The Impact of Wind-Driven Mixing in the Barents Sea

Abstract

The Arctic is changing rapidly, and the characteristic sea ice cover that defines
the Arctic Ocean is retreating rapidly. Despite extensive study and
investigation of many parameters and processes, it remains uncertain how a
future Arctic will respond to reduction of sea ice. The loss of multi-year and
high concentration sea ice cover is predicted to increase the extent of wind
forcing on the Arctic Ocean, compounded by the projected increase in
frequency of storms. Increased wind forcing is likely to lead to enhanced
mixing and associated fluxes of nutrients and heat to the surface from warmer
and nutrient rich waters below. However, there remains a lack of
understanding and quantification of how wind forcing will impact these fluxes
and productivity over short timescales of days. This thesis investigates how
wind, sea ice and stratification influence the surface mixing layer and its
impact on nitrate fluxes.
The study is based on a set of turbulence observations from an MSS 90 freefall
profiler, taken over 2 cruises in spring and summer 2018 in the Barents Sea
and on the shelf north of Svalbard. Additionally, profile data of temperature
and salinity, nitrate concentrations and uptake rates and a 2-year
temperature time series data from 2 moorings deployed on the shelf are also
used. This in situ data is supplemented by satellite remote sensing data for sea
ice and reanalysis data for wind and surface heat fluxes and empirically
derived values for the air-ocean transfer of energy from the literature.
In Chapter 3 profiles from the MSS are used to quantify the mixing depth
parameter and investigate how it varies with wind, sea ice and stratification.
We develop a new empirical model linking the input of energy from wind at the
surface of the ocean with the mixing depth, modified by sea ice and
stratification. We derived a new linear relationship based on measurements
between wind speeds of 2 and 11.4 ms-1 in open water under both stratified
and unstratified conditions.
We then investigate the depth relationship between mixing depth and the
nitracline during the summer post-bloom period in Chapter 4, and the impact on nitrate resupply, uptake and f-ratio. We define a new parameter, the
mixing – nitracline overlap, which quantifies the depth relationship between
mixing depth and the depth of the nitracline and is indicative of the potential
magnitude of the nitrate flux. We estimate the mixing – nitracline overlap for
a south-north transect at 30°E through the Barents Sea using the new
empirical model of mixing depth from Chapter 3. We found that where the
mixing – nitracline overlap is positive the flux of nitrate to the surface is
enhanced by up to an order of magnitude. Periods where the mixing depth
overlaps with the nitracline occur sporadically throughout the summer, but
only north of the Polar Front. The strength and frequency of strong wind
events, and the local stratification environment control how often these periods
lead to an associated enhancement of nitrate flux.

Date of Award	11 Jul 2022
Original language	English
Awarding Institution	University of the Highlands and Islands
Sponsors	NERC
Supervisor	Finlo Cottier (Supervisor), Gareth Davies (Supervisor) & Mark Inall (Supervisor)