Impact of Madden Julian Oscillation and boreal summer intraseasonal oscillation on the marine heat waves of East-Asian Marginal Seas

Recent intensification of the west Pacific warmpool has altered the western boundary currents and subtropical gyres, and turned East-Asian marginal seas (EAMS) into the most rapid warming ocean bodies on Earth. Along with the long-term increasing temperature, the EAMS often experience episodic events of incredibly high sea surface temperatures (SST), known as marine heatwaves (MHWs), which significantly adversely affect the vibrant marine ecosystems in these regions. Interannual climate forcing, such as the decaying El Niño Southern Oscillation (ENSO), also creates favourable conditions for MHWs in EAMS. However, in addition to the long-term trend and interannual background conditions, dominant intraseasonal variability in the tropics, such as Madden Julian Oscillation (MJO) in boreal winter and Boreal Summer Intraseasonal Oscillation (BSISO) in boreal summer, may influence the EAMS SST remotely. This study explores the pathways and mechanisms by which MJO and BSISO influence MHWs in the EAMS region. We show that specific phases of MJO (phases 2, 3, and 4) during boreal winter and summer (BSISO phases 5, 6, and 7) create favourable conditions for the occurrence of MHWs in these regions. Using Estimating the Circulation and Climate of the Ocean (ECCOv4) reanalysis datasets, we further separate the relative contribution of heat and salt changes by MJO and BSISO to the MHWs in EAMS, suggesting key factors that are vital in triggering MHWs in different areas of EAMS.