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Kerry H. Cook
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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

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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

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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

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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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Kerry H. Cook

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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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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 and Anand Gnanadesikan

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A comprehensive rhomboidal-15 general circulation model with idealized boundary conditions is used to investigate the effects of interactions between the tropical circulation and continental climate on the precipitation distribution. Sea surface temperatures are fixed and zonally uniform and, along with the solar forcing, establish perpetual solstice conditions. Clouds are also prescribed and zonally uniform. Experiments with dry and saturated land surfaces an compared with an all-ocean control integration.

The winter hemisphere of the saturated continent is cooler than the prescribed ocean surface at the same latitude, and the summer hemisphere is warmer. When the surface is dry, the maximum summer hemisphere warming is four times larger than in the saturated surface case and extends into the winter hemisphere. The ITCZ is shifted farther into the summer hemisphere and enhanced near the coasts over the saturated continent, but it is interrupted in crossing the dry surface.

The modification of the precipitation distribution over the saturated land surface can be understood by considering the low-level flow. Over the dry surface, however, low-level horizontal moisture convergence and precipitation patterns are unrelated. The extreme dryness of the surface and the atmosphere below 830 mb eliminates condensation in the lower troposphere despite the increased instability of the tropical atmosphere. Condensation in the middle troposphere also decreases over the western half of the continent.

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Naresh Neupane and Kerry H. Cook

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The response over West Africa to uniform warming of the Atlantic Ocean is analyzed using idealized simulations with a regional climate model. With warming of 1 and 1.5 K, rainfall rates increase by 30%–50% over most of West Africa. With Atlantic warming of 2 K and higher, coastal precipitation increases but Sahel rainfall decreases substantially. This nonlinear response in Sahel rainfall is the focus of this analysis. Atlantic warming is accompanied by decreases in low-level geopotential heights in the Gulf of Guinea and in the large-scale meridional geopotential height gradient. This leads to easterly wind anomalies in the central Sahel. With Atlantic warming below 2 K, these easterly anomalies support moisture transport from the Gulf of Guinea and precipitation increases. With Atlantic warming over 2 K, the easterly anomalies reverse the westerly flow over the Sahel. The resulting dry air advection into the Sahel reduces precipitation. Increased low-level moisture provides moist static energy to initiate convection with Atlantic warming at 1.5 K and below, while decreased moisture and stable thermal profiles suppress convection with additional warming. In all simulations, the southerly monsoon flow onto the Guinean coast is maintained and precipitation in that region increases. The relevance of these results to the global warming problem is limited by the focus on Atlantic warming alone. However, confident prediction of climate change requires an understanding of the physical processes of change, and this paper contributes to that goal.

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Bing Pu and Kerry H. Cook

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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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