ANDREX

The Earth's climate is changing, as it has done in the past. One of the great challenges faced by scientists today is to understand the causes and consequences of these changes. The way many scientists seek this understanding is by running computer-based climate simulations that mimic the complex interactions between the ocean, atmosphere, ice and living beings that are thought to be responsible for driving climate change. It is partly thanks to these simulations that we now know that ocean circulation plays a key role in modulating climate. One of the most important elements of ocean circulation is what scientists know as the 'meridional overturning circulation' (MOC). This term describes the cooling and resulting sinking of surface water masses in high-latitude regions, their journey through the deep ocean and their eventual warming and return to the surface, after many decades or centuries. The MOC is important to climate because the water masses involved in this long circuit through the ocean carry with them heat, carbon and other significant subtances such as plant nutrients, which in this way are distributed around the planet and locked away in the deep ocean for long periods of time. One of the stages of the MOC that puzzles scientists the most, and one of the most uncertain processes in climate simulations, is the formation near the Antarctic continent of Antarctic Bottom Water (AABW). AABW is the water mass that fills the deepest layers of the MOC, flushing the global ocean abyss and sequestering carbon and nutrients in the deep ocean. Yet in spite of its key place in the MOC and climate, AABW is surrounded by many basic questions. This is because AABW is formed in remote regions that are only rarely visited by oceanographic ships. As a result, we currently know little about how much AABW is formed, how it is exported from the Antarctic seas to the rest of the world ocean, and what quantity of carbon and nutrients is carried by AABW into the global ocean abyss. In order to begin answering these questions, we plan to conduct an experiment in which we will measure the rate of AABW production and export, and the associated transports of carbon and nutrients, in the Weddell gyre. This is an oval-shaped current that occupies the southern rim of the South Atlantic and Southwest Indian oceans, and is believed to be the main region in which AABW is formed. We will do this by (1) measuring the distributions of temperature, salinity, current velocity, dissolved gases, nutrients and carbon along the northern rim of the Weddell gyre; and (2) teaming up with two groups of American and German scientists that will make similar measurements along the eastern rim of the gyre and across the gyre's southwestern corner, respectively. We will use this unprecedented richness of observations to construct a budget of the water masses, carbon and nutrients entering and leaving the Weddell gyre. In this way, we will be able to answer key questions such as 'How much AABW is formed in the Weddell gyre?' and 'What quantity of carbon and nutrients is locked away by this AABW into the global ocean abyss?'. Our answers to these questions will serve as a benchmark to evaluate the skill of state-of-the-art climate and ocean models, and to identify future climate change.