Variability and Changes to the Mean Meridional Circulation in Isentropic Coordinates

Abstract

We examine the climatology, variability and change in the global mean meridional circulation (MMC) as measured in a dry isentropic coordinate system from 1979–2017 using the ERA-Interim reanalysis. The methodology presents a zonal-mean view of the MMC as a single thermally direct circulation cell in each hemisphere. The circulation is decomposed into 'steady' and 'transient' components which allows us to identify and quantify several MMC features, including the Intertropical Convergence Zone, the descending branches of the Hadley circulation and a 'transient updraft' associated with the extratropical storm track. Large changes were identified in the Southern Hemisphere (SH) in both the Hadley Cell and the extratropical storm track in the late-1990s. These changes intertwine with the Interdecadal Pacific Oscillation that changed from a warm to a cold phase around 2000. Less significant changes were observed in the Northern Hemisphere, although high rates of tropical expansion during boreal summer may have been exacerbated by volcanic eruptions in the 1980s and 1990s. Further to those changes, tropical expansion was observed in autumn, with little change in the extratropical storm track. While potential inhomogeneities in the reanalysis limit the certainty about the magnitude of the identified changes, multiple non-reanalysis-based datasets suggest that large changes did occur in the 1990s in the SH, supporting the presented analysis.

Investigating Tropical Versus Extratropical Influences on the Southern Hemisphere Tropical Edge in the Unified Model

“…Lucas et al. (2021) documented inhomogeneities in the ERA‐I data set, which were found to affect the reproduction of the mean meridional circulation. Their comparison to other reanalysis products yielded similar constraints, suggesting a general issue of reanalysis products and not specific to ERA‐I.…”

Metrics of the Hadley circulation strength and associated circulation trends

“…Wu et al, 2019) or dry isentropic coordinates (e.g. Lucas et al, 2021) that yield a different perspective on the mean meridional circulation. Overall, we believe that future discussions of HC strengthening or weakening would benefit from using a unified metric.…”