Simulation of the planktonic ecosystem response to pre- and post-1976 forcing in an isopycnic model of the North Pacific

• Published in: Canadian Journal of Fisheries and Aquatic Sciences
• Main Authors: Haigh, S P, Denman, K L, Hsieh, W W
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2001
• Subjects: Aquatic Science; Ecology, Evolution, Behavior and Systematics; Pacific; Subarctic
• Online Access: http://dx.doi.org/10.1139/f01-010; http://www.nrcresearchpress.com/doi/pdf/10.1139/f01-010

To investigate the hypothesis that the 1976 "regime shift" in North Pacific fish populations resulted from climatic change propagating up the fisheries food web, we have embedded a four-component planktonic ecosystem model in an ocean general circulation model. The Miami isopycnic model (MICOM) has been implemented on a 2° grid over the domain from 18°S to 61°N, with a Kraus–Turner-type mixed layer model overlaying 10 isopycnal layers. An initial baseline run with forcing for the period 1952–1988 reasonably reproduces the spatial patterns and seasonal changes in SeaWiFS images. Estimates of annual net and export production compare well with contemporary observations of primary and export production at Ocean Station Papa in the subarctic North Pacific but are low by a factor of 8–10 at station ALOHA near Hawaii. Two subsequent runs with forcing for the periods 1952–1975 and 1977–1988 show the main gyres to strengthen after 1976 with large areas of increased mixed layer depth. In the light-limited subarctic, limited areas of shallower spring mixed layer produced increased phytoplankton biomass, whereas in the nutrient-limited subtropical gyre, increased nutrients (or migration of the subarctic front and the equatorial current system into the gyre) after 1976 correlated with increased plankton biomass.