Physical Oceanography of the Northeastern Chukchi Sea: A Preliminary Synthesis I. Hanna Shoal Meltback Variability (causes?) II. Hydrography: Interannual Variability III. Aspects of Hanna Shoal Hydrographic Structure IV. Circulation
ACW Bering Sea Summer Waters ACW: Alaskan Coastal Water BSW: Bering Shelf Water MW T f BSW CSW Freezing Products: WW: Winter Water CSW: Chukchi Summer Water (warmed WW) Melting Product MW: Meltwater WW
Northeasterlies 2nd lowest 1 Aug 2011 11 21 31 10 Sep 20 30 10 Oct 20 30 9 Nov 19 29 Southerlies and Northeasterlies 3rd lowest 1 Aug 2012 11 21 31 10 Sep 20 30 10 Oct 20 30 9 Nov 19 29 Northeasterlies Aug- Sep HS Ice not related to local winds or basin ice cover!
Why? An Ocean Connection Bering Strait Summer 2011 Heat Flux: Anomalously large & ~twice that of 2012 (Woodgate et al., 2012 and 2015) Barrow Canyon 2011: Normal transport and temps (bottom only, though). The Winter Ice-Advection Hypothesis Summer 2012 and 2013: Thick, grounded ice pervaded Hanna Shoal. Eicken and Mahoney (2015): 3 Ingredients 1. Thick ice (~20m) source: Beaufort stamukhi zone (not the basin) 2. Breakout stamukhi 3. Transport it westward via winds (tested next) Weingartner Ice Trap addendum: Once grounded; additional ice is caught, deformed, thickened so that HS ice volume increases.
Jan Mar Mean Winds and Sea Level Pressure (thick ice develops after December) 2011 2012 2013 Southerly Northeasterly Northeasterly
2012 & 2013 Ice Drift: Monthly Means and Variances (Industry Moorings) Jan: southward Feb-Apr 2012: Westward Jan-Apr 2013: Westward
2011 Jan: Eastward Feb & Mar: Northeastward April: Westward Winds and Ice Drift support the hypothesis, but sample size is small.
2011 vs 2012 CSESP Hydrography Plan Views Surface & Bottom Transects
2012 Plan views: Upper 10 m (Surface) Lower 10 m (Bottom) HS HS Temperature Salinity
2011 Temperature Salinity
2011 Distance (km) Distance (km) 2012 Distance (km) Distance (km) MW: Strong stratification & mesoscale features (eddies and fronts) WW: perennial feature of HS
Surface Currents: Weekly Mean 5 September 2012 Surface Currents: σ 2 2011 = 0.5*σ2 2012 Winds: σ 2 2011 = σ2 2012
Healy COMIDA 2012 Station Map and Vertically averaged currents 0.04 ms -1 unsheared
BSW MW + BSW HS HS HS I II III WW HS HS WW HS WW 1. Strongly stratified MW/WW or MW/BSW/WW 2. Surface Temp and salinities decrease along flow path from West to East. 3. Ocean Heat loss ~50 W m -2 (melt rate: ~2 cm day -1 ) Clear sky solar melt rate ~ 5 cm day -1
Vertically-averaged currents from absolute geostrophic velocities 0.04 ms -1 unsheared (Barotropic) 0.02 ms -1 sheared
Northeast and East Sides of Hanna Shoal S N S Z<30m Large dρ/dx Leads to large velocity shear HS
Pressure (db) 0 10 20 30 40 50 60 Pressure (db) 0 10 20 30 40 50 60 Pressure (db) 0 10 20 30 40 50 60 Similar hydrographic structure and velocity shear in 2013 in the same region
Hanna Shoal: A year-round mooring perspective
Northwest Side of Hanna Shoal Steady, Mean = 7 cm s -1 EASTWARD T max T f 47 m Stratified Unstratified?
By contrast... Northeast Side of Hanna Shoal Variable, Mean = 0 cm s -1 T max T f 23 m 47 m T f T/S noise suggests IsCat in pycnocline Year-round Stratification on East Hanna Shoal affects circulation!!
MEAN ANNUAL MEANS and VARIANCE ELLIPSES 2011-12 HS Mean Wind CC Barotropic flow EVERYWHERE except EHS where mean flows are very weak ~ 1 cms -1
MEAN Near-bottom water flow XXX: Dynamic boundary that limits exchange between the East Hanna Shoal and shelf to the south
MEAN Near-surface water flow XXX: Dynamic boundary that limits exchange between the East Hanna Shoal and shelf to the south
Hanna Shoal Weak CorrelaMon Spatial Correlation Patterns In- phase Mode Barrow Canyon Central Shelf Correlated with Canyon winds Out- of- phase Mode Barrow Canyon Central Shelf vs Central Channel AnM- correlated
Examine Annual Cycles Cj Bu BC CPAI01
Barrow Canyon, Burger, and CPAI01 experience mean monthly reversals. Central Channel is ALWAYS northward.
Implications of Barrow Canyon Transport Climatology: Central Channel and Herald Valley absorb ALL of the Bering Strait Transport October April May explain the steady flow along NW Hanna Shoal
Summary 1. Hanna Shoal Ice cover affects NE Chukchi Sea hydrographic/circulation Controls on Chukchi ice cover are poorly understood 2. NE Hanna Shoal bottom flow matches clockwise flow in models. Near-surface flow is counterclockwise and counter to models cause is horizontal density gradients 3. East Hanna Shoal shelf appears isolated from the rest of the shelf, 4. Chukchi Sea Circulation: undergoes a massive re-organization on seasonal time scales.