Fast northward energy transfer in the Atlantic due to Agulhas rings
Erik van Sebille and Peter Jan van Leeuwen
In Journal of Physical Oceanography, 2007, volume 37, pages 2305-2315, doi:10.1175/JPO3108.1.
In Journal of Physical Oceanography, 2007, volume 37, pages 2305-2315, doi:10.1175/JPO3108.1.
Abstract
The adiabatic transit time of wave energy radiated by an Agulhas ring released in the South Atlantic Ocean to the North Atlantic Ocean is investigated in a two-layer ocean model. Of particular interest is the arrival time of baroclinic energy in the northern part of the Atlantic, since this is related to variations in the Meridional Overturning Circulation. The influence of the Mid-Atlantic Ridge is also studied, since it allows for conversion from barotropic to baroclinic wave energy and the generation of topographic waves.
Barotropic energy from the ring is present in the northern part of the model basin within 10 days. From that time, the barotropic energy keeps rising to attain a maximum 500 days after initiation. This is independent of the presence or absence of a ridge in the model basin. Without a ridge in the model, the travel time of the baroclinic signal is 1300 days. This is similar to the transit time of the ring from the eastern to the western coast of the model basin. In the presence of the ridge, the baroclinic signal arrives in the northern part of the model basin after approximately 10 days, which is the same time scale as that of the barotropic signal. Apparently, the ridge can facilitate the energy conversion from barotropic to baroclinic waves and the slow baroclinic adjustment can be bypassed.
The Meridional Overturning Circulation, parameterized in two ways as either a purely barotropic or a purely baroclinic phenomenon, responds also after 1300 days. The ring temporarily increases the overturning strength. The presence of the ridge does not alter the time scales.
Barotropic energy from the ring is present in the northern part of the model basin within 10 days. From that time, the barotropic energy keeps rising to attain a maximum 500 days after initiation. This is independent of the presence or absence of a ridge in the model basin. Without a ridge in the model, the travel time of the baroclinic signal is 1300 days. This is similar to the transit time of the ring from the eastern to the western coast of the model basin. In the presence of the ridge, the baroclinic signal arrives in the northern part of the model basin after approximately 10 days, which is the same time scale as that of the barotropic signal. Apparently, the ridge can facilitate the energy conversion from barotropic to baroclinic waves and the slow baroclinic adjustment can be bypassed.
The Meridional Overturning Circulation, parameterized in two ways as either a purely barotropic or a purely baroclinic phenomenon, responds also after 1300 days. The ring temporarily increases the overturning strength. The presence of the ridge does not alter the time scales.
Key figure
Figure 3: Four snapshots of the interface elevation in the model run without a ridge. The scale runs from -0.5% to 0.5% of the initial maximum interface depression (200 m). The ring slowly moves westward and as it hits the western boundary, Kelvin waves transport mass along the equator to the eastern coast where a Rossby basin mode emerges.
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