It is said that the thermohaline (temperature, salinity) stratification in the tropical Atlantic ocean reflects the winter surface water characteristics from higher latitudes. The reason: dense surface water formed during the winter slips into the deep ocean along approximately surfaces of equal density, eventually reaching the subsurface layers of the tropical Atlantic. Of course, mixing en route attenuates the surface water characteristics a bit, so a perfect correspondence of stratification to surface water is not expected, but lets see if you can match features in the tropical Atlantic water column to likely surface water sources.
End of winter (March in northern hemisphere; September in southern hemisphere) surface (20 meter) water temperature and salinity as well as water column stratification at 10°S and 25°W are provided as excel files: see the 5 sheets of excel workbook "surf_wc_lab".
The temperature of the water column is given in potential temperature, we will discuss this in the lectures, but for now: potential temperature is the temperature of a parcel of water carried adiabatically from its in situ depth to the sea surface. Potential temperature is a few tenths of a degree cooler than the in situ temperature. Use of potential temperature makes it a little easier to relate water at depth to its surface origin.
The objective of this lab is to try to match winter surface water temperature and salinity with subsurface temperature and salinity in the tropical South Atlantic. A good match is an indicator that the surface water acts as a source for the tropical subsurface water. As surface water subducted into the deep interior of the ocean likes to spread on surfaces of equal density (isopycnals) you may use this as a way to guide you to the best fit (that is: follow the isopycnals on the T/S diagram to determine the closest fit between subsurface and surface water). Caution: as the surface data files represent regional and temporal averages, they may not fully capture the densest surface water formed in the area, so the fit with the tropical Atlantic would be only approximate.
The purpose of the hands on experiment is for you to figure out how to drive ocean circulation. You will also determine how water masses are formed and how currents develop. This will give you insights into the nature of ocean circulation. This lab exercise differs from previous labs because you will decide which materials and methods to use.