• Cane, M., , and P. Gent, 1984: Reflection of low-frequency equatorial waves at arbitrary western boundaries. J. Mar. Res., 42 , 487502.

  • Chambers, D. P., , B. D. Taley, , and R. H. Stewart, 1999: Anomalous warming in the Indian Ocean coincident with El Niño. J. Geophys. Res., 104 , 30353047.

    • Search Google Scholar
    • Export Citation
  • Conkright, M. E., , R. A. Locarnini, , H. E. Garcia, , T. D. O’Brien, , T. P. Boyer, , C. Stephens, , and J. I. Antonov, 2002: World Ocean Atlas 2001: Objective Analysis, Data Statistics, and Figures: CD-ROM Documentation. National Oceanographic Data Center, 17 pp.

    • Search Google Scholar
    • Export Citation
  • Han, W., , P. J. Webster, , R. Lukas, , P. Hacker, , and A. Hu, 2004: Impact of atmospheric intraseasonal variability in the Indian Ocean: Low-frequency rectification in equatorial surface current and transport. J. Phys. Oceanogr., 34 , 13501372.

    • Search Google Scholar
    • Export Citation
  • Han, W., , T. Shinoda, , L-L. Fu, , and J. P. McCreary, 2006: Impact of atmospheric intraseasonal oscillations on the Indian Ocean dipole during the 1990s. J. Phys. Oceanogr., 36 , 670690.

    • Search Google Scholar
    • Export Citation
  • Han, W., , D. Yuan, , W. T. Liu, , and D. J. Halkides, 2007: Intraseasonal variability of Indian Ocean sea surface temperature during boreal winter: Madden-Julian oscillation versus submonthly forcing and processes. J. Geophys. Res., 112 , C04001. doi:10.1029/2006JC003791.

    • Search Google Scholar
    • Export Citation
  • Huang, B., , and J. L. Kinter III, 2002: Interannual variability in the tropical Indian Ocean. J. Geophys. Res., 107 , 3199. doi:10.1029/2001JC001278.

    • Search Google Scholar
    • Export Citation
  • Jury, M. R., , and B. Huang, 2004: The Rossby wave as a key mechanism of Indian Ocean climate variability. Deep-Sea Res. I, 41 , 21232136.

    • Search Google Scholar
    • Export Citation
  • Kessler, W. S., , and R. Kleeman, 2000: Rectification of the Madden–Julian oscillation into the ENSO cycle. J. Climate, 13 , 35603575.

    • Search Google Scholar
    • Export Citation
  • Le Blanc, J. L., , and J. P. Boulanger, 2001: Propagation and reflection of long equatorial waves in the Indian Ocean from TOPEX/Poseidon data during the 1993–1998 period. Climate Dyn., 17 , 547557.

    • Search Google Scholar
    • Export Citation
  • Levitus, S., and Coauthors, 1998: Introduction. Vol. 1, World Ocean Database 1998, NOAA Atlas NESDIS 18, 346 pp.

  • Liu, H., , Y. Yu, , W. Li, , and X. Zhang, 2004: LASG/IAP climate system ocean model (LICOM1.0): User manual (in Chinese). Science Publication, 107 pp.

    • Search Google Scholar
    • Export Citation
  • Liu, H., , W. Li, , and X. Zhang, 2005: Climatology and variability of the Indonesian Throughflow in an eddy-permitting oceanic GCM. Adv. Atmos. Sci., 22 , 496508.

    • Search Google Scholar
    • Export Citation
  • Masumoto, Y., , and G. Meyers, 1998: Forced Rossby waves in the southern Indian Ocean. J. Geophys. Res., 103 , 2758927602.

  • McCreary, J. P., 1981: A linear stratified ocean model of the equatorial undercurrent. Quart. J. Roy. Soc. London, 298A , 603635.

  • Murtugudde, R., , J. P. McCready, , and A. J. Busalacchi, 2000: Oceanic processes associated with anomalous events in the Indian Ocean with relevance to 1997–1998. J. Geophys. Res., 105 , 32953306.

    • Search Google Scholar
    • Export Citation
  • Packnowski, R. C., , and S. G. H. Philander, 1981: Parameterization of vertical mixing in numerical models of the tropical ocean. J. Phys. Oceanogr., 11 , 14421451.

    • Search Google Scholar
    • Export Citation
  • Rao, S. A., , and T. Yamagata, 2004: Abrupt termination of Indian Ocean dipole events in response to intraseasonal oscillations. Geophys. Res. Lett., 31 , L19306. doi:10.1029/2004GL020842.

    • Search Google Scholar
    • Export Citation
  • Saji, N. H., , B. N. Goswami, , P. N. Vinayachandran, , and T. Yamagata, 1999: A dipole mode in the tropical Indian Ocean. Nature, 401 , 360363.

    • Search Google Scholar
    • Export Citation
  • Vinayachandran, P. N., , S. Iizuka, , and T. Yamagata, 2002: Indian Ocean dipole mode events in an ocean general circulation model. Deep-Sea Res. II, 49 , 15731596.

    • Search Google Scholar
    • Export Citation
  • Webster, P. J., , A. M. Moore, , J. P. Loschnigg, , and R. R. Leben, 1999: Coupled ocean–atmosphere dynamics in the Indian Ocean during 1997–1998. Nature, 401 , 356360.

    • Search Google Scholar
    • Export Citation
  • Xie, S-P., , H. Annamalai, , F. A. Schott, , and J. P. McCreary, 2001: Structure and mechanisms of south Indian Ocean climate variability. J. Climate, 15 , 864878.

    • Search Google Scholar
    • Export Citation
  • Yu, L., , and M. M. Rienecker, 2000: Indian Ocean warming of 1997–1998. J. Geophys. Res., 105 , 1692316939.

  • Yuan, D., 2005: Role of the Kelvin and Rossby waves in the seasonal cycle of the equatorial Pacific Ocean circulation. J. Geophys. Res., 110 , C04004. doi:10.1029/2004JC002344.

    • Search Google Scholar
    • Export Citation
  • Yuan, D., , and W. Han, 2006: Roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial Indian Ocean. J. Phys. Oceanogr., 36 , 930944.

    • Search Google Scholar
    • Export Citation
  • Yuan, D., , M. M. Rienecker, , and P. S. Schopf, 2004: Long wave dynamics of the interannual variability in a numerical hindcast of the equatorial Pacific Ocean circulation during the 1990s. J. Geophys. Res., 109 , C05019. doi:10.1029/2003JC001936.

    • Search Google Scholar
    • Export Citation
  • View in gallery

    Comparison of averaged vertical profiles of density and buoyancy frequency square over the equatorial Indian Ocean between 5°S and 5°N (dashed) with the WOA01 (solid) data.

  • View in gallery

    Comparison of averaged sea level anomalies between 5°S and 5°N of the OGCM simulation with the TOPEX/Poseidon altimeter data. The altimeter anomalies are based on a climatology of 1993–2001.

  • View in gallery

    Decomposed coefficients of equatorial Kelvin and Rossby waves of the first baroclinic mode from the OGCM simulation. The contour units correspond to 0.33 m for Kelvin wave sea level and 0.14 m for the first meridional-mode Rossby wave sea level on the equator. The coefficients have been smoothed with the 1–2–1 filter.

  • View in gallery

    (top) Coefficients of the decomposed (thin solid curve) and linear reflected (dotted curve) Kelvin waves of the first baroclinic mode at the western boundary. The thick solid curve represents the difference between the two. The time-shifted first meridional-mode Rossby wave coefficient multiplied by its reflection ratio is also shown with the dashed curve in the upper panel. (bottom) Kelvin wave (solid) and the first meridional-mode Rossby wave divided by the Kelvin wave reflection ratio (dotted) at the eastern boundary.

  • View in gallery

    Coefficients of the decomposed (thin solid curve) and linear reflected (dotted curve) Kelvin waves of the first baroclinic mode at the western boundary for (a) main run, (b) test run 1, and (c) test run 2 of the linear continuously stratified model. The thick solid curve represents the difference between the decomposed and the reflected Kelvin wave.

  • View in gallery

    Zonal wind stress anomalies (Pa) along the equator during 1996–98.

  • View in gallery

    Decomposed coefficients of equatorial Kelvin and Rossby waves of the first baroclinic mode from the OGCM simulation for the period of 1996–98. The theoretical first baroclinic Kelvin wave speed is plotted as a solid line at the bottom of the left panel.

  • View in gallery

    Sea level (contours) and wind stress (vectors) anomalies during (a) May–December 1997 and (b) January–August 1998.

  • View in gallery

    As in Fig. 6, but for the period 1993–95.

  • View in gallery

    As in Fig. 7, but for the period of 1993–95.

  • View in gallery

    Sea level (contours) and wind stress (vectors) anomalies for the 1994 IOD.

  • View in gallery

    (left) Zonal wind stress anomalies (Pa) along the equator and (right) OGCM-simulated sea level anomalies (cm) averaged between 5°N and 5°S during 1990–92.

  • View in gallery

    As in Fig. 10, but for the period of 1990–92.

  • View in gallery

    Sea level anomalies along the equator averaged between 5°N and 5°S for (left) EXP01 and (right) the difference between EXP00 and EXP01.

  • View in gallery

    Decomposed coefficients of equatorial Kelvin and Rossby waves of the first baroclinic mode for the results of EXP00–EXP01 for the period of 1993–95.

  • View in gallery

    (left) Daily high-passed wind stress (Pa) averaged between 5°N and 5°S with the mean for October–December 1994 removed and (right) the forced upper-layer thickness (m) along the equator in the simple nonlinear model.

  • View in gallery

    Decomposed coefficients of equatorial Kelvin and Rossby waves of the first baroclinic mode of the results of EXP00–EXP01 for the period of 1996–99.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 111 111 11
PDF Downloads 86 86 5

Long-Wave Dynamics of Sea Level Variations during Indian Ocean Dipole Events

View More View Less
  • 1 Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China
  • | 2 LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
© Get Permissions
Full access

Abstract

Long-wave dynamics of the interannual variations of the equatorial Indian Ocean circulation are studied using an ocean general circulation model forced by the assimilated surface winds and heat flux of the European Centre for Medium-Range Weather Forecasts. The simulation has reproduced the sea level anomalies of the Ocean Topography Experiment (TOPEX)/Poseidon altimeter observations well. The equatorial Kelvin and Rossby waves decomposed from the model simulation show that western boundary reflections provide important negative feedbacks to the evolution of the upwelling currents off the Java coast during Indian Ocean dipole (IOD) events. Two downwelling Kelvin wave pulses are generated at the western boundary during IOD events: the first is reflected from the equatorial Rossby waves and the second from the off-equatorial Rossby waves in the southern Indian Ocean. The upwelling in the eastern basin during the 1997–98 IOD event is weakened by the first Kelvin wave pulse and terminated by the second. In comparison, the upwelling during the 1994 IOD event is terminated by the first Kelvin wave pulse because the southeasterly winds off the Java coast are weak at the end of 1994.

The atmospheric intraseasonal forcing, which plays an important role in inducing Java upwelling during the early stage of an IOD event, is found to play a minor role in terminating the upwelling off the Java coast because the intraseasonal winds are either weak or absent during the IOD mature phase. The equatorial wave analyses suggest that the upwelling off the Java coast during IOD events is terminated primarily by western boundary reflections.

Corresponding author address: Dongliang Yuan, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao, 266071, China. Email: dyuan@ms.qdio.ac.cn

Abstract

Long-wave dynamics of the interannual variations of the equatorial Indian Ocean circulation are studied using an ocean general circulation model forced by the assimilated surface winds and heat flux of the European Centre for Medium-Range Weather Forecasts. The simulation has reproduced the sea level anomalies of the Ocean Topography Experiment (TOPEX)/Poseidon altimeter observations well. The equatorial Kelvin and Rossby waves decomposed from the model simulation show that western boundary reflections provide important negative feedbacks to the evolution of the upwelling currents off the Java coast during Indian Ocean dipole (IOD) events. Two downwelling Kelvin wave pulses are generated at the western boundary during IOD events: the first is reflected from the equatorial Rossby waves and the second from the off-equatorial Rossby waves in the southern Indian Ocean. The upwelling in the eastern basin during the 1997–98 IOD event is weakened by the first Kelvin wave pulse and terminated by the second. In comparison, the upwelling during the 1994 IOD event is terminated by the first Kelvin wave pulse because the southeasterly winds off the Java coast are weak at the end of 1994.

The atmospheric intraseasonal forcing, which plays an important role in inducing Java upwelling during the early stage of an IOD event, is found to play a minor role in terminating the upwelling off the Java coast because the intraseasonal winds are either weak or absent during the IOD mature phase. The equatorial wave analyses suggest that the upwelling off the Java coast during IOD events is terminated primarily by western boundary reflections.

Corresponding author address: Dongliang Yuan, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao, 266071, China. Email: dyuan@ms.qdio.ac.cn

1. Introduction

The Indian Ocean zonal dipole mode, also referred simply as the Indian Ocean dipole (IOD) event, refers to anomalous cooling off the Java coast in the tropical southeastern Indian Ocean and the associated anomalous warming in the central and western equatorial Indian Ocean at interannual time scales (Saji et al. 1999; Webster et al. 1999). Observations have shown that the IOD events are phase locked to the seasonal cycle with peak anomalies in fall–winter. The anomalous warming in the western basin is found to lag the anomalous cooling in the eastern basin during the IOD event (Yu and Rienecker 2000; Huang and Kinter 2002). Examination of recent satellite altimeter data of the Ocean Topography Experiment (TOPEX)/Poseidon suggests that oceanic dynamics play an important role in the interannual variations of the tropical Indian Ocean sea surface temperature (Chambers et al. 1999; Yu and Rienecker 2000; Le Blanc and Boulanger 2001). During the 1997–98 IOD event, the warming in the western basin is coincident with the arrival of positive sea level anomalies at the western boundary, while the cooling and anomalous depression of sea level off the Java coast persist until the late spring of 1998. The sea level anomalies suggest that oceanic upwelling in the eastern basin and downwelling in the western basin have an effect on the interannual SST variations of the equatorial Indian Ocean (Murtugudde et al. 2000; Yu and Rienecker 2000; Vinayachandran 1999). The satellite data thus indicate anomalous tilting of the sea level, which suggests a thermocline dipole, associated with the SST dipole in the equatorial Indian Ocean.

So far, detailed analyses of the dynamics that control sea level and thermocline depth variations during the IOD events have been lacking. Le Blanc and Boulanger (2001) used the altimeter data from the TOPEX/Poseidon mission to study the long equatorial wave dynamics of the seasonal-to-interannual variations of the Indian Ocean circulation. They assumed linear dynamics of one baroclinic mode and decomposed the altimeter sea level data into the long equatorial waves. Their analysis shows that the propagation and reflection of long equatorial Kelvin and Rossby waves are important dynamical processes in the interannual variations of the Indian Ocean circulation. The analysis of heat content anomalies of the tropical Indian Ocean by Huang and Kinter (2002) suggests that the propagation of Rossby waves in the southern Indian Ocean during the IOD events is important for maintaining the SST anomalies in the western basin, the reflection of which at the western boundary may play a role in terminating the SST anomalies in the eastern basin. The wave generation, propagation, and coupling with the atmosphere of these off-equatorial Rossby waves in the southern Indian Ocean have been studied by Masumoto and Meyers (1998), Xie et al. (2001), and Jury and Huang (2004). However, their reflection at the western boundary and the role of the reflection in the IOD evolution have not been demonstrated explicitly.

The atmospheric circulation over the tropical Indian Ocean exhibits large variations at the intraseasonal time periods of 30–60 days. Recent studies by Han et al. (2004, 2006) suggest that the intraseasonal wind forcing can produce significant SST anomalies at the interannual time scales through nonlinear rectification of the oceanic dynamics. Rao and Yamagata (2004) pointed to the coincidence of an intraseasonal oscillation event with the early termination of the 1994 IOD event. On the other hand, the analyses of Han et al. (2006, 2007) suggest that the atmospheric intraseasonal oscillations do not play a significant role during the development and termination of the IOD event because the intraseasonal oscillations are either very weak during the IOD events (e.g., the 1997 IOD event) or are after the anomalous SST evolution (e.g., the 1994 IOD event). So far, the effects of the intraseasonal wind forcing on the low-frequency equatorial wave dynamics of the IOD events have not been examined.

In this paper, we focus on the sea level anomalies of the IOD events, the evolution of which is related to but different from the SST dipole index defined by Saji et al. (1999). The dynamics of the IOD events are investigated using decomposed equatorial waves from an ocean general circulation model (OGCM) simulation. We use three-dimensional data of both zonal velocity and pressure from the OGCM simulation to extract the equatorial wave coefficients, which treat the nonlinear terms of the OGCM as forcing terms and ensure a rigorous dynamic consistency, in the long-wave approximation, of the decomposed waves with the OGCM dynamics by making use of the orthonormal relation of the base-mode functions of the equatorial waves (Yuan et al. 2004; Yuan 2005; Yuan and Han 2006). The wave coefficients in this paper have shown some difference in structure from that in Le Blanc and Boulanger (2001). In the next section, the OGCM, a simple linear model, and a simple nonlinear model used in this study are introduced. The model simulation is validated in section 3, based on which the reflections of the Rossby waves at the western boundary are investigated. The dynamics of the 1997–98, 1994, and 1991 IOD events are discussed in detail using the decomposed waves from section 4. The effects of the intraseasonal wind forcing on the interannual sea level anomalies are investigated in section 5. The final section 6 contains conclusions.

2. Models

The OGCM used in this study is the LICOM 1.0 model developed by the Institute of Atmospheric Physics of the Chinese Academy of Sciences. The model has a free sea surface and nearly leveled coordinates in the deep ocean (the η coordinates). The domain of the model covers the global ocean between 75°S and 65°N with a horizontal resolution of 0.5° × 0.5°. There are 30 levels in the vertical with a grid size of 25 m near the ocean surface. The Packnowski and Philander (1981) mixing scheme is used in the model to represent the enhanced mixing in the surface. The model has been spun up using climatological forcings for 900 years. A benchmark experiment (hereafter EXP00) is forced with the assimilated daily 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) wind stress and heat flux from the ECMWF between 1990 and 2001. The surface salinity for this experiment is relaxed to the climatological field of Levitus et al. (1998). Detailed configuration and forcing in this experiment can be found in Liu et al. (2004, 2005).

To isolate the effects of intraseasonal wind forcing, a controlled experiment called EXP01 is conducted to force the same ocean model with low-pass filtered wind stress of ERA-40. The cutoff period of the low-pass filter is set at 90 days. The difference between EXP00 and EXP01 is the circulation forced by the intraseasonal winds by definition.

The analysis of this paper is based on monthly averaged output of the OGCM. The interannual anomalies for this study are relative to a climatological seasonal average from 1990 to 2001. Notice that the intraseasonal forcing of the atmosphere is included in the EXP00 simulation through the use of daily wind stress forcing. Therefore, the lower-frequency energy resulting from nonlinear rectification of the intraseasonal oceanic variations is still imbedded in the monthly output.

Besides the OGCM, which contains the complete dynamics of the ocean circulation, a simple linear continuously stratified model (LCSM) is also used to study the Rossby wave reflection at the western boundary. This model is based on an averaged density profile of the Indian Ocean between 5°S and 5°N in the OGCM simulation and is similar to the one described by McCreary (1981). The model basin includes only the Indian Ocean north of 30°S with the same horizontal resolution as the OGCM. Only the first baroclinic mode simulation of the LCSM is discussed in the following text.

To investigate the rectification dynamics of the intraseasonal oceanic variations, the linear continuity equation of the first baroclinic mode of the LCSM is replaced with a fully nonlinear one to produce a simple 1.5-layer reduced-gravity nonlinear model. This simple nonlinear model is able to represent the asymmetric response of the equatorial sea level to fluctuating winds.

3. Results

a. Validation of the EXP00 simulation

The vertical profiles of the EXP00 density and buoyancy frequency squared are compared with the WOA01 data (Conkright et al. 2002) in Fig. 1. The averaged vertical profiles of the simulated stratification and buoyancy frequency are in very good agreement with those derived from the WOA01 data, suggesting that the decomposed vertical modes represent well the true baroclinic processes in the real ocean.

The simulated sea level anomalies of EXP00 have been validated with the satellite data of TOPEX/Poseidon altimeter. Figure 2 shows the comparison of the sea level anomalies averaged between 5°S and 5°N over the equatorial Indian Ocean with those of the altimeter data. The altimeter anomalies are relative to a climatological mean seasonal cycle from 1993 to 2001 and have been adjusted with the atmospheric surface pressure of National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis through an inverse barometer response. Clearly, the depressed (elevated) sea level anomalies in the eastern (western) equatorial Indian Ocean during the 1994 and 1997–98 IOD events are simulated well by the OGCM. During the 1997–98 IOD event, the sea level in the eastern basin started to decrease in spring 1997, reaching the lowest level in November 1997, and recovered in spring 1998. The positive sea level anomalies in the western basin appeared to originate from the central basin and did not peak until the end of 1997. In contrast, during the 1994 IOD event, the negative sea level anomalies in the eastern basin ended abruptly in January 1995, and the positive anomalies are the largest in the central basin. The good comparisons of the simulated stratification and sea level anomalies with observations suggest that the model can be used to study the dynamics of the equatorial Indian Ocean circulation during the 1994 and the 1997–98 IOD events.

b. Equatorial waves in EXP00

The EXP00 solution is decomposed into equatorial waves to study the IOD dynamics. The eigenfunctions and wave speeds of the baroclinic modes are first calculated based on the averaged density profile over 5°S–5°N. The three-dimensional dynamic height and zonal velocity referenced to 2500 m depth are then projected onto the eigenfunctions to extract the coefficients associated with each baroclinic mode. These coefficients are further decomposed into equatorial Kelvin and Rossby waves according to the procedure described by Yuan et al. (2004). The horizontal structures of the decomposed wave coefficients are not sensitive to the density profile used in the calculation. Uses of the density profiles in the western and eastern equatorial Indian Ocean yield essentially the same structure as the decomposed wave coefficients.

The decomposed Kelvin and first meridional-mode Rossby wave coefficients of the first baroclinic mode are shown in Fig. 3. The structure of the coefficients is reminiscent of the sea level anomalies in the altimeter data. The eastward propagation of upwelling Kelvin waves in 1991, 1994, and 1997 and of downwelling Kelvin waves in 1992, 1995, and 1998 from the western boundary is clearly expressed by the decomposed wave coefficients. The westward propagation of the downwelling Rossby waves from the central basin to the western boundary at the ends of 1991, 1994, and 1997 and of the upwelling Rossby waves in the eastern basin during the 1994 and 1997–198 IOD events is also clearly identified. The wave propagation explains the time lag of the peak sea level anomalies in the western basin relative to the southeastern basin, as seen in Fig. 2. The downwelling Kelvin waves from the western boundary during each of the 1991, 1994, and 1997–98 IOD events actually consist of two consecutive wave pulses. This is seen more clearly in the time series plot of the decomposed wave coefficients at the western boundary in Fig. 4.

Figure 4 (upper panel) compares the reflection of the first baroclinic Rossby waves at the western boundary with the linear theory of Cane and Gent (1984). Due to the low frequency nature of the problem, the reflection of the antisymmetric Rossby waves contributes very little to the total reflected Kelvin wave. According to Cane and Gent (1984), the reflection ratios of antisymmetric Rossby waves decrease with frequencies of the incoming waves. At annual period, the ratio for the second meridional-mode Rossby wave is calculated by Yuan and Han (2006) to be 0.06, about one order of magnitude smaller than the reflection ratio of 0.4 for the first meridional-mode Rossby wave. At interannual time scales, these ratios are even smaller; therefore, their effects can be neglected. The dot curve in the upper panel of Fig. 4 is the linearly reflected Kelvin wave calculated from the decomposed symmetric Rossby waves at the western boundary according to the method described by Yuan and Han (2006). The time-shifted, prorated, first meridional-mode Rossby wave, which represents the contribution of that Rossby wave to the linearly reflected Kelvin wave, is also drawn in a dashed curve for comparison. The close agreement between the dotted and dashed curves indicates that the linearly reflected Kelvin wave comes primarily from the first meridional-mode Rossby wave. The difference between the decomposed (thin solid curve) and linearly reflected (dotted curve) Kelvin waves is plotted with a thick solid curve with some smoothing. Evidently, significant differences exist between the linear theory and the OGCM simulation. During late 1997 through early 1998, two strong downwelling Kelvin waves are generated at the western boundary in the OGCM. The first one is evidently reflected from the equatorial Rossby waves, as suggested by the agreement of the thin solid curve with the dotted curve. The second one is clearly not produced by the linear reflection. Similar kinds of double downwelling Kelvin waves are also found at the end of the 1991 and the 1994 IOD events (see the thick solid curve), suggesting that the phenomenon is a general characteristic of IOD event dynamics. Comparison also shows that, in the years after 2000, significant upwelling Kelvin waves are generated at the western boundary apparently not only through the linear reflection process. These differences from linear reflection might be generated by nonlinear processes near the western boundary, by the alongshore wind forcing along the East African coasts, or by the reflection of off-equatorial Rossby waves (Yuan and Han 2006), the dynamics of which is beyond the scope of this study.

At the eastern boundary, the reflection of the Kelvin waves is generally in agreement with the linear theory (Fig. 4, lower panel), although some differences exist, primarily in the amplitudes.

The second and the third baroclinic-mode waves behave much like the first baroclinic-mode waves, with double Kelvin waves emitting from the western boundary during the terminating phase of the IOD events, which arrive at the eastern boundary later than the first baroclinic Kelvin waves (not shown). The reconstructed sea level anomalies based on the first four or five baroclinic-mode waves can reproduce the basic characteristics of the Indian Ocean sea level anomalies shown in Fig. 2, as demonstrated by Yuan (2005) and Yuan and Han (2006). The discussion of these wave dynamics is omitted here.

c. Reflection of Rossby waves at the western boundary

To understand the reflection process at the western boundary, three experiments are conducted using the LCSM: the main run (MR) is forced by the same ERA-40 winds over the Indian Ocean north of 30°S; test run 1 (TR1) is forced with the ERA-40 winds east of 60°E; test run 2 (TR2) is forced with the winds east of 60°E and equatorward of 5°S and 5°N. The results of each experiment are decomposed into equatorial waves just as the OGCM decomposition is conducted.

The western boundary reflection of the first baroclinic-mode Rossby waves in the MR is similar to that in the OGCM, with double downwelling Kelvin waves at the ends of the 1991, 1994, and 1997–98 IOD events (Fig. 5a). The first Kelvin wave is reflected from the equatorial Rossby waves; the second is not. When the winds near the western boundary are shut off in TR1, the reflection stays about the same except for a slightly smaller amplitude of the Kelvin waves (Fig. 5b), suggesting that the difference from the linear reflection is not due to the alonghore winds near the western boundary. This result is in contrast to the seasonal cycle dynamics in which the alongshore winds along the east coast of Africa drive significant equatorial Kelvin waves to the eastern basin (Yuan and Han 2006). When the winds west of 60°E and poleward of 5°S and 5°N are shut off in the TR2, the second Kelvin wave pulse at the ends of the 1991, 1994, and 1997–98 IOD events has essentially disappeared and the reflection is in good agreement with the linear reflection of the equatorial Rossby waves at the western boundary (Fig. 5c). These results suggest that the difference from the linear theory in the MR is primarily due to the reflection of the off-equatorial Rossby waves. The complete elimination of the off-equatorial Rossby waves is difficult in the control runs due to the proximity of these waves to the equator and the fact that TR2 contains a discontinuity of the wind stress at 5°S and 5°N. Nevertheless, the comparisons of the western boundary reflections in these controlled experiments suggest that the second downwelling Kelvin wave during the IOD events is generated from the reflection of the off-equatorial Rossby waves. Further experimentation suggests that both Northern and Southern Hemisphere off-equatorial Rossby waves are reflected into the second Kelvin wave at the western boundary, with the contribution from the southern Indian Ocean larger than from the northern Indian Ocean (not shown).

4. Dynamics of the IOD sea level

In this section, we use the decomposed waves in the OGCM simulation to study the dynamics of the IOD circulation in 1997–98, 1994, and 1991. The focus is on the roles of the equatorial waves and the western boundary reflection in the evolution of the IOD sea level anomalies.

a. The 1997–98 IOD event

The 1997–98 IOD is a strong anomalous event according to the dipole mode index (Saji et al. 1999; Han et al. 2006) and is coincident with the strongest ENSO event of this century in the Pacific Ocean. Its dynamics are still not fully understood to date. The decomposed wave coefficients suggest that this IOD event begins in early 1997 (Fig. 2) when the equatorial Indian Ocean is dominated by easterly wind anomalies (Fig. 6). However, the negative sea level anomalies in the eastern basin are interrupted by an intraseasonal westerly event in April of 1997 (Figs. 2 and 7). Since May 1997 the entire equatorial Indian Ocean has been dominated by easterly wind anomalies, which peak in September in the central basin (Fig. 6). The maximum slope of the sea level anomalies, however, is not established until November–December 1997. Associated with the easterly wind anomalies, upwelling equatorial Kelvin waves and downwelling equatorial Rossby waves propagate to the eastern and western basins, respectively (Fig. 7). The Kelvin waves are reflected into upwelling equatorial Rossby waves at the eastern boundary, which propagate across the basin to reach the western boundary in January–February 1998. The downwelling equatorial Rossby waves, on the other hand, reach the western boundary in November–December 1997 and are reflected into a downwelling equatorial Kelvin wave, which propagates across the basin to reach the eastern boundary in December 1997 through January 1998. The equatorial wave propagation explains the time lag between the maximum anomalies of the easterly winds and of the sea level slope. The Kelvin wave reflected from the downwelling equatorial Rossby waves evidently does not terminate this IOD event because the negative sea level anomalies in the eastern basin persist until the spring season 1998 (Fig. 2). The sea level anomalies off the Java coast in the 1997–98 IOD event vanish, however, as the second downwelling Kelvin wave arrives at the eastern boundary in April 1998. This Kelvin wave is reflected from the off-equatorial Rossby waves in the southern Indian Ocean, as suggested by the LCSM experiments in the previous section.

The role of the two Kelvin waves can also be identified from the total sea level anomalies over the Indian Ocean. Figure 8 shows the evolution of the total sea level anomalies associated with the propagation of the two Kelvin waves. In May–June 1997, an intraseasonal wind event is just ended and the sea level off the Java coast is adjusting from elevation to depression associated with the easterly wind forcing. The sea level depression develops as the upwelling Kelvin waves from the central basin arrive at the eastern boundary (Fig. 7). At the same time, two centers of positive sea level anomalies straddling the equator at about 5°S and 5°N are developing in July 1997 and propagating to the western boundary. These are the equatorial Rossby waves identified in the decomposed wave coefficients (Fig. 7). These downwelling Rossby waves reach the western boundary in November–December 1997 and are reflected into the first downwelling equatorial Kelvin wave at the end of 1997, which arrives at the eastern boundary in January 1998 (Fig. 7). However, the sea level anomalies off the Java coast remain negative at the arrival of this Kelvin wave because the southeasterly wind west of the Sumatra coast is still strong. At the same time, a significant wave of positive sea level anomalies in the latitude range of 10°–15°S is propagating to the western boundary. These are the off-equatorial Rossby waves forced by the anticyclonic wind stress curl in the southern Indian Ocean associated with the 1997–98 easterly wind anomalies over the equatorial basin. These off-equatorial Rossby waves are reflected into the second equatorial Kelvin wave, which reaches the eastern boundary in April–May 1998. The fact that the equatorial winds in April are still dominated by easterlies by the time this Kelvin wave reaches the eastern boundary suggests that the propagation of this Kelvin wave is primarily an oceanic process. The analysis here also suggests that the duration between the peak and demise phases of the Java upwelling during the 1997–98 IOD event is determined by the difference between the equatorial and off-equatorial Rossby wave speeds.

b. The 1994 IOD event

The 1994 IOD event is different from the 1997–98 IOD event in that it ends abruptly in early 1995 (Fig. 2). The easterly wind anomalies begin in April 1994 (Fig. 9) with an intraseasonal westerly wind event in November 1994 (Rao and Yamagata 2004; Han et al. 2006). The altimeter data in Fig. 2 show that the anomalous upwelling off the Java coast begins in March 1994 before the easterly wind anomalies in the central basin begin. The decomposed wave coefficients in Fig. 10 suggest that the sea level anomalies before April 1994 are induced by the Kelvin waves reflected from the upwelling equatorial Rossby waves forced by westerly wind anomalies in late 1993. The upwelling Kelvin waves are interrupted briefly by an intraseasonal wind event in the eastern basin in January–February 1994, which delays the onset of the Java upwelling until March of 1994. In the summer of 1994, upwelling Kelvin waves and downwelling equatorial Rossby waves are generated by the easterly wind anomalies, which propagate to the eastern and western boundaries, respectively. The Kelvin waves are reflected into equatorial Rossby waves at the eastern boundary, but only the Rossby waves in October–December make their way to the western boundary in January–February 1995. Other upwelling equatorial Rossby waves are overwhelmed by the easterly wind anomalies in the central basin and do not arrive at the western boundary. A strong downwelling Kelvin wave reflected from the downwelling equatorial Rossby waves at the western boundary arrives at the eastern boundary in December 1994–January 1995. This Kelvin wave evidently damps the negative sea level anomalies off the Java coast in February 1995. Although the second Kelvin wave reflected from the off-equatorial Rossby waves in the southern Indian Ocean also arrives at the eastern boundary in May 1995, the sea level anomalies off the Java coast have long become positive at that time.

Why do Kelvin waves play different roles in the 1997–98 and 1994 IOD events? The answer lies in the local wind forcing off the Java coast. Figure 11 shows the evolution of the sea level anomalies during the 1994 IOD event, which is similar to that in 1997 in terms of equatorial wave generation and propagation. By December 1994, before the first downwelling Kelvin wave arrives at the eastern boundary, the negative sea level anomalies off the Java coast have already become very weak compared to those at the end of 1997. The southeasterly winds along the west coast of Sumatra at the end of 1994 are much weaker than those in late 1997 (Han et al. 2006). Because of the weaker winds and sea level anomalies, the Java upwelling during the 1994 IOD event is damped by the first Kelvin wave reflected from the downwelling equatorial Rossby waves at the western boundary. The comparison here suggests that both local wind forcing off the west coast of Sumatra and the delayed negative feedback from the western boundary reflection are important for the evolution of the Java upwelling during the IOD events.

c. The 1991 IOD event

The 1991 IOD is a weak anomalous event according to the dipole index of SST (Saji et al. 1999). The simulated sea level anomalies show that the eastern equatorial Indian Ocean is dominated by anomalous upwelling currents since early 1991, which are an effect of complicated wind forcing (Fig. 12). The decomposed wave coefficients suggest that the sea level anomalies off the Java coast are terminated by a westerly wind burst in September 1991, which produces downwelling Kelvin waves at the eastern boundary (Fig. 13). The sea level dipole is tilted up again to the western basin in early 1992, evidently forced by the easterly wind anomalies during that period. The event is not terminated until the second Kelvin wave reaches the eastern boundary.

5. Role of the intraseasonal wind forcing

To isolate the effects of the intraseasonal wind forcing, the difference between EXP00 and EXP01 is analyzed and decomposed into equatorial waves. Figure 14 shows the sea level anomalies in EXP01 and in EXP00–EXP01. The sea level anomalies in the left panel compare well with the altimeter data in Fig. 2, suggesting that the interannual sea level anomalies can be reproduced well by the low-frequency winds alone. The anomalies in the right panel are all less than 4 cm, suggesting that the rectified sea level anomalies due to the intraseasonal wind forcing are generally small. The decomposed equatorial waves show that a weak monthly mean downwelling Kelvin wave is generated by the intraseasonal wind event in November 1994 followed by a stronger monthly mean upwelling Kelvin wave in December over the central basin (Fig. 15). The wind stress difference between EXP00 and EXP01, indeed, shows the strong intraseasonal westerly wind events in November 1994, as seen in Rao and Yamagata (2004) (Fig. 16, left panel). We have integrated the simple nonlinear model forced with the high-passed intraseasonal winds from October through December 1994 with the mean stress of that period removed. The upper-layer thickness of the simple nonlinear model indeed shows weak sea level elevation in the central basin associated with the intraseasonal westerly wind bursts in November 1994, followed by stronger sea level depression over the equatorial central Indian Ocean in December (Fig. 16, right panel), the structure of which is similar to the OCGM simulation in Fig. 15. Because the only nonlinearity of the simple nonlinear model is in its continuity equation, the results suggest that the dynamics of the stronger sea level depression in December 1994 is due to the asymmetric response of the thermocline depth to the intraseasonal wind forcing. This kind of dynamics has been discussed in detail by Kesseler and Kleeman (2000) and Han et al. (2004). The daily output of the OGCM shows that the forced intraseasonal sea level anomalies have kept the phase of the altimeter data faithfully, although with a weaker amplitude (not shown) probably due to the overestimated surface mixed layer depth owing to the imperfect surface salinity condition and the mixing scheme. The results of both the OGCM and the simple nonlinear model simulations suggest that the November 1994 intraseasonal westerly wind bursts produce only a weak and short impact on the coastal upwelling off the Java coast, which is quickly overwhelmed by a stronger upwelling Kelvin wave in December 1994. Therefore, the November 1994 intraseasonal wind event is not the primary process that terminates the Java upwelling during the 1994–95 IOD event.

The minor role of the intraseasaonal wind forcing in the termination of the IOD events is generally consistent with the fact that the atmospheric intraseasonal events are weak or absent during the IOD mature phase (Han et al. 2006, 2007). On the other hand, the intraseasonal winds appear to impact the onset and initial evolution of the IOD events, as suggested by our wave dynamics analyses in the previous section. The decomposed waves show sizable equatorial Kelvin waves forced by the intraseasonal winds in early 1997 (Fig. 17), which evidently have affected the timing of the onset of the Java upwelling during the 1997–98 IOD event. Thus, the analyses here suggest that the atmospheric intraseasonal events can impact the initial development of the Java upwelling during the IOD events significantly, but do not play a significant role in the termination of the Java upwelling in general.

6. Conclusions

The dynamics of the sea level anomalies during the Indian Ocean dipole events in the 1990s are studied using an OGCM forced with daily ERA-40 wind stress and heat flux from the European Centre for Medium-Range Weather Forecasts during 1990–2001. The simulated results are validated with the satellite altimeter data and are decomposed into equatorial waves to study the generation, propagation, and delayed feedback of the IOD events. The analyses suggest that the western boundary reflections of the equatorial and off-equatorial Rossby waves play an important role in the development and termination of the IOD events. The time lag between the maximum anomalies of the easterly wind and the maximum slope of the sea level anomalies is due to the propagation of equatorial Kelvin and Rossby waves to the eastern and western boundaries, respectively, from the central basin. Two strong downwelling Kelvin waves are generated at the western boundary at the end of the IOD events. The first is reflected from the equatorial Rossby waves forced by the easterly wind anomalies. The second is reflected from the off-equatorial Rossby waves forced by the anticyclonic wind curl anomalies in the southern Indian Ocean, which in turn are associated with the easterly wind anomalies over the equatorial basin. The sea level anomalies off the Java coast during the 1997–98 IOD event are found to be weakened by the first Kelvin wave and terminated by the second. In comparison, sea level anomalies off the Java coast during the 1994 IOD event is terminated by the first Kelvin wave. The different dynamics of the two events are attributed to the different winds off the west Java coast during the two IOD events. The upwelling forced by the southeasterly winds off the Java coast in the 1997–98 IOD event is strong, so a second downwelling Kelvin wave is needed to terminate the event. In comparison, the local forcing of upwelling currents off the Java coast in late 1994 is weak, so the first downwelling Kelvin wave reverses the phase of that event.

The intraseasonal oscillations of the atmospheric circulation are found to modify the amplitudes and phases of the Java upwelling significantly at the early stage of the IOD events. The onset of the Java upwelling during the 1997 IOD event is found to be interrupted by an intraseasonal westerly wind burst in April 1997. The intraseasonal wind forcing in September 1991 reverses the negative sea level anomalies off the west coast of Sumatra. However, the intraseasonal westerly winds are found to play a minor role in terminating the Java upwelling at the end of the IOD events. In particular, our numerical experiments suggest that the intraseasonal wind forcing in November 1994 is not the primary process that terminates the Java upwelling of the 1994 IOD event.

The delayed feedback dynamics disclosed in this study suggest that the demise of the Java upwelling during the IOD events is not primarily a zonal air–sea coupled process because the western boundary reflection and the eastward propagation of the equatorial Kelvin waves are primarily oceanic processes. Given the close connection between the SST and upwelling anomalies off the Java coast, the delayed feedback mechanism offers potential predictability of the IOD termination. The results suggest that models with long-wave dynamics will have better interannual forecast skills than the ones with a slab-ocean mixed layer.

Acknowledgments

Funding support from the National Basic Research Program of China (“973 program”) (Number 2006CB403600), from the CAS key project (KZCX2-YW-218), from the NSF of China (Numbers 40676020 and 40405017), and from the “Hundreds-Talent Program” of the Chinese Academy of Sciences are gratefully acknowledged. The authors thank the TOPEX/Poseidon project for the altimeter data and the NCAR–NCEP reanalysis project for the atmospheric surface pressure data. D. Yuan gratefully acknowledges the support of K.C. Wong Education Foundation, Hong Kong. Suggestions from two anonymous reviewers are gratefully acknowledged.

REFERENCES

  • Cane, M., , and P. Gent, 1984: Reflection of low-frequency equatorial waves at arbitrary western boundaries. J. Mar. Res., 42 , 487502.

  • Chambers, D. P., , B. D. Taley, , and R. H. Stewart, 1999: Anomalous warming in the Indian Ocean coincident with El Niño. J. Geophys. Res., 104 , 30353047.

    • Search Google Scholar
    • Export Citation
  • Conkright, M. E., , R. A. Locarnini, , H. E. Garcia, , T. D. O’Brien, , T. P. Boyer, , C. Stephens, , and J. I. Antonov, 2002: World Ocean Atlas 2001: Objective Analysis, Data Statistics, and Figures: CD-ROM Documentation. National Oceanographic Data Center, 17 pp.

    • Search Google Scholar
    • Export Citation
  • Han, W., , P. J. Webster, , R. Lukas, , P. Hacker, , and A. Hu, 2004: Impact of atmospheric intraseasonal variability in the Indian Ocean: Low-frequency rectification in equatorial surface current and transport. J. Phys. Oceanogr., 34 , 13501372.

    • Search Google Scholar
    • Export Citation
  • Han, W., , T. Shinoda, , L-L. Fu, , and J. P. McCreary, 2006: Impact of atmospheric intraseasonal oscillations on the Indian Ocean dipole during the 1990s. J. Phys. Oceanogr., 36 , 670690.

    • Search Google Scholar
    • Export Citation
  • Han, W., , D. Yuan, , W. T. Liu, , and D. J. Halkides, 2007: Intraseasonal variability of Indian Ocean sea surface temperature during boreal winter: Madden-Julian oscillation versus submonthly forcing and processes. J. Geophys. Res., 112 , C04001. doi:10.1029/2006JC003791.

    • Search Google Scholar
    • Export Citation
  • Huang, B., , and J. L. Kinter III, 2002: Interannual variability in the tropical Indian Ocean. J. Geophys. Res., 107 , 3199. doi:10.1029/2001JC001278.

    • Search Google Scholar
    • Export Citation
  • Jury, M. R., , and B. Huang, 2004: The Rossby wave as a key mechanism of Indian Ocean climate variability. Deep-Sea Res. I, 41 , 21232136.

    • Search Google Scholar
    • Export Citation
  • Kessler, W. S., , and R. Kleeman, 2000: Rectification of the Madden–Julian oscillation into the ENSO cycle. J. Climate, 13 , 35603575.

    • Search Google Scholar
    • Export Citation
  • Le Blanc, J. L., , and J. P. Boulanger, 2001: Propagation and reflection of long equatorial waves in the Indian Ocean from TOPEX/Poseidon data during the 1993–1998 period. Climate Dyn., 17 , 547557.

    • Search Google Scholar
    • Export Citation
  • Levitus, S., and Coauthors, 1998: Introduction. Vol. 1, World Ocean Database 1998, NOAA Atlas NESDIS 18, 346 pp.

  • Liu, H., , Y. Yu, , W. Li, , and X. Zhang, 2004: LASG/IAP climate system ocean model (LICOM1.0): User manual (in Chinese). Science Publication, 107 pp.

    • Search Google Scholar
    • Export Citation
  • Liu, H., , W. Li, , and X. Zhang, 2005: Climatology and variability of the Indonesian Throughflow in an eddy-permitting oceanic GCM. Adv. Atmos. Sci., 22 , 496508.

    • Search Google Scholar
    • Export Citation
  • Masumoto, Y., , and G. Meyers, 1998: Forced Rossby waves in the southern Indian Ocean. J. Geophys. Res., 103 , 2758927602.

  • McCreary, J. P., 1981: A linear stratified ocean model of the equatorial undercurrent. Quart. J. Roy. Soc. London, 298A , 603635.

  • Murtugudde, R., , J. P. McCready, , and A. J. Busalacchi, 2000: Oceanic processes associated with anomalous events in the Indian Ocean with relevance to 1997–1998. J. Geophys. Res., 105 , 32953306.

    • Search Google Scholar
    • Export Citation
  • Packnowski, R. C., , and S. G. H. Philander, 1981: Parameterization of vertical mixing in numerical models of the tropical ocean. J. Phys. Oceanogr., 11 , 14421451.

    • Search Google Scholar
    • Export Citation
  • Rao, S. A., , and T. Yamagata, 2004: Abrupt termination of Indian Ocean dipole events in response to intraseasonal oscillations. Geophys. Res. Lett., 31 , L19306. doi:10.1029/2004GL020842.

    • Search Google Scholar
    • Export Citation
  • Saji, N. H., , B. N. Goswami, , P. N. Vinayachandran, , and T. Yamagata, 1999: A dipole mode in the tropical Indian Ocean. Nature, 401 , 360363.

    • Search Google Scholar
    • Export Citation
  • Vinayachandran, P. N., , S. Iizuka, , and T. Yamagata, 2002: Indian Ocean dipole mode events in an ocean general circulation model. Deep-Sea Res. II, 49 , 15731596.

    • Search Google Scholar
    • Export Citation
  • Webster, P. J., , A. M. Moore, , J. P. Loschnigg, , and R. R. Leben, 1999: Coupled ocean–atmosphere dynamics in the Indian Ocean during 1997–1998. Nature, 401 , 356360.

    • Search Google Scholar
    • Export Citation
  • Xie, S-P., , H. Annamalai, , F. A. Schott, , and J. P. McCreary, 2001: Structure and mechanisms of south Indian Ocean climate variability. J. Climate, 15 , 864878.

    • Search Google Scholar
    • Export Citation
  • Yu, L., , and M. M. Rienecker, 2000: Indian Ocean warming of 1997–1998. J. Geophys. Res., 105 , 1692316939.

  • Yuan, D., 2005: Role of the Kelvin and Rossby waves in the seasonal cycle of the equatorial Pacific Ocean circulation. J. Geophys. Res., 110 , C04004. doi:10.1029/2004JC002344.

    • Search Google Scholar
    • Export Citation
  • Yuan, D., , and W. Han, 2006: Roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial Indian Ocean. J. Phys. Oceanogr., 36 , 930944.

    • Search Google Scholar
    • Export Citation
  • Yuan, D., , M. M. Rienecker, , and P. S. Schopf, 2004: Long wave dynamics of the interannual variability in a numerical hindcast of the equatorial Pacific Ocean circulation during the 1990s. J. Geophys. Res., 109 , C05019. doi:10.1029/2003JC001936.

    • Search Google Scholar
    • Export Citation

Fig. 1.
Fig. 1.

Comparison of averaged vertical profiles of density and buoyancy frequency square over the equatorial Indian Ocean between 5°S and 5°N (dashed) with the WOA01 (solid) data.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 2.
Fig. 2.

Comparison of averaged sea level anomalies between 5°S and 5°N of the OGCM simulation with the TOPEX/Poseidon altimeter data. The altimeter anomalies are based on a climatology of 1993–2001.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 3.
Fig. 3.

Decomposed coefficients of equatorial Kelvin and Rossby waves of the first baroclinic mode from the OGCM simulation. The contour units correspond to 0.33 m for Kelvin wave sea level and 0.14 m for the first meridional-mode Rossby wave sea level on the equator. The coefficients have been smoothed with the 1–2–1 filter.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 4.
Fig. 4.

(top) Coefficients of the decomposed (thin solid curve) and linear reflected (dotted curve) Kelvin waves of the first baroclinic mode at the western boundary. The thick solid curve represents the difference between the two. The time-shifted first meridional-mode Rossby wave coefficient multiplied by its reflection ratio is also shown with the dashed curve in the upper panel. (bottom) Kelvin wave (solid) and the first meridional-mode Rossby wave divided by the Kelvin wave reflection ratio (dotted) at the eastern boundary.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 5.
Fig. 5.

Coefficients of the decomposed (thin solid curve) and linear reflected (dotted curve) Kelvin waves of the first baroclinic mode at the western boundary for (a) main run, (b) test run 1, and (c) test run 2 of the linear continuously stratified model. The thick solid curve represents the difference between the decomposed and the reflected Kelvin wave.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 6.
Fig. 6.

Zonal wind stress anomalies (Pa) along the equator during 1996–98.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 7.
Fig. 7.

Decomposed coefficients of equatorial Kelvin and Rossby waves of the first baroclinic mode from the OGCM simulation for the period of 1996–98. The theoretical first baroclinic Kelvin wave speed is plotted as a solid line at the bottom of the left panel.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 8.
Fig. 8.

Sea level (contours) and wind stress (vectors) anomalies during (a) May–December 1997 and (b) January–August 1998.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 9.
Fig. 9.

As in Fig. 6, but for the period 1993–95.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 10.
Fig. 10.

As in Fig. 7, but for the period of 1993–95.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 11.
Fig. 11.

Sea level (contours) and wind stress (vectors) anomalies for the 1994 IOD.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 12.
Fig. 12.

(left) Zonal wind stress anomalies (Pa) along the equator and (right) OGCM-simulated sea level anomalies (cm) averaged between 5°N and 5°S during 1990–92.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 13.
Fig. 13.

As in Fig. 10, but for the period of 1990–92.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 14.
Fig. 14.

Sea level anomalies along the equator averaged between 5°N and 5°S for (left) EXP01 and (right) the difference between EXP00 and EXP01.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 15.
Fig. 15.

Decomposed coefficients of equatorial Kelvin and Rossby waves of the first baroclinic mode for the results of EXP00–EXP01 for the period of 1993–95.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 16.
Fig. 16.

(left) Daily high-passed wind stress (Pa) averaged between 5°N and 5°S with the mean for October–December 1994 removed and (right) the forced upper-layer thickness (m) along the equator in the simple nonlinear model.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Fig. 17.
Fig. 17.

Decomposed coefficients of equatorial Kelvin and Rossby waves of the first baroclinic mode of the results of EXP00–EXP01 for the period of 1996–99.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3900.1

Save