Atmosphere Ocean Science Friday Seminar

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A data-driven analysis of the Quasi-Biennial Oscillation with Koopman modes

Speaker: Claire Valva, CAOS

Location: Warren Weaver Hall 1314

Date: Friday, February 3, 2023, 4 p.m.

Synopsis:

The Quasi-Biennial Oscillation (QBO) is the leading mode of variability of the equatorial stratosphere, characterized by downward propagating easterly and westerly wind regimes with a period of approximately 28 months. The QBO is known to affect other regions of the atmosphere, including a warming of the boreal polar stratosphere during easterly QBO (the Holtan-Tan effect). In this talk, I will present Koopman operator formalism as an approach for coherent feature extraction in dynamical systems with the QBO as a test case.

Koopman operator theory allows one to translate nonlinear dynamical systems into a linear framework. Data-driven approximations of Koopman operators can therefore provide decompositions of the climate system without assuming linearity.

We use a Koopman decomposition of zonal winds from ERA5 reanalysis to formulate an objective index for the QBO and compare it to conventional QBO indices. We then use this new data-driven index to investigate the interactions between the QBO and the seasonal cycle as well as diagnose teleconnections between the QBO and the extratropics, including the Holtan-Tan effect.