The identification of the MMC is most often done by vertically integrating the zonal mean meridional wind in isobaric coordinates, resulting in a mass streamfunction whose contours are streamlines that quantify the flow. This calculation produces the familiar ‘three-cell’ model, with the majority of the overturning circulation found in the Hadley Cells (HCs), two counter-rotating cells found between ~30° in each hemisphere. In this view, the extratropical circulation (i.e. the Ferrel and Polar cells) is comparatively weak and little interaction is suggested between the tropics and extratropics.
Alternatively, the MMC can be examined in isentropic coordinates, where (dry) potential temperature is the vertical coordinate. In this case, the zonal-mean mass-weighted meridional wind is vertically integrated. This MMC in this view differs from the three-cell view in that it generally depicts a single circulation cell encompassing the entire hemisphere, with a strong extratropical component that arises from within mass transports due to amplifying baroclinic waves. Further, vertical motion in this coordinate system, proportional to the meridional derivative of the streamfunction, reflects the location of large-scale areas of diabatic heating and cooling which provides relevant information on the processes driving the MMC.
The aim of this work is to document and understand any changes to the MMC that have occurred in the recent historical period. The isentropic MMC is computed using daily 1.5° x 1.5° ERA Interim reanalysis fields of meridional wind (v) and temperature (T) over 1979-2015, then averaged monthly and seasonally (i.e. DJF, MAM, etc.) to get the 'full' isentropic MMC. The 'steady' component of the MMC is similarly computed using monthly mean fields of v and T. The steady component resembles aspects of the isobaric three-cell circulation and allows for an identification of the HC. A 'transient' circulation is determined by subtracting the steady circulation from the full circulation. The transient circulation is dominated by an overturning cell centred in the extratropics, representing the cumulative mass transports due to baroclinic waves.
For the steady and transient circulations, the vertical mass transport (VMT) is computed along selected isentropes at all latitudes and all seasons. From a 7-point smoothed field of VMT, broad currents of upward and downward motion (i.e. diabatic heating and cooling) can be identified. In the steady circulation, a near-equatorial upward current corresponding to the global ITCZ and two downward currents near 30° latitude representing the subsiding branches of the HC are dominant. In the transient circulation, a 'transient updraft' and 'transient downdraft' broadly centred at 30° and 60° latitude respectively are identified in each hemisphere. The upward branch is broadly consistent with the location of hemispheric mean 'storm track'. Seasonal metrics, including the width, the total VMT and the VMT-weighted mean position of these seven MMC features are computed. The temporal variations of these metrics form the basis of the evaluation in this work.
For the Southern Hemisphere (SH) HC, the position of the downward branch has been moving poleward since 1979 in all seasons except SON; statistically significant trends vary between 0.2 and 0.6° decade-1, comparable with other estimates of tropical expansion. The largest trend is identified during MAM. Coincident with this expansion has been an increase in the strength of the downward branch, implying that there is greater radiative cooling taking place. This trend is statistically significant at the 90% level in all seasons, with the strongest increase during SON when no trend in position is identified. In the Northern Hemisphere (NH), significant expansion trends are noted in JJA and SON (0.4 -1.3° decade-1), with weaker, non-significant expansion trends (~0.2° decade-1) during DJF and MAM. The trends in intensity are towards slight weakening during JJA and SON and a strengthening during DJF and MAM, although these trends are not significant in any season. The linkage (if any) between the HC downward-branch position and its intensity is unclear.
The strengthening of the SH is unexpected; most theoretical studies suggest that the strength of the MMC should weaken as the globe warms. One explanation may lie in the equator-to-pole temperature gradient. Surface potential temperature trends show that temperatures near Antarctica have decreased, increasing that gradient. To the degree that this gradient determines the strength of the MMC, the circulation strength could be enhanced. A second explanation may lie in changes to the ITCZ. The ITCZ shows little change in position, but strong (but not statistically significant) increases in strength in all seasons. These changes indicate a higher heating rate, presumably related to observations of increased precipitation and greater latent heating in the ITCZ. The strength of the SH HC downward branch and the ITCZ strength are highly correlated, implying a linkage between the two.
The SH transient circulation, particularly the upward branch, shows some significant changes. There is an equatorward trend in position and a significant weakening in most seasons. The two are related; a weakening of the upward mass transport is noted between 30° and 45°S, while more equatorward portions remain unchanged, resulting in the shift of the centroid of heating. The changes are not linear, but show a ~15% drop between approximately 1998 and 2010, after which they return towards the pre-1998 values.
The causes and consequences of these changes to the MMC continue to be investigated. One particular thread is in the diagnosis of how the MMC changes in relation to modes of climate variability such as ENSO, the Southern Annular Mode (SAM) and the Interdecadal Pacific Oscillation (IPO). There is also interest in the how these broader MMC changes translate into impacts on smaller spatiotemporal scales.
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