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The Subtropical Ridge as Represented in CMIP5 Climate Models

Paper Reviewed
Grose, M., Timbal, B., Wilson, L., Bathols, J. and Kent, D. 2015. The subtropical ridge in CMIP5 models, and implications for projections of rainfall in southeast Australia. Australian Meteorological and Oceanographic Journal 65: 85-101.

Grose et al. (2015) introduce their paper by noting that "the subtropical ridge (STR) is the mean pressure ridge in the mid-latitudes, and is one of the key features affecting climate variability and change in southeast Australia." Hence, it was only natural that they focused their attention on this unique climatic connection. But after completing their study, they were forced to report a large number of unfortunate findings, including the facts that (1,2) "both the absolute values and the inter-annual variability are too low in most models compared to the observations although [3] some models are higher."

In addition, the five Australian scientists discovered that "the spatial pattern of the correlation between April to October rainfall and the STR-I [ridge intensity] in April to October [4] varies considerably between models with [5] some considerable differences to the observed pattern." And they further report that (6) the trend in southeast Australian April-October rainfall in AWAP -- the Australian Water Availability Project climate data set of 1948-2002 -- "is underestimated in all CMIP5 models." In addition they note that (7) "the relationship between change in STR-I and STR-P [ridge position] and rainfall change is not consistent with observations in each model."

In further discussing their findings, Grose et al. report that (8) "there is no systematic difference or improvement between Earth System Models and Coupled General Circulation Models" as used in their study, and that (9) "there is no consistently better simulation in models with high spatial resolution." In addition, they state that (10) "projections from GCMs have the potential to underestimate the rainfall change," such that (11) "larger changes than the CMIP5 range are possible," while, last of all, they state that (12) "many models are likely to have an incorrect projection of the spatial pattern of rainfall change associated with an intensification of pole-ward movement of the STR in the cool season."

So once again we have another shining example of just how phenomenal are the many failures of today's most advanced climate models.

Posted 5 April 2016