Equatorial Waves - Connection To El Nino Southern Oscillation

Connection To El Nino Southern Oscillation

Kelvin waves have been connected to El Nino (beginning in the Northern Hemisphere winter months) in recent years in terms of precursors to this atmospheric and oceanic phenomenon. Many scientists have utilized coupled atmosphere–ocean models to simulate an El Nino Southern Oscillation (ENSO) event and have stated that the Madden–Julian oscillation (MJO) can trigger oceanic Kelvin waves throughout its 30–60 day cycle or the latent heat of condensation can be released (from intense convection) resulting in Kelvin waves as well; this process can then signal the onset of an El Nino event. The weak low pressure in the Indian Ocean (due to the MJO) typically propagates eastward into the North Pacific Ocean and can produce easterly winds. These easterly winds can transfer West Pacific warm water toward the east, thereby exciting a Kelvin wave, which in this sense can be thought of as a warm-water anomaly that travels under the ocean’s surface at a depth of about 150 meters. This wave can be observed at the surface by a slight rise in sea surface height of about 8 cm (associated with a depression of the thermocline) and an SST increase that covers hundreds of square miles across the surface of the ocean.

If the Kelvin wave hits the South American coast (specifically Ecuador), its warm water gets transferred upward, which creates a large warm pool at the surface. That warm water also starts to flow southward along the coast of Peru and north towards Central America and Mexico, and may reach parts of Northern California; the wave can then be tracked primarily using an array of 70 buoys anchored along the entire width of equatorial Pacific Ocean, from Papua New Guinea to the Ecuador coast. Temperature sensors are placed at different depths along the buoys' anchor-lines and are then able to record sub-surface water temperature. The sensors send their data in real-time using a satellite to a central processing facility. These temperature measurements are then compared and contrasted to historically- and seasonally-adjusted average water temperatures for each buoy location. Some results indicate deviations from the 'normal' expected temperatures. Such deviations are referred to as anomalies and can be thought of as either warmer-than-normal (El Nino) or cooler-than-normal (La Nina) conditions.

The overall ENSO cycle can be explained as follows (in terms of the wave propagation throughout the Pacific Ocean): ENSO begins with a warm pool traveling from the western Pacific to the eastern Pacific in the form of Kelvin waves (the waves carry the warm SSTs) that resulted from the MJO. After approximately 3 to 4 months of propagation across the Pacific (along the equatorial region), the Kelvin waves reach the western coast of South America and interact (merge/mix) with the cooler Peru current system. This causes a rise in sea levels and sea level temperatures in the general region. Upon reaching the coast, the water turns to the north and south and results in El Nino conditions to the south. Because of the changes in sea-level and sea-temperature due to the Kelvin waves, an infinite number of Rossby waves are generated and move back over the Pacific. Rossby waves then enter the equation and, as previously stated, move at lower velocities than the Kelvin waves and can take anywhere from nine months to four years to fully cross the Pacific Ocean basin (from boundary to boundary). And because these waves are equatorial in nature, they decay rapidly as distance from the equator increases; thus, as they move away from the equator, their speed decreases as well, resulting in a wave delay. When the Rossby waves reach the western Pacific they ricochet off the coast and become Kelvin waves and then propagate back across the Pacific in the direction of the South America coast. Upon return, however, the waves decrease the sea-level (reducing the depression in the thermocline) and sea surface temperature, thereby returning the area to normal or sometimes La Nina conditions.

In terms of climate modeling and upon coupling the atmosphere and the ocean, an ENSO model typically contains the following dynamical equations:

• 3 primitive equations for the atmosphere (as mentioned above) with the inclusion of frictional parameterizations: 1) u-momentum equation, 2) v-momentum equation, and 3) continuity equation

• 4 primitive equations for the ocean (as stated below) with the inclusion of frictional parameterizations:

• u-momentum,

• v-momentum,

• continuity,

• thermodynamic energy,

.

Note that h is the depth of the fluid (similar to the equivalent depth and analoguous to H in the primitive equations listed above for Rossby-gravity and Kelvin waves), K_T is temperature diffusion, K_E is eddy diffusivity, and τ is the wind stress in either the x or y directions.