Dynamical mechanisms that account for the response of the Southern Annular Mode to natural variability and forced climate change

The main goal of this project is to improve the understanding of the dynamical mechanisms that account for the response of the SAM to the combined influence of natural variability and forced climate change (including both increasing GHG, and ozone depletion/recovery), and its influence on the climate of southern South America.

Why do we care about this?

Climate variability in southern South America from intra-seasonal to multi-decadal time scales is strongly influenced by remote sources of variability located in both tropical and polar regions (e.g. Vera et al., 2006). In each frequency range of variability, large-scale patterns alter the atmospheric circulation and the activity of the weather regimes in the vicinity of the continent. In particular, in the extratropical zones of the Southern Hemisphere (HS), the Southern Annular Mode (SAM) is the main pattern of atmospheric circulation variability, characterized by intensification or weakening of mid-latitude westerly winds, in association with anomalies of pressure or geopotential height of opposite sign between polar and extratropical regions (eg Rogers and van Loon 1982, Thompson and Wallace 2000). The SAM is mainly maintained by the internal variability of the atmosphere and plays an important role in the coupling of tropospheric and stratospheric dynamics (e.g. Baldwin and Thompson, 2009). The SAM exhibits variability on a wide range of time scales, especially from intraseasonal to longer time scales (Flateau and Kim 2013, Pohl et al., 2010). In this way, SAM activity can coincide with that associated with tropical climate patterns (such as the Madden-Julian Oscillation (MJO) on intra-seasonal time scales, or the El Niño-Southern Oscillation (ENSO) on seasonal time scales) and alter the influence of the latter on the dynamics of the circulation of SH in general and on the climate of southern South America in particular. These combined influences have not received enough attention from the scientific community despite the potential consequences that can arise from either reinforcing or inhibiting the influences that each pattern can individually exert.

In the past decade, great advances in the knowledge of how the mentioned elements of the climate system interact and influence the Southern Annular Mode. However, a lot of work has been done in one-on-one interactions (i.e., trop-strat coupling or ENSO-SAM interactions) the challenge of this project is to propose methods that allow looking at these proceses as part of a complex connected system, to then develop a framework for understanding the change of this system under antrhopogenic forcing. For this, the storyline approach, bayesian networks and causal effect networks are innovative and powerful methods.

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The hypothesis underpinning the proposed research are:

  1. SAM activity results mainly from the internal variability of the climate system and it can be modulated by external forcing of climate such as that associated with GHG increases and stratospheric ozone depletion.
  2. SAM variability mainly results from the convergence/divergence of atmospheric eddy momentum transport. Eddy activity is associated with westerly jet dynamics that in turn can be influenced by the tropical climate variability.
  3. There is vertical coupling of the SH circulation whereby stratospheric vortex anomalies in the spring season affect the timing of the summertime regime transition of the SAM A3. nthropogenic climate forcing (GHG increases, ozone depletion) can induce both symmetric (jet intensification/weakening) and asymmetric (jet latitude shift) responses of the circulation in the SH.
  4. Climate projections are associated with three sources of uncertainties (internal variability, model differences, scenario differences). The storyline approach is a powerful tool to provide a physical basis for addressing these uncertainties.

I am having a great time working on this project and part of the results can be found in these peer-reviewed published articles. Additionally, to keep track of my own advances and because I find this part so interesting, I am posting as a personal blog some elements of my current research.