Abstract
Air–sea heat fluxes are essential climate variables, required for understanding air–sea interactions, local, regional and global climate, the hydrological cycle and atmospheric and oceanic circulation. In situ measurements of fluxes over the ocean are sparse and model reanalysis and satellite data can provide estimates at different scales. The accuracy of such estimates is therefore essential to obtain a reliable description of the occurring phenomena and changes. In this work, air–sea radiative fluxes derived from the SEVIRI sensor onboard the MSG satellite and from ERA5 reanalysis have been compared to direct high quality measurements performed over a complete annual cycle at the ENEA oceanographic observatory, near the island of Lampedusa in the Central Mediterranean Sea. Our analysis reveals that satellite derived products overestimate in situ direct observations of the downwelling short-wave (bias of 6.1 W/m2) and longwave (bias of 6.6 W/m2) irradiances. ERA5 reanalysis data show a negligible positive bias (+1.0 W/m2) for the shortwave irradiance and a large negative bias (−17 W/m2) for the longwave irradiance with respect to in situ observations. ERA5 meteorological variables, which are needed to calculate the air–sea heat flux using bulk formulae, have been compared with in situ measurements made at the oceanographic observatory. The two meteorological datasets show a very good agreement, with some underestimate of the wind speed by ERA5 for high wind conditions. We investigated the impact of different determinations of heat fluxes on the near surface sea temperature (1 m depth), as determined by calculations with a one-dimensional numerical model, the General Ocean Turbulence Model (GOTM). The sensitivity of the model to the different forcing was measured in terms of differences with respect to in situ temperature measurements made during the period under investigation. All simulations reproduced the true seasonal cycle and all high frequency variabilities. The best results on the overall seasonal cycle were obtained when using meteorological variables in the bulk formulae formulations used by the model itself. The derived overall annual net heat flux values were between +1.6 and 40.4 W/m2, depending on the used dataset. The large variability obtained with different datasets suggests that current determinations of the heat flux components and, in particular, of the longwave irradiance, need to be improved. The ENEA oceanographic observatory provides a complete, long-term, high resolution time series of high quality in situ observations. In the future, more similar sites worldwide will be needed for model and satellite validations and to improve the determination of the air–sea exchange and the understanding of related processes.
Highlights
The ocean interacts with the atmosphere by means of exchanges of momentum, heat, water, gases and particles at the air–sea interface, defining its energy and mass balance and its role in the regional and global climate system at all spatial and temporal scales [1,2].Air–sea heat fluxes (Figure 1) are essential drivers of the climate system, controlling interaction processes from sub-diurnal to multi-decadal time scales and from regional to global scales
1, red line) or bulk formulae with in situ meteorological parameters. (b) Simulations imposing air–sea heat and momentum fluxes obtained by ERA5, by ERA5 except for the shortwave irradiance, which is from in situ measurements and by ERA5 except for the shortwave irradiance, which is estimated by Spinning Enhanced Visible and Infrared Imager (SEVIRI). (c) Simulations imposing air–sea heat and momentum fluxes obtained by ERA5, by ERA5 except for the longwave irradiance, which is form in situ observations at the buoy, by ERA5 except for the longwave irradiance, which is estimated by SEVIRI
If we focus on the first month of simulations, when the role of horizontal advection is expected to have produced a limited impact, the use of the measured shortwave irradiance produced a smaller bias with respect to temperature at 1 m of depth (T1m), of only −0.08 ◦ C, while using ERA5 or SEVIRI the bias reached 0.21 ◦ C and 0.53 ◦ C, respectively
Summary
The ocean interacts with the atmosphere by means of exchanges of momentum, heat, water, gases and particles at the air–sea interface, defining its energy and mass balance and its role in the regional and global climate system at all spatial and temporal scales [1,2]. The most common approach, even within numerical climate modeling, is to use empirical methods to estimate the various components of air–sea heat (or momentum) exchange by linking micro-scale turbulent transfer to more measurable macroscopic quantities, such as near-surface wind intensity, humidity or temperature. Both have synoptic coverage but different temporal and spatial resolutions, e.g., the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis 5th Generation (ERA5; https://confluence.ecmwf.int/display/CKB/ERA5%3A+data+documentation (last accessed on 21 March 2019) data are globally distributed at hourly frequency and spatial resolution of 0.25 × 0.25 degrees, while Meteosat Second Generation (MSG) surface shortwave and longwave irradiance data are distributed on hourly basis but at 0.05 × 0.05 degrees resolution In such a context, the comparison between heat flux components estimates and direct measurements is crucial to assess the goodness of heat flux existing datasets. The work concludes with some remarks based on these findings
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
