The influence of large-scale environmental changes on carbon export in the North Pacific Ocean using satellite and shipboard data

Abstract

The subarctic Pacific Ocean experiences strong climate-modulated seasonal, interannual to decadal variations in meteorological and physical oceanographic conditions, which can have a profound influence on biological processes and carbon cycling in the region. Inorganic nitrate, a major nutrient controlling phytoplankton growth, is key to understanding the export of organic matter out of the euphotic zone. Its supply to the region is driven largely by winter convective mixing. Using satellite data for a 5-year period beginning in 1997, we provide evidence of strong interannual variations in the supply of inorganic nitrate and new production in the subarctic Pacific in association with the El-Nino of 1997 and the transition to La-Nina conditions thereafter. These satellite based climatologies allowed us to view and describe large changes in nitrate distribution and new production along the entire breadth of the subarctic Pacific basin. In addition, our accessibility to a 25-year database of shipboard measurements focused primarily in the Oyashio waters, a region representative of the western subarctic Pacific, enabled us to demonstrate that El-Nino/La-Nina changes in this region differed from those observed in the eastern subarctic Pacific. Thus, in addition to the primary motive of verifying the changes that we observed in our satellite-derived maps, this exercise allowed us to obtain a clear picture of the mechanistic connections between the atmosphere and the oceans and the biological response to these changes. The results from this study make a compelling case that the primary driver for the observed interannual variations in biological production in the western subarctic Pacific is the strength of the wintertime monsoonal winds. This anomalous intensification of the southeastward wind stress appears to be particularly strong during El-Nino years when the Aleutian Low intensifies and moves southeastwards, causing disturbances in the pressure gradient between the Siberian high and the Aleutian Low. An abrupt shift in oceanographic conditions follows this change in pressure gradient, among them the most prominent being a reduction in sea surface temperature, a southward migration of the belt of zero wind-stress curl, and the anomalous southward penetration of the Oyashio Current. In tandem, these changes, contribute to an increase in nutrient inputs in winter and a southward displacement of the boundary of the subarctic gyre. The spring following an El-Nino event is characterized by reduced wind stress and a resulting increase in water column stability as well as the elevated solar radiation leads to blooms. Conversely, in the winter of 2000, the subarctic gyre experienced the weakest winds of all 5 years. These weak wind conditions were associated with increased wind speeds in spring of that year and the lowest annual rates of new production of all 5 years.