Persistent chlorophyll maxima in the Eastern Tropical North Pacific

TL;DR

This study investigates how physical oceanographic processes, especially Long Rossby Waves, influence persistent chlorophyll maxima in the Eastern Tropical North Pacific, revealing their responses to nutrient and light variations.

Contribution

It provides new insights into the mechanisms by which Rossby Waves and related physical processes modulate chlorophyll maxima and nutrient dynamics in the ETNP.

Findings

Shallower chlorophyll maximum intensifies with nutrient-rich denser waters.

Deeper chlorophyll maximum is mechanically displaced and less affected by nutrients.

Rossby Waves influence thermocline and nitracline positions, affecting phytoplankton growth.

Abstract

This study aims to describe the response of two persistent chlorophyll-a maxima to physical processes affecting the thermocline and nitracline position in the Eastern Tropical North Pacific (ETNP). We focused on Long Rossby Waves given their relevance to the ETNP circulation and its potential as a mechanism introducing nutrients into the euphotic zone. We found the shallower chlorophyll-a maximum in oxygenated waters became more intense when denser waters (more nutrients) moved toward the surface. It suggests that isopycnals and nitracline displacements modify the nutrient supply in the euphotic zone, which produces changes in phytoplankton growth. The suboxic and deeper chlorophyll a maximum showed a strong association with the 26 kg m^{-3} isopycnal being only mechanically displaced, and its chlorophyll-a content does not seem to covary with irradiance or nutrients. The different responses of the chlorophyll-a maxima could be explained if different phytoplankton groups are associated with them. Long Rossby Waves can affect the position of the thermocline, nitracline and isopycnals in an annual cycle, but it seems to be a background signal modulated by higher frequency processes such as mesoscale eddies and other Rossby waves. The co-occurrence of processes can control the nitracline depth, and hence the input of nutrients into the euphotic zone that can cause sporadic enhancements of the chlorophyll-a concentration of one maximum.