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North Atlantic Oscillation & El Niño Southern Oscillation:
Planetary-Scale Climate Patterns and Ecology


Although ecological processes are influenced by local climate conditions, such as temperature and precipitation, planetary-scale climate patterns also play an influential role. The North Atlantic Oscillation (NAO) and the El-Niño Southern Oscillation (ENSO) are the primary large-scale patterns of concern.

Over the last 30 years there has been a trend towards more positive North Atlantic Oscillation phases that result in stronger winter storms across the Atlantic Ocean. (Stenseth et al. 2002) Additionally, warm, wet winters prevail in Europe; cold, dry winters are present in northern Canada and Greenland; and mild, wet winters occur along the eastern United States. The frequency and duration of ENSO events have increased over the last few decades and coupled global climate models predict there will be stronger and more frequent El-Niño Southern Oscillation events in the future (Rajagopalan et al (1997), Biodiversity and Climate Change).


Although there are a number of uncertainties with regard to assertions about changes in ENSO and NAO patterns, if they are correct, the ecological responses to ENSO events will be exacerbated. Some examples of these responses include:
  • Large fish populations dying near the coast of South America due to lack of nutrient upwelling. (ENSO) (Stenseth et al. 2002).
  • Fluctuations in cod recruitment (NAO) (Stenseth et al. 2002).
  • Germination of annuals in South America. (ENSO) (Stenseth et al. 2002).
  • Rodent outbreaks in South America. (ENSO) (Stenseth et al. 2002).
  • Vertebrate predator responses over arid and semi-arid regions of South America. (ENSO) (Stenseth et al. 2002).
  • Climate affects the hunting behavior and success of the lynx via snow conditions in Canada. (NAO) (Stenseth et al. 2002).
  • Earlier breeding for plants, amphibians, and birds in Europe because of warmer and wetter winter conditions associated with long term changes in NAO. (Stenseth et al. 2002).
  • Changes in temporal and spatial population of zooplankton (NAO) (Stenseth et al. 2002).
  • Changes in food availability, fish larva feed on phytoplankton, which determines recruitment success and size of various fish populations (NAO) (Stenseth et al. 2002, Walther et al. 2002).
  • Changes in biological characteristics of sardine, anchovy, and herring (NAO). Fluctuations in cod recruitment (NAO) (Stenseth et al. 2002).
  • Migration patterns and spatial distributions of large fish altered indirectly through climate-induced changes in prey abundance. (NAO and ENSO) (Walther et al. 2002).
  • Positive NAO values are suspected to affect the availability of plank tonic food for fish larvae, which determines the recruitment success and consequently the size of fish populations. (Stenseth et al. 2002).
  • Mortality of Soay sheep and red deer due to increased precipitation and thus reduced resistance to heat flux across Scotland (positive NAO) Walther et al. 2002).
  • Early blooming of vegetation in northern and central Europe (NAO) Walther et al. 2002).