The culprit for the dry weather that has parched much of the Midwest may be fading, a University of Missouri atmospheric scientist said. But don't expect much relief, yet. The dry soil will have an impact on this summer's temperatures.
Tony Lupo, department chair of Soil, Environmental and Atmospheric Sciences at the College of Agriculture, Food and Natural Resources, said that the La Niña weather pattern – the Midwest's toasty weather influencer for more than two years – has diminished.
That's the good news. The not-so-good news is that while La Niña has faded, it hasn't been fully replaced by El Niño – the warming of waters along the equator in the Eastern Pacific Ocean.
Through its influence on the atmosphere, El Niño shifts tropical rainfall patterns which causes further shifts in weather around the globe, including milder winters in western Canada and parts of the northern United States and wetter winters in the some southern states.
Lupo says El Niño will bring more moisture and cooler temperatures to the Midwest, causing industry sectors, like agriculture, to be interested in how El Niño will affect their costs.
El Niño-influenced weather can affect fuel oil demand and growing patterns, and weather forecasters have long known that El Niño events can throw seasonal climate patterns off kilter, particularly during winter months.
Lupo said because El Niño hasn't fully replaced La Niña, some growing areas have entered a neutral period that could persist through late summer 2013.
This will bring more moisture to much of the Midwest, but at still below normal levels. A late summer pickup in normal rain will be too late for much of the next growing season, Lupo said.
A Change in the Pacific
La Niña and El Niño are regularly occurring climatic features, often changing places every few years.
During La Niña, sea surface temperatures across the equatorial Pacific Ocean are lower than normal by 3–5 °C. This typically directs the jet stream from the Pacific on a northeastern path over Canada. Rain producing storms follow the jet stream, leaving the central and south-central states dry. This route of the jet stream also blocks arctic air from moving south into the American midsection, resulting in higher temperatures.
Even if a neutral or El Niño weather patterns mutes the warmer trend in the Midwest, the dry soil left over from multiple years of drought will still result in higher-than-average summertime temperatures, Lupo said. Soil moisture influences air temperature. High soil moisture produces high evaporation, producing evaporative cooling.
Without this evaporative cooling, daytime temperatures could be as much as 10 degrees higher, Lupo said.
Ongoing research underway to expand El Niño planning
Because of the ongoing shifts, researchers from the National Oceanic and Atmospheric Administration and the University of Washington have developed a plan to detect El Niño patterns and help forecasters predict the unusual weather it causes.
A network of buoys that spans the Pacific, the TAO-Triton array, observes conditions in the upper ocean and is essential for forecasting El Niño months in advance, and for monitoring it as it grows and decays. A new study, just published in the February issue of the Journal of Climate, describes an atmospheric El Niño signal that is very strongly associated with U.S. winter weather impacts.
Ed Harrison, Ph.D. of the NOAA Pacific Marine Environmental Laboratory in Seattle and Andrew Chiodi, Ph.D., of the NOAA Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington, co-authored the paper.
"When it comes to El Niño's weather impacts, we are always looking for ways to improve our forecasting skill," said Harrison. "Our goal is to extract the most useful information to predict El Niño seasonal weather anomalies."
Harrison and Chiodi looked at all El Niño events that were identified by sea surface temperature measurements since 1979. They then examined satellite imagery for these events and found that a subset of the events showed a sharp dip in heat radiating from the tops of deep convective clouds, an indicator known as outgoing long-wave radiation or OLR. When comparing the El Niño events to historical weather records, the scientists found that the El Niño events with drops in OLR were the ones most likely to play havoc with winter weather.
They also found that El Niño events with no corresponding drop in OLR did not produce statistically significant anomalies in weather patterns. The dip in heat from deep convective clouds usually occurred before winter, so the timing of the signal could help forecasters improve winter seasonal outlooks, the scientists said.
"By sorting El Niño events into two categories, one with OLR changes and one without, forecasters may be able to produce winter seasonal outlooks with more confidence than previously thought possible," Harrison said.