Weakening and Shifting of the Saharan Shallow Meridional Circulation During Wet Years of the West African Monsoon

The correlation between increased Sahel rainfall and reduced Saharan surface pressure is well established in observations and global climate models, and has been used to imply that increased Sahel rainfall is caused by a stronger shallow meridional circulation (SMC) over the Sahara. This study uses two atmospheric reanalyses to examine interannual variability of Sahel rainfall and the Saharan SMC, which consists of northward near-surface flow across the Sahel into the Sahara and southward flow near 700 hPa out of the Sahara. During wet Sahel years, the Saharan SMC shifts poleward, producing a drop in low-level geopotential and surface pressure over the Sahara. Statistically removing the effect of the poleward shift from the low-level geopotential eliminates significant correlations between this geopotential and Sahel precipitation. As the Saharan SMC shifts poleward, its mid-tropospheric divergent outflow decreases, indicating a weakening of its overturning mass flux. The poleward shift and weakening of the Saharan SMC during wet Sahel years is reproduced in an idealized model of West Africa; a wide range of imposed sea surface temperature and land surface albedo perturbations in this model produce a much larger range of SMC variations that nevertheless have similar quantitative associations with Sahel rainfall as in the reanalyses. These results disprove the idea that enhanced Sahel rainfall is caused by strengthening of the Saharan SMC. Instead, these results are consistent with the hypothesis that the a stronger SMC inhibits Sahel rainfall, perhaps by advecting mid-tropospheric warm and dry air into the precipitation maximum.Comment: Submitted to Journal of Climat

Sensitivity of the South Asian monsoon to elevated and non-elevated heating

Elevated heating by the Tibetan Plateau was long thought to drive the South Asian summer monsoon, but recent work showed this monsoon was largely unaffected by removal of the plateau in a climate model, provided the narrow orography of adjacent mountain ranges was preserved. There is debate about whether those mountain ranges generate a strong monsoon by insulating the thermal maximum from cold and dry extratropical air or by providing a source of elevated heating. Here we show that the strength of the monsoon in a climate model is more sensitive to changes in surface heat fluxes from non-elevated parts of India than it is to changes in heat fluxes from adjacent elevated terrain. This result is consistent with the hypothesis that orography creates a strong monsoon by serving as a thermal insulator, and suggests that monsoons respond most strongly to heat sources coincident with the thermal maximum.Engineering and Applied Science

The Effect of Moist Convection on the Tropospheric Response to Tropical and Subtropical Zonally Asymmetric Torques

Tropospheric winds can be altered by vertical transfers of momentum due to orographic gravity waves and convection. Previous work showed that, in dry models, such zonally asymmetric torques produce a pattern of tropical ascent that is well described by linear dynamics, together with meridional shifts of the midlatitude jet. Here a series of idealized models is used to understand the effects of moisture on the tropospheric response to tropical and subtropical zonally asymmetric, upper-tropospheric torques. The vertical motion response to a torque is shown to be amplified by the reduction in effective static stability that occurs in moist convecting atmospheres. This amplification occurs only in precipitating regions, and the magnitude of subsidence in nonprecipitating regions saturates when clear-sky radiative cooling balances induced adiabatic warming. For basic states in which precipitation is concentrated in an intertropical convergence zone (ITCZ), most of the vertical motion response is thus confined within the basic-state ITCZ, even when the torque is remote from the ITCZ. Tropical and subtropical torques perturb the extratropical baroclinic eddy field and the convectively coupled equatorial wave field. Resulting changes in momentum flux convergence by transient eddies induce secondary meridional overturning circulations that modify the zonal-mean response to a torque. The net effect allows tropical torques to merge a double ITCZ into a single equatorial ITCZ. The response of tropical transient eddies is highly sensitive to the representation of convection, so the zonal-mean response to a torque is similarly sensitive, even when the torque is located in the subtropics

Diapycnal mixing transience and the MOC

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002.Includes bibliographical references (leaves 27-28).Diapycnal mixing of ocean waters is crucial to the dynamics and associated heat transport of the meridional overturning circulation, yet uncertainty exists regarding the distribution and physical mechanisms of this mixing. This study uses a highly-idealized, single-hemisphere model of buoyancy-forced flow to examine the examine the effects of the transience of diapycnal mixing on the MOC. The strength of the MOC was found to be insensitive to mixing transience when mixing occurred uniformly on basin boundaries. For mixing that was highly localized in space, a ten-fold increase in transience, as compared with the time-invariant control, resulted in a decrease by about 20% of MOC mass and heat transport. The degree of sensitivity in the highly localized case is likely to be a strong function of the surface restoring timescale for temperature. The circulation dynamics associated with transient mixing displayed large-scale, complex oscillations that increased in amplitude with the transience of mixing. Attempts to quantify the relationship between mixing transience, MOC strength, and the power expended in mixing were inconclusive and merit further investigation.by William R. Boos.S.M

Wind-evaporation feedback, angular momentum conservation, and the abrupt onset of monsoons

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2008.Includes bibliographical references (p. 167-174).This thesis examines the mechanisms responsible for the abrupt onset of monsoon circulations, focusing on the role played by wind-evaporation feedback and its interaction with angular momentum conserving flow. The first half of the thesis examines the effect of wind induced surface heat exchange (WISHE) on nonlinear, axisymmetric Hadley circulations in a convective quasi-equilibrium framework. For thermal forcings localized off the equator, WISHE is found to reduce the critical forcing amplitude needed to produce angular momentum conserving flow. For forcings that are subcritical even with the effects of WISHE, the combination of WISHE and momentum advection is shown to nonlinearly enhance the circulation strength for all but the weakest forcings. These results hold for the time-dependent response to seasonally varying forcings: for forcings of intermediate strength, WISHE produces an abrupt onset of solsticial flow when only a linear response would otherwise occur, while for strong forcings WISHE shifts the abrupt onset to an earlier time in the seasonal cycle. The second half of the thesis examines the consistency of these idealized results with the onset of the South Asian monsoon in both observations and a detailed three-dimensional model. Observational composites of monsoon onset are consistent with a wind-evaporation feedback in that the increase in baroclinic flow during onset is accompanied by a large increase in surface enthalpy flux over the off-equatorial ocean. This increase in surface enthalpy flux is collocated with the peak increase in deep tropospheric ascent. Results from the three-dimensional numerical model were less conclusive in that this model did not successfully simulate an abrupt monsoon onset even with WISHE, although WISHE did strongly control the intensity and spatial structure of the model's mean summer circulation.(cont) In particular, a version of the model integrated without WISHE failed to produce a strong, angular momentum conserving monsoon circulation. Combined with the axisymmetric model results from the first half of the thesis, this suggests that the thermal forcing of the South Asian land mass may not be sufficiently strong in the absence of WISHE to produce angular momentum conserving monsoon flow.by William Ronald Boos.Ph.D

Weakening of the North American monsoon with global warming

Future changes in the North American monsoon, a circulation system that brings abundant summer rains to vast areas of the North American Southwest, could have significant consequences for regional water resources. How this monsoon will change with increasing greenhouse gases, however, remains unclear, not least because coarse horizontal resolution and systematic sea-surface temperature biases limit the reliability of its numerical model simulations. Here we investigate the monsoon response to increased atmospheric carbon dioxide (CO_2) concentrations using a 50-km-resolution global climate model which features a realistic representation of the monsoon climatology and its synoptic-scale variability. It is found that the monsoon response to CO_2 doubling is sensitive to sea-surface temperature biases. When minimizing these biases, the model projects a robust reduction in monsoonal precipitation over the southwestern United States, contrasting with previous multi-model assessments. Most of this precipitation decline can be attributed to increased atmospheric stability, and hence weakened convection, caused by uniform sea-surface warming. These results suggest improved adaptation measures, particularly water resource planning, will be required to cope with projected reductions in monsoon rainfall in the American Southwest

A genesis index for monsoon disturbances

Synoptic-scale monsoon disturbances produce the majority of continental rainfall in the monsoon regions of South Asia and Australia, yet there is little understanding of the conditions that foster development of these low pressure systems. Here a genesis index is used to associate monsoon disturbance genesis in a global domain with monthly mean, climatological environmental variables. This monsoon disturbance genesis index (MDGI) is based on four objectively selected variables: total column water vapor, low-level absolute vorticity, an approximate measure of convective available potential energy, and midtropospheric relative humidity. A Poisson regression is used to estimate the index coefficients. Unlike existing tropical cyclone genesis indices, the MDGI is defined over both land and ocean, consistent with the fact that monsoon disturbance genesis can occur over land. The index coefficients change little from their global values when estimated separately for the Asian–Australian monsoon region or the Indian monsoon region, suggesting that the conditions favorable for monsoon disturbance genesis, and perhaps the dynamics of genesis itself, are common across multiple monsoon regions. Vertical wind shear is found to be a useful predictor in some regional subdomains; although previous studies suggested that baroclinicity may foster monsoon disturbance genesis, here genesis frequency is shown to be reduced in regions of strong climatological vertical shear. The coefficients of the MDGI suggest that monsoon disturbance genesis is fostered by humid, convectively unstable environments that are rich in vorticity. Similarities with indices used to describe the distribution of tropical cyclone genesis are discussed

Histologic Correlates of Angiographic Chronic Total Coronary Artery Occlusions Influence of Occlusion Duration on Neovascular Channel Patterns and Intimal Plaque Composition

AbstractObjectives. Age-related changes in histologic composition and neovascular channel (NC) pattern of angiographic chronic total coronary artery occlusions (CTOs) were studied to define histologic correlates of age-related revascularization profiles and neovascular channel formation.Background. Revascularization of CTOs is frequently characterized by inability to cross or dilate the lesion and a high incidence of reocclusion or restenosis but low periprocedural ischemic complication rates. Little is known about the histopathologic basis of these observations.Methods. Ninety-six angiographic CTOs from autopsy studies in 61 patients who had undergone coronary angiography within 3 months of death were studied. Abrupt plaque rupture was excluded. Occlusion segments were analyzed for 1) histologic composition as a function of lesion age; and 2) NC pattern as a function of lesion age and intimal plaque (IP) composition.Results. Cholesterol and foam cell–laden IP was more frequent in younger lesions (p = 0.0007), whereas fibrocalcific IP increased with CTO age (p = 0.008). IP NCs arose directly from adventitial vasa vasorum and were anatomically and quantitatively related in terms of number and size (p = 0.0001) to the extent of IP cellular inflammation. IP cellular inflammation exceeded that found in the adventitia (p < 0.001) or media (p = 0.0001) across all CTO ages. In CTOs <1 year old, the adventitia was associated with a larger number and size of NCs relative to the IP (p = 0.0006 and p = 0.009), media (p = 0.0001 and p = 0.002) and recanalized lumen (p = 0.0001 and p = 0.001). In CTOs >1 year old, the adventitia and IP NC numbers were similar and exceeded NC numbers found in the media (p = 0.0001) and recanalized lumen (p = 0.0001 and p = 0.003).Conclusions. Angiographic CTO frequently corresponds to less than complete occlusion by histologic criteria. Age-related changes in IP composition from cholesterol laden to fibrocalcific may explain the adverse revascularization profile of older CTOs. IP NC growth derived from the adventitia increases with age and is strongly associated with IP cellular inflammation. IP NC formation may protect against the flow-limiting effects of IP growth.(J Am Coll Cardiol 1997;29:955–63)© 1997 by the American College of Cardiolog

Weakening of the North American monsoon with global warming

Future changes in the North American monsoon, a circulation system that brings abundant summer rains to vast areas of the North American Southwest, could have significant consequences for regional water resources. How this monsoon will change with increasing greenhouse gases, however, remains unclear, not least because coarse horizontal resolution and systematic sea-surface temperature biases limit the reliability of its numerical model simulations. Here we investigate the monsoon response to increased atmospheric carbon dioxide (CO_2) concentrations using a 50-km-resolution global climate model which features a realistic representation of the monsoon climatology and its synoptic-scale variability. It is found that the monsoon response to CO_2 doubling is sensitive to sea-surface temperature biases. When minimizing these biases, the model projects a robust reduction in monsoonal precipitation over the southwestern United States, contrasting with previous multi-model assessments. Most of this precipitation decline can be attributed to increased atmospheric stability, and hence weakened convection, caused by uniform sea-surface warming. These results suggest improved adaptation measures, particularly water resource planning, will be required to cope with projected reductions in monsoon rainfall in the American Southwest