Vorticity Waves

From the above equations, allowing either f to vary as a function of y (the b-plane) or allowing H to vary as a function of x and y allows a similar class of waves to exist. In the ocean, where f does vary, a class of waves called Rossby waves (or planetary Rossby waves) exist because of the conservation of angular momentum.

In inland waters such as lakes, these effects can be ignored as they are generally smaller than 500 km and f can be assumed to be constant. However, variations in water depth H result in a similar type of wave being possible, again due to the conservation of angular momentum. The structure of these motions is typically more complex than planetary Rossby waves as variations in water depth can occur in all directions, whereas variations in fare limited to the north-south

Total

Anti-cyclonic (clockwise) Cyclonic (anti-clockwise)

10-5

10-4

Frequency (Hz)

Figure 6 Spectra of currents along the 24C isotherm in Lake Kinneret during summer 1998 at station T3 on the western margin, showing the total spectrum, the component due to anticyclonic motion ('Poincare waves'), and the component due to cyclonic motion ('Kelvin waves'). The arrows denote periods of 24 and 12 h from left to right. Adapted from Antenucci JP, Imberger J, and Saggio A (2000) Seasonal evolution of the basin-scale internal wave field in a large stratified lake. Limnology and Oceanography 45: 1621-1638.

Total

Anti-cyclonic (clockwise) Cyclonic (anti-clockwise)

10-6

10-5

10-4

10-3

Frequency (Hz)

Figure 6 Spectra of currents along the 24C isotherm in Lake Kinneret during summer 1998 at station T3 on the western margin, showing the total spectrum, the component due to anticyclonic motion ('Poincare waves'), and the component due to cyclonic motion ('Kelvin waves'). The arrows denote periods of 24 and 12 h from left to right. Adapted from Antenucci JP, Imberger J, and Saggio A (2000) Seasonal evolution of the basin-scale internal wave field in a large stratified lake. Limnology and Oceanography 45: 1621-1638.

direction. In this section, we use the term 'vorticity waves' to describe these motions, though they are also called 'topographic waves,' 'vortical modes,' 'second class waves,' or 'quasi-geostrophic waves.' These waves have been observed in large lakes such as Lake Ontario, Lake Michigan, Lake Zurich, and Lake Lugano.

The frequency of these motions is always subiner-tial (i.e., less than the inertial frequency at that latitude), and the frequency depends primarily on the topography of the basin as it is the topography (through the variation in H(x,y)) that causes changes in angular momentum. Importantly, the frequency of these motions is not a function of stratification, and so does not vary on a seasonal basis. This simplifies the measurement of these waves as they are existing at the same frequency year-round.

These waves propagate their phase cyclonically (anticlockwise in the Northern Hemisphere); however, the currents measured rotate both cyclonically and anticyclonically dependent on the horizontal structure of the wave. The currents induced by these waves in the bottom layer consist of a barotropic component only, whereas in the surface layer both a barotropic and a baroclinic current component exists, provided the upper layer is relatively thin than the lower layer. The exact structure of these waves is difficult to determine as there are multiple solutions that have similar frequencies, though it is recognized that the fundamental modes are the most likely to be generated.

Was this article helpful?

0 0
Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Post a comment