Long-Term Weather Cycles Big Drought Drivers

Long-Term Weather Cycles Big Drought Drivers

Role of ocean wind current, pressure cycles major for dry cycles in southern Plains; current conditions indicate long period of drought

Kansas farmers have opinions all over the map on the topic of global climate change.

But they have no doubts about the impact that year-to-year,  growing season weather can have on the ability to grow food for the human population of Earth. If anyone needs examples, they need look no further than the turbulent year of 2011.

Worse yet, many forecasters are warning that the southern Great Plains could be headed for a multi-year drought with more years like 2011 in the coming decade.

At the same time, several encouraging rain events in December have farmers hoping that the region won't see a repeat of last summer's drought in next year's growing season.

Researching the factors that drive weather patterns such as the cyclical droughts with which Kansans are all too familiar can be confusing because of the vast amount of things that scientists don't know about what drives the weather patterns that, bundled together, can be called climate.

What they do know is that cycles of drought have always been part of the climate of North America, and long-term cycles of warming and cooling and of increases and decreases in atmospheric pressure in the Atlantic and Pacific Oceans coincide with periods of wet weather or dry weather across the United States.

Much has been reported about the El Nino, La Nina phenomena and their impact on drought the southern Great Plains and across the Southern U.S.

More information  is available about La Nina, which is currently occurring, on the National Oceanic and Atmospheric Administration's Web site.

But even as La Nina is the most talked about of the weather drivers, there are more patterns, some short-term and some long-term that affect the cycles of weather in the Great Plains.

Scientists, including those at the National Weather Service NOAA are quick to say that there is a lot about what drives those patterns that remains unknown. In general, what is known is that changes in the wind patterns over the ocean precede changes in the surface temperature of the water. Winds from the north tend to cause an upwelling of cooler waters while winds from the south push warmer water in the pool.

It is also well-documented that changes in water temperatures, surface pressure and winds occur more often in the Atlantic because it is smaller and changes occur more quickly than in the larger Pacific.

The North Atlantic Oscillation is a key driver of weather along the east coast. The daily index reported for the NAO is based on the difference between a surface pressure of the subtropical high pressure, recorded at a reporting station in the Azores, to the polar low, reported in Iceland.

In a positive phase, the NAO shows strong high pressure and strong low pressure, a difference that creates more and stronger storms moving across the Atlantic on a track pushed to the north. It generally means milder, wetter weather in the northeast U.S. and Europe and cold, dry winters in Canada and Greenland.

The negative phase is a weaker high and a weaker low, which means fewer, weaker storms and more opportunity for cold air to move down the U.S. east coast, bringing cold, snowy winter weather.

The NAO vacillates between positive and negative phases, but also shows prolonged periods that are predominately positive or negative. Since 2007, it has been predominately positive.

The other big driver in the Atlantic is the North Atlantic Multi-Decadal Oscillation. Like El Nino and LaNina, it is a temperature event, characterized as either cold or warm. Since the mid-1990s, it has been in a warm phase, which is often associated with more severe droughts in the Great Plains. It was in a warm phase from 1925 to 1965, which both the Dust Bowl of the 1930s and the severe drought of the 1950s occurred.

In the Pacific, there is the Pacific Decadal Oscillation, which is also an ocean surface temperature event, that like El Nino and La Nina, has an impact on the position of the jet stream that crosses the North American continent from east to west.

In periods of warmer temperatures, the jet stream moves south, entering the U.S. over southern California and bringing more storms across Kansas. In periods of cooler temperatures, it is pushed north, entering the U.S. over Oregon or Washington and moving the storm track across the northern Plains.

Phases tend to last for years or even decades, thought the last two or three cycles have been of only three or four years in duration.

Weather forecasters say they don't know if that's a temporary blip or a signal that bigger changes in the Earth's climate might change the cadence of smaller cycles. What they do know is how to make fairly reliable predictions about what happens when those cycles occur.

Here's what one forecaster told Kansas Farmer:

"If I were buying seed to plant south of I-70 in Kansas all the way into Oklahoma and Texas, I'd buy nothing but the most drought-tolerant crop I could find. And if I were planting north of I-70, even up into Nebraska, I'd try experimenting with a few things that do well when there's plenty of rain."

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