Weekly Tropical Climate Note

Indian Ocean tropical cyclones in both hemispheres

Two cyclones have straddled the equator in the Indian Ocean during the past week, but are of no threat to the Australian region. The official start of the tropical cyclone season in the southern hemisphere (November to April for the Australian region) coincides with a transitional pattern of broadscale wind flow by which well-defined low pressure troughs can be observed across tropical regions on both sides of the equator. This has led to twin tropical cyclones simultaneously active over both the northern and southern Indian Ocean during the past week. Cyclonic storm Gaja, over the Bay of Bengal, remains active: it is a relatively weak system that may develop further to an intensity comparable to an Australian category 2 tropical cyclone. Gaja is predicted to make landfall on the southeast coast of India in the next few days. The southern hemisphere storm, ex-tropical cyclone Bouchra, developed to the west of Sumatra, south of Gaja.

Tropical cyclone warnings and information for India can found at the Indian Meteorological Department.

Madden–Julian Oscillation moves over Maritime Continent

A moderately strong pulse of the Madden–Julian Oscillation (MJO) continued to track eastwards across the Indian Ocean during the past week, and now lies over the Maritime Continent. As this sustained pulse of the MJO tracked over the Indian Ocean to its current position, it likely contributed to the formation of the tropical cyclones which formed over the Indian Ocean. More generally, this MJO pulse contributed to enhanced rainfall at many locations across the Indian Ocean, South-East Asia and the western Maritime Continent.

The MJO pulse also assisted the formation of a related tropical atmospheric wave called an Equatorial Rossby wave. These waves have been observed across the Indian and Pacific oceans in recent days. Equatorial Rossby waves are often associated with twin tropical lows straddling the equator and, along with the MJO pulse, assisted in the formation of Gaja and Bouchra. Interestingly, twin tropical lows are also apparent over the Pacific Ocean, just to the west of the Date Line. However, neither of the Pacific systems is expected to intensify to a tropical cyclone.

International climate models predict the MJO pulse will move further east but weaken in the coming days. Prior to it weakening, the MJO is likely to contribute to further enhanced tropical rainfall about the Maritime Continent. At this time of the year, an MJO over the Maritime Continent has only a minor influence on rainfall across Australia's far north.

See the Bureau's current MJO monitoring for more information on the MJO.

El Niño ALERT continues—dry start to wet season likely

The Bureau's ENSO Outlook remains at El Niño ALERT, meaning there is approximately a 70% chance of El Niño occurring in the coming months. Sea surface temperatures in the tropical Pacific Ocean have exceeded El Niño thresholds, with models predicting further warming of the ocean is likely. However, ENSO atmospheric indicators (such as the Southern Oscillation Index and cloudiness near the Date Line) are yet to indicate that the ocean and atmosphere have coupled and are reinforcing each other. Coupling is needed to sustain an El Niño event.  

Positive values of the Indian Ocean Dipole (IOD) index persist, continuing the positive IOD event which has been in effect since about mid-September. Models predict a return to a neutral IOD during November.

El Niño and a positive IOD at this time of the year increase the likelihood of a drier than usual start to the northern wet season for Queensland and the Northern Territory. El Niño is typically also associated with a later-than-usual monsoon onset for northern Australia.

See the Bureau's current ENSO Wrap-Up for more information.

Product code: IDCKGEW000

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