A new ocean GCM for tropical ocean and ENSO studieshorizon.documentation.ird.fr/exl-doc/pleins_textes/doc34-08/30231.pdf · RESUME iii ACKNOWLEDGMENTS vi INTRODUCTION 1. Motivation

445

A New Ocean GeM for Tropical Ocean and ENSO Studies

Peter R. GENT

NCAR, P.O. Box 3000,Boulder, CO 80307 - U.SA.

ABSTRACf

A new reduced gravity, primitive equation model of the upper equatorial ocean has beendeveloped by Gent and Cane (1989). It has been configured to simulate the annual cycle in the tropicalPacific Ocean. It is forced by the monthly winds from Rasmusson and Carpenter (1982) and a new heatflux formulation from Seager et al. (1988). The coefficients of vertical eddy viscosity and conductivitydepend upon the Richardson number of the flow. With this mixing a deep warm pool forms in the WesternPacific, increasing the west-east gradient of the thermocline and equatorial undercurrent. The model hasbeen diagnosed as to what factors maintain this deep warm pool.

1. Heat flux parameterization.

Heat Flux =Incoming Solar - Latent - (Sensible + Longwave)

* Incoming Solar Flux =(1 - A) (1 - 0.62 C + 0.(0198) QoA is albedo, C is % cloud cover, 8 is solar altitude, Qo is clear sky flux

* Latent Heat Flux = Pa CE L [u] (1-8) qs(SST)[u] is wind speed with minimum of 4 m s', q (SST) is saturationhumidity, and humidity factor 8 is related to relative humidity, 8rh,

8 qs(SST) =8rh qs(Ta)

* Sensible + Longwave Flux =Cl (SST - T*)

[A =0.06,8 =0.78, Cl =1.8 m s', 1'* =2.8°C]

2. Vertical mixing coefficients.

* Diffusivity: v = 1()3 (1+lORJ2 + VB

* Conductivity: K = 1()3 (1+lORJ3 + KB

~B (z) varies between 1 and 2 cm- s-lKB (z) varies between 1/2 and 1 cm2 s-l

In the COARE volume, Ri is nearly always> 3, so that the vertical mixing isdone solely by the background values VB and KB•

Sections of average temperature and zonal velocity in January along the equatordown to 400 m and along 160° E down to 500 m are shown in Figures 1-4.

&0.101

lDS.W

130.W

155.W

lBO.W

155.E

130.E

446

o. so. 100. lSO. 200. 250. 300. 350. too.

FIG.I. Average JanuaryTemperature Cc) alongEquator.

80.1J

lDS.H

130.W

155.W

lBO.W

155.E

130.Eo. so. 100. lSO. ZOO. 250. 300. 350. too.

FIG.2.Average JanuaryZonal velocity (cm.s') alongEquator.

2O.N

15.N

10.N

5.N

EQ

5.5

10.5

15.5

20.5O. 100. 200.

447

300. 400.

FIG.3. Average January Temperature ("e) along 160oE.

2O.N

15.N

10.N

S.N

EQ

5.5

10.5

15.5

20.5

---- -- -- -----;..-10-- __ .J

'-._--~"----------...]0 J_.....__- __ -- lo'

:;f.~__ -

------=-=20--':- -- -~1O- _:._------ ---

1------0r-------u---------- ----;JO

-- -141- -- __ --

o

O. 100. 200. 300. 400. 500.

FIG.4. Average January Zonal Velocity (cm.s') along 160oE.

448

3. Heat balance over the TOGA-COARE domain.

1400E-1800E; lOoS-lOoN; 0-150 m (average)Advection and temperature gradients in the domain are small, and the advection

tends to be parallel to the isotherms, so that the 3-D heat advection in the domain issmall. Thus, if the vertical heat diffusion is small, then the heat flux must also be small.This is confirmed in the model, where over the COARE domain Ri mixing andhorizontal diffusion are negligible. Thus, over a year

d/dt(hT) = -3-D Advection + Heat Flux - Vertical diffusiono == -3.9 + 13.5 - 9.4 W m-2

The model SST for January and the SST such that Q "" 0 (Figures 5 and 6) showthat, over the COARE area, SST is set by Q(SST) "" O.

+ d/dy(hvT) + d/ds(wT)+ 202.1 + 48.1 W m-2

Heat Flux =13,5 =

V.(huT)3.9

Incoming Solar189,8

= a/ax(huT)= -246.3

- Latent- 117,6

- (Sensible + Longwave)- 58,7 W m-2

Thus the 3-D heat advection is a small residual of large component terms and,therefore, will be very difficult to measure. Alternatively, a small error in the very smallangle between velocity and isotherms will cause a large error in the 3-D advective heatflux estimate. Quantitatively

IV.(huT) 1/ { ~[a/axi(huiT)f pn = 0.012

4. What sets the SST and depth of the warm pool?

The hypothesis that 0 < Q(SST) < 20 W m-2 was confirmed by setting theminimum value of lul to 3 m s-l (not 4) in the latent flux. This increases the temperature,such that Q =0, from about 29.5 ~ 30°C (min =4) to about 33 ~ 33.5°C (min =3) onthe equator between BooE and 1500E. The actual model SST in this region increasesfrom about 29.5°C (min =4) to about 32°C (min = 3). When the minimum is set to 3m.s', the outgoing latent flux is reduced by 1/4 in the region, or about 30 W m-2• Thus,the sensitivity of SST in this region to errors in the heat flux is

Sensitivity == dT/dQ = 2.5/30 = 1/12 °C W-l m-2

Thus, an error of 16 W m-2 in Q will lead to an error of 1°C in SST in this region of themodel. With lul min =3 m s-l, the heat balance over a year is

0"" -5.5 (3 - D A) + 17.7 (H F) - 10.5 (V D) W m-2

This similar to the min =4m s-l case, but is not so close to equilibrium.

The depth of the warm pool in this model is set by KB• In a run where KB and vwere increased by factors of 2 and 10, with no R, mixing, the depth of the warm poo'(water>27°C) was reduced by about 5Om. Thus the run with smaller KB, which cansustain a sharper thermocline, has a deeper warm pool. With a larger KB, there is morevertical diffusion of heat, so that the COARE region SST is reduced by close to 1°C toproduce a larger heat flux. With increased VB and KB, the heat balance over a year is

0"" 3.2 (3 - D A) + 25.0 (H F) - 21.3 (V D) W m-2.

449

lO.N

IS.N

10.N

S.N

Ea

S.S

10.5

15.5

20.5130.E 155.E 180.101 155.101 130.101 105.101 10.11

FIG.5. Average January SeaSurfaceTemperature ('C).

2O.N

15.N

10.N

S.N

Ea

5.5

10.5

15.5

20.5130.E 155.E 180.101 155.101 130.104 105.101 to.lJ

FIG.6.Average January Temperature ('C) such thatQ= O.

450

REFERENCES

Gent, P.R. and M.A. Cane, 1989: A reduced gravity, primitive equation model of theupper equatorial ocean. J. Comp. Phys., 81, 444'-480.

Rasmusson, E.M. and T.H. Carpenter, 1982: Variations in tropical sea surfacetemperature and surface wind fields associated with the Southern oscillation/ElNino. Mon. Wea. Rev., 110,354-384.

Seager, R., S.E. Zebiak and M.A. Cane, 1988: A model of the tropical Pacific sea surfacetemperature climatology. J. Geophys.Res., 93, 1265-1280.

WESTERN PACIFIC INTERNATIONAL MEETING

AND WORKSHOP ON TOGA COARE

Noumea, New Caledonia

May 24-30, 1989

edited by

Joel Picaut *Roger Lukas **

Thierry Delcroix *

* ORSTOM, Noumea, New Caledonia** JIMAR, University of Hawaii, U.S.A.

INSTITUT FRANCAIS DE RECHERCHE SCIENTIFIQUEPOUR LE DtVELOPPEMENT EN COOPtRATlON

Centre de Noumea

vii

TABLE OF CONTENTS

ABSTRACT i

RESUME iii

ACKNOWLEDGMENTS vi

INTRODUCTION

1. Motivation 12. Structure ..... .......................... ... ... .......... ............. ......... .... ...... .. ...... . .. 2

LIST OF PARTICIPANTS 5

AGENDA 7

WORKSHOP REPORT

1. Introduction ............................... ............. .. .......... .. ....... ...... .... ... ...... .. 192. Working group discussions, recommendations, and plans 20

a. Air-Sea Fluxes and Boundary Layer Processes 20b. Regional Scale Atmospheric Circulation and Waves 24c. Regional Scale Oceanic Circulation and Waves 30

3. Related prograDlS ................. ......... ......... ............ .......... ...... .... . ........ . . 35a. NASA Ocean Processes and Satellite Missions .. . .. .. . 35b. Tropical Rainfall Measuring Mission .. . .. . . 37c. Typhoon Motion Program 39d. World Ocean Circulation Experiment .. . .. .. . 39

4. Presentations on related technology ....... ............ .. .. ..... ... ..... ... .. ...... .. . 405. National reports 406. Meeting of the International Ad Hoc Committee on TOGA COARE 40

APPENDIX: WORKSHOP RELATED PAPERS

Robert A. WeUer and David S. Hosom: Improved MeteorologicalMeasurements from Buoys and Ships for the World OceanCirculation Experiment ............. .. .... ............. .......... .. ........ ....... .... . ....... .... 45Peter H. Hildebrand: Flux Measurement using Aircraftand Radars 57-Waiter F. Dabberdt, Hale Cole, K. Gage, W. Ecklund and W.L. Smith:Determination of Boundary-Layer Fluxes with an IntegratedSounding System 81·

viii

MEETING COLLECTED PAPERS

WATER MASSES. SEA SURFACE TOPOGRAPHY. AND CIRCULATION

KJaus Wyrtki: Some Thoughts about the West Pacific Warm Pool.................. 99Jean Rene Donguy, Gary Meyers, and Eric Lindstrom: Comparison ofthe Results of two West Pacific Oceanographic Expeditions FOC (l971)and WEPOCS (1985-86) 111Dunxin Hu, and Maochang Cui: The Western Boundary Current in theFar Western Pacific Ocean 123Peter Hacker, Eric Firing, Roger Lukas, Philipp L. Richardson. andCurtis A. Collins: Observations of the Low-latitude Western BoundaryCirculation in the Pacific during WEPOCS ill ................ .. . . .. .. .. 135Stephen P. Murray, John Kindle, Dharma Arief, and Harley Hurlburt:Comparison of Observations and Numerical Model Results in the IndonesianThroughflow Region 145Christian Henin: Thermohaline Structure Variability along 165eEin the Western Tropical Pacific Ocean (January 1984 - January 1989) 155David J. Webb. and Brian A. King: Preliminary Results fromCharles Darwin Cruise 34A in the Western Equatorial Pacific 165Warren B. White, Nicholas Graham. and Chang-Kou Tai: Reflection ofAnnual Rossby Waves at The Maritime Western Boundary of the TropicalPacific ..... .......... ... .. .. .... .... ... .............................. ............ ........ ... .... .... .... 173William S. Kessler: Observations ofLong Rossby Waves in the NorthernTropical Pacific .......................... ..... .. .. ... . .. ... . ........... .. .. ......... .... . .. .. ... ... .. 185Eric Firing, and Jiang Songnian: Variable Currents in the WesternPacific Measured During the US/PRC Bilateral Air-Sea Interaction Programand WEPOCS 205John S. Godfrey, and A. Weaver: Why are there Such StrongSteric Height Gradients off Western Australia? 215John M. Toole, R.C. Millard, Z. Wang, and S. Po: Observationsof the Pacific North Equatorial Current Bifurcation at the Philippine Coast 223

EL NINO/SOUTHERN OSCILLATION 1986-87

Gary Meyers, Rick Bailey, Eric Lindstrom, and Helen PhiUips:Air/Sea Interaction in the Western Tropical Pacific Ocean during1982/83 and 1986/87 229Laury Miller, and Robert Cheney: GEOSAT Observations of SeaLevel in the Tropical Pacific and Indian Oceans during the 1986-87El Nino Event 247Thierry Delcroix, Gerard Elmn, and Joel Picaut: GEOSAT SeaLevel Anomalies in the Western Equatorial Pacific duringthe 1986-87 El Nino. Elucidated as Equatorial Kelvinand Rossby Waves 259Gerard Eldin. and Thierry Delcroix: Vertical Thermal StructureVariability along 165eE during the 1986-87 ENSO Event 269Michael J. McPhaden: On the Relationship between Winds andUpper Ocean Temperature Variability in the Western EquatorialPacific ..... ..... ...... ... .. .... ... ........................................... ..... .. .. .... .. .... ........ 283

i"'{

John S. Godfrey, K. Ridgway, Gary Meyers, and Rick Bailey:Sea Level and Thennal Response to the 1986-87 ENSO Event in theFar Western Pacific 291Joel Picaut, Bruno Camusat, Thierry Delcroix, MichaelJ. McPhaden, and Antonio J. Busalacchi: Surface Equatorial FlowAnomalies in the Pacific Ocean during the 1986-87 ENSO using GEOSATAltimeter Data 301

TIlEORETICAL AND MODELING STUDIES OF ENSOAND RELATED PROCESSES

Julian P. McCreary, Jr.: An Overview of Coupled Ocean-AtmosphereModels of El Nino and the Southern Oscillation 313Kensuke Takeuchi: On Wann RossbyWaves and their Relationsto ENSO Events 329Yves du Penhoat, and Mark A. Cane: Effect of Low Latitude WesternBoundary Gaps on the Reflection of Equatorial Motions 335Harley Hurlburt, John Kindle, E. Joseph Metzger, and Alan Wallcraft:Results from a Global Ocean Model in the Western Tropical Pacific 343John C. Kindle, Harley E. Hurlburt, and E. Joseph Metzger: On theSeasonal and Interannual Variability of the Pacific to Indian OceanThroughflow 355Antonio J. Busalacchi, Michael J. McPhaden, Joel Picaut, and ScottSpringer: Uncertainties in Tropical Pacific Ocean Simulations: TheSeasonal and Interannual Sea Level Response to Three Analyses of theSurface Wind Field 367Stephen E. Zebiak: Intraseasonal Variability - A Critical Componentof ENSO? 379Akimasa Sumi: Behavior of Convective Activity over the "Jovian-type"Aqua-Planet Experiments 389Ka-Ming Lau: Dynamics of Multi-Scale Interactions Relevant to ENSO 397Pecheng C. Chu and Roland W. Garwood, Jr.: Hydrological Effectson the Air-Ocean Coupled System 407Sam F. Iacobellis, and Richard CJ. Somerville: A one DimensionalCoupled Air-Sea Model for Diagnostic Studies during TOGA-COARE 419AlIan J. Clarke: On the Reflection and Transmission of Low FrequencyEnergy at the Irregular Western Pacific Ocean Boundary - a PreliminaryReport 423Roland W. Garwood, Jr., Pecheng C. Chu, Peter Muller, and NiklasSchneider: Equatorial Entrainment Zone: the Diurnal Cycle 435Peter R. Gent: A New Ocean GCM for Tropical Ocean and ENSO Studies 445Wasito Hadi, and Nuraini: The Steady State Response of IndonesianSea to a Steady Wind Field .......................................................... ............ 451Pedro Ripa: Instability Conditions and Energetics in the Equatorial Pacific 457Lewis M. Rothstein: Mixed Layer Modelling in the Western EquatorialPacific Ocean 465Neville R. Smith: An Oceanic Subsurface Thermal Analysis Scheme withObjective Quality Control 475Duane E. Stevens, Qi Hu, Graeme Stephens, and David Randall: Thehydrological Cycle of the Intraseasonal Oscillation , 485Peter J. Webster, Hai-Ru Chang, and Chidong Zhang: TransmissionCharacteristics of the Dynamic Response to Episodic Forcing in the WannPool Regions of the Tropical Oceans .. _ 493

x

MOMENWM, REAT, AND MOISlURE FLUXES BETWEENATMOSPHERE AND OCEAN

W. Timothy Liu: An Overview of Bulk Parametrization and RemoteSensing of Latent Heat Flux in the Tropical Ocean ...................................... 513E. Frank Bradley, Peter A. Coppin, and John S. Godfrey: Measurementsof Heat and Moisture Fluxes from the Western Tropical Pacific Ocean 523Richard W. Reynolds, and Ants Leetmaa: Evaluation of NMC'sOperational Surface Fluxes in the Tropical Pacific 535Stanley P. Hayes, Michael J. McPhaden, John M. Wallace, and JailPicaut: The Influence of Sea-Surface Temperature on Surface Wind in theEquatorial Pacific Ocean 543T.D. Keenan, and Richard E. Carbone: A Preliminary Morphology ofPrecipitation Systems In Tropical Northern Australia 549Phillip A. Arkin: Estimation of Large-Scale Oceanic Rainfall for TOOA 561Catherine Gautier, and Robert Frouin: Surface Radiation Processes inthe Tropical Pacific 571Thierry Delcroix, and Christian Henin: Mechanisms of SubsurfaceThermal Structure and Sea Surface Thermo-Haline Variabilities in the SouthWestern Tropical Pacific during 1979-85 - A Preliminary Report 581Greg. J. Holland, T.D. Keenan, and MJ. Manton: Observations from theMaritime Continent: Darwin, Australia 591Roger Lukas: Observations of Air-Sea Interactions in the Western PacificWarm Pool during WEPOCS 599M. Nunez, and K. Michael: Satellite Derivation of Ocean-Atmosphere HeatFluxes in a Tropical Environment ............................................................. 611

EMPIRICAL SlUDIES OF ENSO AND SHORT-TERM CLIMATE VARIABILITY

Klaus M. Weickmann: Convection and Circulation Anomalies over theOceanic Warm Pool during 1981-1982 623Claire Perigaud: Instability Waves in the Tropical Pacific Observed withGEOSAT 637Ryuichi Kawamura: Intraseasonal and Interannual Modes of Atmosphere;.Ocean System Over the Tropical Western Pacific 649David Gutzler, and Tamara M. Wood: Observed Structure of ConvectiveAnomalies 659Siri Jodha Khalsa: Remote Sensing of Atmospheric Thermodynamics inthe Tropics 665Bingrong Xu: Some Features of the Western Tropical Pacific: Surface WindField and its Influence on the Upper Ocean Thermal Structure 677,Bret A. Mullan: Influence of Southern Oscillation on New ZealandWeather 687Kenneth S. Gage, Ben Basley, Warner Ecklund, D.A. Carter, andJohn R.McAfee: Wind Profiler Related Research in the Tropical Pacific 699John Joseph Bates: Signature of a West Wind Convective Event inSSM/I Data 711David S. Gutzler: Seasonal and Interannual Variability of the Madden-Iulian Oscillation 723Marie-H~lene Radenac: Fine Structure Variability in the Equatorial WesternPacific Ocean 735George C. Reid, Kenneth S. Gage, and John R. McAfee: The Oimatologyof the Western Tropical Pacific: Analysis of the Radiosonde Data Base 741

xi

Chung-Hsiung Sui, and Ka-Ming Lau: Multi-Scale Processes in theEquatorial Western Pacific , 747Stephen E. Zebiak: Diagnostic Studies of Pacific Surface Winds 757

MISCELLANEOUS

Rick J. Bailey, Helene E. Phillips, and Gary Meyers: Relevance to TOGAof Systematic XBT Errors 775Jean Blanchot, Robert Le Borgne, Aubert Le Bouteiller, and MartineRodier: ENSO Events and Consequences on Nutrient, Planktonic Biomass,and Production in the Western Tropical Pacific Ocean 785Yves Dandonneau: Abnonnal Bloom of Phytoplankton around weN in theWestern Pacific during the 1982-83 ENSO 791Ceclle Dupouy: Sea Surface Chlorophyll Concentration in the South WesternTropical Pacific, as seen from NIMBUS Coastal Zone Color Scanner from1979 to 1984 (New Caledonia and Vanuatu) 803Michael Szabados, and Darren Wright: Field Evaluationof Real-Time XBT Systems 811Pierre Rual: For a Better XBT Bathy-Message: Onboard Quality Control,plus a New Data Reduction Method 823