Development of an operational coastal

ocean observing system for the South

Atlantic Bightor

Operational modeling – what’s it gonna take? An

attempt in the SAB

The SABLAM group

SABLAM Group (NOPP, 2000)

• Dartmouth: Dan Lynch, [Chris Naimie], Keston Smith, Jeff Proehl

• UNC-CH: Cisco Werner, Rick Luettich, Brian Blanton, Alfredo Lopez de Aretxabaleta, Luke Stearns, Harvey Seim

• WHOI: Dennis McGuillicuddy• SkIO: Jim Nelson, Trent Moore• [MCNC: Eric Sills]• JAX, CHA WFO: Pat Welsh, [Stephen

Brueske][ ] – no longer participating

SABLAM Objective

To develop a portable, limited-area modeling system that provides an operational forecast of conditions in the coastal ocean that includes the influence of tides, local air-sea interactions, buoyancy and remote forcing.

Tall order. Requires….

A coastal ocean observing system

Modeling perspective

•Climatology•Eventually, (soon) HYCOM/GODAE

Initialization

Nested Meteorological/Ocean Models

SAB Climatology

HYCOM or COFSBaroclinic/Gulf Stream Forcing

ADCIRC DomainFar-Field Tide and

Wind-Band Forcing

SABLAM DomainQUODDY

TRUXTON/CASCOLimited-Area Shelf Models,

Data Assimilative

AWIP 32 Domain10 km ETAMet. Model

High Resolution, RegionalETA Model

Improved Air/Sea Interaction

Will examine some of the stumbling blocks encountered in SABLAM:

Getting a good priorBarotropic dynamics: tidal, weather band, lower

frequency

Baroclinic dynamics (density field): from climatology, SST/in-situ obs, basin-scale models

Assimilation - a technique to minimize difference between sparse observations and simulated fields

Three components: frequency domain inverse,time domain inverse, objective analysis

Barotropic tides: should be straightforward…

•Finite Element•2D (ADCIRC)•Time-dependent•Fully Nonlinear•Elev. BCs from Global FES95D

Performswell exceptin SAB

Problem: typical coastal tide station

is not “in” the typical shelf model domain

Fort Pulaski, GA

Landward Bndy of Operational models

In the SAB large sections of the coastline are backed by extensive

estuaries

No Estuary

Boundary

This coastal geometry is concentrated between central

SC and north FL

“No Estuary”

Boundary

•Finite Element •Nonlinear•2D (ADCIRC)•Western North Atl.•Crossshelf Amplification•Equatorward phase propagation •Latest phase along GA/FL border

•Shelf response sensitive

NC

SC

FL

GA

M2 Elevation Prior without estuaries – tide experiences

two-fold amplitude increase and notable phase change

M2 Obs Vs. Prior, without Estuaries

Larger phase error closer to shore

Amplitude (m) Phase (deg)

M2 Obs Vs. Prior, with Estuaries

Substantially reduced phase error closer to shore

Amplitude (m) Phase (deg)

M2 Phase Comparison

RED = without EstuariesBLUE = with Estuaries

FL

RED = without estuariesBLUE = with estuaries

Implication: different M2 energy flux required to support estuarine dissipation

Operationally:need unstructuredgrids or true two-way coupling toaccurately representtide along thistype of coastline

Nontidal prior response – from 2D wind-forced model of western North Atlantic. Comparison of detided CSL at Mayport, FL

Observed CSL Modeled CSL

Weather-band (<15 day) comparison favorable; some under-estimate during large events…

Observed CSL Modeled CSL

Consistentwith under-estimate oflongshorewinds seenin ETA predictions(cross-shoreand tempslook OK)

Observed CSL Modeled CSL

At lower frequencies (>15 day) the comparison is less favorable; see some 25 cm offsets; partly steric but…

Blaha, JGR ’84 (?)found coherent monthly averagedsea level variationsover SAB (’55-’75 period, heatingand atmos. presseffects removed).Can be more than 20 cm variation annually. Postulated due toGulf Stream transportvariations.

Noble/Gelfenbaum – modeled coastal SL impact of GS transport variations.

Coast

Shelf

Gulf Stream

Average transport

Low transport

Offshore Fixed “Hinge”

Coast

Shelf

Gulf Stream

High transport

Average transport

Offshore Fixed “Hinge”

Low transport,higher CSL

High transport,lower CSL

Baringer/Larsen

Climatology (3)

Objective analysis digital analog to Atkinson et.al. (1983)

TEMP

SALN

t

Climatology (5)Cross-shelf Structure from Objective

Analysis

Climatology (7)Monthly Mean COADS Winds Only

Monthly BaroclinicSolution

Winds + BC

Climatology …

Charleston Bump½ x ½

deg squares

Bottom depth:

<400m

>400m

GODAE into SABLAM

Micom D180

•Mid summer

•Reanalysis

•Unrealistic

upwelling

Data Assimilation System

Wind+Tide

Data Assimilative Loop (1)

Data Assimilative Loop (2) Far-Field computation of Wind+Tide

Data Assimilative Loop

SABSOON/SABLAM Data

East Coast Domain for Tidal/Wind-Driven BCs for Limited-Area Mesh

Nested SABLAM Mesh for Hindcast/Forecast System

Obs. Locations:

Water Level

SABSOON ADCP

NC

SC

GA

FL

3000m

1000m

200m

50m25m

M2 Phase Comparison

GA

FL

RED = W/O EstuariesBLUE = W/ Estuaries

>3

Phas

e D

iff.

Lower Mean Sealevel

Coast

Shelf

Gulf Stream

Higher Transport

Average Transport

Offshore Fixed “Hinge”

xnV

Increased Transport

Increased Cross-stream Slope

LOWER Coastal Sea level

Noble/Gelfenbaum

Higher Mean Sealevel

Coast

Shelf

Gulf Stream

Average Transport

Lower Transport

Offshore Fixed “Hinge”

xnV

Decreased Transport

Decreased Cross-stream Slope

HIGHER Coastal Sealevel

Noble/Gelfenbaum