Meteorological modulation of the North Atlantic spring bloom

Abstract

Using ocean time-series observations and remote chlorophyll estimates derived from SeaWiFS ocean-color observations, we examine and illustrate the relationships between changes in the intensity of the spring bloom and changes in weather patterns, mediated by upper-ocean mixing. A simplifed two-layer model provides the conceptual framework, predicting regional-regimes of differing biological response to vertical mixing anomalies in the ocean-surface boundary layer. The meteorological anomalies may be derived from re-analyzed meteorological data. We examine two regimes of regional and interannual sensitivity to meteorological forcing, defined by the ratio of the spring critical layer depth and the winter mixed layer depth, hc/hm. Regions of large hc/hm (subtropics) are characterized by an enhanced bloom in response to enhanced mixing, both across the region and from year to year. The subtropics exhibit consistent, interannual changes that are coordinated over large regions, and local interannual changes are comparable in magnitude to the regional variations in each bloom. In the low hc/hm regime (subpolar), regional variations reflect retardation of the bloom by enhanced mixing. Local interannual changes in the subpolar region, however, are small relative to the regional variations and do not show a clear and consistent response to interannual variability in the local meteorological forcing. We infer that other factors, including changes in insolation, local mesoscale variability, and grazing exert a stronger infuence on local interannual variability of the subpolar bloom. We discuss the implications of these relationships for the implications of decadal climate changes on biological productivity.