Regulation of atmospheric circulation controlling the tropical Pacific precipitation change in response to CO2 increases.

The spatial pattern of precipitation responses to CO2 concentration increases significantly influences global weather and climate variability by altering the location of tropical heating in a warmer climate. In this study, we analyze the Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model projections of tropical Pacific rainfall response to quadrupled increase of CO2. We found that the precipitation changes to the CO2 concentration increase cannot be interpreted by a weakening or strengthening of large-scale east-west coupling across the tropical Pacific basin, i.e., Walker circulation. By calculating the water vapor transport, we suggest instead that different responses of the Walker and Hadley circulations to the increasing CO2 concentration shape the details of the spatial pattern of precipitation in the tropical Pacific. Therefore, more regionally perturbed circulations over the tropical Pacific, which is influenced by the mean state change in the tropical Pacific and the enhanced precipitation outside the tropical Pacific, lead to greater increases in precipitation in the western equatorial Pacific as compared to the eastern tropical Pacific in a warmer climate.

Origin, Maintenance and Variability of the Asian Tropopause Aerosol Layer (ATAL): The Roles of Monsoon Dynamics.

Using NASA MERRA2 daily data, we investigated the origin, maintenance and variability of the Asian Tropopause Aerosol Layer (ATAL) in relation to variations of the Asia Monsoon Anticyclone (AMA) during the summer of 2008. During May-June, abundant quantities of carbon monoxide (CO), carbonaceous aerosols (CA) and dusts are found in the mid- and upper troposphere over India and China, arising from enhanced biomass burning emissions, as well as westerly transport from the Middle East deserts. During July-August, large quantities of dusts transported from the deserts are trapped and accumulate over the southern and eastern foothills of the Tibetan Plateau. Despite strong precipitation washout, ambient CO, CA and dust are lofted by orographically forced deep convection to great elevations, 12-16 km above sea level, via two key pathways over heavily polluted regions: a) the Himalayas-Gangetic Plain, and b) the Sichuan Basin. Upon entering the upper-troposphere-lower-stratosphere, the pollutants are capped by a stable layer near the tropopause, advected and dispersed by the anticyclonic circulation of AMA, forming the ATAL resembling a planetary-scale "double-stem chimney cloud". The development and variability of the ATAL are strongly linked to the seasonal march and intraseasonal (20-30 days and higher frequency) oscillations of the Asian monsoon.

Impact of snow-darkening by deposition of light-absorbing aerosols on snow cover in the Himalaya-Tibetan-Plateau and influence on the Asian Summer monsoon: A possible mechanism for the Blanford Hypothesis.

The impact of snow darkening by deposition of light absorbing aerosols (LAAs) on snow cover over the Himalaya-Tibetan-Plateau (HTP) and influence on the Asian summer monsoon are investigated using the NASA Goddard Earth Observing System Model Version 5 (GEOS-5). We find that during April-May-June, deposition of LAAs on snow leads to a reduction in surface albedo, initiating a sequence of feedback processes, starting with increased net surface solar radiation, rapid snowmelt in HTP and warming of the surface and upper troposphere, followed by enhanced low-level southwesterlies and increased dust loading over the Himalayas-Indo-Gangetic Plain. The warming is amplified by increased dust aerosol heating, and subsequently amplified by latent heating from enhanced precipitation over the Himalaya foothills and northern India, via the Elevated Heat Pump (EHP) effect during June-July-August. The reduced snow cover in the HTP anchors the enhanced heating over the Tibetan Plateau and its southern slopes, in conjunction with an enhancement of the Tibetan Anticyclone, and the development of an anomalous Rossby wavetrain over East Asia, leading to weakening of the subtropical westerly jet, and northward displacement and intensification of the Mei-Yu rainbelt. Our results suggest that atmosphere-land heating induced by LAAs, particularly desert dust play a fundamental role in physical processes underpinning the snow-monsoon relationship proposed by Blanford more than a century ago.

How light absorbing properties of organic aerosol modify the Asian summer monsoon rainfall?

Light absorbing aerosols not only contribute to Earth's radiative balance but also influence regional climate by cooling the surface and warming the atmosphere. Following recent suggestions that organic aerosols (OAs) absorb substantial amount of solar radiation, we examine the role of light absorbing properties of OA on Asian summer monsoon rainfall redistribution using observational data and an atmospheric general circulation model (AGCM) experiment. Results suggest that the enhanced light absorption by OA in Southeast Asia and Northeast Asia are associated with the advance of the Indian summer monsoon in May and the southward shift of East Asian summer monsoon rain band in June. The rainfall redistribution in May is induced by elevated orographic effect with a warm-core upper-level anticyclone and surface warming of 1-2°C over the Tibetan Plateau whereas that of the East Asian summer monsoon in June is formed by stable conditions associated with surface cooling and atmospheric warming around 30°N.

Competing influences of greenhouse warming and aerosols on Asian Summer Monsoon circulation and rainfall.

In this paper, we have compared and contrasted competing and amplifying influences on the global and regional drivers, circulation and rainfall responses of the Asian monsoon under global greenhouse warming (GHG) and aerosol forcing, based on CMIP5 historical simulations. Under GHG-only forcing, the land warms much faster than the ocean, magnifying the pre-industrial climatological land-ocean thermal contrast and hemispheric asymmetry, i.e., warmer northern than southern hemisphere. A steady increasing warm-ocean-warmer-land (WOWL) trend has been in effect since the 1950's substantially increasing moisture transport from adjacent oceans, and enhancing rainfall over the Asian monsoon regions. However, under GHG warming, increased atmospheric stability due to strong reduction in mid-tropospheric and near surface relative humidity coupled to an expanding subsidence areas, associated with the Deep Tropical Squeeze (DTS, Lau and Kim, 2015b) strongly suppress monsoon convection and rainfall over subtropical and extratropical land, leading to a weakening of the Asian monsoon meridional circulation. The inclusion of aerosol emissions strongly masks WOWL, by over 60% over the northern hemisphere, negating to a large extent the rainfall increase due to GHG warming, and leading to a further weakening of the monsoon circulation, through increasing atmospheric stability, most likely associated with aerosol solar dimming and semi-direct effects. Overall, we find that GHG exerts stronger positive rainfall sensitivity, but less negative circulation sensitivity in SASM compared to EASM. In contrast, aerosols exert stronger negative impacts on rainfall, but less negative impacts on circulation in EASM compared to SASM.

Impacts of aerosol-monsoon interaction on rainfall and circulation over Northern India and the Himalaya Foothills.

The boreal summer of 2008 was unusual for the Indian monsoon, featuring exceptional heavy loading of dust aerosols over the Arabian Sea and northern-central India, near normal all-India rainfall, but excessive heavy rain, causing disastrous flooding in the Northern Indian Himalaya Foothills (NIHF) regions, accompanied by persistent drought conditions in central and southern India. Using NASA Unified-physics Weather Research Forecast (NUWRF) model with fully interactive aerosol physics and dynamics, we carried out three sets of 7-day ensemble model forecast experiments: 1) control with no aerosol, 2) aerosol radiative effect only and 3) aerosol radiative and aerosol-cloud-microphysics effects, to study the impacts of aerosol-monsoon interactions on monsoon variability over the NIHF during the summer of 2008. Results show that aerosol-radiation interaction (ARI), i.e., dust aerosol transport, and dynamical feedback processes induced by aerosol-radiative heating, plays a key role in altering the large-scale monsoon circulation system, reflected by an increased north-south tropospheric temperature gradient, a northward shift of heavy monsoon rainfall, advancing the monsoon onset by 1-5 days over the HF, consistent with the EHP hypothesis (Lau et al. 2006). Additionally, we found that dust aerosols, via the semi-direct effect, increase atmospheric stability, and cause the dissipation of a developing monsoon onset cyclone over northeastern India/northern Bay of Bengal. Eventually, in a matter of several days, ARI transforms the developing monsoon cyclone into meso-scale convective cells along the HF slopes. Aerosol-Cloud-microphysics Interaction (ACI) further enhances the ARI effect in invigorating the deep convection cells and speeding up the transformation processes. Results indicate that even in short-term (up to weekly) numerical forecasting of monsoon circulation and rainfall, effects of aerosol-monsoon interaction can be substantial and cannot be ignored.

Changing circulation structure and precipitation characteristics in Asian monsoon regions: greenhouse warming vs. aerosol effects.

Using model outputs from CMIP5 historical integrations, we have investigated the relative roles of anthropogenic emissions of greenhouse gases (GHG) and aerosols in changing the characteristics of the large-scale circulation and rainfall in Asian summer monsoon (ASM) regions. Under GHG warming, a strong positive trend in low-level moist static energy (MSE) is found over ASM regions, associated with increasing large-scale land-sea thermal contrast from 1870's to present. During the same period, a mid-tropospheric convective barrier (MCB) due to widespread reduction in relative humidity in the mid- and lower troposphere is strengthening over the ASM regions, in conjunction with expanding areas of anomalous subsidence associated with the Deep Tropical Squeeze (DTS) [Lau and Kim, 2015]. The opposing effects of MSE and MCB lead to enhanced total ASM rainfall, but only a partial strengthening of the southern portion of the monsoon meridional circulation, coupled to anomalous multi-cellar overturning motions over ASM land. Including anthropogenic aerosol emissions strongly masks MSE but enhances MCB via increased stability in the lower troposphere, resulting in an overall weakened ASM circulation with suppressed rainfall. Rainfall characteristics analyses indicate that under GHG, overall precipitation efficiency over the ASM region is reduced, manifesting in less moderate but more extreme heavy rain events. Under combined effects of GHG and aerosols, precipitation efficiency is unchanged, with more moderate, but less extreme rainfall.

West African monsoon decadal variability and surface-related forcings: Second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II).

The second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II) is designed to improve understanding of the possible roles and feedbacks of sea surface temperature (SST), land use land cover change (LULCC), and aerosols forcings in the Sahel climate system at seasonal to decadal scales. The project's strategy is to apply prescribed observationally based anomaly forcing, i.e., "idealized but realistic" forcing, in simulations by climate models. The goal is to assess these forcings' effects in producing/amplifying seasonal and decadal climate variability in the Sahel between the 1950s and the 1980s, which is selected to characterize the great drought period of the last century. This is the first multi-model experiment specifically designed to simultaneously evaluate such relative contributions. The WAMME II models have consistently demonstrated that SST forcing is a major contributor to the 20th century Sahel drought. Under the influence of the maximum possible SST forcing, the ensemble mean of WAMME II models can produce up to 60% of the precipitation difference during the period. The present paper also addresses the role of SSTs in triggering and maintaining the Sahel drought. In this regard, the consensus of WAMME II models is that both Indian and Pacific Ocean SSTs greatly contributed to the drought, with the former producing an anomalous displacement of the Intertropical Convergence Zone (ITCZ) before the WAM onset, and the latter mainly contributes to the summer WAM drought. The WAMME II models also show that the impact of LULCC forcing on the Sahel climate system is weaker than that of SST forcing, but still of first order magnitude. According to the results, under LULCC forcing the ensemble mean of WAMME II models can produces about 40% of the precipitation difference between the 1980s and the 1950s. The role of land surface processes in responding to and amplifying the drought is also identified. The results suggest that catastrophic consequences are likely to occur in the regional Sahel climate when SST anomalies in individual ocean basins and in land conditions combine synergistically to favor drought.

Scale Dependence of Land-Atmosphere Interactions in Wet and Dry Regions as Simulated with NU-WRF over the Southwestern and South-Central United States.

Large-scale forcing and land-atmosphere interactions on precipitation are investigated with NASA-Unified WRF (NU-WRF) simulations during fast transitions of ENSO phases from spring to early summer of 2010 and 2011. The model is found to capture major precipitation episodes in the 3-month simulations without resorting to nudging. However, the mean intensity of the simulated precipitation is underestimated by 46% and 57% compared with the observations in dry and wet regions in the southwestern and south-central United States, respectively. Sensitivity studies show that large-scale atmospheric forcing plays a major role in producing regional precipitation. A methodology to account for moisture contributions to individual precipitation events, as well as total precipitation, is presented under the same moisture budget framework. The analysis shows that the relative contributions of local evaporation and large-scale moisture convergence depend on the dry/wet regions and are a function of temporal and spatial scales. While the ratio of local and large-scale moisture contributions vary with domain size and weather system, evaporation provides a major moisture source in the dry region and during light rain events, which leads to greater sensitivity to soil moisture in the dry region and during light rain events. The feedback of land surface processes to large-scale forcing is well simulated, as indicated by changes in atmospheric circulation and moisture convergence. Overall, the results reveal an asymmetrical response of precipitation events to soil moisture, with higher sensitivity under dry than wet conditions. Drier soil moisture tends to suppress further existing below-normal precipitation conditions via a positive soil moisture-land surface flux feedback that could worsen drought conditions in the southwestern United States.

Robust Hadley Circulation changes and increasing global dryness due to CO2 warming from CMIP5 model projections.

In this paper, we investigate changes in the Hadley Circulation (HC) and their connections to increased global dryness (suppressed rainfall and reduced tropospheric relative humidity) under CO2 warming from Coupled Model Intercomparison Project Phase 5 (CMIP5) model projections. We find a strengthening of the HC manifested in a "deep-tropics squeeze" (DTS), i.e., a deepening and narrowing of the convective zone, enhanced ascent, increased high clouds, suppressed low clouds, and a rise of the level of maximum meridional mass outflow in the upper troposphere (200-100 hPa) of the deep tropics. The DTS induces atmospheric moisture divergence and reduces tropospheric relative humidity in the tropics and subtropics, in conjunction with a widening of the subsiding branches of the HC, resulting in increased frequency of dry events in preferred geographic locations worldwide. Among various water-cycle parameters examined, global dryness is found to have the highest signal-to-noise ratio. Our results provide a physical basis for inferring that greenhouse warming is likely to contribute to the observed prolonged droughts worldwide in recent decades.

Possible mechanism of abrupt jump in winter surface air temperature in the late 1980s over the Northern Hemisphere.

Possible cause of an abrupt warming in winter mean surface air temperature in the midlatitudes of the Northern Hemisphere in the late 1980s is investigated using observation and reanalysis data. To determine the timing of abrupt warming, we use a regime shift index based on detection of the largest significant differences between the mean values of two contiguous periods. Results show that the abrupt warming occurred in association with a regime shift after the 1980's in which the zonal mean sea level pressure (SLP) is significantly increased (decreased) at the latitude 25-35°N (60-70°N), in the form of north-south dipole-like SLP anomaly spanning the subtropics and high latitude. The dipole SLP anomaly can be attributed to a northward expansion of Hadley cell, a poleward broadening and intensification of the Ferrel cell, coupled with a collapse of polar cell. During the abrupt warming, strong anomalous southerly warm advection at the surface was induced by an enhanced and expanded Ferrel circulation, in association with a northward and downward shift of maximum center of northward eddy heat flux over the midlatitudes. An intensification of polar jet subsequent to regime shift may be instrumental in sustaining the warming up to more than 5 years.

Native valve Bacillus cereus endocarditis in a non-intravenous-drug-abusing patient.

Bacillus cereus is a rare cause of endocarditis, typically associated with intravenous drug abuse, rheumatic heart disease, prosthetic heart valves, pacemakers, or immunodeficiency. We present the first case of native valve Bacillus cereus endocarditis with no apparent risk factors. The patient had a fulminant course requiring emergent valve replacement.

Randomized, open-label, comparative study of piperacillin-tazobactam administered by continuous infusion versus intermittent infusion for treatment of hospitalized patients with complicated intra-abdominal infection.

The purpose of this randomized, multicenter, open-label study was to compare the continuous infusion of piperacillin-tazobactam with the standard intermittent infusion in 262 hospitalized patients with complicated intra-abdominal infections. Within 1 day of surgical intervention, eligible patients were randomized (1:1) to piperacillin-tazobactam 12 g/1.5 g administered continuously over 24 h or 3 g/0.375 g administered over 30 min intermittently every 6 h for 4 to 14 days. The demographics of the patients in the groups were similar, with a median APACHE II score of 7 and a median length of hospitalization of 7 days. Among 167 clinically evaluable patients, 86.4% and 88.4% of the patients treated with the continuous infusion and the intermittent infusion, respectively, were clinically cured or improved at the test-of-cure visit (P = 0.817). Bacteriological success was observed in 83.9% and 87.9% of patients (P = 0.597) in the two groups, respectively, and no differences in bacteriological response by pathogen were noted. Defervesence and white blood cell count normalization occurred in the majority of patients within 3 days and were similar between patients receiving the continuous infusion and those receiving the intermittent infusion. Drug-related adverse events were generally mild and were reported in similar numbers of patients in each arm of the trial. The results of this study support continuous infusion as a safe and reasonable alternate mode of administration of piperacillin-tazobactam for the treatment of complicated intra-abdominal infection.

Telavancin versus standard therapy for treatment of complicated skin and soft-tissue infections due to gram-positive bacteria.

BACKGROUND: Telavancin, a novel lipoglycopeptide, exerts concentration-dependent, rapid bactericidal activity on account of its multiple mechanisms of action. Telavancin is highly active against gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-intermediate, and vancomycin-resistant strains. METHODS: We conducted a randomized, double-blind, controlled, phase-2 clinical trial. Patients > or = 18 years of age with a diagnosis of complicated skin and soft-tissue infection caused by suspected or confirmed gram-positive organisms were randomized to receive either intravenously administered telavancin once daily or standard therapy (antistaphylococcal penicillin 4 times daily or vancomycin twice daily). RESULTS: For the study, 167 patients were randomized and received at least 1 dose of study medication. Success rates were similar in all analysis populations at the test-of-cure evaluation. Of patients with S. aureus infection at baseline (n = 102), 80% of the telavancin group were cured and 77% of the standard therapy group were cured. For patients with MRSA infection at baseline (n = 48), cure rates were 82% for the telavancin group and 69% for the standard therapy group. Microbiologic eradication in patients with MRSA infection was 84% for the telavancin group versus 74% for the standard therapy group. MIC90 values were lower for telavancin in all tested strains of S. aureus (< or = 0.25 ug/mL) compared with the MIC90 values for vancomycin and oxacillin. Similar proportions of patients discontinued therapy for adverse events in both treatment groups (approximately 5%). Fewer serious adverse events were reported in the telavancin group (4 events) than were for the standard therapy group (9). CONCLUSION: Clinical and microbiological results of this study support the further development of telavancin, especially for treatment of infection due to MRSA.