Internal Tides in the Bab el Mandab Strait Ewa Jarosz and Cheryl Ann Blain Naval Research Laboratory, Stennis Space Center, MS
Project Objectives Numerical simulation of tides and exchange flow in the Bab el Mandab Strait with the ELCIRC model Dynamics of the internal tides in the Bab el Mandab Strait, in particular their generation, propagation, energy fluxes, and dissipation
Bab el Mandab Strait 14.5N Red Sea -500-200 -100 30N 28N 26N 24N Hanish Islands 14N 18N R e d S e a 22N 20N 16N 13.5N Hanish Sill 14N 12N 10N 8N Bab el Mandab Strait Gulf of Aden Indian Ocean Mocha 6N 31E 33E 35E 37E 39E 41E 43E 45E 47E 49E 51E 53E 55E 57E 59E Latitude 13N Assab 12.5N Ras Dumeira Perim Narrows Ras Siyan Ras Bab al Mandab Perim 12N Length: 150 km Width: 20 110 km Max depth: 160 300 m Gulf of Aden 11.5N 41E 41.5E 42E 42.5E 43E 43.5E 44E 44.5E 45E Longitude
Exchange Flow and Stratification in the Bab el Mandab Strait Velocity (cm/s) -100-80 -60-40 -20 0 20 40 60 0-20 -40 Red Sea Surface Water Outflow Gulf of Aden Water Inflow σ θ a(kg/m 3 ) 22 23 24 25 26 27 28 29 σ θ A(kg/m 3 ) 21 22 23 24 25 26 27 28 29-60 -80 Depth (m) -100-120 Gulf of Aden Intermediate Water Inflow -140 Red Sea Water Outflow -160-180 Red Sea Water Outflow -200-220 Two-layer circulation Three-layer circulation Winter stratification (two-layer circulation) Summer stratification (three-layer circulation)
Water Level Fluctuations G108 G89 G14 0.5 0 [m] -0.5 0.5 0 [m] -0.5 0.5 0 [m] Variance: G89 (Perim Narrows) 47% for semidiurnal band and 49% for diurnal band; G108 (Hanish Sill) 81% for semidiurnal band and 5% for diurnal band. Semidiurnal constituents: M 2 (principal lunar), S 2 (principal solar), and N 2 (larger lunar elliptic); S 2 amplitudes: ~ 50% of M 2, N 2 amplitudes: ~ 39% of M 2 ; 06/05/95 06/15 06/25 07/05 07/15 07/25 08/04 08/14-0.5 Diurnal constituents: K 1 (principle luni-solar), O 1 (principle lunar), and P 1 (principle solar); O 1 amplitudes: ~ 54% of K 1, P 1 amplitudes: ~ 33% of K 1.
Tidal Currents Semimajor Axis (cm/s) 4 8 12 16 20 24 28 32 36 40 44 48 0-20 Two-Layer Flow Semimajor Axis (cm/s) 4 8 12 16 20 24 28 32 36 40 Three-Layer Flow Semimajor Axis (cm/s) 8 12 16 20 24 28 32 36 40 Two-Layer Flow -40-60 -80 Depth (m) -100-120 -140-160 -180-200 A2b mooring C mooring K 1 B2b mooring K 1 M 2-220 Tidal currents have both barotropic and baroclinic components. The vertical structure differs between semidiurnal and diurnal constituents, and depends on the location and stratification. M 2 and K 1 are the dominant constituents.
Model Mesh Latitude 26821 horizontal nodes 71 vertical nodes 51004 elements in the horizontal grid 2 open boundaries Horizontal resolution between 0.1 km - 26 km Vertical resolution between 0.9 m 350 m Bathymetry DBDB-V, charts Longitude
Barotropic Tides Forcing: 1) amplitudes and phases of eight principle tidal constituents K 1, O 1, P 1, Q 1, M 2, S 2, N 2, and K 2 ) provided at the open boundaries; 2) tidal potential for the same eight constituents. Model-data comparison: 1) 16 locations with tidal amplitudes and phases; 2) semidiurnal constituents: elevation rms: M 2 8.8 cm S 2 4.9 cm N 2 3.6 cm K 2 1.4 cm 3) diurnal constituents: elevation rms: K 1 5.0 cm O 1 2.3 cm P 1 4.1 cm Q 1 3.1 cm Latitude 18N 16N 14N 12N Harmil Island Massawa Subsurface Pressure Gauges Water Level Stations G108 Kamaran Ras Khathib Mocha Assab G109 G14 G89 Perim Djibouti Saylac 10N 38E 40E 42E 44E 46E Longitude Berbera Aden
Exchange Flow Model Parameters: lock-exchange set-up for initial T and S distributions; two different T and S were used: 1) two vertically and horizontally homogenous water masses: one having T/S of the Red Sea Waters and second with T/S of Gulf of Aden; 2) two water masses, horizontally uniform but vertically stratified: one characteristic for the southern Red Sea and second characteristic for the Gulf of Aden when the two-layer flow is present in the Strait; additional forcing constant elevation along the open boundary located in the Gulf of Aden; minimum depth 3 m; turbulent closure model MY2.5 or UB; horizontal diffusion Smagorinsky s scheme; quadratic bottom friction with C d =0.0025; external and internal time steps 180 sec; variable Coriolis parameter; advection terms included; implicitness parameter of 0.6; no wind input and heat conservation model.
Exchange Flow 17 16 15 Latitude 14 13 (1) (2) 12 11 (3) 40 41 42 43 44 45 Longitude Dam Locations red lines Constant Elevation: 0.50 m 0.05 m Transect Nodes green dots
Exchange Flow Subsurface Currents 17 Latitude 16 15 14 Red Sea Waters S=40 psu, T=22 o C 13 12 11 Gulf of Aden Waters S=36 psu, T=26 o C Latitude 40 41 42 43 44 45 1 m/s Longitude Time = 5014 h, Depth=0.1 m Longitude
Exchange Flow Subsurface Currents 17 16 15 Red Sea Waters S=40 psu, T=22 o C Latitude 14 13 12 Gulf of Aden Waters S=36 psu, T=26 o C 11 Latitude 40 41 42 43 44 45 Longitude 1 m/s Time = 966 h, Depth=0.1 m Longitude
Exchange Flow Along-Strait Velocity Model Data
Exchange Flow Salinity Distribution 0 Salinity at 966 h -50 Depth (m) -100-150 -200-250 Red Sea Gulf of Aden -300-350 -400 300 350 400 450 500 550 600 650 700 750 800 850 Distance (km)
Exchange Flow Subsurface Currents 17 16 15 Red Sea Waters S=40 psu, T=22 o C Latitude 14 13 Latitude 12 11 40 41 42 43 44 45 Longitude Gulf of Aden Waters S=36 psu, T=26 o C Elevation=0.05 m Time = 720 h, Depth=0.1 m Longitude
Exchange Flow Along-Strait Velocity Model Data
Exchange Flow Salinity Distribution 0 Salinity at 720 h -50 Depth (m) -100-150 -200-250 Red Sea Gulf of Aden -300-350 -400 300 350 400 450 500 550 600 650 700 750 800 850 Distance (km)
Internal Tides Model Parameters: horizontally uniform but vertically varying initial T and S; forcing: tidal potential and amplitudes and phases of eight principle tidal constituents K 1, O 1, P 1, Q 1, M 2, S 2, N 2, and K 2 ) provided at the open boundaries; minimum depth 3 m; turbulent closure model MY2.5 or UB; horizontal diffusion Smagorinsky s scheme; quadratic bottom friction with C d =0.0025; external and internal time steps 180 sec; variable Coriolis parameter; advection terms included; implicitness parameter of 0.6; no wind input; heat conservation model was not used. Depth (m) Initial Salinity (red) and Temperature (blue) S (psu) /T ( o C)
Internal Tides - Salinity Distribution 0-50 -100-150 Node 19925 (southern end of the Strait) -200-250 Depth (m) 0 1 101 201 301 401 501 601 701 801 901 1001-50 -100 Node 17203 (middle of the Strait) -150-200 1 101 201 301 401 501 601 701 801 901 1001 Time (h)
Internal Tides - Salinity Distribution 0 Salinity at 1010 h -100-200 Depth (m) -300 Red Sea Gulf of Aden -400-500 -600 0 100 200 300 400 500 600 700 800 900 Distance (km)
Future Work Further simulations with different model setups to reproduce the observed exchange flow in the Bab el Mandab Strait as close as possible Concurrent simulations of the tides and exchange flow to examine internal tides dynamics in the Strait