The North Atlantic is characterized by diatom-dominated spring blooms that results in significant transfer of carbon to higher trophic levels and the deep ocean. These blooms are terminated by limiting silicate concentrations in summer. Numerous regional studies have demonstrated phytoplankton community shifts to lightly-silicified diatoms and non-silicifying plankton at the onset of silicate limitation. However, to understand basin-scale patterns in ecosystem and climate dynamics, nutrient inventories must be examined over sufficient temporal and spatial scales. Here we show, from a new comprehensive compilation of data from the subpolar Atlantic Ocean, clear evidence of a marked pre-bloom silicate decline of 1.5–2 µM throughout the winter mixed layer during the last 25 years. This silicate decrease is primarily attributed to natural multi-decadal variability through decreased winter convection depths since the mid-1990s, a weakening and retraction of the subpolar gyre and an associated increased influence of nutrient-poor water of subtropical origin. Reduced Arctic silicate import and the projected hemispheric-scale climate change-induced weakening of vertical mixing may have acted to amplify the recent decline. These marked fluctuations in pre-bloom silicate inventories will likely have important consequences for the spatial and temporal extent of diatom blooms, thus impacting ecosystem productivity and ocean-atmosphere climate dynamics.

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A global climate scenario run based on the IPCC future emission scenario SRES A1B was dynamically downscaled for the North Sea, using an uncoupled meso-scale model system. The atmospheric forcing shows an increase in air temperature during the 21st century with stronger warming in winter than in summer. These trends are reflected by an increase in annual mean sea surface temperature of ~1.8°C. The strong warming in winter is the main driver for the total warming of the North Sea, as indicated by the analysis of surface heat flux components. The thermocline weakens in intensity according to the changing temperature profiles, and its mean depth slightly decreases due to lower wind speeds. The salinity in the North Sea drops by ~0.6 psu because of decreasing salinity of the inflowing Atlantic waters, increasing Baltic discharge, and higher downward surface fresh water fluxes. Some of our results disagree with other dynamical downscaling studies, as the results depend on the parent global model. In general, the downscaled projected trends qualitatively agree with the results of the coarser global model. Nevertheless, in particular, the magnitudes of the surface heat fluxes, the formation of the frontal system, and the strength and structure of the general circulation are more realistically represented in the regional meso-scale model. KEY WORDS: Dynamical downscaling · North Sea · IPCC SRES A1B · Climate changeResale or republication not permitted without written consent of the publisher CLIMATE RESEARCH

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