Mesoscale variability of phytoplankton and of new production: Impact of the large‐scale nutrient distribution

Concomitance at the mesoscale between doming isopycnals and phytoplankton maxima is often observed. A different situation is sometimes revealed by satellite images, where chlorophyll varies at the submesoscale. These two situations are rationalized using numerical simulations of interactive oceanic eddies, in the standard situation where they result from baroclinic instability. The driving parameter is found to be the nutrient distribution in the region where the eddies are formed. It is shown that the existence of a large‐scale horizontal gradient is a sufficient condition for observing phytoplankton maxima within the eddies. The proposed mechanism is based on the direct horizontal cascade and involves horizontal transport. This mechanism is different from eddy pumping, which involves vertical transport, but leads to similar distributions of nutrients, phytoplankton, and density at the eddy scale. When vertical gradients are dominant, the regime is that of small‐scale nutrient injection. In this regime, eddy‐induced upwelling and associated nutrient transport occur on a scale one order of magnitude below the eddy scale. In both regimes, new production resembles vorticity and displays energetic small‐scale features. On the contrary, phytoplankton distribution shows different characteristics depending on the regime. In the absence of horizontal gradients, submesoscale upwelling and subsequent stirring lead to enhanced phytoplankton in vorticity filaments. Otherwise, when horizontal gradients prevail, phytoplankton is primarily driven by the horizontal cascade and resembles the mesoscale temperature field. Another important result is that small‐scale subduction can contribute to the export and decrease new production.