Decadal and long-term variability of sea level in the Southwestern Pacific (SWP) is investigated with tide gauge measurements near New Zealand ranging from 1948 to 2018 and a 1.5-layer reduced gravity model. After removing the global mean sea level, SWP sea level still exhibits a prominent increasing trend accompanied by strong decadal oscillations, which is attributed to the Southern Annular Mode (SAM) and Interdecadal Pacific Oscillation (IPO), respectively. On the long-term time scale, the intensification of local wind stress curl associated with strengthening SAM contributes to the SWP sea level rise through Ekman convergence. On the decadal time scale, locally generated and westward propagating signals induced by IPO exert comparable influences on the sea level variations in the SWP. Multi-variable linear regression analysis suggests that SAM and IPO account for approximately 83% and 58% of the tide gauge sea level variability on long-term and decadal time scales, respectively.

Interannual variation of subthermocline eddy kinetic energy (EKE) east of the Philippines is investigated based on mooring measurements during 2015-2019 and ocean state estimates during 1995-2017 from the Oceanic General Circulation Model for the Earth Simulator (OFES). Prominent interannual variation of EKE is detected below the thermocline east of the Philippine coast, which is closely related to the El Niño and Southern Oscillation (ENSO) events and generally lags the Nino3.4 index by 14 months. Further energy diagnostic analysis indicates that the interannual variation of subthermocline EKE is controlled by both baroclinic and barotropic instability of the background flows and dominated by the barotropic instability especially. Barotropic instability in the southern part of the Philippine coast is associated with the subsurface component of the quasi-permanent anticyclonic eddy Halmahera Eddy (HE), while that in the northern part is closely related to the Mindanao Undercurrent (MUC). Both HE and MUC are modulated by the ENSO events. When El Niño occurs, negative sea surface height anomalies appear near the dateline and propagate westward in the form of the first mode baroclinic Rossby wave, exerting delayed impacts upon the western boundary currents east of the Philippine coast and further modulating the interannual variation of subthermocline EKE. Moreover, the barotropic energy conversion rate and its corresponding subthermocline EKE at lower latitudes responds relatively faster to ENSO due to the higher Rossby wave phase speed there.