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

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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·

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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

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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

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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

329

On Warm Rossby Waves and their Relations to ENSO Events

Kensuke TAKEUCHI

Department ofGeophysicsHokkaido UniversitySapporo, 060 Japan

ABSTRACf

The hypothesis suggested by several simple air-sea coupled models is examined with an oceannumerical modeldriven by FSU wind stress for the period from 1963 to 1987.The examined hypothesis isthat warm Rossby waves generated by stronger trades associating with cold events following warmepisodesare responsible for triggering the next warmevents. As results, followings are found.

1) Warm Rossby waves are always found approaching the western boundary of the tropical Pacificin the precedingwintersof the warm events.

2) However, some winters are not followed by warm episodeseven though warm Rossby waves arefound near the westernboundary.

3) The warm Rossby waves are mostly generated in the central and western tropical Pacific in thepreceding yearsand no clear relations havebeen found between the previouswarm events.

1. Introduction

Is ENSO cyclic or random '/ This is the most fundamental question about ENSO.If ENSO is cyclic, the previous warm event is the causes of the next cold event which inturn gives rise to the next warm event. On the other hand, if ENSO is essentially random,the trigger of an event has no relation to the previous events.

Recently, several simple atmosphere-ocean coupled models succeeded toreproduce ENSO-like vacillations (Cane and Zebiak, 1985; Schopf and Suarez, 1987;Xie, 1988). One notable common feature of their results is that warm events are triggeredby warm Kelvin waves which are generated by reflection of warm Rossby waves at thewestern boundary. The warm Rossby waves are forced by stronger than usual Trades inthe cold events following the previous warm events. So, these results suggest ENSO iscyclic.

It has been already pointed out by several authors that the warm water pile-ups inthe western tropical Pacific are nearly always found in the previous northern winter ofthe beginning of warm events (White et al., 1985; Matsumoto and Takeuchi, 1988).However, relations between these pile-ups and the previous events are still not clear.Observational data are not enough to examine them.

Therefore, in the present study, a numerical model is used to examine theformation process of the warm water pile-ups in the western tropical Pacific, for theperiod from 1969 to 1984.

2. MODEL

The model simulates the tropical Pacific Ocean between 30·S and 30·N. Themodel ocean has realistic coastline at western and eastern sides of the model, but nobottom topography. The numerical procedure for the integration is similar to thatdescribed in Bryan (1969). The model specifications are:

Coordinate :Horizontal resolution:Vertical resolution:Horizontal mixing :Vertical mixing :

Salinity:Sea surface heat flux:Wind stress :

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Cartesian delta-plane2/30 in both direction15 levels, 25 m in upper 250 mBi-hannonicRichardson number dependent mixingcoefficient, similar to Pacanowskiand Philander (1981)Not includedHaney (1971)' s typeFSU (Goldenberg and O'Brien 1981)

b

72 73 711 75 76 77 78 79 80 81· 82 83 811 85TIHE (rEAR)

O~u.;.;.:~L..L..L......JL--L.L.J"--.L1-L=.L.1-a-LLL;(;;';';';~==~;t.;.;.;L.&....-.a:.;;.:.;;a;.&;.;,;:.:a.:;;.;.:.x.:a:a...........J

72 73 711 75 76 77 78 79 80 81 82 83 811TIHE (YEAR)

FIG.I. Surfacedynamic heightanomaly (re. 400db) along I37°E, as derived from (a) observeddata takenby JapanMeteorological Agency, and (b) the model.

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EQ-HRVE C~HPONENT

lESS SEASONALHDDE 3

VT-MODE

HODE 3

78

77"

8011

l~OE 160E 180M 160W I~OM 120H I OOH 80MnOE 160E 180W 16011 IQOW 12011 100H 80H

71

751176.

FIG.2. Time versus longitude diagram showing the amplitude of the 3rd meridional first vertical modeRossby wave. Big tip-marks on the vertical axis are boundaries between years. Darker shade indicatewarm Rossby wave.

332

At the northern and southern boundaries, the meridional volume transport iscalculated assuming Sverdrup relation. Otherwise, there is no derivative in north andsouth direction. Newtonian dumping is applied for temperature near the boundaries tothe climatology.

Initially, the ocean is at rest and uniformly stratified . First, the model is driven byannual mean wind for more than ten years, using one degree resolution model, then threeyears using 2/3 degree resolution model. The integration is continued for three yearsusing climatological seasonal wind, and then interannual run take over the integration.

3. Warm water pile-up in the model and the real ocean

Analyses of the model results are meaningless unless the model simulates the realocean well enough. As the purpose of the present study is to examine the pile upprocesses of warm water in the western tropical Pacific, comparison between model andobservation is made for dynamic height anomaly along the 137"E (Fig. 1). Observeddynamic height is calculated from data taken by Japanese Meteorology Agency incruises carried out routinely twice a year.

Quasi -biannual variations can be seen in both figures in lower latitudes in the70's. Positive anomalies at the beginning of 1972 and 1976 are followed by warmevents. For the 1982-1983 event, preceding positive anomalies are not strong, but arestill visible. Therefore, as far as these periods are concerned, all warm events arepreceded by positive anomaly in dynamic height, which is a consequence of the warrnwater pile up. However, the contrary is not true. For example, strong positive anomalyfound at the beginning of 1974 is not followed by a warm event.

Two conclusions can be derived. First, warm water pile up is a necessaryprecondition for a warm event, but not a sufficient condition. Second, the model simulatethe warm water pile up well enough to suggest that analyses of the model is meaningful.

4. Equatorial wave analysis

The piling up process can be either dynamical or thermal. However, the thermalprocess is suggested to be not essential as the positive dynamic height anomalies are alsoreproduced in 1.5 layer model which doesnot include any thermal process at all(Takeuchi, 1987).

If the process is dynamical, and if non-linear effects are not too strong, theprocess may be understood in terms of wave dynamics. From the wave dynamic point ofview, the warm water piling up process can be seen as formation process of warmRossby wave, thus generation and propagation can better be seen.

For the purpose, each equatorial wave component is isolated . First, the meandensity field is calculated, and vertical mode functions are computed at every locationusing the mean density distribution. Next, pressure anomaly distribution is decomposedonto vertical modes. Finally, each meridional mode is isolated using hyperboliccylindrical functions.

Fig. 2 shows time versus longitude distribution of the third Rossby wavecomponent of the first vertical mode. In this figure, we can see warm Rossby waves areapproaching the western boundary in the periods when positive dynamic heightanomalies are found in Fig. I. Other lower mode Rossby wave show similar behavior. Itindicates that the warm water pile up is not formed locally. Then the problem is whereand when the warm Rossby waves are generated. The figure shows that the formationarea is the central Pacific, say, between the date line and 150·W. They are generated inthe previous years of the beginning of warm events.

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Also very notable is the similarity between the one to tow year periods precedingthe major warm events, 1972-1973 and 1982-1983. Three warm Rossby packets arepropagating from central Pacific to the western boundary in each period. The firstpackets are strongest but tend to die out before reaching the western boundary. Thesecond ones are responsible for the positive anomaly appeared in Fig. 1. The similarityalso can be seen to some extent for the 1976 minor warm event.

Another common feature is that each warm event is followed by positivedynamic height anomaly in the western tropical Pacific, as seen in Fig. 1. Warm Rossbywaves are also found in Fig. 2. However, in the period analyzed in the present study, nowarm event follow the warm water piling up.

5. Discussion and conclusion

Through the present study, the followings are found.

1) Piling up of warm water in the western tropical Pacific is a necessaryprecondition for a warm event to develop, but not an efficient condition.

2) The piling up of warm water is formed in the central Pacific in the previous yearof the beginning of each warm event and propagates to the west as warm Rossbywave.

3) . The longitude versus time distribution of Rossby wave component shows similarpattern in each 1 to 2 year period previous to the beginning of warm event.

4) In the following years of warm events, formation of warm Rossby waves arealways found, but not followed by the next warm event.

These results suggest that ENSO cycle is not as simple as simple coupled modelsindicate, even if it is a cycle. After a warm event terminates, a cold phase appears andstronger than usual trade winds generate warm Rossby waves in the central Pacificwhich propagate westward resulting in an anomalous warm water piling up in thewestern Tropical Pacific. However, at least in the period examined in the present study,no warm event developed in such a situation.

On the other hand, generation of warm Rossby waves in the previous year of eachwarm event seems to have no apparent relation with the previous warm event. We stillhave missing rings to be found. The similarity of the behavior of central to westerntropical Pacific in the previous year of 1972-73 and 1982-83 events seems to be verysuggestive.

REFERENCES

Bryan, K., 1969: A Numerical Method for the Study of the Circulation of the WorldOcean. J. Compt. Phys., 4, 347-376

Cane, M.A. and S.E. Zebiak, 1985: A theory for El Nino and the Southern Oscillation.Science, 228, 1085-1087

Goldenberg, R.L. and J.J. O'Brien, 1981: Time and space variability of tropical Pacificwind stress. Mon. Wea. Rev., 109, 1190-1207

Haney, RL., 1971: Surface Thermal Boundary Condition for Ocean Circulation Models.J. Phys. Oceanogr., 1,241-248

Matsumaoto, Y. and K. Takeuchi, 1988: Interannual Variation in the Western TropicalPacific Ocean and Its Relation to ENSO, seen in the Japan MeteorologicalAgency 137°ELine Observations. Geophys. But.Hokkaido Univ., 51,63-75

Pakanowski, R and S.G.H. Philander, 1981: Parameterization of vertical Mixing inNumerical Models of Tropical Oceans. Geophys. Res. Lett., 11,802-804

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Pazan, S.E., W.B. White, M. Inoue and J.1. O'Brien, 1986: Off-equatorial Influence uponpacific Equatorial Dynamic Height Variability during the 1982-1983 ElNino/Southem Oscillation Event. J. Phys. Oceanogr., 91,8437-8449

Schopf, P.S. and M.J. Suarez, 1987: Vacillations in a coupled ocean-atmosphere model.J. Atmos. Sci., 45, 549-566

Takeuchi, K., 1987: Forerunners of El Nino/Southem Oscillation Appearing in the FSUModel, Geophys. Bul. Hokkaido Univ, 49,381-386.