Interannual variability of phytoplankton abundances in the North Atlantic

A framework is developed for examining spatial patterns of interannual variability in springtime chlorophyll concentrations as a response to physical changes. A simplified, two-layer bio-physical model reveals regional responses to interannual variability of convective mixing. Vertical mixing can promote productivity in the surface waters through enhanced nutrient supply, but also can retard productivity due to the transport of phytoplankton below Sverdrup's critical depth. The balance of these processes determines the regimes of response in the two-layer model. The regimes may be identified by the ratio of the thickness of Sverdrup's critical layer during spring and the end of winter mixed layer, h(c)/h(m). The responses predicted by the simplified model are found in a more sophisticated four-compartment, nitrogen-based ecosystem model, driven by a general circulation model of the North Atlantic. Anomalously strong convective mixing leads to enhanced chlorophyll concentrations in regions of shallow mixed layers (h(c)/h(m) similar to 1), such as the subtropics. In contrast, in the subpolar regions, where mixed layers are deeper (h(c)/h(m) much less than 1), the sensitivity to convective mixing is weaker, and increased mixing can lead to lower phytoplankton abundances. The numerical model also reveals regions of more complex behavior, such as the inter-gyre boundary, where advective supply of nutrients plays a significant role on interannual timescales. Preliminary analyses of in situ and remote observations from the Bermuda Atlantic Time-Series, Ocean Weather Station "India" and the Coastal Zone Color Scanner also show qualitative agreement. The conceptual framework provides a tool for the analysis of ongoing remote ocean-color observations. (C) 2001 Elsevier Science Ltd. All rights reserved.