Ocean Temperatures Offer Clues to Crop Season Weather

March 5, 2014 02:07 AM
ocean temps

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.

PDO MDO Climate Trend Maps

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:

PDO (+) and AMO (+)
PDO (+) and AMO (-)
PDO (-) and AMO (+)
PDO (-) and AMO (-)

The negative sign indicates a cold phase, while the positive sign indicates a warm phase. The graphic uses 5% contour intervals; the 25% contour would be considered normal risk. For each contour increase or decrease of 5%, drought risk increases or decreases by 15% compared to random occurrence. Therefore, a 40% contour indicates a 60% drought risk increase, while a 15% contour indicates a 30% drought risk decrease.

These charts indicate that the best case scenario for U.S. grain production is when the Pacific Ocean is in the warm phase and the Atlantic Ocean is in the cold phase. The worst case U.S. grain production scenario occurs when the Pacific Ocean is in the cold phase and the Atlantic Ocean is in the warm phase, which happened during the 2012 drought.

You can view global animations of the sea surface temperature deviations for the last six months on a National Oceanic and Atmospheric Administration (NOAA) website.

It appears that the Pacific Ocean is beginning to signal a movement toward the warm phase, while the Atlantic Ocean is beginning to show signs of entering a cold phase.

If the cold pocket of water in the eastern Atlantic continues to build westward over the next couple of months, risk analysis would indicate that drought frequency across the entire Corn Belt would decrease. However, if the eastern Atlantic cold pocket disappears, drought frequency would increase dramatically across a substantial portion of the U.S. Corn Belt.

Please remember that these relationships do not mean that the event is likely to occur, only that the odds of the event are more or less favorable depending on the phase of each ocean basin. If your drought risk increases, the effect could be partially offset by above normal soil moisture recharge. However, if your soil moisture recharge is below normal and your drought risk is high, you may want to take defensive positions to protect your farm income stream.


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