Basins are regions of enclosed water semi-isolated from the open oceans.

• Estuarine basins: where riverine input flows out to fill an estuaries before connecting with the sea. Characterized by a gain in freshwater (buoyancy) where fresh water flows out at the surface and saltier water flows in at depth.

• Evaporative basins: where the basin separated from the ocean by a sill/strait. Characterized by a loss of freshwater, or buoyancy, where freshwater flows in at the surface, and saltier water flows out at depth.

By considering the mass & salt budgets over the basin, we can predict the size of exchange flow at the sill/strait or mouth.

1. Estuarine basin

where:

• R is the riverine input;
  
• Q_U and Q_L are the transport of mass at the upper (U) and lower (L) layers;
• S_U and S_L are the salinity of the upper and lower layers.

For a steady circulation, we have that:

Solving the system for Q_U and Q_L, we obtain:




The relations for Q_U and Q_L, known as Knudsen relations, allow us to draw some conclusions about the 2-layer circulation:

• For (S_L - S_U) > 0, the lower layer flow has opposite sign to the upper layer;
• Mixing reduces (S_L - S_U) and increases (Q_L + Q_U) along the estuary. By measuring salinity profiles in different coordinates along the estuarine channel we can quantify the mixing that occurs in the zone between the profiles;
• Near the ocean (S_L - S_U) will be smaller and hence the fluxes will be larger;
• The along-estuary difference in the inflowing water, Q_L, must equal the amount of water mixed vertically into the upper layer.
  

2. Evaporative basin



Here, there is a net evaporation E_net equal to difference between the loss of freshwater due to evaporation (E) and the input by land runoff (R) and precipitation (P):





Analogously to the estuarine basin case, we have:




Solving for Q_U and Q_L, we obtain:




Now, let’s use the Knudsen relations to estimate the Q_L and Q_U exchanged between the Atlantic ocean and the Mediterranean sea across the Gibraltar Strait, given E_net, S_L and S_U:

• Net evaporation (E_net ) = 0.55 x 10⁵ m³s^-1~ 0.05 Sv;
• Upper layer salinity (S_U) = 36.15;
• Lower layer salinity (S_L) = 37.90.

Note that, because (SL - SU) is small, the inflow/outflow transports are 20 times larger than the net loss of mass due to evaporation on the Mediterranean basin.


Practical problems on for this estimation:

• Estimating the net evaporation;
• Deciding on where/how to measure S_U, S_L;
• Sensitivity of fluxes to the accuracy of E_net , S_U + S_L.

3. Physical differences between estuarine and evaporative basins:

• Estuarine basins (e.g. Baltic sea, Black sea): Stagnant deep water. Deep water is anoxic because it is not ventilated. A thin surface layer of freshwater separated from the stagnant saltier deep water by a sharp halocline;

• Evaporative basins (e.g. Mediterranean sea, Red sea): Vigorous deep water circulation driven by buoyancy loss from net
  evaporation at the surface. Waters sink or connect to form deep water.

Last modified: Sep 2014