Air-Sea Fluxes With a Focus on Heat and Momentum

Abstract

Turbulent and radiative exchanges of heat between the ocean and atmosphere (hereafter heat fluxes), ocean surface wind stress, and state variables used to estimate them, are Essential Ocean Variables (EOVs) and Essential Climate Variables (ECVs) influencing weather and climate. This paper describes an observational strategy for producing 3-hourly, 25-km (and an aspirational goal of hourly at 10-km) heat flux and wind stress fields over the global, ice-free ocean with breakthrough 1-day random uncertainty of 15 W m −2 and a bias of less than 5 W m −2. At present this accuracy target is met only for OceanSITES reference station moorings and research vessels (RVs) that follow best practices. To meet these targets globally, in the next decade, satellite-based observations must be optimized for boundary layer measurements of air temperature, humidity, sea surface temperature, and ocean wind stress. In order to tune and validate these satellite measurements, a complementary global in situ flux array, built around an expanded OceanSITES network of time series reference station moorings, is also needed. The array would include 500-1000 measurement platforms, including autonomous surface vehicles, moored and drifting buoys, RVs, the existing OceanSITES network of 22 flux sites, and new OceanSITES expanded in 19 key regions. This array would be globally distributed, with 1-3 measurement platforms in each nominal 10 • by 10 • box. These improved moisture and temperature profiles and surface data, if assimilated into Numerical Weather Prediction (NWP) models, would lead to better representation of cloud formation processes, improving state variables and surface radiative and turbulent fluxes from these models. The in situ flux array provides globally distributed measurements and metrics for satellite algorithm development,

Ship-Based Contributions to Global Ocean, Weather, and Climate Observing Systems

“…Research vessels also measure variables necessary to interpret flux observations and develop parameterizations such as mean conditions (wind speed and direction, air and sea temperatures, humidity, pressure, pCO 2 , and salinity) and sea-state parameters (such as wave heights, periods, lengths, directions, spectra, and whitecap fraction). Although capability to make comprehensive flux measurements on other types of ships, moored buoys, or autonomous surface vessels is developing rapidly (Cronin et al, 2019;Swart et al, 2019), research vessels will be the primary platform used to evaluate data from these emerging technologies.…”

Observing Requirements for Long-Term Climate Records at the Ocean Surface

“…However, to date only SST has extended the satellite record back in time using in situ observations. Air temperature and humidity are hard to derive from space (e.g., Andersson et al, 2011;Prytherch et al, 2015) but doing so would be valuable for estimation of air-sea exchanges (Weller, 2018;Cronin et al, 2019).…”

“…Whilst it does not strictly conform to the definition of a reference network used for observations over land (Thorne et al, 2017b) the maintenance of a high quality array of moored observations is critical for the maintenance of long-term records. The number of OceanSITES currently providing most of the ECVs in the scope of this paper is about 20, but an extension of similar size has been recommended in support of air-sea fluxes (Cronin et al, 2019), sampling regions thought to be particularly important for understanding the mechanisms of air-sea interaction and providing validation observations for satellites. Such an extension to approximately 40 sites would be extremely valuable for the construction of CDRs.…”

Observing Sea States

“…Still, these data are very useful but the observing system has not been optimized to sample storms in space and time. Wave observations are particularly useful for the investigation of air-sea fluxes of momentum and heat (Cronin et al, 2019), gas, aerosols (Veron, 2015) and parameterizations in weather predictions or climate models. Similarly, the evergrowing network of seismic stations on land (Romanowicz et al, 1984;Tytell et al, 2016) are providing opportunities for long-term sea state monitoring, even in remote locations (e.g., Bromirski et al, 1999;Ardhuin et al, 2012;Retailleau et al, 2017), or, at the very least, some independent data for validating trends of other observing systems.…”