Current North Atlantic Oscillation (NAO) Index, courtesy of Climate Prediction Center
Figure 1. Current North Atlantic Oscillation (NAO) Index.

Why So Hot This Summer of 2009?
Maybe, Blame it on El Niño and the North Atlantic Oscillation

Frequently Asked Questions and Answers to Guide You

What Is El Niño?
El Nino, an abnormal warming of surface ocean waters in the eastern tropical Pacific, is a part of what's called the Southern Oscillation. The Southern Oscillation is the see-saw pattern of reversing surface air pressure between the eastern and western tropical Pacific. For example, higher surface pressure in the eastern tropical Pacific is counteracted by lower pressure in the western tropical Pacific; conversely, lower surface pressure in the eastern tropical Pacific is counteracted by higher presure in the western tropical Pacific. Because ocean warming and pressure reversals are for the most part simultaneous, scientists call this phenomenon the El Niño/Southern Oscillation, or ENSO for short.

The term El Niño was coined when coastal residents of Peru noticed an unusual, early season warming of the nearshore waters and resultant impacts on flora, fauna, and fish. Fishermen noticed a sharp decline in marine life, while those on land would see increased rainfall, leading to greater crop production but also dangerous flooding. Occurring around Christmastime, the phenomenon was named for the Christ Child – El Niño in Spanish. Click here for much more.

What is the North Atlantic Oscillation (NAO)?
The NAO consists of anomalous pressure differences (known as "dipoles", with one center located over Greenland and the other center of opposite sign spanning the central latitudes of the North Atlantic between 35°N and 40°N. The positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States and western Europe. The negative phase reflects an opposite pattern of height and pressure anomalies over these regions. Both phases of the NAO are associated with basin-wide changes in the intensity and location of the North Atlantic jet stream and storm track.

While the NAO is best correlated with winter season weather, there are lesser, but still notable, relationships with summer weather. During the winter, the positive phase (NAO+) increases the strength, incidence, and pathway of winter storms crossing the North Atlantic Ocean between the central latitudes and Greenland. This phase tends to support stormy conditions across northern Europe. Across eastern North America, wetter and milder conditions tend to prevail. During the negative phase (NAO–), weaker subtropical high surface pressure centered closer to the Azores combined with weaker low pressure between Greenland and Iceland leaves fewer and weaker winter storms crossing on an east to west path, increasing rainfall to central and southern Europe. Across eastern North America, drier conditions prevail, but colder temperatures and exact location of individual mid and upper level troughs can increase the snow and ice accumulation.

Relationship Between El Niño and the NAO
Studies have found that when the NAO is negative, as it is now (Figure 1, above), it can worsen an ongoing summer drought in South Texas. Overall, significantly different ridge/trough patterns develop over North America, depending on the combined phase of ENSO and the NAO. Specifically, winters with El Niño/NAO+ conditions feature positive height anomalies (ridging) over the Great Lakes region, while negative height anomalies troughing) occur from Alaska’s Aleutian Islands to the west coast of North America. This contrasts with the large-scale flow observed in El Niño/NAO– winters, which are characterized by ridging over the western third of North America. The NAO may be an important modulating feature on El Niño impacts in the southwestern United States, namely, variability of winter season precipitation totals.

Data show that winter season precipitation is significantly greater, statistically, when El Niño episodes occur during NAO+ than when El Niño occurs during NAO–. For example, Arizona statewide precipitation during El Niño/NAO+ winters averages 56% above normal, but just 9% above normal when El Niño occurs in conjunction with NAO–. This modulation of El Niño winter precipitation by the NAO also occurs in California, Colorado, New Mexico, Oklahoma, Texas, and southern Nevada. Modulation of La Niña winter precipitation by the NAO occurs throughout Texas and Oklahoma. For instance, Oklahoma statewide precipitation during La Niña/NAO- winters averages 20% below normal, while winters featuring La Niña/NAO+ conditions result in slightly above normal precipitation. Click here for local details.

El Niño, the NAO, and the RGV Heat Wave
There are several reasons that might explain the Deep South Texas heat spell. Many recent studies discuss a connection between El Niño and the NAO as possible causes of hot and dry conditions extending from the Midwest states to northern and central Mexico during the summer months preceding, or at the onset of, an El Niño event. During the summer preceding El Niño, the Pacific Inter–Tropical Convergence Zone (ITCZ) shifts southward from its typical location [where], notably reducing showers and thunderstorms over Mexico. One reason for this shift is warming waters of the Equatorial Pacific Ocean associated with El Niño refocus shower and thunderstorm development equatorward, rather than poleward, during neutral or La Niña episodes.

Other studies have shown that the blocking pattern of the westerlies that occurs in the transition between the summer and fall over the South Central U.S. and northern Mexico is enhanced during the summer preceding El Ninño episodes. The enhanced pattern triggers more dry weather in August and September from the South Central U.S. into northern Mexico, including the Rio Grande Valley. This high pressure is referred to as "La Canícula". During weak El Niño episodes, a moderate to strong mid level high pressure ridge may set up across the Southwest U.S., with a fairly vigorous trough across the eastern third of North America (Figure 2). Moderate to Strong El Niños shift this ridge westward, blocking out Gulf moisture across Deep South Texas and northern Mexico, creating or intensifying drought through late summer (Figure 3). The pattern shown in Figure 2 match closely with a number of instances this June and July. Recent model trends suggest a return of the Bermuda High to the Southeast U.S. during August, which could signal a transition to the pattern shown in Figure 3. In either case, drought relief for South Texas is not in the cards, at least not yet.

While strong correlations between La Canícula and the NAO are not evident in historical data, there is some indication that either a slight NAO–, or a trend toward a NAO– is evident in the historical data. In 1998, the last summer with persistent heat and rain free weather across South Texas, both a strong NAO– and a formerly strong El Niño existed.

Typical pattern for a Canicula during a weak El Nino episode in late Summer
Figure 2. Average 500 mb heights during a weak El Niño episode in late summer (August 12-20 average) across North America. The high pressure ridge is referred to as La Canícula in northern Mexico.
Typical pattern for a Canicula during a strong El Nino episode in late Summer
Figure 3. Average 500 mb heights during a moderate/strong El Niño episode in late summer (August 12-20 average) across North America. The high pressure ridge is referred to as La Canícula in northern Mexico.

What is the El Niño Forecast?
Figure 4 below shows predictions for Sea Surface Temperature Anomalies in the Niño 3+4 Regions of the eastern Tropical Pacific through early 2010. The heavy blue line is the Coupled Forecast System Ensemble Mean, and shows an intensification of the current conditions to moderate (+1.0 to +1.9°C) over the next few months, rising toward or above strong levels (+2.0°C) before the end of calendar year 2009.

Current El Nino Coupled Forecast System ensemble forecasts, courtesy of Climate Prediction Center (click to enlarge)
Figure 4. Coupled Forecast System Ensemble for the 2009/10 El Niño. Three month spans are depicted after August. "ASO"=August through October, "SON"=September through November, etc. Click to enlarge.

References
Alcalá, J., A. Meulenert, O. García H. Ramírez, M. García, 2004: Análisis de la sequía de medio verano (canícula) en el estado de Jalisco, México. Convención: Trópico 2004. 1er taller de Meteorología Tropical, La Habana Cuba. Abr. 2004.

Magaña, V., J. A. Amador, and S. Medina, 1999: The midsummer drought over México and Central America. J. Climate, 12, 1577-1588.

Miller, James A., 2008: The combined role of ENSO and the NAO on winter precipitation and summer drought in the southwestern United States. 17th Conf. on Applied Climatology, British Columbia, Canada, 11-15 Aug. 2008.

Peña, M. and M. W. Douglas, 1999: Characteristics of Central America wet and dry spells. Preprints, Twenty–third Conference on Hurricanes and Tropical Meteorology, Dallas, TX, 819-822.

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