The sea surface temperature (SST) front in the Gulf Stream (GS) extension region is important to synoptic variations in atmosphere. In winter, large amounts of heat and moisture are released from the SST front, modulating the baroclinicity and humidity of the atmosphere, which is important for extratropical cyclones and atmospheric rivers (ARs). In this study, the variation of SST in the North Atlantic in winters since 1981 is investigated using satellite and reanalysis datasets, and a 23-year (1997 to 2019) warming trend of SST in the GS extension region is detected. The increase of SST is mainly distributed along the SST front, with more than 2 °C warming and a northward shift of the SST gradient from 1997 to 2019. Connected with the SST warming, significant increases in turbulent heat flux and moisture release into the atmosphere were found along the ocean front. As a result, baroclinic instability, upward water vapor flux and AR occurrence frequency increased in recent decades. Meanwhile, there was an increase in extreme rainfall along with the increase in AR landfalling on continental Western Europe (especially in the Iberian Peninsula and on the northern coast of the Mediterranean Sea).
A velocity gradient tensor decomposition method based on a normal frame is introduced in this paper. The velocity gradient tensor is decomposed into a compression–stretching tensor, pure rotation tensor, and pure shear tensor. The analysis shows that both the strain rate tensor and vorticity tensor in Helmholtz velocity decomposition contain shear tensor components, and the total pure shear tensor is the combination of shear components in the two tensors. Based on this decomposition and the physical meaning of each tensor term, the energy dissipation of the channel flow with or without a pressure gradient and a turbine passage flow are analyzed. The results show that the energy dissipation is caused by shear deformation and expansion and contraction deformation of the motion fluid, and pure rotation does not cause energy dissipation. In particular, the pure shear is the primary factor of energy dissipation. Shear accounts for 99.9% of energy dissipation in the fully developed turbulence of zero-pressure gradient channel flow, 99% of the energy dissipation in the separated boundary layer flow is caused by the pure shear, and 81% of the energy dissipation in the turbine stage flow is caused by pure shear.
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