Impact of tropical Atlantic warming on the Pacific Walker circulation with numerical experiments of CGCM

Abstract

The Pacific Walker circulation (PWC) was weak in the20th century, but its strength increased in an interdecadal scale in the late 1990s. Previous studies have suggested that it could be caused by the warming of the tropical Atlantic Ocean, or induced by the warming of the tropical Indian Ocean. The tropical Atlantic Ocean would not only directly affect the PWC through the equatorial east Pacific to the west, but also produce an indirect effect to the east through the equatorial west Indian Ocean. Using a coupled general circulation model, we designed a series of tropical Atlantic Heating and Heating_Shut experiments with different heating rates, to detect the mechanism of the impact of tropical Atlantic warming on the PWC. Results show that the tropical Atlantic heating weakens the Atlantic Walker circulation but strengthens the PWC. Diagnostics of multiple physical variables with coherent lower–upper troposphere structure show the responses of the Indian Ocean to the Atlantic heating play a critical role in the strengthening of the PWC. The Atlantic–linked atmosphere over the tropical Indian Ocean exerts a significantly positive heat flux onto the ocean there, greatly warming the tropical Indian Ocean, especially on the west part. This produces strong convectively ascending at the equatorial West Indian Ocean, but descending at the East-central Indian Ocean, corresponding to a ‘Walker’ circulation and an ‘anti–Walker’ circulation situated at the West and East equatorial Indian Ocean respectively. Meanwhile, the convergence (divergence) of the lower (upper) troposphere over the Indo‒Pacific region is also strengthened. In this way, the tropical Atlantic heating is linked to the PWC through the circulation over the equatorial Indian Ocean. This study serves as a preliminary step to understand the impact of tropical Atlantic warming on the PWC, more Atlantic heating sensitivity studies with multi-model experiments are required to further reveal the linkage of the Pacific and Atlantic.