The Less Popular, but Potent Atmospheric Kelvin Wave Influences Enhanced Local Rains

Atmospheric Kelvin waves are not as popular as tropical waves, tropical storms, and hurricanes but they do occur and enhance extreme weather events across tropical areas, including Trinidad and Tobago. This was the case during October 2018, when the characteristics of a strong Kelvin wave passed across our region combined with the effects of a tropical wave and the Inter-Tropical Convergence Zone (ITCZ) influenced a record torrential rain event that caused devastating flooding consequences in Trinidad and Tobago.

Not to be confusing, there are both oceanic and atmospheric Kelvin waves. Oceanic Kelvin waves drive up- and down-welling of warm and cold blobs of water in the Tropical Pacific Ocean, which initiates and/or affects the El Niño–Southern Oscillation. However, this blog is not about oceanic Kelvin waves.

In this post, we will try to provide some details on what atmospheric Kelvin waves are, how they influence our weather extremes, and why they are important in weather forecasting and warnings. Unlike tropical waves which are westward-moving in our region, atmospheric Kelvin waves are invisible wave-like features in the atmosphere that are vertically penetrating, as they move through the global equatorial tropics from west to east and are thus eastward-moving waves. Often, there are significant ocean-atmosphere interactions during the passage of Kelvin waves that can considerably modify the tropical convection on small to large scales.

Figure1. A schematic diagram of the vertical structure of a CCKW. The red arrow points towards the direction of propagation [adapted from Fig. 6 in Straub and Kiladis (2003)]

Unlike the much talked about tropical waves that originate over Africa, which are often the embryo for tropical cyclones as they move westward across the Atlantic Basin, these Kelvin Waves can be initiated by, and at times are fixed or coupled with convective activity or large areas of thunderstorm activity, or what meteorologists call storm systems. They can also become convectively active as an independent weather system. As a result, they are called Convectively Coupled Kelvin Waves (CCKW). They typically originate, mostly over the Indian and the western Pacific Oceans, and circumnavigate the equator in about 15 to 20 days, which makes them relatively fast-moving waves.

Figure 2. Negative-cold-colored contours blue Velocity Potential anomalies represent upper-level divergence and Positive (warm-colored contours) represent upper-level convergence. The convectively active phases of all Kelvin waves are represented by the blue areas and the convectively inactive phase by the reds.

The basic dynamics of atmospheric Kelvin waves can strongly influence rainfall in Trinidad and Tobago and other tropical areas. They consist of opposing zones of sinking and rising air that is more commonly called positive or negative Velocity Potential (VP) areas. In the rising-air phase, strong thunderstorms and enhanced rainfall events are typical, whereas at the same time, in the sinking-air region of the Kelvin wave there is very little to no rainfall in the tropics.

Figure 3. This is a Hovmoeller (Time/Longitude) diagram showing large areas of rising air in blues and pink colours and sinking air browns and yellows near the equator as a function of time. Convectively suppressed phase of the Kelvin wave: Red solid contours. Convectively active phase of the Kelvin wave: Black dashed contours

In essence, Kelvin waves have both an active phase and inactive phase, which influences increased or decreased rainfall anomalies. This influence on rainfall activity occurs because each phase of the wave temporarily produces large-scale environmental conditions, such as vertical wind shear or atmospheric moisture, more conducive or less conducive conditions for thunderstorm formation, and enhanced rainfall activity.  Some of the active phases of Kelvin waves are weak and therefore not every wave’s active phase will enhance thunderstorm and rainfall activity; however, there is a greater likelihood of enhanced and prolonged rainfall events with the passage of a convectively coupled Kelvin Wave in its active phase in our region.

High-activity periods of enhanced rainfall under the influence of Kelvin Waves can last as long as three to five days or even six days but are generally followed by a similar number of days with little to no rainfall activity during the inactive phases of these waves when large-scale sinking-air conditions become unfavorable for rainfall development. 

The Kelvin wave is of great practical and operational importance. Through using this knowledge, local meteorologists know that thunderstorm development and rainfall activity are very sensitive to atmospheric conditions. This means that strong thunderstorm activity and enhanced rainfall events can be directly related to the presence of a Convectively Coupled Kelvin Wave as it has a major influence on the state of the atmosphere.

Figure 4. A schematic diagram representing the modification of the African environment and African Easterly Wave (AEW) activity by the passage of a CCKW when the convective envelope of the CCKW interacts with the tropical wave (AEW)

Interactions between the eastward-propagating Kelvin waves and the westward-propagating tropical waves, northward/southward migrating Inter-Tropical Convergence Zone (ITCZ) or other areas of collocated tropical disturbed weather can lead to Kelvin wave-driven enhancements. Such interactions are known to contribute to prolonged rainfall events or the formation of a tropical depression/tropical storm activity in waters in close proximity to the east of the island chain. Depending on the state of the background winds, Kelvin waves can provide favorable conditions, when taken together with topography and terrain effects, can initiate, drive and enhance thunderstorm and rainfall activity. Kelvin waves and their associated small to large-scale interactions, therefore, provide useful references for weather monitoring and forecasting in Trinidad and Tobago and the southern Caribbean region in general.

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