Ag Weather Forum
Bryce Anderson DTN Ag Meteorologist and DTN Analyst

Friday 09/28/12

The "So What" Of A Weak El Nino

The Australia Bureau of Meteorology is hedging its outlook regarding El Nino formation. Here are some highlights from the department's latest El Nino/Southern Oscillation (ENSO) discussion this week:

"Tropical Pacific Ocean temperatures have generally cooled over the past fortnight, easing towards neutral values (neither El Nino nor La Nina). Other ENSO indicators such as the Southern Oscillation Index (SOI) and tropical cloud patterns have remained at neutral levels. Given September is the time of year when El Nino events consolidate, this recent cooling is considered somewhat unusual, hence the risk of an El Nino event remains. (my emphasis)

Despite the shift towards neutral conditions, the tropical Pacific remains warmer than average. When combined with the patterns of cloud and ocean temperatures in the Indian Ocean, conditions continue to favour below average spring rainfall over much of Australia.

Climate models surveyed by the Bureau of Meteorology suggest sea surface temperatures in the tropical Pacific Ocean will maintain values around typical El Nino thresholds for the remainder of 2012.

The Indian Ocean Dipole (IOD) is currently positive, with weekly values of the IOD index consistently above positive thresholds for the past two months. Outlooks from the Bureau’s climate model indicate the IOD will most likely remain positive throughout the remainder of spring. A positive IOD is typically associated with decreased winter and spring rainfall over parts of southern, central and northern Australia."

The highlighted line is the main feature of this discussion, because September is usually the month when either El Nino (warm Pacific) trends or La Nina (cool Pacific) trends really get going. (I say usually, because there have been times when these features intensified at other times of the year.) But, this is an important feature for several reasons. First, a weakening El Nino implies less support for subtropical jet-stream winds and support for rainfall in southern Brazil and Argentina. That, of course, could mean a drier period in the southern hemisphere midsummer, and possible stressful conditions for corn and soybeans during their reproductive phases.

Secondly--a weaker El Nino could hinder the development of follow-up moisture for the southern Plains and southeastern Midwest. These areas have had some very good rains during the past month, bringing two full stages of drought improvement to Illinois and Indiana; and in the Texas Panhandle, producers have seen in some cases the best rains in the last two years. But more moisture is still needed to produce long-lasting benefit. El Nino wind patterns offer a better chance for such a trend; but if the Pacific temperature and air pressure patterns are around neutral, not so much. And then there's the persistently dry region of the western and northern Corn Belt through the northern and central Plains along with the developing dry pattern in the Canadian Prairies that has not shown any sign of letting up.

There is of course a six-month period or so before spring 2013. So, we'll see how things play out. But right now, on the verge of October 1, it still appears that dryness concerns have got to be the big item in thinking about next season.

Bryce

Twitter-- @BAndersonDTN

Posted at 2:59PM CDT 09/28/12 by Bryce Anderson
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