Water mass analysis of the 2018 US GEOTRACES Pacific Meridional Transect (GP15)

A water mass analysis is a tool for interpreting the effect of ocean mixing on the distributions of trace elements and isotopes (TEI’s) along an oceanographic transect. The GEOTRACES GP15 transect along 152°W covers a wide range in latitude from Alaska to Tahiti. Our objective is to present the nutrients and hydrography of GP15 and quantify the distributions of water masses to support our understanding of TEI distributions along GP15. We used a modified Optimum Multiparameter (OMP) analysis to determine the distributions of water masses with high importance to nutrient and hydrographic features in the region. In the thermocline, our results indicated the dominance of Pacific Subarctic Upper Water (PSUW) in the subpolar gyre, Eastern North Pacific Central Water (ENPCW) in the northern subpolar gyre, and Equatorial Subsurface Water (ESSW) in the equatorial region. South Pacific Subtropical Water (SPSTW) dominated the top of the thermocline in the southern subtropical gyre, while South Pacific Central Water (SPCW) dominated the lower thermocline. Antarctic Intermediate Water (AAIW), Equatorial Intermediate Water (EqIW), and North Pacific Intermediate Water (NPIW) in the southern hemisphere, equatorial region, and northern hemisphere, respectively, occupied waters just below the thermocline. Dominant water masses in the deep waters of the southern hemisphere include Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) with minimal contributions from Antarctic Bottom Water (AABW). Pacific Deep Water (PDW) dominated the deep water in the northern hemisphere. Our results align well with literature descriptions of these water masses and related circulation patterns.