Spitz, Yvette H., Mark R. Abbott and James G. Richman

Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis OR 97331, Tel: 541-737-3227, Fax: 541-737-2064, E-mail: yvette@coas.oregonstate.edu

Before and after 1996 in the North Pacific basin

The long term times series observations at the Hawaii Ocean Time series (HOT) station show that the ecosystem underwent some changes in 1996. From 1988 (beginning of the time series observations) until 1996, we observe a continuous slow decrease of chlorophyll a in the upper 140 m while a continuous increase of the DOC stock is present. In 1996, an abrupt increase of chlorophyll a is observed, followed by a constant annual chlorophyll a until the present time. The DOM pool stops increasing after 1996, which indicates that production and consumption are in balance. We also note that primary productivity does not display any change from before 1996 to after 1996. The causes of the changes in the various pools of nitrogen, carbon and phosphorus are not yet well understood. Based on the results of a coupled ecosystem model with an Ocean General Circulation Model (OGCM) for the North Pacific basin and with a one-dimensional mixed-layer model, we can speculate that these changes are due to non-local effects. Indeed, while the changes can be reproduced in a 3D mode, they cannot be reproduced in a 1D mode. In this study, we will analyze in details the causes of the 1996 changes in terms of the North Pacific circulation as well as of the ecosystem response.