The Behavior of the Atmospheric Boundary Layer in the Vicinity of the Gulf Stream Sea Surface Temperature Front

Abstract

The evolution of the marine atmospheric boundary layer (MABL) in the vicinity of a sea surface temperature (SST) front is of particular research interest, as the large air-sea temperature and humidity differences at the surface fuels various physical processes inside the boundary layer, causing intense heat and momentum exchange. Such processes make the mesoscale MABL an ocean-drive-atmosphere scenario. Dominant mechanisms, although having been studied intensively, are still yet to be fully understood due to the highly turbulent nature of the MABL. Previous studies often relied on satellite-derived SST and wind fields to investigate boundary layer dynamics, yet the coarse spatial and temporal resolution of such a method limits the understanding of the MABL evolution on shorter timescales.

In this thesis, a combination of in situ data and model simulations is used to investigate the MABL response to the SST front in the Gulf Stream region on a timescale of one day or less. Analysis of MABL structure is divided into three categories depending on the background wind strength and its direction relative to the front: cold to warm, parallel/weak, and warm to cold. Two mechanisms identified in previous studies, vertical mixing and thermally induced pressure gradient, and their role in MABL evolution, are studied quantitatively. A comparison between observations and model simulations allows further analysis of the contribution of moist processes that were often considered to be of secondary importance in the past even over the ocean. Results show that vertical mixing is responsible for the majority of the MABL deepening, while the pressure adjustment’s effect is more significant when the cross-frontal wind is weak. Sensitivity tests conducted in the Weather Research and Forecast (WRF) also show that moisture processes, including surface latent heat, boundary layer transport of moist, and cloud formation, further enhance the mixing that drives MABL changes.