资源环境科技发展态势分析平台

Detailed mechanisms of reinforcement/relaxation of a sea surface salinity front, i.e. frontogenesis/frontolysis, in the northeastern subtropical Pacific region are investigated using observational datasets. Throughout the year, meridional shears of zonal wind induce meridional ageostrophic convergence around the front. Saltier water is advected on the southern side than the northern side, and thus the convergence contributes to frontogenesis. The evaporation and precipitation gradients also strengthen the front, because higher sea surface temperatures on the southern side induce stronger evaporation through the formation of higher surface saturated specific humidity, and because the precipitation rate is low in the southern region in association with the subtropical high of the atmosphere. However, in summer-autumn when the mixed layer is seasonally thin and evaporation exceeds precipitation, the frontogenesis by the freshwater flux gradient is damped by the mixed layer depth (MLD) gradient. This is attributed to the thicker mixed layer south of the front being less sensitive to freshwater fluxes. During the mixed-layer deepening phase, the mixed layer on the southern side entrains lower salinity water from the lower layer, and thus the entrainment relaxes the front. Therefore, it is shown that the gradients of ageostrophic advection, evaporation, and precipitation result in frontogenesis, while those of MLD and entrainment cause frontolysis. Furthermore, a metric that quantifies the relative importance of the horizontal gradients of freshwater fluxes and MLD for salinity frontogenesis/frontolysis is proposed. Over the large domain, contribution from the horizontal gradient of freshwater fluxes dominates over that of MLD.