Decoupling spatial and temporal variability of CH4, N2O and CO2 concentrations in a coastal upwelling zone using high-frequency ship-board measurements and numerical analysis.

We used continuous ship-board measurements of surface water pCO2, CH4 and N2O, in combination with depth profile analysis and numerical model output, to examine the factors driving greenhouse gas variability in the California upwelling system. The highest gas concentrations (682% CH4 saturation, 220% N2O saturation and ΔpCO2 of 943 µatm) were observed in the northern upwelling region, near Cape Blanco, with significantly lower values south of Cape Mendocino (average values of 160% CH4 saturation, 109% N2O saturation and ΔpCO2 of 228 µatm). Stable carbon isotopic data identify variable water mass contributions to sub-surface source waters and suggest that surface water CO2 distributions were controlled by sea-air exchange & primary productivity. Surface distributions of N2O were driven by upwelling of sub-surface waters, where N2O was mainly produced through nitrification. Relative to CO2 and N2O, sources of CH4 are more varied, with contributions from biogenic and thermogenic inputs, as well as a potential phytoplankton source in the upper water column. Using backward trajectory analysis, we show that coastal circulation patterns north of Cape Mendocino were consistent with expected trends for the coastal upwelling, while 2 surface waters south of the cape consistently flowed northward, even during periods with consistent northerly winds. This local circulation acted to offset the effects of upwelling, and decreased surface water gas concentrations. Our work highlights the utility of high-resolution measurements and numerical models to disentangle spatial and temporal variability in dynamic coastal waters. We discuss our results in the context of projected future climate change.