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

Variability in the Eastern Equatorial Pacific Ocean During 1986-1988

Michael J. McPhaden and Stanley P. Hayes

NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

Journal of Geophysical Research, 95(C8), 13,195-13,208 (1990)
Not subject to U.S. copyright. Published in 1990 by the American Geophysical Union.

6. Comparison of ENSO Variations During 1982-1983 and 1986-1987

A general discussion of the 1982-1983 ENSO is given by Cane [1983], and a detailed description of moored wind, current, and temperature measurements in the eastern equatorial Pacific during 1982-1983 is given by Halpern [1987a]. The purpose of this section is to point out some of the most significant similarities and differences between the 1982-1983 and 1986-1987 ENSO events in the eastern equatorial Pacific, relying primarily on the long time series at 0, 110W (Plate 2).

Figure 10 shows 3-year segments of data (1982-1984, 1986-1988) overplotted on the mooring climatologies for 110W. These data show that in terms of thermal and surface height anomalies the 1982-1983 ENSO was more intense than the 1986-1987 ENSO. Table 1, for example, indicates that the maximum monthly mean SST anomaly was 5.6C (January 1983) compared to 3.4C (September 1987) and the maximum depression of the thermocline was 96 m (December 1982) compared to 67 m (January 1987). Zonal winds were close to or slightly stronger than normal during the 1986-1987 event, whereas they were weaker than normal during late 1982 (Figure 10). Moreover, according to Halpern [1987a], zonal winds became westerly in early 1983 as far east as 95W. This local relaxation and reversal of the trade winds may have contributed to the intensity of thermal and flow anomalies that developed during the 1982-1983 ENSO at 110W.

Figure 10. Time series of zonal winds, 10-m zonal velocity, SST, dynamic height 0/250 dbar, and 20C isotherm depth for 1986-1988 (solid curves) and 1982-1984 (dashed curves). Daily data have been smoothed with a 51-day Hanning filter to remove variability at periods shorter than about 1 month. SST, 20C isotherm depths, and 10-m zonal velocities are identical to those plotted in Plate 2. Monthly mean mooring climatologies are indicated by dotted lines.

If your browser cannot view the following table correctly, click this link for a GIF image of Table 1.
TABLE 1.  Selected Monthly or Seasonally Averaged Anomaly Extrema During the 1982–1983
ENSO and the 1986–1987 ENSO

1982–1983 1986–1987

SST, C     5.6 (Jan. 1983)        3.4 (Sept. 1987)
20C isotherm depth, m 96 (Dec. 1982) 67 (Jan. 1987)
Dynamic height 0/250 dbar, dyn. m 36 (Dec. 1982) 21 (Jan. 1987)
SEC, cm s          32 (Aug.–Dec. 1982)            32 (Sept.–Nov. 1986)

   All values are positive, implying elevated SST, depressed 20C isotherm depth, elevated dynamic
height, and weaker South Equatorial Current (SEC). Time of the anomalies is indicated in parentheses.

During both events, warm thermal anomalies began to develop in midyear, prior to the South American coastal warming [Kousky and Leetmaa, 1989]. The 1982-1983 ENSO was of shorter duration, though, terminating after about 12 months in mid-1983, whereas the 1986-1987 ENSO lasted for about 18 months from mid-1986 to early 1988. Following both ENSOs, the ocean rebounded to significantly colder than normal conditions, with SSTs plummeting to <20C and dynamic heights dropping to 10 dyn. cm below normal.

Near-surface flow variations show a weakening of the South Equatorial Current during August-December 1982 and September-November 1986. In both cases the average anomaly relative to climatology is 32 cm s though during 1982 it lasts for 5 months whereas during 1986 it lasts for only 3 months. Compared to the 1986-1987 ENSO, prolonged weakening of the South Equatorial Current, combined with a much deeper thermocline, implies more pronounced anomalous zonal and vertical heat advection. This in turn may account for the larger SST anomalies observed during the 1982-1983 ENSO vis--vis the 1986-1987 ENSO at 110W.

It is interesting to note the opposite behavior of the springtime reversal in the South Equatorial Current near the onset of these two events. In March-June 1982, flow is weaker than normal by 19 cm s, whereas in March-June 1986 it is stronger than normal by 31 cm s. Warm SST anomalies appear during both periods, so that there is no consistent argument relating advective warming and zonal flow anomalies during the spring season prior to both events. Similarly, the strongest eastward flow (in both an absolute sense and an anomaly sense) for the 6 years of data shown in Figure 10 occurs in March-May 1983 (average anomaly of 54 cm s). During this period, SST changes little because the SST gradient across the Pacific is nearly zero [Kousky and Leetmaa, 1989].

We noted in section 4.5 that the 1986-1987 ENSO terminated in the eastern equatorial Pacific with surface cooling related to remotely forced uplift of the thermocline. Figure 10 shows that at the end of the 1982-1983 ENSO, failing surface temperatures were also associated with a shoaling thermocline (and decreasing surface heights). The precipitous drop in SST in June-July 1983, however, coincided with the strongest westward flow in the 6 years of data shown in Figure 10. Average anomalies of 49 cm s to the west were observed during this period, suggesting that westward advection of cold upwelled water may have contributed to the termination of the 1982-1983 ENSO. Local winds were weaker than normal throughout 1983 at 110W [Inoue and O'Brien, 1984], indicating that like the 1986-1987 ENSO event, nonlocal forcing was involved in the termination of the 1982-1983 ENSO in the eastern Pacific.

The Equatorial Undercurrent disappeared and was replaced by westward flow in the thermocline at 159W in September 1982 [Firing et al., 1983] and in January-February 1983 at 110W (Plate 2 and Figure 7; see also Halpern, [1987a]). This followed several months of nearly flat or reversed sea surface slope along the equator in the central and eastern Pacific [Wyrtki, 1984]. There were also periods of nearly flat or reversed pressure gradient at the surface and in the thermocline in the eastern Pacific during 1986-1988. During some of these periods (e.g., 1988 at 110W) the Undercurrent was significantly weaker than normal, though it did not disappear at either 110W or 140W during 1986-1988. The difference may be due to the duration, intensity, and/or zonal scale of the pressure gradient anomalies, which are likely to have been smaller in 1986-1988 than during 1982-1983. As noted above, westerly winds, which favor the establishment of a westward zonal pressure gradient force along the equator, penetrated to at least 95W in 1983 [Halpern, 1987a], whereas they were confined to the west of Christmas Island (157W) during the 1986-1988 (Figure 8). The Undercurrent disappeared in the western Pacific during October-November 1987 in response to these westerlies when the zonal pressure gradient at the surface and in the thermocline reversed [McPhaden et al., 1990a].

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