Winds and cyclones
Measurements of winds, both from tracking weather balloons and from satellite observations, also tell us about ENSO. When trade winds blowing from the east increase at the surface, and winds around 10 km above the ground strengthen in the opposite direction, it is clear that the Walker Circulation has strengthened – typical of La Niña. If the Walker Circulation shifts into an El Niño phase, the surface easterlies weaken, or even reverse, and the westerly winds aloft do likewise.
With a stronger Walker Circulation comes a shift westward in the typical area of the Pacific affected by tropical cyclones. Hence the only wet seasons in which Queensland has experienced multiple tropical cyclones crossing its coast have been during La Niña periods.
Changes in the location of convection over the Pacific mean that the location and level of cloudiness can be used to track the intensity and phase of the Walker Circulation, much as the SOI tracks changes in atmospheric pressure. Radiation ('heat') released into space acts as a good proxy (an indirect measure) for cloudiness as cloud tops are generally far cooler than the earth's surface, and hence more long-wave radiation is released from cloud-free regions, and less in cloudy regions. This radiation can be easily and accurately detected by satellites. Cloudiness along the equator – particularly near the Date Line – typically increases during an El Niño and decreases during a La Niña. Likewise, rainfall for equatorial countries near the Date Line, such as Kiribati, is lower during La Niña and higher during El Niño – the opposite of Australian impacts.
No two events are the same
Each El Niño and La Niña event is different from region to region, and event to event. Variations in the timing, location and magnitude of ocean temperature anomalies and wind patterns cause differences both in the strength and extent of climate impacts.
In addition, a chaotic 'weather' factor is involved in any event. For instance, flooding in western Victoria during the 2010–11 La Niña event originated from two tropical cyclones (Anthony and Yasi) which decayed over central Australia before moving south. While both were 'random' weather events, more cyclones and tropical depressions occur in the Australian region and more cross the eastern coast during La Niña events. ENSO also interacts with other drivers of climate variability, such as the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM), which increases the likelihood of each event differing from the last.
Impacts during La Niña:
- stronger easterly trade winds
- increased risk of tropical cyclones around Australia
- increased cloudiness over Australia.
Impacts during El Niño:
- weakened easterly trade winds
- reduced risk of tropical cyclones around Australia
- reduced cloudiness over Australia.
A tropical depression is a low pressure system of clouds and thunderstorms with a defined circulation but does not usually have an eye or the spiral shape associated with cyclones or more powerful storms.