ENSO/NAO

Global short-term climate variability is associated with oceanic and atmospheric phenomena including the El Nino Southern Oscillation (ENSO) and The North Atlantic Oscillation (NAO). These large-scale changes in the atmospheric wave and jet stream patterns influence temperature, rainfall, storm tracks, and jet stream location and intensity over vast areas. Research shows that both ENSO- and NAO-induced seasonal weather conditions can affect primary productivity and insect abundance.

What is ENSO? The El Nino Southern Oscillation (ENSO) describes climate conditions fluctuating between the extremes of El Nino years (e.g. 1992, 1997, 1998) and La Nino years (e.g. 1994-96, 1999). During El Nino winters the surface waters of the subtropical and tropical Pacific Ocean between South America and the international dateline are abnormally warm. As a result, the weather between February and April in western Mexico tends to be cooler and wetter (or hotter and drier in La Nina years).

This effect diminishes as you move north into the wintering ranges of temperate wintering birds where weather patterns are more influenced by the North Atlantic Oscillation.

Typica l Effects of La Nia

Impacts of La Nina

Cold Episode Relation ships December - February

Abnormally heavy monsoons on the Indian subcontinent. In the United States, colder and snowier winter in the northern states west of the Great Lakes. Drier than normal winter in the Southeast. Warmer and drier winters in the Southwest. Enhanced hurricane activity around the mid-Atlantic states. Torrential rains leading to floods in Southeast Asia. Wet weather in eastern Australia. Cool and wet winter in southeastern Africa.

What is NAO? The North Atlantic Oscillation (NAO) is a large scale seesaw in atmospheric mass between the subtropical high and the polar low, which influences weather conditions at northerly latitudes. When the surface waters of the North Atlantic are warmer than normal (warm-phase NAO), the northerly latitudes of the United States can experience mild, wet winters. Conversely, they are more likely to experience cold, dry, snowy winters during cold-phase NAO, especially in the northeast region of North America.

Q ic kTime and a u TIFF (L ZW) d ec ompres so r are n eed ed to se e th is pi cture.

The negative NAO index phase shows a weak subtropical high and a weak Icelandic low. The reduced pressure gradient results in fewer and weaker winter storms crossing on a more west-east pathway. They bring moist air into the Mediterranean and cold air to northern Europe The US east coast experiences more cold air outbreaks and hence snowy weather conditions.

Greenland, however, will have milder winter temperatures

Q ic kTime and a u TIFF (L ZW) d ec ompres so r are n eed ed to se e th is pi cture.

The Positive NAO index phase shows a stronger than usual subtropical high pressure center and a deeper than normal Icelandic low. The increased pressure difference results in more and stronger winter storms crossing the Atlantic Ocean on a more northerly track. This results in warm and wet winters in Europe and in cold and dry winters in northern Canada and Greenland The eastern US experiences mild and wet winter conditions

Rasmussen, 2000 NAO has two phases, like El Nio and La Nia Like ENSO, the NAO has in recent years been changing in frequency, for reasons that are not yet understood, although climate change is a possibility. ENSO and NAO both have an effect on the ocean and therefore impact on fisheries.

Differences between ENSO and NAO The main difference between the ENSO and NAO is that whilst NAO is largely an atmospheric model, ENSO is both atmospheric and oceanic NAO is a fluctuation in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high. A positive NAO index shows a stronger than usual

subtropical high pressure centre and a deeper than normal Icelandic low. The negative NAO index phase shows a weak subtropical

high and weak Icelandic low.

Whereas ENSO involves variations in the temperature of the surface of the tropical eastern Pacific Ocean and in air surface pressure in the tropical western Pacific.With the warm oceanic phase, El Nino, accompanied by high air surface pressure in the western Pacific, while the cold phase, La Nina, is accompanied by low air surface pressure in the western Pacific El Nino is an unusual warming of the tropical Pacific Ocean that occurs in response to large scale weakenings of the trade winds that normally blow westward from South America toward Asia.

Normally, the trade winds produce cool surface water in the eastern Pacific, through evaporation and the upwelling of colder water from below the surface, and warm surface waters over in the far western Pacific.As the trade winds weaken, so does the containment of the warm water in the west and the maintenance of the coolness in the east. As a result, there is relatively warm water all across the Pacific from New Guinea to South America. http://www.esrl.noaa.gov/psd/people/joseph.barsugli/anim.html

Areas they affect NAO - mainly North America and Europe ENSO South America, Australia and Asia

Rasmussen, 2000 The National Centre for Atmospheric Research, has calculated that 47% of the variance in the December-to-March Northern Hemisphere temperatures can be accounted for by the combination of the NAO, ENSO, and the Pacific-North American teleconnection. Rogers, J. C., 1984: The association between the North Atlantic Oscillation and the Southern Oscillation in the Northern Hemisphere.

Sea level pressure patterns in winter According to Pozo-Vazquez et al. 2001, no relationship of significance was found between sea level pressure (SLP) during El Nio and those during NAO. However, the SLP anomaly pattern during La Nia closely resembles the positive phase of the NAO

Pozo-Vaquez et al. 2001 For La Nia years, the mean value of the NAO index in winter was 0.98, while for El Nio 0.47, and for normal 0.22 During the spring, the value of the NAO index for La Nia is still slightly greater than for normal springs, and still statistically significant at the 90% confidence level

Sea level pressure patterns in spring During spring, there seems to be no relationship in sea level pressure between El Nio and NAO yet again There is still a relationship discovered however between La Nia and NAO SLP indices.

Similar findings Dong, B. W., R. T. Sutton, S. P. Jewson, A. ONeill, J. M. Slingo, 2000: Predictable winter climate in the North Atlantic sector during the 19971999 ENSO cycle Fraedrich, K., 1990: European Grosswetter during the warm and cold extremes of the El Nio/Southern Oscillation.

Sea surface temperature patterns Huang et al. 1998 - 70% of the total 27 El Nio events between 1900 and 1995 show significant coherence at the 95% confidence level. Watanabe and Kimoto (1999) - tropical Atlantic sea surface temperature anomalies enhance the NAO selectively among other circulation patterns in the North Atlantic. Rajagopalan et al. (1998) evidence for the influence of the tropical Atlantic SST on the NAO state.