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    Progress in Earth and Planetary Science

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    Atmospheric and hydrospheric sciences


    Spatiotemporal vertical velocity variation in the western tropical Pacific and its relation to decadal ocean variability

    Nagano A, Hasegawa T, Wakita M

    Western tropical Pacific, ENSO, PDO, Vertical velocity, P-vector inverse method, AGEM

    Schematic representation of the relationship between upwelling intensity in the North Equatorial Current region and temperature anomalies at the sea surface (solid shades) and subsurface (dotted shades) layers for positive (left panel) and negative (right panel) PDO phases. Anomalously high (low) temperature regions are colored red (blue). Solid (dashed) arrows indicate the Kuroshio and Mindanao Current (subsurface subtropical gyre interior flow). Lower panel shows the relationship between PDO phases and temperature and its temporal tendency due to the intensity change of the upwelling.

    Prior to the cold phase of the Pacific decadal oscillation (PDO), sea surface water in the western tropical North Pacific is heated. To evaluate the impact of the subsurface water on the upper-ocean (<100 m depth) temperature variation, we examined the vertical velocity variation. We applied altimetry-based gravest empirical mode (AGEM) method to Argo hydrographic profile (2001–2017) and altimetric sea surface height (1993–2017) data in the western tropical Pacific and obtained a time-varying three-dimensional (3D) potential density field in the layer from 100 to 1000 m depth. From the AGEM-derived potential density, we derived a 3D field of vertical velocity below 100 m depth by the P-vector inverse method, which is based on potential density and vorticity conservations, and examined the spatiotemporal characteristics, focusing on the seasonal to quasi-decadal (QD) variations. In the mean state, substantial upward current (>10−6m\,s−1) is observed in the region of 10°–17°N, 140°E–180° in the North Equatorial Current, southern part of the North Pacific subtropical gyre. Possibly besides the Ekman upwelling, the upward current contributes to the cooling of the upper ocean in the western tropical North Pacific through the upward advection of the deep cold water. The upward current was found to be intensified (attenuated) by the strengthening (weakening) of the subtropical gyre interior southward flow on the QD timescale in association with frequent occurrences of La Niña (El Niño) events. The weakening of the upward current is responsible for the QD increase in upper-ocean temperature in the western tropical North Pacific, which is equivalent to or larger than 0.2 °C, several years after the warm phase of the PDO. The QD temperature variation may affect the phase reversal of the PDO through the heat transport of the western boundary current of the subtropical gyre, i.e., the Kuroshio.