Arctic Oscillations

The climate of the Arctic is also periodically influenced by driving forces that are cyclic or quasicyclic (e.g., sunspots and El Nino-Southern Oscillation). One of these climate-forcing cycles is the North Atlantic Oscillation (NAO), which has major impacts on the weather of northern Europe, Russia, and even Central Asia. The NAO is an oceanographic switch in the North Atlantic that apparently modifies atmospheric pressure gradients and causes changes in the climate. Typically, cyclonic circulation over Iceland and anti-cyclonic conditions near the Azores produce strong westerly winds across the latitudes in between. These westerly winds blow over warm Gulf Stream waters, as they did in the NAO high-index years in 1980-1995, delivering heat to Eurasia and creating unusually mild winters there. The oscillations have been observed to affect animal and plant populations in the UK and Norway.

When the NAO swings to the other extreme, as it did in 1995, air pressure builds up in Iceland, weakening the pressure gradient and reducing the warm airflow to Eurasia. The effects in 1995/1996 were a very cold winter in Europe and unusual calm near Greenland. In 1996/1997, the NAO reverted to neutral again. Although the shifts in the NAO over the past century and their effects on climate have been observed in detail, the origins of the oscillations are still largely unknown. Some hypotheses have associated them with the occurrence of anomalously cold or warm water masses. On the Pacific side, there is a similar oscillation called the Pacific Decadal Oscillation (PDO), and the NAO and PDO have more recently been identified as linked phenomena, associated with a broader Arctic Oscillation (see Climate Oscillations).

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