A New Equatorial Oscillation Index for Better Describing ENSO and Westerly Wind Bursts
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Volume 34
OCTOBER 2020
A New Equatorial Oscillation Index for Better Describing ENSO and Westerly Wind Bursts Yunhao SHI1,2 and Jingzhi SU1* 1 Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100081 2 College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049 (Received November 11, 2019; in final form March 20, 2020)
ABSTRACT An Equatorial Oscillation Index (EOI) is defined, based on the zonal gradient of sea surface pressure between the western Pacific and eastern Pacific along the equator, to describe the distribution of wind and pressure within the equatorial Pacific. The EOI has a stronger correlation with the Niño3.4 sea surface temperature anomaly (SSTA), as well as with westerly/easterly wind bursts (WWBs/EWBs), showing a superiority over the Southern Oscillation Index (SOI). In general, the EOI is consistent with the SOI, both of which reflect large-scale sea level pressure oscillations. However, when there are inconsistent SSTAs between the equator and subtropical regions, the SOI may contrast with the EOI due to the reverse changes in sea level pressure in the subtropical regions. As a result, the SOI fails to match the pattern of El Niño, while the EOI can still match it well. Hence, the EOI can better describe the variability of the Niño3.4 SSTA and WWBs/EWBs. The correlation between the SOI and Niño3.4 SSTA falls to its minimum in May, due to the large one-month changes of sea level pressure from April to May in the subtropical southern Pacific, which may be related to the spring predictability barrier (SPB). The newly defined EOI may be helpful for monitoring El Niño–Southern Oscillation (ENSO) and predicting ENSO. Key words: Equatorial Oscillation (EO), Southern Oscillation (SO), westerly wind bursts (WWBs), EOF, ENSO prediction, spring predictability barrier (SPB) Citation: Shi, Y. H., and J. Z. Su, 2020: A new Equatorial Oscillation Index for better describing ENSO and westerly wind bursts. J. Meteor. Res., 34(5), 1025–1037, doi: 10.1007/s13351-020-9195-6.
1.
Introduction
When El Niño events occur, the sea surface temperature (SST) in the eastern equatorial Pacific can be several degrees Celsius higher than normal. These rising temperatures in the vast waters of the Pacific Ocean are associated with changes in the trade winds. The phenomenon most directly responsible for this is the inverse relationship of the sea level pressure between the eastern Pacific and the western Pacific, namely, the Southern Oscillation (SO; Julian and Chervin, 1978; Wright, 1989; Walker, 1933). During El Niño, the pressure in the southeastern Pacific decreases significantly, while the pressure near Indonesia and Australia increases; this pressure distribution follows the opposite trend during La Niña. The interaction between the ocean and atmosphere is collectively referred to as El Niño–Southern Oscillation
(ENSO) in meteorology (Bjerknes, 1969; Timmermann et al., 2018). ENSO shows many regularities, which may be explained by high-fre
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