East of New Zealand, an unusually warm patch of water the size of Australia has caused a decade-long drought in Chile. Dr Kyle Clem looks at what could happen as the climate continues to rise.
Comment: Climate change is affecting weather patterns and millions of people far downstream from where the areas of greatest oceanic and atmospheric heating are happening.
The oceans have absorbed about 90 percent of the heat gained by our planet from increasing greenhouse gases. As a result, there has been a global average increase in sea surface temperature (SST) of around 0.5°C during the past 40 years (1979-2018), though much of this added heat is being stored at depth below the ocean surface. The warming, however, is not evenly distributed across the world’s oceans.
In our study, published today in the Journal of Climate, we have identified a “blob” of intensely warming SSTs off the east coast of New Zealand, in the subtropical southwest Pacific Ocean, that has warmed by 1.5°C during the past 40 years in the winter season of May to September. That’s warming that is about three times faster than the global average rate.
This “Southern Blob” of extreme warming is about the size of Australia and reaches depths of about 100m. Incredibly, despite covering only about 1 percent of the total global ocean surface, another recent study suggests this blob accounts for about one-quarter of the total global ocean heat absorption in recent decades.
The development of the “blob” is linked to a recent multi-decadal reduction in convective rainfall over the central tropical Pacific. This triggers an atmospheric circulation pattern across the South Pacific which favours the transport of warm water into the blob.
Climate-model simulations without anthropogenic greenhouse gases can reproduce an SST structure similar to the current pattern. However, they show the extreme rate at which this patch has been warming over the past 40 years far exceeds any rate of 40-year warming that could occur under natural variability alone.
Therefore, while the initiation of the blob is likely of natural origin, anthropogenic increases in greenhouse gases have contributed to the remarkable rate of warming and heat uptake observed in recent decades.
The blob has had a significant influence on the Southern Hemisphere’s climate. Using observational analyses and climate model experiments designed to isolate its direct effect on the atmosphere, we show the blob builds a strong ridge of high pressure across the sub-tropical South Pacific, extending downstream from New Zealand all the way to South America.
This high-pressure belt has shifted winter storm systems south towards Antarctica, blocking them from reaching the subtropical west coast of South America, a region that relies on winter storm systems to replenish freshwater sources before the summer dry season, similar to other Mediterranean climates around the world.
This poleward shift in the track of winter storms has caused a decade-long, uninterrupted sequence of drought years across central Chile and adjacent portions of the Andes Mountains and Argentina. This has been called the “Central Chile Megadrought”, given its unprecedented longevity.
With an average 30 percent rainfall deficit, although some years have had rainfall 70 percent below normal, the megadrought has diminished freshwater supplies across central Chile, affecting drinking water in rural communities, hydro-electric generation, agriculture and many other activities in a highly populated area that includes the capital city of Santiago, which has seven million people.
Given humankind’s contribution to the blob’s remarkable characteristics, it is unclear when or if it will dissipate and the megadrought in central Chile will break. Current rainfall observations show this year will almost certainly be yet another dry year in the region.
Our study highlights that, in climate change, what happens in one place does not necessarily stay in that place.
The Southern Blob, though likely natural in its formation, has reached extreme levels of warming due to increasing greenhouse gases, with cascading effects on the climate system of the Southern Hemisphere affecting millions of people seemingly far-removed geographically from the source.
