Evidence of Episodic Nitrate Injections in the Oligotrophic North Pacific Associated With Surface Chlorophyll Blooms

Abstract

AbstractSummer blooms of chlorophyll develop in the oligotrophic North Pacific Ocean between Hawaii and 30°N. It is thought that episodic injections of subsurface nutrients fuel these blooms, but the exact mechanism is unknown. Here data from Biogeochemical‐Argo floats are examined to look for evidence of subsurface mixing that could stimulate these surface chlorophyll features. Data are examined from 25 floats that measured 5,569 profiles near Hawaii between September 2002 to April 2021. Since not all floats had nitrate sensors, but all floats had oxygen sensors, nitrate concentrations were calculated from oxygen measurements using the “CArbonate system and Nutrients concentration from hYdrological properties and Oxygen using a Neural‐network” model. There were 14 times when the nitracline, defined as the depth where nitrate is 1 µmol kg−1, reached 100 m during the summer (June–September). In six of these 14 episodes, the nitracline anomaly was the result of shoaling isopycnals. During the other eight episodes nitrate increased along isopycnal surfaces, indicating diapycnal mixing was occurring. In these events, referred to as injection events, perturbations to the vertical nitrate distribution extended all the way down to 300 m depth, and lasted up to 3.5 months, spanning hundreds of kilometer. Seven of the eight injection events occurred within cyclonic eddies, one occurred within an anticyclonic eddy. The timing of the injection events often coincided with a change in eddy dynamics, either eddy speed or eddy transformation. The injection events in cyclonic eddies were followed by a surface increase in chlorophyll in the surrounding area as observed in satellite data.