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Bing Pu and Qinjian Jin

Abstract

High concentrations of dust can affect climate and human health, yet our understanding of extreme dust events is still limited. A record-breaking trans-Atlantic African dust plume occurred during June 14–28, 2020, greatly degrading air quality over large areas of the Caribbean Basin and U.S. Daily PM2.5 concentrations exceeded 50 μg m−3 in several Gulf States, while the air quality index reached unhealthy levels for sensitive groups in more than 11 States. The magnitude and duration of aerosol optical depth over the tropical North Atlantic Ocean were the greatest ever observed during summer over the past 18 years based on satellite retrievals. This extreme trans-Atlantic dust event is associated with both enhanced dust emissions over western North Africa and atmospheric circulation extremes that favor long-range dust transport. An exceptionally strong African easterly jet and associated wave activities export African dust across the Atlantic toward the Caribbean in the middle to lower troposphere, while a westward extension of the North Atlantic subtropical high and a greatly intensified Caribbean low-level jet further transport the descended, shallower dust plume from the Caribbean Basin into the U.S. Over western North Africa, increased dust emissions are associated with strongly enhanced surface winds over dust source regions and reduced vegetation coverage in the western Sahel. While there are large uncertainties associated with assessing future trends in African dust emissions, model-projected atmospheric circulation changes in a warmer future generally favor increased long-range transport of African dust to the Caribbean Basin and the U.S.

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Bing Pu and Robert E. Dickinson

Abstract

Diurnal variations of the Great Plains low-level jet (GPLLJ) and vertical motions have been related to the development of summer precipitation individually, but their underlying connection and consequences for the nocturnal and afternoon precipitation peaks are less discussed. This paper examines how together they help explain the spatial pattern of the frequency of summer convective precipitation over the Great Plains. A one-layer linearized boundary layer model is used to reproduce the diurnal cycle of the GPLLJ. Its periodic rising and sinking motions compare favorably with those of the North American Regional Reanalysis (NARR) climatology.

Its development of rising motion is also consistent with the enhanced occurrence of nocturnal convective precipitation over the central and eastern Great Plains (90°–100°W) and afternoon maximum over the western Great Plains (100°–105°W). The diurnal phasing of the vertical motions can be captured by the model only if the diurnal oscillation of the jet is forced by both near surface geopotential gradients and friction with observed diurnal variability.

The diurnal variation of the vertical velocity (or boundary layer convergence and divergence) is explained by local vorticity balance; that is, following the diurnal oscillation of the jet, the zonal gradient of the meridional wind oscillates and, thus, relative vorticity and its tendency. The slowing down of the jet after midnight decreases the anticyclonic (cyclonic) vorticity and consequently gives a positive (negative) vorticity tendency to the east (west) of the jet core; anomalous rising (sinking) motions occur to balance these positive (negative) vorticity tendencies. The pattern reverses when the jet is relatively weak.

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Bing Pu and Kerry H. Cook

Abstract

The West African westerly jet (WAWJ) is a low-level westerly jet located at 8°–11°N over the eastern Atlantic and the West African coast. It is clearly distinguished from the monsoon westerly flow by its structure and dynamics, and plays an important role in transporting moisture from the tropical eastern Atlantic to Sahelian West Africa during boreal summer.

The WAWJ develops in early June, sustains maximum wind speeds of 5–6 m s−1 from late July to early September, and weakens and dissipates by mid-October. In its mature stage, the WAWJ is located within the Atlantic ITCZ. It extends from the surface to 700 hPa, with maximum speed at 925 hPa. The jet has a weak semidiurnal cycle, with maxima at 0500 and 1700 local time.

A momentum budget analysis reveals that the WAWJ forms when a region of strong westerly acceleration is generated by the superposition of the Atlantic ITCZ and the westward extension of the continental thermal low. The WAWJ is supergeostrophic at its maximum, with zonal pressure gradient and Coriolis accelerations both pointing eastward. While much of the WAWJ’s seasonal variation can be explained by the geostrophic wind, the ageostrophic wind contributes more than 40% of the wind speed during the jet’s formation and demise.

The westward extension of the thermal low is associated with the formation of an offshore low, which is related to seasonal warming of the ocean between 6° and 18°N along the coast. The coastal SSTs vary in response to a net surface heating pattern with warming to the north and cooling to the south, which is mainly controlled by solar radiative and latent heat fluxes.

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Bing Pu and Kerry H. Cook

Abstract

The West African westerly jet is a low-level feature of the summer climatology that transports moisture from the eastern Atlantic onto the African continent at 8°–11°N. This study examines the relationship between the jet and Sahel precipitation variability in August, when both the jet and rainfall reach their seasonal maxima.

Variations of the West African westerly jet are significantly positively correlated with precipitation variations over the Sahel on both interannual and decadal time scales. Three periods are identified (1958–71, 1972–87, and 1988–2009), corresponding to times with a wet Sahel–strong jet, dry Sahel–weak jet, and relatively wet Sahel–strong jet. In wet (dry) periods, enhanced (decreased) westerly moisture fluxes associated with a strong (weak) jet increase (decrease) the low-level moisture content over the Sahel, decreasing (enhancing) the stability of the atmosphere. This association between the jet and Sahel rainfall is also found in case studies of 1964, 1984, 1999, and 2007.

The southerly moisture flux associated with the West African monsoon has less pronounced decadal variability than the westerly moisture flux of the West African westerly jet and weaker correlations with Sahel rainfall. When the monsoon flow is weak, for example, 1999 and 2007, the Sahel may still experience positive precipitation anomalies in association with strong westerly moisture transport by the jet.

The West African westerly jet is also important for stabilizing the regional vorticity balance by introducing strong relative vorticity gradients. Northward flow advects low relative vorticity south of the jet to balance positive vorticity tendencies generated by midtropospheric condensation.

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Bing Pu, Edward K. Vizy, and Kerry H. Cook

Abstract

Paleo-proxy and modeling evidence suggest that a shutdown of the Atlantic meridional overturning circulation (AMOC) would decrease North Atlantic Ocean sea surface temperatures and have far-reaching climate impacts. The authors use a regional climate model to examine the warm season response over North America to a hypothetical late-twenty-first-century shutdown of the AMOC with increased atmospheric CO2. In the future simulation, precipitation decreases over the western and central United States by up to 40% and over eastern Mexico by up to 50%. Over the eastern United States rainfall generally increases except during July. Variations in the moisture convergence associated with large-scale circulation changes dominate the rainfall variations, while evaporation plays a critical role over the northeastern United States in spring and the north-central United States in summer. During April–June the westward extension of the North Atlantic subtropical high enhances southwesterly moisture fluxes from the Gulf of Mexico into the eastern and south-central United States. Increases in low-level moisture content reduce the stability of the atmosphere. Enhanced southerly winds promote convergence over the eastern United States through the Sverdrup vorticity balance and precipitation increases. In July–August anomalous anticyclonic moisture fluxes associated with an anomalous high over the Gulf of Mexico and eastern Pacific decrease the moisture supply into the United States and Mexico. Over the central United States decreases in evaporation support decreases in low-level moisture content and increases in atmospheric stability. Over the eastern United States the Sverdrup balance weakens in summer and anomalous moisture convergence is mainly located over the East Coast.

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