oceanographic Institution

The Objectively Analyzed air-sea heat Fluxes (OAFlux) project aims at establishing a one-stop
data shop for high-quality, long-term, climate data record of global ocean surface forcing
functions, including air-sea heat (latent, sensible, shortwave, and longwave), momentum, and
freshwater (evaporation) fluxes. The objectives and scientific rationale for developing and
maintaining a long-term global surface flux climate record are of broad interest and great
importance to the research and modeling activities for the ocean and climate community. Air-sea fluxes play a critical role in establishing feedbacks between the atmosphere and the ocean; and they are a key variable of the global climate system. A reliable surface flux time series is needed for characterizing long-term change and variability in surface forcing conditions, attributing the cause of change in the atmosphere and ocean circulation, assessing global water and energy budget, and quantifying the oceanic role in the global climate change and variability.

Direct measurements of air-sea fluxes are limited. Global air-sea fluxes are commonly
constructed from bulk flux parameterizations using air-sea observables (e.g., wind speed,
temperature, humidity, cloud cover, etc) as inputs. Satellite observations have been increasingly relied upon as a crucial input data source for better quality surface fluxes due to their combined strength in global coverage, temporal resolution, and measurement accuracy. Nevertheless, satellite retrievals are not error free and require ground truth for calibration and validation. There is also the need for auxiliary data, often provided by numerical weather prediction operational analyses/reanalyses, to supply the missing information in case there are no direct measurements of flux-related variables (e.g. the near-surface thermal and moisture structures over the oceans) and/or there are data gaps. To address these technical challenges, two critical components were developed and implemented in the OAFlux framework. One is a validation database that integrates more than 130 in situ surface flux buoys including the tropical moored arrays (TAO/TRITON (Tropical Atmosphere Ocean/Triangle Trans-Ocean Buoy Network), PIRATA (Prediction and Research Moored Array in the Atlantic), and RAMA (The Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction)) and the global network of OceanSITEs. The other component is an objective analysis approach that takes into account errors in both satellite and auxiliary data sources and seeks an optimal combination through minimization. The validation database provides a benchmark reference for quality control input variables, statistical characterization of input data errors needed in the objective optimization, and evaluation of the resulting flux estimates. It has been shown that the OAFlux framework leads to improved estimates of air-sea variable fields and improved flux products. The benefit and value of integrating the ocean observing system with the satellite Earth observing system in obtaining quality global surface fluxes are demonstrated.

The OAFlux global 1° gridded, daily/monthly analysis of ocean evaporation, air-sea latent and
sensible heat fluxes, and related surface meteorological variables is available from 1958 to the
FY2012 Work Plan [Global Ocean Surface Heat Flux Analysis] Page 3 of 13
present. The time series is updated twice per year and disseminated online at the project website (http://oaflux.whoi.edu/ data.html). At the same time, persistent and steadfast efforts are made to enhance and expand the existing OAFlux data products and to move toward providing consistent climate data record for surface full physical flux components. The OAFlux new datasets,developed in coordination with other projects, are the global 0.25° and 1° resolution analyses of daily ocean vector wind fields covering the period of satellite passive/active sensors from 1987 to the present. The significantly improved representation of the response of synoptic wind variability to SST fronts by the high-resolution vector wind analysis, together with recent advances in satellite observing capability, provides compelling incentive and opportunity to strive toward high-resolution surface latent and sensible fluxes. Work on the latter has been ongoing. Meanwhile, improving surface shortwave and longwave radiation estimates to achieve a globally balanced net heat budget has been conducted in parallel, with work at this stage focusing on using buoy measurements to understand and characterize errors in existing satellite products.

The OAFlux data user base is growing rapidly, with increasing recognition and appreciation on
the quality and accuracy of the OAFlux time series. Users of the OAFlux products include but
are not limited to: 1) climate modeling groups and centers, including National Centers for
Environmental Prediction (NCEP) Global Ocean Data Assimilation System (GODAS), the
National Aeronautics and Space Administration (NASA) Modern Era Retrospective‐Analysis for
Research and Applications (MERRA), and Coupled Model Intercomparison Project (CMIP),
using the global OAFlux fields as a base reference for validating the performance of a suite of
climate models developed by many countries; 2) investigators and researchers working under
NASA Aquarius mission, the European Space Agency's (ESA) Soil Moisture and Ocean Salinity
(SMOS) mission, the NASA field experiment of the Salinity Processes in the Upper Ocean
Regional Study (SPURS), and the NASA Energy and Water Cycle Study (NEWS), using
OAFlux 50+year time series of evaporation to study the global freshwater budget and the
relationship between the water cycle and ocean salinity; 3) investigators under targeted field
programs, including CLIVAR (Climate Variability research program) Mode Water Dynamic
Experiment (CLIMODE) and Dynamics of the Madden-Julian Oscillation (DYNAMO), using
OAFlux products to identify key air-sea interaction processes and feedback mechanisms; 4)
those working on the ocean carbon cycle, using OAFlux products to identify global "hot spots"
where the ocean releases heat to the atmosphere and absorbs carbon dioxide from the
atmosphere, as air-sea heat flux drives air-sea carbon flux; 5) those working to quantify the role of the ocean in climate variability and change by using the OAFlux time series to help identify the source or sink of heat and freshwater; 6) those working with ocean models, who use the OAFlux product as the forcing fields for model runs; 7) those developing alternate air-sea fluxfields from remote sensing and/or in situ data, who compare their products to the OAFlux fields;8) those researching new and renewable energy sources, using OAFlux wind and near-surfacemeteorological parameters to estimate global offshore wind power potential; 9) educatorsinvolved in the Education and Outreach programs, e.g., the education program of the NASA Aquarius mission, the NOAA National Weather Service (NWS) online weather school -
JetStream, and the Cooperative Program for Operational Meteorology, Education and Training
(COMET) established by the University Corporation for Atmospheric Research (UCAR) and
NWS, who use OAFlux climatology as course materials; and 10)the OAFlux products are used
in the IPCC 5th assessment report (AR5) to provide an assessment of the role of the ocean in the change of the global water cycle during the past warm decades.