Whether a water parcel sinks or not depends on its temperature and salinity — both of which affect its density. This is why the ocean circulation is classified as Thermo (temperature) — Haline (salt content). Cold water is denser than warm water and salty water is denser than fresh water.
If Agulhas Leakage is large, more warm, salty water from the Indian Ocean finds its way into the South Atlantic Ocean. Via the surface arm of the THC, this water mass travels all the way up to the North Atlantic and Arctic Oceans. While the water parcels quickly lose their excess heat to the atmosphere the salt remains, making them on average denser than surrounding water. With increased density, these parcels will sink more easily in the Arctic, accelerating the MOC and warming climate. Conversely, if the Agulhas Leakage decreases, the water traveling up to the Arctic will have less salt. This decreased salinity and density inhibits the water’s ability to sink, thereby slowing the MOC and cooling the regional climate.
As described in the scope of the expedition, better understanding the variability of the Agulhas Current is a vital component of climate change science. A recent study using a global ocean model shows how an increase in Agulhas Leakage (green tracks) may have occurred between 1970 and 2002 as a result of global warming. Marked in red in the picture are virtual floats released in the leakage which find their way into the northern hemisphere via the thermohaline circulation. The salinity of these floats has increased since 1970, in agreement with some observations off Brazil (yellow box). Paleaoceanographers, who study deep marine sediments for clues about past climates, have also found evidence that changes in Agulhas Leakage affect climate. Increases in leakage coincided with the end of the last several ice ages.
This figure shows paleaoclimate data stretching over the past 560,000 years (time goes backwards in this plot!). It comes from an ocean sediment core collected underneath the Agulhas leakage in the southeast Atlantic Ocean near Africa. Ocean sediments are made up of layer-upon-layer of shells and detritus of marine creatures that have dropped to the ocean floor over millions of years. By identifying and counting these shells in the layers of the sediment core, and by analyzing their chemistry, paleaoceanographers can build up a record of past climate.
The red curve shows the amount of shells that were found from plankton that live in Agulhas waters — in other words it gives a measure (or proxy) for the strength of Agulhas leakage in the past. If the number of Agulhas leakage fauna is high, then they must have been carried by a stronger leakage and in contrast, if the number is low, the leakage must have weakened. Hence, when the red curve peaks, Agulhas leakage is at a maximum and the amount of warm, salty water transported into the Atlantic is a maximum.
Coincident with peaks in Agulhas leakage are temperature changes and ice volume changes, shown by the blue and black curves at the top of the figure. In fact, whenever leakage peaks, temperature rises, and subsequently the global ice volume decreases.
Does this signify that the Agulhas acts as some kind of trigger for climate change? Each time Agulhas leakage strengthens there is a transition from glacial to interglacial climate! If the Agulhas isn’t a trigger, it is certainly an important component of the climate system.
In terms of what we know theoretically about the controls on Agulhas leakage it makes sense that leakage would be gradually increasing today, with global climate change. This figure shows how the Agulhas leakage flows through a “gateway” between the bottom of the African continent and the Subtropical Front, which separates the Indian and Atlantic Oceans from the Southern Ocean. In the simplest terms, if the front shifts northward, the “gateway” shrinks and there is less leakage of waters from the Indian into the Atlantic Ocean. If the front shifts southward, the “gateway” is enlarged and there is more leakage.
There is evidence from both atmosphere and ocean data that the westerly winds and the Subtropical Front have shifted southward with global climate change. This makes sense — as the planet warms, the warm subtropical regions expand. Hence, we can expect Agulhas leakage to increase.
The increased leakage can feed back on climate through its influence on the Atlantic thermohaline circulation, just as the palaeodata suggest. A larger Agulhas leakage would lead to climate warming, because the additional salty waters in the Atlantic will tend to increase deep water formation there, which in turn will enhance the Atlantic THC and its heat transport. An increased heat transport to high northern latitudes can melt Arctic sea ice which, through the ice-albedo feedback further enhances warming.
There is much more to learn about the Agulhas and its role in climate. With the Agulhas Current Time-series experiment we will learn more about this important current system and its variability over the last twenty years.