Lecture Nine

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Lecture 9: Basin budgets and two layer exchange flows

Link to Lecture 9 slide show: 9_basinbudgets.pdf

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

beal labwhere:

  • R is the riverine input;
  • QU and QL are the transport of mass at the upper (U) and lower (L) layers;
  • SU and SL are the salinity of the upper and lower layers.

For a steady circulation, we have that:

Solving the system for QU and QL, we obtain:

The relations for QU and QL, known as Knudsen relations, allow us to draw some conclusions about the 2-layer circulation:
  • For (SL - SU) > 0, the lower layer flow has opposite sign to the upper layer;
  • Mixing reduces (SL - SU) and increases (QL + QU) 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 (SL - SU) will be smaller and hence the fluxes will be larger;
  • The along-estuary difference in the inflowing water, QL, must equal the amount of water mixed vertically into the upper layer.
2. Evaporative basin

Here, there is a net evaporation Enet 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 QU and QL, we obtain:

Now, let’s use the Knudsen relations to estimate the QL and QU exchanged between the Atlantic ocean and the Mediterranean sea across the Gibraltar Strait, given Enet, SL and SU:
  • Net evaporation (Enet ) = 0.55 x 105 m3s-1~ 0.05 Sv;
  • Upper layer salinity (SU) = 36.15;
  • Lower layer salinity (SL) = 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 SU, SL;
  • Sensitivity of fluxes to the accuracy of Enet , SU + SL.
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