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Recent findings from oceanographic field studies conducted on both sides of the North Atlantic have dramatically altered our view of how zooplankton populations respond to climate-driven changes in ocean circulation. Shelf populations of the copepod species Calanus finmarchicus dominate springtime secondary production in many shelf ecosystems throughout the North Atlantic. Despite their seasonal dominance, these shelf populations must be supplied by advection from oceanic sources or else they would be driven to local extinction. It now appears that the supply of external recruits is not only important to the long-term persistence of these shelf populations, but also central to their dynamics on seasonal to interdecadal time scales. Here, we present a synthesis of findings from the U.S. Global Ocean Ecosystem Dynamics (GLOBEC) Northwest Atlantic Field Study. This synthesis reveals that climate-driven changes in the circulation of the Northwest Atlantic can account for a large amount of the variability in C. finmarchicus abundance observed in the Gulf of Maine/Georges Bank region during recent years and over the past half century.