Bing Pu

Role of the West African Westerly Jet in Sahel Rainfall Variations

AbstractThe 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 200...

Dynamics of the West African Westerly Jet

The West African westerly jet (WAWJ) is a low-level westerly jet located at 88‐118N over the eastern AtlanticandtheWestAfricancoast.Itis clearlydistinguishedfromthemonsoonwesterlyflowbyitsstructure 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 21 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 generatedbythesuperpositionoftheAtlanticITCZandthewestwardextensionofthecontinentalthermallow. The WAWJ is supergeostrophic at its maximum, with zonal pressure gradient and Coriolis accelerations both pointingeastward.WhilemuchoftheWAWJ’sseasonalvariationcanbeexplainedbythegeostrophicwind,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 68 and 188N 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.

Diurnal Spatial Variability of Great Plains Summer Precipitation Related to the Dynamics of the Low-Level Jet

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 onelayer 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 (908‐1008W) and afternoon maximum over the western Great Plains (1008‐1058W). The diurnal phasing of the vertical motionscan 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.

Projection of American dustiness in the late 21 st century due to climate change

Climate models project rising drought risks over the southwestern and central U.S. in the twenty-first century due to increasing greenhouse gases. The projected drier regions largely overlay the major dust sources in the United States. However, whether dust activity in U.S. will increase in the future is not clear, due to the large uncertainty in dust modeling. This study found that changes of dust activity in the U.S. in the recent decade are largely associated with the variations of precipitation, soil bareness, and surface winds speed. Using multi-model output under the Representative Concentration Pathways 8.5 scenario, we project that climate change will increase dust activity in the southern Great Plains from spring to fall in the late half of the twenty-first century – largely due to reduced precipitation, enhanced land surface bareness, and increased surface wind speed. Over the northern Great Plains, less dusty days are expected in spring due to increased precipitation and reduced bareness. Given the large negative economic and societal consequences of severe dust storms, this study complements the multi-model projection on future dust variations and may help improve risk management and resource planning.

Why do summer droughts in the Southern Great Plains occur in some La Niña years but not others

Droughts in the Southern Great Plains (SGP) have been attributed to the cold phase of El Nino–Southern Oscillation or La Nina. While La Nina events have been clearly linked to winter droughts, their link to summer droughts has remained unclear. We analyze the difference in precipitation between dry and nondry summers over the SGP during La Nina years. Anomalously high geopotential height and subsidence over the SGP occur in spring, along with an intensified northward moisture flux that advects moisture away to the northern plains and Midwest. The dependence of SGP drought on La Nina is statistically significant only in winter and becomes insignificant in spring and summer. The drought development in La Nina years is related to an anomalous warming over the tropical North Atlantic in spring and an anomalous negative North Atlantic Oscillation (NAO) in summer, both of which suppress precipitation by strengthening the anomalous high over the SGP and displacing the subtropical jet streams. In years with relatively large precipitation anomalies (i.e., the 27% driest and wettest La Nina years over the SGP), up to 45% of the variances of summer precipitation can be explained by a linear combination of the Nino 3.4 sea surface temperature (SST), Pacific Decadal Oscillation (PDO), tropical North Atlantic SST, and NAO indices. An anomalously strong dry summer appears to be largely a result of a superposition of these factors.

Dynamical connection between Great Plains low‐level winds and variability of central Gulf States precipitation

The Great Plains low-level jet has been related to summer precipitation over the northern Great Plains and Midwest through its moisture transport and convergence at the jet exit area. Much less studied has been its negative relationship with precipitation over the southern Great Plains and the Gulf coastal area. This work shows that the southerly low-level winds at 30°–40°N over the southern Great Plains are significantly correlatedwith anticyclonic vorticity to its east over the central Gulf States (30°–35°N, 85°–95°W) fromMay to July. When the low-level jet is strong in June and July, anomalous anticyclonic vorticity over the central Gulf States leads to divergence and consequent subsidence suppressing precipitation over that region. In contrast, an enhanced southerly flow at the entrance region of the jet over the Gulf of Mexico, largely uncorrelated with the meridional wind over the southern Great Plains, is correlated with increased precipitation over the central Gulf States. Precipitation is large over the central Gulf States when themeridional wind over the southern Great Plains is weakest and over the Gulf of Mexico is strongest. This increase is consistent with the increased moisture transport and dynamic balance between loss of vorticity by advection and friction and gain by convergence.

Warm Season Response over North America to a Shutdown of the Atlantic Meridional Overturning Circulation and CO2 Increases

AbstractPaleo-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 t...