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IOVWST Meeting
Portland, Maine, USA, 2019

Deep atmospheric structure anchored by surface wind convergence near major ocean fronts

Presenting Author: Professor Shang-Ping Xie

Significant surface wind modulation by sea surface temperature (SST) fronts has been extensively documented from QuikSCAT. While such SST frontal effects are often trapped in the atmospheric boundary layer (ABL), our recent studies show that they may extend into as deep as the upper troposphere, along the Gulf Stream in the North Atlantic, the Kuroshio Extension and subtropical countercurrent in the Northwest Pacific. The Gulf Stream affects the entire troposphere. In the ABL, atmospheric pressure adjustments to sharp SST gradients lead to surface wind convergence that anchors a narrow precipitation band along the Gulf Stream. In this rain band, upward motions and cloud formation extend into the upper troposphere as corroborated by the frequent occurrence of very cold cloud-top temperatures. These mechanisms offer a direct pathway by which the Gulf Stream can affect the atmosphere both locally and possibly in remote regions via planetary wave propagation. Similarly, the Kuroshio Extension anchors a surface convergence zone, along which the cloud top is significantly higher in altitude than the surroundings. The North Pacific subtropical countercurrent (STCC) is an eastward current, against not only the southeast trades as well as the westward flow predicted by the Sverdrup theory. Potential vorticity gradients of thermocline water subducted in winter are considered to cause this peculiar current. The QuikSCAT climatology reveals that the SST front of the STCC anchors a zonal band of weakly positive curls in the midst of prevailing negative curls. This band of positive wind curls is associated with a deep moist layer in the atmosphere and frequent genesis of sub-synoptic cyclones, possibly due to enhanced atmospheric baroclinicity. The deep atmospheric influence is most pronounced during April-May when SST gradients are still strong and SST on the warm flank of the STCC front is high enough and conducive to atmospheric convection.

2019 International OVWST Meeting
May 29-31 in Portland, Maine, USA
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