Short-Term Variabilities of Upper Ocean Current in the Warm Pool Region during TOGA/COARE IOP

Journal of Oceanography, Vol. 54, pp. 227 to 240. 1998 Short-Term Variabilities of Upper Ocean Current in the Warm Pool Region during TOGA/COARE IOP IWAO UEKI 1, KUNIO KUTSUWADA 1, HIDEO INABA 1 and ARATA KANEKO 2 1 School of Marine Science and Technology, Tokai University, 3-20-1, Orido, Shimizu, Shizuoka 424-8610, Japan 2 Department of Environmental Sciences, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739-0046, Japan (Received 26 August 1997; in revised form 16 January 1998; accepted 23 January 1998) Oceanic current data in the warm pool region of the western equatorial Pacific measured by upward-looking moored Acoustic Doppler Current Profilers at two equatorial sites (147 E and 154 E) and two off-equatorial sites (2 N and 2 S, 156 E) during TOGA/ COARE Intensive Observing Period (IOP) from November 1992 to February 1993 are used to examine short-term variabilities in the upper layer above 160 240 m. In time series of the zonal and meridional currents in many layers, spectral peaks are found at periods around 2 days and 4 days in addition to high energies in a period range longer than 10 days. The signal with the period of about 2 days has significantly high energies at all sites, and its magnitude is higher for the meridional current than for the zonal one. This signal is especially active in the first half of IOP from November to December in 1992. In this period, the quasi-2-day signal in the current field is coherent between northern (2 N) and southern (2 S) stations, but it has no evident relationship with that in the surface wind field around the stations. The quasi-4-day signal with the period of about 4 days has highest energies in layers above 160 m at the southern station, and is coherent between northern and southern stations. Besides, the signal at the station of 2 S has a significantly high coherence with that in the wind at the southern station, suggesting that it is a local phenomenon. Keywords: Warm-pool, variabilities, oceanic current, TOGA/COARE IOP, quasi-2-day signal. 1. Introduction It is well known that the warm pool region in the western Pacific is a great energy source for driving the convection system in the tropical region; it is also a key area for understanding of the global-scale air-sea coupled system. A typical phenomenon in the tropical Pacific is the El Niño/ Southern Oscillation (ENSO) event which has a time scale of several years (e.g. Philander, 1990). On the other hand, variabilities with shorter time scales have been studied by many oceanographers and meteorologists, because they are considered to play important roles in the triggering and/or evolution of an ENSO event. For the purpose of elucidating these mechanism of the system, the Tropical Ocean and Global Atmosphere (TOGA)/Coupled Ocean and Atmosphere Research Experiment (COARE) project has been planned internationally (Webster and Lukas, 1992; McPhaden, 1993; Lukas et al., 1995), with enhanced measurements having been performed by various types of equipment during an Intensive Observing Period (IOP) from November in 1992 to February in 1993 (Suzuki et al., 1995; Huyer et al., 1997). As a part of this project, the Japanese Pacific Climatic Studies (JAPACS) project has been conducted to contribute to the TOGA/COARE project thanks to funding by the Science Technology Agency of Japan. In this project we have carried out long-term measurements of oceanic currents in the near-surface layer of the warm-pool region using upward-looking Acoustic Doppler Current Profilers (ADCP). Some of these results were described by Kutsuwada and Inaba (1995), who noted long-term variabilities with time scales longer than several days. Recently, Zhu et al. (1998), using the ADCP data at the same stations, detected a variation with the period of 14 days and attempted to interpret it as an equatorial wave. In this paper, we describe short-term variabilities with time scales shorter than several days. Some previous studies detected oceanic signals with the periods of several days in the tropical Pacific (Wunsch and Gill, 1976; Luther, 1980; Ripa and Hayes, 1981). These are derived from sea level data at island stations. Recent studies, using current data at some depths at numerous TOGA/Tropical Atmosphere Ocean (TAO) buoys, examined variabilities with time scales from a few days to several Copyright The Oceanographic Society of Japan. 227

months in the tropical Pacific (McPhaden and Hayes, 1990; McPhaden et al., 1990, 1992). The ADCP supplies us with time series of current data at multiple layers, which permits us to analyze the vertical structure of current variabilities. In this paper we describe variabilities of surface oceanic currents with the periods of several days. Time series analyses are made for time series of zonal and meridional currents by ADCP data at four stations in the warm pool region. The major aim of this paper is to describe a general feature of current variabilities and to clarify the existence of signals with periods of about 2 and 4 days which are prominent during IOP. Details of the signal with the period of 2 days will be given in our subsequent paper (Kutsuwada et al., 1998). In the next section we describe the data source and procedures. In Section 3, features of the current structure during our measurement period are shown to reveal some prominent signals, and their periodic features are described in Section 4 using frequency spectra and cross spectra. Section 5 summarizes the results and propose future work. 2. Data In this study we use time series of current data in the Fig. 1. Location of ADCP moorings used in this study. upper ocean which have been obtained by upward-looking ADCP moored at the following four stations during TOGA/ COARE IOP; Site W (EQ, 147 E), Site E (EQ, 154 E), Site N (2 N, 156 E) and Site S (2 S, 156 E). These observation sites are depicted in Fig. 1. Raw current data derived every 15 or 20 minutes are corrected using the depth, water temperature and salinity data which have been measured at the same time (see Kutsuwada and Inaba, 1995). These current data are interpolated vertically to derive each 10 m depth and averaged temporally to make hourly data. Periods and depth ranges for these data are indicated in Table 1. At all sites, there are no data in the near-surface layer, because output data by ADCP sensors are contaminated by side-lobe effects. The resultant hourly data cover 2430 2613 hours from 6 November in 1992 to 24 February in 1993 at four sites (see Table 1). For the purpose of comparing the current variabilities among different stations, we use time series of the hourly current data in a period in which there are no missing data at all stations: for 99 days from 8 November 1992 to 14 February 1993. As an additional data set, we use wind and sea temperature data which have been measured by ATLAS (Autonomous Temperature Line Acquisition System) buoys in the TOGA/TAO array (Hayes et al., 1991). In this study, hourly data of the zonal and meridional components of surface wind at 2 N and 2 S, 156 E and daily data of the sea temperature at EQ, 154 E are used. For examinations of periodicities for current variations, frequency spectra are calculated by the Fast Fourier Transformation (FFT) method as followings. First, time average and linear trend are calculated and removed for each time series. Next, for the detrended time series, spectra are calculated by the FFT method with the degree of freedom of 16. Calculations of cross spectra between currents at different stations and between current and surface wind are made by similar procedures. Table 1. Information on ADCP measurements at four mooring stations in this study. Location Depth range Period EQ, 147 E (Site W) 30 230 m (21 layers) 1992/11/7 11:00 1993/2/24 08:00 Bottom depth 4174 m EQ, 154 E (Site E) 30 240 m (22 layers) 1992/11/6 00:00 1993/2/15 16:00 Bottom depth 2991 m 2 N, 156 E (Site N) 30 160 m (14 layers) 1992/11/6 00:00 1993/2/15 05:00 Bottom depth 2590 m 2 S, 156 E (Site S) 30 210 m (19 layers) 1992/11/6 00:00 1993/2/17 04:00 Bottom depth 1750 m 228 I. Ueki et al.

Fig. 2. Vertical profiles of time means and standard deviations of zonal (U) and meridional (V) currents at (a): Site W, (b): Site E, (c): Site N and (d): Site S. 3. Features of Current Structure during IOP Vertical profiles of time averaged zonal and meridional currents during IOP are shown in Fig. 2, together with their standard deviations (SD). Similar figures were found by Kutsuwada and Inaba (1995), who calculated time average and SD for about one year including pre-iop (from January or March in 1992) at Site W and Site E. In the year-long averages, the zonal mean flow is dominated by the eastward flow in almost the entire layer between the near-surface layer and 200 m at both sites. On the other hand, the zonal mean flow in IOP is relatively weak with maxima of about 0.3 m s 1 at 220 m, corresponding to core depth of the Equatorial Undercurrent (EUC). In the upper layer, the mean zonal flow is westward with maxima of about 0.1 m s 1 around 100 m at Site W and of about 0.2 m s 1 around 90 m at Site E, corresponding to the South Equatorial Current (SEC). At Site N and Site S, the mean zonal flow has a westward maximum of 0.15 m s 1 around 120 m and of 0.04 m s 1 at 100 m, respectively, which are deeper than those of the sites on the equator. At Site S, there is an eastward mean flow in the lower layer with a maximum of about 0.3 m s 1 around 180 m, while at Site N there are no eastward mean flows in the lower layer. This suggests that the EUC has no latitudinal coverage until 2 N. The magnitudes of SD for the zonal flow have maxima in the uppermost layer at all sites, which are similar to those of the year-long SD (Kutsuwada and Inaba, 1995). Another maxima are found in the subsurface layer around 110 130 m, except at Site N, corresponding to the upper part of the EUC. The magnitudes of the mean meridional flow are smaller than those of the zonal flow at four sites, and their maximum values never exceed 0.1 m s 1. However, the magnitudes of SD are not different from those for the zonal flow at all sites. These contrast with those for the year-long SD, which suggests features of short-term variabilities during IOP. Time-depth diagrams of the zonal currents at four sites are shown in Fig. 3. In November 1992, the core of an eastward-flowing Equatorial Undercurrent (EUC) is found in a layer deeper than 180 m at three sites except at Site N. Thereafter, the EUC became shallower with time, reaching about 100 m in January 1993. In the upper layer above the Variabilities of Oceanic Current in the Warm-Pool 229

230 I. Ueki et al. Fig. 3. Contour plots of the zonal currents at (a): Site W, (b): Site E, (c): Site N and (d): Site S. Contour interval is 0.2 m s 1.

EUC, there are the westward-flowing South Equatorial Current (SEC) around 110 m at four stations. Except at Site S, the SEC also extended upward with time and reached the uppermost layer in the middle of January 1993. During the initial stage of IOP we can see the eastward flow in the nearsurface layer above about 100 m at all sites. Figure 4 shows a time-depth diagram of the sea temperature at EQ, 154 E from 1 November 1992 to 18 January 1993. At the beginning of November 1992, the main thermocline is found between about 100 m and 150 m. Compared with current variabilities at same station (Site E), we can see that the depth of the main thermocline is almost identical with that of the SEC. The thermocline depth becomes shallower with time, as the SEC becomes shallower with time, while the surface eastward flow tends to be weaker with time, changing to a westward Fig. 4. Contour plots of the sea temperature at EQ, 154 E from 1 November 1992 to 18 January 1993. Contour interval is 1 C. flow in the middle and end of January. Kutsuwada and Inaba (1995) suggested that these strong eastward flows in the near-surface layer are driven by a westerly wind burst. Variabilities of near-surface flow observed at this time might be due to some forcing of surface wind field in the western equatorial Pacific. 4. Quasi-2-Day and Quasi-4-Day Signals 4.1 Spectral features Time series of hourly data for the zonal currents at 50 m are shown in Fig. 5, where we can see the existence of signals with the periods shorter than a day. Similar signals are recognized in the meridional currents (not shown). These are mainly related to tidal variabilities such as diurnal and/or semidiurnal cycles in this region. Since in this paper we pay attention to variabilities with periods longer than these tidal signals, we removed the tidal signals using a lowpass filter with a 25-hour running mean. Figure 6 shows low-pass filtered time series of the zonal and meridional currents in the uppermost layer (50 m) at each site. The amplitudes of the zonal and meridional currents at each station are almost the same as each other, which is consistent with the features in Fig. 2. These amplitudes are not constant during IOP, and are related to some changes with time scales from several days to several months. When we compare the variations at the two equatorial sites (W and E) with those at the two off-equatorial sites (N and S), we can find that shorter period variabilities are more marked at Site N and Site S. As an additional event, a strong westward flow is found out in the middle of January 1993. This event may have propagated eastward from site W to Site E. Other events are found for the strong westward Fig. 5. Time series of the zonal currents in 50 m layer at Site W. Unit is m s 1. Variabilities of Oceanic Current in the Warm-Pool 231

Fig. 6. Time series (25-hour running mean) of the zonal and meridional currents in 50 m layer at (a): Site W, (b): Site E, (c): Site N and (d): Site S from 8 November 1992 to 14 February 1993. Fig. 7. Spectra of the zonal and meridional currents at (a): Site W, (b): Site E, (c): Site N and (d): Site S for a period from 8 November 1992 to 5 February 1993.Vertical bar is 90% confidence interval. 232 I. Ueki et al.

flow at the end of January 1993 at Site N and the strong northward flow at the beginning of January 1993 at Site W. Using the FFT method with 16 degrees of freedom, frequency spectra were calculated for each time series of hourly current data with record length of 2400. There are some spectral peaks in Fig. 7 for the time series at 50 m of all sites. Although some of these peaks are not statistically significant for a 90% confidence limit, they are classified into three period ranges; at periods around approximately a day (24 26 hours), 2 days (42 54 hours) and 4 5 days (90 120 hours). In particular, spectral peaks at the periods of about 2 days are dominant for the meridional currents at Site N and Site S. Using spectral densities calculated for the time series at each layer, we derive variance spectra which are defined by the product of the spectral density and frequency. Perioddepth diagrams of the variance spectra for the zonal and meridional currents at Site E are depicted in Fig. 8. Maxima of the energies for both the current components are found at three period ranges: longer than 300 hours, around 100 hours (4 days) and around 50 hours (2 days). These are Fig. 8. Contour plots of variance spectra (spectral density multiplied by frequency) for the (a): zonal and (b): meridional currents at Site E for a period from 8 November 1992 to 5 February 1992. Contour intervals are 20 10 4 m 2 s 2. referred to as low-frequency (LF), quasi-4-day and quasi-2- day signals, respectively, hereafter. The LF energies are greatest between the near-surface to 100 m for the both current components. The former feature is related to the changes of the EUC and/or SEC with the time scale of several days (Fig. 3). The quasi-4-day signal has maxima at the near-surface (above 90 m) and subsurface (around 150 m) layers for the zonal current and at the near-surface (above 50 m) and the subsurface (around 110 m) layers for the meridional current. On the other hand, the quasi-2-day signal has maxima around 70 150 m for the zonal current and around 200 m for the meridional current. In particular, it should be noted that the energies of quasi-2-day period for the meridional current are relatively large in most of the layers, which suggests that the quasi-2-day signal has a large vertical scale. From spectral analysis, it was discovered that the quasi-2-day and quasi-4-day signals are dominant in the upper oceanic current field of the warm-pool region during TOGA/COARE IOP. To inspect temporal and spatial features of these signals, we derive their time-depth diagrams of energies for the current components. In these diagrams, the energies are defined as weighted means of 1:2:3:2:1 from spectral density values at five period ranges around the 4 and 2 days. Calculations are made from time series of each data lengths of 480 hours and 960 hours for the quasi-2-day and quasi-4-day signals, respectively. In the diagrams for the quasi-2-day signal of the zonal current (Fig. 9), energy maxima are found in the subsurface layer between 90 and 200 m from the end of November to the middle of December 1992 at Site W and Site E. At Site E, and other maxima are found around 80 m from the middle of December 1992 to the beginning of January 1993. At Site N the energies of the quasi-2-day signal are relatively low (below 0.1 m 2 s 2 ), and at Site S high energies are found in almost the entire layers, with maxima of about 0.5 m 2 s 2 in the near-surface layer (about 70 m) from the end of November to the middle of December 1992. The diagram of the quasi-2-day signal for the meridional current (Fig. 10) at Site W exhibits energy maxima at two subsurface layers (about 80 m and 140 m) from the end of November to the middle of December 1992. The high energy in the upper layer persists until January 1993. At the other sites, the energies are high in almost all layers from the end of November to the middle of December and there are energy maxima exceeding 0.6 m 2 s 2 in the lower layer (about 200 m) at Site E and in the near surface (about 50 m) at Site N and Site S. Furthermore, at these off-equatorial sites, high energies are also found above 60 m from the middle of January to the beginning of February in 1993. Comparison between the energy values for the zonal and meridional currents exhibits that the energy of the quasi-2-day signal for the meridional current is higher than that for the zonal current, especially in the former part of IOP. Variabilities of Oceanic Current in the Warm-Pool 233

Fig. 9. Time-depth diagram of energies for the zonal currents at periods around 2 days during TOGA/COARE IOP. Unit is 10 4 m 2 s 2 /cph. Similarly, the energy diagrams of the quasi-4-day signal for the zonal current are depicted in Fig. 11. High energies of the zonal currents are found in the near-surface and subsurface layers above about 160 m at Site S. The magnitude of the energy at this site persisted during almost the entire IOP with values higher than twice that at the other sites, except for that in the near-surface layer at Site W in December 1992. Similar features are found in the diagrams for the quasi-4-day signal of the meridional current (Fig. 12). In other words, the energy magnitude at Site S is much higher than those at the other sites in the almost entire IOP. Another high energy is found in the near-surface layer at Site W after the middle of December 1992. These features suggests that the quasi-4-day signal is a local phenomenon in a region around Site S and is driven by some forcing in the overlying atmospheric field. Comparison between the energy 234 I. Ueki et al.

Fig. 10. The same as Fig. 9, except for the meridional currents. values for the zonal and meridional currents exhibits no distinct differences in many of the measuring layers. 4.2 Cross-spectral features In the previous section it was shown that the quasi-2- day signal is dominant from the end of November to the middle of December, and the energies for the meridional current are higher than for the zonal current. To examine the spatial relationships of the signals, we calculated cross spectra among the time series of the meridional currents at different stations. Calculations are made for two 50-day periods: one begins from 8 November, containing the dominance of quasi-2-day signal, and the other from 28 December in 1992. Examples of the coherence and the phase for time series of the meridional currents in the near-surface layer (50 m) are shown in Fig. 13. For the east-west rela- Variabilities of Oceanic Current in the Warm-Pool 235

Fig. 11. Time-depth diagram of energies for the zonal currents at periods around 4 days during TOGA/COARE IOP. Unit is 10 4 m 2 s 2 /cph. tionship (Fig. 13(a)), the coherence of the meridional current between Site W and Site E is not significantly high at the period range around 2 days and 4 days in both the two periods. On the other hand, for the north-south relationship, the coherence between Site N and Site S (Fig. 13(b)) is significantly high in the period range around 2 days (46-58 hour) with the phase difference of 100 to 140 and around 4 days (90-96 hour) with phase difference of about 180 during the first 50-day period. Since, wind changes on the sea surface are considered as a factor driving any signals in the upper ocean, spectral densities of the surface wind at Site N and Site S during same period have been calculated (Fig. 14). Calculations have been done for the record length of 2400 and 1200 hours at 236 I. Ueki et al.

Fig. 12. The same as Fig. 11, except for the meridional currents. Site S and at Site N, respectively, because many data points are missing after December at Site N. At both stations there are spectral peaks at period ranges around 2 days (42 56 hour) and around 4 days (96 120 hour). To reveal relationships of the signals between the current and wind, cross spectra have been calculated for two 50-day period between the near-surface current and surface wind at Site S. The coherence between both components of the near-surface (50 m) current and the wind during the first 50-day period beginning 8 November in 1992 (Fig. 15(a)) has significantly high values in the period range around 4 days. These features suggest that the signal of the current at Site S is driven by the surface wind forcing during November and December. For the quasi-2-day signal, there is no significant coherence around 2 days. During the second 50-day period beginning 28 December in 1992 (Fig. 15(b)), high coherence values are not found at the period range around 4 days, but in the period range around 2 days; at the periods around 52 hours for the Variabilities of Oceanic Current in the Warm-Pool 237

Fig. 13. Coherence and Phase of the meridional currents in nearsurface layer (50 m) between (a): Site W and E and (b): Site N and Site S. Thin line (circle marks) and thick lines (triangle marks) depict values for each 50-day periods beginning from 8 November and from 28 December 1992, respectively. Broken lines depict 90% confidence levels. Fig. 14. Spectra of the zonal and meridional wind at (a): Site N and (b): Site S for a period from 8 November 1992 to 5 February 1993. Vertical bar is 90% confidence interval. meridional component. It should be noted that, in the period range around 2 days, the coherence has significantly high values between the current and wind during the second 50- day, in spite of no evidence of the dominant signal in the current. 5. Summary and Discussion In this study, current variabilities with periods of several days were examined using moored ADCP data at four sites of the western equatorial Pacific warm-pool region. The major results of this study are summarized as follows: (1) Time averaged (about four months mean) flow had an eastward maximum in the lower layer (about 200 m) at three sites, except Site N, and a westward maximum in the subsurface layer (about 100 m) at all sites. The EUC appeared in layers below 180 m in November 1992, and became shallower with time, and in January 1993 the depth of the EUC reached 100 m. On the other hand, the SEC were dominant at about 110 m from the beginning of November 1992 to the beginning of January 1993. In the middle of January 1993, the SEC was recognized from near the surface to 110 m. (2) The time series of the zonal and meridional currents indicate that, in period ranges shorter than several days, there are signals which have the periods of a few days in addition to tidal variabilities with the diurnal and semidiurnal periods. The spectra of the currents demonstrate that they have periods of about 2 days and 4 days at all sites. (3) The quasi-2-day signal has higher energy for the meridional current than for the zonal current at all sites. The signal of the meridional current is dominant in the almost entire layers, especially in a period from the end of November to the middle of December. This signal has a significant correlation between the two off-equatorial sites. (4) For the quasi-4-day signal, the energies of both current components have almost the same magnitudes as each other. The signals are dominant in the near-surface layers at three sites, except Site S where the energy is higher than the other sites and is not low in the lower layers. The signals in the near-surface currents are not coherent between two off-equatorial sites. In November to December 1992, a high correlation is found between the signals in the nearsurface current and surface wind at Site S. During TOGA/COARE IOP, it was pointed out by 238 I. Ueki et al.

for any signals. In fact, it may be difficult to interpret all the results consistently for correlation calculated in this study. In post IOP, the Tropical Ocean Climate Study (TOCS) project has been started with the support of the Japan Marine Science and Technology Center (JAMSTEC). Current data including ADCP supplied by this project will be very helpful for further studies. Acknowledgements We would like to express our grateful thanks to Drs. Yoshifumi Kuroda, Kei Muneyama and Kentaro Ando at the Japan Marine Science and Technology Center for providing invaluable technical support for our mooring installation and deployment. The assistance of the captains and crews of the R/V Natsushima and R/V Kaiyo is also gratefully appreciated. We also thank Dr. M. J. McPhaden, the director of the TOGA-TAO Project Office for kindly supplying wind data from ATLAS buoys. This study was supported by the Science and Technology Agency, Japan through the Japanese Pacific Climatic Studies program of the Special Coordination Funds for Promoting Science and Technology. Fig. 15. Coherence and Phase between current in near-surface layer (50 m) and surface wind for each 50-day period beginning (a):from 8 November 1992 and (b):from 28 December 1992. Thin line (circle marks) and thick lines (triangle marks) depict values for the zonal and meridional one. Suzuki et al. (1995) that variabilities of sea temperature had a period of about 2 days. Moreover, Takayabu et al. (1996), using infrared equivalent brightness temperature data and the other data, demonstrated the dominance of the quasi-2- day signals in the cloud field during IOP. Furthermore, they pointed out that the quasi-2-day signals can be interpreted as a westward-propagating inertia-gravity wave. In oceanic variabilities, high-frequency disturbances were indicated by Wunsch and Gill (1976), Luther (1980) and Ripa and Hayes (1981). They also interpreted these variabilities as theoretical equatorial waves. There are some possibilities that the signals detected in this study can be interpreted as any linear equatorial waves. Investigations including these possibilities will be discussed in the subsequent paper (Kutsuwada et al., 1998). On the other hand, the quasi-4-day signal was recognized in some time series of the nearsurface currents, but its dominance is not persistent. The high coherence between the current and surface wind in a limited region (surrounding Site S) suggests that the signal is a local phenomenon driven by wind forcing. The moored ADCP data used in this study supply continuous time series for us but never give much information about non-persistent features and spatial relationships References Hayes, S. P., L. J. Mangum, J. Picaut, A. Sumi and K. Takeuchi (1991): TOGA-TAO: A moored array for real-time measurements in the tropical Pacific Ocean. Bull. Amer. Met. Soc., 72, 339 347. Huyer, A., P. M. Kosro, R. Lukas and P. Hacker (1997): Upper ocean thermohaline fields near 2 S, 156 E, during tropical ocean-global atmosphere-coupled ocean-atmosphere response experiment, November 1992 to February 1993. J. Geophys. Res., 102, 12749 12784. Kutsuwada, K. and H. Inaba (1995): Year-long measurements of upper-ocean currents in the western equatorial Pacific by Acoustic Doppler Current Profilers. J. Meteor. Soc. Japan, 73, 665 675. Kutsuwada, K., H. Inaba, I. Ueki and A. Kaneko (1998): Quasi-2- day signal of surface oceanic current in the warm-pool region during TOGA/COARE IOP. J. Geophys. Res. (submitted). Lukas, R., P. J. Webster, M. Ji and A. Leetmaa (1995): The largescale context for the TOGA coupled ocean-atmosphere response experiment. Meteor. Atoms. Phys., 56, 3 16. Luther, D. (1980): Observations of long period waves in the tropical oceans and atmosphere. Tech. Rep. WHOI-80-17, Woods Hole Oceanogr. Inst., Woods Hole, Mass. McPhaden, M. J. (1993): TOGA-TAO and the 1991 93 El Niño Southern Oscillation event. Oceanogr., 6, 36 44. McPhaden, M. J. and S. P. Hayes (1990): Variability in the eastern equatorial Pacific Ocean during 1986 88. J. Geophys. Res., 95, 13195 13208. McPhaden, M. J., S. P. Hayes, L. J. Mangum and J. M. Toole (1990): Variability in the western equatorial Pacific Ocean during the 1986 87 El Niño Southern Oscillation event. J. Phys. Oceanogr., 20, 190 208. McPhaden, M. J., F. Bahr, Y. du Penhoat, E. Firing, S. P. Hayes, P. P. Niiler, P. L. Richardson and J. M. Toole (1992): The Variabilities of Oceanic Current in the Warm-Pool 239

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