Covariability of Southern Hemisphere tropical edge metrics with the SAM

The tropical edge is predicted to continue its expansion toward the poles in future climate model scenarios. Observations show the intensity and position of the subtropical jet and subtropical ridge are correlated with one another, to the Hadley cell edge and to Southern Annular Mode (SAM). In this study, we use a multivariate linear independence approach to identify partial correlations and conditional independence between tropical edge metrics and with the SAM to identify which aspects interact directly and indirectly in Era-Interim and a subset of CMIP5 models.

The observed subtropical ridge position mediates relationships between tropical edge metrics. A poleward displaced ridge is associated with a stronger subtropical ridge, a poleward displaced and weaker subtropical jet. The subtropical jet position is conditionally independent of its intensity in each season except summer. Interactions with the SAM vary seasonally and the SAM is linked to at least one tropical edge metric in each season. The subtropical jet intensity is conditionally independent of the SAM. CMIP5 models are generally able to reproduce the observed results, however the modelled subtropical jet, ridge and especially the SAM interact more with one another than is seen in observations.

A simple physical interpretation is offered to explain why a poleward subtropical ridge is strong and how the poleward subtropical ridge is linked to a weak subtropical jet. Understanding and correctly modelling the interactions between the subtropical jet, ridge and SAM will lead to increased understanding of Southern Hemisphere atmospheric variability and precipitation variability in subtropical regions.