Ocean nutrient pathways associated with the passage of a storm

Anna Rumyantseva,Adrian Martin,Tom Rippeth,Stephanie Henson,Natasha Lucas,Stuart C Painter,Timothy J Boyd

doi:10.1002/2015gb005097

Abstract

AbstractStorms that affect ocean surface layer dynamics and primary production are a frequent occurrence in the open North Atlantic Ocean. In this study we use an interdisciplinary data set collected in the region to quantify nutrient supply by two pathways associated with a storm event: entrainment of nutrients during a period of high wind forcing and subsequent shear spiking at the pycnocline due to interactions of storm‐generated inertial currents with wind. The poststorm increase in surface layer nitrate (by ~20 mmol m−2) was predominantly driven by the first pathway: nutrient intrusion during the storm. Alignment of poststorm inertial currents and surface wind stress caused shear instabilities at the ocean pycnocline, forming the second pathway for nutrient transport into the euphotic zone. During the alignment period, pulses of high‐turbulence nitrate flux through the pycnocline (up to 1 mmol m−2 d−1; approximately 25 times higher than the background flux) were detected. However, the impact of the poststorm supply was an order of magnitude lower than during the storm due to the short duration of the pulses. Cumulatively, the storm passage was equivalent to 2.5–5% of the nitrate supplied by winter convection and had a significant effect compared to previously reported (sub)mesoscale dynamics in the region. As storms occur frequently, they can form an important component in local nutrient budgets.

Highlights

Primary production in the temperate and subpolar North Atlantic Ocean plays a crucial role in the global carbon cycle [Sabine et al, 2004]
In this study we use an interdisciplinary data set collected in the region to quantify nutrient supply by two pathways associated with a storm event: entrainment of nutrients during a period of high wind forcing and subsequent shear spiking at the pycnocline due to interactions of storm-generated inertial currents with wind
This paper presents direct observations of the flux of nutrients to the surface layer resulting from the passage of an autumn extratropical storm in the open North Atlantic Ocean

Summary

Introduction

Primary production in the temperate and subpolar North Atlantic Ocean plays a crucial role in the global carbon cycle [Sabine et al, 2004]. The precise physical mechanisms that supply nutrients to support observed levels of annual primary production and the relative importance of these mechanisms are still under debate [McGillicuddy et al, 2003; Oschlies, 2002a]. In temperate and high-latitude oceans, strong wind forcing can be important during the postspring bloom period when the surface ocean is nitrate-depleted and a well-established pycnocline inhibits the upward flux of nutrients to the euphotic zone. Under these conditions primary production is predominantly fuelled by regenerated forms of nitrogen such as ammonium and urea unless physical resupply of nutrients occurs. Satellite observations of episodic storm events in summer and autumn have been linked to subsequent phytoplankton increases in otherwise nutrient-limited conditions [Babin et al, 2004; Son et al, 2006; Wu et al, 2008]

Results

Discussion

Conclusion