4.0 Results

4.1 CTD profiling data

Continuous profiles of temperature, salinity, and potential density ( )at each CTD station are presented in Figs. 9.1-3 for each of the EQ cruises. The results of bottle determination of salinity are also shown.

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4.2 Hydrography and upper ocean current sections

CTD and velocity data collected during the cruises are presented in contour plots of potential temperature (), salinity, and (Figs. 10-29) along the sections shown in Figs. 1-3. Two different vertical scales of pressure (0-200 dbar, 0-1000 dbar) for the hydrographic data are used, and salinity is also contoured against . Also included with the 0-200 dbar CTD contours are the near-surface temperature (NST, EQ-1, 2 and 3) and near-surface salinity (NSS, EQ-2 and 3) measured with the thermosalinograph along the sections. The NST and NSS plots show overlapping in some sections (e.g. Fig. 18a). These are instances in which the ship re-traced its trajectory back and forth along the section. The zonal and meridional ADCP velocities were averaged and interpolated onto a 10-m by 0.25° latitude or longitude grid, and contoured over the 0-300 m-depth interval. The velocity vectors were averaged in various layers: between 20-25 m, and over 50-m layers centered at 50, 100, 150, 200 and 250 meters depth.The resulting vectors are shown along the cruise track in Figs. 30-33.

In the following paragraphs, major features of temperature, salinity and velocity structures in each section are briefly described.

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4.2.1 EQ-1

The EQ-1 cruise was conducted in April-May 1992. In early 1992, the warm pool was displaced substantially eastward, and from April through October 1992, it retreated back (Lukas et al., 1994). The upper isothermal layer in the western Pacific is relatively thinner than usual during the EQ-1 cruise as shown in the results (e.g. Figs. 10a, 11a).

Potential temperature, salinity, potential density, and the velocity along the equator during the EQ-1 cruise are shown in Fig. 10. Surface waters are warmer than 29°C within this longitudinal range. Strong salinity stratification is evident in the upper 50 dbar. From 154°E to 170°E, the surface flow is toward the west, with the strongest flows exceeding 80 cm s¯ ¹ farther east.

The contours along the 143°E meridional section (Fig. 11) show a high salinity tongue from the southern hemisphere extending to the northern hemisphere around depths of 100-200 dbar, and water saltier than 35 is found near 3°N. The salinity distribution around this depth range agrees with that observed in previous cruises. The average salinity along the 143°E section from three cruises of the Western Equatorial Pacific Ocean Circulation Study (WEPOCS) and along a 141.5°E section from six United States/People's Republic of China (US-PRC) cruises indicated that the water saltier than 35 from the southern hemisphere extends to 3°N (Gouriou and Toole, 1993). During WEPOCS 1 and 2 cruises, water saltier than 35 was found farther north around 4°N-5°N (Tsuchiya et al., 1989).

The thickness of the upper isothermal layer is about 60 m near the equator. This is thinner than the climatology in this region (Gouriou and Toole, 1993, their Fig. 5a). During WEPOCS 1 and 2 cruises, the upper isothermal layer near the equator along 143°E was about 80 m. Surface water near the equator is warmer than 30°C, which is warmer than the climatology (Levitus, 1982). Surface water warmer than 29°C is found between 1°S and 5°N.

Fig. 11b shows the zonal and meridional velocity along 143°E. A strong eastward surface current between the equator and 3°S is evident and is associated with the fresh water near the surface in this region (Fig. 11a). The surface South Equatorial Current (SEC) is seen between the equator and 4.5°N. The maximum current speed is 30-40 cm s¯ ¹. The North Equatorial Countercurrent (NECC) is weak (about 20 cm s¯ ¹ near the surface) and narrow and found poleward of 4°N. There is a subsurface eastward flow maximum found near 4°N at a depth of about 150 m. This eastward flow is connected to the NECC. The meridional velocity is mostly northward in the upper 100 m.

The New Guinea Coastal Undercurrent (NGCUC) is also observed in this section southward of 1.5°S. The maximum zonal velocity is about 40 cm s¯ ¹ at 200-250 m-depth. The velocity and depth of the NGCUC agrees with the climatology (Gouriou and Toole, 1993, their Fig. 5c), but it is weaker than the one observed during the 3 WEPOCS cruises, conducted in the same area between 1985 and 1988 (Lindstrom et al., 1987, Santiago-Mandujano and Firing, 1988). A 60 cm s¯ ¹ core in zonal current was observed during the WEPOCS 3 cruise (E. Firing, pers. comm.).

The results along the 147°E meridional section (Fig. 12) show that surface waters are warmer than 29°C in this latitudinal range. There are three distinct local salinity maxima at the equator near 50-250 dbar as a result of intrusions of different water masses.

The zonal current structure along 147°E is similar to that along 143°E (Fig. 12b). The NECC in this section is weaker than at 143°E. The meridional velocity is mostly southward.

The westward North Equatorial Current (NEC) was observed during the transit from Guam to the first station (Fig. 13). Surface currents are between 40-50 cm s¯ ¹ just southwest of Guam.

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4.2.2 EQ-2

The mature warm episode conditions returned in early 1993. The time series of sea surface temperature along the equator indicate that the warmest water was found east of the dateline during the period of the EQ-2 cruise in March 1993 (Lukas et al., 1994). Potential temperature, salinity, and potential density along the equator during the EQ-2 cruise are shown in Fig. 17. The gradual slope of the sharp upper thermocline just below the isothermal layer downward from the west is seen in this longitudinal range. A sharp salinity gradient corresponding to the sharp temperature gradient of the upper thermocline is also evident.

The velocity contours along the equator (Fig. 17b) show that an eastward surface current is evident between 155°E and 169°E, and the westward subsurface SEC is evident. The Equatorial Undercurrent (EUC) exists in this longitudinal range below 150 dbar, but it is very weak between 160°E and 170°E.

Contours along 165°E (Fig. 18) show that the thickness of the upper isothermal layer is about 100 m near the equator, and a sharp temperature gradient is evident just below the top of the thermocline between 3°S and 2°N. A maximum salinity of about 35.8 is found in this section around 5°S near 180 m.

Note that there is an apparent spike of high temperature measured by the thermosalinograph near the equator (Fig. 18a). This may be explained by the maneuvering of the ship during the mooring operation, which caused the surface warm water to mix with the colder water underneath, increasing the water temperature at the depth where the sensor was located.

Fig. 18b shows the velocity along 165°E. The eastward surface current and the westward subsurface current are evident between 1°N and 2°S. The NECC is seen between 4°N and 8°N.

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4.2.3 EQ-3

Fig. 22 shows potential temperature, salinity and potential density along 165°E during the EQ-3 cruise. Surface temperature was more than 29°C in the southern hemisphere which was warmer than in the same section during the EQ-2 cruise.The climatological data show that the surface waters are warmer than 29°C during this season in this area (Levitus, 1982). Surface water saltier than 35 near the equator is evident, which was not seen during EQ-2. Strong salinity stratification in the upper isothermal layer is seen between 4°S and 8°S. Surface salinity in this region is less than 34.

The velocity contours along 165°E (Fig. 22b) show that an eastward surface current is evident between 2°S and 7°S, associated with fresh near-surface waters (Fig. 22a). Eastward near-surface currents dominate near the surface in this section, and the SEC is weak. The core of the EUC is displaced southward between 1°S and 2°S. The meridional velocity is mostly southward at the surface and below.

In the section along 156°E (Fig. 23), the upper 100 dbar is stratified by salinity. The high salinity water near the equator observed in the 165°E section is not seen in this section, suggesting that equatorial upwelling was occurring only at the eastern edge of the warm pool. The thickness of the upper isothermal layer near the equator is about 100 m. Temperature inversions near both 1°S and 1°N are evident (Fig. 23a). The depth of the inversions corresponds to the depth of the strong salinity stratification. The velocity along 156°E (Fig. 23b) shows that the SEC is stronger here than in the 165°E section, and it has maximum velocity near 75-m depth. A 40 cm s¯ ¹ core of the EUC is located between 1.5S° and 1°N.

Along the equator (Fig. 24), the temperature of the upper isothermal layer is near 30°C. The upper 100 dbar is stratified by salinity in this section. A temperature inversion is found at 40-60 dbar between 151°E and 154°E. The water at about 60 dbar is approximately 0.4°C warmer than the water near the surface. A decrease in the surface salinity is also seen between 150.5°E and 152.5°E, with salinities as low as 33 observed near 152°E.

The velocity along the equator (Fig. 24b) shows that the SEC is strong westward from 148°E, reaching near the surface. The eastward surface current and the westward subsurface SEC are seen between 149°E and 154°E.

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4.3 Thermosalinograph time series

Time series of NSTs and NSSs measured by thermosalinograph during these cruises are presented in Figs. 34-39.

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4.3.1 EQ-1

Time series of NST measured by thermosalinograph during this cruise are shown in Fig. 34. NSS data were not obtained during this cruise because the conductivity sensor failed at the beginning of the cruise. NSTs ranged from 27.5°C to 30.2°C. Diurnal warming is evident most of the time during the cruise, and it was sometimes as large as 1°C above the night time minimum NSTs.

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4.3.2 EQ-2

Fig. 36 shows the time series of NST and NSS during the EQ-2 cruise. A strong (0.5°C amplitude) diurnal cycle is seen during the middle of the cruise. This period corresponds to the light wind condition. NSS varied from 33.7 to 34.7 during the cruise.

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4.3.3 EQ-3

Fig. 38 shows the time series of NST and NSS during the EQ-3 cruise. NSS decreased from the middle of leg 1, down to less than 34. Diurnal warming is evident in the middle of leg 2 when the wind was weak.

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4.4 Meteorology time series

The meteorological data collected during these cruises by the science group (EQ-1 and 3) and by the ship's bridge personnel (EQ-1, 2, and 3) are presented in Figs. 40-42.

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4.4.1 EQ-1

A wide range of meteorological conditions were encountered during the cruise. Winds were often lighter than 6 m s¯ ¹, with several days of calm conditions (Fig. 40n). Winds exceeding 10 m s¯ ¹ were observed for short periods on several days. A surge of the Northwest Monsoon was observed during the first week of the cruise, with westerlies of 6-10 m s¯ ¹. The subsequent light and variable winds gave way to strong Southeast Trades for a few days, followed by light winds from the northwest quadrant. Air temperatures were quite variable, ranging from 24°C to 33°C (Fig. 40c).

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4.4.2 EQ-2

Wind speed and direction also varied substantially during this cruise (Figs. 41m-n). Westerly winds dominated early in the cruise, and easterlies dominated near the end of the cruise. Winds were usually weak in the middle of the cruise except for a period of strong westerly wind for a couple of days. Air temperatures ranged between 23.5°C-32°C (Fig. 41c).

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4.4.3 EQ-3

Fig. 42n shows the wind vector along the cruise track. In the beginning of leg 1 (April 12-14), the north-east winds were dominant in the northern part of the 165°E section. A westerly wind event with speeds of 10-15 m s¯ ¹ was observed in the southern hemisphere along 165°E. The wind then became weak along the section from 8°S, 165°E to Pohnpei.

During leg 2, winds were mostly calm, except near the end of the leg when a tropical depression was encountered. Strong westerly wind was observed along 137°E.

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4.5 XBT

The stack plots of temperature profiles measured by XBT during the EQ-1 cruise are shown in Fig. 43.

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