Atmosphere-Ocean Momentum Exchange by Extra-Tropical Storms

• Main Author: Hell, Momme Claus
• Other Authors: Miller, Arthur J., Gille, Sarah T.
• Format: Other/Unknown Material
• Language: English
• Published: eScholarship, University of California 2020
• Subjects: Atmospheric sciences; Geophysics; Environmental science; angular momentum conservation; Atmosphere-Ocean fluxes; supervised machine learning; surface waves; surface wind; wave generation; Pacific; Ross Ice Shelf; Southern Ocean; Ice Shelf
• Online Access: https://escholarship.org/uc/item/8qf3t9wk

The earth's climate warms with increasing greenhouse gases in the atmosphere. The Southern Ocean (SO) mixed layer dampens the speed and intensity of global warming by storing a large fraction of the anthropogenic CO2 and heat. However, the mechanisms and hence the SO's future capabilities to store heat and CO2 remain uncertain. This thesis aims to understand better how atmospheric wind forcing drives mid-latitude mixed-layer variability. It focuses on the wind forcing and swell generation under extra-tropical cyclones and links these to the large-scale atmospheric circulation.\parA supervised machine learning method is developed to characterize events in wave's spectrograms of Ross Ice Shelf seismometers. The events are used to show that wave origins under SO storms are systematically displaced compared to the highest wind speeds. This result is further explored by extending the optimization method to multiple wave buoys in the North Pacific to derive a common set of parameters that describe the origin and intensity of waves. The triangulated wave source location motivates developing an idealized swell generation model that mimics the time and spatially varying wind forcing as a 2D-Gaussian distribution that moves with a constant speed over the ocean. It shows that the location where wind stress and wave forcing are the strongest is not the same as the identified swell source location because non-linear wave-wave interaction prohibits wave dispersion. The Gaussian moving wind model reveals the sensitivity of the waves spectral energy and peak frequency on extreme winds under storms because they influence the spatial gradients of the moving wind field.\parFinally, an SVD decomposition on surface wind probability distributions from reanalysis and scatterometer winds over the SO is used to link changes in the extremes of the joint wind and stress probability density functions over the SO to the Southern Annular Mode. It reveals how the planetary-scale circulation drives surface wind extremes through storm intensity over the SO and suggests how the swell climate, related surface stress pattern, and mixed-layer ventilation may change with a drifting large-scale atmospheric circulation