Significant Diurnal Warming Events Observed by Saildrone at High Latitudes

Abstract

The sea surface temperature (SST) is one of the essential parameters needed to understand the climate change in the Arctic. Saildrone, an advanced autonomous surface vehicle, has proven to be a useful tool for providing 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 1 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 and hence less dense surface layer was likely created by precipitation or melting sea ice, providing favorable conditions for the formation of upper ocean stratification. Comparisons with two prognostic diurnal warming models showed the simulations match reasonably well with Saildrone measurements for moderate wind speeds but exhibit large differences at low winds. Both schemes show significant negative biases in the early morning and late afternoon. It is necessary to improve the model schemes when applied at high latitudes.

Plain Language Summary

At high latitudes, satellite remote sensing retrieval of sea surface temperature (SST) is challenging and the number of drifting buoys measuring in situ temperatures for atmospheric correction algorithm refinement and validation is sparser than elsewhere. Two Saildrone cruises in summer 2019, carrying a suite of scientific instruments onboard, can offer both accurate skin SST and subsurface SST measurements in the Pacific sector of Arctic. This study concentrates on those data along with associated atmospheric parameters to demonstrate the characteristics of diurnal variability of temperatures in the upper 2 m of the ocean. That the Arctic is warming much faster than elsewhere as a result of temperature‐dependent feedbacks, emphasizes the need for a better understanding of the thermal structure of the upper Arctic Ocean. Furthermore, comparisons with model simulations reveal that the diurnal warming schemes for high latitudes still need to be improved.

Key Points

• Using skin and subsurface ocean temperatures from two Saildrones to study the 2 m upper ocean thermal stratifications at high latitudes

• Several diurnal warming events with significantly large amplitudes, some even with long persistence, are documented and discussed

• Model schemes for diurnal warming applied at high latitudes necessarily need to be improved