SEASONAL-TO-DECADAL VARIABILITY OF THE PACIFIC NORTH EQUATORIAL CURRENT:
A DOWNSTREAM PREDICTABILITY AND IMPACTS ASSESSMENT
Roger Lukas
Department of Oceanography
University of Hawaii
1000 Pope Road
Honolulu, Hawaii 96822
USA
ABSTRACT
It is well-established that the substantial westward mass and heat transport of the North Equatorial Current (NEC) in the western Pacific Ocean is subject to pronounced seasonal and year-to-year variations. Some of these variations are caused by local wind forcing, such as the monsoon-related variations of the western North Pacific trade winds. Some variations are forced by remote seasonal and interannual wind variability in the central and eastern North Pacific Ocean. Still oilier variations may be associated with variable buoyancy forcing of the subtropical ocean, and with the associated shallow overturning cell connecting the subtropical and tropical gyres.
Much less certain are the impacts of this variability on downstream ocean circulation elements, which include the Kuroshio, the Mindanao Current, the North Equatorial Countercurrent, the Indonesian Throughflow, and the circulation of the South China Sea. Even less certain are the impacts of related ocean beat content and transport variability on the regional monsoon circulations in the atmosphere. Models and observations suggest that the downstream impacts of NEC variability are complicated by the processes controlling the splitting of the NEC at the coast of the Philippine Islands.
In order to observe, understand and model these important processes, it is proposed to begin a multi- year process study under CLIVAR with the overall objective of determining the predictability of downstream ocean circulation variations associated with NEC seasonal-to-decadal variability and the associated impacts on the Asian monsoons. The field program would consist of enhanced western Pacific monitoring and its component currents, with special focus on the NEC bifurcation region. Short-term, intensive observing periods might be appropriate to obtain additional observations of critical processes. A modeling program is proposed to parallel the field program, with the objective of capturing all regional predictability inherent in the natural coupled ocean-atmosphere-land system.
This process study would provide a reasonable oceanographic emphasis under CLIVAR/GOALS to complement the ongoing GEWEX Asian Monsoon Experiment, and the planned South China Sea Monsoon Experiment.
INTRODUCTION
The western Pacific/East Asia region has a disproportionate influence on global climate because the heating of the atmosphere associated with the heavy rains of the Asian-Australian monsoon system drives mean and variable global-scale circulations (Yasunari and Seki, 1992). For example, the fluctuations of the monsoon rains strongly affect the midlatitude jet stream in the upper atmosphere of the winter hemisphere, and these impacts ripple all the way around the planet. The western Pacific is also a major source for the interannual variations of the global climate system associated with the El Nino/Southern Oscillation (ENSO) phenomenon, which affects the ocean and atmosphere throughout the entire planet.
The monsoon system is a coupled ocean-atmosphere-land phenomenon. Thermal contrasts between the land masses (including the Maritime Continent) and the adjacent western Pacific and Indian Oceans (Li
