Internal Tides in the Bab el Mandab Strait

Ewa Jarosz and Cheryl Ann BlainNaval 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

41E 41.5E 42E 42.5E 43E 43.5E 44E 44.5E 45E

Longitude

11.5N

12N

12.5N

13N

13.5N

14N

14.5NLa

titud

e

Gulf of Aden

Red Sea

Hanish Sill

Perim Narrows Perim

Mocha

Assab

-100

-200-500

Ras Siyan

Ras Bab al Mandab

Hanish Islands

Ras Dumeira

31E 33E 35E 37E 39E 41E 43E 45E 47E 49E 51E 53E 55E 57E 59E

6N

8N

10N

12N

14N

16N

18N

20N

22N

24N

26N

28N

30N

Indi

an O

ceanGulf

of Aden

R e d S e a

Bab el Mandab Strait

Length: 150 km Width: 20 – 110 kmMax depth: 160 – 300 m

Exchange Flow and Stratification in the Bab el Mandab Strait

Velocity (cm/s)

Dep

th (m

)

-100 -80 -60 -40 -20 0 20 40 60

-220

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

Two-layer circulationThree-layer circulation

a

22 23 24 25 26 27 28 29σθ(kg/m

3) A21 22 23 24 25 26 27 28 29

σθ(kg/m3)

Winter stratification(two-layer circulation)

Summer stratification(three-layer circulation)

Red Sea Surface Water Outflow

Gulf of Aden Water Inflow

Gulf of Aden Intermediate Water Inflow

Red Sea Water Outflow

Red Sea Water Outflow

Water Level Fluctuations

-0.5

0 [m]

0.5

-0.5

0 [m]

0.5

-0.5

0 [m]

0.5

G14

G89

G108

06/05/95 06/15 06/25 07/05 07/15 07/25 08/04 08/14

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:M2 (principal lunar), S2 (principal

solar), and N2 (larger lunar elliptic);S2 amplitudes: ~ 50% of M2, N2

amplitudes: ~ 39% of M2;

Diurnal constituents:K1 (principle luni-solar), O1

(principle lunar), and P1 (principle solar);

O1 amplitudes: ~ 54% of K1, P1amplitudes: ~ 33% of K1.

Tidal CurrentsSemimajor Axis (cm/s)

4 8 12 16 20 24 28 32 36 40 44 48

-220

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

A2b mooringC mooringB2b mooring

Semimajor Axis (cm/s)4 8 12 16 20 24 28 32 36 40

Semimajor Axis (cm/s)8 12 16 20 24 28 32 36 40

Dep

th (m

)

Tidal currents have both barotropic and baroclinic components.The vertical structure differs between semidiurnal and diurnal constituents, and

depends on the location and stratification.M2 and K1 are the dominant constituents.

Two-Layer Flow Two-Layer FlowThree-Layer Flow

K1 K1 M2

Model Mesh

Longitude

Latit

ude

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

Barotropic Tides

Forcing:1) amplitudes and phases of eight principle

tidal constituents K1, O1, P1, Q1, M2, S2, N2, and K2) 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: M2 – 8.8 cmS2 – 4.9 cmN2 – 3.6 cmK2 – 1.4 cm

3) diurnal constituents: elevation rms:

K1 – 5.0 cmO1 – 2.3 cmP1 – 4.1 cmQ1 – 3.1 cm

38E 40E 42E 44E 46ELongitude

10N

12N

14N

16N

18N

Latit

ude

Assab

Berbera

Djibouti

Harmil Island

Massawa

Saylac

Aden

Kamaran

Mocha

Perim

Ras Khathib

G14G109G89

G108

Subsurface Pressure Gauges

Water Level Stations

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 characteristicfor the Gulf of Aden when the two-layer flow is present in the Strait;

additional forcing – constant elevation along the open boundary locatedin the Gulf of Aden;minimum depth 3 m;turbulent closure model – MY2.5 or UB;horizontal diffusion – Smagorinsky’s scheme;quadratic bottom friction with Cd=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

(1)

(2)

(3)

40 41 42 43 44 45

11

12

13

14

15

16

17

Latit

ude

Dam Locations – red lines

Constant Elevation: 0.50 m – 0.05 m

Transect Nodes – green dots

Longitude

Exchange Flow – Subsurface Currents

40 41 42 43 44 45

11

12

13

14

15

16

17

Longitude

Latit

ude

Longitude

Latit

ude

1 m/s

Time = 5014 h, Depth=0.1 m

Red Sea Waters S=40 psu, T=22oC

Gulf of Aden WatersS=36 psu, T=26oC

Exchange Flow – Subsurface Currents

40 41 42 43 44 45

11

12

13

14

15

16

17

Red Sea Waters S=40 psu, T=22oC

Gulf of Aden Waters S=36 psu, T=26oC

Longitude

Latit

ude

Longitude

Latit

ude

1 m/s

Time = 966 h, Depth=0.1 m

Exchange Flow – Along-Strait Velocity

Model

Data

Exchange Flow – Salinity Distribution

Salinity at 966 h

300 350 400 450 500 550 600 650 700 750 800 850-400

-350

-300

-250

-200

-150

-100

-50

0

Dep

th (m

)

Red SeaGulf of Aden

Distance (km)

Exchange Flow – Subsurface Currents

Longitude

Latit

ude

Time = 720 h, Depth=0.1 m

40 41 42 43 44 45

11

12

13

14

15

16

17

Longitude

Latit

ude

Gulf of Aden Waters S=36 psu, T=26oC

Red Sea Waters S=40 psu, T=22oC

Elevation=0.05 m

Exchange Flow – Along-Strait Velocity

Model

Data

Exchange Flow – Salinity Distribution

Salinity at 720 h

300 350 400 450 500 550 600 650 700 750 800 850-400

-350

-300

-250

-200

-150

-100

-50

0

Red SeaGulf of Aden

Dep

th (m

)

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 K1, O1, P1, Q1, M2, S2, N2, and K2) 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 Cd=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.

S (psu) /T (oC)

Initial Salinity (red) and Temperature (blue)

Dep

th (m

)

Internal Tides - Salinity Distribution

1 101 201 301 401 501 601 701 801 901 1001

-250

-200

-150

-100

-50

0

1 101 201 301 401 501 601 701 801 901 1001

-200

-150

-100

-50

0

Dep

th (m

)

Node 19925(southern end of the Strait)

Node 17203(middle of the Strait)

Time (h)

Internal Tides - Salinity Distribution

Salinity at 1010 h

0 100 200 300 400 500 600 700 800 900-600

-500

-400

-300

-200

-100

0

Gulf of AdenRed Sea

Dep

th (m

)

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

Internal Tides in the Bab el Mandab StraitProject ObjectivesBab el Mandab StraitExchange Flow and Stratification in the Bab el Mandab StraitWater Level FluctuationsTidal CurrentsModel MeshBarotropic TidesExchange FlowExchange FlowExchange Flow – Subsurface CurrentsExchange Flow – Subsurface CurrentsExchange Flow – Along-Strait VelocityExchange Flow – Salinity DistributionExchange Flow – Subsurface CurrentsExchange Flow – Along-Strait VelocityExchange Flow – Salinity DistributionInternal TidesInternal Tides - Salinity DistributionInternal Tides - Salinity DistributionFuture Work