Phil Richardson Interview

Q. A lot of the material I've been reading about the southern tip of Africa describes how the Agulhas rings introduce warm salty water from the Indian Ocean into the cooler and less salty Atlantic and its influence on the overturning circulation. How significant an input is this, say compared to water from the Mediterranean?

A. Models have shown that it is significant. The Med increases the heat and salt at around 1000 m depth and below which increases the salinity of the deep water flowing southward. I think the leakage salinity is important because when the water eventually reaches the northern North Atlantic the salinity is important for creating deep mixing and the formation of deep water. If the near surface water were fresher (say from melting of glaciers and heavy rains or from fresher MOC) then the deep water formation could be shut down or at least reduced. If the near surface water were salty then the deep water formation could be enhanced in that when cooled the salt results in dense water and therefore deep water--given lots of wintertime cooling. Of course there are ocean-atmosphere feedbacks and this subject is very complicated, but I would say that having more salt flux from the Indian Ocean does affect the MOC and deep water formation, but it is not simple and is being researched especially in light of climate warming.

Q. Is it a major source of salt in the Atlantic? Does this saltier water eventually get channeled up through the Gulf Stream, or does it get mixed into the wider ocean?

A. The warm and salty leakage water disperses into the South Atlantic especially in the very vigorous mixing and stirring environment in the Cape Basin (off Cape Town) where energetic Agulhas current rings and cyclones interact and exchange water. Some rings translate across the mid-Atlantic Ridge carrying Agulhas water a long way. The blended mixture of waters in the Cape Basin is also carried by currents across the mid-Atlantic Ridge. Near the western boundary of the South Atlantic the flow divides, part (~15 Sv) going north toward the equator and eventually into the Caribbean and Gulf Stream and part (~15 Sv) recirculates southward in the Brazil Current. So, yes the leakage does get channeled up through the Gulf Stream but often in complicated paths--western boundary currents, eddies, upwellings, etc. And it can get sidetracked for various amounts of time going around the South Atlantic subtropical gyre and back into the Benguela Current again.

Q. Without this input, would the overturning circulation be effected?

A. People have modelled the ocean circulation with and without the leakage and they find that the leakage does have a strong effect on the MOC. Of course the whole circulation is very complicated and models need to be verified. One way to think of it is that given a MOC, you can get water from the Drake Passage or from the Agulhas leakage. Drake passage water is cooler and fresher than Agulhas water. So, more Agulhas water means more heat and salt flowing through the Atlantic, Drake Passage water means less heat and salt. As the leakage goes north, it looses some heat in Cape Basin during winter due to cold weather, but then it gains heat on its way to the equator and near the equator. So as the MOC goes northward it increases in heat and the heat flux is northward (warm water going north, cold water going south underneath)throughout the Atlantic. And there is more evaporation than precipitation (E-P) in the subtropics so salt increases as the MOC goes north, and part of the warm salty water subducts under the surface and flows equatorward. Near the equator there is lots of upwelling and the cooler upwelled water warms up due to strong heating there. This water is carried north the the North Brazil Current and its current rings toward the Caribbean.

Q. I'm a little confused about why some of the water off the coast of Brazil gets subducted and heads north. Is it due to some properties of the water, or is it because a lot of currents are coming together, and the water has nowhere else to go.

A. The wind stress in the subtropics, both north and south Atlantic subtropics, generates a near surface convergence of water and near the equator divergence. Its a pull push question. Water is subducted because of the convergence and because of buoyancy forcing and it upwells near the equator because of the strong divergence there. So water from the South Atlantic is partly subducted and partly it flows from the Cape Basin and is modified as it goes by stirring and mixing. Most of the MOC flows northward along the coast of South America 5S-10S as a swift western boudary undercurrent--under a near surface layer heading south and driven by local winds--the Ekman flow. Maximum northward currents are located at a depth around 200-300 m but northward flow extends down to around 1000 m. Near the equator a lot of the 200-300 m water upwells and eventually continues north (after going eastward and westward in the equatorial jets) in the North Brazil Current which is surface intensified.  The deeper water--intermediate water--also continues northward along the boundary. The NBC breaks up into rings which carry over half of the water up the coast. (The MOC is driven in part by density of the water--the thermohaline circulation--and in part by the wind. They both are important but the ratios differ in different places.)