The Impact of Wind-Driven Mixing in the Barents Sea

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

Name dc:type.qualificationname

Doctor of Philosophy (awarded by UHI)

Level dc:type.qualificationlevel

Doctoral Thesis

Grantor dc:publisher.institution

University of the Highlands and Islands

Year dc:date.issued

2022

Author dc:creator

• Rodgers, James

Advisors dc:contributor.advisor

• Davies, Gareth
• Inall, Mark

Language dc:language

eng

Identifier

oai:pure.atira.dk:studenttheses/8ca41181-8ac2-483f-b4d2-3de9ea0c489c

OAI identifier oai:identifier

oai:pure.atira.dk:studenttheses/8ca41181-8ac2-483f-b4d2-3de9ea0c489c