U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

Direct measurements of upper ocean currents and water properties across the tropical Pacific during the 1990's

Gregory C. Johnson, Bernadette M. Sloyan1, William S. Kessler, Kristene E. McTaggart

NOAA/Pacific Marine Environmental Laboratory, Seattle, WA 98115-6349, USA
1Present address: Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1050, USA

Prog. Oceanogr. , 52 (1), 31-36, 2002.
Copyright 2002 Elsevier Science Ltd. Further electronic distribution is not allowed.

4. Zonal current properties and their variation

The EUC, NECC, and both branches of the SEC are analyzed for each synoptic section. Quantities evaluated include zonal volume transport as well as velocity-weighted latitude, depth, potential temperature, and salinity for each current at each longitude. These quantities are estimated only where the sections have sufficient meridional and vertical coverage in currents and water properties to sample fully the current. The NECC was the least frequently sampled current by the synoptic sections, followed by the SEC(S). These quantities are then fitted in the same manner as the section data discussed above.

In this analysis, the EUC is defined as all eastward flow between 2°N and 2°S (Gouriou & Toole, 1993) for potential densities of 23 < < 26.5 kg m3. In the west, the latitudinal limits help separate the EUC from the NSCC and the SSCC at depth, as well as from the surface NECC. The lighter limit helps to distinguish the EUC from transient eastward-flowing surface jets driven by local westerly wind events (Cronin et al., 2000), especially in the west. The denser limit attempts to distinguish the EUC from deeper eastward flows that are sometimes present below the EUC, usually separated by a velocity reversal (the EIC) or a minimum in eastward velocity.

The NECC is here defined as all eastward flow north of 2°N for < 26.0 kg m3. To be used for NECC estimates, the velocity and density fields had to suggest that the northern edge of the NECC was sampled. This restriction meant that the sections usually had to reach to 8°N or beyond, which unfortunately, but unavoidably, creates some potential for a biased sample. The 26.0 kg m3 density limit is near the base of the thermocline and helps to distinguish the NECC from the NSCC. The current separation is on the whole a bit denser than previous ones at 165°E based on the minimum in eastward velocity (Gouriou & Toole, 1993), and at 110°W based on (Hayes, Toole, & Mangum, 1983).

The SEC(N) is defined as all westward flow between the equator and the NECC, and again for < 26.0 kg m–3. The SEC(S) is defined as all westward flow between 8°S (because most of the sections stop at that latitude) and the equator for < 26.0 kg m–3. In the far west, around 143°E, New Guinea is substituted for 8°S as the southern limit of the SEC(S). In this region, upper portions of the NGCUC are counted as part of the SEC(S). The density limit for both branches of the SEC is consistent with that for the NECC.

The synoptic analysis presents a picture different from that of the mean section analysis in at least three ways. First, in the synoptic analysis each section is reduced to a set of single parameters before fitting. Thus the fits have less structure than the mean analysis, since the mean analysis allows independent variations in the properties on isopycnals and the depth of those isopycnals. Deviations of current properties with respect to the ENSO cycle in the synoptic analysis are just linear perturbations of the mean properties regressed against the SOI and the seasonal cycle is a pure annual harmonic. Thus, only El Niño fits are shown, since La Niña fits contain no additional information. For each current and quantity the means and the variations with the ENSO cycle (shown as a strong El Niño by evaluating the fits at SOI = –1.5) are presented as functions of longitude. The seasonal cycle is shown separately.

The second difference is that the synoptic section analysis exactly follows the currents as they move horizontally and vertically, within the density and latitude limits noted above: a Lagrangian approach. However, while the mean section analysis is quasi-Lagrangian in the vertical, since it is done on isopycnals, it is closer to Eulerian in the meridional. The annual harmonics and regression against the SOI in the mean section analysis accounts for some of the currents’ meridional migrations, but not all of them. That is to say, currents are not completely followed as they move north or south with various higher frequency motions such as TIWs and other disturbances.

The third difference is that the synoptic section analysis defines currents as all flow in a given direction and region as noted above. With this definition, there are never any current reversals portrayed in individual sections, just absences of currents, which are fitted as zero transport and omitted from the other property fits. This situation contrasts with the mean section analysis, which includes velocity reversals at any given latitude and isopycnal. These last two differences combine for stronger current transports in the fits of the synoptic section analysis than in the mean section analysis.

4.1. Zonal current volume transports

Variations of the equatorial zonal currents are closely connected to seasonal and ENSO cycle changes in the equatorial easterlies. Equatorial zonal winds affect the surface currents primarily through downwind frictional forcing and setup of a zonal pressure gradient, which exerts an upwind forcing below the frictional layer. Thus, when the easterly trade winds are strong, they tend to spin up both the westward SEC at the surface and, after time, the eastward EUC beneath. By elevating the equatorial thermocline (through upwel­ling caused by local Ekman divergence and non-local wave adjustments), strong trades also tend to produce meridional pressure gradients that foster westward off-equatorial currents. During El Niño, or during the phase of the annual cycle with westerly anomalies, the opposite changes occur.

The mean volume transport of the EUC (Figs. 17 and 5) is larger in the central Pacific than in the west or the east. In the central Pacific the trade winds are strongest, the zonal thermocline slope and thus the zonal pressure gradient is the biggest, and so the EUC is the largest. The seasonal cycle in EUC transport is substantial at all longitudes. The phasing is such that EUC transport peaks in mid-May in the east and late June in the west, as the relaxed trade winds during this time allow increased eastward flow in the upper half of the EUC (Philander, Hurlin, & Seigel, 1987). This westward phase propagation in the EUC agrees with model results (Blanke & Raynaud, 1997) and empirical analyses (Yu & McPhaden, 1999). The weakening or disappearance of the EUC during the height of El Niño was observed during the events of 1982–83 (Firing, Lukas, Sadler, & Wyrtki, 1983), 1986–87 (McPhaden, Hayes, Mangum, & Toole, 1990), 1991–92 (Kessler & McPhaden, 1995) and 1997–98 (Johnson et al., 2000).

Fig. 17. Zonal current volume transport (106 m3 s–1) for (from top to bottom) the EUC, NECC, SEC(N), and SEC(S). Left panels show the quantity with error bars versus longitude for means (GRAPHIC_HERE joined by solid lines), SOI = –1.5 strong El Niño (GRAPHIC_HERE joined by dashed lines), and raw data (+). Right panels show contours of the seasonal cycle versus longitude. Contour interval in the right panels is 5 × 106 m3 s–1 and the y-axis tick marks on the left panels are at twice that interval.

The mean transport of the NECC is fairly constant over the western and central Pacific, but decreases in the east, becoming smallest around 110°W (Fig. 17). This minimum at 110°W is as expected from the Sverdrup relation (Kessler, 2002). The sampling in the far west is sparse and the transport uncertainties are correspondingly large. The seasonal cycle is a large fraction of the mean transport of the NECC. The NECC has its largest transport in August at 95°W, but at and west of 125°W the maximum in transport occurs in October–December, except near 143°E, which probably sees the influence of the western bound­ary. This shift with longitude is again a signature of westward phase propagation. Around February the NECC is very weak east of 125°W. During El Niño events, the NECC is important for export of water eastward from the warm pool, and its transport increases by about 25% (Picaut, Ioualalen, Menkes, Delcroix, & McPhaden, 1996). Central Pacific transport NECC variations with the SOI are aliased by TIWs as discussed in Section 3.2.

Transport estimates for the westward-flowing SEC(N) are difficult and rarely direct (Wyrtki, 1974a; Wyrtki & Kilonsky, 1984; Picaut & Tournier, 1991). The mean volume transport of this current peaks in amplitude in the central Pacific (Fig. 17). Throughout the basin, but especially in the western Pacific where the NECC is closest to the equator, the SEC(N) is sometimes absent. The current transport estimated through the synoptic analysis in the west is clearly large compared with the mean fields estimated on isopycnals (Fig. 8). This example best highlights this difference between the synoptic current transport analysis and the mean fit fields as discussed in Section 4.

Much like the NECC, the seasonal cycle of the SEC(N) volume transport is such that the current is weakest in February–March in the east, but this lull transitions to around July in the west (Fig. 17; Wyrtki, 1974a). Depth fluctuations of the thermocline trough near 4°N–5°N affect both the SEC(N) to the south and the NECC to the north, by simultaneously modulating the pressure gradients across the two currents. As a result, their transport variations tend to be approximately in phase (Kessler & Taft, 1987). The local manifestation of an annual equatorial Rossby wave at 5°N (Yu & McPhaden, 1999) accounts for most of the annual variability of the depth of the trough (Kessler, 1990). This wave is forced by the wind stress curl resulting from the annual north–south migration of the ITCZ. The amplitude of the seasonal cycle is large throughout the basin. Unfortunately, the SEC(N) and NECC transport estimates around 140°W are aliased by energetic TIWs sometimes present in the central Pacific as discussed in Section 3.2.2. The TIW signal is reflected by the relatively wide distribution of transport estimates for these currents in that region (Fig. 17).

West of 110°W, the SEC(N) is greatly reduced during El Niño, at least partly because of the reduction in the trades. The deepening of the equatorial thermocline reduces the meridional pressure gradient and results in anomalously eastward surface geostrophic velocities during El Niño, with a reduced SEC and an increased NECC. Momentum advection driven by cross-equatorial southerly winds may also be important in the SEC(N) dynamics in the central to eastern Pacific (Philander et al., 1987).

The SEC(S) mean volume transports (Fig. 17) build from fairly small values at 95°W to nearly constant values west of 140°W, before diminishing again west of 165°E. As mentioned before, at 143°E the upper part of the NGCUC is folded into these SEC(S) transport estimates. The amplitude of the seasonal cycle is large only between 165°E and 155°W, with the current being strongest in February–March, roughly in opposite phase with the SEC(N). The volume transport decreases significantly only in the west during El Niño (except at 143°E where the NGCUC is strong during that phase of ENSO). This change is a different, but much less dramatic, response to the reduction in trade wind forcing than that seen in the SEC(N). The zonal asymmetry may result from an enhancement of the South Pacific Convergence Zone (SPCZ) during El Niño, which increases the SECC and reduces the SEC(S) in the western Pacific (Kessler & Taft, 1987). It is important to remember that there may be SEC(S) fluctuations beyond 8°S that are missed by this data set.

4.2. Zonal current latitudes and depths

The mean latitude of the EUC (Figs. 18 and 2) is near the equator except at 143°E where the NGCUC is strong. The EUC core shifts slightly south of the equator in the east, owing to the meridional circulation induced by predominantly southerly winds in that region (Philander & Delecluse, 1983; Kessler, Rothstein, & Chen, 1998). Eastward shoaling of the EUC is apparent in the mean from 143°E to 125°W, but the velocity-weighted EUC is relatively constant in depth east of that longitude (Fig. 19) while the peak velocity continues to shoal slightly (Fig. 3).

Fig. 18. Zonal velocity-weighted current latitude (°) for (from top to bottom) the EUC, NECC, SEC(N), and SEC(S). Contour interval in the right panels is 0.5° and the y-axis tick marks on the left panels are at twice that interval. Details follow Fig. 17.

Fig. 19. Zonal velocity weighted current depth (m) for (from top to bottom) the EUC, NECC, SEC(N), and SEC(S). Contour interval in the right panels is 10 m and the y-axis tick marks on the left panels are at twice that interval. Details follow Fig. 17.

The seasonal cycle of EUC depth is very smooth and well defined. The current shoals in March at 95°W, and this shoaling generally lags to the west (except closest to the western boundary), with maximum shoaling in August by 165°E. This westward propagation is a signature of an annual Rossby wave (Lukas & Firing, 1985; Kessler & McCreary, 1993). Shallower depths are generally associated with large transports in the seasonal cycle, although there are some phase differences between seasonal EUC transport and depth. This correlation arises as the EUC picks up eastward transport in the shallower layers with relaxed trade winds. The seasonal cycle of EUC latitude is very small except in the far west. During El Niño the EUC shifts northward toward the equator in the central Pacific. During this phase of ENSO the EUC deepens considerably in the east, and shoals in the west, consistent with the reduced zonal slope of the equa­torial thermocline.

The mean latitude of the NECC moves gradually northward as the current progresses from west to east, with an interruption of this tendency at 110°W (Figs. 18 and 2), where the mean transport is weakest (Fig. 17). At this longitude the NECC and NSCC are difficult to distinguish because the NECC is very weak, so there is little eastward velocity above the thermocline. The NECC also shoals from west to east (Fig. 19) with the shoaling thermocline. The seasonal cycle of latitude is such that the NECC is generally found furthest south when its volume transport is at a minimum. The seasonal cycle of depth is notable only west of 125°W, with the current being slightly deeper in February–April. During El Niño the NECC is found south of its usual latitude in the western and central Pacific, and it shoals in the western Pacific with a hint of deepening in the east, as expected from the migration of the warm pool to the east during this phase of ENSO.

The mean latitude of the SEC(N) starts near 2°N in the east, approaches 3°N in the central Pacific (Figs. 18 and 2), where the current is strongest, and then tends southward to 1°N in the west where the current weakens. The SEC(N) also deepens to the west (Fig. 19), again with the westward deepening of the thermocline until it reaches the warm pool, where the depth remains constant. The seasonal cycle of latitude is such that the SEC(N) is generally furthest north when its volume transport is at a minimum, in contrast with the seasonal cycle of the NECC. The current is shallowest in January–February in the east, with a westward lag toward shallowest values in May–June in the west. Deeper values are correlated with larger transports. During El Niño the current moves southward as it weakens in transport, again consistent with eastward migration of the warm pool. Interestingly, during El Niño it also deepens in the west as its transport decreases, because surface flow tends to be eastward during this time and the SEC(N) is sometimes limited to a subsurface expression.

The SEC(S) is located from 4°S to 5°S in the mean (Fig. 18), except near the western boundary where it is forced northward by New Guinea and joins the NGCUC. Of course, this latitude is somewhat influenced by the southern limit of 8°S in this analysis. Like the rest of the currents, the SEC(S) deepens to the west (Fig. 19) with the thermocline. In the seasonal cycle, the SEC(S) also generally shifts south when transports are weakest. The seasonal cycle of the SEC(S) depth is such that it is shallowest in May–August, roughly in phase with the SEC(N). The current shifts southward in the west and weakens under El Niño conditions. During El Niño, like the SEC(N), the current deepens (except in the far west where the NGCUC influences the analysis) for the same reason: a tendency toward increased eastward surface flows.

4.3 Zonal current temperatures and salinities

In the mean the EUC is warmest (Fig. 20) and saltiest (Fig. 21) near the dateline. These changes occur because the current adds warm and salty water from the southern hemisphere (Johnson & McPhaden, 1999) as it moves eastward, but then as it surfaces in the east its warmer and saltier waters upwell into the SEC. The seasonal cycle is such that the EUC is warmer and saltier when the transport is largest, because the upper layers of the EUC, which are warmer and saltier, make up most of the seasonal variability. During El Niño there is some warming of the EUC in the eastern Pacific, owing to more near-surface eastward flow under the relaxed trades. During El Niño there is also some freshening of the EUC in the western Pacific, perhaps associated with eastward migration of the fresh pool in the upper portions of the EUC.

Fig. 20. Zonal velocity-weighted current temperature (°C) for (from top to bottom) the EUC, NECC, SEC(N), and SEC(S). Contour interval in the right panels is 1°C and the y-axis tick marks on the left panels are at twice that interval. Details follow Fig. 17.

Fig. 21. Zonal velocity-weighted current salinity (PSS–78) for (from top to bottom) the EUC, NECC, SEC(N), and SEC(S). Contour interval in the right panels is 0.1 and the y-axis tick marks on the left panels are at twice that interval. Details follow Fig. 17.

The NECC temperature is relatively warm (Fig. 20) and fresh (Fig. 21), but it cools a little and freshens a lot in the mean as it flows eastward under the high precipitation of the ITCZ. The seasonal cycle is such that the NECC is warmest and freshest when the transports are largest, because near-surface intensification accounts for much of the seasonal cycle. During El Niño the NECC tends to be warmer in the east, and fresher in the central Pacific. The latter change is probably a result of large-scale precipitation changes and eastward advection of the fresh pool.

The SEC(N) is cool (Fig. 20) and fresh (Fig. 21) in the east where it flows adjacent to the ITCZ and to the fresh coastal surface waters. It warms and becomes increasingly salty to the west. The seasonal cycle of temperature is such that larger transports are generally associated with cooler temperatures. The seasonal cycle of salinity is really only large in the east, where the current is freshest in January, a time when its transport is low and the ITCZ has moved south over the current. As might be expected, the SEC(N) is slightly warmer in the east during El Niño, when the cold equatorial upwelling that feeds it is suppressed. The salinity shows no change with variations of the ENSO cycle.

Like the SEC(N), the SEC(S) warms (Fig. 20) and gets saltier (Fig. 21) as it transits from east to west, except that it cools slightly and freshens in the far west, because of the presence of the SPCZ. The exception is the western boundary where the cold, relatively fresh influence of the NGCUC is seen. The current is generally warmest around April throughout the basin. However, it is saltiest in the eastern Pacific around July when the ITCZ is far to the north, and freshest in the western Pacific around the same time. During El Niño the current warms slightly in the east; this is again not surprising, since it is fed at least in part from equatorial and coastal upwelling, both of which are much weaker and warmer during El Niño. The SEC(S) also gets a bit saltier in the central Pacific during El Niño, mostly because the current deepens, and additionally, is found only well south of the equator. As a result of these changes, the current carries a lower fraction of fresher surface equatorial water.


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