A framework is developed for examining spatial patterns of interannual vari
ability in springtime chlorophyll concentrations as a response to physical
changes. A simplified, two-layer bio-physical model reveals regional respon
ses to interannual variability of convective mixing. Vertical mixing can pr
omote productivity in the surface waters through enhanced nutrient supply,
but also can retard productivity due to the transport of phytoplankton belo
w Sverdrup's critical depth. The balance of these processes determines the
regimes of response in the two-layer model. The regimes may be identified b
y 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 s
implified model are found in a more sophisticated four-compartment, nitroge
n-based ecosystem model, driven by a general circulation model of the North
Atlantic. Anomalously strong convective mixing leads to enhanced chlorophy
ll 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 mix
ed layers are deeper (h(c)/h(m) much less than 1), the sensitivity to conve
ctive mixing is weaker, and increased mixing can lead to lower phytoplankto
n abundances. The numerical model also reveals regions of more complex beha
vior, 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, Oce
an Weather Station "India" and the Coastal Zone Color Scanner also show qua
litative agreement. The conceptual framework provides a tool for the analys
is of ongoing remote ocean-color observations. (C) 2001 Elsevier Science Lt
d. All rights reserved.