Global energetics and local physics as drivers of past, present and future monsoons

Abstract

Global constraints on momentum and energy govern the structure of the zonal mean tropical circulation and rainfall. The continental-scale monsoon systems are also facets of a momentum-and energy-constrained global circulation, but their modern and paleo variability deviates substantially from that of the longitudinal mean through mechanisms neither fully understood nor well simulated. A framework grounded in global constraints yet encompassing the complexities of monsoon dynamics is needed to identify the causes of mismatch between theory, models, and observations and, ultimately, improve regional climate projection. In a first step towards this goal, disparate regional processes must be distilled into gross measures of energy flow in and out of continents and from the surface to the tropopause, so that monsoon dynamics may be coherently diagnosed across modern and paleo observations and across idealized and comprehensive simulations. Accounting for zonal asymmetries in the circulation, land/ocean differences in surface fluxes, and the character of convective systems, such a monsoon framework would integrate our understanding at all relevant scales: from the fine details of how moisture and energy are lifted in the updrafts of thunderclouds, up to the global circulations. Most tropical precipitation, whether steady rain or intense showers, falls from cloud clusters where individual, small-scale updrafts are organized over a few hundred kilometers in discrete weather systems. These, in turn, are orchestrated by planetary-scale circulation features: the monsoons and the Inter-Tropical Convergence Zone (ITCZ, Figure 1a and b). The clustering of individual clouds is the visible signature of an otherwise invisible global stirring. The notion that a link exists between smaller and larger scales underpins our understanding of tropical rain belt dynamics and is the basis of their representation in global climate models. Our understanding, however, remains incomplete, as evidenced by our inability to achieve reliable predictions of how the ITCZ and monsoons respond to external forcings.

Rainfall trends in the African Sahel: Characteristics, processes, and causes

“…The direct effect of GHGs is to wet the Sahel for all models (Biasutti, ; Gaetani et al, ). This is consistent with the interpretation of the monsoon as a facet of the energetically direct tropical circulation (Biasutti et al, ) and, mechanistically, with the warming over the Sahara and the strengthening of the monsoonal circulation (Biasutti, Sobel, & Camargo, ; Haarsma, Selten, Weber, & Kliphuis, ). The key aspect of the anomalous circulation is not the deepening of the heat low and an intensification of the inflow at low level, but the low's shift northward and the weakening of the dry intrusion by the return flow of the shallow meridional circulation (Gaetani et al, ; Shekhar & Boos, ).…”

Competing Topographic Mechanisms for the Summer Indo‐Asian Monsoon

“…This comparison allows us to answer the question of whether the Himalayas and the Iranian Plateau are necessary or sufficient conditions for the IAM. In the process, we answer the more general question of whether the insulation mechanism is important for causing the IAM as implied in various studies (Biasutti et al, 2018;Boos, 2015;Ma et al, 2014).…”

“…The basic core of our study is to understand how the large-scale Asian topographies such as the Himalayan Mountains, the Tibetan Plateau, and the Iranian Plateau impact monsoon onshore flow, redistribute the high-enthalpy monsoon air, and redirect the rainfall distribution. Along the way, we evaluate the three main hypotheses whether topographic thermal forcings He et al, 2018;Wu et al, 2007; or the insulation mechanism (Biasutti et al, 2018;Boos, 2015;Boos & Hurley, 2013;Ma et al, 2014), or mechanical and orographic forcing are necessary or sufficient conditions for generating a modern-like IAM. the models' biases and brings the regional circulation in better agreement with observations is necessary for understanding monsoon-topography interaction (Acosta & Huber, 2017;Johnson et al, 2015;Meehl et al, 2006;Roe et al, 2016;Shields et al, 2016).…”

Diversity of ENSO Events Unified by Convective Threshold Sea Surface Temperature: A Nonlinear ENSO Index

“…With its direct connection to convective heating, ELI reflects the emerging energetic perspective on climate dynamics, which is leading to a powerful unified understanding of diverse circulations patterns including the Intertropical Convergence Zone and monsoons (Biasutti et al, ; T. Schneider et al, ). The simplicity and physical basis of ELI makes it useful for operations and applied research, including the evaluation of coupled climate models.…”

“…With its direct connection to convective heating, ELI reflects the emerging energetic perspective on climate dynamics, which is leading to a powerful unified understanding of diverse circulations patterns including the Intertropical Convergence Zone and monsoons (Biasutti et al, 2018;T. Schneider et al, 2014).…”