LESSON 2b: CLIMATE EFFECTS ON CROPS

CLIMATE VARIABILITY

Around 1960 the average annual precipitation for the state of Iowa was 31-31.5 inches. Something on the order of 33.5 inches is now typical of the precipitation for the state of Iowa (Fig. 2.13).

Fig. 2.13 Statewide average precipitation trend (30 yr running average).

This 10% increase in annual precipitation over a 30-year period represents a very dramatic change in climate. By any standard, it is a major climate shift. That change in the amount of precipitation can be directly seen as responsible for some very marked effects on regional yields in the state of Iowa.

Records showed the yield trend in western Iowa increasing in a linear manner (Fig. 1.5); whereas, it has leveled off in the eastern part of the state (Fig. 1.5). It may be no surprise then that this increase of 10% in precipitation has had a great effect in the western part of the state; perhaps the greatest effect is in that area.

The most important weather pattern is shown in Fig. 2.14. It shows a high pressure over Bermuda, appropriately named the Bermuda high pressure. It is not a permanent feature. The Bermuda high pressure appears during the summer. It may appear as early as February, but usually becomes established in April and May lasting through the summer. The Bermuda high pressure follows the sun. As the sun moves back toward the Southern Hemisphere (the apparent position of the sun), the high pressure moves south of the Bermuda-centered position.

Fig. 2.14 Influence of the Bermuda High on Midwest weather patterns. This causes warm, moist air to be brought from the Gulf of Mexico.

A high pressure can be visualized as a pile of air on the surface. Air sinking from high in the atmosphere piles up on the ocean, in this case, or on the earth's surface. That pile of air spreads out in all directions, as piles of things tend to do. Because of the rotation of the earth, that pile of air rotates in a clockwise direction, and that rotation, in the clockwise direction, moves warm, moist air from the Gulf of Mexico into the Midwest.

When the southerly air flow begins with the onset of the Bermuda High, spring and summer weather develops in the Midwest. These southerly winds become the prevailing winds for summers in the Midwest. Warm, humid nights in the spring when the temperature does not fall below 40°F become common. An influx of insects occurs in the spring with the change of wind direction. Black cutworms move from the Texas-Mexico area into the Midwest, and many related things happen. But mainly it brings the moisture. If it were not for the Bermuda high pressure, the Corn Belt of the United States would have a different name. It would be called The Great Desert of North America. The Bermuda high pressure is the great singular aspect of the meteorological environment that makes the Midwest what it is, at least as far as summer weather conditions are concerned.

Midwest rain results from a combination of things. The warm, moist air moving up from the Gulf of Mexico combines with disturbances crossing the Rockies, perhaps traveling along a jet stream flow that comes from near the state of Washington and Oregon, across the Rockies. The disturbances lose most of their moisture when they cross the mountain ranges. When they reach the Midwest, they are just a trough aloft or an atmospheric low pressure. When that disturbance encounters warm, moist air in the Midwest, it triggers spring storms and can also contribute to the development of scattered summer thunderstorms.

Occasionally a droughty weather pattern develops, as exemplified by Fig. 2.15, that represents the weather pattern in 1988. High pressure over Bermuda was missing. There was a high pressure over Colorado and perhaps out further in the Atlantic. Low pressure disturbances were not crossing the Rockies because of low pressure that was a persistent feature in the Gulf of Alaska. The disturbances were being deflected up into the Yukon. During the drought of 1988, the Yukon had a record wet year. With no Bermuda high pressure to put moisture up into the plains, it was a record dry year. It was drier, actually, than the Dust Bowl years. It was not as hot, however, so it was not as stressful as those years.

Fig. 2.15 Weather conditions during the drought of 1988. Moisture from the Gulf of Mexico was shifted westward.

There are two significant weather patterns, the droughty pattern, and the normal, or sometimes "super- normal" pattern (when the El Niño is going on). The drought pattern (Fig. 2.15) tends to develop the year after an El Niño ends. This drought pattern has become a more common occurrence during the past 15 or 20 years than we know of it being before that. Why is it becoming more common? I do not know, but it is a significant feature of changing climate.

There are four main factors influencing large scale weather predictability in the Midwest. Most important is global warming. The El Niņo cycle is second, followed by the 20-year cycle. The 20-year cycle is probably third in overall importance, but most important when discussing droughts. Persistence is the last factor to account for in forecasting.