simulation of the bohai sea circulation and thermohaline structure using a coupled...
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Simulation of the Bohai Sea Circulation and Thermohaline Structure
Using a CoupledHydrodynamical-Ecological Model
by
LCDR Rodrigo ObinoBrazilian Navy
Thesis Presentation – Naval Postgraduate School
Outline
Objectives Background Model Features Experiments Case Analysis Turbulence Study Conclusions Recommendations
Objectives
Simulation of the Bohai Sea using COHERENS model
Sensitivity studies with different forcing functions
Physical mechanisms for the Bohai Sea circulation and thermohaline structure
Comparison between two turbulence schemes
Background
BohaiSea
China
KoreanPeninsulaYellow
Sea
East ChinaSea
Eastern China
Mid latitude
Semi-Enclosed sea
Connected to the Yellow Sea
The Bohai Sea
Liaodong Gulf
HuangheRiver
Central Basin
Haihe R.
Huanhe R.
Laizhou Bay
Yellow Sea
Bohai Strait
Bohai Gulf
LiaodongPeninsula
Liaohe R
Surrounded by the Chinese mainland and Liaodong Peninsula
Connected to the northern Huanghai Sea (Yellow Sea) through the Bohai Strait
Divided in four parts: Liaodong Gulf, Bohai Gulf, Laizhou Bay and Central Basin
Area of 80,000 km square, width of 300 km and length of 500 km
Relatively shallow waters
Characteristics
Topography
Average depth – 18 m Maximum depth at Bohai Strait – 60 m Open boundary relatively deep Gulfs are shallow
Circulation
Winter current – surface mainly wind-driven transport
Anticyclonic patternin central basin
winter monsoon
Driven by strong monsoon winds, large buoyancyforces, active tidal mixing, strong open ocean forcing
Circulation
summer monsoon
Weaker in summerthan in winter due toweaker winds
Counterclockwise gyrein central-northern partof Bohai Sea
Monsoon
summer monsoonJun - Sep
winter monsoonNov - Mar
Siberian High
Relatively strong, cold and dry NW-NE winds
Low over East Asia
Relatively weak, warm and moist SE-SW winds
Tidal Harmonics
Model Features
COHERENS – Coupled Hydrodynamical- Ecological Model for Regional and Shelf Seas
3-D hydrodynamical model coupled to sediment, contaminant, and biological models
Flexibility
Developer: EU Marine Science and Technology (MAST)
Hydrohynamical Equations Governing Primitive Equations derived from
Navior-Stokes Equations
Boussinesq approximation, hydrostaticequilibrium and incompressibility
Mode-splitting technique – coupling betweenexternal and internal modes
Sea surface elevation and depth-integratedvelocities – external mode
Three dimensional currents, temperatureand salinity – internal mode
Discretization
Formulation in spherical coordinates (λ,φ,z)
Vertical terrain-following coordinate – σ
Sigma coordinate – 0 at the bottom and1 at the surface – 16 levels
, -h z
Horizontal differencing – Arakawa staggeredC-scheme – 2nd order centered
Horizontal grid – 62x50 points - 9 km
z h
h
Other Features
Coastline and bottom topography – DBDB55’ resolution
External time step – 15 sec Internal time step – 10 min
Free surface BC and slip bottom BC Zero gradient open BC
Spinup – Jul 01 1999 to Jan 01 2000 - 0000Z
Simulations – Jan 01 2000 to Dec 31 2000
Initialization
Initial conditions for Spinup – GDEM climatological data, and zero velocities and sea surface elevation
Initial conditions for simulations – last information obtained in the spinup for all scalar and vector parameters
Initial sea surface temperature
Initial sea surface salinity
Forcings
Tidal harmonics at open boundary phase and amplitude: M2, S2, N2, K2,
K1, O1, P1 and Sa
Climatological data at open boundary monthly GDEM temperature and salinity Atmospheric forcing over the sea surface (Full flux forcing)
No river runoff
Atmospheric Forcing Function
National Center for Environmental Prediction (NCEP) Reanalysis Data – 2.5° global grid (4 times daily) and interpolated to model grid
Parameters: wind components at 10 m, air temperature, sea surface pressure, relative humidity, precipitation rate and cloudiness
Interpolated on each time step
Air Temperature at Sea Surface15 January 2000
15 July 2000
Wind Field at 10 m15 January 2000
15 July 2000
Sea Surface Pressure
15 January 2000
15 July 2000
Relative Humidity15 January 2000
15 July 2000
Cloudiness
15 January 2000
15 July 2000
0
Precipitation Rate
15 January 2000
15 July 2000
Experiments
Control Run – all forcing functions
Non-Fluxes Run – exclude heat and salt fluxes
Non-Tidal Run – tide effect not considered
Non-Wind Run – no surface stress due to winds
Adopted same settings for all runs – types ofturbulence scheme, advection and diffusion
Control Run
Most complete case
Analysis based on T, S and V fields
Plots only January and July
Zonal and Meridional Vertical Cross-Sections
surface
Horizontal Temperature and Velocity Vectors
Head of Gulfs are colder Northern Bohai Strait warmer Velocities are S-SE and strong Inflow at open boundary and outflow at the southern part
January 2000
July 2000 Head of Gulfs are warmer N Bohai Strait relatively cold Open boundary is colder Currents flow NE Still strong current at N Bohai Strait
mid-depth
January 2000
July 2000
Confirms warmer N Bohai Strait Relatively low temperature at central basin Currents weaker than at surf
Warm region at central basin Penetration of sub-surface cold water mass from YS Currents weaker than at surf Anticyclonic gyre at central basin
bottom
January 2000
July 2000
Temperature almost the same as the SST field Currents are more N-NE, but weaker
Temperature different from the SST field Presence of cold water mass from YS – North YS Bottom Cold Water
Vertical Temperature Cross-Sections
Along meridian 121º01’E
January 2000
July 2000
Vertically uniform Shallow regions are colder
Some stratification North YS Bottom Cold
Water Surface and shallow regions are warmer
Along parallel 38º35.5’N
January 2000
July 2000
No stratification
Some stratification
Horizontal Salinity Field
surface
15 January 2000
15 July 2000
Fresher region near Huanghe River delta Saltier at central basin
Saltier at Bohai Gulf head Values have increased slowly along the year No river runoff
Vertical Salinity Cross-Sections
Along meridian 121º01’E
Vertically uniform
Little stratification
15 January 2000
15 July 2000
Along parallel 38º35.5’N
Vertically uniform
Little stratification
15 July 2000
15 January 2000
Effects of Surface ThermohalineForcing (Control – No Fluxes)
Winter (January): cooling, reduction of the circulation, minor effect on salinity. The effects are vertically uniform.
Summer (July): warming, saline, enhancement of the circulation. The effects decrease with depth except in the shallow water regions. There is no effect on temperature in the deeper layer connecting to the Yellow Sea.
Temperature and Velocity Differences
July 2000 July 2000
January 2000 January 2000
surface bottom
In winter vertically uniform, while in summer some stratification
July 2000 July 2000
January 2000 January 2000
Salinity Differences
surface
Differences increase along the year
Head of Gulfs present highest differences
Bohai Strait and eastern boundary have smaller differences
15 July 2000
15 January 2000
Surface layer more affected by salt fluxes and even more in July
15 July 2000 15 July 2000
15 January 2000 15 January 2000
Wind Effect (Control – No Wind)
Winter (January): cooling in deeper region, warming at southern Bohai Strait, enhancement of the circulation, presence of salty and fresher spots in the central basin. The effects are vertically uniform.
Summer (July): warming in central basin and in shallow regions, cooling in deeper region, enhancement of the circulation, fresher at surface layer. There is some variability in the surface layer.
Temperature and Velocity Differences
July 2000July 2000
January 2000January 2000
surface
Salinity Differences
15 July 200015 July 2000
surface
15 January 200015 January 2000
Tidal Mixing (Control - No Tides)
Winter (January): reduction of the circulation in the central basin, variable effect on temperature. The effects are vertically uniform.
Summer (July): warming close to the bottom and cooling in surface layer, enhancement of the circulation in the central basin. There is no effect on temperature in the deeper layer connecting to the Yellow Sea.
Temperature and Velocity Differences
July 2000July 2000
surface
January 2000January 2000
Salinity Differences
15 July 200015 July 2000
15 January 200015 January 2000
surface
Turbulence Study Vertical eddy viscosity and diffusion coefficients parameterized by turbulence scheme
Study = “k-l” x “k-”
Spatial and Seasonal comparisons
Observed parameter – TKE
Selected 6 points
January and July
July – Sta # 3
January – Sta # 2
Spatial and Seasonal Comparison
“k-l” (green) > “k-” (blue)
Diurnal and Seasonal Variation
“k-” Turbulence Closure Scheme
15 January 2000
15 July 2000
Summer reaches higher values Summer has weak turbulence in deeper layer
Sta # 4
15 January 2000
15 July 2000
Sta # 5
15 January 2000
15 July 2000
“k-l” Turbulence Closure Scheme
Sta #1
Sta # 615 January 2000
15 July 2000
Summer is highly turbulent at mid-depth Summer has weak turbulence in deeper layer
Pattern of TKE Profiles
Conclusions Seasonal circulation patterns and temperature fields are reasonably well simulated
Salinity is not as well simulated as temperature, probably due to no river runoff
Winter monsoon season presents vertically uniform thermohaline structure, while summer monsoon season presents a multi-layer structure
Wind effect is the major forcing for driving surface currents
The Heat fluxes are the predominant driving force for the thermal structure
Conclusions
Tidal mixing is responsible for deep layer characteristics. It cools the surface layer and warms the deeper layer in the summer (i.e., vertical mixing)
“k-l” scheme provides higher TKE than “k-” scheme
Summer has weaker turbulence at deeper layer than in winter
Recommendations
Extend simulation to Yellow Sea, East China Sea and South China Sea
Include river runoff
Assimilate MCSST and Scatterometer Winds
Detailed study of the tidal effect on surfaceelevation and main harmonics
Questions ?
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