457

Instability Conditions and Energetics in the Equatorial Pacific

Pedro RIPA

CICESE, Espinoza 843,22800 Ensenada, BC - Mexico

ABSTRACf

Instability conditions applicable to zonal flows in the equatorial oceans are presented. Amulti-layer model, with either rigid bottom (including the effects of topography) or "reduced gravity"lower boundary, is employed. Preliminary use of these theoretical results is illustrated with the zonalgeostrophic velocity profiles calculated from the Hawaii-Tahiti Shuttle Data (Wyrtki and Kilonsky, 1984).Sufficient stability conditions - or necessary instability ones - come in two different sets : the second one isrelated to the sign of the energy of growing perturbations.

1. Introduction

In a 1Ill-layer model, if there exists a value a. such that

and,

[V(y)-a.] (dQ/dy) < 0

g'H(y) > [V(y)-a.]2

(la)

(2a)

for all y, then the zonal flow (u, v, h)=(V, 0, H) is stable (Ripa 1983)1. There aresufficient stability conditions; necessary instability conditions are, therefore, that for anyvalue of a., either (la) or (2a) or both must be violated.

In a model with two layers (Ripa, 1987, 1989a-b; McPhaden and Ripa, 1990),stability condition (la) is simply replaced by

[VI -cl (dQ1/dy) < 0 and [U2 -a.] (dQfdy) < 0, (I b-e)

whereas the second condition, (2a), changes into

g' > [U1- a.]2/H1+ [V2 - a.]2/H2

if the system has a rigid bottom (which might include topography), or into

g'l < [g'l+g'2 - (U, - a.)2/Htl [g'2- (V2-a.)2/H2]

(2b)

(2c)

in the "reduced gravity" case (i.e., when the lower boundary is the interface with a third,motionless layer); these last two systems are called the "2-layer" and "2l/2-layer" model,respectivel y.

Two peculiarities of the stability conditions might call the attention:Firstly, an arbitrary constant a. may be subtracted from the fields Ujy) : the

variations of V is what matters. Thus uniform flow, V. = constant is stableJ to finiteamplitude perturbations. In particular, there is no "barodinic instability" in a 11/2-layer

1.The notation is standard, e.g., g' is the reduced gravity and Q = (f - dU/dy)IH, denotes potentialvorticity, The basic flow is in geostrophic balance: fU +g'dH/dy =O.

458

model. This powerful property follows from zonal homogeneity of the system equationsand boundary conditions (e.g., -plane or the sphere, with topography function only oflatitude, and with coasts along parallels), as well as of the basic flow underconsideration.

Secondly, there are two different stability conditions, or two sets of them. Eventhough this is by no means novel, it is because of the absence of the second condition inthe quasi-geostrophic models (not applicable to the equatorial zone) that some readersfind it difficult to accept. The method used to find the stability conditions, and therelationship of the second one with the sign of perturbation energy is discussed next.

2. Stability conditionsand integrals of motion.

The procedure devised by Amol'd (l965, 1966) consists in finding conditionssuch that if satisfied by a particular basic flow, [u.h] =[V,H], then a certain integral ofmotion, S[u,h], has an extreme at the basic solution, i.e., .

S[u,h] =constant,

for any initial condition, and

BS =S[U+u', H+h'] - S[U,H] > 0,

for all finite perturbations, [u' .h'], which are different from zero and sufficiently small.Now, BS is an exact, finite-amplitude, constant of motion (N.B., S[U,H] = constant,because the basic flow is steady) which, to lowest order, is quadratic in the perturbation.This lowest order is composed of two parts : the first one involves the s~uare of theperturbation potential vorticity, and the second one is the perturbation energy. The latteris not necessarily positive, because there might exist perturbations that diminish the totalkinetic energy more than they increase the total potential energy, by means ofanti-correlated changes of velocity and layer thickness. The two stability conditions, e.g.(la, b, or c) and (2a, b, or c), are precisely those that guarantee that both quadratic partsof BS are positive definite.

Since BS is constant, in the case of an unstable basic flow a small perturbationmust have a vanishing net value of the quadratic part of BS (Just think of an infinitesimalwave whose amplitude grows exponentially with time.) If the first condition is notviolated (for instance, when the basic flow has uniform potential vorticity), then theperturbation energy must be negative (e.g., Marinone and Ripa, 1982) or zero (e.g.,Hayashi and Young, 1987).

Non-positive wave energy is present in a well-known phenomenon, namely,Kelvin-Helmholtz instability (e.g., see Miles 1980). However this seems to be ignored bymany researchers, particularly those with prejudices formed in the realm of thequasi-geostrophic models, for which the energy of a growing perturbation is alwayspositive (that is why, incidentally, only the first condition is needed in thequasi-geostrophic case). In Kelvin-Helmholtz instability theory, there are no gradients ofpotential vorticity involved and the energy of a growing perturbation must be exactlyzero, whereas the energy of a neutral wave can have either sign.

2. Moreprecisely, "energy - Cl (zonal momentum)", but I call it "energy", for short.

459

3. Stability conditions for the N-Iayermodel.

Consider a general N-Iayer model. A value of N = nI/2-layers with integer n,actually means a (n+1) - layer model in which the deepest layer is at rest; it is, then, an-Iayer, system with a "soft" bottom. On the other hand, in a regular N-Iayer model, i.e.with integer N, the deepest layer has a rigid bottom, which may include topography. I amworking with the primitive equations, and making use of the hydrostatic, Boussinesq and"rigid lid" approximations; one can easily deal without the last two, but that onlycomplicates the algebra without much gain in physical insight. The model is completelyspecified by the total volume of each layer (or the mean thickness, in the case of anunbounded horizonal domain) and the buoyancy jumps across the interfaces, g'j' Thedynamical variables are the thickness, hj , and horizonal current in each layer Uj; thelatter is independent of the vertical coordinate.

The stability of a general basic zonal flow, (Uj = Uj(y), Vj =: 0, Hj = Hj(y) isguaranteed if there exists a value a such that':

and[Uj - a] (dQJdy) < 0

Jl. > [U - a]2/H.J J J

Od)

(2d)

where the fields Jlj are defined in the following paragraph. These sufficient stabilityconditions are the generalization of (la) and (2a) for the multi-layer case. Necessaryinstability conditions are that,for any a, either (Id) or (2d) or both must be violated.

First define 1j =: (Uj.-a)2 / Hj. For the reduced gravity case, nI/2-layer model, firstmake Jln:: g' and then J.1j-l :: g'j-l - Jl"Yj / (Jlr1} for j decreasing from n-2 down to 2.For the rigid Rottom case, n-Iayer model one starts from Jln-l :: g'n-l - 1n and then onedoes a similar thing, decreasing j from n-3 down to 2. As a check, notice that one goesfrom N = n+l to N = nI/2 by making ~+l ~ 00, which implies 'Y _j ~ O. Similarly, oneformally goes from N = nI/2 to N = n by making g' ~ 00; I sayI\tformally" because then-Iayer model might have topography, and therefore is not truly derivable from thenI/2-layer one.

Let us review the second condition, for systems with few layers. Perturbationenergy must be positive definite if

N =1:

N = 11/2:

N =2:

N = 21/2:

N=3:

N = 31/2:

where 1j:: [Ura]2/Hj'

always

g' >1

g'>11+12

g'l < (g'l + g'2 - 11)(g'2 - 12)

(g'2 - 13)2 < (g'l + g' 2 - 13 - 11)(g'2 - 13 - 12)

Jl2 < [g'l +Jl-11] [Jl-12] Jl=g'2-13g'3/(g' r13)

3. In all three cases, the first condition, (Id), must be satisfied in every layer U= 1, n), whereas thesecond one, (2d). must be fulfilled for U= 1..... n) in the reduced gravity case or U= 1, n-I) in the rigidbouom case,

460

o

?,,\O-If 1-- ---.;. ~(___=a...J_. 600 ~

o

Cb)-~"Il)1j '-- -'--.

110$ 'looN

FIG.I. (a) 2 Ill-layer model of the equatorial Pacific,defined by the surface and the (Jo=23.20kg.nr? and(Jo=26.35 kg.m-3 isopycnals (the third layer is assumed infinitely deep and motionless). The isopycnalsmean depths were calculated from the data of Wynki and Kilonsky (1984). (b) Meridional derivatives ofthe layer thicknesses.

tJ Eec/-._~-\.. .:....... _-_..... - _-...._----.:~--- ~

' _-, <,/

""Ee.

u·J

---.--------------'-- -J

FIG.2. Geostrophic currents in each layer (solid lines) and average velocities calculated from WyrLlci andKilonsky (1984)continuous field(dashedlines).

461

4. From one to infinity: the ultraviolet problem.

The second condition (perturbation energy positive definite) is more restrictiveon the shear of the basic flow as the number of layers is increased. Moreover, assumethat one builds successive approximations to a continuously stratified medium byNslayer models, with increasing N: As the layer thicknesses are diminished, say H~eH,with e < 1, so are the buoyancy jumps, g' ~eg', but the fields "( are increased, "( ~ "(/e,and thus the second condition is harder to satisfy, by a factor of l/e2. The inevitableconclusion is that there are no conditions on the basic flow that make the energy of anarbitrary perturbation positive definite: one has to resort to conditions that also involveproperties of the disturbance.

For instance, following Holm and Long (1988), one can use as the "secondcondition" in a continum system.

(3)

where N is the Brunt-Vaisala frequency (N2 is the vertical gradient of the basic buoyancyprofile) and m is a local vertical wavenumber of the perturbation'[. Unlike conditions(2a-d), which only involve the basic flow, (3) also bounds the vertical scale of theperturbations : short enough disturbances will violate it (as happenes with the horizontalscale in the case of Kelvin-Helmholtz instability). Therefore, (3) is indeed a condition forlinear stability : in a fully nonlinear problem, there is no way to assure that m2 will bebounded at all times, even if it so initially.

S. Preliminary results of an application to the equatorial Pacific

A model of equatorial currents with some pretense of realism must, at least, havetwo layers. Prof. Klaus Wyrtki has been kind enough to provide me with the meanannual data shown in the Wyrtki and Kilonsky (1984) paper; figure la shows a 21/2-layermodel (i.e., the third one is assumed infinitely deep and motionless) with interfacesdefined as the 0 0=23.20 kg.rrr-' and 0 0=26.35 kg.m-3 isopycnals, respectively. Themeridional derivatives of the layer thicknesses, figure 1b, vanish right at the equator,which is a necessary condition for geostrophic balance.

Geostrophic currents in each layer (solid lines in figure 2) show, not surprisingly,good agreement with the average velocities (dashed lines) calculated from Wyrtki andKilonsky "continuous" field, using buoyancy jumps equal to g'l = 27 mm.s-2 and g:z =15 mm.s·2. The south equatorial current (SEC), north equatorial countercurrent (NECC)and current (NEC) are clearly seen in the first layer. The equatorial undercurrent (EVC)is observed in the second layer, along with the SEC, NECC (or the subsurface one), andNEC.

The meridional gradient of potential vorticity is always positive everywhere inthe first layer and only south of lOON in the second layer. The first stability condition isviolated at the site of NECC, in the first layer, and of the EUC, in the second one, if avalue a = 0 is used. However, with a = 60 cm.sl, the first condition is not violated inthose regions, but rather, north of lOON in the second layer. The second stabilitycondition is found to be satisfied for both values of a.

In summary, the second condition is satisfied (i.e., the vertical resolution is "otrich enough to allow for negative energy perturbations) but the first one is violated; theplace where this occurs depends on the value of a. A normal mode calculation is

4. The generalization of the "firstcondition" to the continuum case is straightforward.

462

underway. I expect to find the flow to be unstable, much like in the work of Philander(1976, 1978), and it will be interesting to compare the localization of a growingperturbation and the regions where (1c) is violated, depending on the value of a.

6. Finale

Laplace tidal equations are probably the archetype of ocean dynamical models.When they are linearized in the deviation from a resting ocean, two types of waves arefound: Poincare and Rossby ones (these become gravity waves and vertical modes,respectively, in the absence of Coriolis effects). Two is also the number of conditionsthat guarantee the stability of the steady solutions of those equations, as discussed hereand elsewhere.

It is my conjecture that growing perturbations from an unstable flow that violates(1) but satisfies (2) are Rossby-like. Conversely, an unstable steady solution that fulfills(1) but not (2), is presumed to have growing perturbations which are Poincare-Iike.Marinone and Ripa (1982) studied unstable easterly equatorial jets in a Ita-layer model,finding that the narrower jet violated (1a) but not (2a); the opposite was true for one witha width equal to the deformation radius. The structure of the perturbations in each casewas like as described above (Rossby and Poincare-like, respectively). Of course, I expectmy conjecture to hold true not only for the Ita-layer model, but also for cases withricher vertical resolution, for which the second condition is harder to satisfy.

In the strictly two-dimensional (non-divergent) case as well as in thequasi-geostrophic models, there is only one type of linear waves (i.e., Rossby ones), andalso there is only one stability condition, namely that corresponding to (1), in accordancewith that expressed above. For all other cases two sets of stability conditions are found.The second condition is associated to the possibility of perturbations with negative orvanishing energy. It is important to examine this concept further, particularly in theequatorial region, where Poincare waves cannot be avoided in any truly non-linear model.

REFERENCES

Amol'd, V.I., 1965: Condition for nonlinear stationary plane curvilinear flows of anideal fluid, Dokl. Akad. Nauk. USSR, 162,965-978; (English transl: Soviet Math.,6 : 773-777, 1965).

Amol'd, V.I., 1966: On an a priori estimate in the theory of hydrodynamical stability,Izv. Vyssh. Uchebn, Zaved. Maiematika; 54, 3-5; (English transl. Amer. Math.Soc. Transl., Series 2,79,267-269, 1969).

Hayashi, Y.Y., and W.R. Young, 1987. Stable and unstable shear modes of rotatingparallel flows in shallow water. J. Fluid Mech., 184: 477-504.

Holm, D.D., and B. Long, 1988: Lyapunov stability of ideal stratified fluid equilibria inhydrostatic balance., Nonlinearity, In press.

Marinone, S.G. and P. Ripa, 1982: Energetics of the instability of a depth-independentequatorial jet, Geophys. Astrophys. Fluid Dyn.; 30, 105-130.

McPhaden, MJ. and P. Ripa, 1990: Wave-mean flow interactions in equatorial oceans,Ann. Rev. Fluid Mech. In press.

Miles, I.W., 1980: On Kelvin-Helmholtz instability, Phys. Fluids, 23, 1915-1916.Philander, S.G.H., 1976: Instabilities of Zonal Equatorial Currents. J. Geophys. Res., 81,

3725-3735.Philander, S.G.H., 1978. Instabilities of Zonal Equatorial currents, 2. J. Geophys. Res.,

83. 3679-3682.

463

Ripa, P., 1983: General stability conditions for zonal flows in a one-layer model on thebeta-plane or the sphere, J. Fluid Mech., 126: 463-487.

Ripa, P., 1987: Flow stability in equatorial oceans. In Further Progress in EquatorialOceanography: A report of the U.S. TOGA Workshop on the Dynamics of theequatorial Oceans, E. Katz & 1. White, Eds. Honolulu, Hawaii, 1986. NovaUniv. Press, Ft. Lauderdale, Florida. pp. 351-356.

Ripa, P., 1989a: On the stability of ocean vortices. In Mesoscalelsynoptic coherentstructures in geophysical turbulence. J.1. Nihoul and B.M. Jamart Eds., ElsevierOceanographic series, Amsdertam, 167-179.

Ripa. P., 1989b: General stability conditions for a multi-layer model. J. Fluid Mech.,submitted.

Wyrtki, K. and B. Kilonsky, 1984: Mean water and current structure during theHawaii-to-Tahiti Shuttle Experiment. J. Phys. Oceanogr., 14,242-254.

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