Biogeochemical variability in the California Current System

Abstract

The biological productivity and diversity of the California Current System (CCS) is at the leading edge of major emerging climate trends, including hypoxia and acidification. We present results from a hindcast simulation (reanalysis) of an eddy-resolving oceanic physicalbiogeochemical model of the CCS, to characterize its mean state and its patterns and drivers of variability in marine biogeochemical and ecosystem processes from 1995-2010. This is a companion paper to a physical analysis in Renault et al. (2019). The model reproduces longterm mean distributions of key ecosystem metrics, including surface nutrients and productivity and subsurface O 2 and carbonate undersaturation. The spatial patterns of Net Primary Productivity (NPP) are broadly consistent with measured and remotely sensed rates, and they reflect a predominant limitation by nitrogen, with seasonal and episodic limitation by F e nearshore in the central CCS, and in the open ocean northern CCS. The vertical distribution of NPP is governed by the trade-off between nutrient and light limitation, a balance that reproduces and explains the observed spatial variations in the depth of the deep Chl maximum. The seasonal to interannual variability of biogeochemical properties and rates is also well captured by model simulations. Because of the prevailing nutrient limitation, fluctuations in the depth of the pycnocline and associated nutricline are the leading single factor explaining interannual variability in the interior biogeochemical state, and the relationships between density and biogeochemical rates and tracers are consistent between model and observations. The magnitude and relationship between density structure and biogeochemical processes is illustrated by the 1997-98 El Niño event, which faithfully reproduces the single largest deviation from the mean state in the simulated period. A slower decadal shoaling of the pycnocline also accounts for the concomitant trends in hypoxic and corrosive conditions on the shelf. The resulting variability is key to understanding the vulnerability of marine species to oceanic change, and to the detection of such changes, soon projected to exceed the range of conditions in the past century.

Climate-driven aerobic habitat loss in the California Current System

“…Distributions of hydrographic variables such as temperature and O 2 over the CCS were reconstructed from Regional Ocean Modeling System (ROMS) simulations that were validated with historical hydrographic observations [ fig. S2 and Supplementary Materials; (15)(16)(17)(18)].…”

“…We used a regional model of the CCS circulation (ROMS) with a Biogeochemical Elemental Cycling model to hindcast physical and biogeochemical variables over the years 1995-2010 (15)(16)(17)(18). The hydrographic variability was extended further back in time by combining high-resolution model fields with long-term, lowerresolution hydrographic observations starting in 1956.…”

Evaluation of high-resolution atmospheric and oceanic simulations of the California Current System

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12This paper is the first of two that present a 16-year reanalysis solution from a coupled physi-13 cal and biogeochemical model of the California Current System (CCS) along the U. S. West

14Coast and validate the solution with respect to mean and seasonal fields and, to a lesser de-15 gree, eddy variability. Its companion paper is Deutsch et al (2019a). The intent is to construct 16 and demonstrate a modeling tool that will be used for mechanistic explanations, attributive 17 causal assessments, and forecasts of future evolution for circulation and biogeochemistry, with 18 particular attention to the increasing oceanic stratification, deoxygenation, and acidification.

19A well-resolved mesoscale (dx = 4 km) simulation of the CCS circulation is made with the 20 Regional Oceanic Modeling System over a reanalysis period of 16 years from 1995 to 2010.

21The oceanic solution is forced by a high-resolution (dx = 6 km) regional configuration of 22 the Weather and Research Forecast (WRF) atmospheric model.

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“…Coast and validate the solution with respect to mean and seasonal fields and, to a lesser de-15 gree, eddy variability. Its companion paper is Deutsch et al (2019a). The intent is to construct 16 and demonstrate a modeling tool that will be used for mechanistic explanations, attributive 17 causal assessments, and forecasts of future evolution for circulation and biogeochemistry, with 18 particular attention to the increasing oceanic stratification, deoxygenation, and acidification.…”

Carbon Export and Fate Beneath a Dynamic Upwelled Filament off the California Coast

“…In coastal regions, carbon export needs to be understood both laterally and vertically. A future expanded scope of autonomous observations during process studies and surveys would provide a 3D view of mechanisms dictating export in these regions and inform the new class of eddy resolving simulations of biogeochemical processes in the California Current System such as recently described by Deutsch et al (2020).…”