DFG Research Unit 1740: Atlantic Freshwater Cycle

The overarching theme of the research unit's work will be the analysis and understanding of the freshwater budget of the ocean, including the role, which surface freshwater fluxes (net evaporation minus precipitation plus run-off and sea ice melting) play in changing the salt content in the upper ocean.

Hereby, the research unit will focus on the Atlantic Ocean, one of the key climate relevant regions. Specifically, the questions will be addressed of how and how much the upper Atlantic Ocean contributes to the global hydrological cycle.

These questions are of fundamental and long-standing scientific concern. Moreover the subject 'water' is central to many questions dealt with under climate impact studies, including precipitation over land, the availability of water for drinking and irrigation, but also sea level rise. The research topic is therefore essential for many climate-related and socio-economic questions.

The research unit is guided by several key questions:

  • How variable is salinity in the Atlantic on sub-seasonal to decadal time scales and what role plays surface forcing (E-P-R) vs. ocean processes (e.g. lateral transports, diapycnal mixing) in modulating the salt content of the Atlantic, on basin scale and locally?

  • How does surface freshwater forcing influence ocean mixed layer dynamics in both the tropics and high latitudes of the Atlantic, how does it regulate heat exchange with the atmosphere, and how do these processes feedback on ocean-atmosphere coupling?

  • What is the role of the observed changes in surface freshwater forcing and sea ice for the regional and basin scale salinity budget and how can we improve our estimates of those processes?

  • How do varying surface fluxes of freshwater and heat generate temperature-salinity anomalies in mid-latitude central waters and how are such anomalies incorporated and transported in the low-latitude sub ducted water masses?

  • Which processes transfer freshwater from the surface to subsurface layers and drive the mixing at submesoscale?


Why now?

The opportunity to establish the Research Unit at this point in time is given by the fact that for the first time in history many new data sets are available now, that can be combined with numerical models (with and without data assimilation) and thereby enable us to finally approach the problem with sufficient observational backing - a situation that never existed before in history. 

Among those new observational capabilities are:

  1. the global Argo float system delivering about 3000 high-precision near-surface salinity measurements every 10 days,
  2. salinity measurements provided by the global surface-drifter network, moorings and other autonomous platforms,
  3. new or planned underway measurements obtained by research vessels (including German research vessels) providing high-precision surface salinity data along cross-ocean transects or during local process studies,
  4. new satellite-based measurements of surface salinity from the two new missions SMOS (ESA) and AQUARIUS (NASA), providing measurements of the surface salinity globally every 3 days with unprecedented coverage in space and time.

Jointly, the new observations will allow us for the first time in history:

  • to estimate the changing ocean salinity and transports of freshwater anomalies,

  • to document the resulting changes in the oceans circulation and transport properties as well in the CO2 uptake in the ocean,

  • to estimate ocean mixing processes and the surface freshwater fluxes (E-P, run-off, and sea ice) required to establish those changes,

  • to quantify freshwater budgets.