The Heat Balance Of Air-sea Boundary Layer In A Coupled Regional Ocean-atmosphere Model System And The Assessment Of Climate Change In North Sea In The Current Century Under The IPCC-A1B Scenario

Using the coupler OASIS, a regional coupled ocean-atmosphere model systemwas employed based on the two models HAMSOM (HAMburg Shelf Ocean Model)and REMO (REgional climate MOdel). The heat balance of the air-sea boundary layerin this coupled system could be improved significantly compared with that of theun-coupled model simulations. Noteworthy, surface latent heat flux contributes mostto the balance.It was demonstrated that there was a positive convection-wind-evaporationfeedback in the ocean-atmosphere boundary layer. When the sea surface temperature(SST) raises, the surface air temperature (SAT) follows, leading to an increasedhumidity in the atmospheric bottom layer. Both the heating of air parcels and a higherhumidity lowers the density of the surface air and help to strengthen the atmosphericvertical convection. At the same time, strong convection brings surface water vapor tothe upper atmosphere, thus the ocean surface evaporation rate can be accelerated.Unstable surface air convection will amplify the surface wind as a result of the massconservation. Interestingly, the initial energy of the convection-wind-evaporationfeedback comes from the surface latent heat flux, and the SST controls the intensity ofthis feedback.It was found that the difference between SST and SAT in the coupled model issmaller than that of the un-coupled model, especially during winter time. In thecoupled model, SST and SAT are closely connected due to the interaction of bothparameters in the sensible and latent heat flux formulations. They follow each otherclosely and affect each other. However, in the un-coupled model there is no feedbackmechanism working, and pre-described values are used on either side. This unrealistictreatment will cause errors in the simulation of SST. Note that the error will be largerin winter due to the fact that the sea water is well mixed from the top to the bottom inNorth Sea. Hence the influence of the SAT on the SST is reduced.It is also shown in this study that different coupled time steps (1h\3h\6h) in the coupled model system have an impact on the heat balance of air-sea boundary layer.Although the long-term trend of SST can not be reversed, the average SST differencecan reach as much as2degree during one year. This is mainly because of theincoming net-solar radiation is not the same when using different coupling time steps.With an increasing coupling frequency, the chances to resolve an extreme wind eventare also increasing, which will helps to strengthen the vertical mixing of ocean surfacelayer.Future100years climate change in North Sea was investigated using thecoupled HAMSOM and REMO system based on the global coupled MPI-OM andECHAM5result under the IPCC-A1B scenario. It is shown that the sea surface height(SSH) will rise as much as about0.6-0.7m, annual precipitation will increase as muchas100mm and total cloud cover will increase from69%to72%. Average SST inNorth Sea will have an increase of2.1degree while the SAT will increase by2.5degree. Noteworthy, the temperature increase during night time is larger than duringday time, and in winter it is larger than in summer.