  We estimate the frequency of Saharan mineral dust outbreak events observed
over the North Atlantic island of La Palma based on in situ nightly atmospheric
extinction measurements recorded almost continuously since 1984 by the
Carlsberg Meridian Telescope at the Roque de los Muchachos observatory. The
outbreak frequency shows a well-defined seasonal peak in the months of July to
September, during which time the occurrence of Saharan dust events (SDEs) is
approximately 28\pm6%. We find considerable year-to-year variability in the
summertime SDEs frequency, observing a steady reduction between 1984 and 1997,
followed by a period of relative mean stability from 1999 to 2012. We
investigated changes in the atmospheric extinction of the SDEs as an indicator
of strength of the episodes and found that this parameter approximately follows
the SDE frequency, however, instrumental limitations prevented us from deriving
precise conclusions regarding their long-term changes. A lagged correlation
analysis between SDE properties and the El Ni\~{n}o Southern Oscillation
(ENSO), North Atlantic Oscillation (NAO), and Sahel rainfall index (SRI) was
performed. We found that 55\pm4% of the year-to-year variations in
July-September SDE frequency may be reproduced by a lagged relationship to the
NAO conditions during the preceding October-December period, and 45\pm4% may be
reproduced by a negative correlation to the SRI during the preceding
February-April period. Based on these relationships it may be possible to
obtain an approximate indication of the strength of the upcoming summertime
dust season over the North Atlantic around half a year in advance.
