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Pacific problems for climate models

Dr David Whitehouse, Science editor

State-of-the-art climate models generally predict that the Walker circulation will weaken bringing enhanced warming in the eastern equatorial Pacific and warming in the Southern Ocean, contrary to what is observed.

The warmest water on Earth can be found on the edge of the Pacific and is getting warmer due, according to some reports, anthropogenic climate change. But it’s not that straightforward. In fact this vital region is showing us just how wrong climate models can be. The Indo-Pacific Warm Pool (IPWP) is one of the most climatically important regions on our planet with a global influence yet, as observations and recent studies show, climate models fail to explain its behaviour with implications for all forecasts of climate change.

Waters from the western Pacific Ocean move into the eastern Indian Ocean via many passages that thread the Indonesian Archipelago. Many of those seas are shallow and straddle the equator resulting in very warm water all year. The IPWP is huge, covering a surface area equivalent to that of the entire USA, and is crucial to the regulation of climate globally.

The region is affected by monsoons and their seasonally alternating wind directions. It is the in the region of the Intertropical Convergence Zone (ITCZ) – a belt of low pressure that traverses the equator. Here trade winds from the northern and southern hemispheres converge. The ITCZ is formed as a result of the vertical motion of convective clouds, most of which the rising component of the Hadley Cell, and of great importance to global atmospheric circulation.

It is a region of change. Observations over recent decades show a significant warming of the IPWP and slight cooling in the eastern equatorial Pacific, consistent with strengthening of the so-called Walker circulation, and also cooling in the Southern Ocean. The relative warmth of the warm pool in the western Indo-Pacific compared to the cold tongue in the eastern equatorial Pacific drives the Walker circulation in the tropical atmosphere which through its interaction on the upper tropospheric drives large-scale atmospheric Rossby waves that propagate into higher latitudes and impact climate around the globe.

The problem is that state-of-the-art climate models generally predict that the Walker circulation will weaken bringing enhanced warming in the eastern equatorial Pacific, and warming in the Southern Ocean, contrary to what is observed. Sea-level pressure (SLP) observations show a weakening of the Walker circulation over the twentieth century, though some maintain that the Walker circulation has strengthened since 1979 contradicting climate model hindcasts over this period.

New research looks into these discrepancies. A team of scientists from the Universities of Washington, Columbia and Illinois investigate the ability of 16 climate model large ensembles to reproduce observed sea-surface temperature and sea-level pressure trends over 1979-2020 through a combination of externally forced climate change and internal variability.

They find that the models don’t work and don’t work by a great margin. They find the large-scale differences between observed and modelled trends are very unlikely (<5% probability) to occur due to internal variability as represented in models. Conflicting trends are found even in regions with weak multi-decadal variability, suggesting that the climate model inadequacy is profound.

We show that the spatial pattern of observed temperature changes since 1979 is highly unusual, and many aspects of it cannot be reproduced in current climate models, even when accounting for the influence of natural variability. We find a particularly large discrepancy in the rate of warming within the western Pacific Ocean and eastern Indian Ocean, which suggests that models have systematic biases,” say the researchers.

The IPWP is adjacent to a highly populated region and also located at a crucial place on the globe, at the confluence of two important oceans, where atmospheric processes are significant and also where biodiversity on land and in the oceans is unique. What this research points to is a failing of climate models in this vital region indicating a huge source of uncertainty in multi-decadal projections of regional and global climate.