Abstract
Variability in surface currents is mainly induced by the time-varying wind forcing. Recent studies have revealed robust pronounced changes in the atmospheric circulation over the Pacific came with the Pacific Decadal Oscillation (PDO). However, the PDO is a climate index based on sea surface temperature (SST) variations, and may not be appropriately considered as a climate forcing. Here, we suggest the Atlantic SST variability is the ultimate forcing leading to changes in the atmospheric circulation and surface winds over the North Pacific. Anomalous warm North Atlantic and cold South Atlantic leads to weak Hadley cell in the Northern Hemisphere, resulting in a northward displacement of the ITCZ as well as a positive wind stress curl anomaly in the Pacific subtropical region, which would reduce the North Pacific subtropical gyre (NPSG). Associated with reduced sea surface height in the subtropics by the weakened NPSG, the North Equatorial Current (NEC) is weakened based on geostrophy. Changes in basin-scale winds further result in the southward migration of the tropical gyre and consequential downstream ocean circulation.
Highlights
Several studies based on model outputs and satellite altimeter measurements revealed a similar seasonality of the North Equatorial Current (NEC) bifurcation latitude (NECBL), migrating southward in summer and northward during wintertime[9,10]
The current study presents the transbasin influence of the Atlantic climate variability on the Pacific NEC bifurcation latitude (NECBL) fluctuation while Fig. 5 shows a schematic representation of the sequence among atmospheric/oceanic variabilities in the Atlantic and Pacific Oceans
In the mid-1990s, the North Atlantic undergoes a remarkable warming, which leads to weak Hadley cell in the Northern Hemisphere but strong Hadley cell in the Southern Hemisphere, resulting in a northward displacement of the Inter-tropical Convergence Zone (ITCZ) in the Atlantic and in the Pacific
Summary
Variability in surface currents is mainly induced by the time-varying wind forcing. Recent studies have revealed robust pronounced changes in the atmospheric circulation over the Pacific came with the Pacific Decadal Oscillation (PDO). Beyond the seasonal timescale which is mainly sun-driven, the time-varying ITCZ could be understood by a global energetic framework proposed by Frierson et al.[45] By both observations and model simulations, Frierson et al.[45] demonstrated that the meridional overturning circulation contributes significantly to the hemispheric asymmetry in tropical rainfall by transporting heat from the Southern Hemisphere to the Northern Hemisphere, and thereby pushing the tropical rain band north. The southward raised SSH implies the southward migration of the tropical gyre and the southward migrating trend of the NEC, which agrees with the previous studies[48,49] The southward migration of the NECBL will lead to enhance the Kuroshio transport east of Luzon, and the Kuroshio has tended to bypass the Luzon Strait without significant westward encroachment[49,50]
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