Remotely modulated tropical-North Pacific ocean–atmosphere interactions by the South Asian high

Abstract The tropical Pacific exhibits pronounced interannual fluctuations, which are predominantly characterized by the El Nino-Southern Oscillation (ENSO). Studies have demonstrated various atmospheric and oceanic circulations associated with ENSO. However, many aspects of how ENSO links the downstream Asian monsoon systems remain unclear. In this study, we aimed to improve understanding of these interactions and linkages and to explore underlying physical processes and mechanisms. Our results suggest that the summer South Asian high (SAH) appears to play a modulating role in the tropical-North Pacific large-scale ocean–atmosphere interactions and serves to transmit signals linking ENSO. When an El Nino develops in the central and eastern tropical Pacific in winter, an anomalous high appears over the midlatitudes of East Europe and West Asia, with locally less precipitation and a drier land surface from spring to summer. This helps retain the atmospheric signal excited by the winter ENSO until summer. The drier land surface increases tropospheric temperature at the midlatitudes of Eurasia during summer, favoring the strengthening of the summer SAH. The strengthened summer SAH emanates anomalous wave energy downstream along the westerly jet stream, which is manifested by the eastward-propagating high-frequency disturbances peaking at 6–9-day and quasi-two-week periods. This affects the extratropical North Pacific, where an intensified surface subtropical high occurs. Accordingly, the meridional wind anomalies occur away from the equator and the easterly wind anomalies occur over the tropical eastern Pacific. These changes in the atmospheric circulation then induce the development of a negative ENSO cycle. As a consequence, the North Pacific sea surface temperature decreases and the ENSO phase begins to change.

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