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Diurnal Seasonal

Diurnal Seasonal

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1 Diel variability in the mixed depth of a subtropical reservoir (Wellington Reservoir, Western Australia), reflecting the net heat exchanges with the atmosphere. The top panel shows the depth distribution of isotherms through a single day. The left-hand column of smaller graphs shows features of the evolving temperature structure (based on Imberger, 1985). The right-hand column proposes stages in the season-long development of enduring stratification. Redrawn with permission from Reynolds (1997a).

where Apw is the density difference between the surface mixed layer and the water beneath the thermocline. Imberger and Hamblin (1982) divided Rib by the aspect ratio (i.e. the horizontal length of the layer, L, by, divided by its thickness, hm) in order to test the robustness, in any given system, of the density gradients detectable. This (still dimensionless) ratio, they named after another atmosphere scientist, Wedderburn.

Working in meters, values of W > 1 are held to describe stable structures, resistant to further down-mixing and incorporation of deeper water into the surface mixed layer, without either a significant diminution in the value of Apw (e.g. through convectional heat loss across the surface) or the sharp increase in the turbulent intensity, (u*2). Structures in which W is significantly <1 are liable to modification by the next phase of wind stress.

This relationship is especially sensitive to the onset of thermal stratification and, equally, simulates the occurrence of mixing events. The insets in Fig. 2.18 show the onset of an early-season thermocline, when net strong daily heating and the absence of sufficient wind action or nighttime convection overcome full column mixing. A series of days with net warming compounds the stability which, in acquiring increased resistance, halts the downwelling mechanical energy at lesser and lesser depths. The stepped gradient of 'fossil' thermoclines is typical and explicable. It is only following a change (lesser heat income, greater net heat loss or the onset of storms, W diminishing) that deeper penetration by turbulence eats into the colder water and sharpens the thermocline at the base of the mixed layer.

This is the basic mechanism for the onset and eventual breakdown in temperate lakes and seas. It also serves to track the seasonal behaviour of many more kinds of system other than those of middle- to high-latitude lakes. It applies to very deep lakes and seas, which may remain incompletely mixed (meromictic) for years on end. It also covers circumstances of water bodies too shallow or too wind-exposed to stratify for more than a few hours or days at a time (polymictic). It explains the patterns of seasonal stratification in tropical lakes wherein stable density structures are precipitated by relatively small gains in heat content but are correspondingly liable to major mixing episodes for a relatively small drop in suface temperature (atelomictic lakes). It can be used to test the contribution of ionic strength (e.g. salt content in reinforcing density gradients).

Examples of all these kinds of stratification, classified by Lewis (1983), may equally be viewed from the opposite standpoint as a series describing variability in the extent and duration of turbulent mixing. The intriguing consequence is that the depth of the turbulent mixed layer (hm) may remain nearly constant, when it is the full depth of the water (H) in a shallow, wind-exposed site. In a large, deep lake, it may fluctuate between <1m and >100 m, in some instances, within a matter of a few hours.

The Wedderburn formulation equation has also been used to determine whether lakes will stratify at all. Putting W = 1 and interpolating the observed summer-thermocline depths of a series of temperate lakes, Reynolds (1992c) rearranged Eq. (2.34) to determine the density difference, Apw, between the waters separated by the seasonal thermocline. In most instances, the outcome was not less than 0.7 to 0.9 kg m-3. At the depths of the respective thermoclines, the density difference would resist erosion by surface-layer circulations generated by winds up to ~20 ms-1. Winds much stronger than these would cause deepening of the mixed layer and depression of the thermocline. Interpolating the corresponding values for Apw and u*, Eq. (2.34) was solved for hm against various nominated values for L. The resultant slope separated almost perfectly the dataset of stratifying and non-stratifying lakes assembled by Gorham and Boyce (1989) (Fig. 2.19).

This outcome is a satisfying vindication of theoretical modelling. Its principal virtue in plankton biology is to empiricise the relationships by which familiar environmental components govern the entrainment and transport of plankton-sized particles and how often the various conditions might apply.

Figure 2.19

Depth, H, plotted against L, the length across various lakes of routinely stratifying (•) and generally unstratified lakes (o) considered by Gorham and Boyce (1989). The line corresponds to Reynolds' (1992c) prediction of the wind stress required to overcome a density difference of 0.7kgm-3 (equivalent to u* = 0.025 ms-1). The diamond symbols refer to lakes said to stratify in some years but not in others Redrawn with permission from Reynolds (1997a).

Figure 2.19

Depth, H, plotted against L, the length across various lakes of routinely stratifying (•) and generally unstratified lakes (o) considered by Gorham and Boyce (1989). The line corresponds to Reynolds' (1992c) prediction of the wind stress required to overcome a density difference of 0.7kgm-3 (equivalent to u* = 0.025 ms-1). The diamond symbols refer to lakes said to stratify in some years but not in others Redrawn with permission from Reynolds (1997a).

This is a suitable point at which to emphasise an important distinction between the thickness of the mixed layer and the depth of the summer thermocline. As demonstrated here, the latter really represents the transition between the upper parts of the water column (in lakes, the epilimnion) that are liable to frequent wind-mixing events and the lower part that is isolated from the atmosphere and the effects of direct wind stress (the hypolimnion). The thickness of the intermediate layer (in lakes, the metalimnion) is defined by the steepness of the main vertical gradient of temperature (the thermocline) or density (pycnocline) between the upper and lower layers, though neither layer is necessarily uniform itself. The top of the thermocline may represent the point to which wind mixing and/or convection last penetrated. Otherwise, the mixed layer is entirely dynamic, its depth and structure always relating to the current or very recent (the previous hour) balance between Jb and Jk. The mixed layer can be well within the epilimnion or its full extent. Any tendency to exceed it, however, results in the simultaneous deepening of the epilimnion, the surface circulation shearing off and entraining erstwhile metalimnetic water and simultaneously lowering the depth of the thermocline.

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