Low frequency winter rainfall variability in the north west of Morocco

Abstract

The main characteristic of the rainfall climate in the North West of Morocco is its strong interannual variability which is due to its eccentric latitudinal position on the fringe of the North Atlantic mid-latitude baroclinic region. Linked to the North Atlantic large scale circulation variation and winter extra-tropical disturbance, the rainfall events in our region are closely connected to the NAO regime as the dominant winter mode of low-frequency variability over the North Atlantic sector. The swings between one phase to another of this regime produce large changes in storminess and precipitation over our region. The temporal rainfall evolution over the last 60 years shows two characteristic sub-periods: the first one is dominated by more rainy winters from 1950-1951 to 1978-1979, and the second one has frequently experienced more dry winters, particularly between 1979-19 80 and 2008-2009 usually with many persistent and severe long drought episodes. Both sub-periods exist in a causal relationship with one of the two phases of the NAO. In the first sub-period, under the effect of a negative phase of this pattern, a southward shift in the storm-track is produced, which can easily reach our latitude and give rise to rainier winters. The second sub-period is subject to an increase in frequent persistent and intensive positive phase events, which generate enhance a northward shift of the mid-latitude storm-track and deficiency conditions, and which can affect our region substantially.