Tide‐induced vertical mixing in the Laptev Sea coastal polynya

Abstract

[1] Enhanced semidiurnal-band velocity shear across the shelf halocline layer (SHL) was found during land-fast ice edge mooring-based acoustic Doppler current profiler (ADCP) and conductivity-temperature-depth (CTD) observations over the eastern Laptev Sea shelf ($74 N, 128 E) in April-May 2008 and April 2009. In 2008, the major axis amplitude for the lunar semidiurnal M 2 tidal ellipses demonstrated intermediate maximum in the SHL at 11-13 m (15 AE 3 cm/s), gradually decreasing to subice and near-bottom layers to $9 AE 3 cm/s (at 7 m) and 7 AE 2 cm/s (at 19 m), respectively. In 2009, the semidiurnal tidal flow exhibited similar patterns, but velocities were reduced by about factor of 2. Our estimates of gradient Richardson numbers suggest that the velocity shear associated with semidiurnal baroclinic tidal flow may be strong enough to play a role in water mass modification, promoting shear instabilities, turbulence, and vertical mixing of seawater properties across the SHL. This suggestion is consistent with near-homogeneous water layers episodically occurring in the SHL. Differences in the background stratification and local tidal dynamics between 2008 and 2009, together with rapid responses of the semidiurnal motion to polynya openings, suggest that the baroclinic tide is locally generated.

Semidiurnal tides in the Laptev Sea Shelf zone in the summer season

“…The result is in agreement with observational data on the Laptev Sea Shelf (Dmitrenko et al, 2008b(Dmitrenko et al, , 2012. The number of simulated periods was dictated by the time of complete system equilibration.…”

“…Whereas the Russian Arctic coast zones, and the Laptev Sea in particular, are getting more and more in the spotlight, the still insufficient amount of observational data as well as the lack of modeling efforts with fine resolution over the shelf leaves many challenges. However, certain observational evidence has already been accumulated, leading to valuable insights in tidal dynamics (Dmitrenko et al, 2012; Janout and Lenn, 2013; Lenn et al, 2011).…”

Arctic Ocean outflow and glacier–ocean interactions modify water over the Wandel Sea shelf (northeastern Greenland)

“…This is comparable to the Laptev Sea shelf where strong salinity stratification is maintained by the river runoff from the Lena River, the second largest river flowing into the Arctic Ocean (e.g., Dmitrenko et al, 2010). Dmitrenko et al (2012) showed that significant velocity shear over the Laptev Sea shelf, attributed to the lunar semidiurnal baroclinic internal tide M2 with velocity ∼ 15 cm s −1 at 11 m and ∼ 7 cm s −1 at 19 m depth is required to transform the vertically stratified water into a locally mixed layer. In contrast, over the Wandel Sea shelf velocities hardly exceed 3-5 cm s −1 , and the most energetic M2 tidal currents are weak, decreasing from ∼ 2 cm s −1 beneath the ice to ∼ 1 cm s −1 below 60 m depth (Kirillov et al, 2017).…”

The penetrative mixing in the Laptev Sea coastal polynya pycnocline layer

“…In Section 3.2 we indicated that the observed water dynamics infer the shear across the density interface separating eastward surface flow and nearbottom westward current. Another source of shear instability at intermediate depths is connected to the baroclinic mode of internal tidal motions in the area under study (Dmitrenko et al, 2005(Dmitrenko et al, , 2012. The shear-driven instability of both, the mean flow and tidal oscillations can result in entrainment at density interface depending upon the local Richardson number.…”