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.
Highlights
The role of the mean meridional circulation (MMC) in the global climate is to alleviate the global heating imbalance between the equator and poles (Oort and Yienger 1996)
The Hadley Cells (HC) metric considered here is based in the mid-troposphere and the results show a similarity with other reanalysis-based studies using the widely used ’PSI500’ metric
This study investigates changes to the global mean meridional circulation (MMC) using the dry isentropic streamfunction Ψ, which presents a different view of the flow compared to the more common isobaric streamfunction calculation
Summary
The role of the mean meridional circulation (MMC) in the global climate is to alleviate the global heating imbalance between the equator and poles (Oort and Yienger 1996). While the dry isentropic circulation does not fully capture the energy transformations as well as the moist isentropic or thermodynamic coordinate systems (Kjellsson et al 2014), it has the advantage that it is well defined in space, since the θ is vertically stratified in both the tropics and the extratropics In this framework, the MMC is defined by a single thermally direct circulation cell in each hemisphere (Fig. 1b), as opposed to the isobaric “three-cell” view. Throughout the paper seasons will be referred to as that relevant to the hemisphere under discussion, if this is not entirely clear the terms ’austral’ or ’boreal’, or the series of months, will be added
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