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 ?