Coastal Air-Ocean Coupled System (CAOCS) for the East Asian Marginal

Seas (EAMS)by

LCDR Mike RothThesis Presentation

07SEP01

SignificanceFocus of METOC support for the littoral region at the mesoscale level

Emphasis on Air-sea interaction

EAMS is a critical operating area of the USN, especially 7th Fleet

The objective of METOC’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) developed by NRL

Purposes

To provide further support that CAOCS does perform well in simulating EAMS surface current circulation, SST structure, and SSS structure.

To provide support that CAOCS does perform well in simulating EAMS surface wind stress and low level atmospheric forcing.

Purposes (cont.)

Through analysis of CAOCS output: To show how the atmosphere and ocean behave in a way that cannot be described climatologically due to the small temporal scales of numerous mesoscale features present at the surface of the ocean and in the lower levels of the atmosphere even during a period following the onset of the summer monsoon. This will provide support regarding the usefulness of CAOCS over an uncoupled, climatologically forced ocean or atmospheric model.

Purposes (cont.)

Through analysis of CAOCS output: To show the significance of the air-sea interaction processes that occur between the lower atmosphere and the surface of the ocean and that CAOCS is indeed handling these air-sea interaction processes.

To emphasize the near-real time capability of CAOCS.

Purposes (cont.)

To show that CAOCS is an excellent tool for USN METOC community personnel because the accurate, near-real time model output will contribute to increased meteorological, oceanographic, and acoustic forecasting skill in a littoral environment.

The EAMSThe EAMS is comprised of:

Japan/East Sea (JES)

Yellow Sea/East China Sea (YES)

South China Sea (SCS)

The EAMS

YS/ECS (YES)

SCS

JESBohai Sea

Japan

China

KoreanPeninsula

Russia

Taiwan

Philippines

Borneo

Indonesia

Malaysia

Gulf ofThailand

Vietnam

Gulf ofTonkin

Components of the the EAMS

JESOceanography

JES

KoreanStrait

TsugaruStrait

Soya Strait

Tatar Strait

Honshu

KoreanPeninsula

Vladivostok Hokkaido

Kyushu

JESViewed as a miniature prototype ocean:

Basin wide circulation pattern

Boundary currentsA Subpolar Front (SPF)Mesoscale eddy activity

Deep water formation

JES CurrentsTsushima Warm Current (TsWC)

Flows northward from the ECS through the Korean Strait

Carries warm water into the JES

Separates north of 35°N into eastern/western channels

JES CurrentsJapan Nearshore Branch (JNB)

Flows northward as the eastern branch of the TsWC along the Japanese west coast

JES CurrentsEast Korean Warm Current (EKWC)

Flows northward as the western branch of the TsWC

Bifurcates at 37°N into an eastern and western branch

The western branch makes a cyclonic turn in the East Korean Bay

JES CurrentsLiman Current and North Korean Cold Current

(NKCC)Flows southward from the

Sea of Okhotsk through the Tatar Strait and along the Russian and North Korean west coast

Brings cold water into the JES

JES CurrentsThe Subpolar Front (SPF)

The southward flowing NKCC and the northward flowing eastern branch of the EKWC converge at approx. 38°N

The SPF stretches across the JES in a northeasterly direction and extends to the west coast of Hokkaido

YESOceanography

YES

Ryuky

u Isla

nds

Taiwan Strait

Yangtze R.

Yellow R.

Han R.

Liao R.

YES BathymetryYS quite shallow

Most water depth < 50 m

N-S oriented trench in central portion of YS

Broad/shallow continental shelf – water readily affected by varying atmospheric forcing (heating, cooling, wind stress)

YES BathymetryE/W asymmetry:

Extensive shoals <20 m in western YS and and not in eastern YS

50-m isobath > 100 km from Chinese coast but only 50 km from South Korean coast

Plays a crucial role in the shoaling of the MLD

YES Thermal StructureMonsoon atmospheric

forcing greatly alters SST and MLD depth:

Winter:

Cold northerly winds

SAT<SST

Surface heat lost from ocean to atmosphere resulting in upward buoyancy flux

YES Thermal StructureWinter (continued):

Thermal Forcing (cooling) and Mechanical Forcing (wind stress) generate turbulence

Mixing of surface water with deep water

Deepening of MLD that often extends to bottom

YES Thermal StructureSummer:

Warm southerly winds

SAT>SST

Strong downward net radiation

Leads to downward buoyancy flux

MLD shoals

Multi-layer structure (MLD, thermocline, and sublayer)

YES CurrentsKuroshio Current (KUC)

Strong WBC

Flows northward along the shelf break in the southern ECS

YES CurrentsTaiwan Warm Current (TWC)

Enters ECS through the Taiwan Strait

Flows northward inshore of the KUC.

YES CurrentsTsushima Warm Current (TsWC)

Flows northward from the KUC west of Kyushu and passes through the Korean Strait

Splits in the vicinity south of Cheju Island

YES CurrentsYellow Sea Warm Current (YSWC)

Flows northward beneath the surface into the YS

Brings warm water into the YS

YES CurrentsKorean Coastal Current

Flows southward along the Korean Peninsula

YES CurrentsChinese Coastal Current

Flows southward around the tip of the Shandong peninsula and along the Chinese coast

SCSOceanography

SCSGulf ofTonkin

LuzonStrait

TaiwanStrait

Balabac Strait

Mindoro Strait

SCS BathymetryStraits are relatively

shallow except the Luzon Strait (sill depth = 2,400 m)

Broad shallows of the Sunda shelf in the S/SW

Continental shelf in the N extends from Gulf of Tonkin to the Taiwan Strait

LuzonStrait

TaiwanStrait

SCS BathymetryExtensive continental

shelves (< 100 m deep) in W and S

Deep slopes w/ almost no shelves in the E

Deep eliptical shaped basin in the center of the SCS extends to over 4,000 m

Numerous reef islands and underwater plateaus scattered throughout SCS

LuzonStrait

TaiwanStrait

SCS CurrentsComplex dynamics

involved in the flow of the SCS are related to:

geometry of the SCS

its connectivity with the Pacific Ocean

strongly variable atmospheric forcing

water exchange between the SCS/ECS via the Taiwan Strait

LuzonStrait

TaiwanStrait

SCS CurrentsKuroshio Current (KUC) – bifurcation regime

Originates from the North Equatorial CurrentFlows northward as a WBC east of LuzonEnters ECS through the Luzon Strait, bifurcates into northward and northwestward branches to the northeast of a cyclonic eddy that is located northwest of Luzon (NWL eddy)

E

SCS CurrentsKuroshio Current (KUC) – bifurcation regime

The northward branch flows northward along the western coast of Taiwan

EThe northwestward branch makes a cyclonic turn around the NWL eddy

SCS CurrentsKuroshio Current (KUC) – loop regimeOriginates from the North

Equatorial Current

Flows northward as a WBC east of Luzon

Enters ECS through the southern Luzon Strait, loops around an anticyclonic eddy northwest of Luzon, and exits through the northern Luzon Strait

E

SCS CurrentsWinter upper ocean circulation

A southward coastal jet off the Vietnam coast and a cyclonic circulation throughout the SCS

SCS CurrentsSummer upper ocean circulation

A northward coastal jet off the Vietnam coast and an anticyclonic circulation throughout the SCS

SCS CurrentsSCS Eddies

Several cold core and warm core eddies are often found in the SCS

Generally, cold core are more common

Bottom topography is a key factor in their lifetime/trajectory

EAMSAtmospheric Forcing – the winter and summer

monsoon

YES

Atmospheric Forcing – Winter MonsoonNovember through March

Siberian High over East Asia continent

Polar Front positioned north of the Philippines

Relatively stronger, cold, and dry NW/N/NE winds flow over the EAMS

Equatorial Trough located south of equator

H JES

SCS

Polar Front

YES

Atmospheric Forcing – Transition Period

Polar Front moves northward toward Korea

Winter to Summer: March through May

YS SST increases by 10°C

The Siberian High rapidly weakens in April

Frontally generated events often occur in the YES during late April and May that cause highly variable winds, cloud amount, and precipitation (Mei-Yu Trough due to cyclonic shear between NE and SW).

Yellow dessert sand is often carried into the YS by eastward migrating surface lows originating in Mongolia

An atmospheric low pressure system forms in the north YS in late May/early June and migrates westward over Manchuria

Atmospheric Forcing – Summer Monsoon

Heat Lows over East Asia continent due to high solar insolation

Mid-May through Mid-September

Higher pressure over Pacific Ocean but subtropical ridge is displaced poleward

Equatorial Trough lies over central Philippines and extends NW to Tibetan Plateau.

JES

YES

SCS

H

L

L

Atmospheric Forcing – Summer Monsoon

JES

YES

SCS

L

LAir flows SE south of equator and turns SW over the SCS due to Coriolis Force

Polar Front moves north ivo 30-35°N

Relatively weaker, warm, and moist SW/S/SE winds flow over the northern SCS and the remainder of the EAMS

H

A Tropical Easterly Jet is found at 125-mb between the subtropical ridge and the Equatorial trough

Atmospheric Forcing – Transition Period

Polar Front begins to move southward away from the Korean Peninsula

Summer to Winter: Mid-September through October

SST steadily decreases

Southerly winds weaken as the Manchurian Low is replaced by the Siberian High

The Atmospheric Component of the

CAOCS

Mesoscale Model Fifth Generation (MM5)

Developed by Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR)

Limited-area, non-hydrostatic, terrain-following sigma- coordinate model

Designed to simulate or predict mesoscale and regional-scale atmospheric circulation

Area for Atmospheric Model

Distribution of Vegetation

The Oceanic Component of the

CAOCS

Princeton Ocean Model (POM)Developed at Princeton University

Time dependent, primitive eqn circulation model on a 3-D

Specifically designed to accommodate mesoscale phenomena, including the often non-linear processes commonly found in estuarine and coastal environments

Includes realistic topography and a free surface

Ocean Bottom

CAOCS Numerics

• MM5V3.4– Resolution

• Horizontal: 30 km• Vertical: 16 Pressure Levels

– Time step: 2 min• POM

– Resolution• Horizontal: 1/6o × 1/6o

• Vertical: 23 σ levels– Time Steps: 25 s, 15 min

Coupling of the Oceanic and Atmospheric

Components of the CAOCS

Ocean-Atmospheric Coupling

• Surface fluxes (excluding solar radiation) are of opposite signs and applied synchronously to MM5 and POM

• MM5 and POM Update fluxes every 15 min

• SST for MM5 is obtained from POM • Ocean wave effects (ongoing)

Lateral Boundary Conditions

• MM5: ECMWF T42

• POM: Lateral Transport at 142oE from the climatological data

MM5 Initialization

• Initialized from: 30 April 1998 (ECMWF T42)

Three-Step Initialization of POM• (1) Spin-up

– Initial conditions: annual mean (T,S) + zero velocity– Climatological annual mean winds + Restoring type thermohaline

flux (2 years)• (2) Climatological Forcing

– Monthly mean winds + thermohaline fluxes from COADS (3 years)

• (3) Synoptic Forcing– Winds and thermohaline fluxes from NCEP (1/1/96 – 4/30/98)

• (4) The final state of the previous step is the initial state of the following step

Reality Check of the Oceanic Output of the

CAOCS

Reality Check of the Oceanic Output of the

CAOCS

Liman/NKCC

JNBEKWC

SPF

Reality Check of the Oceanic Output of the

CAOCS

Reality Check of the Atmospheric Output of

the CAOCS

Reality Check of the Atmospheric Output of the CAOCS

JES

YES

SCS

L

LH

H

L

850-mb Winds and GHTFor 12Z July 19, 1998

JES

YES

SCS

Reality Check of the Net Radiation Output of the

CAOCS

Reality Check of the Net Radiation Output of the

CAOCS

Reality Check of the Net Radiation Output of the

CAOCS

Surface Long Wave Radiation Flux did not verify in position nor in magnitude.

This discrepancy will be corrected in future work with CAOCS.

APPROACH

APPROACHCAOCS model output was examined for the entire May through July 1998 with the intention of identifying the following:A time period prior to the onset of the summer monsoon that involved:

A significant weather event over the EAMS as well asan oceanic event that could be forcing flow at the lower levels of the atmosphere

APPROACHA time period after the onset of the summer monsoon that involved:

A significant weather event over the EAMS as well asan oceanic event that could be forcing flow at the lower levels of the atmosphere

Results Using the JES as an Example

Regions of JES

Example of Air-Sea Interaction:

Low level Atmospheric Wind Stress Driving the

Oceanic Surface Currents in the JES

12Z MAY 16 through12Z MAY 17, 1998

Example of Air-Sea Interaction:

LH

L

L

L

H

L

00Z MAY 30 through12Z MAY 31, 1998

L

H

L

H

L

H

L

H

L

00Z MAY 24 through12Z MAY 25, 1998

L

H

L

H

L

L

L

00Z through12Z MAY 27, 1998

Coastal Upwelling off the Russian Coast inthe Northern JES

Due to strong southerlies leading to cyclonic turning and offshore flow of the

normally southwestward, along-shore flowing Liman Current

L

H

15°C isotherm

26 MAY 1998

Old 15°C isotherm

1527 MAY 1998

00Z through12Z July 24, 1998

Warm Currents enforcingupward vertical motion of a

developing cyclonein the YES

Weaknesses of CAOCSCAOCS possesses an erroneous Surface

Longwave Radiation FluxCAOCS has trouble with the open ocean

boundary

Conclusions

In general, the oceanic component of CAOCS performs well in simulating the EAMS surface current circulation, SST structure, and SSS structure.

Surface winds of the atmospheric component of CAOCS verified well against NCEP surface wind fields during May through July 1998.

Conclusions (cont.)

The impact of the atmosphere on the ocean sea surface temperature is also significant but to a lesser degree.

The impact of wind stress on surface current is significant.

Oceanic SSS fields are altered due to atmospheric forcing but to a lesser degree than SST and surface velocity fields.

Conclusions (cont.)CAOCS atmospheric and oceanic output is indicative of the impact of ocean thermal structure on the lower level of the atmosphere.

Conclusions (cont.)CAOCS output clearly demonstrates the presence of numerous atmospheric mesoscale features that either develop over the EAMS or transit over the EAMS on relatively small temporal scales both during periods prior to summer monsoon onset and during periods following summer monsoon onset.

Conclusions (cont.)CAOCS output clearly demonstrates the presence of numerous oceanic mesoscale features that develop over the EAMS with a relatively small temporal scale both during periods prior to summer monsoon onset and during periods following summer monsoon onset.

Conclusions (cont.)Results clearly show that a climatologically forced atmospheric (oceanic) model will be far less representative of the actual atmosphere (ocean) than a coupled system because air-sea interaction plays such a crucial role at a relatively short temporal scale. The climatologically forced model will be misrepresentative of the low-level atmospheric wind stress and the oceanic surface velocity, SST, and SSS fields.

Conclusions (cont.)Although an atmospheric (oceanic) model that is forced with previously analyzed oceanic (atmospheric) model output is useful for research purposes, the experienced delay during the process is insufficient for METOC support to the Fleet.

Conclusions (cont.)CAOCS has the potential to be an extremely useful tool for USN METOC personnel because of its verification and near-real time capability at the mesoscale level of a littoral region.

CAOCS support the concept behind NRL’s COAMPS future capability.

Recommendations for further researchComparison of winter and summer monsoon using the CAOCS

The inclusion of an acoustic prediction system as part of the CAOCS and comparison with an uncoupled acoustic prediction system

Impact of air-sea interaction at lower depths of the ocean using the CAOCSA detailed study that focuses solely on the comparison of coupled model output versus uncoupled model output