Asymmetric Changes in Intraseasonal Oscillation Intensity over the Tropical Western North Pacific in El Niño and La Niña Developing Summers

Abstract

The intraseasonal oscillations (ISOs) over the tropical western North Pacific (WNP) modulate atmospheric convection and heating and affect weather and climate in remote regions through atmospheric teleconnection. Present study unravels that the ISO intensity increase over the tropical WNP in El Niño developing summers is larger, with the center located eastward, compared with the decrease in La Niña developing summers. The asymmetric ISO intensity changes are attributed to the eastward shift of regions of anomalous low-level westerly winds, ascent, easterly shear of zonal winds, and large moisture in El Niño developing summers and westward shift of regions of opposite anomalies in La Niña developing summers, respectively. The asymmetric atmospheric mean anomalies, in turn, are due to the westward shift of anomalous cooling in La Niña developing summers compared to anomalous warming in El Niño developing summers. The 10–20-day and 30–60-day ISOs show different patterns of intensity variations due to their different source regions and propagation paths. Atmospheric model simulations confirm the asymmetric response of boreal summer ISO intensity over the tropical WNP to El Niño and La Niña events and the role of asymmetric atmospheric background field changes. Sensitivity experiments with illustrate that asymmetric changes in the ISO intensity and atmospheric background fields over the tropical WNP are due to their asymmetric response to opposite tropical central-eastern Pacific SST anomalies. The asymmetry in tropical central-eastern Pacific SST anomalies in El Niño and La Niña events has a small impact on asymmetric ISO intensity changes over the tropical WNP.

Diverse Impacts of ENSO on the Intensity of Tropical Western North Pacific Synoptic‐Scale Disturbances During Boreal Summer

“…Hence, the effects of asymmetric SST anomalies during EN and LN years are relatively small on asymmetric SSD intensity changes over the TWNP. Wang et al (2022) reached a similar conclusion for the ISO intensity changes.…”

“…Based on both the evolution of Niño3.4 SST anomalies during the year and the magnitude of Niño3.4 SST anomalies in summer (larger than 0.5 standard deviation), 11 EN developing summers, 8 LN developing summers and 14 normal summers are selected during the analysis period (Table 1), which is the same as Wang et al. (2022). The normal summers include EN and LN decaying summers, but exclude ENSO persisting summers.…”

“…More details for asymmetric anomalous seasonal mean dynamic and thermodynamic fields were presented by Wang et al. (2022). We note that the horizontal structure displays a difference between the SSD intensity change and anomalous seasonal mean background fields.…”

“…As described by Wang et al. (2022), the differences of quantities between four sensitivity experiments and CLM‐SST run represent atmospheric response to anomalous tropical CEP SST forcing. In the following, the difference between CEP‐EN and CEP‐LNrev (or CEP‐ENrev and CEP‐LN) experiments is used to represent response of atmospheric mean fields and SSD intensity to asymmetric SST anomalies, and the difference between CEP‐EN and CEP‐ENrev (or CEP‐LN and CEP‐LNrev) experiments is used to represent asymmetric response of atmospheric mean fields and SSD intensity to completely opposite tropical CEP SST anomalies.…”

“…The asymmetric atmospheric response is associated with the longitudinal location of EN and LN induced anomalous heating and cooling (Wang et al., 2019, 2022). Previous studies have indicated that atmospheric deep convection occurs only when total SST reaches a certain threshold (e.g., 27ºC) (Graham & Barnett, 1987; Lau et al., 1997; Waliser & Graham, 1993; Zhang, 1993).…”