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{degree sign}N-30{degree sign}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.