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Michael R. Grose
,
Jonas Bhend
,
Sugata Narsey
,
Alex Sen Gupta
, and
Josephine R. Brown

Abstract

Climate warming has large implications for rainfall patterns, and identifying the most plausible pattern of rainfall change over the next century among various model projections would be valuable for future planning. The spatial pattern of projected sea surface temperature change has a key influence on rainfall changes in the tropical Pacific Ocean. Here it is shown that simple indices of the size of the equatorial peak in the spatial pattern of warming and to a lesser extent the hemispheric asymmetry in warming are useful for classifying the surface temperature change in different models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Models with a more pronounced equatorial warming show a fairly distinct rainfall response compared to those with more uniform warming, including a greater “warmer-get-wetter” or dynamical response, whereby rainfall increases follow the surface warming anomaly. Models with a more uniform warming pattern project a smaller rainfall increase at the equator and a rainfall increase in the southern tropical Pacific, a pattern that is distinct from the multimodel mean of CMIP5. Thus, the magnitude of enhanced equatorial warming and to some extent the hemispheric asymmetry in warming provides a useful framework for constraining rainfall projections. While there is not a simple emergent constraint for enhanced equatorial warming in models in terms of past trends or bias in the current climate, further understanding of the various feedbacks involved in these features could lead to a useful constraint of rainfall for the Pacific region.

Full access
David J. Karoly
,
Mitchell T. Black
,
Andrew D. King
, and
Michael R. Grose
Full access
Michael R. Grose
,
James S. Risbey
,
Mitchell T. Black
, and
David J. Karoly
Full access
Sugata Narsey
,
Josephine R. Brown
,
Francois Delage
,
Ghyslaine Boschat
,
Michael Grose
,
Rob Colman
, and
Scott Power

Abstract

The South Pacific convergence zone (SPCZ) is evaluated in simulations of historical climate from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and phase 6 (CMIP6) models, showing a modest improvement in the simulation of South Pacific precipitation (spatial pattern and mean bias) in CMIP6 models but little change in the overly zonal position of the SPCZ compared with CMIP5 models. A set of models that simulate a reasonable SPCZ are selected from both ensembles, and future projections under high emissions (RCP8.5 and SSP5–8.5) scenarios are examined. The multimodel mean projected change in SPCZ precipitation and position is small, but this multimodel mean response obscures a wide range of future projections from individual models. To investigate the full range of future projections a storyline approach is adopted, focusing on groups of models that simulate a northward-shifted SPCZ, a southward-shifted SPCZ, or little change in SPCZ position. The northward-shifted SPCZ group also exhibit large increases in precipitation in the equatorial Pacific, while the southward-shifted SPCZ group exhibit smaller increases in equatorial precipitation but greater increases within the SPCZ region. A moisture budget decomposition confirms the findings of previous studies: that changes in the mean circulation dynamics are the primary source of uncertainty for projected changes in precipitation in the SPCZ region. While uncertainty remains in SPCZ projections, partly due to uncertain patterns of sea surface temperature change and systematic coupled model biases, it may be worthwhile to consider the range of plausible SPCZ projections captured by this storyline approach for adaptation and planning in the South Pacific region.

Significance Statement

The South Pacific convergence zone is a band of intense rainfall that influences the weather and climate of many Pacific Island communities. Future changes in the SPCZ will therefore impact these communities. We examine climate model representations of future climate to find out how the SPCZ might change in a warmer world. While the models disagree on future changes in the SPCZ, we suggest that it may be useful to consider groups of models with common “storylines” of future change. The changes in the position of the SPCZ in a warmer world correlate strongly to the amount of rainfall change locally. Some models suggest a northward movement of the SPCZ, while others suggest a southward movement. Consideration of the full range of possible future behavior of the SPCZ is needed to better prepare for the impacts of a warmer climate.

Open access
Michael R. Grose
,
Mitchell Black
,
James S. Risbey
,
Peter Uhe
,
Pandora K. Hope
,
Karsten Haustein
, and
Dann Mitchell
Full access
Michael R. Grose
,
James S. Risbey
,
Aurel F. Moise
,
Stacey Osbrough
,
Craig Heady
,
Louise Wilson
, and
Tim Erwin

Abstract

Atmospheric circulation change is likely to be the dominant driver of multidecadal rainfall trends in the midlatitudes with climate change this century. This study examines circulation features relevant to southern Australian rainfall in January and July and explores emergent constraints suggested by the intermodel spread and their impact on the resulting rainfall projection in the CMIP5 ensemble. The authors find relationships between models’ bias and projected change for four features in July, each with suggestions for constraining forced change. The features are the strength of the subtropical jet over Australia, the frequency of blocked days in eastern Australia, the longitude of the peak blocking frequency east of Australia, and the latitude of the storm track within the polar front branch of the split jet. Rejecting models where the bias suggests either the direction or magnitude of change in the features is implausible produces a constraint on the projected rainfall reduction for southern Australia. For RCP8.5 by the end of the century the constrained projections are for a reduction of at least 5% in July (with models showing increase or little change being rejected). Rejecting these models in the January projections, with the assumption the bias affects the entire simulation, leads to a rejection of wet and dry outliers.

Full access
Carly R. Tozer
,
James S. Risbey
,
Michael Grose
,
Didier P. Monselesan
,
Dougal T. Squire
,
Amanda S. Black
,
Doug Richardson
,
Sarah N. Sparrow
,
Sihan Li
, and
David Wallom
Free access