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Kerry H. Cook

Abstract

GCM simulations are used to investigate how forcing that originates over land surfaces influences the Hadley circulation. The presence of continental surfaces is found to approximately double the intensity of the winter hemisphere Hadley cell and to halve the intensity of the summer cell. The strengthening of the winter cell occurs because the increase in surface friction associated with land enhances the angular momentum flux into the atmosphere. The development of strong monsoon circulations in the Northern Hemisphere summer and the convergence zones of the Southern Hemisphere (South Pacific, South Atlantic, and South Indian convergence zones) shifts mass out of the subtropics, lowers the zonal mean subtropical highs, and weakens the summer cells. The responses of the summer and winter cells are different signs and occur by different processes, because heating of the land surfaces in the summer is effectively communicated through the depth of the atmosphere, whereas cooling in the winter is not. Also, the higher surface wind speeds and shears of the winter hemisphere trade winds make the winter cell more sensitive to surface friction than the summer cell. These results suggest that the axisymmetric models that have provided a theoretical basis for the understanding of the Hadley circulation do not capture some of the important physical processes that determine the intensity of the mean meridional circulation.

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Kerry H. Cook

Abstract

Ensemble GCM simulations with an imposed, idealized warming of the eastern Pacific Ocean reveal two wave anomalies in the Southern Hemisphere, one in the eastern and one in the western hemisphere. Both are statistically significant at the 99% confidence level. Application of a steady-state linear model and a Rossby wave source analysis is used to diagnose the causes of the waves. The western hemisphere wave is forced by the advection and stretching of planetary vorticity by the divergent flow from the Southern Hemisphere component of the central Pacific “twin anticyclones” that straddle the equator during warm events. The eastern hemisphere wave is a result of the northeastward shift of the South Indian convergence zone (SICZ) that, in turn, is forced from the upper troposphere by convergence to the north. An upper-level convergence maximum over the equatorial Indian Ocean induces divergence to the south, encouraging vertical motion and precipitation to the northeast of the SICZ's normal position. The resulting anomalous upper-level convergence in the climatological position of the SICZ, as well as the anomalous vorticity flux convergence by the transients associated with an equatorward shift of the storm track behind the SICZ, force the eastern hemisphere Rossby wave.

Since a shift of the SICZ is a fairly robust observed consequence of ENSO events, these results suggest the mechanism by which drought conditions develop over southern Africa at the height of many warm events. Seasonal prediction capabilities in this region can be improved by monitoring and understanding the details and consequences of the adjustment of the Walker circulation near the equator outside of the Pacific Ocean basin.

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Kerry H. Cook

Abstract

Observations show a broad band of precipitation across northern Africa, with maxima evident in some analyses on either side of the continent. A low-resolution GCM with simple boundary conditions produces such a band and, by producing a double-maximum structure, suggests the operation of distinct mechanisms for generating rainfall in the east and west. The precipitation, moisture convergence, and low-level wind convergence anomalies are very similar, indicating that an understanding of the low-level dynamics is essential for understanding the precipitation perturbation over the land surface. A linear model analysis shows that the anomalous low-level convergence is primarily forced by condensational heating in the middle and upper troposphere over East Africa. Low-level condensation and dry convection are also important for driving convergence in the west.

Understanding the response of the low-level flow is key for understanding how inhomogeneity at the surface is communicated into the precipitation field. Midtropospheric condensational heating stretches vortex columns and induces a positive vorticity tendency in the lower troposphere. To establish a climatology, the low-level dynamics must adjust to balance this tendency in a way that maintains moisture convergence. The balance is accomplished by the meridional advection of low absolute vorticity air from the south and frictional effects.

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Kerry H. Cook

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 divergence between 600 and 800 mb overlies low-level convergence over West Africa to the south of the African easterly jet. This moisture divergence is important for determining the total column moisture convergence. Since the moisture divergence is closely tied to the jet dynamics, and the jet’s magnitude and position are sensitive to SST and land surface conditions, a mechanism by which the West African precipitation field is sensitive to surface conditions is suggested.

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Kerry H. Cook
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Kerry H. Cook

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 activity, and moisture convergence associated with moisture advection. This fact suggests that interactions between transient and stationary eddy features and between tropical and midlatitude disturbances are key to understanding variability of the SICZ. In a GCM ensemble simulation of the response to ENSO-like warming in the eastern Pacific, the SICZ shifts northeastward because of a weakening of the western portion of the South Indian high. This shift results in the dipole precipitation pattern, with higher precipitation to the northeast and lower precipitation to the southwest, that is observed in connection with drying over southern Africa during warm events.

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Kerry H. Cook

Abstract

The observed precipitation climatology in austral summer has a pronounced longitudinal gradient across Africa and South America. A low-resolution general circulation model (GCM) with a simple continent centered on the equator is used to understand how the presence of the land surface generates this gradient, and the role of surface wetness in determining its magnitude. In the model, precipitation is enhanced on the east coast of the continent in the summer hemisphere tropics with magnitude and location independent of surface wetness. Precipitation rates are lower in the continental interior and in the west as the surface becomes drier, resulting in a longitudinal precipitation gradient that is similar to observations.

Modeled low-level moisture convergence and wind convergence anomalies mimic the precipitation anomalies over all but the driest land surfaces. A linearized primitive equation model is used to identify the physical mechanisms responsible for the GCM's dynamical response. Dry convection and condensational heating force most of the anomalous convergence over the land surface in the GCM, with sensible heating and transient eddies playing more minor roles. At the latitude of the intertropical convergence zone (ITCZ), dry convection drives anomalous convergence at low levels, and this convergence is larger over drier (warmer) surfaces. Anomalous divergence develops in response to decreased condensational heating below 680 mb. The dependence on surface wetness arises because the relative strength of these opposing responses depends on the degree of warming over the land surface.

Low-level convergence over the eastern portion of the land surface in the model is forced by condensational heating in the middle and upper troposphere. Here, diabatic heating is balanced by adiabatic cooling, and the positive vertical velocities induce convergence below 830 mb by continuity. The magnitude of the response is largely independent of land surface drying and warming. The longitudinal precipitation gradient develops when even moderate surface drying affects precipitation in the continental interior and west, but not in the east.

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Jen-Shan Hsieh
and
Kerry H. Cook

Abstract

The relationship between African easterly waves and the background climatology in which they form is studied using a regional climate model. The surface and lateral boundary conditions in the model are manipulated to modify the background climatology, especially the African easterly jet and the ITCZ, and the behavior of the waves in these different settings is evaluated.

Three climate simulations are presented, with monthly mean lateral and surface boundary conditions. One has a strong jet and a strong ITCZ, the second has a strong jet and a weak ITCZ, and the third has a weak jet and a strong ITCZ. In these simulations, the presence of wave activity is more closely associated with the concentration of the ITCZ than the strength of the African easterly jet. In particular, the simulation with a strong jet accompanied by a weak ITCZ does not produce significant wave activity, but a weak jet with a strong ITCZ has realistic wave disturbances. Both the Charney–Stern and the Fjörtoft necessary conditions are satisfied in all three simulations, suggesting that combined barotropic and baroclinic instability contributes to the generation of waves. Near the origin of the waves, meridional gradient reversals of isentropic potential vorticity result from potential vorticity anomalies generated by convective heating within the ITCZ, implying that the unstable zonal flow may be caused by cumulus convection within the ITCZ and not by shear instability associated with the jet.

Two additional simulations with 1988 lateral boundary conditions demonstrate that 3–5-day wave disturbances can be generated in the absence of the African easterly jet, but with unrealistically small wavelengths. These results suggest that African easterly waves are initiated by cumulus convection within the ITCZ, and not by barotropic instability associated with the jet.

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Jen-Shan Hsieh
and
Kerry H. Cook

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 is strong. The eddy available potential energy consumed by the baroclinic overturning is compensated directly by the conversion of zonal to eddy available potential energy and the generation of eddy potential energy by diabatic heating. These direct conversions of latent heat and zonal available potential energy suggest that interactions across space scales, from convective space scales to the large scales, are important for generating African waves. The convectively induced barotropic instability may enhance baroclinic overturning through the resonance between these two instabilities. This leads to the nonlinear interaction of the waves with convection, corresponding to the formation of organized precipitation migrating with the waves.

A space–time spectral analysis shows that the dispersion characteristics of African easterly waves with wavelengths between 2650 and 4000 km do not follow the dispersion relation of the shallow water waves, indicating that these waves, similar to other easterly waves in the Tropics, possess significant nonlinearity, and cannot be fully explained by linear wave theory.

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Edward K. Vizy
and
Kerry H. Cook

Abstract

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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