Review and assessment of latent and sensible heat flux accuracy over the global oceans

For over a decade, several research groups have been developing air-sea heat flux information over the global ocean, including latent (LHF) and sensible (SHF) heat fluxes over the global ocean. This paper aims to provide new insight into the quality and error characteristics of turbulent heat flux estimates at various spatial and temporal scales (from daily upwards). The study is performed within the European Space Agency (ESA) Ocean Heat Flux (OHF) project. One of the main objectives of the OHF project is to meet the recommendations and requirements expressed by various international programs such as the World Research Climate Program (WCRP) and Climate and Ocean Variability, Predictability, and Change (CLIVAR), recognizing the need for better characterization of existing flux errors with respect to the input bulk variables (e.g. surface wind, air and sea surface temperatures, air and surface specific humidities), and to the atmospheric and oceanic conditions (e.g. wind conditions and sea state). The analysis is based on the use of daily averaged LHF and SHF and the asso- ciated bulk variables derived from major satellite-based and atmospheric reanalysis products. Inter-comparisons of heat flux products indicate that all of them exhibit similar space and time patterns. However, they also reveal significant differences in magnitude in some specific regions such as the western ocean boundaries during the Northern Hemisphere winter season, and the high southern latitudes. The differences tend to be closely related to large differences in surface wind speed and/or specific air humidity (for LHF) and to air and sea temperature differences (for SHF). Further quality investigations are performed through comprehensive comparisons with daily-averaged LHF and SHF estimated from moorings. The resulting statistics are used to assess the error of each OHF product. Consideration of error correlation between products and observations (e.g., by their assimilation) is also given. This reveals generally high noise variance in all products and a weak signal in common with in situ observations, with some products only slightly better than others. The OHF LHF and SHF products, and their associated error characteristics, are used to compute daily OHF multiproduct-ensemble (OHF/MPE) estimates of LHF and SHF over the ice-free global ocean on a 0.25° × 0.25° grid. The accuracy of this heat multiproduct, determined from comparisons with mooring data, is greater than for any individual product. It is used as a reference for the anomaly characterization of each individual OHF product

Multispectral analysis of Northern Hemisphere temperature records over the last five millennia

Aiming to describe spatio-temporal climate variability on decadal-to-centennial time scales and longer, we analyzed a data set of 26 proxy records extending back 1,000–5,000 years; all records chosen were calibrated to yield temperatures. The seven irregularly sampled series in the data set were interpolated to a regular grid by optimized methods and then two advanced spectral methods—namely singular-spectrum analysis (SSA) and the continuous wavelet transform—were applied to individual series to separate significant oscillations from the high noise background. This univariate analysis identified several common periods across many of the 26 proxy records: a millennial trend, as well as oscillations of about 100 and 200 years, and a broad peak in the 40–70-year band. To study common NH oscillations, we then applied Multichannel SSA. Temperature variations on time scales longer than 600 years appear in our analysis as a dominant trend component, which shows climate features consistent with the Medieval Warm Period and the Little Ice Age. Statistically significant NH-wide peaks appear at 330, 250 and 110 years, as well as in a broad 50–80-year band. Strong variability centers in several bands are located around the North Atlantic basin and are in phase opposition between Greenland and Western Europe

Evidence of Time-dependent Sverdrup Circulation in the South Pacific from the Seasat Scatterometer and Altimeter

Seasat scatterometer and altimeter data are analyzed to investigate time-dependent Sverdrup dynamics in the Southern Ocean (40°S to 60°S) over seasonal time scales. Sverdrup dynamics are shown to be inadequate to describe the circulation in the South Atlantic and Indian oceans. The Sverdrup circulation in the South Pacific is reasonable north of 55°S. The changes in Sverdrup circulation from July to September 1978 indicate an eastward acceleration along 55°S and westward acceleration along 40°S, suggesting a southward shift in the subpolar eastward flow. Sea level in the South Pacific is estimated for July and September 1978 from scatterometer vector wind data based on Sverdrup dynamics assuming a flat-bottom ocean with barotropic flow. The changes in Sverdrup sea level are compared with the changes in sea level observed by the altimeter for the same time period. Both estimates indicate a rise in sea level along a zonal band centered at about 50°S. This sea level rise inferred from both the scatterometer and altimeter data is supported by a similar rise in sea level observed from tide gauge measurements at two locations in New Zealand. The spatial correlation between the two satellite estimates of sea level change is about 0.5. This agreement suggests that time-dependent Sverdrup dynamics may account for about ¼ of the spatial variance of sea level change in the South Pacific over the 3-month Seasat mission

Improvement in air–sea flux estimates derived from satellite observations

International audienc

Differences between two estimates of air‐sea turbulent heat fluxes over the Atlantic Ocean

Uncertainties in turbulent ocean‐atmosphere heat flux estimates, both among the estimates and between them and ground truth, suggest that further comparisons are needed. We analyze estimates from the French Research Institute for Exploitation of the Sea (IFREMER) and the Woods Hole Oceanographic Institution's Objectively Analyzed air‐sea Fluxes (WHOI OAFlux). The IFREMER products are based on satellite observations and the WHOI OAFlux ones on data from satellites, buoys, and ships assimilated into numerical analyses. We focus on the Atlantic sector (70°W–30°E, 45°S–45°N) during 1996–2005, where the variables that enter the bulk formulae for computing fluxes (wind speed, sea surface and air temperature, and specific humidity) can be evaluated against buoys in the Prediction and Research Moored Array in the Atlantic (PIRATA). Since WHOI assimilates PIRATA observations, we have added two independent buoy data sets: FETCH and ROMEO. To examine how each variable contributes to the difference between estimated and buoy fluxes, the method of Bourras (2006) is applied. His so‐called Q terms showed that specific air humidity and air temperature contributed the most to the biases of IFREMER latent and sensible heat fluxes, respectively, at both independent buoys. For WHOI OAFlux products, deviations from FETCH values were mainly due to wind speed and sea surface temperature differences, while in comparison with ROMEO fluxes, WHOI OAFlux biases were primarily due to specific humidity and sea surface temperature estimates. Modified estimates of turbulent fluxes with the IFREMER approach using the 10 m specific humidity and air temperature products of Jackson et al. (2009) show significant improvement in three test cases at PIRATA buoys. Key Points Turbulent air‐sea fluxes can be obtained globally quite well Uncertainties in two methods compared are still large Better sampling and new satellite instruments show promise for improvement

Differences between two estimates of air-sea turbulent heat fluxes over the Atlantic Ocean

Uncertainties in turbulent ocean-atmosphere heat flux estimates, both among the estimates and between them and ground truth, suggest that further comparisons are needed. We analyze estimates from the French Research Institute for Exploitation of the Sea (IFREMER) and the Woods Hole Oceanographic Institution's Objectively Analyzed air-sea Fluxes (WHOI OAFlux). The IFREMER products are based on satellite observations and the WHOI OAFlux ones on data from satellites, buoys, and ships assimilated into numerical analyses. We focus on the Atlantic sector (70 degrees W-30 degrees E, 45 degrees S-45 degrees N) during 1996-2005, where the variables that enter the bulk formulae for computing fluxes (wind speed, sea surface and air temperature, and specific humidity) can be evaluated against buoys in the Prediction and Research Moored Array in the Atlantic (PIRATA). Since WHOI assimilates PIRATA observations, we have added two independent buoy data sets: FETCH and ROMEO. To examine how each variable contributes to the difference between estimated and buoy fluxes, the method of Bourras (2006) is applied. His so-called Q terms showed that specific air humidity and air temperature contributed the most to the biases of IFREMER latent and sensible heat fluxes, respectively, at both independent buoys. For WHOI OAFlux products, deviations from FETCH values were mainly due to wind speed and sea surface temperature differences, while in comparison with ROMEO fluxes, WHOI OAFlux biases were primarily due to specific humidity and sea surface temperature estimates. Modified estimates of turbulent fluxes with the IFREMER approach using the 10 m specific humidity and air temperature products of Jackson et al. (2009) show significant improvement in three test cases at PIRATA buoys