New paper in QJRMS!
Decadal-scale drivers of South Pacific hydroclimate variability is now available to read in the Quarterly Journal of the Royal Meteorological Society. In this new paper we use the IGCM4 cliamte model to show the effects of different ocean states on the South Pacific Convergence Zone (SPCZ).
We uncovered two key findings in the paper:
1. The position of the SPCZ is governed primarily by thermodynamic processes
The SPCZ forms when Rossby waves refracted north-eastward from the Southern Hemisphere jet trigger convection in the central South Pacific.
We find that changes in the source location of Rossby waves preopagating into the central South Pacific plays little role in setting the position of the SPCZ. Due to the role of the westerly duct as a wave guide, the Rossby waves converge on a similar path, regardless of starting point. The factor governing the position of the SPCZ is the point at which these Rossby waves meet sufficient moisture to trigger deep convection.
Moisture is transported into the central South Pacific from the tropical East Pacific by the South Pacific Gyre (which is in turn closely related to the Pacific Walker Circulation). A stronger transport allows mositure to reach more deeply into the central South Pacific, meanning that Rossby waves will trigger deep convection earlier on their path. The net result of this change in the location of convection is a shift in the SPCZ to the south west.
2. The IPO is the primary driver of decadal-scale SPCZ variability
The Interdecadal Pacific Oscillation (IPO) is a large-scale mode of decadal variability in Pacific sea surface temperatures. When the IPO is in its positive phase, we see warmer temperatures in the tropical East Pacific, the western North Pacific, and the western South Pacific.
The warming of the tropical East Pacific weakens the temperature gradient across the tropical Pacific, in turn weakening the Pacific Walker circulation and South Pacific Gyre. With reduced south-westward transport, moisture cannot reach as far into the central South Pacific before meeting north-eastward propagating Rossby waves. These waves trigger deep convection, where the moisture is rained out. Since convection is occuring closer to the tropical East Pacific moisture source, we see a north-eastward shift in the SPCZ. Conversely, when the IPO is in its negative phase, we see a south-westward shift in the SPCZ.
Shifts in the Southern Hemisphere jet can cause changes in rainfall across the South Pacific, but these changes do not constitute a homogeneous shift in the SPCZ. When the jet shifts northwards, it compresses the Hadley and Ferrel cells. The northward shift in regions of ascent and descent cause similar shifts in regions of precipitation, however these are on a much smaller scale than those driven by the IPO.
Despite being the largest mode of decadal-scale variability in global terms, the Atlantic Multi-decadal variability (AMV) has very little direct effect on the SPCZ. We do, though, note that the atmosphere-only modelling approach we take may hide some oceanic teleconnections that could drive SPCZ changes. Whilst we do see a very weak rainfall shift in the South Pacific as a result of the AMV, this is actually due to its connection to the IPO, meaning that we are actually seeing an aliasing of the IPO-SPCZ relationship.
Overall the IPO exerts by far the greatest control over the SPCZ on decadal time scales.
This paper marks the final part of the PROMS project, a collaboration between the University of East Anglia and the University of Southampton. You can find out more about the PROMS project below: