Analysis of longitudinal variations in North Pacific alkalinity to improve predictive algorithms

The causes of natural variation in alkalinity in the North Pacific surface ocean need to be investigated to understand the carbon cycle and to improve predictive algorithms. We used GLODAPv2 to test hypotheses on the causes of three longitudinal phenomena in Alk*, a tracer of calcium carbonate cycling. Thesephenomenaare (a)an increase fromeast towestbetween45°Nand55°N, (b) an increase fromwest toeast between25°N and40°N, and (c) aminor increase fromwest to east in the equatorial upwelling region. Between 45°N and 55°N, Alk* is higher on the western than on the eastern side, and this is associated with denser isopycnals with higher Alk* lying at shallower depths. Between 25°N and 40°N, upwelling along the North American continental shelf causes higher Alk* in the east. Along the equator, a strong east-west trendwas not observed, even though the upwelling on the eastern side of the basin is more intense, because the water brought to the surface is not high in Alk*. We created two algorithms to predict alkalinity, one for the entire Pacific Ocean north of 30°S and one for the easternmargin. The Pacific Ocean algorithm is more accurate than thecommonlyusedalgorithmpublishedbyLeeetal. (2006), of similaraccuracy to thebestpreviouslypublished algorithm by Sasse et al. (2013), and is less biased with longitude than other algorithms in the subpolar North Pacific. Our eastern margin algorithm is more accurate than previously published algorithms.