PDO Modulates Co-Occurring Summertime Marine Heatwaves in the Extratropical North Pacific and Atlantic

Marine heatwaves (MHWs), defined as extreme sea surface temperature (SST) anomalies, significantly impact marine ecosystems and regional climate. While past research has focused on the regional driver, this study reveals a novel co-occurrence phenomenon between the northwest Pacific (NWP) and northwest Atlantic (NWA) during summer seasons (JJAS) from 1982 to 2022. We find significant correlations in MHW occurrence and intensity across these regions using three independent SST data sets. Concurrent high-pressure anomalies, reducing cloud cover and enhancing net shortwave radiation, are identified as the key driver of this co-occurrence. The Pacific decadal oscillation (PDO) emerges as the large-scale modulator. During negative PDO phases, tropical Pacific cooling weakens the meridional temperature gradient and westerlies around 30°N, inducing anomalous easterlies and subtropical high pressure. The Coriolis effect steers these easterly anomalies southward, generating anomalous southerlies and upper tropospheric convergence, ultimately triggering strong mid-latitude subsidence. This co-occurrence of high pressure and subsidence leads to reduced cloud cover, and amplified net shortwave radiation, driving rapid SST warming and the observed co-occurring MHWs in the NWP and NWA. Our North Pacific pacemaker experiment successfully replicates these PDO-induced features, including anomalous easterlies, high-pressure systems, and vertical motions in the subtropics, providing strong support for the proposed mechanisms. This study emphasizes the significant role of large-scale atmospheric-ocean teleconnections in shaping basin-scale MHW co-occurrence.