![]() Through long Rossby wave dynamics this decrease results in an anomalous westward equatorial flow that tends to push the warm pool westward and often results in the generation of a La Niña during March–June. With the wind forcing that causes El Niño in the eastern Pacific removed, the eastern equatorial Pacific sea level and thermocline anomalies decrease. Specifically, after westerly equatorial wind anomalies in a coupled ocean–atmosphere instability push the warm pool eastward during El Niño, the westerly anomalies follow the warmest water south of the equator in the Southern Hemisphere summer in December–February. In contrast to previous discharge–recharge oscillator theory, here it is shown that anomalous zonal flow acceleration right at the equator and the movement of the equatorial warm pool are crucial to understanding WWV–El Niño dynamics and the ability of WWV to predict ENSO. Previous work has shown that warm water volume (WWV), usually defined as the volume of equatorial Pacific warm water above the 20☌ isotherm between 5°S and 5°N, leads El Niño. The wind stress data are downloaded from, the OLR data are available from, and the SST is downloaded from. In (c), the correlation between the 29☌ and −40-mPa time series is r = 0.89, r crit (95%) = 0.42, between the 29☌ and 250 W m −2 time series is r = 0.87, r crit (95%) = 0.39, and between the −40-mPa and 250 W m −2 time series is r = 0.87, r crit (95%) = 0.95. ![]() The wind stress was averaged between 2.25°S and 2.25°N, the OLR between 2.5°S and 2.5°N, and the SST between 2°S and 2°N. All data have been filtered with a 5-month running mean. ![]() The break in the red and black curves near the end of 1997 occurs because the −40-mPa and 250 W m −2 values did not occur in the equatorial Pacific then. (a) Time–longitude plot of westerly wind stress anomalies (mPa) along the equator in the equatorial Pacific with the thick yellow line denoting the equatorial location of the 29☌ isotherm, a proxy for the eastern edge of the warm/fresh pool (b) as in (a), but for −OLR and (c) equatorial longitudinal location of the 29☌ isotherm (yellow), −40-mPa eastward equatorial wind stress isoline (black), and the 250 W m −2 OLR isoline (red). Here the Niño-3.4 index is from, and the monthly anomaly is calculated by removing the mean of January 1993–December 2016. The value in the bracket is the critical correlation coefficient at the 95% level based on Ebisuzaki (1997). The correlation between these two series is. (c) Niño-3.4 (black line) and S( m) Y( a) (red line). When | S( m) Y( a)| < 0.55☌, the year is neutral (green). When | S( m) Y( a)| ≥ 0.55☌ for at least 3 months of the year, the year is an El Niño year ( Y > 0, red) or a La Niña year ( Y < 0, blue). The structure function has been normalized so that. The first EOF mode represents 87.3% of the Niño-3.4 index variance. The product S( m) Y( a) was determined as the first mode of an EOF analysis of the April, May, June, …, February, March Niño time series. Representation of the Niño-3.4 index (☌) as the product of S( m) Y( a), where S( m) ( m = 1 for April, m = 2 for May, …, m = 12 for March) is the calendar-year structure function and Y( a) ( a = 1950, 1951, …, 2015) is the annual time series amplitude function.
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