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Towards sustained ocean observations for the next decade
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My PhD aims to investigate the Agulhas Current transport variability with respect to local and remote winds.
This research is considered important due to the vital role the Agulhas plays in transporting heat and salt from the Indian to the Atlantic Ocean, acting as a key link in the global ocean conveyor belt.
Ocean models predict a seasonal cycle of the Agulhas Current where the flow is strongest in winter months; however, observations from the Agulhas Current time series have shown that the current is in fact strongest in summer.
The question is - what is driving this opposite seasonal cycle to that predicted? A strong contender is the wind. My project is designed to identify this link and the details of the connection.
I started my PhD in January 2015 and after nine months of working on the data, my supervisors and I decided to plan a visit to Europe where I would spend one month networking and collaborating, broadening my horizons and absorbing new ideas. The first half of my trip was spent in Ireland at a conference, and the second leg in France working with a well-respected scientist to develop a shallow water model.
I was overjoyed when my abstract was accepted for presentation at the GAIC "Sustained Ocean Observation for the next decade" 2015 conference in Galway, Ireland. This meeting was hosted by three major ocean observing organisations: Go-Ship (ship-based observations), Argo (autonomous floats) and IOCCP (carbon measurements).
An ocean monitoring strategy for the next decade
The reason I was so excited, is that this was the first time that these three major ocean observing groups all met in one place to strategise together how to better plan and coordinate an ocean monitoring strategy for the next decade. Very important issues were raised, such as the fact that even though we are doing a very good job in increasing the number of platforms that measure the changes occurring in the seas, perhaps we have failed in being strategic in the placement of some of our measurements.
Some criticisms were made that yearly full-depth Conductivity-Temperature-Depth (CTD) rosette deployments in the middle of an ocean gyre are unnecessary. Instead, we should better use our time and capital by measuring the boundary currents and important areas of the Arctic and Antarctic Oceans where we are noticing key changes.
As the youngest member of this conference, I tried to absorb all the information like a sponge, and make as many academic acquaintances as possible. My presentation was very well received and I managed to convince the Go-Ship committee to consider the repeat ocean monitoring lines that South Africa is responsible for under their badge as “associated Go-Ship”.
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Up until now the three lines occupied by the SA Agulhas (Good Hope to Antarctica, the crossing of the Agulhas retroflection on the way back from the Prince Edward Islands, and the SAMBA line halfway across the South Atlantic) were not official Go-Ship accredited lines as they are either not coast to coast or perform full-depth CTDs. However, after some discussion and my motivation that even though these lines do not fill all Go-Ship criteria they are sampling areas of vital water mass exchanges and transformations, the Go-Ship committee agreed to include our lines under their badge.
It was a wonderful experience being part of this monumental planning meeting, and as the official representative of South Africa, I was honoured to represent my country.
The next phase of my journey brought me to Brittany in France to collaborate with Dr Pierrick Penven, a ROMS expert. The Regional Ocean Modelling System (ROMS) is a free-surface, terrain-following, primitive equations ocean model widely used by the scientific community.
Even though my PhD thesis is not designed as a modelling project, we have come to the realisation that it is necessary to use a simple idealised version of a model to obtain a better understanding of the relationship between the wind-driven transport integrated over the Indian Ocean and the Agulhas Current flow along the western boundary of the basin (South Africa’s east coast).
Learning to develop a shallow water model
Upon arrival at the Ifremer Marine Institute in Brest, where Dr Penven is located, I was immediately thrown into the deep end deriving the shallow water equations from scratch. While this was daunting at first, it gave me an in-depth understanding of the fundamental dynamics of the system.
From this strong physics grounding I was able to start using a simple one layer barotropic model and understand the basic response of the ocean to wind forcing. From there things got increasingly complicated, but thanks to Dr Penven’s patience and mentorship I was able to keep up, and eventually we ended up with a working one and a half layer reduced gravity shallow water model!
I forced this model with various climatologies and inter-annual winds and the results have been fascinating. As I start to filter through the model output, I am noticing things that would not have been evident from observations alone. I have come to realise the necessity of using models in tandem with observations to obtain a better understanding of the mechanisms driving the variability in the ocean-atmosphere system.
My one-month trip to Europe was very intense but extremely productive. Instead of feeling exhausted after receiving all these new ideas and information, I feel energised and excited to tackle the second year of my PhD.
* Katherine is a Professional Development Programme (PDP) student with Dr Juliet Hermes at SAEON’s Egagasini Node, working as part of the ASCA team. 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.
