Transient upwelling hot spots in the oligotrophic North Pacific

Vertical exchange between deep, nutrient-rich and nutrient-depleted surface waters controls the physical supply of nutrients into the euphotic zone in the oligotrophic waters of the subtropical North Pacific Ocean. Sea surface temperature and ocean color satellite images are used in this study to show that submesoscale chlorophyll patches are locally intensified. These patches are associated with surface temperature fronts and regions of strong stretching of the eddying flow. The latter are identified by calculating the finite-size Lyapunov exponents of the geostrophic flow derived from the sea surface height field from altimetry. Surface frontogenesis and nonlinear Ekman pumping (NLEP) arise as natural candidates for the sporadic generation of large vertical velocities that could favor the formation of these features. A high-resolution regional numerical model simulation confirms that large vertical velocities are indeed associated with strong density fronts and gradients of relative vorticity. Using the Q vector divergence as a proxy for frontogenetically generated vertical velocities yields good agreement with the model vertical velocities, particularly in regions where they are large. The pattern and magnitude of NLEP suggests that it is also contributing to the strong vertical motions, although NLEP and model vertical velocities are not necessarily in phase. Surface frontogenesis and NLEP provide a possible explanation for the episodic injection of nutrients at the submesoscale, which are then horizontally stirred and modulated by the unstable manifolds of the flow field. Given that these surface-intensified processes would be biogeochemically more effective when the pycnocline is shallow, they could also serve as one possible explanation for the late summer chlorophyll blooms that are known to occur in the region.