Mini mooring and Wirewalker data collected over the Southern California Bight during winter 2020

Wind-driven coastal upwelling delivers nutrient to the surface waters and
enhances primary productivity. In contrast, downwelling transports
nutrient-depleted water away from the surface and is usually presumed to
decrease primary production. Here, we show that processes during
downwelling can actually pump nutrients into previously nutrient-depleted
subsurface layers to fuel primary production. Our in-situ data collected
from the Southern California Bight showed that downwelling-favorable winds
deepened near-surface isopycnals to the bottom boundary layer, followed by
a rapid nitrate increase (4 uM within days) on those isopycnals. This
increase in nitrate and changes in temperature and salinity were
consistent with elevated diapycnal mixing. Excess nitrate was subsequently
advected from the boundary into the ocean interior, replenishing offshore
concentrations. Together, these findings reveal a mechanism of nutrient
redistribution by wind forcing, marginal mixing, and along-isopycnal
transport, highlighting the importance of bottom boundary layer mixing and
lateral exchange to nutrient dynamics in wind-forced regions.