Momentum fluxes and wind gradients in the marine boundary layer — a multi-platform study

Oceans cover about 70% of the earth’s surface. They are the largest source of the atmospheric water vapour and act as enormous heat reservoirs. Thus in order to predict the future weather and climate it is of great importance to understand the processes governing the exchange of water vapour and heat between the ocean and atmosphere. This exchange is to a large extent mediated by turbulent eddies. Current numerical climate and weather forecast models are unable to resolve the turbulence, which means that the turbulent exchange needs to be simplified by using parameterizations. Tower based measurements at the Ostergarnsholm Island in the Baltic Sea have been used to study the air-sea turbulent exchange of latent and sensible heat and the heat flux parameterizations. Although the measurements are made at an island, data obtained at this site is shown to represent open ocean conditions during most situations for winds coming from the east-south sector. It is found that during conditions with small air-sea temperature differences and wind speeds above 10 m s-1, the structure of the turbulence is re-organized. Drier and colder air from aloft is transported to the surface by detached eddies, which considerably enhance the turbulent heat fluxes. The fluxes where observed to be much larger than predicted by current state-of-the-art parameterizations. The turbulence regime during these conditions is termed the Unstable Very Close to Neutral Regime, the UVCN-regime.The global increase of the latent and sensible heat fluxes due to the UVCN-regime is calculated to 2.4 W m-2 and 0.8 W m-2 respectively. This is comparable to the current increase of the radiative forcing due to anthropogenic emissions of greenhouse gases, reported in Intergovernmental Panel on Climate Change fourth assessment report (IPCC AR4). Thus the UVCN-effect could have a significant influence when predicting the future weather and climate.