Miami missions

The Rosenstiel School for Marine and Atmospheric Science in the foreground, with the Miami skyline behind. Fly fishing with fellow students Jerry Lin (left) and Mark Portman (right) in the Everglades

By Katherine Hutchinson, PhD Student, SAEON Egagasini Node

The Rosenstiel School for Marine and Atmospheric Science is situated on an island just offshore of Miami, linked to the mainland by a causeway. It has exquisite views over the ocean, and its own private beach.

Surprisingly though, this was not my motivation to visit the institute, and I didn't know how stunning the campus was until I arrived there. Professor Lisa Beal of RSMAS was the main attraction, as she is possibly one of the most knowledgeable people on the Agulhas Current, which happens to be the focus of my PhD.

I am a Professional Development Programme^* (PDP) student with Dr Hermes at SAEON’s Egagasini Node working as part of the ASCA team. My study is co-supervised by Prof. Ansorge at UCT and by Prof. Beal, who led the Agulhas Current Time-series experiment (ACT, which has now been extended into the ASCA array).

In this ground-breaking study, seven full-depth current meter moorings along with four current pressure inverted echo sounders were placed across the current to follow the trajectory of the descending TOPEX/Jason ground track that leaves the South African coastline at 33.4^°S and stretches out to sea approximately perpendicular to the continental slope. The mooring data spans the period 2010-2013, thereby providing 34 months of velocity and transport measurements at an unprecedented resolution.

Valuable dataset

This data is extraordinarily valuable as it can provide insight into the variability of a current which is thought to play a vital role in the meridional overturning circulation, a system of surface and deep currents encompassing all ocean basins. It transports large amounts of water, heat, salt, carbon, nutrients and other substances around the globe, and connects the surface, ocean and atmosphere with the huge reservoir of the deep sea.

By coupling the mooring data with the overlaid satellite altimetry measurements, Prof. Beal's team at RSMAS were able to extend the transport data back in time using a proxy, thereby producing 20 years of transport estimates for the Agulhas Current from 1993-2013. This dataset will be the foundation of my thesis and was the motivation to work at RSMAS for the very first two months of my PhD.

Initially I was apprehensive about spending an extended period of time in Miami as my impression was that the city was all about glitz, glam and superficiality. Never before have I been proven so wrong! The people I met and the places I visited were truly impressive, from the natural beauty of the Everglades and Florida Keys to the mind-blowing creativity of the hipster art district, Wynwood. The impressive sights were complemented by the delicious Cuban food and Latino flair.

A meeting of bright scientific minds

However, my favourite part of the trip was, surprisingly, not the sightseeing and the tasty food, but the Wednesday morning group meetings with Prof. Beal's research team. This group of extraordinarily bright minds meets once a week to discuss a paper, present their latest research results, or simply brainstorm ideas or challenges for the road ahead.

Being given the opportunity to participate and absorb the ideas flying around the room once a week was an incredible opportunity and education. I have come to realise that being a scientist is not something you can learn by just reading academic journals or processing data, but is better achieved by exercising your curiosity and approaching all scientific statements and findings with a critical mind. “How did they get this result? What processing was undertaken? Why is this different to previous literature? How do we replicate the methodology?” From data analysis techniques, the formation of robust scientific key questions, and the art of finding a signal amidst all the noise, I received a whirlwind education on how to be a scientist.

Research topic

During my time at RSMAS I came up with a very exciting topic for my PhD - how local and remote winds affect Agulhas Current Transport variability.

Figure 1 shows the mean wind speeds for the Indian Ocean from 1993 to 2015 and the position of the ACT /ASCA mooring array. As can be seen from the image, there are two patches of very high wind speeds, one centred around 15S known as the Trade winds, and another south of 50S called the Westerlies. These two maximums in wind speed, and thus wind stress, create a positive wind stress curl between them which, in turn, creates a net northward transport across the basin. This is known as the Sverdrup transport as it is the ocean current pattern produced by the wind induced (Ekman) movement of water.

This northward transport must be balanced by a flow out of the basin - a task that is largely undertaken by the Agulhas Current. The Agulhas is the western boundary flow of the South Indian subtropical gyre and dominates what may be the highest meridional heat flux in the world’s oceans. The leakage of waters from the Agulhas into the South Atlantic is a critical link in the global thermohaline circulation, feeding warm and salty waters into the upper limb of the global overturning circulation, and therefore playing a vital role in the climate system.

Figure 1: Mean wind speed (m/s) from 1993-2015 over the Indian Ocean with vectors showing direction overlaid. The position of the ACT/ASCA mooring array off the east coast of South Africa is shown in black.

Regionally, the Agulhas Current exerts a strong control on rainfall and climate over South Africa, acting as a major source of latent heat for onshore wind systems. Furthermore, the current is also of fishing (and thus economic) importance to South Africa, as upwelling and high levels of productivity are induced when it separates from the shelf during a periodic meander event.

Wind-driven dynamics have been shown to have a critical influence on the variability of western boundary currents elsewhere, but this relationship has yet to be addressed in the Agulhas Current. Decadal trends of surface wind stress have indicated an increase in both the Trade and Westerly winds over the Indian Ocean basin.

A variation in the winds across the Indian Ocean basin would result in a modification in the flow of the Agulhas. An alteration in strength of the Agulhas would have a variety of implications, ranging from local effects on the climate of the east coast of South Africa, an adjustment of upwelling affecting fisheries, and on a global scale, an alteration of the volume flux of warm salty water from the Indian to the Atlantic Ocean.

My PhD will endeavour to gain insight into this and shed some light into what has been happening with winds and western boundary current responses in the Indian Ocean over the past 20 years.

My two-month trip in Miami was the perfect kick start to my PhD. I return home to Cape Town with a topic that I am very passionate about and a strong drive to understand and learn more. Even though it was a reasonably short period of time, it was jam-packed with experiences and lessons.

^* The Professional Development Programme of the Department of Science and Technology and the National Research Foundation aims to accelerate the development of scientists and research professionals in key research areas.