Kerry H. Cook

Evaporation-Wind Feedback and Low-Frequency Variability in the Tropical Atmosphere

Abstract A mechanism by which feedback between zonal wind perturbations and evaporation can create unstable, low-frequency modes in a simple two-layer model of the tropical troposphere is presented. The modes resemble the 30–50 day oscillation. A series of general circulation model experiments designed to test the effect of suppressing this feedback on low-frequency variability in the model tropics is described. The results suggest that the evaporation-wind feedback can be important to the amplitude of the spectral peak corresponding to the 30–50 day oscillation in the model, but that the existence of the oscillation does not depend on it. The feedback is found to have a much more dramatic effect on low-frequency variability when sea surface temperatures are fixed than when the lower boundary is a zero heat capacity “swamp”.

Generation of the African Easterly Jet and Its Role in Determining West African Precipitation

Abstract An examination of analyses and model simulations is used to show that the African easterly jet forms over West Africa in summer as a result of strong meridional soil moisture gradients. In a series of GCM experiments, the imposition of realistic surface wetness contrasts between the Sahara and equatorial Africa leads to strong positive meridional temperature gradients at the surface and in the lower troposphere; the associated easterly shear in the atmosphere is strong enough to establish easterly flow—the African easterly jet—above the monsoon westerlies at the surface. Positive temperature gradients associated with the summertime distributions of solar radiation, SSTs, or clouds are not large enough to produce the easterly jet in the absence of soil moisture gradients. A thermally direct ageostrophic circulation is identified that can accelerate the largely geostrophic zonal flow and maintain the jet. While moisture converges throughout the lower troposphere over East Africa, moisture divergenc...

Coupled Model Simulations of the West African Monsoon System: Twentieth- and Twenty-First-Century Simulations

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 behav...

Mechanisms by Which Gulf of Guinea and Eastern North Atlantic Sea Surface Temperature Anomalies Can Influence African Rainfall

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% effec...

The South Indian Convergence Zone and Interannual Rainfall Variability over Southern Africa

Abstract The South Indian convergence zone (SICZ) is identified in this paper as a region of enhanced precipitation extending off the southeast coast of southern Africa during austral summer. Unlike the South Pacific convergence zone, the SICZ is a land-based convergence zone (LBCZ), with position and intensity at least partially determined by surface conditions over southern Africa. An idealized GCM simulation is used to explore the basic dynamics of LBCZs in the Tropics and subtropics. Output from a realistic GCM simulation and the National Centers for Environmental Prediction–National Center for Atmospheric Research 40-Year Reanalysis are analyzed to apply this basic dynamical framework to the case of the SICZ. In contrast to the intertropical convergence zone where column moisture convergence is primarily due to meridional wind convergence in the moist environment, precipitation within the SICZ and the LBCZs in general is also supported by zonal wind convergence, moisture convergence by transient eddy...

Dynamics of the west african monsoon jump

The observed abrupt latitudinal shift of maximum precipitation from the Guinean coast into the Sahel region in June, known as the West African monsoon jump, is studied using a regional climate model. Moisture, momentum, and energy budget analyses are used to better understand the physical processes that lead to the jump. Because of the distribution of albedo and surface moisture, a sensible heating maximum is in place over the Sahel region throughout the spring. In early May, this sensible heating drives a shallow meridional circulation and moisture convergence at the latitude of the sensible heating maximum, and this moisture is transported upward into the lower free troposphere where it diverges. During the second half of May, the supply of moisture from the boundary layer exceeds the divergence, resulting in a net supply of moisture and condensational heating into the lower troposphere. The resulting pressure gradient introduces an inertial instability, which abruptly shifts the midtropospheric meridional wind convergence maximum from the coast into the continental interior at the end of May. This in turn introduces a net total moisture convergence, net upward moisture flux and condensation in the upper troposphere, and an enhancement of precipitation in the continental interior through June. Because of the shift of the meridional convergence into the continent, condensation and precipitation along the coast gradually decline. The West African monsoon jump is an example of multiscale interaction in the climate system, in which an intraseasonal-scale event is triggered by the smooth seasonal evolution of SSTs and the solar forcing in the presence of land–sea contrast.

Ocean Warming and Late-Twentieth-Century Sahel Drought and Recovery

Abstract The influences of decadal Indian and Atlantic Ocean SST anomalies on late-twentieth-century Sahel precipitation variability are investigated. The results of this regional modeling study show that the primary causes of the 1980s Sahel drought are divergence and anomalous anticyclonic circulation, which are associated with Indian Ocean warming. The easterly branch of this circulation drives moisture away from the Sahel. By competing for the available moisture, concurrent tropical Atlantic Ocean warming enhanced the areal coverage of the drought. The modeled partial recovery of the precipitation in the 1990s simulations is mainly related to the warming of the northern tropical Atlantic Ocean and an associated cyclonic circulation that supplies the Sahel with moisture. Because of the changes in the scale and distribution of the forcing, the divergence associated with the continued Indian Ocean warming during the 1990s was located over the tropical Atlantic, contributing to the recovery over the Sahel...

Effects of Twenty-First-Century Climate Change on the Amazon Rain Forest

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.

Springtime Intensification of the Great Plains Low-Level Jet and Midwest Precipitation in GCM Simulations of the Twenty-First Century

Abstract Simulations from 18 coupled atmosphere–ocean GCMs are analyzed to predict changes in the climatological Great Plains low-level jet (GPLLJ) and Midwest U.S. hydrology resulting from greenhouse gas increases during the twenty-first century. To build confidence in the prediction, models are selected for analysis based on their twentieth-century simulations, and their simulations of the future are diagnosed to ensure that the response is reasonable. Confidence in the model projections is also bolstered by agreement among models, in a so-called multimodel ensemble, and by analogy with present-day interannual variability. The GCMs agree that the GPLLJ will be more intense in April, May, and June in the future. The selected models even agree on the reason for this intensification, namely, a westward extension and strengthening of the North Atlantic subtropical high (the Bermuda high) that occurs when greenhouse gas–induced warming over the continental United States exceeds that of the subtropical Atlant...

A Study of the Energetics of African Easterly Waves Using a Regional Climate Model

Abstract The evolution and spatial distribution of the energetics of African waves are studied. Complete eddy energy equations for an open system are derived for the computation of energy transformations during wave generation and dissipation. It is found that baroclinic overturning is the dominant energy source, although barotropic conversions can be almost equally important when there is concentrated moist convection south of the jet or shallow cumulus convection beneath the jet. The generation of active waves usually results from the nearly in-phase evolution of baroclinic and barotropic conversions, which are associated with significant rainfall over Africa. Significant barotropic instability associated with the horizontal shear is usually induced by concentrated deep convection on the southern flank of the jet. Barotropic conversions associated with the vertical wind shear may attain even greater magnitudes than that associated with the horizontal shear when shallow cumulus convection beneath the jet...