Atmosphere Ocean Science Colloquium

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How annular modes are linked surface wave events and wind-driven mixing

Speaker: Momme Hell, Brown

Location: Warren Weaver Hall 1302

Date: Wednesday, March 30, 2022, 3:30 p.m.

Synopsis:

Southern Ocean (SO) surface winds are essential for ventilating the upper ocean by bringing heat and CO2 to the ocean interior. Synoptic wind variability is also known to drive the generation of large swell events, while it remains unclear how their characteristic scales affect the generation of swell. 

In the first part of the talk, observed time-varying 5-day PDFs of ERA5 surface winds and stresses over the SO are used in a singular value decomposition to derive a linearly independent set of empirical basis functions. The first modes of wind and stress are highly correlated with the momentum flux divergence in the atmospheric column (SAM) and reflect the SAM's role in driving cyclone intensity and, in turn, extreme westerly winds on short time scales. The joint PDFs of zonal and meridional wind show that southerly and less westerly winds associated with strong mixed-layer ventilation are more frequent during short and distinct negative SAM phases. 

The second part of the talk describes the dynamics of wave generation under a moving storm by using a simple parametric model of wave development, forced by a temporally and spatially varying moving wind field. This framework reveals why swell systems are expected to originate from locations different than the moving high-wind forcing regions. We confirm this by a physically informed optimization method that back-triangulates the common source locations of swell using their dispersion slopes, simultaneously measured at five wave-buoy locations. This model then describes swell generation as a three-stage process that outlines a focus area where swell energy and subsequent wave growth are enhanced.