Evaluation of Processes Related to Stratosphere–Troposphere Coupling in GEFSv12 Subseasonal Hindcasts

Author:

Lawrence Zachary D.12ORCID,Elsbury Dillon13,Butler Amy H.3,Perlwitz Judith2,Albers John R.12,Ciasto Laura M.4,Ray Eric13

Affiliation:

1. a Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado

2. b NOAA/Physical Sciences Laboratory, Boulder, Colorado

3. c NOAA/Chemical Sciences Laboratory, Boulder, Colorado

4. d National Center for Environmental Prediction, Climate Prediction Center, College Park, Maryland

Abstract

Abstract The representation of the stratosphere and stratosphere–troposphere coupling processes is evaluated in the subseasonal Global Ensemble Forecast System, version 12 (GEFSv12), hindcasts. The GEFSv12 hindcasts develop systematic stratospheric biases with increasing lead time, including a too strong boreal wintertime stratospheric polar vortex. In the tropical stratosphere, the GEFSv12 winds and temperatures associated with the quasi-biennial oscillation (QBO) tend to decay with lead time such that they underestimate the observed amplitudes; consistently, the QBO-associated mean meridional circulation is too weak. The hindcasts predict extreme polar vortex events (including sudden stratospheric warmings and vortex intensifications) about 13–14 days in advance, and extreme lower-stratospheric eddy heat flux events about 6–10 days in advance. However, GEFSv12’s ability to predict these events is likely affected by its zonal-mean circulation biases, which increases the rates of false alarms and missed detections. Nevertheless, GEFSv12 shows stratosphere–troposphere coupling relationships that agree well with reanalysis and other subseasonal forecast systems. For instance, GEFSv12 reproduces reanalysis relationships between polar vortex strength and the Northern Annular Mode in the troposphere. It also exhibits enhanced weeks 3–5 prediction skill of the North Atlantic Oscillation index when initialized during strong and weak polar vortex states compared to neutral states. Furthermore, GEFSv12 shows significant differences in Madden–Julian oscillation (MJO) amplitudes and enhanced MJO predictive skill in week 4 during easterly versus westerly QBO phases, though these results are sensitive to the level used to define the QBO. Our results provide a baseline from which future GEFS updates may be measured.

Funder

National Weather Service

National Oceanic and Atmospheric Administration

Publisher

American Meteorological Society

Subject

Atmospheric Science

Reference136 articles.

1. Influence of the QBO on the MJO during coupled model multiweek forecasts;Abhik, S.,2019

2. Subseasonal predictability of the North Atlantic Oscillation;Albers, J. R.,2021

3. Andrews, D. G., J. R. Holton, and C. B. Leovy, 1987: Middle Atmosphere Dynamics. Academic Press, 489 pp.

4. A critical comparison of stratosphere–troposphere coupling indices;Baldwin, M. P.,2009

5. The quasi-biennial oscillation;Baldwin, M. P.,2001

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