Blame La Niña for Brisbane Floods

Heavy rains in Eastern Australia have been in the news lately. Rivers near Brisbane have jumped their banks, flooding over 30,000 homes and businesses and killing at least 23 people. This local extreme weather has its roots in a much larger phenomenon, taking place across the entire Pacific Ocean.

Pacific SST anomaly

Sea-surface temperature (SST) anomaly over the Pacific, January 10, 2011. The SST anomaly is the difference between the current temperature and the long-term average: oranges indicate warmer-than-usual water, blues indicate colder-than-usual. Data from the AVHRR satellite, courtesy of NOAA.

The equatorial Pacific is currently experiencing La Niña, the flipside of the more well-known El Niño. During neutral conditions, a large pool of warm water sits near Indonesia and the Philippines in the western Pacific. This water mass, called the Western Warm Pool, is heaped up there by the prevailing easterly trade winds, actually sitting as much as two meters higher than the sea level at the other side of the ocean, off South America.

But during El Niño, the trade winds weaken and this slug of warm water sloshes eastward along the equator, bringing heat, water vapor, and rain along with it. El Niños are typically marked by rainy weather in Peru, Ecuador, Central America, and California, while Australia experiences drought.

La Niña is the opposite. The trade winds strengthen, the Warm Pool retreats, and a tongue of cold water extends west from South America along the equator. The warm water is pushed up against Australia, bringing heat energy, evaporation, water vapor, and rain along with it. And ladies and gentlemen, we are in the middle of a strong La Niña.

The most commonly-used index of the El Niño/La Niña cycle is the Southern Oscillation Index, or SOI. In fact, the phenomenon is usually referred to by scientists as “ENSO,” which stands for “El Niño-Southern Oscillation.” The SOI is simply the difference in air pressure between Tahiti and Darwin, Australia, adjusted for the long-term averages. During El Niño, air pressure is higher over Darwin, an the index is negative. During La Niña, pressure is higher over Tahiti, and the index is positive. Below is a graph of the SOI for the past 40 years—notice how the index was higher at the end of 2010 than it has been since 1973-74. Not coincidentally, 1974 was also a year of major flooding in Brisbane.

Southern Oscillation Index 1970-2010

Southern Oscillation Index (SOI) from 1970 to 2010. Red portions of the curve are La Niña periods, blue portions are El Niños. Notice the large La Niña events in 1973-74 and at the end of 2010. Data from the Australian Bureau of Meteorology.

There is light at the end of the tunnel: La Niña won’t last forever, and will typically trail off with the onset of the austral winter. Unfortunately, that probably won’t happen until April or thereabouts. In the meantime, western Australia should brace for more rain.

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