The North Atlantic Oscillation: A Climate Phenomena with Significant Impacts on Society

The strength and trajectories of winter storms crossing the Atlantic Ocean vary on interannual to multi-decadal time scales. They are part of a hemispheric sea level pressure signal with reduced or enhanced pressure gradient between the polar low and subtropical high, a phenomena known as the North Atlantic Oscillation (NAO).

During its high index phase, which is defined by an enhanced pressure gradient, warmer and wetter air is advected towards Europe resulting in increased winter mean temperatures and precipitation. For counties like Scottland, Denmark and Norway this means a reduction in the demand for energy (e.g. heating oil) and a surplus of rain water supply available for power generation by hydroelectric power plants. Thus the last decade of a generally high NAO index results in low energy stress with reduced demand and increased supply. However, a single year with a negative NAO index (1995-96) caused a dramatic increase in demand and even bigger loss in water supply for hydroelectric power generation. This resulted in a mayor energy crisis in Norway that will be discussed in more detail.

In the US the low index phase is associated with a weaker polar jet stream and consequently more cold air outbreaks occur in the Northeast. This yields to enhanced snow accumulation and overall better conditions for hydroelectric power generation.

A third case study involves precipitation in Turkey and its impact on stream flow and the irrigation dependent agriculture in Syria and the Middle East. A decade of high NAO index together with increased demand due to population growth have caused a dramatic stress on water supply to the Middle East region.

Although the prospects for skill full NAO predictions are low an improved understanding of its socioeconomic impacts will help decision makes to plan ahead. The implications of the recently apparent trend in the NAO index and its relation to "global warming" will be discussed.