Kelvin Waves Quiz: Ocean Dynamics and Thermocline Shifts

Oceanic Kelvin waves are large-scale gravity waves trapped along the equator and coastlines by the Coriolis effect. During El Nino development, a downwelling Kelvin wave is triggered by the weakening of trade winds in the western Pacific. It propagates eastward along the equator, deepening the thermocline and warming the surface in the eastern Pacific, suppressing upwelling of cold deep water and amplifying El Nino conditions.

The thermocline is the zone of rapid temperature decrease with depth that separates the warm, wind-mixed surface layer from the cold deep ocean. Across the tropical Pacific, thermocline depth varies significantly east to west. Under normal conditions, trade winds push warm water westward, making the thermocline deeper in the west and shallower in the east where cold water upwells. This east-west thermocline tilt is a key feature of the ENSO system.

Eastern Pacific sea surface temperatures are partly controlled by coastal and equatorial upwelling, which draws water from below the thermocline. When the thermocline is shallow, upwelled water is cold, keeping the surface cool. When a Kelvin wave deepens the thermocline, the water brought to the surface is less cold, reducing the cooling effect of upwelling. This thermocline deepening is a key mechanism by which Kelvin waves warm the eastern Pacific surface during El Nino.

During normal conditions, trade winds maintain the warm pool in the western Pacific. When trade winds weaken, the wind stress on the ocean surface is reduced and the warm water held in the west is released. This perturbation propagates eastward along the equator as a downwelling Kelvin wave. The wave carries the signal of thermocline deepening from the western Pacific to the eastern Pacific, contributing to the development of warm SST anomalies characteristic of El Nino.

The terminology is reversed in the statement. Downwelling Kelvin waves deepen the thermocline and carry warm anomalies that suppress upwelling and warm sea surface temperatures, contributing to El Nino. Upwelling Kelvin waves shoal the thermocline, bringing the cold deep water closer to the surface and enhancing the cooling effect of upwelling, which contributes to the termination of El Nino or the development of La Nina.

Equatorial Kelvin waves travel eastward at approximately 2 to 3 meters per second, taking roughly two to three months to cross the Pacific basin from west to east. The Coriolis effect acts in opposite directions on either side of the equator, creating a restoring force that traps and guides these waves along the equatorial waveguide. Kelvin waves propagate eastward, not westward. Westward-propagating equatorial waves are Rossby waves, which play a complementary role in ENSO dynamics.

Equatorial Rossby waves propagate westward along the equator, slower than Kelvin waves. When a downwelling Kelvin wave reflects off the South American coast, it generates upwelling Rossby waves that travel westward. When these reflect off the western Pacific boundary as upwelling Kelvin waves propagating east, they contribute to the termination of El Nino. This Kelvin-Rossby wave interaction is central to the delayed oscillator theory of ENSO periodicity.

Under normal conditions, trade winds push warm water westward, creating a deep thermocline in the west of approximately 150 to 200 meters and a shallow thermocline in the east of approximately 50 meters. This pronounced tilt is maintained by wind stress. During El Nino, weakened trade winds and eastward-propagating Kelvin waves reduce or reverse this tilt, deepening the eastern thermocline and allowing the warm surface layer to extend farther eastward across the Pacific.

The equatorial undercurrent, known as the Cromwell Current, flows eastward beneath the ocean surface along the equator, not westward. It flows against the direction of the trade winds, transporting warm water from the west toward the east at depths of approximately 100 to 200 meters. During El Nino, the undercurrent weakens along with the trade winds and the thermocline deepens in the east, reducing its influence on upwelling and eastern Pacific sea surface temperatures.

Downwelling Kelvin waves are detected in the eastern Pacific through several observational signatures. Moored buoys show thermocline deepening as isotherms descend. Subsurface temperatures at 100 to 200 meters depth warm in advance of surface temperature changes. Satellite altimetry shows elevated sea surface heights as warm water accumulates below the surface. Coastal upwelling along Peru weakens rather than strengthens during El Nino because the thermocline deepening reduces the cold deep water available for upwelling.

The delayed oscillator theory proposes that ENSO is maintained by a balance between positive feedbacks that grow the warm anomaly and a delayed negative feedback. When trade winds weaken, downwelling Kelvin waves warm the eastern Pacific. Simultaneously, Rossby waves propagate westward, reflect off the western boundary, and return as upwelling Kelvin waves that arrive months later and terminate El Nino. The delay in this negative feedback determines the approximate periodicity of ENSO.

Satellite altimeters measure sea surface height with millimeter precision. Because warm water is less dense and expands more than cold water, regions with deeper thermoclines and thicker warm surface layers have elevated sea surface heights. As downwelling Kelvin waves travel eastward, they raise sea surface height anomalies, allowing their propagation speed and amplitude to be tracked in near real-time by satellites such as TOPEX/Poseidon, Jason-1, Jason-2, and Sentinel-6.

The Tropical Atmosphere Ocean and Triangle Trans-Ocean Buoy Network is a system of approximately 70 moored buoys spanning the equatorial Pacific from the South American coast to the western Pacific near Australia. The buoys continuously measure sea surface temperature, subsurface temperature profiles, surface winds, and ocean currents. This real-time data is essential for detecting Kelvin waves, monitoring thermocline depth changes, and providing early warning of developing El Nino and La Nina events.

During El Nino, warm water moves eastward from the western Pacific warm pool, causing the thermocline to shoal in the west. This shoaling generates westward-propagating upwelling Rossby waves. When these waves reflect off the western Pacific boundary as eastward-propagating upwelling Kelvin waves, they carry a cold signal toward the eastern Pacific. This delayed negative feedback contributes to the eventual termination of El Nino and is a key element of the delayed oscillator mechanism.

The recharge-discharge oscillator theory proposes that ENSO is driven by the buildup and release of warm water in the equatorial Pacific heat reservoir. During the recharge phase before El Nino, trade winds pile up warm water across the equatorial Pacific, increasing heat content. During El Nino, this warm water is discharged poleward. After discharge, the equatorial Pacific is heat-depleted and primed for La Nina. This framework complements the delayed oscillator and emphasizes integrated heat content over wave dynamics.