Dominant Processes for Dependence of Boreal Summer Intraseasonal Oscillation on El Niño Phases

Abstract

Boreal summer intraseasonal oscillation (BSISO) strongly interacts with background mean fields and tends to be stronger and longer in its northward propagation during La Niña than El Niño summers. However, BSISO dependence on El Niño‐Southern Oscillation phase has not been well understood. Here, we show that BSISO‐related convections are stronger and more organized with northward propagation on 30–60‐day timescales during El Niño developing (E‐DV) than decaying (E‐DC) summers over the western Pacific. The easterly vertical shear, responsible for the next vorticity generation north of the BSISO convection, in the E‐DV (E‐DC) remarkably increases (decreases) due to the enhanced (weakened) upper‐level easterly anomaly. The air‐sea interaction in E‐DV gets stronger due to the larger meridional gradient of sea surface temperature, amplifying northward propagation than in E‐DC.

Intraseasonal Variability of Surface Circulation in the Indo‐Pacific Warm Pool Induced by Summer Monsoon Intraseasonal Oscillations

“…On seasonal and interannual time scales, ocean currents can be significantly influenced by ENSO, IOD, and Indian Ocean basin‐wide warming by modulating equatorial winds and oceanic internal variations (Nyadjro & McPhaden, 2014; Sachidanandan et al., 2017; Wang, 2019). The multiscale variabilities in these currents can lead to heat convergence/divergence in the IPWP, which not only likely feedbacks to the MISO over the Pacific Ocean (Lee et al., 2022; Yang et al., 2020) but also has profound implications for the climate and local environment. For example, warm water volume in the western Pacific (WWV W ; the volume of water above the 20°C isotherm within 120°E−155°W, 5°S–5°N) is the main precursor of ENSO events and plays a crucial role in these events by propagating eastward and triggering sea–air interactions (Ballester et al., 2016; Izumo et al., 2019; Larson & Kirtman, 2019; Petrova et al., 2017; Planton et al., 2018).…”