Brazil’s southern grain and oilseed producing state of Rio Grande do Sul has struggled with dryness this summer. The yield losses have surprised many producers and market analysts, but the nation is still expected to produce a huge soybean crop. The yield losses are the greatest seen since 2012, and raises a flag for possible US dryness in the coming summer.
How in the world does Rio Grande do Sul have anything to do with US summer weather? Well, there is an association between the two. The relationship has not been proven well enough to use it as a great forecasting tool, yet, but each time Rio Grande do Sul experiences notable dryness that harms production it raises the need for further research. The reason for all of this dates back to 2005, when another drought hit southern Brazil. At that time, World Weather, Inc. started doing research on correlating that dryness with summer dryness that seemed to follow in the US summer season in a part of key production areas east of the Rocky Mountains.
A gauge for dryness and its significance was needed for the scientific study to have verifiable parameters, so World Weather, Inc. chose to look at soybean and corn yields in Rio Grande do Sul. Every time a significant deviation from trend yields has taken place in Rio Grande do Sul it has resulted from dryness at reproduction.
Soybean production in southern Brazil was not always a big attraction, and production was minimal for a long time. In recent decades Rio Grande do Sul took a liking to soybean planting, and, as time moved along, it has proven to be a very successful crop in the nation, and one that has made many farmers rich. Data prior to 1970 has proven to be largely useless because of poor data, but the data collected since that time has proved useful and has provided greater insight to the relationship between dryness in southern Brazil and that in the United States.
This year’s drought in Rio Grande do Sul will be the eighth time that a significant drop in soybean yield has occurred since 1970. The other seven years were 1979, 1986, 1988, 1991, 2004, 2005 and 2012. Do some of those years sound familiar? They should because 1988 and 2012 were significant dry years in key US crop areas. Dryness also was noted in 1991 in the US Midwest and in the southeastern United States in 1986.
US dryness also developed in 2005 after a wet start to the calendar year, and 2005 is already highly correlated with 2020 because of ocean temperature anomalies in the Gulf of Alaska, El Niño-like conditions that gave way to La Niña-like conditions over the calendar year (as expected in 2020) and a split jet stream pattern over North America during the winter and spring similar to this year. Areas from eastern Texas and the western Delta to Michigan and Wisconsin were in a regional drought starting in May and continued into December 2005. We are not expecting that to repeat, but some dryness is possible this summer.
Rio Grande do Sul yield losses also occurred in both 1978 and 1979, but the greater departure from trend yield occurred in 1979. Dryness was hard to come by in the United States during 1979, making that year the greatest failing in the proposed relationship between poor soybean yields in Rio Grande do Sul and dryness in a part of the US key summer crop region.
The 1978 deviation from yields in Rio Grande do Sul was not as significant as that in 1979, but it does fit the criteria for this study. Weather in 1978 in the United States was only dry biased in the southern Plains, and that dryness was already in place at the start of 1978, which may disqualify that year for verification in the relationship.
We also looked at 2004, which also had Rio Grande do Sul soybean yields down significantly from the previous trend, but 2005 was the greater anomalous year for the study. The year 2005 fit our study and theory quite well, but 2004 dryness was mostly confined to the northern Plains and western United States, leaving most other areas favorably moist. The dryness was already in place when the year began.
Overall, we looked at 1978, 1979, 1986, 1988, 1991, 2004, 2005 and 2012, all of which had soybean yields in Rio Grande do Sul significantly lower than the trend for that year. Out of those years, 1986, 1988, 1991, 2005 and 2012 verified with dryness in a part of key US crop areas east of the Great Plains. That makes five out eight years fitting the scenario, meaning 63% of the time dryness occurs in a part of the key US crop areas in the summer that follows a below-trend soybean yield in Rio Grande do Sul. If we include the summers of 2004 and 1978 in which dryness occurred in southern Texas and from the northern Plains in the western United States, respectively, then we end up with six out eight years there was some verification, which is 75% of the time.
Even though the statistics are more supportive of the theory if we include 2004 and 1978, we are opting to not include them because the US weather in those summers really did not fit well. We also determined that 1978 yields in Rio Grande do Sul did not deviate far enough from trend to really qualify for this study.
By tossing out 1978 completely we are left with seven below-average trend yield years for soybeans and corn in Rio Grande do Sul and five out of the seven years had dryness in the United States in the same growing season. That is 71.4% of the time this relationship seems to verify. The sample size is too small for a good study, but the study does offer a little more support for a drier bias in at least a part of US crop areas this year.
That does not mean drought like that of 2012 or 1988 will evolve, but enough dryness for crop stress to evolve and a threat to production “might” emerge. World Weather, Inc. already was predicting a change to drier weather this summer prior to the renewal of this study on Rio Grande do Sul, with July and August to be notably drier and warmer. A change in weather trends may occur sooner than July and August in the Delta and Tennessee River Basin based on other studies.
There is also some influence supporting the development of dryness in a part of the United States from both the solar cycle and what World Weather, Inc. calls the 18-year cycle. Only time will tell. For now, the bigger concern is over the delay in spring planting, which could play into the drier biased summer and hurt yields a little more.