By Dr Pauhla McGrane & Prof Karen Wiltshire |
Life on-board the RV Polarstern continues to revolve around the daily ocean sampling, data crunching and mealtime routines as we continue to transit south on our Floating University. We passed the 20° South parallel and are steering a south-easterly course for Cape Town, South Africa, escorted by the usual shoals of flying fish.
Crossing the Atlantic from North to South has allowed us to witness, first hand, exceptional ocean phenomena, from Saharan iron-rich dust clouds seeding the oceans to diverging equatorial waters and gyrating eddies which can slow a ship in its track.
In order to investigate the currents and water masses in the region, each morning our oceanographic team deploys an expendable bathythermograph or XBT probe to measure the thermal structure of the upper 2000 metres of the water column. Water masses are bodies of water with physical and chemical values characteristic of being formed in a particular area of the ocean. These bodies can spread thousands of miles at depth, mixing with other water masses at their outer edges, but retaining their characteristic temperature and salinity signatures. This oceanic ‘fingerprint’ allows us to build a picture of the origin of deep water masses, their circulation around the Globe and ultimately their impact on the Earth’s climate.
Some water masses are easier to recognise then others. The warm, salty Mediterranean Outflow Water (MOW) flows through the straits of Gibraltar and into the cold North Atlantic. Due to intense heating and evaporation Mediterranean water is warm (~11°C), extremely salty (36.5) and heavier than the Atlantic surface waters and, therefore, sinks. It is still however less dense than the cold (<2°C), salty (34.9) North Atlantic Deep Water (NADW) flowing south from the Arctic and so it settles and spreads at a depth of around 1000 metres. Another major source of deep water in the Atlantic is Antarctic Intermediate Water (AIW) produced near the Polar Front where surface currents converge and sink. AIW has a temperature of and can be seen far north in the Atlantic, Pacific and Indian oceans.
As we transit further towards the Cape we encounter the Benguela upwelling system, one of the most productive marine ecosystems in the world, which extends along the west coast of South Africa from the tip of the Cape to the north at the Angola Front. Driven by the prevailing south-easterly trade wind, this upwelling of cold, nutrient rich water stimulates vast blooms of phytoplankton which in turn fuel the growth of zooplankton, fish and so on up the food chain to the larger mammals and sea birds. Due to the relatively cool temperatures and increased chlorophyll signals, upwelling systems can be easily detected from space by satellite remote sensing.
Our last CTD sent down to 5,000 metres detected signs of the Benguela upwelling system, as we had hoped.This added to the excellent data set collected so far and which our young ocean scholars will work up until arrival in Cape Town. After almost four weeks at sea we are looking forward to seeing land and our friends and families. However, we will miss working as a team to delve into the ocean´s secrets. The North South Atlantic Transect was for teachers, crew and scholars alike a unique experience. Until the next time…..
Dr Pauhla McGrane, Coordinator of the Strategic Marine Alliance for Research and Training (SMART) at Galway-Mayo Institute of Technology, Ireland
Prof Karen Wiltshire, Vice Director of Alfred Wegener Institute, Germany and Chair of Partnership for Observation of the Global Ocean