Do submesoscale processes inject nitrate pulses into
the euphotic zone around station ALOHA?


The upper ocean around station ALOHA, 100 km north of Oahu, is on average depleted in nutrients but it sustains nonetheless a net primary production. Several mechanisms have been proposed to explain how nutrient can be made available within the euphotic zone. The two main mechanisms are the fixing of atmospheric nitrogen by organisms and the episodic vertical lifting of nutrients by mesoscale (100-200 km radius) eddies. The estimated contribution of these mechanisms, however, does not completely close the nutrient budget.

Recently, it has been suggested that surface-intensified submesoscale processes with scales of 1 day and 1-10 km might provide the rest of the necessary nutrients. This hypothesis is supported by idealized numerical simulations and is a priori consistent with pulses of elevated shallow nitrate measured recently by four profiling floats.

In this talk, we review the literature in the context of the oceanic regime specific to station ALOHA. Then we analyze the float observations in conjunction with satellite sea surface height (SSH) and a high-resolution time series of the density field from a mooring. Output from a high-resolution numerical simulation of the ocean around the Hawaiian Archipelago provides additional insight.

We conclude that the submesoscale processes may contribute to the vertical entrainment of nitrate into the surface mixed layer from January to April but are not the cause of the deeper nitrate pulses. We propose that the pulses are instead the signature of the convoluted shape of mesoscale eddies.