Waiting on El Nino
May 9, 2014
by Drew Lerner
A large part of China’s summer grain and oilseed production region was wetter than usual in April. The precipitation has brought winter wheat and rapeseed development along at a much faster than usual pace and the same can be said of the planting of corn and a host of other early-season summer crops. The abundance of precipitation noted in April will contrast sharply with a drying bias that is expected to evolve during the summer across much of the same region. China’s drier bias will come after a similar bout of below average precipitation impacts a large part of Southeast Asia over the next few weeks.
Weather conditions during the month of April were wetter than usual over a huge region of China including more than 60% of its rapeseed production region and 85% of its wheat production country. The wet conditions occurred while temperatures were near to above average. Some of these tendencies began in late March and brought winter crops out of dormancy quickly while supporting aggressive early-season development. Yield potentials in this year’s winter wheat and rapeseed will be above average barring no significant changes in weather during May.
The fine early spring weather also supported a quick start to the planting of corn, rice, early sunseed and groundnut planting. Summer crop planting is often more aggressive in May and June than in April, but this year’s fieldwork is likely well ahead of average because of the favorable rainfall and temperature regime that has prevailed recently. The situation will bode well for summer crop development if it can be sustainable.
Weather conditions up until now have been largely dominated by a prevailing weather pattern that was last seen dominating the Northern Hemisphere in 1996 and 1978. If the weather patterns from these two years were to prevail, a large part of northern and east-central China would be wetter and cooler biased in the summer of 2014. However, a huge influence is expected to evolve soon from El Niño.
HOW EL NIÑO WORKS
El Niño occurs when sea surface temperatures in the eastern equatorial Pacific Ocean are at least 0.5 degrees C warmer than usual. Once the ocean temperature becomes anomalously warm, the changes start to influence the atmosphere not unlike a warming pot of water in your kitchen changes the relative humidity and temperatures of the room. When sea surface temperatures become significantly warmer than usual the air above the warmest water will have a tendency to warm and rise upward just like warming the air beneath a hot air balloon results in the balloon’s rise upward into the atmosphere. Once this rising moisture is established and is consistent, the atmospheric circulation across the tropical Pacific Ocean begins to change as well.
Rising air over the eastern equatorial Pacific Ocean creates a low pressure center in the region. A low pressure center is nothing more than a region in which the air is steadily rising, reducing the weight of air over that region. Since the air over the region is moving upward, there is less weight of air pushing down toward the ocean’s surface which is just another way of talking about a low pressure system. Once the rising air over the eastern equatorial Pacific reaches a certain altitude, it will not be able to rise any longer. But the air below is still rising, forcing the warmer air aloft to spread out in all directions as it moves away from the rising column of air. As the warm air aloft spreads out it will begin to cool because it gets away from the warm source of rising air. As the air cools it begins to sink because that is one of the physical properties of air. As air cools it becomes denser, and the heavier the air becomes, the more it sinks.
When El Niño warms the eastern equatorial Pacific Ocean, it tends to create a low pressure center between the International Dateline and the coast of South America. As the air rises it will spread out and cool. The sinking motion that evolves as the air aloft cools will form a high pressure center because of the greater amount of air pushing downward toward the ocean surface. This sinking motion of cooler air creates higher pressure, and higher pressure produces drier conditions. That explains why El Niño events produce drought in Indonesia, Malaysia, the Philippines, northern portions of South America and Central America.
ASIA MAY TREND DRIER
The development of El Niño significantly alters the weather in the tropical Pacific Ocean. When El Niño events are significant, they generate huge low and high pressure systems. These weather systems are so large that they cause a chain reaction of weather pattern changes in many other areas in the world. In the case of Asia, the changes often include a progressively drier tendency for east-central China. As El Niño matures, the drier tendency in China occurs while much of South Asia trends drier as well.
Indonesia, Malaysia, the Philippines, mainland portions of Southeast Asia and eastern Australia all tend to become drier biased in the early weeks of El Niño along with northern parts of South America, Central America and a part of southern Mexico. As the summer wears on, the drier bias does expand from Southeast Asia into the Indian subcontinent, and China’s drier bias tends to come a little later in the summer and often reaches a peak of significance in the early autumn.
Developing dryness in each of these Asian locations will greatly change crop production potential in significant El Niño events. Severe droughts do not always evolve in El Niño events and it would be a mistake to make a blanket statement about such a potential association. However, rainfall does fall below average, and in many cases there is enough of a decline in precipitation that production is curtailed for many crops.
Lower sugarcane, citrus, coffee, corn, rubber, tea and palm oil output tends to occur in each year in which a moderate to strong El Niño evolves. Countries that often report reduced agricultural income because of El Niño tend to be many of these Asian nations, and this year is not expected to be much different.
In the case of China, droughts can evolve because of El Niño, but quite often the rain intensity slips below average while the timing remains favorable. Well-timed rainfall that is lighter than usual can still support crops favorably, raising the debate over how significant El Niño events will get over China. The question of impact on most southern Asia nations is far less debatable since there is a high correlation between below average rainfall and reduced agricultural production, which explains why the decree of a developing El Niño event can have significant influence on global commodity futures trade long before rainfall and crop production are impacted.
By the way, there is a tendency for the middle latitudes of the Northern Hemisphere to trend a little wetter and slightly milder than usual during the summer seasons of El Niño years. If that is true, there will likely be favorable crop production coming from portions of Europe, the Commonwealth of Independent States and across portions of the United States. The significance of this year’s developing El Niño is not fully known yet, although speculators are noting the ocean water temperature anomalies might become as significant as those in the 1997-98 event, which was the strongest El Niño event to occur since record keeping began in 1950.
Drew Lerner is a senior agricultural meteorologist with World Weather, Inc. He may be reached at firstname.lastname@example.org. World Weather, Inc. forecasts and comments pertaining to present, past and future weather conditions included in this report constitute the corporation’s judgment as of the date of this report and are subject to change without notice.