U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
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°, 110°W (Plate 2).
Figure 10 shows 3-year segments of data (1982-1984, 1986-1988) overplotted on the mooring climatologies for 110°W. 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 5.6°C (January 1983) compared to 3.4°C (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 95°W. 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 110°W.
Figure 10. Time series of zonal winds, 10-m zonal velocity, SST, dynamic height 0/250 dbar, and 20°C 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, 20°C isotherm depths, and 10-m zonal velocities are identical to those plotted in Plate 2. Monthly mean mooring climatologies are indicated by dotted lines.
| 1982–1983 | 1986–1987 | |
|---|---|---|
| SST, °C | 5.6 (Jan. 1983) | 3.4 (Sept. 1987) |
| 20°C 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 20°C 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 <20°C 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-à-vis the 1986-1987 ENSO at
110°W.
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 110°W [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 159°W in September 1982 [Firing et al., 1983] and in January-February 1983 at 110°W (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 110°W) the Undercurrent was significantly weaker than normal, though it did not disappear at either 110°W or 140°W 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 95°W in 1983 [Halpern, 1987a], whereas they were confined to the west of Christmas Island (157°W) 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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