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Equatorial waves and warm pool displacements during the 1992–1998 El Niño Southern Oscillation events: Observation and modeling
Abstract
In the equatorial Pacific, zonal displacements of the eastern edge of the warm pool represent an intrinsic manifestation of El Niño Southern Oscillation (ENSO) events, with numerous dynamical and biogeochemical consequences. Following a previous work dedicated to the 1986-1989 Geosat period, we focus on the 1992-1998 zonal displacements of the warm pool using mainly TOPEX/Poseidon data. We also used a simple linear model forced by monthly ERS winds to help in the interpretation of the results. We found that the 1992-1998 zonal displacements of the warm pool resulted mainly from horizontal advection by zonal current anomalies, through a combination of interannual equatorial Kelvin and first meridional mode Rossby waves. The interannual equatorial Kelvin waves were essentially wind forced in the western and central equatorial Pacific, with some minor contribution from reflected Rossby waves on the western Pacific boundary. In particular, westerly wind anomalies and the resulting downwelling Kelvin waves (entailing eastward surface current anomalies and thermocline deepening) contributed strongly to the onset of the 1993, 1994-1995 and 1997-1998 El Niño events. In contrast, easterly wind anomalies and the resulting upwelling Kelvin waves (with westward surface current anomalies and thermocline shoaling) played a role in stopping the 1993 El Niño and in shifting the 1994-1995 and 1997-1998 El Niño into La Niña events. Consistently with the 1987-1988 El Niño-La Niña scenario, two main downwelling Rossby wave packets, originating from eastern boundary reflections and wind forcing, crossed the entire basin in 1993 and 1994-1995. These waves favored the decay of the corresponding El Niño events, in the sense that their associated current anomalies contributed to shifting the displacements of the eastern edge of the warm pool from eastward to westward. Unlike what happened for the termination of the 1993 and 1994-1995 El Niño events, downwelling Rossby wave packets, mostly reflected from impinging Kelvin waves, did not propagate all the way to the western Pacific during the 1997-1998 El Niño. They stopped propagating in the central basin where they met unfavorable eastward migrating westerly wind anomalies which generated upwelling Rossby waves. Hence reflected downwelling and wind-forced upwelling Rossby waves opposed each other for shifting the eastern edge of the warm pool. The rapid demise of the 1997-1998 El Niño and its shift into La Niña in mid-1998 are interpreted as resulting mainly from the effect of upwelling Kelvin waves forced by easterly wind anomalies occurring in the west from the end of 1997. The associated thermocline shoaling was further enhanced by the wind-forced upwelling Rossby waves in the central basin in mid-1998, strongly influencing the fast sea surface temperature (SST) cooling at times when the thermocline was very close to the surface at the end of the mature phase of the 1997-1998 El Niño.
... Ocean-atmosphere feedback mechanisms are emphasized by the first three theories, but coupled climate models with ocean-atmosphere feedbacks still have difficulty in simulating ENSO and are quite sensitive to different physical parameterizations. Free ocean waves, including equatorial Kelvin and Rossby waves, are emphasized by the other three theories, and have been found in both observations [45][46][47][48][49] and models 41,[50][51][52][53][54] . The phase speeds of the ...
... Ocean-atmosphere feedback mechanisms are emphasized by the first three theories, but coupled climate models with ocean-atmosphere feedbacks still have difficulty in simulating ENSO and are quite sensitive to different physical parameterizations. Free ocean waves, including equatorial Kelvin and Rossby waves, are emphasized by the other three theories, and have been found in both observations [45][46][47][48][49] and models 41,[50][51][52][53][54] . The phase speeds of the free Kelvin waves are generally 2-3 m/s, while those of the free Rossby waves are 0.5-1 m/s. ...
The El Nino-Southern Oscillation (ENSO) is the dominant interannual variability of Earth’s climate system, and strongly modulates global temperature, precipitation, atmospheric circulation, tropical cyclones and other extreme events. However, forecasting ENSO is one of the most difficult problems in climate sciences affecting both interannual climate prediction and decadal prediction of near-term global climate change. The key question is what cause the switch between El Nino and La Nina. For the past 30 years, ENSO forecasts have been limited to short lead times after ENSO sea surface temperature (SST) anomaly has already developed, but unable to predict the switch between El Nino and La Nina. Here, we demonstrate that the switch between El Nino and La Nina is caused by a subsurface ocean wave propagating from western Pacific to central and eastern Pacific and then triggering development of SST anomaly. This is based on analysis of all ENSO events in the past 136 years using multiple long-term observational datasets. The wave’s slow phase speed and decoupling from atmosphere indicate that it is a forced wave. Further analysis of Earth’s angular momentum budget and NASA’s Apollo Landing Mirror Experiment suggests that the subsurface wave is likely driven by lunar tidal gravitational force.
... The high-frequency oceanic Kelvin waves might influence the evolution of the ENSO by moving warm or cold water eastward (Harrison and Schopf 1984;Fedorov and Melville 2000;Roundy and Kiladis 2006). The highfrequency eastward-propagating equatorial Kelvin waves can be excited by the westerly wind bursts (WWBs; e.g., McPhaden and Yu 1999;Delcroix et al. 2000;Kessler 2002;Roundy and Kiladis 2006;Fedorov et al. 2015). The WWBs are commonly associated with the onset and growth of El Niño events (McPhaden 2004), while the easterly wind surges (EWSs) play an important role in the onset and maintaining phase of La Niña (Chiodi and Harrison 2015). ...
... The high-frequency equatorial oceanic Kelvin waves can be excited by westerly wind bursts or easterly wind surges (e.g., McPhaden and Yu 1999;Delcroix et al. 2000;Kessler 2002;Roundy and Kiladis 2006;Fedorov et al. 2015;Tseng et al. 2017a). The WWBs and EWSs, synoptic-scale disturbances that occur near the equator, are considered to have a large impact on ENSO evolution. ...
This study investigates the sudden reversal of anomalous zonal equatorial transport above thermocline at the peak phase of ENSO. The oceanic processes associated with zonal transport are separated into low-frequency ENSO cycle and high-frequency oceanic wave processes. Both processes can generate a reversal of equatorial zonal current at the ENSO peak phase, which is a trigger for the rapid termination of ENSO events. For the low-frequency process, zonal transport exhibits slower and basinwide evolution. During the developing phase of El Niño (La Niña), eastward (westward) transport prevails in the central-eastern Pacific, which enhances ENSO. At the peak of ENSO, a basinwide reversal of the zonal transport resulting from the recharge-discharge process occurs and weakens the existing SST anomalies. High-frequency zonal transport presents clear eastward propagation related to Kelvin wave propagation at the equator, reflection at the eastern boundary, and the westward propagating Rossby waves. The major westerly wind bursts (easterly wind surges) occur in late boreal summer and fall with coincident downwelling (upwelling) Kelvin waves for El Niño (La Niña) events. After the peak of El Niño (La Niña), Kelvin waves reach the eastern boundary in boreal winter and reflect as off-equatorial Rossby waves; then, the zonal transport switches from eastward (westward) to westward (eastward). The high-frequency zonal transport can be represented by equatorial wave dynamics captured by the first three EOFs based on the high-pass-filtered equatorial thermocline. The transport anomaly during the decaying phase is dominated by the low-frequency process in El Niño. However, the transport anomaly is caused by both low- and high-frequency processes during La Niña.
... These two branches are separated at subsurface by the westward-flowing equatorial under current centred around 150 m underneath the Equator, while the NECC is deeper and flows westward from the surface to around 400 m (Godfrey et al. 2001). The Tropical Pacific current variability has been extensively studied as it is constitutive of ENSO variability (see, for instance, Delcroix et al. 2000;Meinen and Mcphaden 2001). In 2015 the El Niño event (see also Section 4.1) had a strong signature on surface currents in the Tropical Pacific. ...
... In 2015 the El Niño event (see also Section 4.1) had a strong signature on surface currents in the Tropical Pacific. The Tropical Pacific current system experienced a large positive eastward anomaly in 2015, associated with the slowing down of the trade winds, eastward propagating downwelling Kelvin waves and associated transfer of heat from the western Tropical Pacific Ocean warm pool towards the Central and Eastern Tropical Pacific as described for previous El Niño events for instance in Meinen and Mcphaden (2001) or Delcroix et al. (2000). This resulted in a slowing down of the westward, northern and southern branches of the SEC, and in the strengthening of the NECC as shown by Figure 19(b). ...
The Copernicus Marine Environment Monitoring Service (CMEMS) Ocean State Report (OSR) provides an annual report of the state of the global ocean and European regional seas for policy and decision-makers with the additional aim of increasing general public awareness about the status of, and changes in, the marine environment. The CMEMS OSR draws on expert analysis and provides a 3-D view (through reanalysis systems), a view from above (through remote-sensing data) and a direct view of the interior (through in situ measurements) of the global ocean and the European regional seas. The report is based on the unique CMEMS monitoring capabilities of the blue (hydrography, currents), white (sea ice) and green (e.g. Chlorophyll) marine environment. This first issue of the CMEMS OSR provides guidance on Essential Variables, large-scale changes and specific events related to the physical ocean state over the period 1993–2015. Principal findings of this first CMEMS OSR show a significant increase in global and regional sea levels, thermosteric expansion, ocean heat content, sea surface temperature and Antarctic sea ice extent and conversely a decrease in Arctic sea ice extent during the 1993–2015 period. During the year 2015 exceptionally strong large-scale changes were monitored such as, for example, a strong El Niño Southern Oscillation, a high frequency of extreme storms and sea level events in specific regions in addition to areas of high sea level and harmful algae blooms. At the same time, some areas in the Arctic Ocean experienced exceptionally low sea ice extent and temperatures below average were observed in the North Atlantic Ocean.
... Satellite SSH observations have also revealed the importance of the combined 977 effects of wind forced equatorial Kelvin and Rossby waves and their reflections at the 978 eastern and western boundaries on the ENSO cycle (e.g., Delcroix et al., 2000). The 979 reflection of Rossby waves at western boundaries is key to the termination of El Niño in 980 the Delayed Action Oscillator theory. ...
The development of the technologies of remote sensing of the ocean was initiated in the 1970s, while the ideas of observing the ocean from space were conceived in the late 1960s. The first global view from space revealed the expanse and complexity of the state of the ocean that had perplexed and inspired oceanographers ever since. This paper presents a glimpse of the vast progress made from ocean remote sensing in the past 50 years that has a profound impact on the ways we study the ocean in relation to weather and climate. The new view from space in conjunction with the deployment of an unprecedented amount of in situ observations of the ocean has led to a revolution in physical oceanography. The highlights of the achievement include the description and understanding of the global ocean circulation, the air–sea fluxes driving the coupled ocean–atmosphere system that is most prominently illustrated in the tropical oceans. The polar oceans are most sensitive to climate change with significant consequences, but owing to remoteness they were not accessible until the space age. Fundamental discoveries have been made on the evolution of the state of sea ice as well as the circulation of the ice-covered ocean. Many surprises emerged from the extraordinary accuracy and expanse of the space observations. Notable examples include the determination of the global mean sea level rise as well as the role of the deep ocean in tidal mixing and dissipation.
... As noted in section 3.4.1, the use of geostrophy to characterize the surface velocity field from SSH may be questionable, in particular, for near-equatorial regions. Comparisons made in the Pacific Ocean between direct equatorial current measurements and geostrophic velocities derived from SSH indicate that the variability of the equatorial currents are reasonably well represented by geostrophy for periods greater than one month and with a 1-2°latitudinal scale (e.g., Delcroix et al., 2000;Picaut et al., 1990). These comparisons, right at the equator, represent the most stringent tests for geostrophy given the high sensibility of the calculation to small SSH errors. ...
The signature of westward propagating mesoscale eddies in sea surface salinity (SSS) is analyzed for the tropical Pacific by collocating 7 years (2010–2016) of Soil Moisture and Ocean Salinity SSS satellite data with coherent mesoscale eddies automatically identified and tracked from altimetry‐derived sea level anomalies. First, the main characteristics of the long‐lived coherent eddies are inferred from sea level anomalies maps. Then, the mean signature of the mesoscale eddies on SSS is depicted for the whole tropical Pacific before focusing in regions centered around the central and eastern parts of the tropical North Pacific. In these areas, composite analyses based on thousands of eddies reveal regionally dependent eddy impacts with opposite SSS anomalies for cyclonic and anticyclonic eddies. In the central region, where the largest meridional SSS large‐scale gradients and smallest eddy amplitudes are observed, results show dipole‐like SSS changes with maximum anomalies on the leading edge of the composite eddy. In contrast, in the eastern region, where the largest near‐surface vertical salinity gradients and largest eddy amplitudes are observed, the composite eddy shows monopole‐like SSS changes with maximum anomalies near the composite eddy center. These distinct dipole/monopole SSS patterns suggest the dominant role of horizontal advection and vertical processes in the central and eastern regions, respectively. Other possible explanations, notably one involving the contrasted eddy amplitudes of the two regions, are discussed.
... Les anomalies TJ sont définies ici par rapport à une moyenne calculée sur la période 10/92-9/96. Nous allons ici juste rappeler les principales étapes qui ont permis d'aboutir, à partir des données AVISO, aux données telles qu'elles sont utilisées dans mon étude (pour plus de détails, voir Delcroix et al., 2000) : ...
Although juvenile anguillid eels live in freshwater/estuarine habitats, and marine eels live in diverse ocean environments ranging from shallow-to-deep continental shelf areas and around islands to deep-benthic habitats and deeper meso- and bathy- pelagic zones, the larvae (leptocephali) of all species mix together in the ocean surface layer. All types of eel habitats are present in the western South Pacific (WSP), so it is a unique region for studying long-lived leptocephali, especially because the westward flowing South Equatorial Current (SEC) and several countercurrents pass through many different WSP island groups and deep waters where both anguillid and marine eels live and spawn. Large mouth-opening IKMT sampling surveys for leptocephali were conducted in the southwest Pacific extending to French Polynesia in Jan-Mar 2013 (99 tows, 78 stations, 1052 larvae) and Jul-Sept 2016 (187 tows, 111 stations, 3976 larvae) that collected about 152 species of 18 anguilliform and elopomorph families. The larvae of mesopelagic serrivomerid eels were the most abundant taxa in all oceanic areas, and they were particularly abundant at northern SEC or equatorial latitudes. Australian and New Zealand anguillid eels had spawned in the western SEC areas, as previously detected, and the larvae of tropical anguillids were also only caught in western areas. The larvae of the mesopelagic nemichthyid and derichthyid eels were also widely distributed at lower abundances and with more patchy distributions, but larvae of Eurypharyngidae, Cyematidae, and Mongnathidae were rare. Shallow-water eel larvae were most abundant west of New Caledonia near the banks of the Chesterfield Islands, or near other island-groups, but they were rare in the 2 easternmost 2016 transects passing by both sides of Tahiti. Some conger eels were suggested to have spawned in offshore areas in the western region. Congrid Ariosoma and various shallow-water or slope eels had spawned in the region near the Chesterfield Islands or near New Caledonia where current jets can transport larvae westward, and eastward countercurrents exist. Some taxa of larvae of coastal species (muraenesocids, and elopomorphs) were extremely rare, all non-mesopelagic eel larvae were rare in the far-eastern transects, but the New Caledonia region with large shelf areas appears to be a high biodiversity region for marine eels, as it is for reef/shore fish in general.
Observations of the ocean‐atmosphere system sustained over decades are essential for describing, understanding, modeling, and predicting variations associated with El Niño and the Southern Oscillation (ENSO). Here we discuss the history of observing system development in the tropical Pacific and profile the mix of satellite and in situ components that currently make up the sustained observing system. To illustrate key physical processes that give rise to ENSO variations, we focus on the period 2014–2019, which encompassed the first extreme El Niño of the 21st century in 2015–2016. We also provide an overview of model‐based oceanic and atmospheric reanalysis products that are used for monitoring climate variability and initializing seasonal to decadal timescale climate forecasts. In addition, we highlight new developments in coupled ocean‐atmosphere data assimilation and century‐long reanalyses. We conclude by addressing some of the challenges in sustaining and evolving the observing system over time.
The equatorial wave dynamics of interannual sea level variations between 2014/2015 and 2015/2016 El Niño events are compared using the Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics Climate Ocean Model (LICOM) forced by the National Centers for Environmental Prediction (NCEP) reanalysis I wind stress and heat flux during 2000–2015. In addition, the LICOM can reproduce the interannual variability of sea surface temperature anomalies (SSTA) and sea level anomalies (SLA) along the equator over the Pacific Ocean in comparison with the Hadley center and altimetric data well. We extracted the equatorial wave coefficients of LICOM simulation to get the contribution to SLA by multiplying the meridional wave structure. During 2014/2015 El Niño event, upwelling equatorial Kelvin waves from the western boundary in April 2014 reach the eastern Pacific Ocean, which weakened SLA in the eastern Pacific Ocean. However, no upwelling equatorial Kelvin waves from the western boundary of the Pacific Ocean could reach the eastern boundary during the 2015/2016 El Niño event. Linear wave model results also demonstrate that upwelling equatorial Kelvin waves in both 2014/2015 and 2015/2016 from the western boundary can reach the eastern boundary. However, the contribution from stronger westerly anomalies forced downwelling equatorial Kelvin waves overwhelmed that from the upwelling equatorial Kelvin waves from the western boundary in 2015. Therefore, the western boundary reflection and weak westerly wind burst inhibited the growth of the 2014/2015 El Niño event. The disclosed equatorial wave dynamics are important to the simulation and prediction of ENSO events in future studies.
An intermediate coupled tropical Pacific Ocean-atmosphere model is used to study the sensitivity of 1997-1998 El Nino forecasts to the vertical structure of the ocean. The coupled model consists of a tropical Pacific Ocean model taking into account three baroclinic modes and the Gill tropical atmosphere model [1]. Experiments based on a model using the observed wind stress are initially considered. The sea-level and surface-current fields obtained in the experiment based on a multimode ocean model are in better agreement with observational data than the experimental results obtained by a single-mode ocean model. A series of forecasting experiments with model integration over two years during the 1970-1998 period are conducted. These forecasts are used to obtain statistical estimates for the predictive ability of the model. In order to assess the feasibility of employing altimetric data to improve the quality of the initial conditions, the model is tested for its sensitivity to the vertical structure of the ocean in 1996-1998. the tests show that the consideration of the second baroclinic mode in the ocean model is necessary for the reproduction of an anomalous increase in temperature in April 1997 and also for a correct estimation of the amplitude of this phenomenon in its culmination. On the other hand, the first baroclinic mode is more important for the reproduction of cool ocean conditions formed since early January 1998.
The vertical structure of the variability in the equatorial Pacific in a high-resolution ocean general circulation model (OGCM) simulation for 1985-94 is investigated. Near the equator the linear vertical modes are estimated at each grid point and time step of the OGCM simulation. The characteristics of the vertical modes are found to vary more in space than in time. The contribution of baroclinic modes to surface zonal current and sea level anomalies is analyzed. The first two modes contribute with comparable amplitude but with different spatial distribution in the equatorial waveguide. The third and fourth modes exhibit peaks in variability in the east and in the westernmost part of the basin where the largest zonal gradients in the density field and in the vertical mode characteristics are found. Higher-order mode (sum of third to the eighth mode) variability is the largest near the date line close to the maximum in zonal wind stress variability. Kelvin and first-meridional Rossby components are derived for each of the first three baroclinic contributions by projection onto the associated meridional structures. They are compared to equivalent ones in multimode linear simulations done with the projection coefficients and phase speeds derived from the OGCM simulation. This suggests that in addition to the first-mode-forced equatorial Kelvin and Rossby waves earlier found in the data, forced waves of higher vertical modes should also be observable. For the first two vertical modes, the anomalies in the linear and the OGCM simulations have a similar magnitude and usually present similar propagation characteristics. Phase speed characteristics are however different in the eastern Pacific with larger values for the OGCM. The effect of zonal changes in the stratification is tested in the linear model for a stratification change located either in the eastern or in the western Pacific. This results in a significant redistribution of energy to higher modes via modal dispersion. In particular the third mode increases to a magnitude closer to the one in the OGCM simulation. Gravest modes are also affected. This suggests that modal dispersion plays an important role and should be considered when interpreting data as a combination of linear long equatorial waves.
The authors examine the sensitivity of the Battisti coupled atmosphere-ocean model - considered as a forecast model for the E1 Niño-Southern Oscillation (ENSO) - to perturbations in the sea surface temperature (SST) field applied at the beginning of a model integration. The spatial structures of the fastest growing SST perturbations are determined by singular vector analysis of an approximation to the propagator for the linearized system. Perturbation growth about the following four reference trajectories is considered: (i) the annual cycle, (ii) a freely evolving model ENSO cycle with an annual cycle in the basic state, (iii) the annual mean basic state, and (iv) a freely evolving model ENSO cycle with an annual mean basic state. Singular vectors with optimal growth over periods of 3, 6, and 9 months are computed. The magnitude of maximum perturbation growth is highly dependent on both the phase of the seasonal cycle and the phase of the ENSO cycle at which the perturbation is applied and on the duration over which perturbations are allowed to evolve. However, the spatial structure of the optimal perturbation is remarkably insensitive to these factors. The structure of the optimal perturbation consists of an east-west dipole spanning the entire tropical Pacific basin superimposed on a north-south dipole in the eastern tropical Pacific. A simple physical interpretation for the optimal pattern is provided. In most cases investigated, there is only one structure that exhibits growth. Maximum perturbation growth takes place for integrations that include the period June-August, and the minimum growth for integrations that include the period January-April. Maxima in potential growth also occur for forecasts of ENSO onset and decay, while minima occur for forecasts initialized during the beginning of a warm event, after the transition from a warm to a cold event, and continuing through the cold event. The physical processes responsible for the large variation in the amplitude of the optimal perturbation growth are identified. The implications of these results for the predictability of short-term climate in the tropical Pacific are discussed.
Weekly and monthly wind fields over the global ocean are calculated from ERS-1 scatterometer measurements starting in August 1991 and extending through April 1996. The spatial resolution of the resultant wind fields is 1 deg. in longitude and 1 deg. in latitude. The scatterometer wind observations are derived from IFREMER ERS-1 wind products. The calculation is based on a geostatistical approach named the kriging method. This technique averages the scatterometer wind vectors in time and space using spatial and temporal structure functions for surface wind parameters. The aim of this paper is to provide some results about the accuracy of these new surface wind data. It is evaluated by comparison with averaged buoy wind measurements, for local validation, or with surface wind analyses from the European Center for Medium Range Weather Forecasts (ECMWF), for global validation. The statistics calculation indicates that the rms. difference between the weekly buoy and scatterometer wind speed, zonal component and meridional component are 0.92 m/s, 1.64 m/s and 1.54 m/s, respectively. Annual and seasonal means of wind stress, and wind stress curl are presented and compared with previous climatological means. The seasonal and latitudinal variation of the weekly winds are studied using Weibull distribution. The known results obtained in the Northern Hemisphere are recovered. The most important contribution of this wind analysis is found in Southern Hemisphere. For instance, it was found that the seasonal variability is larger than those described by previous climatologies.
The annual and interannual variability in the subthermocline equatorial Pacific is studied in a simulation of the tropical Pacific for the 1985-94 decade using a primitive equation high-resolution model. The study focuses on temperature variability and vertical energy fluxes. Similar to the observations made by W. S. Kessler and J. P. McCreary, the annual harmonic of vertical isotherm displacements presents phase lines sloping downward from east to west with off-equatorial maxima. Estimates of zonal and vertical phase speeds, the location of the maxima of isotherm displacements, and an analysis of ernergy fluxes suggest the presence of l 5 1 Rossby waves. The simulated field is somewhat more trapped toward the equator than the observations. A linear simulation is carried out for the vertical standing modes with characteristics derived from the OGCM simulation. In the linear simulation, high-order meridional mode Rossby waves are more prominent than in the OGCM simulation. However, the (l 5 1) Rossby wave in the linear model shares many characteristics with the one in the OGCM simulation. It is the result of both the reflection of surface-forced Kelvin waves on the eastern boundary and of direct forcing at the surface. On interannual timescales the simulation also presents vertical propagation of Rossby waves from the eastern boundary, but not as deep as for the annual cycle and in a much thinner beam. The variability of vertical displacements exhibits an asymmetry with a larger amplitude north than south of the equator. The zone of large interannual variability originating at the eastern boundary extends westward following ( l 5 1) or (l 5 2) WKB ray paths and reaches the western Pacific near 400 m at 38-48 north but not south of the equator. The 3.3-yr harmonic is prominent in isotherm vertical displacements for this particular simulation. The analysis also suggests the presence of (l 5 1) Rossby wave, but the phase line characteristics are also indicative of the contribution of higher-order meridional modes. At this frequency, the solution of the linear model for the ( l 5 1) Rossby mode agrees well with the OGCM (l 5 1) Rossby wave contribution.
A fast, efficient numerical procedure for modeling the linear low-frequency motions on an equatorial beta plane is developed. The model is capable of simulating the seasonal and interannual variability in realistically shaped ocean basins forced by realistic winds. The timestep allowed is the order of days rather than hours as allowed by more conventional schemes. The numerical method is designed around the special characteristics of low-frequency equatorial waves. A crucial element is the formulation of proper boundary conditions, including those for a partial boundary such as the western end of the Gulf of Guinea.
The response of an Atlantic-shaped basin to a periodic wind is compared with the analytic results of Cane and Sarachik for a meridionally unbounded ocean. The response along the equator is essentially the same. A narrow boundary layer forms along the Guinea coast, too narrow to be a single coastally trapped wave: it is the sum of many modes. The near lack of phase variation along the coast and the smallness of the phase difference between the equator and the coast are additional contrasts to the results for an initial value problem studied by Moore and others. The implication is that the annually recurring upwelling in the Gulf of Guinea cannot be adequately modeled as the response to an impulsively applied wind stress.
The western boundary of the tropical Pacific is not continuous, and leakage of low-frequency energy from the Pacific to the Indian Ocean is possible. At low frequencies, equatorial Kelvin and Rossby waves have very large east-west scales compared with the east-west scale of the land masses in the region. Consequently, these land masses may be treated as islands that are infinitesimally thin in the east-west direction. By generalizing previous theory for a single island, the leakage and multiple reflection of low-frequency energy through the seven major ``islands'' forming the boundary of the western Pacific can be studied. The results obtained depend on continuity of mass and large-scale balances and not on the details of nonlinear and/or frictional flow near island eastern boundaries. The major results are as follows: (1) When a mode 1 low-frequency Rossby wave is reflected at the discontinuous western Pacific boundary, the eastward reflected Kelvin wave energy flux is about one third of the incoming energy flux, or about two thirds of that expected for a solid meridional wall. In other words, the reflected Kelvin wave amplitude is 83% of that which would be reflected from a solid meridional wall. The reflection mainly occurs from the Indonesia/Borneo/Asia land mass and very little energy gets into the Indian Ocean. (2) Sea levels in the western equatorial Pacific and on the western boundaries of the major western Pacific land masses should be in phase and of a similar amplitude. In particular, in-phase interannual sea levels should occur along Australia's western coast and be highly correlated with sea levels in the western equatorial Pacific. Sea level data at Truk Island and on Australia's western coastline confirm this prediction.(3) The interannual exchange of water between the Pacific and Indian oceans due to interannual oscillations in the Pacific is about 6 Sv (1 Sv=106 m3 s-1), is highly correlated with El Niño/Southern Oscillation (ENSO) events, and bears definite phase relationships to sea level stations at locations in the western Pacific. For example, for a high sea level at Darwin, the transport is westward from the Pacific Ocean into the Indian Ocean. It seems that interannual transport may be approximately monitored using coastal sea level. (4) Strong narrow low-frequency currents are predicted to occur westward of some island tips. (5) Interannual Indian Ocean Kelvin wave signals are largely blocked at the Indonesia/Borneo/Asia land mass. (6) In terms of net transport, reflection, and transmission properties, the seven island Pacific western boundary can be reduced to a simpler two-island problem, in which one island is the Indonesia/Borneo/Asia land mass and the other New Guinea/Australia. (7) The comparatively small Halmahera Sea between Halmahera and New Guinea is dynamically important. Closing this sea for the incident first meridional mode Rossby wave case reduces the interannual transport between the Pacific and Indian Oceans by 40%. ©1991 American Geophysical Union
