Significant Diurnal Warming Events Observed by Saildrone at High Latitudes

Abstract

The sea surface temperature (SST) is one of the essential parameters to understand the climate change in the Arctic. Saildrone, an advanced autonomous surface vehicle (ASV), has proven to be a useful tool for providing sufficiently accurate SST data at high latitudes. Here, data from two Saildrones, deployed in the Arctic in the summer of 2019, are used to investigate the diurnal variability of upper ocean thermal structure. An empirical cool skin effect model with dependence on the wind speed with new coefficients was generated. Several local large diurnal warming events were observed, the amplitudes of warming in the skin layer > 5 K, rarely reported in previous studies. Furthermore, the warming signals could persist beyond one day. For those cases, it was found surface warm air suppressed the surface turbulent heat loss to maintain the persistence of diurnal warming under low wind conditions. Salinity also plays an important role in the formation of upper ocean density stratification during diurnal warming at high latitudes. A less salty surface layer was likely created by precipitation or melting sea ice, providing favorable conditions for the formation of upper ocean stratification. By comparing with two prognostic diurnal warming models, the simulations match reasonably well with those from the Saildrone for moderate wind speed conditions but exhibit large differences at low winds. Both schemes show significant negative biases in the early morning and late afternoon. It is necessary to further improve the model schemes when applied at high latitudes.