Intraseasonal Variability of Sea Level in the Western North Pacific

Sea levels in the Western North Pacific (WNP) are presented with anomalous intraseasonal variations. This study examines the response of sea level in the WNP to the atmospheric Intraseasonal Oscillation modes, namely the Madden-Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation (BSISO), using the 25 years (1993-2017) of satellite altimetry and barotropic model output. In winter, the MJO has significant effects on the component of sea level due to the instant wind and atmospheric pressure effects (high-frequency), showing an eastward propagation pattern in most regions, with the strongest effects in the western marginal seas. The MJO-associated pattern of dynamical (low-frequency) component of sea level propagates southward, with the significant effects mostly in the tropics. In summer, the BSISO-associated pattern of the high-frequency component of sea level moves from southwest to northeast, with the largest anomalies in the middle of WNP (20 degrees N-30 degrees N), while the strongest BSISO effects on the low-frequency component are detectable mostly in the coasts of China and east of the Philippines. The MJO and BSISO can also modulate the probability of extreme sea level events. In winter, during phases 2-5, MJO increases the chance of extreme high events in the high-frequency component of sea level by 100%-200% in the western coasts and the tropics. In summer, in BSISO phases 6-7, the chance of extreme high events in the high-frequency component of sea level is enhanced by >300% in the South China Sea and east of the Philippines. Plain Language Summary The coastal regions of the Western North Pacific (WNP) are densely populated areas, which are exposed to tremendous oceanic hazards. The intraseasonal variability of sea level may alter the occurrence of extreme sea level events when superimposing on other factors under extreme conditions. Thus, understanding the variations of sea level in the WNP on intraseasonal timescales is helpful for marine disaster prevention and mitigation. The atmospheric Intraseasonal Oscillation (ISO) is one of the dominant modes of climate variability on intraseasonal timescales, including the Madden-Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation (BSISO). In this study, we investigate the relationship between the atmospheric ISO and WNP sea level over the satellite era (1993-2017) using observational data. Our analysis suggests that the atmospheric ISO significantly modulates the intraseasonal variations of WNP sea level both in winter and summer seasons, including the probability of extreme sea level events, through altering the convection and surface wind circulations associated with, or originated from, the atmospheric ISO. These findings imply the potential for developing a statistical approach to predict the intraseasonal variability of sea level extremes.

Automated summary

This study investigates the response of sea level in the Western North Pacific to atmospheric Intraseasonal Oscillation modes. The Madden-Julian Oscillation (MJO) and Boreal Summer Intraseasonal Oscillation (BSISO) have significant effects on sea level components in winter and summer, modulating the probability of extreme sea level events. The study highlights the importance of understanding intraseasonal variations of sea level for marine disaster prevention and mitigation.