Thursday, September 29, 2011

Repeat of the Drought of Record?

Excerpt from the TWDB Newsletter September 2011...

Ocean Temperatures and Predicting Drought in Texas
The current drought in Texas is capturing national and international attention. While it's not unusual for Texas to experience dry spells, the current dry spell is notable for its fierce intensity and state-wide impact resulting in record-breaking low rainfalls and record-breaking high temperatures across the state. The state climatologist has already declared the current dry spell the worst one-year drought in Texas' history. Given how drought prone Texas is, that's saying something.

There's an old saw among ranchers that one more day of drought means one day closer to rain. But the question remains: When will this drought break? Ultimately, no one knows. However, scientists often look to sea surface temperatures for clues on long-term weather trends.

Oceans impact the atmosphere and thus the weather (short-term variations) and the climate (long-term variations). A dramatic example is a hurricane where warm sea surface temperatures feed large swirling storms. When a hurricane moves from ocean to land thereby breaking the ocean-atmosphere connection, it rapidly weakens. More subtle changes in ocean temperatures also affect the weather and climate.

A well-known example of how subtle changes in ocean temperatures impact the weather in Texas is the El Niño Southern Oscillation (known by the cool kids as "ENSO"). This oscillation is represented by sea surface temperatures in the eastern equatorial region of the Pacific Ocean. Sometimes the temperatures are warmer than usual (El Niño conditions), sometimes they are cooler than usual (La Niña conditions), and sometimes they are in-between (neutral conditions). Any particular ENSO phase tends to persist for 6 to 18 months. Scientists have noticed that El Niño conditions tend to result in wetter conditions in Texas during the winter months and a general suppression of tropical storm activity and that La Niña conditions tend to result in dryer conditions in Texas with a greater number of hurricanes with greater intensity in the Atlantic. The current drought started with the onset of La Niña conditions. We are currently in a neutral phase with a 50 percent chance for a return of La Niña conditions this fall and winter. A return of La Niña means the probable persistence of the current drought.

There are other less talked about sea surface temperature oscillations that appear to relate to the climate of Texas. One is the Pacific Decadal Oscillation and the other is the Atlantic Multidecadal Oscillation. The Pacific Decadal Oscillation (PDO) is represented by a seesaw of sea surface temperatures between the central Pacific and the Pacific coast of North America. It typically lasts for 20 to 30 years. Currently the PDO is in a "cool" phase with cooler sea surface temperatures along the Pacific Coast. Similar to ENSO, the PDO's cool phase is associated with below average rainfall and higher temperatures for the southern United States and northern Mexico. "Warm" PDO events are associated with the opposite: lower temperatures and higher rainfall.

The Atlantic Multidecadal Oscillation (AMO) is represented by sea surface temperatures in the northern Atlantic Ocean. AMO is associated with the frequency and severity of drought in North America and the frequency of severe hurricanes in the Atlantic. Two of the most severe droughts in North American recorded history, the Dust Bowl and the drought of the 1950s, occurred when a warm AMO that lasted from 1925 to 1965 lined up with La Niña and PDO events. A cool AMO persisted from 1965 up until the mid-1990s when warm conditions returned.

There are other oscillations besides the PDO, ENSO, and AMO that are known to affect climate in Texas. Some of these include the North Atlantic Oscillation (NAO), the Madden Julian Oscillation, the Indian Ocean Dipole, and the Southern Annular Mode. The North Atlantic Oscillation (NAO) has a major influence on winter climate in the northern hemisphere. How the NAO may interact with ENSO, PDO, and AMO in influencing the climate over Texas is still unclear. Factors such as changes in the "preferred" locations of atmospheric high pressure systems and storm tracks also influence climate over Texas. Some of these factors are directly influenced by the oscillations. Some are the result of a dryer land surface leading to warmer temperatures leading to less rainfall that further dries out the land surface and increases warming....and so on in a feedback loop. Some of these other oscillations are known to act in concert with ENSO and may have some (as yet undefined) role to play in the occurrence and severity of drought in Texas.

While the phase of any of these oscillations, individually or collectively, doesn't guarantee drought or non-drought conditions, certain phases appear to increase the chances that we will see certain climatic conditions. During the drought of the 1950s, a warm AMO, a cold PDO, and a La Nina lined up for the hotter and drier conditions that defined our drought of record. Interestingly (and perhaps alarmingly), with warmer sea surface temperatures in the north Atlantic, cooler temperatures along the Pacific Coast, and recent and projected La Nina conditions, all three seem to be lining up again...

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