Simulation of the Bohai Sea Circulation and Thermohaline Structure Using a Coupled Hydrodynamical-Ecological Model by LCDR Rodrigo Obino Brazilian 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 Bohai Sea China Korean Peninsula Yellow Sea East China Sea  Eastern China  Mid latitude  Semi-Enclosed sea  Connected to the Yellow Sea

The Bohai Sea Liaodong Gulf Huanghe River Central Basin Haihe R. Huanhe R. Laizhou Bay Yellow Sea Bohai Strait Bohai Gulf Liaodong Peninsula 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 pattern in central basin winter monsoon  Driven by strong monsoon winds, large buoyancy forces, active tidal mixing, strong open ocean forcing

Circulation summer monsoon  Weaker in summer than in winter due to weaker winds  Counterclockwise gyre in central-northern part of Bohai Sea

Monsoon summer monsoon Jun - Sep winter monsoon Nov - 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, hydrostatic equilibrium and incompressibility  Mode-splitting technique – coupling between external and internal modes  Sea surface elevation and depth-integrated velocities – external mode  Three dimensional currents, temperature and salinity – internal mode

Discretization  Formulation in spherical coordinates (λ,φ,z)  Vertical terrain-following coordinate – σ  Sigma coordinate – 0 at the bottom and 1 at the surface – 16 levels, -h  z    Horizontal differencing – Arakawa staggered C-scheme – 2 nd order centered  Horizontal grid – 62x50 points -  9 km

Other Features  Coastline and bottom topography – DBDB5 5’ resolution  External time step – 15 sec Internal time step – 10 min  Free surface BC and slip bottom BC Zero gradient open BC  Spinup – Jul to Jan Z  Simulations – Jan to Dec

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 Surface 15 January July 2000

Wind Field at 10 m 15 January July 2000

Sea Surface Pressure 15 January July 2000

Relative Humidity 15 January July 2000

Cloudiness 15 January July 2000

0 Precipitation Rate 15 January 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 of turbulence 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 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 July 2000

Along parallel 38º35.5’N  Vertically uniform  Little stratification 15 July January 2000

Effects of Surface Thermohaline Forcing (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 January 2000 surfacebottom

 In winter vertically uniform, while in summer some stratification July 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 January 2000

 Surface layer more affected by salt fluxes and even more in July 15 July 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 2000 January 2000 surface

Salinity Differences 15 July 2000 surface 15 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 2000 surface January 2000

Salinity Differences 15 July 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 July 2000  Summer reaches higher values  Summer has weak turbulence in deeper layer Sta # 4 15 January July 2000 Sta # 5

15 January July 2000 “k-l” Turbulence Closure Scheme Sta #1 Sta # 6 15 January 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 surface elevation and main harmonics

Questions ?