Variability of Currents off the Northern Coast of New Guinea

Transcription

1 Journal of Oceanography, Vol. 56, pp. 103 to Variability of Currents off the Northern Coast of New Guinea YOSHIFUMI KURODA Ocean Research Department, Japan Marine Science and Technology Center, 2-15 Natsushima, Yokosuka , Japan (Received 18 May 1998; in revised form 20 May 1999; accepted 24 May 1999) The variability of the New Guinea Coastal Current (NGCC) and New Guinea Coastal Undercurrent (NGCUC) were examined from one year time series of current data from ADCP moorings at 2 S, 142 E and 2.5 S, 142 E. Change in the hydrographic structure induced by monsoonal wind forcing was also examined from hydrographic data along the 142 E covering consecutively two winter seasons and two summer seasons. The westward NGCUC was observed to persist year around. The annual mean depth of the current core was 220 m, the mean speed of the zonal component was 54 cm/s with a standard deviation of 15 cm/s at the 2.5 S site. Velocity fluctuations at day period were observed year around. Seasonal reversal of the surface intensified NGCC was clearly observed. In the boreal summer characterized by the southeasterly monsoon, westward currents of over 60 cm/s were dominant in the surface layer. The warm, low-salinity layer thickened at this time and sloped down toward the New Guinea coast from the equator. This surface water accumulation may be caused by onshore Ekman drift at the New Guinea coast, combined with weak Ekman upwelling at the equator. In the boreal winter, an eastward surface current developed to 100 cm/s extending down to 100 m depth in response to the northwesterly monsoonal winds. Coastal upwelling was indicated in this season and the surface water accumulated at the equator due to Ekman convergence. Shipboard ADCP data indicated that the NGCUC intensified in boreal summer as the width and depth of the NGCUC increased. Keywords: The New Guinea Coastal Current, The New Guinea Coastal Undercurrent, ADCP moorings, seasonal variation, hydrography, monsoonal wind, Ekman drift, volume transport. 1. Introduction The Pacific western boundary currents at low latitude play an important role in maintaining the warm water pool of the western tropical Pacific Ocean; the key area of El Niño/Southern Oscillation (ENSO) (cf. Lukas et al., 1996). In particular, the New Guinea Coastal Current (NGCC) and the New Guinea Coastal Undercurrent (NGCUC) are considered central to the mass, heat and salt budgets of the warm pool. Wyrtki (1961) depicted the surface layer flow off the north coast of New Guinea as changing its direction seasonally with the monsoonal winds along the coast. The NGCC flows northwestward with the southeasterly winds in boreal summer, and southeastward with the northwesterly winds in boreal winter. A steady northwestward flow below the surface NGCC has been observed by Lindstrom et al. (1987). Since then, Corresponding author kuroday@jamstec.go.jp Copyright The Oceanographic Society of Japan. the NGCUC has been described by several authors using ship survey observations (e.g. Toole et al., 1988; Tsuchiya et al., 1989; Lindstrom et al., 1990; Gouriou and Toole, 1993; Butt and Lindstrom, 1994; Fine et al., 1994). These studies showed that the NGCUC carries high salinity South Pacific Tropical Water. These waters originate in the South Equatorial Current, cross the Solomon Sea and pass through Vitiaz Strait and the St. George Channel to form the NGCUC that in turn feeds the Equatorial Undercurrent (EUC). Despite these efforts, the mean state and temporal variability of the NGCC and NGCUC from intraseasonal to interannual are not fully understood. In order to investigate the variability of the upper ocean equatorial current system in the western tropical Pacific Ocean, the Japan Marine Science and Technology Center (JAMSTEC) deployed an array of subsurface moorings equipped with upward looking acoustic Doppler current profilers (ADCP). Moorings have been successfully deployed at two sites (0, 142 E and 0, 147 E) since Janu- 103

2 Fig. 1. Location of subsurface ADCP moorings. Cross indicates the mooring location. ary 1995 and at four sites (0, 138 E; 2 S, 142 E; 2.5 S, 142 E; and 0, 156 E) since July 1995 (Fig. 1). The two moorings at 2 S, 142 E and 2.5 S, 142 E were deployed especially to observe the NGCC and NGCUC. In this paper, one-year-long time series of currents at these two sites are examined together with hydrographic data along the 142 E section obtained during four cruises: January 1995, July 1995, February 1996 and July 1996, covering consecutively two winter seasons and two summer seasons. The intraseasonal, seasonal and interannual changes of the NGCC and NGCUC will be described in the following sections. 2. Data and Methods Fig. 2. Configuration of the subsurface ADCP mooring. 2.1 Subsurface ADCP moorings The southern mooring site at 2.5 S, 142 E was located in 3436 m of water on the steep continental slope, about 50 km from the Papua New Guinea coast. The mooring at 2 S, 142 E was in 3609 m. The mooring sites were selected based on shipboard ADCP data obtained during the January 1995 cruise. The subsurface moorings (Fig. 2) were equipped with an upward looking ADCP (150 kh Broad Band ADCP of RD Instruments) at a depth of m (Ito et al., 1995). Data were sampled every hour by averaging 16 pings at 2-sec interval in 40 bins with 8 m vertical resolution and rotated to earth coordinates based on standard formula (IGRF1990, National Geophysical Data Center). Due to movement of the mooring line, the depth of ADCP may change requiring correction of the bin depths. A CTD (Conductivity-Temperature- Depth; SBE16, SEA-Bird Electronics) was attached 1.5-m below the ADCP and the pressure was sampled every 30-minute with 1-dbar resolution. Finally, the current data at each bin was linearly interpolated to a 10-m vertical grid for subsequent analysis. 2.2 Shipboard measurements Beginning in 1993, JAMSTEC has conducted two cruises each year using R/V Kaiyo as the Tropical Ocean Climate Study (TOCS) programme in the western tropical Pacific Ocean (cf. Kuroda et al., 1996). Hydrographic data were obtained during the four cruises of January 1995, July 1995, February 1996 and July In particular, the section along 142 E from the equator to 2 40 S near the New Guinea coast has been occupied during each cruise to investigate the seasonal and interannual changes of temperature, salinity, and currents in the NGCC and NGCUC. The CTD casts were carried out using a Sea-Bird Electronics CTD (SBE9/11) usually to below 1,000-m 104 Y. Kuroda

3 depth at lowering speeds of between 1 m/s 1.5 m/s with a sample rate of 24 Hz. The sensor calibrations were performed before and after each cruise by the manufacturer. As the drifts of temperature and conductivity sensors were small, the calibration coefficients before each cruise were used to calculate the temperature and salinity data. Insitu water samples were collected using a 12-position sampler. Reversing digital thermometers and pressure gage were used to confirm sample depths. The water sample conductivity data from approximately 1000 m depth were analyzed with a Guildline Auto-Sal (model 8800) to check the salinity offset at each cast. R/V Kaiyo is equipped with a RDI 75 khz Narrow Band ADCP. Current velocities and backscattering amplitude were recorded in 40 bins with a 16-m vertical resolution. The first bin was centered at 30 m depth. Navigation data were obtained with a Magnavox MX4400 Global Positioning System (GPS) receiver to determine instantaneous ship velocity in earth coordinates. Performance of the ADCP was tested by sampling on reciprocal courses. This test indicated that the ADCP data were unreliable below about 600 m where the percent-good value decreased to about 50%. The raw data at 2.5-sec ping interval were vector averaged every 5 minutes and subsequently gridded to 10 km in distance along the ship tracks. 3. Climate Conditions and Surface Winds The Southern Oscillation Index (SOI) showed negative anomalies from the end of 1990 to early 1995 with a few moderate peaks during (Climate Diagnostics Bulletin, 1997). The January 1995 cruise was thus conducted during a weak warm phase when westerly Fig. 3. Time series of 12-hourly surface wind data at 2.25 S, E from January 1995 to July 1996 by ECMWF objective analysis: (a) Zonal component; (b) Meridional component. Thin solid line indicates daily data and thick solid line indicates 15-day running mean data. The arrows indicate the period at 142 E section during the four cruises of January 1995, July 1995, February 1996 and July Variability of Currents off the Northern Coast of New Guinea 105

4 winds prevailed as shown by the time series of surface wind data produced at the European Center for Medium Range Weather Forecast (ECMWF) (Fig. 3). The SOI indicated normal condition in mid 1995 and turned positive in early This indicates La Niña conditions occurred through As the SOI turned positive, easterly wind anomalies were induced. The southeasterly winds in July August 1995 were enhanced, while the westerly Fig. 4. Time series of ADCP velocities: (a) zonal component whose contour interval is 10 cm/s and (b) meridional component whose contour interval is 5 cm/s at 2 S, 142 E; and (c) zonal component and (d) meridional component at 2.5 S, 142 E. Data have been smoothed with lowpass Butterworth filter of 10 day. Thick solid line indicates velocity zero contour. 106 Y. Kuroda

5 winds were not so strong in October 1995 March 1996 (Fig. 3). Thus, in the context of ENSO, current data from July 1995 to July 1996 and hydrographic data during the cruises of July 1995, February 1996 and July 1996 covered the transition period from normal conditions in mid 1995 to La Niña conditions in early-mid Variation of the NGCC and NGCUC 4.1 Current time series The most striking feature in the time series of surface layer currents is the seasonal reversal near the New Guinea coast (Figs. 4(a) and (c)). In boreal summer (July September 1995 and May July 1996), the westward current was dominant with a maximum speed over 60 cm/s. In contrast, during boreal winter eastward flow was observed beginning in October 1995 that persistent through December 1995 January The eastward current reached down to 100 m with a maximum speed over 100 cm/s at 30 m in early January 1996 at 2 S. This eastward current was weaker at 2.5 S. On the equator at the same longitude, the surface eastward current was very weak compared with the NGCC region (not shown). Thus the eastward NGCC appear to have an offshore velocity maximum. Late in 1995, the eastward NGCC was interrupted a few times by westward flows and diminished in April This is the first observation showing the time development of seasonal reversal of the NGCC with monsoonal wind forcing, although, in the past, the eastward NGCC in boreal winter had been observed by discrete cruise data. Lindstrom et al. (1987) for example observed an eastward surface current of 70 cm/s around 1.5 S, 143 E by shipboard ADCP. In contrast, below 100 m depth, the westward NGCUC was persistent all year at both sites (Figs. 4(a) and (c)). 4.2 Annual- and monthly-mean profiles In order to investigate the vertical structure of these low-latitude currents and their seasonal changes, annualmean and monthly-mean profiles were constructed (Figs. 5 and 6). The annual-mean core depth of the NGCUC at 2.5 S was 220 m. The mean speed at this depth was 54 cm/s with standard deviation of 15 cm/s. The velocity core at 2.5 S was slightly deeper and stronger than at 2 S (210 m and 46 cm/s) (Fig. 5). In the mean, the NGCUC is thus strongest at the coast. The monthly-mean profiles at 2 S and 2.5 S (Fig. 6) (a) (b) Fig. 5. Annual mean current profiles at 2 S, 142 E (a) and at 2.5 S, 142 E (b). Thick solid (dashed) line indicates zonal (meridional) component and thin solid (dashed) line indicates standard deviation. Variability of Currents off the Northern Coast of New Guinea 107

6 Fig. 6. Monthly mean zonal current profiles at (a) 2 S, 142 E and at (b) 2.5 S, 142 E. Fig. 7. Power spectrum density of zonal current at 2 S, 142 E. (a) at 40 m and (b) at 210 m. show the dramatic changes in the surface currents in contrast to relatively steady subsurface currents. The maximum surface westward current with a speed of 52 cm/s was observed at 2 S in August The surface currents subsequently decreased gradually and reversed direction; the maximum eastward speed reached 67 cm/s at 40 m in January 1996 where the eastward surface current speed increased monotonically from 70 m depth. 108 Y. Kuroda

7 day variability The power spectrum density was calculated to quantify the dominant fluctuation in these current data (Fig. 7). The peaks about 0.5 day and at 1 day periods of semidiurnal and diurnal tidal currents were prominent at shorter time scales. At longer time scale, a peak near 20 day period is predominant that manifests the current fluctuation at day period as seen in Fig. 4. The characteristics of this day period fluctuation will be discussed in Section 6. The values of power spectrum den- Fig. 8. Five day mean surface wind vectors based on ECMWF objective analysis data: (a) August 6 10, 1995; (b) January 1 5, Fig. 9. Time series of 10 day lowpassed surface wind data of ECMWF (dashed and dotted line; unit = m/s) at 2.25 S, E, zonal current at 30 m (solid line; unit = cm/s) and 200 m (dashed line; unit = cm/s) at 2 S, 142 E. The time series of zonal currents were shifted by 2.5-day ahead which period showed the highest correlation coefficient between zonal current and surface wind in the surface layer. Variability of Currents off the Northern Coast of New Guinea 109

8 sity at 40 m depth in periods longer than 30-days were much larger than those at 210 m, and it may result from large seasonal variation of the surface NGCC. Similar day oscillations have been found in the eastern Pacific (e.g. Wilson and Leetmaa, 1988), central Pacific (Kuroda and McPhaden, 1993; Weisberg and Hayes, 1995) and also in the western Pacific (McPhaden et al., 1990). 4.4 Relationship between the NGCC and surface wind forcing The periods of August 6 10 and January 1 5 marking the two extremes of surface currents in the low pass filtered data (Fig. 4) were selected for detailed study. Surface wind vector maps for these periods (Fig. 8) show typical boreal summer monsoon winds from the northwest along the north coast of New Guinea and winter monsoon winds from the southeast. Southeasterly winds were dominant during June September 1995 with maximum speed of the zonal component of exceeding 5 m/s in August. Northwesterly wind Fig. 10. Correlation coefficient between surface zonal wind and 12-hour lowpassed zonal current velocities at 30 m, 50 m, 70 m, 100 m, and 150 m. Dashed line at 0.07 indicates significance level of non correlation. Fig. 11. Zonal component of current velocities obtained with shipboard ADCP along meridional sections at 142 E during four cruises of January 1995, July 1995, February 1996, and July Solid (dashed) contours indicate eastward (westward) flow. (a) January 1995 cruise; (b) July 1995 cruise; (c) February 1996 cruise; and (d) July 1996 cruise. 110 Y. Kuroda

9 appeared first in October 1995, and developed to 1 2 m/ s during December 1995 March 1996, although it relaxed in early February Then, in May 1996, southeasterly winds strengthened again. The 30 m zonal currents data at 2 S is well correlated with the zonal wind at 2.25 S, Fig. 9. The correlation coefficient between the zonal current at 30 m and the surface wind is remarkably high at 0.67 with 2.5-day lag (Fig. 10). The correlation coefficients decreases with depth: 0.65 at 50 m, 0.51 at 70 m, 0.23 at 100 m, 0.11 at 150 m respectively. These Fig. 12. (a) Temperature, (b) salinity and (c) potential density sections along 142 E during the January 1995 cruise. Contours are at intervals of 1 C (temperature), 0.1 psu (salinity), and 0.5 kg m 3 (density). Dense shading indicates salinities greater than 35.5 psu, and light shading indicates less than 34.4 psu. Fig. 13. (a) Temperature, (b) salinity and (c) potential density sections along 142 E during the July 1995 cruise. Contours are at intervals of 1 C (temperature), 0.1 psu (salinity), and 0.5 kg m 3 (density). Dense shading indicates salinities greater than 35.5 psu. Variability of Currents off the Northern Coast of New Guinea 111

10 high correlation coefficients imply that the currents were directly forced by the local surface winds from the surface to m depths. Peaks of correlation coefficients near 20 days and 40 days indicates also the current fluctuation at near 20 day period. 5. Seasonal Oceanic Changes at the 142 E Section Hydrographic sections of current, temperature, salinity and potential density observed during the cruises of January 1995, July 1995, February 1996, and July 1996 are shown in Figs. 11, 12, 13, 14, and 15. As these data Fig. 14. (a) Temperature, (b) salinity and (c) potential density sections along 142 E during the February 1996 cruise. Contours are at intervals of 1 C (temperature), 0.1 psu (salinity), and 0.5 kg m 3 (density). Dense shading indicates salinities greater than 35.5 psu, and light shading indicates less than 34.4 psu and densities less than 21.5 kg m 3. Fig. 15. (a) Temperature, (b) salinity and (c) potential density sections along 142 E during the July 1996 cruise. Contours are at intervals of 1 C (temperature), 0.1 psu (salinity), and 0.5 kg m 3 (density). Dense shading indicates salinities greater than 35.5 psu. 112 Y. Kuroda

11 Table 1. Volume transports obtained by shipboard ADCP between 1 S, 142 E and 2 40 S, 142 E (2 30 S only in the July 1996 cruise) in the layer of 22 m 598 m (unit = 10 6 m 3 s 1 = Sv). January 1995 July 1995 February 1996 July 1996 Average Westward component Eastward component Zonal net transport were obtained during similar periods in boreal summer and winter over the two years, they reveal both seasonal and year to year changes in the hydrographic structures. 5.1 Oceanic response to southeasterly winds in boreal summer During the boreal summer season cruises, the surface layer of warm water was observed with increasing thickness from the equator toward the New Guinea coast. The 22.0 kg/m 3 isopycnal and 35.1 psu isohaline during the July 1996 cruise sloped down toward the coast from the equator. The 29 C isotherm sloped down to the south between 1 S and the coast (Fig. 15). During the July 1995 cruise, this structure was not as clear; those contours at the bottom of the surface layer were relatively flat and tended to be deepest around 2 S (Fig. 13). This accumulation of surface water may be caused by Ekman convergence due to shoreward drift near the coast, while at the equator weak Ekman upwelling forced by southeasterly winds may have occurred. The equatorial Ekman upwelling is evident from the fact that vertical gradients of density, temperature and salinity near the base of the surface layer were weak at the equator during the boreal summer cruises compared to those during the winter cruises. The shipboard ADCP data reveal the spatial structure of the NGCC, NGCUC and EUC. The strength and extent of the NGCC and NGCUC changed remarkably between the four cruises (Fig. 11). The strongest NGCUC was observed during the July 1995 cruise. The northern boundary of the NGCUC, defined as where the westward current velocities were over 20 cm/s at 200 m, extended further north to 1 10 S in the July 1995 cruise from 1 50 S in the January 1995 cruise. The width of the NGCUC increased from 90 km to 165 km, (where a fixed southern boundary of the current is assumed at 2 40 S, the end of hydrographic section). The NGCUC also became deeper, from 350 m to over 600 m. This deep structure was similar in the July 1996 cruise. Thus, the NGCUC was well developed during the two boreal summer cruises. In boreal winter, the portion of NGCUC having speeds over 20 cm/s was much shallower (300 m in January 1995, and 500 m in February 1996). The core speed region shrunk, and was located offshore. These seasonal structure changes cause large variation of volume transport in this section. The transports were estimated in the region of the NGCC and NGCUC by integrating the shipboard ADCP currents on the 142 E meridional section between 22 m (top depth of the first bin layer of the shipboard ADCP) and 598 m (bottom depth of the 36th bin layer) from 1 S to 2 40 S, Table 1. The estimates of total westward flow (i.e. not including regions of eastward flow) show large variation among the four cruises, as well as seasonal variation. In boreal summer, the transports were large, about 32 Sv in July 1995 and 20 Sv in July 1996, whereas in winter these were small, about 11 Sv in January 1995 and 15 Sv in February Gouriou and Toole (1993) estimated the mean transport south of 1 S down to 400 m along 142 E at about 23.8 Sv by direct measurement. Their figure is somewhat biased towards boreal summer because most of the cruises were carried out in boreal summer. 5.2 Oceanic response to northwesterly winds in boreal winter The most striking features of the oceanic structure found during the cruises of January 1995 and February 1996, the boreal winter seasons, was an accumulation of warm, low salinity surface water at the equator and an indication of upwelling to the south. The 28 C isotherm, 22.0 kg/m 3 isopycnal, and 34.6 psu isohaline were deepest at the equator during the January 1995 cruise (Fig. 12), i.e. those contours sloped up toward the New Guinea coast from the equator. This tendency was similarly observed during the February 1996 cruise (Fig. 14). During these cruises, northwesterly winds were dominant (Fig. 3). Therefore, it is likely that this surface water accumulation on the equator is due to Ekman convergence, while the shallow isotherm, isopycnal and isohaline near the coast are due to coastal upwelling. The shoaling of isopycnals in the surface layer near the coast is consistent with the observed surface eastward currents in boreal winter in terms of geostrophic balance. 6. Discussion 6.1 Characteristics of velocity fluctuation at day period Characteristics of day fluctuation described in Subsection 4.3 are examined in terms of seasonal de- Variability of Currents off the Northern Coast of New Guinea 113

12 Fig. 16. Time series of day band passed ADCP velocities: (a) zonal component and (b) meridional component at 2 S, 142 E whose contour interval is 5 cm/s. Data have been bandpassed by Butterworth filter. Thick solid line indicates velocity zero contour. Fig. 17. Time series of 60 day lowpassed ADCP velocities: (a) zonal component whose contour interval is 10 cm/s and (b) meridional component at 2 S, 142 E whose contour interval is 5 cm/s. Data have been lowpassed by Butterworth filter. Thick solid line indicates velocity zero contour. Following year s data, which have been obtained by successive mooring operations but not shown in previous figures, were included to be able to indicate seasonal variations as long as possible. 114 Y. Kuroda

13 pendency, relationship with surface winds, and its vertical structure. In order to extract the day fluctuation and the seasonal change, day bandpass filter (Fig. 16) and a 60 day lowpass filter (Fig. 17) were applied to the velocity time series data at 2 S. The 60 day lowpass data (Fig. 17) show large seasonal changes of the surface current. The surface westward current was dominant in the boreal summer and reversed eastward current was intensified in the boreal winter as already described in Section 4. Below about 150 m depth, the westward NGCUC intensified in the boreal summer particularly in September 1995 and June July 1996, but weakened in November December 1995 and in April The day band pass data demonstrates clearly the day fluctuation (Fig. 16). The fluctuation is dominant in alongshore direction because amplitude of meridional component is much smaller than that of zonal component. In the surface layer, the fluctuation is intensified in the boreal winter especially in February March associated with periodic surface current reversals. In general, maximum speed of zonal component appeared fast in the deeper layer below 150 m and then did in the surface layer, i.e. the phase tends to propagate upward. The phase delay is large at the pycnocline near 100 m. Such phase propagation indicates that there is some energy exchange mechanism between the surface layer and subsurface layer to develop the day fluctuation. 6.2 Seasonal change of volume transports in 142 E section As described in Subsection 5.1, large seasonal change of volume transports was observed in 142 E hydrographic section; 20 and 32 Sv in the boreal summer cruises, and 11 and 15 Sv in the boreal summer. Several model studies on seasonal cycle of the transports in this region and the Indonesian seas using climatological wind forcing have been conducted (e.g. Godfrey, 1989; Inoue and Welsh, 1993; Miyama et al., 1995; Masumoto and Yamagata, 1996; etc). In these studies, a major part of seasonal transport variations in the Pacific western boundary is associated with seasonal change of Sverdrup circulation induced by surface winds in the Pacific and Indian Oceans. The annual cycle of the transport of Indonesian Throughflow from the Pacific Ocean to the Indian Ocean increases in boreal summer and decreases in boreal winter. Inoue and Welsh s (1993) model showed that the volume transport through Vitiaz Strait varied between 4 Sv in boreal winter and 8 Sv in summer. The increased transport in boreal summer in their study is consistent with this study although their values are small because their model did not allow annual mean net transport of the Indonesian Throughflow. 7. Summary In order to investigate the variability of the NGCC and NGCUC, one year time series of current data from the ADCP moorings at 2 S, 142 E and 2.5 S, 142 E close to the New Guinea coast were examined with hydrographic data along the 142 E section. The conclusions drawn from this investigation can be summarized as follows. (1) The annual mean core depth and mean speed of zonal component of the NGCUC were 220 m and 54 cm/s with a standard deviation of 15 cm/s at 2.5 S which were slightly deeper and stronger than those 210 m, 46 cm/s and similar standard deviation at 2 S. (2) Seasonal reversal of the surface intensified NGCC was clearly observed. In the boreal summer characterized by the southeasterly monsoon, westward currents of over 60 cm/s were dominant in the surface layer, whereas, in the boreal winter, an eastward surface current developed to 100 cm/s extending down to 100 m depth in response to the northwesterly monsoonal winds. (3) Velocity fluctuation at day period was observed year around. In the surface layer, the fluctuation was intensified and modulated by the local monsoon winds. Vertical phase difference in zonal current speed suggesting the existence of some coupling process between waves in surface layer and in the deeper layer was observed. (4) Seasonal hydrographic structure changes were also observed which may be caused by monsoonal winds. In the boreal summer, the warm, low-salinity layer thickened and sloped down toward the New Guinea coast from the equator due to onshore Ekman drift associated with the southwesterly monsoon. In the boreal winter, warm water accumulated at the equator due to equatorial Ekman convergence combined with coastal upwelling by northwesterly monsoon. (5) Shipboard ADCP data indicated that the NGCUC intensified in boreal summer as the width and the depth of the NGCUC increased. The transport of westward flow (based on shipboard ADCP data not taking the eastward flow into account) reached 32 Sv in July 1995 and 20 Sv in July In the boreal winter, the westward transport was relatively small, 11 Sv in January 1995 and 15 Sv in February As described above this paper successfully depicted the annual mean status, seasonal and intraseasonal changes of the NGCC and NGCUC. Further dynamical analysis, however, must be carried out with numerical models to fully understand the mechanism of the seasonal change and day fluctuation in that region. Similar measurements have been continued and current data over 3 years from this moored ADCP current meter array already been obtained. The longer time series may enable Variability of Currents off the Northern Coast of New Guinea 115

14 us to analyze further how the NGCC and NGCUC vary on ENSO time scales, and how they relate to the equatorial current system. These studies will contribute for the better understanding of variations and maintenance of warm water pool. Acknowledgements The author greatly acknowledges the colleagues of K. Muneyama for leading the TOCS project, and K. Ando and K. Yoneyama for conducting the measurements during those cruises. The July 1995, February 1996, July 1996 cruises were conducted as joint cruises between BPP Teknologi, Indonesia and JAMSTEC. He acknowledges Dr. Indroyono Soesilo and his staff of BPPT who enabled us to carry out the joint cruises, especially to Mr. Djoko Hartoyo who participated in these cruises. He would like to thank the Captains H. Hyodo, S. Ishida, H. Tanaka, and crew members of R/V Kaiyo. He is indepted to K. Takao, A. Ito, H. Yamamoto, M. Hayashi, M. Fujisaki, T. Katayama, N. Komai and M. Hanyu of Nippon Marine Enterprise/NME, and T. Shiribiki and K. Komine of Sanyo Techno Marine, Inc. for carrying out the observational works with professional skill. He thanks H. Matsuura of JAMSTEC and K. Kutsuwada of Tokai University for the analysis of moored ADCP data and ECMWF wind data, Y. Kashino of JAMSTEC for processing the shipboard ADCP, S. Sawato of International Meteorological and Oceanographic Consultants Co., Ltd. for the data processing of moored ADCP data and K. Sano of Sena Co. for computer drafting of the CTD data. K. Ando, T. Matsuura, K. Kutsuwada and K. Shimada gave helpful comments to early manuscripts. The comments of two anonymous reviewers led to substantial revision and improvement of the manuscript. He thanks Prof. T. Matsuno of Hokkaido University and other members of a supervisory committee who advised us on the observational plan of TOCS program. The TOCS cruises were supported by the Science and Technology Agency, Japan. References Butt, J. and E. Lindstrom (1994): Currents off the east coast of New Ireland, Papua New Guinea, and their relevance to regional undercurrents in the western equatorial Pacific Ocean. J. Geophys. Res., 99, Climate Prediction Center (1997): Climate Diagnostics Bulletin March U.S. Dep. of Commer., Washington, D.C., pp. 78. Fine, R. A., R. Lukas, F. Bingham, M. Warner and R. Gammon (1994): The western equatorial Pacific: A water mass crossroads. J. Geophys. Res., 99, Godfrey, J. S. (1989): A Sverdrup model of the depth-integrated flow for the world ocean allowing for island circulations. Geophys. Astrophys. Fluid Dyn., 45, Gouriou, Y. and J. Toole (1993): Mean circulation of the upper layer of the western equatorial Pacific Ocean. J. Geophys. Res., 98, Inoue, M. and S. E. Welsh (1993): Modeling seasonal variability in the wind-driven upper-layer circulation in the Indo- Pacific region. J. Phys. Oceanogr., 23, Ito, A., K. Takao, K. Yoneyama, K. Ando, T. Kawano and Y. Kuroda (1995): Design for subsurface mooring with an acoustic Doppler current profiler and its motions in the open sea. JAMSTECR, 32, Kuroda, Y. and M. McPhaden (1993): Variability in the western equatorial Pacific ocean during Japanese Pacific Climate Study Cruises in 1989 and J. Geophys. Res., 15, Kuroda, Y., K. Ando, K. Yoneyama and K. Muneyama (1996): TOCS Data Report 1; JAMSTEC, pp Lindstrom, E., R. Lukas, R. Fine, E. Firing, S. Godfrey, G. Meyers and M. Tsuchiya (1987): The western Equatorial Pacific Ocean Circulation Study. Nature, 330, Lindstrom, E., J. Butt, R. Lukas and S. Godfrey (1990): The flow through Vitiaz Strait and St. George s Channel, Papua New Guinea. p In The Physical Oceanography of Sea Straits, ed. by L. Pratt, Kluwer Academic Publisher. Lukas, R., T. Yamagata and J. P. McCreary (1996): Pacific lowlatitude western boundary currents and the Indonesian throughflow. J. Geophys. Res., 101, Masumoto, Y. and T. Yamagata (1996): Seasonal variations of the Indonesian Throughflow in a general ocean circulation model. J. Geophys. Res., 101, McPhaden, M. J., S. P. Hayes, L. J. Mangum and J. M. Toole (1990): Variability in the Western Equatorial Pacific Ocean during the El Niño/Southern Oscillation Event. J. Phys. Oceanogr., 20, Miyama, T., T. Awaji, K. Akitomo and N. Imasato (1995): Study of seasonal transport variations in the Indonesian seas. J. Geophys. Res., 100, Toole, J. M., E. Zou and R. C. Millard (1988): On the circulation of the upper waters in the western equatorial Pacific Ocean. Deep-Sea Res., 35, Tsuchiya, M., R. Lukas, R. A. Fine, E. Firing and E. Lindstrom (1989): Source waters of the Pacific Equatorial Undercurrent. Prog. Oceanogr., 23, Weisberg, R. H. and S. P. Hayes (1995): Upper ocean variability on the equator in the Pacific at 170 W. J. Geophys. Res., 100, Wilson, D. and A. Leetmaa (1988): Acoustic Doppler current profiling in the equatorial Pacific in J. Geophys. Res., 93, Wyrtki, K. (1961): Physical oceanography of the southeast Asian waters. NAGA Report, 2, 195 pp. 116 Y. Kuroda