Wind and Heat Forcings of the Seasonal and Interannual Sea Level Variabilities in the Southwest Pacific

Sea level variabilities in the southwest Pacific contribute to the variations of equatorial current bifurcation and the Indonesian Throughflow transport. These processes are closely related to the recharge/discharge of equatorial heat content and dynamic distribution of anthropogenic ocean heating over the Indo-Pacific basin, thus being of profound significance for climate variability and change. Here we identify the major features of seasonal and interannual sea level variabilities in this region, confirming the dominance of the first baroclinic mode in the tropics (contributing 60%-80% of the variances) and higher baroclinic modes in the extratropics (40%-60% of the seasonal variance). Seasonally, except in the western Coral Sea where the Ekman pumping is significant, the wind-driven first-mode baroclinic Rossby waves originating to the east of the date line control the sea level variations over most tropical Pacific regions. In the domain where the 1.5-layer reduced gravity model becomes deficient, the surface heat fluxes dominate, explaining ;40%-80% of sea level variance. For interannual variability, ;40%-60% of the variance are El Nino-Southern Oscillation (ENSO) related. The wind-driven Rossby and Kelvin waves east of the date line explain ;40%-78% of the interannual variance in the tropical Pacific. Outside the tropics, small-scale diffusive processes are presumed critical for interannual variability according to a thermodynamic analysis using an eddy-permitting ocean model simulation. Further process and predictive understandings can be achieved with the coupled climate models properly parameterizing the subgrid-scale processes.

Automated summary

Variations in sea level in the southwest Pacific play an important role in the bifurcation of equatorial currents and the Indonesian Throughflow transport, which are significant for climate variability and change. The study reveals that the first baroclinic mode dominates the sea level variations in the tropics, while higher baroclinic modes affect the extratropics. Seasonally, wind-driven first-mode baroclinic Rossby waves control the sea level variations in most tropical Pacific regions. El Nino-Southern Oscillation (ENSO) is found to be the main contributor to interannual variability. Wind-driven Rossby and Kelvin waves explain a significant portion of interannual variance in the tropical Pacific. In the non-tropical regions, small-scale diffusive processes are believed to be critical for interannual variability.