Prof. Dr. Monika Rhein

Physical Oceanography

Scientific interests:
Physical oceanography, ventilation and circulation of the ocean; temporal and regional variability of these processes

Homepage:
http://www.ocean.uni-bremen.de/index.html

Email:
mrhein@physik.uni-bremen.de

Ocean and Climate

The ocean plays an important role in the global distribution of solar heat. About half of the meridional polar heat transport occurs in the ocean, the other half in the atmosphere. The ocean transports heat by mainly supplying warm surface water from the Pacific to the northern North Atlantic, where it cools and – owing to its greater density – sinks and flows as cold deep water into the South Atlantic and around Antarctica into the Indian and Pacific Oceans. The formation of deep water in the northern North Atlantic (Labrador Sea and north of Iceland) thus has a key function for the so-called `thermohaline circulation' and the capability of the ocean to transport heat and influence the climate. In these regions, the upper ocean and the atmosphere react with the deep sea, and changes in climate might influence the intensity of the deep water formation. Furthermore, trace gases like CO 2 and
chlorofluorocarbons (CFCs) enter the deep ocean in these regions.

Methods

To study the ventilation and circulation of the deep ocean, a tracer is needed whose distribution is mainly governed by advection and turbulent diffusion. Furthermore, a means to obtain temporal information about the above mentioned processes is required. Since the early 1980s the CFC components CFC-11 and CFC-12 have been employed as the most suitable tracers. Their origin is completely anthropogenic, and their input into the atmosphere has been well known from the beginning of their production (1930). Except for their destruction in the stratosphere (ozone hole) the CFCs in troposphere and ocean behave like noble gases. The necessary temporal information is contained in the CFC-11/CFC-12 ratios and/or the temporal increase of the atmospheric CFC concentrations. Newly formed deep water is loaded each year with a higher CFC concentration, and the inventory of this tracer in the deep ocean increases. The CFCs have a further advantage in that they can be analysed directly after sampling, on board the research vessel, by a gas-chromatographic technique which detects them efficiently in very small amounts (femtomoles) by an electron capture detector. I have used this method since 1989. We also determine the density stratification of the ocean by measuring vertical profiles of Conductivity, Temperature and pressure (Depth) with a so-called CTD probe. Besides other things these data allow us to calculate the relative horizontal pressure gradient. Geostrophic calculations then yield the mean relative velocity profile between two locations and hence the volume transport. Direct velocity measurements are carried out with Acoustic Doppler Current Profilers (ADCP), which are either lowered from the ship to the ocean bottom together with the CTD probe, or are vessel mounted.

Questions

The combined tracer, CTD and velocity data are used to estimate the intensity and variability of the formation of deep water in the Greenland Sea, the Labrador Sea and the Western Mediterranean, to study the role of turbulent vertical diffusivity for the renewal of deep water, to infer the spreading of deep water masses and their recirculation in the northern North Atlantic and the subtropical/tropical Atlantic. With data from moored instruments, we estimate the long-term variability of the basin wide thermohaline circulation.

Most of our data were and are obtained as part of international (WOCE, CLIVAR, MAST) and national programmes. The interpretation of the data is supported by model results in co-operation with the respective participating groups.

In the future, we will deploy moored instruments to obtain time series (basin wide) of velocity, tracer and density fields. A helium mass spectrometer for the analysis of water samples is available. In addition we will provide a facility to analyse the noble gas concentrations of rocks in the near future.

Fig. 1 Distribution of CFC-11 in deep water (ca. 600 m – 1800 m), 1997. The deep water is formed in the Labrador Sea (dark red spot) and spreads northeast into the Irminger Sea, towards the east into the eastern Atlantic and south along the continental slope.

Fig. 2 Meridional velocity component from acoustic measurements along 16° N, western Atlantic. red: northward, blue: southward