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Kerry H. Cook and Edward K. Vizy

Abstract

Evaluation of three reanalyses (ERA-Interim, NCEP-2, and MERRA) and two observational datasets [CRU and Global Historical Climatology Network (GHCN)] for 1979–2012 demonstrates that the surface temperature of the Sahara Desert has increased at a rate that is 2–4 times greater than that of the tropical-mean temperature over the 34-yr time period. While the response to enhanced greenhouse gas forcing over most of the globe involves the full depth of the atmosphere, with increases in longwave back radiation increasing latent heat fluxes, the dryness of the Sahara surface precludes this response. Changes in the surface heat balance over the Sahara during the analysis period are primarily in the upward and downward longwave fluxes. As a result, the warming is concentrated near the surface, and a desert amplification of the warming occurs. The desert amplification is analogous to the polar amplification of the global warming signal, which is concentrated at the surface, in part, because of the vertical stability of the Arctic atmosphere. Accompanying the amplified surface warming of the Sahara is a strengthening of both the summertime heat low and the African easterly jet and a weakening of the wintertime anticyclone and the low-level Harmattan winds. Potential implications of the desert amplification include decreases in mineral dust aerosols globally, decreases in wintertime cold air surge activity, and increases in Sahel rainfall.

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Kerry H. Cook and Edward K. Vizy

Abstract

A regional atmospheric model with 60-km resolution is asynchronously coupled with a potential vegetation model to study the implications of twenty-first-century climate change for the tropical and subtropical climate and vegetation of South America. The coupled model produces an accurate simulation of the present day climate and vegetation. Future climate is simulated by increasing atmospheric CO2 levels to 757 ppmv and imposing lateral and surface boundary conditions derived from a GCM simulation for 2081–2100 from the Canadian Climate Center GCM.

The coupled regional model simulation projects a 70% reduction in the extent of the Amazon rain forest by the end of the twenty-first century and a large eastward expansion of the caatinga vegetation that is prominent in the Nordeste region of Brazil today. These changes in vegetation are related to reductions in annual mean rainfall and a modification of the seasonal cycle that are associated with a weakening of tropical circulation systems.

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Edward K. Vizy and Kerry H. Cook

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Changes in rainfall and temperature extremes are predicted by many global climate models as a response to greenhouse gas increases, and such changes will have significant environmental and social impacts. A regional climate model is used to predict changes in extremes across tropical and northern Africa for 2041–60 under a midline emissions forcing scenario. Six indicators are examined, including annual extreme and daily diurnal temperature ranges, heat wave days, number of dry days, number of extreme wet days, and extreme wet day rainfall intensity. Confidence in the projections is evaluated by examining the ensemble spread and the validation of extreme events in the twentieth-century simulation.

Despite an increase in both daily minimum and maximum temperatures, diurnal temperature ranges decrease from West Africa to Ethiopia during spring and fall, over the Sahel during summer, and over the Congo basin during winter and spring. Diurnal temperature ranges increase over the Horn of Africa during boreal winter and over Kenya and Tanzania during boreal summer. The number of heat wave days increases north of 8°N with the largest increase (60–120 days) over the western Sahel. The number of dry days decreases over the Congo and the central Sahel but increases over East Africa, the latter associated with a reduction in the springtime long rains. The number of extreme wet rainfall days is projected to increase over West Africa, the Sahel, and the Ethiopian Highlands but decrease over the Congo. The predicted changes in extreme wet rainfall intensity are highly regional.

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Kerry H. Cook and Edward K. Vizy

Abstract

The ability of coupled GCMs to correctly simulate the climatology and a prominent mode of variability of the West African monsoon is evaluated, and the results are used to make informed decisions about which models may be producing more reliable projections of future climate in this region. The integrations were made available by the Program for Climate Model Diagnosis and Intercomparison for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. The evaluation emphasizes the circulation characteristics that support the precipitation climatology, and the physical processes of a “rainfall dipole” variability mode that is often associated with dry conditions in the Sahel when SSTs in the Gulf of Guinea are anomalously warm.

Based on the quality of their twentieth-century simulations over West Africa in summer, three GCMs are chosen for analysis of the twenty-first century integrations under various assumptions about future greenhouse gas increases. Each of these models behaves differently in the twenty-first-century simulations. One model simulates severe drying across the Sahel in the later part of the twenty-first century, while another projects quite wet conditions throughout the twenty-first century. In the third model, warming in the Gulf of Guinea leads to more modest drying in the Sahel due to a doubling of the number of anomalously dry years by the end of the century. An evaluation of the physical processes that cause these climate changes, in the context of the understanding about how the system works in the twentieth century, suggests that the third model provides the most reasonable projection of the twenty-first-century climate.

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Kerry H. Cook and Edward K. Vizy

Abstract

A regional climate model with 90-km horizontal resolution on a large domain is used to predict and analyze precipitation changes over East Africa caused by greenhouse gas increases. A pair of six-member ensembles is used: one representing the late twentieth century and another the mid-twenty-first century under a midline emissions scenario. The twentieth-century simulation uses boundary conditions from reanalysis climatology, and these are modified for the mid-twenty-first-century simulation using output from coupled GCMs. The twentieth-century simulation reproduces the observed climate well. In eastern Ethiopia and Somalia, the boreal spring rains that begin in May are cut short in the mid-twenty-first-century simulation. The cause is an anomalous dry, anticyclonic flow that develops over the Arabian Peninsula and the northern Arabian Sea as mass shifts eastward near 20°N in response to strong warming over the Sahara. In Tanzania and southern Kenya, the boreal spring's long rains are reduced throughout the season in the future simulation. This is a secondary response to precipitation enhancement in the Congo basin. The boreal fall “short rains” season is lengthened in the twenty-first-century simulation in the southern Kenya and Tanzania region in association with a northeastward shift of the South Indian convergence zone.

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Edward K. Vizy and Kerry H. Cook

Abstract

Prior results indicate an amplified annual mean warming trend over the Sahara, with temperature trends that are 2–4 times that of the tropical mean rate. Trend analysis is conducted using five atmospheric reanalyses and three observational datasets to better understand the seasonality and physical processes of this amplified warming and the implications for Sahel precipitation. The seasonality of the amplified warming is maximum during July–October with a minimum during June. Two processes related to the amplified warming are identified. A “dry process” supports amplified warming over the Sahara when there is limited latent heating and/or evaporation to cool the surface and distribute heat to the atmosphere. In this mechanism, the warming results from changes in the upward longwave and downward longwave fluxes that are tightly coupled to each other. The second, termed a “wet process,” occurs during the summer West African monsoon season. In this mechanism there are increases in the low- and midlevel atmospheric moisture over the Sahara that add to the surface warming by increasing the longwave downward radiation. This additional atmospheric moisture is transported over the Sahara because of a strengthening of the thermal low/Saharan high circulation system. A positive feedback mechanism is discussed in which enhanced moisture transport due to the stronger Saharan warming leads to increased Sahel rainfall that further strengthens the meridional temperature and height gradients by cooling the Sahel surface, further enhancing moisture transport into the region. Both processes contribute to the amplified warming, with the amplification being greater during the summer.

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Kerry H. Cook and Edward K. Vizy

Abstract

The easterly Caribbean low-level jet (CLLJ) is a prominent climate feature over the Intra-America Seas, and it is associated with much of the water vapor transport from the tropical Atlantic into the Caribbean Basin. In this study, the North American Regional Reanalysis (NARR) is analyzed to improve the understanding of the dynamics of the CLLJ and its relationship to regional rainfall variations.

Horizontal momentum balances are examined to understand how jet variations on both diurnal and seasonal time scales are controlled. The jet is geostrophic to the first order. Its previously documented semidiurnal cycle (with minima at about 0400 and 1600 LT) is caused by semidiurnal cycling of the meridional geopotential height gradient in association with changes in the westward extension of the North Atlantic subtropical high (NASH). A diurnal cycle is superimposed, associated with a meridional land–sea breeze (solenoidal circulation) onto the north coast of South America, so that the weakest jet velocities occur at 1600 LT. The CLLJ is present throughout the year, and it is known to vary in strength semiannually. Peak magnitudes in July are related to the seasonal cycle of the NASH, and a second maximum in February is caused by heating over northern South America. From May through September, zonal geopotential gradients associated with summer heating over Central America and Mexico induce meridional flow. The CLLJ splits into two branches, including a southerly branch that connects with the Great Plains low-level jet (GPLLJ) bringing moisture into the central United States. During the rest of the year, the flow remains essentially zonal across the Caribbean Basin and into the Pacific.

A strong (weak) CLLJ is associated with reduced (enhanced) rainfall over the Caribbean Sea throughout the year in the NARR. The relationship with precipitation over land depends on the season. Despite the fact that the southerly branch of the CLLJ feeds into the meridional GPLLJ in May through September, variations in the CLLJ strength during these months do not impact U.S. precipitation, because the CLLJ strength is varying in response to regional-scale forcing and not to changes in the large-scale circulation. During the cool season, there are statistically significant correlations between the CLLJ index and rainfall over the United States. When the CLLJ is strong, there is anomalous northward moisture transport across the Gulf of Mexico into the central United States and pronounced rainfall increases over Louisiana and Texas. A weak jet is associated with anomalous westerly flow across the southern Caribbean region and significantly reduced rainfall over the south-central United States.

No connection between the intensity of the CLLJ and drought over the central United States is found. There are only three drought summers in the NARR period (1980, 1988, and 2006), and the CLLJ was extremely weak in 1988 but not in 1980 or 2006.

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Edward K. Vizy and Kerry H. Cook

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Two successive African easterly waves (AEWs) from August 2006 are analyzed utilizing observational data, the European Centre for Medium-Range Weather Forecasts reanalysis, and output from the National Center for Atmospheric Research–National Oceanic and Atmospheric Administration Weather Research and Forecasting model (WRF) to understand why the first wave does not develop over the eastern Atlantic while the second wave does. The first AEW eventually forms Hurricane Ernesto over the Caribbean Sea, but genesis does not occur over the eastern Atlantic. The second wave, although weaker than the first over land, leaves the West African coast and quickly intensifies into Tropical Storm Debby west of the Cape Verde islands. This study shows that the environmental conditions associated with the first AEW’s passage inhibited development. These conditions include strong low- and midtropospheric vertical wind shear owing to a stronger than normal African easterly jet, lower than normal relative humidity, and increased atmospheric stability. All of these are characteristics of an intensification of the Saharan air layer (SAL), or SAL outbreak, over the eastern Atlantic. The environmental conditions were more favorable for genesis 2½ days later when the second wave left the African coast. Additionally, a strong low-level southwesterly surge develops over the eastern North Atlantic in the wake of the passage of the first wave. This westerly surge is associated with an enhancement of the low-level westerly flow, low-level cyclonic vorticity, large-scale low-level wind convergence, and vertical motion conducive for development over the region. While the initial westerly surge is likely associated with the passage of the first wave, over time (i.e., by 1600 UTC 20 August 2006) the development of the second wave becomes influential in maintaining the low-level westerly surge. Although SAL outbreaks are also associated with the addition of dust, the different cyclogenesis histories of the two systems are simulated without including dust in the regional model.

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Edward K. Vizy and Kerry H. Cook

Abstract

The sensitivity of precipitation over West Africa to sea surface temperature anomalies (SSTAs) in the Gulf of Guinea and the eastern North Atlantic is studied using a GCM. Results from nine perpetual July simulations with various imposed SSTAs are presented and analyzed to reveal associations between the precipitation and SST fields via large-scale circulation and atmospheric moisture anomalies.

Rainfall increases over the Guinean Coast and decreases over the Congo basin when warm SSTAs are present in the Gulf of Guinea. These precipitation perturbations are related to the forcing of a Kelvin and a Rossby wave. The former is associated with a weakening of the Walker circulation, while the latter strengthens the West African monsoon. Rainfall over West Africa is less sensitive to cold SSTAs than to warm anomalies. Three contributing factors are identified as follows: 1) latitude of the SST forcing, 2) background flow, and 3) nonlinearity of the Clausius–Clapeyron equation (no more than a 20% effect). Despite the relative insensitivity to eastern North Atlantic SSTAs alone, a superposition of the individual responses to SSTAs is shown to be a poor predictor of the response to combined SSTAs, especially over central northern Africa.

A comparison of the modeled moisture budget anomalies to the difference between the summer seasons of 1988 and 1994 from the satellite observations and the NCEP reanalysis is conducted. While there may be many causes of precipitation differences between two particular years, the moisture budget anomalies are similar in that enhanced precipitation along the Guinean coast is supported mainly by low-level wind convergence from the south. The role of advection is also similar in the model and the reanalysis. However, the precipitation decrease over the Congo Basin that is associated with the Kelvin wave response to Gulf of Guinea SSTs in the model is not evident in the observations for these 2 yr.

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Edward K. Vizy, Kerry H. Cook, Julien Crétat, and Naresh Neupane

Abstract

Confident regional-scale climate change predictions for the Sahel are needed to support adaptation planning. State-of-the-art regional climate model (RCM) simulations at 90- and 30-km resolutions are run and analyzed along with output from five coupled atmosphere–ocean GCMs (AOGCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to predict how the Sahel summer surface temperature, precipitation, and surface moisture are likely to change at the mid- and late-twenty-first century due to increased atmospheric CO2 concentrations under the representative concentration pathway 8.5 (RCP8.5) emission scenario and evaluate confidence in such projections. Future lateral boundary conditions are derived from CMIP5 AOGCMs.

It is shown that the regional climate model can realistically simulate the current summer evolution of the West African monsoon climate including the onset and demise of the Sahel wet season, a necessary but not sufficient condition for confident prediction.

RCM and AOGCM projections indicate the likelihood for increased surface air temperatures over this century, with Sahara and Sahel temperature increases of 2–3.5 K by midcentury, and 3–6 K by late century. Summer rainfall and surface moisture are also projected to increase over most of the Sahel. This is primarily associated with an increase in rainfall intensity and not a lengthening of the wet season. Pinpointing exactly when the rainfall and surface moisture increase will first commence and by exactly what magnitude is less certain as these predictions appear to be model dependent. Models that simulate stronger warming over the Sahara are associated with larger and earlier rainfall increases over the Sahel due to an intensification of the low-level West African westerly jet, and vice versa.

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