By Al Dutcher, University of Nebraska-Lincoln
As planting season nears, a plethora of weather experts will be commenting on how this year's weather will affect U.S. ag production. If you approach weather prediction as a risk analysis tool, you can eliminate much of this banter as simply information overload.
During the past several decades, my extension presentations have examined weather risk across the country, as well as the type of conditions that may impact agricultural productivity. Since the late 1990s, I have discussed the importance of measuring soil moisture recharge and Rocky Mountain snowpack as tools for understanding whether drought susceptibility is increasing or decreasing prior to any given production season.
More importantly, the Pacific and Atlantic oceans have a direct impact on weather patterns across the United States. Understanding these effects can help you assess which U.S. region(s) is likely to have a drought or favorable weather pattern.
Since the late 1990s, I have spoken frequently about the Pacific Ocean entering a cold phase and that drought risk increases dramatically across the western half of the country during these 20- to 30-year cycles. During these periods, there will be some variation from year to year and individual years may show the opposite pattern, however, overall there will be a long-term, dominant trend.
When it comes to drought risk assessment, researchers are finding that the Atlantic Ocean also goes through distinct cycles that last an average of 20 to 25 years. From 1975 to 1995 the Atlantic Ocean was in a cold phase and low hurricane activity was the norm. Since the mid 1990s the Atlantic has been in a predominately warm phase and tropical activity has been above long-term historical norms.
When the Pacific Decadal Oscillation is in the cold phase, sea surface temperatures will be colder than normal from the Gulf of Alaska south to the Baja Peninsula. In the Atlantic Ocean the warm phase is described as above normal temperatures from Africa eastward to southeastern U.S.
Greg McCabe, Mike Palecki, and Julio Betancourt have examined the interaction between the Pacific Decadal Oscillation (PDO) and Atlantic Multi-Decadal Oscillation (MDO) in regard to drought risk. (See article.) They looked at the percentage of occurrences when precipitation was in the lowest 25% of the precipitation distribution curve. Four interactions were plotted and are shown in Figure 1.
Figure 1. Areas of high (red: more than 25%) and low (blue: less than 25%) drought frequencies associated with complimentary modes of the PDO and AMO. (Source: Pacific and Atlantic Ocean Influences on Multidecadal Drought Frequency in the U.S.)
These combinations include above and below normal surface temperatures for the two oceans: