Near-inertial waves in an Arctic fjord and their impact on vertical mixing of Atlantic water mass

Abstract

With increase in the Arctic warming, fjords in the region are undergoing significant decline in sea-ice, increase in glacier retreat, and changes in primary productivity. Warm and saline Atlantic water (AW) advected from the open ocean is attributed as one of the primary reasons for these changes. Vertical mixing of the subsurface AW with the cold surface Arctic water positively feeds many of these changes. One of the major contributors of vertical mixing in the water column is the near-inertial waves (NIW). In this study, we report prominent NIW activity in Kongsfjorden, an Arctic fjord in west Svalbard, using hydrography observations and numerical simulations. Shallow summer mixed layer depth facilitates generation of strong near-inertial currents in the mixed layer (ML), even with relatively weak storms. During storm events in summer, the near-inertial currents induce large shear at the base of the ML and downward propagating NIW induce shear in the fjord interior leading to an enhanced vertical mixing. These two processes during summertime storms lead to redistribution of the subsurface AW thereby warming the ML. Numerical simulations from the Regional Ocean Modelling System (ROMS) are used to understand the generation, dissipation and energetics of the NIW and their impact on vertical mixing in the fjord. Near-inertial energy budget estimations show that ∼45% of near-inertial energy input in the fjord and continental shelf–slope region dissipates within the upper 150 m of the water column. Numerical experiments are performed to confirm the role of storms in inducing vertical mixing of the AW. When storms are removed from the forcing field, eddy diffusivity reduces and fails to reproduce the upper ocean warming that was present in the simulation with storms. Even though stability in the fjord upper layer is enhanced with an increased glacier discharge, strong vertical mixing is still profound during storms. Consistent with this, in the experiment with double the glacier discharge, near-inertial energy flux increases by nearly 10% whereas the viscous dissipation of the NIW increases by about 13%. The findings of the study underscore the potential significance of the NIW dynamics and their impacts in the Arctic fjords. This is further relevant in understanding the vertical mixing in shallow regions such as shelves, slopes and fjords in the future Arctic with more storms and reduced sea-ice cover as projected by climate models.