Summary

Summary

This case study examined the role of the Madden-Julian Oscillation (MJO) and three types of equatorial waves; Kelvin, equatorial Rossby, and mixed Rossby-gravity waves in tropical variability and cyclone development. First we reviewed the structure of these equatorial waves. Then we used the 30 April to 10 May 2002 period over the Africa/Indian Ocean/Indonesia sector to demonstrate that the favorable large-scale environment provided by the active MJO leads to their development.

On 30 April 2002, the MJO moved from Africa out into the Indian Ocean. By the next day, mixed Rossby-gravity waves had already formed and were moving westward.

Infrared satellite view of Indian Ocean basin and surrounding landmasses, 18 UTC 1 May 2002. Note bright clouds associated with convection and equatorial waves.

The westernmost northern and southern Indian Ocean cyclonic circulations developed into twin tropical cyclones by 6 May 2002.

Infrared satellite view of Indian Ocean basin and surrounding landmasses, 18 UTC 6 May 2002.  Note bright clouds associated with convection and equatorial waves. Graphic is annotated with Mixed Rossby-Gravity wave-generated tropical cyclones.

On 4 May 2002, a linear formation of convection became a Kelvin wave straddling the equator and propagating eastward.

Infrared satellite view of Indian Ocean basin and surrounding landmasses, 18 UTC 4 May 2002.  Red contour labels location of convection that seems to generate Kelvin wave.

Consequently, the convection from the active Kelvin wave generated twin vortices on 6 to 7 May 2002. These represented a pair of westward-moving equatorial Rossby waves.

Infrared satellite view of Indian Ocean basin and surrounding landmasses, 18 UTC 7 May 2002.  Note bright clouds associated with convection and equatorial waves.

These equatorial Rossby waves provided the circulations from which two additional tropical cyclones developed: TC2B and Tropical Cyclone Errol. These are shown as they were on 10 May 2002 at 18 UTC, below.

Infrared satellite view of Indian Ocean basin and surrounding landmasses, 18 UTC 10 May 2002.  Note bright clouds associated with convection and equatorial waves.