Propagating Annular Modes in Reanalysis and CMIP Models: Dynamics and Timescales
Abstract
In recent work, we have questioned the importance of a cross-EOF eddy-zonal flow feedback in variability of the Southern Hemisphere (SH) annular modes. We have shown when the extratropical circulation is in a propagating regime, the persistence of the annular mode is controlled by the cross-EOF feedbacks arising from a more complex interaction of eddies and zonal winds. In this study, we will revisit the low-frequency variability of the Southern Hemisphere extratropical large-scale circulation using reanalysis data and CMIP6 models. In particular, we will investigate (1) the underlying dynamics of the cross-EOF eddy-zonal flow feedbacks (EOF1-onto-EOF2 and EOF2-onto-EOF1 feedbacks), and (2) the implication of propagating regime of the annular mode in the extratropical circulation in CMIP models. Our results so far indicate that the cross-EOF feedbacks are essential components of the propagating annular modes. Using a finite-amplitude wave activity (FAWA) framework, we found that the cross-EOF feedbacks arising from the out-of-phase oscillations of EOF1 (north-south jet displacement) and EOF2 (jet pulsation) leads to an orchestrated combination of equatorward propagation of wave activity (a baroclinic process) and nonlinear wave breaking (a barotropic process), which altogether act to reduce the total eddy forcings. Using the CMIP5/6 database, we have also analyzed the pre-industrial control and RCP8.5 forcing scenario integrations and categorized them by their propagation characteristics. We found that the models with the propagating regime show more realistic the annular mode timescales compared to the models with non-propagating regime (dominated by EOF1). It is also shown that the model tends to have more propagating regime under future RCP8.5 scenario, thus affecting the seasonal extratropical surface climate. Our findings suggest a new metric for evaluating state-of-the-art climate models and potentially improving them.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.A15M1818L