Characteristics of Density Currents

Density currents are driven by differences in water density, which can result from gradients in water temperature, salinity, dissolved uncharged substances, or suspended particles and are also affected by pressure. If a water mass with higher density is situated above a water mass with lower density, the stratification is unstable and buoyancy causes the upper water mass to sink. In the sinking process ambient water is mixed into the sinking water mass and thus alters its density (Figure 1), thereby reducing the density difference between the density plume and the ambient water.

Furthermore, the properties of the ambient water change along the path of the sinking water mass. Hence the buoyancy of the sinking plume changes continuously as it sinks into deeper depth. Eventually, a depth is reached where the density of the density plume and the density of the surrounding water become equal. At this depth the sinking process ceases and the plume water spreads out laterally into the ambient water forming an intrusion (Figure 1). In many cases the sinking plume meets the lake boundary and then continues to sink along the lake boundary (Figure 1). In this case, entrainment of ambient water is limited to the upper side of the density plume and less ambient water is entrained per unit sinking depth. Hence the characteristic properties of the water within the density plume (e.g., temperature, salinity, suspended particles, dissolved oxygen) change more

Figure 1 Schematic illustration of density currents. The shading on the left-hand side of the figure indicates an increase in density with increasing depth.

4.3 4.5 4.7 4.9 6.002 6.004 6.006 8 9 10 80 81 82 83

(a) Potential temperature (°C) Salinity (g kg-1) Dissolved oxygen (mg l-1) Light transmission (%)

Figure 2 Intrusions as indicators of density currents. Vertical profiles of temperature, salinity, dissolved oxygen, and light transmission measured in Lake Issyk-Kul. The distinct features in these profiles suggest intrusions from density currents. Grey bars mark depth regions with high concentrations of dissolved oxygen and low light transmission, suggesting water originating from shallower depth regions. Redrawn from Figure 2 in Peeters FD, Finger M, Hofer M, Brennwald DM, and Livingstone R Kipfer (2003). Deep-water renewal in Lake Issyk-Kul driven by differential cooling. Limnology & Oceanography 48(4): 1419-1431.

4.3 4.5 4.7 4.9 6.002 6.004 6.006 8 9 10 80 81 82 83

(a) Potential temperature (°C) Salinity (g kg-1) Dissolved oxygen (mg l-1) Light transmission (%)

Figure 2 Intrusions as indicators of density currents. Vertical profiles of temperature, salinity, dissolved oxygen, and light transmission measured in Lake Issyk-Kul. The distinct features in these profiles suggest intrusions from density currents. Grey bars mark depth regions with high concentrations of dissolved oxygen and low light transmission, suggesting water originating from shallower depth regions. Redrawn from Figure 2 in Peeters FD, Finger M, Hofer M, Brennwald DM, and Livingstone R Kipfer (2003). Deep-water renewal in Lake Issyk-Kul driven by differential cooling. Limnology & Oceanography 48(4): 1419-1431.

slowly and the density plumes typically can propagate to larger depths than would be possible in the open water.

The occurrence of density plumes propagating from shallow to deep water are indicated by intrusions that can be identified in CTD-profiles (conductivity as measure of salinity, temperature, and depth) and in profiles of dissolved substances and suspended particles. Figure 2 presents an example from Lake Issyk-Kul (Kyrghystan), where intrusions are characterized by a higher dissolved oxygen concentration and a lower light transmission than is observed in the ambient water.

High oxygen levels in the intrusions indicate oxygen-rich surface water that must have been transported recently because oxygen levels have not yet been significantly reduced by degradation processes.

The low light transmission in the intrusions indicates water with a high load of suspended particles suggesting either, that the water that generated the density current was enriched in suspended particles and thus may have originated from river inflow, or, that the density plume responsible for the intrusions has propagated along the lake boundary and caused resuspension of sediments during the sinking process. Temperature is usually not a good indicator of intrusions because it is a key parameter determining plume density. Thus, at the depth of the intrusion, plume water and surrounding water often have about the same temperature. However, in cases where the density plume propagates down to the largest depths, as it is sometimes the case e.g., in Lake Baikal, temperature anomalies at the lake bottom can be used to identify density plumes.

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