Enso

Atmospheric variability: The Southern Oscillation

As Walker noted, the Southern Oscillation shows up very clearly as a "see-saw" in

FIGURE 12.8. Schematic of the feedback inherent in the Pacific Ocean-atmosphere interaction. This has become known as the Bjerknes feedback.

sea level pressure (SLP) across the tropical Pacific basin. When SLP is higher than normal in the western tropical Pacific region, it tends to be low in the east, and vice versa. This can be made evident by showing how SLP at one location is related to that elsewhere.

Bjerknes Feedback
FIGURE 12.9. Correlations (x10) of the annual-mean sea level pressure with that of Darwin (north of Australia). The magnitude of the correlation exceeds 0.4 in the shaded regions. After Trenberth and Shea (1987).

Fig. 12.9 shows the spatial structure of the temporal correlation6 of annual-mean SLP with that of Darwin (northern Australia). The correlation reveals a trans-Pacific dipole, with structure roughly similar to that of the Walker cell. Because of the location of the cell that anticorrelates with Darwin SLP, Tahiti SLP is frequently taken to be representative of this cell. It has become conventional to define a ''Southern Oscillation Index'' (SOI) as we mean the long-term average—Fig. 12.10 makes it clear that the tropical atmosphere is rarely in such a state, but rather fluctuates around it.

Such SLP variability is indicative of variability in the meteorology of the region as a whole as well as (to a lesser degree) of higher latitudes. Note, for example, the hint of an impact outside the tropics (such as the L-H-L pattern across N. America) in Fig. 12.9.

SLPrakiti - SLPd a where a is the standard deviation of the pressure difference. The time series of this index for the period 1951-2000 is shown by the solid curve in Fig. 12.10.

The index shows persistent but irregular fluctuations on periods of 2-7 years, with a few outstanding events, such as 1982-1983 and 1997-1998. Since, by definition, SOI = 0 under normal conditions—if by ''normal''

Oceanic variability: El Nino and La Nina

Figure 12.10 also shows (dashed curve) a time series of SST anomalies (departures from normal for the time of year) in the far eastern equatorial Pacific Ocean. Like the SOI, the SST shows clear interannual fluctuations on a typical time scale of a few years, and shows a dramatic anticorrelation with the SOI. The correlation coefficient between the two time series, over the period shown, is C = -0.66. The negative sign of the correlation, together with the spatial pattern of a

6If two time series are perfectly correlated, then the correlation C = +1; if perfectly anticorrelated C = -1. If they are uncorrelated, C = 0.

FIGURE 12.10. The Southern Oscillation index (solid) and sea surface temperature (SST) anomaly (K) in the equatorial east Pacific Ocean (dashed), for the period 1951—2000. The SST anomaly refers to a small near-equatorial region off the coast of South America. The two time series have been filtered to remove fluctuations of less than about 3 months.

FIGURE 12.10. The Southern Oscillation index (solid) and sea surface temperature (SST) anomaly (K) in the equatorial east Pacific Ocean (dashed), for the period 1951—2000. The SST anomaly refers to a small near-equatorial region off the coast of South America. The two time series have been filtered to remove fluctuations of less than about 3 months.

Fig. 12.9, tells us that warm SST in the east equatorial Pacific approximately coincides with anomalously high pressure in the west and low in the east.

The spatial structure of SST variations is revealed by comparison of the two cases shown in Fig. 12.7. As noted previously, during "normal" or cold conditions, when the SSTs in the eastern equatorial Pacific are at their coldest (illustrated by the case of 1989), the coldest tropical water is concentrated in a narrow tongue extending outward from the South American coast, while the warmest water is found in an extensive warm pool west of the International Date Line. During a warm El Nino event (illustrated by the case of 1998), the warm water extends much further eastward, and the cold tongue is anomalously weak (in a strong event it may disappear). At such times, the eastern ocean, though still no warmer than the western equatorial Pacific waters, is very much warmer than normal for that time of year. While most of the equatorial Pacific Ocean is anomalously warm, the SST anomalies are greatest in the east, where they can be as large as 5 °C. Note from Fig. 12.7 that, for the most part, significant SST variability is concentrated within a few degrees latitude of the equator, consistent with our earlier estimate of the equatorial deformation radius.

Theory ofENSO

The "big picture" of what happens during a warm ENSO event is illustrated schematically in Fig. 12.11. As discussed above, in cold conditions, there is a strong east-west tilt of the thermocline and a corresponding east-west gradient of SST, with cold upwelled water to the east and warm water to the west. Atmospheric convection over the warm water drives the Walker circulation, reinforcing the easterly trade winds over the equatorial ocean. During a warm El Nino event, the warm pool spreads eastward, associated with a relaxation of the tilt of the thermocline. Atmospheric convection also shifts east, moving the atmospheric circulation pattern with it. Pressure increases in the west and decreases in midocean. This adjustment of the Walker circulation, which corresponds to a negative SOI, leads to a weakening or, in a strong event, a collapse of the easterly trade winds, at least in the western part of the ocean. We can summarize the mutual interaction as follows.

First, the atmosphere responds to the ocean: the atmospheric fluctuations manifested as the Southern Oscillation are mostly an atmospheric response to the changed lower boundary conditions associated with El Nin o SST fluctuations. We should expect (on the basis of our previous discussion) that the Walker circulation and its associated east-west pressure gradient, would be reduced, and the Pacific trade winds weakened, if the east-west contrast in SST is reduced as it is during El Nino. There have been many studies using sophisticated atmospheric general circulation models (GCMs) that have quite successfully reproduced the Southern Oscillation, given the SST evolution as input.

Second, the ocean responds to the atmosphere: the oceanic fluctuations manifested as El Nino seem to be an oceanic response to the changed wind stress distribution associated with the Southern Oscillation. This was first argued by Bjerknes, who suggested that the collapse of the trade winds in the west Pacific in the early stages of El Nino (see the lower frame of Fig. 12.11) drives the ocean surface waters

Cold Conditions: "La Niña"

Atmosphere

Ocean

Cold Conditions: "La Niña"

Atmosphere

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