Synergistic effects of the spring tropical Atlantic and Indian Ocean on early-summer Meiyu

Abstract

The early-summer Meiyu over the Yangtze River Basin (YRB) is jointly modulated by multiple factors, and the synergistic effect by the tropical Atlantic and Indian Oceans remains to be systematically revealed. By presenting the convergent empirical and modeling evidence, this study identifies three leading joint modes of the boreal spring tropical Atlantic and Indian Ocean (IO) sea surface temperature (SST)—the warm pool mode, the Atlantic dipole mode, and the Atlantic–IO gradient mode—that synergistically modulate Meiyu through distinct cross-seasonal and cross-basin pathways. The warm pool mode, often phase-locked with a preceding El Niño, acts as an important precondition sustaining the western North Pacific anomalous anticyclone (WNPAC) and leading to enhanced Meiyu. The Atlantic dipole mode emerges independently of ENSO but triggers a summertime developing La Niña, which enhances Meiyu over the lower YRB via triggering an eastward-displaced WNPAC. The Atlantic–IO gradient mode, coupled with a persistent La Niña, intensifies the Indian Summer Monsoon via cross-equatorial flows and upstream wave trains, subsequently exciting the Circumglobal Teleconnection (CGT) wave train. This induces a barotropic high-pressure anomaly over the YRB, resulting in a Meiyu deficit but more frequent heatwaves there. This work should help deepen the understanding of the complex causes of Meiyu and provide a reference for improving prediction capability.