Influence of the North Atlantic Oscillation on the Canary Islands precipitation

Abstract

The aim of this paper is to investigate the relationship between the Canary Island rainfall and the Atlantic large-scale circulation, characterized by the North Atlantic oscillation (NAO) index. The Canary Islands are located in the Atlantic subtropical belt under the direct influence of the Azores high and the trade winds. Their steep orography makes the islands very sensitive to small variations in a synoptic situation, thus providing an excellent natural observatory for the North Atlantic variability associated with changes in pressure patterns. A significant relationship between rainfall and the NAO is found for five of the seven Canary Islands. In order to characterize the physical mechanisms involved, a set of automatic objective techniques for identification and detection of disturbances is applied to the 1000- and 500- hPa geopotential from the NCEP-NCAR reanalysis for the period from 1955 to 1998. This method allows for the identification and detection of four main synoptic systems-either at the surface or at 500 hPa-that are responsible for 80% of the precipitation over the Canary Islands: Atlantic surface lows (ASLs), 500-hPa lows (UALs), 500- hPa troughs over the Canary Islands (TROs), and deep Atlantic lows (DALs), which affect the entire troposphere. Three of the detected disturbances (ASLs, TROs, and DALs) exhibit a dipolar structure with a low pressure center over 35 degreesN associated with a positive anomaly at higher latitudes, resembling the NAO negative pattern. The analysis of the variability of the disturbance occurrence and the changes in their associated anomaly patterns shows that deeper and more frequent ASLs and TROs affect the Canary Islands during the negative phase of the NAO. However, UAL disturbances are less frequent, and DALs do not exhibit significant variations with the NAO phase. The standard deviation of the 2.5-8-day bandpass-filtered geopotential height shows that the maximum variability associated with the NAO occurs over the Canary Island area, confirming its sensitivity to NAO variations. The study provides a comprehensive view of the mechanisms involved in the precipitation generation over the Canary Islands, documenting a sensitivity to the NAO influences for a group of islands that have been poorly studied so far.