The cooling of the Eastern Equatorial Pacific Ocean and its implications for climate change
Climate models have consistently predicted that as greenhouse gas emissions rise, ocean waters will warm, and this has proven to be mostly correct. However, the opposite has been happening for the past 30 years in an area of the east Pacific Ocean extending west from the coast of Ecuador for thousands of kilometres (Figure 1).
Every three to five years, the Pacific reverses from La Niña conditions, where surface water temperatures in its equatorial region are relatively cool, to El Niño conditions, where these waters become warmer than average. This cycle, known as the El Niño-Southern Oscillation, or ENSO, is caused by changes in winds blowing across the ocean and movement of water between the cooler depths and the warmer surface.
Further, there is the Pacific Decadal Oscillation, whose exact cause is unclear, but whose effects mirror those of ENSO, although usually over a timescale of between 20 and 30 years. These changes make it hard to identify long-term trends, so when investigators first identified the cooling eastern Pacific in the 1990s, they attributed it to natural variability. However, in 1997 Cane et al. (1997) demonstrated that the equatorial Pacific was cooling, a trend not seen in climate models. Since then, data on sea surface temperatures has confirmed this view. The eastern Pacific (near the Americas) has always been cooler on average than the western part of the ocean (near Asia) by 5°C or 6°C, but between 1980 and 2019, this temperature difference widened by about 0.5°C (Wills et al., 2022). The observations of Cane et al. (1997) have been confirmed recently by those investigators (Seager et al., 2019).
Although ENSO’s effects are well known, the phenomenon itself is difficult to predict, which is making multi-decade projections of climate difficult to forecast. Many aspects of human-caused climate change are playing out as predicted for several decades, including overall warming of the global atmosphere and oceans as well as the intensification of rainfall extremes and the drying of many subtropical areas. However, they have been less successful in predicting ENSO. Global climate models have predicted for more than 20 years that the tropical Pacific would gradually shift toward an El Niño-like state, with the surface waters warming more rapidly toward the east than toward the west.
Possible causes
Some investigators think that the cause of cooling of the eastern equatorial Pacific could lie in the cold seas of the Southern Ocean around Antarctica. These waters, as in the eastern Pacific, are among the few places where sea surface temperatures have declined in recent decades. Dong et al. (2023) have shown that it is unlikely that the Southern Annular Mode is the main cause of the observed cooling of the Southern Ocean. One likely driver is the melting of Antarctic glaciers as global temperatures rise. Another possibility is that ozone depletion and rising greenhouse gas emissions are strengthening winds in the region, intensifying the movement of cold air from Antarctica to the surface waters of the Southern Ocean. Whatever the cause, it may be having an effect in the tropical Pacific. Climate models do not always include Antarctic meltwater in their calculations and have trouble correctly reflecting changes in sea temperatures, winds, and currents in the Southern Ocean. However, Dong et al. (2022) have shown that when Antarctic meltwater is included in the calculations, climate models start to show cooling in the tropical Pacific and that therefore near-future warming projections by current global climate models may be overestimated.
Another possible explanation for cooling of the equatorial eastern Pacific is that climate change is altering winds and ocean upwelling of colder water from the deep ocean rising to the surface in the Pacific. As described above, the western Pacific is naturally warmer than the east, due to upwelling of cold water in the eastern Pacific, which keeps surface temperatures cooler there and the accumulation of warm surface waters in the west being advected by consistent trade winds (east to west). As climate change advances, this is amplified, so that the prevailing trade winds strengthen. This amplifies the upwelling of cool water in the eastern Pacific, promoting further regional cooling, but that effect is seldom accounted for in climate modeling. It is possible that a combination of these two mechanisms, together with natural variation, is causing the cooling of the eastern equatorial Pacific.
Implications for the climate’s future
This observation has significant implications for the climate’s future, because without knowing why this cooling is happening, it is difficult to know whether or when it might stop, making climate predictions uncertain. Warming waters in the western Pacific and cooling waters in the east lead to more low-lying cloud over vast stretches of the eastern Pacific, which reflect more sunlight, reducing the heat entering Earth’s atmosphere and becoming trapped as a result of greenhouse gases, slowing the rate of global warming. If the current trend continues, the cold ocean region could reduce the level of projected global warming by 30% compared with the predictions from climate models. However, it would mean that the base state of the climate would be more similar to La Niña, increasing the risk of droughts in the Horn of Africa and the United States Southwest, while increasing the risk of extreme rain events and floods in Australia.
On the other hand, if climate models are correct and the eastern Pacific warms, the rate of global warming will be higher and there will be different regional impacts. The base state of the climate will be more like El Niño, increasing coral reef bleaching, making the Amazon hotter and drier, bringing more droughts to Australia and Indonesia and leading to fatal heatwaves in India as monsoons fail. Meanwhile, parts of the Americas will experience more storms, with heavy rains leading to devastating floods and landslides in countries such as Peru and Ecuador.
Which future should we prepare for? If the cooling in the eastern Pacific persists, it will have large implications for regional climate predictions. At present, policymaking and planning for climate change is based on climate change forecasts derived from models that assume a warming eastern Pacific, which may be erroneous. More accurate climate models that are better at simulating cloud cover, ocean currents, winds and melting Antarctic glaciers are needed.