The North Atlantic Oscillation: Effects on freshwater systems in Ireland

The North Atlantic Oscillation (NAO) is a winter phenomenon in which the north-south contrast in barometric pressure in the Atlantic shows interannual variability with approximately decadal cycles. Positive NAO index values are associated with increased wind speeds, temperatures and rainfall in northern Europe. The present study explored the possible impacts of the NAO in the west of Ireland. Data were obtained from two synoptic meteorological stations (Valentia, Co. Kerry, and Belmullet, Co. Mayo) and two lake systems (Lough Leane, Co. Kerry, and Lough Feeagh, Co. Mayo). Mean winter air temperature, wind speed, rainfall, relative humidity and cloud amount were all found to be positively related to the NAO index values. In contrast, the relationship between solar radiation and the NAO index value was negative. Interannual variation in surface water temperatures and soil temperatures was also found to be highly dependent on the NAO index value. An inverse relationship was found between winter chlorophyll in Lough Leane and the NAO. The processes that might link winter phytoplankton biomass to the NAO were not identified. However, the results establish that the signal of the NAO is discernible in lacustrine biological activity in the west of Ireland.