Zhongwei Yuan, Thomas J. Browning, Chuanjun Du, Hui Shen, Lei Wang, Yifan Ma, Zong-Pei Jiang, Zhiyu Liu, Kuanbo Zhou, Shuh-Ji Kao, Minhan Dai

Published in Journal of Geophysical Research, November 2023

Mesoscale eddies are common in the subtropical Northwest Pacific, however, relatively little is known about their spatial variability and temporal evolution, and how these impact upper ocean biogeochemistry. Here we investigate these using observations of a cyclonic eddy carried out along four sequential transects. Consistent with previous observations of cyclonic eddies, the eddy core had doming isopycnals, bringing elevated nutrient waters nearer to the surface. However, we also found that the upper layer of the eddy above the nutricline had significantly lower phosphate concentrations within its core relative to its edge. We attributed this to elevated N2 fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy-enhanced N2 fixation was additionally supported by the elevation of nitrate + nitrite to phosphate ratios below the euphotic zone. Moreover, we observed that while the upward displacement of isopycnals within the eddy core led to an increase in phytoplankton biomass in the lower euphotic zone, there was no significant increase in total phytoplankton biomass across the entire euphotic zone. Cyclonic eddies in the subtropical North Pacific are projected to be becoming more frequent, implying that such dynamics could become increasingly important for regulating nutrient biogeochemistry and ultimately productivity of the region.

Fig. Study area and the evolution of Eddy E2. (a) Map of the western North Pacific showing the sampling locations (open circles) with the context of satellite-derived chlorophyll a in April 2019. Blue line indicates the trajectory of the center of Eddy E2 within the whole lifespan. Boxed regions identify the study locations. (b) Sea level anomaly (SLA) of the center of Eddy E2 within the whole lifespan. The shaded bars indicate the periods of four observations. (c–f) Surface distribution of SLA and the satellite-derived geostrophic current velocities corresponding to the date when sampling campaigns were conducted at the eddy core. Black dots represent the sampling stations along the transects.

Yuan, Z., Browning, T. J., Du, C., Shen, H., Wang, L., Ma, Y., et al. (2023). Enhanced phosphate consumption stimulated by nitrogen fixation within a cyclonic eddy in the Northwest Pacific. Journal of Geophysical Research: Oceans, 128, e2023JC019947. https://doi.org/10.1029/2023JC019947