Modulation of the Convectively Coupled Kelvin Waves over South America and tropical Atlantic Ocean in Association with the Madden-Julian Oscillation

The convectively-coupled equatorial Kelvin wave (CCKW) is one of the major tropical wave modes and plays important roles in local and downstream weather. The CCKW activity is found to exhibit strong variation over the South America and tropical Atlantic region on both climatological and intraseasonal time scales. In this work, the modulation of the CCKW activity over this region by the Madden-Julian Oscillation (MJO) has been investigated. The CCKW activity is found to be enhanced during MJO phase 8, 1, and 2 (most evident in phase 1), and suppressed during MJO phase 4, 5, 6 (most evident in phase 5), while transitional signals occur in phase 3 and 7. The enhancement (suppression) of the CCKW activity is primarily associated with the increased (decreased) CCKW amplitude, with the amplitude modulation evident throughout the troposphere and even into the stratosphere for winds and temperature, while the vertical structure of the CCKWs show only modest variation across MJO phases.

Mechanism examination reveals that the MJO modulates the CCKW activity through its modulation of two large scale background conditions in which the CCKWs are embedded: the vertical zonal wind shear and the lower to middle troposphere moisture content. To be more specific, the enhanced CCKW activity is associated with the westward wind shear anomalies and positive troposphere moisture anomalies, and vice versa. These two physical processes modulated by the MJO are found to have positively (negatively) reinforcing influences in the CCKW activity in phase 1 (4, and 5), while counteracting influences in phases 2, 3, 6, 7, and 8. Combined together, they produce the observed MJO cycle of the CCKW activity anomalies in the study region to a large extent.

The above results have broader implications for the prediction of tropical weather patterns and extreme events as well as the initiation and prediction of the MJO itself.