Energetics and monsoon bifurcations

Seshadri, Ashwin K

doi:10.1007/s00382-016-3094-7

Cited by 7 publications

(15 citation statements)

References 27 publications

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“…As we noted earlier, Boos and Storelvmo (2016a) reported that a bifurcation was seen in their model only when a dominant term in the thermodynamic balance, the dry thermal stratification, was ignored. Consistent with this result is the study of Seshadri (2017), which pointed out that bifurcation arose in the simple models of (Levermann et al, 2009) owing to the neglect of the static stability of troposphere. Seshadri (2017) showed that dry thermal stratification provided a stabilizing effect in the energy balance, which exported dry static energy through a term that is linear in the wind-speed.…”

Section: Introductionmentioning

confidence: 55%

“…Consistent with this result is the study of Seshadri (2017), which pointed out that bifurcation arose in the simple models of (Levermann et al, 2009) owing to the neglect of the static stability of troposphere. Seshadri (2017) showed that dry thermal stratification provided a stabilizing effect in the energy balance, which exported dry static energy through a term that is linear in the wind-speed. In contrast, the stabilizing effect in the absence of this stratification is much weaker, relying on horizontal advection and thus depending quadratically on wind-speed.…”

Section: Introductionmentioning

confidence: 55%

“…The present understanding of this abrupt change in such models is that it is unphysical, being an artefact of neglecting dry thermal stratification (Boos and Storelvmo, 2016a, b;Seshadri, 2017). This stratification, arising from the increase in dry static stability with height, is the dominant contribution in the thermodynamic balance of monsoons.…”

Section: Introductionmentioning

confidence: 99%

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Origins and suppression of bifurcation phenomena in lower-order monsoon models

Seshadri

2022

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Abstract. South Asian monsoon rainfall varies rapidly in the paleoclimate record, and this has been interpreted using simple models as arising from tipping points. This study explores a class of simple monsoon models, based on convective quasi-equilibrium, and the bifurcations permitted by their mathematical forms. Specifically, low-order models are derived starting from the Quasi-equilibrium tropical circulation model (QTCM) to examine the bifurcations present. Previous studies that have pointed to an abrupt transition in low-order monsoon models typically identify a saddle node bifurcation occurring as a result of changes in the radiation budget. The present study shows how such saddle node structures arise across a wide range of modeling assumptions and parameter values, and yet permit a continuous transition into and out of precipitating regimes without any bifurcation being physically manifest. This is because the bifurcation points lie in a regime that is not physically relevant when the dry thermal stratification is sufficiently large. As a result, these low-order models can be interpreted as possessing abrupt transitions that are latent in the equations but do not express themselves physically. However, when the dry thermal stratification is reduced, bifurcations can occur. This paper also shows that these latent saddle-node structures are themselves part of the unfolding of a pitchfork bifurcation. These findings help understand the role of stabilizing phenomena on the general absence of abrupt monsoon transitions despite the presence of nonlinear terms in these models.

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Section: Introductionmentioning

confidence: 55%

Section: Introductionmentioning

confidence: 55%

Section: Introductionmentioning

confidence: 99%

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Origins and suppression of bifurcation phenomena in lower-order monsoon models

Seshadri

2022

Preprint

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“…Further analysis by Schewe et al (2012) suggested that the nonlinear transitions in monsoons could occur on account of a specific humidity threshold due to complex moisture advection feedbacks. Boos and Storelvmo (2016) and Seshadri (2017) criticized the simple model since it did not accurately represent the dominant balance of the dry static energy budget in tropical atmosphere. Boos and Storelvmo (2016) used a global model to show that the Asian monsoon responds linearly to wide variety of radiative forcing.…”

Section: Discussionmentioning

confidence: 99%

The Role of Nonlinear Drying above the Boundary Layer in the Mid-Holocene African Monsoon

et al. 2017

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Paleoclimatic proxies indicate that significant summertime rainfall reached the Sahara region during the mid-Holocene, presumably in response to stronger summertime heating in the Northern Hemisphere. Climate models generally do not replicate the enhanced precipitation. As a step toward understanding the response and possible role of model errors, a series of idealized experiments were conducted with the Community Earth System Model in which local atmospheric heat sources of increasing magnitude were applied in the boundary layer over the Sahel and Sahara. In response to this local heating, the cold and moist southwesterly monsoon inflow encroaches farther northward. A source strength of roughly 1 K day−1 produces responses similar to those in a simulation with mid-Holocene orbital forcing imposed globally, while that of 1.5 K day−1 produces a precipitation response similar to that from paleoproxies. The precipitation increases nonlinearly, with a jump at heating of around 1 K day−1, even though the low-level monsoon inflow increases linearly. Competition at low-to-middle levels between drying by a shallow return flow just above the boundary layer and moistening by vertical advection within the layer affects convection and determines the northward extension of precipitation. When the heating becomes 1.5 K day−1, the boundary layer flow encroaches sufficiently northward to weaken the shallow return flow, further aiding precipitation. This novel nonlinear mechanism operates without biogeophysical feedbacks, and suggests that poor representation of the local thermodynamic processes may hamper a model’s ability to simulate dynamical feedbacks and hence the strength and poleward extension of monsoon rains under forcings like those during the mid-Holocene.

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“…This uncertainty in future projections raises questions about potential mechanisms of change that may be present in some models but not others, and that may be responsible for the large differences between models. In particular, paleoclimatic records suggest that Sahel rainfall is capable of abrupt transitions in response to gradual forcing (DeMenocal et al, 2000;McGee et al, 2013); and theoretical studies have demonstrated that such a non-linear response can in principle arise from internal monsoon dynamics (Levermann et al, 2009;Seshadri, 2016). In this study, we examine Sahel rainfall in state-of-the art climate model simulations and show that in those models that exhibit the strongest rainfall increase, this increase is non-linearly related to the SST warming in the tropical North Atlantic and Mediterranean moisture source region.…”

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confidence: 99%

Non–linear intensification of Sahel rainfall as a dynamic response to future warming

Schewe¹,

Levermann²

2016

Preprint

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Abstract. Projections of the response of Sahel rainfall to future global warming diverge significantly. Meanwhile, paleoclimatic records suggest that Sahel rainfall is capable of abrupt transitions in response to gradual forcing. Here we present climate modeling evidence for the possibility of an abrupt intensification of Sahel rainfall under future climate change. Analyzing 30 coupled global climate model simulations, we identify seven models where central Sahel rainfall increases by 40 % to 300 % over the 21st century, owing to a northward expansion of the West African monsoon domain. Rainfall in these models is non–linearly related to sea surface temperature (SST) in the tropical Atlantic moisture source region, intensifying abruptly beyond a certain SST warming level. We argue that this behaviour is consistent with a self–amplifying dynamic–thermodynamical feedback, implying that the gradual increase in oceanic moisture availability under warming could trigger a sudden intensification of monsoon rainfall far inland of today's core monsoon region.

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Energetics and monsoon bifurcations

Abstract: monsoon over India. The default model with mean parameter estimates does not contain a bifurcation, but the model admits bifurcation as parameters are varied.

Cited by 7 publications

References 27 publications

Origins and suppression of bifurcation phenomena in lower-order monsoon models

Origins and suppression of bifurcation phenomena in lower-order monsoon models

The Role of Nonlinear Drying above the Boundary Layer in the Mid-Holocene African Monsoon

Non–linear intensification of Sahel rainfall as a dynamic response to future warming

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