Stratification in a shelf sea: response to different atmospheric forcings

Continental shelves comprise a small portion of the world ocean, yet they are a key contributor to ocean carbon uptake with their immense biological productivity. On-set of spring stratification is one of the physical factors that influence the productivity of the continental shelves. Atmospheric convective mixing determines the on-set of stratification. This is particularly important in seasonally stratified shelf seas, where stratification inhibits nutrient injection to the upper water column. Higher productivity during the stratified period relies on intermittent diapycnal mixing events. Thus, the on-set and intensity of stratification is important for the functioning of the shelf-sea ecosystem. In this study, we investigate the period of on-set of spring stratification and its relation with atmospheric conditions using high resolution in-situ measurements from 10 glider deployments, spanning over 18 months in the central North Sea. Focusing on two consecutive winter-to-spring periods we investigate year to year variability in the timing and intensity of stratification. We observe an early initiation of stratification in 2018/2019, which is more intense than the previous year of 2017/2018. We identify that reduced wind stress and net air-sea heat fluxes result in an early on-set of stratification in 2018/2019. In February 2019, intermittent increases in chlorophyll are observed, corresponding to a minimum in sea-to-air heat loss. Similarly, in 2019 an earlier spring bloom is observed. The two years are characterised by a shift in winter NAO index, which is positive in 2017/2018 and negative in 2018/2019. We subsequently investigate the connections between variability in seasonal stratification on a continental shelf sea and large-scale atmospheric circulation patterns using the outputs of the 7km NEMO-shelf model (AMM7).