Pycnocline

The pycnocline is the layer where the density changes rapidly with depth. It results from the stratification under the effect of gravity of the mixed layer. In tropical oceans, the base of the pycnocline coincides with the thermocline where the temperature changes rapidly with depth. The top of the pycnocline is at the interface between the surface water, of low density, and the stratified layer.

Profiles of density considered as representative of equatorial oceans to determine the depth of the interfaces (dashed lines) associated with normal modes for the Pacific (1), the Atlantic (2) and the Indian (3) Oceans. Interfaces associated with the first baroclinic modes are determined so that phase speeds are close to those observed, i.e. 2.8, 2.35 and 2.3 m/s, respectively. Interfaces associated with highest modes are positioned at the top of the pycnoclines. Phase speeds that follow allow explaining satisfactorily the resonance of 4-year period waves in the Pacific and the Indian oceans and the 8-year period waves in the Atlantic ocean. However, the resonance of a 4-year period wave in the Atlantic and 1- and 2-year period waves in the Indian oceans that contribute to the IOD suggests the existence of intermediate interfaces located within the pycnocline (their location is approximate).
Profiles of density considered as representative of equatorial oceans to determine the depth of the interfaces (dashed lines) associated with normal modes for the Pacific (1), the Atlantic (2) and the Indian (3) Oceans. Interfaces associated with the first baroclinic modes are determined so that phase speeds are close to those observed, i.e. 2.8, 2.35 and 2.3 m/s, respectively. Interfaces associated with highest modes are positioned at the top of the pycnoclines. Phase speeds that follow allow explaining satisfactorily the resonance of 4-year period waves in the Pacific and the Indian oceans and the 8-year period waves in the Atlantic ocean. However, the resonance of a 4-year period wave in the Atlantic and 1- and 2-year period waves in the Indian oceans that contribute to the IOD suggests the existence of intermediate interfaces located within the pycnocline (their location is approximate).

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