Impact of vertical and horizontal advection on nutrient distribution in the southeast Pacific

Bàrbara Barceló-Llull,Ananda Pascual,Evan Mason,Arthur Capet

doi:10.5194/os-12-1003-2016

Abstract

Abstract. An innovative approach is used to analyze the impact of vertical velocities associated with quasi-geostrophic (QG) dynamics on the redistribution and uptake of nitrate in the southeast Pacific (SEP). A total of 12 years of vertical and horizontal currents are derived from an observation-based estimate of the ocean state. Horizontal velocities are obtained through the application of thermal wind balance to weekly temperature and salinity fields. Vertical velocities are estimated by integration of the QG omega equation. Seasonal variability of the synthetic vertical velocity and kinetic energy associated with the horizontal currents is coincident, with peaks in austral summer (November–December) in accord with published observations. The impact of vertical velocity on SEP nitrate uptake rates is assessed by using two Lagrangian particle tracking experiments that differ according to vertical forcing (ω = ωQG vs. ω = 0). From identical initial distributions of nitrate-tagged particles, the Lagrangian results show that vertical motions induce local increases in nitrate uptake reaching up to 30 %. Such increases occur in low uptake regions with high mesoscale activity. Despite being weaker than horizontal currents by a factor of up to 10−4, vertical velocity associated with mesoscale activity is demonstrated to make an important contribution to nitrate uptake, hence productivity, in low uptake regions.

Highlights

Mesoscale dynamics make an important contribution to biogeochemical cycles through the redistribution of nutrients and passive marine organisms by both horizontal advection and vertical exchange
Horizontal geostrophic velocity and QG vertical velocity are computed from ARMOR3D temperature and salinity in the southeast Pacific
Vertical velocity and kinetic energy in the southeast Pacific (SEP) have similar and intense seasonal variability with maximums in austral summer (November–December), which suggests that these quantities are mostly influenced by the seasonal modulation of Subtropical Countercurrent (STCC)–South Equatorial Current (SEC) vertical shear (Qiu and Chen, 2004; Qiu et al, 2008)

Summary

Introduction

Mesoscale dynamics make an important contribution to biogeochemical cycles through the redistribution of nutrients and passive marine organisms by both horizontal advection and vertical exchange. Frontal areas and mesoscale eddies, the vertical velocity has fundamental importance and can significantly contribute to nutrient supply in the euphotic zone (Mahadevan, 2014). The importance of vertical exchange for phytoplankton growth and chlorophyll a distributions in mesoscale oceanic eddies has been attributed to various mechanisms such as eddy pumping, eddy-induced Ekman pumping or vortex Rossby waves (McGillicuddy et al, 1998; Siegel et al, 1999; Mahadevan et al, 2012; Martin and Richards, 2001; McGillicuddy et al, 2007; Benítez-Barrios et al, 2011; Buongiorno Nardelli, 2013; Gaube et al, 2013, 2015)

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