ENSO combination mode’s role in the maintenance of the anomalous anticyclone over North Pacific in boreal summer

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(a) Regression of the winter SST (shading, unit: ℃) and 850 hPa winds (vectors, unit: m s-1) anomalies on the summer WNPAC index during 1979–1999. (b) as in (a), but in spring. (c) Same as in (a) but in summer. (d) (e) (f) are the same as (a) (b) (c) but for the 2000–2020 period. The blue and red boxes indicate the WNP region and the IO region, respectively. Stippling represents the precipitation anomalies exceeding the 90% confidence level and vectors with values exceeding the 90% confidence level are shown. Credit: Science China Press

In a paper published in Science China Earth Sciences, Prof. Wenjun Zhang from Nanjing University of Information Science and Technology demonstrated that the ENSO combination mode plays an important role in maintaining the anomalous anticyclone over the western North Pacific in boreal summer. New data were provided to resolve the controversial issue involving the key physical mechanism for the maintenance of the anomalous anticyclone over the western North Pacific during the ENSO decaying summer.

It has long been established that anomalous low-level Western North Pacific (WNP) anticyclone (WNPAC) exerts pronounced influences on the weather and climate over East Asia during the boreal summer. Its interannual variations are closely associated with the equatorial Pacific ENSO states in the previous winter; however, the mechanisms for their linkage remain controversial. So far, three possible physical mechanisms have been proposed to explain the maintenance of WNPAC during the ENSO decaying summer, that is, the local air-sea interaction, Indian Ocean capacitor effect and ENSO Combination mode (C-mode).

This study demonstrated that the influences of the local WNP and Indian Ocean (IO) sea surface temperature (SST) anomalies exhibit conspicuous decadal differences in maintaining WNPAC in summer. This decadal difference could be related to changes in the decaying speed of ENSO events. In contrast to El Niño events before the 2000s, El Niño events after the 2000s decay more rapidly, and the associated tropical central-eastern Pacific SST features a La Niña-like condition in the El Niño decaying summer. Concomitantly, no significant SST anomalies appear over the WNP and IO in the ENSO decaying phase, exerting a weak influence on the WNPAC.

Relative to the WNP and IO SST anomalies, the C-mode, originating from the nonlinear interaction between ENSO and the annual cycle, has a stable relationship with the WNPAC during the El Niño decaying summer for both pre- and post-2000 periods. The feasibility of the mechanism appears invulnerable to changes in the ENSO evolution features (i.e., ENSO decaying speed), suggesting a crucial role of the C-mode in the maintenance of the WNPAC. Considering the persistence of the ENSO combination mode, the WNPAC and associated climate variability during the El Niño decaying summer can be skillfully predicted at least one season in advance based on the ENSO C-mode.

Important role of the ENSO combination mode in the maintenance of the anomalous anticyclone over the western North Pacific in bo
17-year running correlations between the winter (blue), spring (orange), summer (red) ENSO combination mode index and the summer WNPAC index during 1979–2020. The horizontal red dashed line indicates the 90% significance level for the correlation. Credit: Science China Press


Study reveals decadal variation of relationship between East Asian summer monsoon and El Nino


More information:
Wenjun Zhang et al, Important role of the ENSO combination mode in the maintenance of the anomalous anticyclone over the western North Pacific in boreal summer, Science China Earth Sciences (2022). DOI: 10.1007/s11430-021-9908-5

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Science China Press


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ENSO combination mode’s role in the maintenance of the anomalous anticyclone over North Pacific in boreal summer (2022, August 26)
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