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The increasing severity of droughts/floods and worsening air quality from increasing aerosols in Asia monsoon regions are the two gravest threats facing over 60% of the world population living in Asian monsoon regions. These dual threats have fueled a large body of research in the last decade on the roles of aerosols in impacting Asian monsoon weather and climate. This paper provides a comprehensive review of studies on Asian aerosols, monsoons, and their interactions. The Asian monsoon region is a primary source of emissions of diverse species of aerosols from both anthropogenic and natural origins. The distributions of aerosol loading are strongly influenced by distinct weather and climatic regimes, which are, in turn, modulated by aerosol effects. On a continental scale, aerosols reduce surface insolation and weaken the land-ocean thermal contrast, thus inhibiting the development of monsoons. Locally, aerosol radiative effects alter the thermodynamic stability and convective potential of the lower atmosphere leading to reduced temperatures, increased atmospheric stability, and weakened wind and atmospheric circulations. The atmospheric thermodynamic state, which determines the formation of clouds, convection, and precipitation, may also be altered by aerosols serving as cloud condensation nuclei or ice nuclei. Absorbing aerosols such as black carbon and desert dust in Asian monsoon regions may also induce dynamical feedback processes, leading to a strengthening of the early monsoon and affecting the subsequent evolution of the monsoon. Many mechanisms have been put forth regarding how aerosols modulate the amplitude, frequency, intensity, and phase of different monsoon climate variables. A wide range of theoretical, observational, and modeling findings on the Asian monsoon, aerosols, and their interactions are synthesized. A new paradigm is proposed on investigating aerosol-monsoon interactions, in which natural aerosols such as desert dust, black carbon from biomass burning, and biogenic aerosols from vegetation are considered integral components of an intrinsic aerosol-monsoon climate system, subject to external forcing of global warming, anthropogenic aerosols, and land use and change. Future research on aerosol-monsoon interactions calls for an integrated approach and international collaborations based on long-term sustained observations, process measurements, and improved models, as well as using observations to constrain model simulations and projections.

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The observed structure and seasonal evolution characteristics of the tropospheric biennial oscillation (TBO) in the warm ocean areas of the Indo-Pacific region are explored using a seasonal-sequence EOF analysis approach. The major convective activity centers associated with the TBO appear in the southeast Indian Ocean (SEIO) and western North Pacific (WNP), accompanied by anticyclonic (or cyclonic) circulation patterns with a first-baroclinic-mode structure. The convection and circulation anomalies have distinctive life cycles in the SEIO and WNP: the former have a peak phase in northern fall and the latter persist from northern winter to subsequent summer. The mechanisms of the TBO in this region are investigated with a hybrid coupled GCM. Numerical results show that air–sea interaction in the warm ocean alone can support TBO variability that has many observed characteristics. Key processes involved in the TBO include the WNP monsoon variability and associated cross-equatorial flows, convective activity over Southeast Asia/the Maritime Continent and associated anomalous Walker circulation, and ocean dynamic responses to anomalous wind stress curl in the western Pacific. The coupled model experiment demonstrates that the essential element of the TBO in this region arises from the monsoon–warm ocean interaction. A possible connection between the TBO and ENSO variability is further studied in another model that excludes the delayed oscillator dynamics. The key in causing the biennial variability of ENSO arises from teleconnections between the tropical Pacific and Indian Oceans, with three “atmospheric bridges”: 1) the north–south teleconnection that connects the WNP monsoon and the SEIO, 2) the east–west teleconnection that connects the Indian Ocean and the Pacific cold tongue, and 3) the El Niño–WNP monsoon teleconnection.

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