Inter-annual variability in the timing of stratification and the spring bloom in the North-western North Sea

A physical–biological 1D numerical model, forced by tidal currents and observed meteorology, has been used to simulate the inter-annual changes in the timing of spring stratification and the spring phytoplankton bloom between 1974 and 2003 in the Marr Bank region of the North-western North Sea. The model successfully simulated the observed long-term variability and warming trend of water temperatures in the region, indicating that the oceanographic climate is influenced primarily by local meteorology rather than by inflows of water from the NE Atlantic Ocean. The spring-neap tidal cycle is shown to affect the timing of the onset of stratification and the spring bloom, with established spring stratification usually beginning as tidal currents decrease from springs to neaps. Spring-neap tidal variability can also produce double spring blooms when stratification that develops after a neap tide is temporarily broken down by the increasing tidal mixing towards the next spring tide. Over the 30 years of model simulations the dominant meteorological control on the timing of the spring stratification and bloom was the spring air temperature. However, there is evidence that control by wind stress variability on the timing of spring stratification was more important before the early 1990s (r2 = 0.32, significant at 95%). After the early 1990s the wind stress was not significantly correlated with the timing of spring stratification, and the spring air temperature became the main control (r2 = 0.33, significant at 95%). Increasing air temperature also appears to have driven a gradual trend in the timing of stratification and the spring bloom since the mid 1990s of an average 1 day earlier per year (r2 = 0.27, significant at 95%). The link between the spring meteorology and the North Atlantic Oscillation (NAO) was investigated, indicating that the NAO played a role in the timing of stratification and the spring bloom prior to 1990, with a significant correlation between the NAO and the spring wind stress, but has had less of a role since 1990, with only a weak connection between the NAO and the spring air temperature.