Discussion: According to Sara Menker, GRO Intelligence quoting the Buenos Aires Grain Exchange an estimated $3.4B economic loss was caused by severe drought in Argentina this past summer season. Huge heat stress amplified by lack of rain affected corn and soybeans. Let’s take a look at the cause of the drought.
During meteorological winter (DEC/JAN/FEB 2017-18) central to northeast Argentina observed approximately 1/3 to 2/3 normal rainfall with <25% of normal rainfall in western Santa Fe (Fig. 1). In March the rainfall pattern dryness peaked averaging about 15 to 45 percent of normal across central to northeast Argentina (Fig. 2). Interestingly, the dry weather was not accompanied by any long-term unusual heat as seasonal temperatures averaged near normal.
Normally, the El Nino southern oscillation (ENSO) is considered the lead catalyst for the prevailing climate pattern across South America. During this past summer a La Nina episode was intact but with limited typical (wet northern Brazil/cool southeast Brazil) impact. Instead the prevailing climate in Argentina was driven by a semi-permanent upper trough of low pressure off the southeast coast of Brazil (Fig. 3).
Presence of the upper trough off the southeast coast of Brazil caused lower atmospheric wind direction to be from non-tropical sources such as across Brazil from the moist northwest South Atlantic or from the east where the subtropical South Atlantic is located. The 700 MB wind vector analysis indicates wind was mostly light from a (dry) land mass trajectory (Fig. 4).
The upper trough was linked to an area of cooler than normal sea surface temperature anomalies (SSTA) east-southeast of Brazil throughout the warm season (Fig. 5).
Fig. 1: DEC/JAN/FEB 2017-18 percent of normal rainfall analysis provided by NOAA.
Fig. 2: March 2018 percent of normal rainfall analysis provided by NOAA.
Fig. 3: JAN/FEB/MAR 2018 400 MB anomaly analysis identifying a persistent low pressure trough off southeast Brazil.
Fig. 4: JAN/FEB/MAR 2018 700 MB wind vector anomaly analysis identifying prevailing air mass trajectory across Argentina (from non-tropical moist source regions).
Fig. 5: DEC/JAN/FEB 2017-18 global SSTA analysis provided by the International Research Institute for Climate and Society identifies an unusual cool pool southeast of Brazil related to a cool trough in the upper atmosphere which caused a dry wind direction across Argentina.
