The Tropics and the Furious Fifties

There’s an old sailor’s expression: “Below 40 degrees south there is no law, below 50 degrees south there is no God.” Since the sixteenth century, sailors have spoken in awe of the violent westerly winds and seas they experienced fighting their way across the Southern Ocean. With few landmasses to slow them down, the winds found across 40 degrees latitude often reach speeds of twenty-five knots—about 40 percent stronger than their northern hemisphere counterparts—earning them the title the “roaring forties.” As shipping pushed farther south, explorers realized that these winds form part of a vast storm belt that includes the “furious fifties” and “screaming sixties,” names more reminiscent of terrible rock bands than a major part of our planet’s circulation system. The early hunters and traders didn’t understand it at the time, but these winds are created by a procession of low-pressure systems carried east by the jet stream, a river of cold air hurtling and twisting round the Antarctic at 10,000 metres. Importantly, something quite profound appears to have been happening in recent decades: The winds seem to be getting even stronger and moving south.

Trying to get a handle on what’s happening in the Southern Ocean, however, is easier said than done. Because the region is notoriously wild, it’s sparse in scientific data. Most of the records we have today come from satellite observations and sporadic records taken by ships as they hastily beat a path to safer latitudes. Fortunately, scattered across the Southern Ocean are a number of tiny pinpricks of land, the so-called subantarctic islands, many of which are home to weather stations that have been taking careful observations since the mid-twentieth century. In the southwest Pacific, the New Zealand subantarctics straddle 48 to 53 degrees south, and lying right under the path of the winds are precious sanctuaries for wildlife, many of whom are suffering a major decline in numbers in recent decades, such as the elephant seals (seen here) and the Rockhopper penguins. Covered in vegetation these islands offer the possibility of finding centuries-old plant and animal remains that preserve a record of the changing impact of the roaring forties and furious fifties.

Elephant seal on the subantarctic islands

On the Australasian Antarctic Expedition 2013-2014, we undertook extensive work on the New Zealand subantarctic islands, with a particular focus on the Auckland and Campbell islands. These islands are home to patches of small native trees called Dracophyllum, a shrub that can grow up to 5 metres high. Their importance for understanding the changing weather is thanks to the genius of Leonardo da Vinci. In the fifteenth century, the Italian polymath realized there was a link between the thickness of tree rings and the growing conditions. Thick rings, he reasoned, must have been when it was a good season for tree growth; narrow rings, terrible. Although we have more sophisticated methods than those available to da Vinci, the principle is still the same. By measuring the ring thickness, we can get a handle on changing climate. Dracophyllum is one of nature’s weather stations, putting down a ring of growth each year.

In a paper recently published in the journal Climate of the Past called ‘Tropical forcing of increased Southern Ocean climate variability revealed by a 140-year subantarctic temperature reconstruction’, we report the changes recorded by the Dracophyllum trees and compare these to climate models. The trees are truly a remarkable natural weather station, more than doubling the length of the observational record kept carefully by scientists working on the islands. But the big discovery is that since the mid-twentieth century, the trees reveal the climate in the southwest Pacific has become increasingly more extreme, apparently caused by changing winds in the Furious Fifties. Sometimes the wind has weakened, allowing warm airmasses to pass over the islands; at other times, the westerly winds have roared, bringing cold southerly blasts.

When we drilled down further into what was happening, we found the pattern of alternating warm and cold winds matched the climate changes seen in the tropical Pacific, a phenomenon known as the El Niño-Southern Oscillation. ENSO, as its commonly abbreviated, describes changes in tropical ocean temperatures every 2 to 8 years that can have a major effect on the global atmospheric circulation system. The remarkable thing is as tropical Pacific Ocean waters have warmed from the mid-twentieth century, the global climate system seems to be changing too. And nowhere is this more apparent than over the New Zealand subantarctic islands. And as the climate swings from one extreme to another it looks like these changes might be hammering the mammal and sea bird populations that call this part of the world home. Why is the big question? One possibility is the increasingly variable climate is impacting food sources in the region; something we’re hoping to test in future work.

You can download our paper ‘Tropical forcing of increased Southern Ocean climate variability revealed by a 140-year subantarctic temperature reconstruction’ from Climate of the Past for free at http://www.clim-past.net/13/231/2017/. Hope you enjoy it.

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