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Marine heatwaves (MHWs) are extreme climatic events that can have a significant impact on marine ecosystems and their services across the world. We examine the spatiotemporal variation of summer MHWs in the north Indian Ocean (NIO) and find that the whole NIO Basin exhibits a pronounced spatial variability as well as a significant increasing trend in MHW frequency. We show that the NIO has two leading MHW modes linked to two distinct sea surface temperature (SST) patterns during summer. The first MHW mode is associated with basinwide warming, which is preconditioned by a decaying El Niño-Southern Oscillation (ENSO) and sustained throughout the summer by anomalous northeasterlies extending from the anticyclonic circulation of the western North Pacific subtropical high (WNPSH). The combined effect of thermocline warming due to downwelling oceanic planetary waves, decreased wind-induced evaporative cooling, and enhanced insolation cause basinwide summer MHWs. The second MHW mode exhibits a zonal dipole pattern, which has unfavorable cooling conditions in the previous seasons. The second MHW mode is associated with a phase change of ENSO and is greatly influenced by the formation of an interhemispheric pressure difference (IHPD) due to strengthening of the Australian high (AH) and weakening of the WNPSH. The IHPD induces cross-equatorial southerly winds across the eastern Indian Ocean. These winds favor the transformation of basinwide cooling conditions into zonal SST patterns via wind-evaporation-SST and thermocline-SST feedback, causing MHWs with a zonal dipole pattern. These MHW modes have a significant influence on the distribution and intensity of summer precipitation in the NIO. [ABSTRACT FROM AUTHOR]

The impact of El Nino –Southern Oscillation (ENSO) on the winter stratospheric polar vortex intensity (PVI) is re-examined, using JRA-55 datasets and the CESM2 model. Our results show that the negative correlation between ENSO and PVI has weakened in recent decades and is no longer statistically significant after the mid-1990s. This weakening ENSO–PVI relationship after the mid-1990s is associated with wave-1 fluxes entering the stratosphere; that is, the ENSO-induced increasing wave-1 fluxes are weaker after the mid-1990s than before the mid-1990s. Except for the Pacific–North America (PNA)-like wave train, the stratospheric wave flux changes are related to ENSO-induced geopotential height changes over East Asia. The ENSO-induced tropical zonal circulation anomalies are enhanced from the central Pacific to East Asia, leading to more substantial wave flux anomalies propagating westward from the central Pacific to East Asia and a more significant high center over Northeast Asia after the mid-1990s than before the mid-1990s. This ENSO-induced high center after the mid-1990s is out of phase with the climatological center of wave 1, leading to a relatively weaker increase in wave 1. Model results show that changes in the ENSO–PVI relationship and ENSO–Asia circulation are associated with changes in ENSO-related SST combined with global mean SST. [ABSTRACT FROM AUTHOR]