My research interests are in atmospheric dynamics, with a special emphasis on understanding the coupling between larger-scale circulations and the hydrological cycle. I am particularly interested in tropical circulations, such as Hadley and monsoonal circulations, because of their intimate coupling with moist convection and rainfall, and the variety of scales they embody. I make use of observations and models of different complexity, and emphasize theoretically-based approaches that allow for conceptual undestanding. Some projects we are actively working on are listed below.

Idealized monsoons

A central, long-standing theme in my research is the exploration of elemental monsoons that can exist even in the absence of land-sea contrast and can be studied with GCMs in aquaplanet setup. This framework has provided insight into the dynamics of seasonal monsoon onset, the response of monsoons to radiative perturbations, such as those associated with orbital precession or greenhouse gas perturbations, and the dynamics of seasonally migrating convergence zones in a broad range of planetary rotation rates. Ongoing work is exploring the dynamics of monsoons over idealized continental configurations, such as zonally symmetric continents or zonally localized but hemispherically symmetric continents

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Dynamics of the East Asian monsoon

Unlike other monsoon systems, the East Asian monsoon reaches maturity in early summer and begins to dissipate in mid-summer. Previous work has argued for a critical role of the Tibetan Plateau (TP) in its existence and seasonality. We have explored this fundamental role by means of observations, numerical experiments and analyses of CMIP5 model output. Importantly, by using observational analyses of the moist static energy budget and by performing a decomposition of all terms into mean, and transient and stationary eddy fluxes, we have shown how stationary flux of moist enthalpy, and in particular, of dry enthalpy, sustains the front in a region of otherwise negative net energy input into the atmospheric column. Numerical simulations in which the TP is either retained or removed show that the TP influences the stationary enthalpy flux, and hence the EAM, primarily by changing the meridional stationary eddy velocity, with reinforced southerly wind over the region and northerly wind to its north, rather than by enhancing zonal temperature contrasts, as might be expected if the TP primarily acted as an elevated heat source.

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Variability of the South Asian monsoon

The theoretical understanding emerging from our more idealized studies motivates revaluation of important features of observed monsoons. One of these is the investigation of patterns of interannual variability of the South Asian monsoon (SAM). Our analyses reveal a number of important and not-yet-identified features, with implications for the SAM predictability: from a large-scale perspective, variations in monsoon precipitation are associated with dynamic changes (i.e., winds) rather than thermodynamics changes (i.e., moisture). Additionally, strong monsoon years are characterized by a poleward shift of the ascending branch of the monsoonal circulation and associated precipitation, a reduced (rather than increased, as the land-sea paradigm would suggest) lower-level meridional temperature gradient, and enhanced large-scale eddy fluxes of moist static energy in the southern hemisphere extratropics. It is also of interest to analyze seasonal transitions in the energy and momentum budgets at SAM onset/withdrawal. In order to do so, we recently introduced an objective index for the onset and withdrawal of the SAM, which is indicative of transitions in the large-scale SAM moisture budget and independent of arbitrarily selected thresholds.

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Coupling between the Hadley cell and regional climate feedbacks

Understanding how climate feedbacks, and associated uncertainties, interact with and influence the circulation response to warming is of primary importance for constraining regional projections. We have recently argued that the atmospheric energy budget provides a useful framework to explore this coupling, through the impact of regional climate feedbacks on the anomalous meridional energy gradient, and hence, the anomalous atmospheric energy transport. This framework has allowed us to quantify changes in Hadley cell strength in CMIP5 coupled simulations in terms of feedbacks, radiative forcing, ocean heat uptake, atmospheric eddies, and gross moist stability. The picture that emerges is one of complex and nonlocal interactions, emphasizing the need for more idealized simulations. For this purpose, we have performed a set of idealized experiments in an aquaplanet model to evaluate the coupling between the surface albedo feedback and other feedbacks and the impact on the Hadley circulation. Only half of the range of Hadley cell weakening exhibited in these experiments is found to be attributable to imposed, systematic variations in the surface albedo feedback. Resulting changes in extratropical clouds explain the remaining spread. These findings provide critical insight into the pathways by which the climate system, including tropical and extratropical circulations and the hydrological cycle, adjusts to high latitude feedbacks.

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Dynamics of the North American monsoon

The North American monsoon (NAM) is one of the smallest-scale and least understood monsoons in the Earth’s atmosphere, but essential for water resources over very arid regions of northwestern Mexico and the Southwestern United States. The NAM is impacted by the region complex geography in fundamental ways: intrusions of moist air masses from the tropical Pacific into the Gulf of California, which are known as gulf surges, are a dynamical mode essential for the observed mean monsoon. To what extent are these transients resolved in state-of-the art GCMs? In the first study to provide a detailed analysis of gulf surges in global coupled GCMs, we have addressed this question in a suite of global models developed at the Geophysical Fluid Dynamics Laboratory (CM2.1, FLOR, CM2.5, CM2.6, HiFLOR) with same physics but varying atmosphere and ocean resolution. One important result is that increasing atmospheric resolution greatly improves the representation of gulf surges and, accordingly, of the NAM climatology. In other words, coarser resolution GCMs, such as those participating to the IPCC assessment reports, do not allow for a faithful representation of the NAM and its variability. Hence, their future projections might be affected by significant biases.

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