A152-0014
Seasonal Prediction of the Boreal Stratospheric Vortex
Seasonal Prediction of the Boreal Stratospheric Vortex
Monday, 14 December 2020
Poster
Abstract:
In winter, seasonal forecasts of mid-latitude weather benefit from a realistic representation of the extratropical stratosphere, particularly in the Northern Hemisphere (NH). Recent advancement is seasonal forecasts reveal unprecedented skill in predicting the extra-tropical flow in the troposphere. Here, with a process oriented approach, we assess the predictability of the extra-tropical flow in the stratosphere and the underlying dynamics. In particular the role of upward-propagating stratospheric waves for the variability and predictability of the polar vortex strength is examined.
Winter predictions of the NH stratosphere (start date: 1st of November from 1993 to 2016) are obtained from the seasonal Copernicus Climate Change Service multi-model system, and compared with ERA-Interim reanalysis. Among the five forecast systems, three predict the strength of the stratospheric vortex with significant skill. Reanalysis data shows that the upward propagation of planetary waves, typically generated by tropospheric wave sources, explain the interannual variability of the vortex wind; such feature is represented by the majority of systems. Using a relationship derived by Hinssen and Ambaum (2010)[1], we separate the relative contribution of the waves in the first month and in the subsequent season. We demonstrate that skilful predictions of the vortex intensity originate from predictability of wave-forcing on two time scales, namely subseasonal and seasonal. Seasonal sources of extratropical-stratosphere variability are also investigated and reveal a role for ENSO, QBO and NAO. Encouraging evidence for seasonal probabilistic prediction of sudden stratospheric warmings is discussed.
Winter predictions of the NH stratosphere (start date: 1st of November from 1993 to 2016) are obtained from the seasonal Copernicus Climate Change Service multi-model system, and compared with ERA-Interim reanalysis. Among the five forecast systems, three predict the strength of the stratospheric vortex with significant skill. Reanalysis data shows that the upward propagation of planetary waves, typically generated by tropospheric wave sources, explain the interannual variability of the vortex wind; such feature is represented by the majority of systems. Using a relationship derived by Hinssen and Ambaum (2010)[1], we separate the relative contribution of the waves in the first month and in the subsequent season. We demonstrate that skilful predictions of the vortex intensity originate from predictability of wave-forcing on two time scales, namely subseasonal and seasonal. Seasonal sources of extratropical-stratosphere variability are also investigated and reveal a role for ENSO, QBO and NAO. Encouraging evidence for seasonal probabilistic prediction of sudden stratospheric warmings is discussed.
[1] Hinssen, Y. B. L. and Ambaum, M. H. P.: Relation between the 100-hPa heat flux and stratospheric potential vorticity, J. Atmos.Sci., 67, 4017–4027, 2010.