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Samson Hagos

Abstract

Rotated EOF analyses are used to study the composition and variability of large-scale tropical diabatic heating profiles estimated from eight field campaigns. The results show that the profiles are composed of a pair of building blocks. These are the stratiform heating with peak heating near 400 hPa and a cooling peak near 700 hPa and the convective heating with a heating maximum near 700 hPa. Variations in the contributions of these building blocks account for the evolution of the large-scale heating profile. Instantaneous top-heavy (bottom-heavy) large-scale heating profiles associated with excess of stratiform (convective) heating evolve toward a stationary mean profile due to exponential decay of the excess stratiform (convective) heating.

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Chidong Zhang and Samson M. Hagos

Abstract

Tropical diabatic heating profiles estimated using sounding data from eight field campaigns were diagnosed to document their common and prevailing structure and variability that are relevant to the large-scale circulation. The first two modes of a rotated empirical orthogonal function analysis—one deep, one shallow—explain 85% of the total variance of all data combined. These two modes were used to describe the heating evolution, which led to three composited heating profiles that are considered as prevailing large-scale heating structures. They are, respectively, shallow, bottom heavy (peak near 700 hPa); deep, middle heavy (peak near 400 hPa); and stratiform-like, top heavy (heating peak near 400 hPa and cooling peak near 700 hPa). The amplitudes and occurrence frequencies of the shallow, bottom-heavy heating profiles are comparable to those of the stratiform-like, top-heavy ones. The sequence of the most probable heating evolution is deep tropospheric cooling to bottom-heavy heating, to middle heavy heating, to stratiform-like heating, then back to deep tropospheric cooling. This heating transition appears to occur on different time scales. Each of the prevailing heating structures is interpreted as being composed of particular fractional populations of various types of precipitating cloud systems, which are viewed as the building blocks for the mean. A linear balanced model forced by the three prevailing heating profiles produces rich vertical structures in the circulation with multiple overturning cells, whose corresponding moisture convergence and surface wind fields are very sensitive to the heating structures.

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Samson Hagos and L. Ruby Leung

Abstract

A survey of tropical divergence from three GCMs, three global reanalyses, and four in situ soundings from field campaigns shows the existence of large uncertainties in the ubiquity of shallow divergent circulation as well as the depth and strength of the deep divergent circulation. More specifically, only two out of the three GCMs and three global reanalyses show significant shallow divergent circulation, which is present in all in situ soundings, and of the three GCMs and three global reanalyses, only two global reanalyses have deep divergence profiles that lie within the range of uncertainty of the soundings. The relationships of uncertainties in the shallow and deep divergent circulation to uncertainties in present-day and projected strength of the hydrological cycle from the GCMs are assessed. In the tropics and subtropics, deep divergent circulation is the largest contributor to moisture convergence that balances the net precipitation (precipitation minus evaporation), and intermodel differences in the present-day simulations carry over onto the future projections. In comparison to the soundings and reanalyses, the GCMs are found to have deeper and stronger divergent circulation. While these two characteristics of GCM divergence affect the strength of the hydrological cycle, they tend to compensate for each other so that their combined effect is relatively modest.

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Samson Hagos and L. Ruby Leung

Abstract

The moist thermodynamic processes that determine the time scale and energy of the Madden–Julian oscillation (MJO) are investigated using moisture and eddy available potential energy budget analyses on a cloud-resolving simulation. Two MJO episodes observed during the winter of 2007/08 are realistically simulated. During the inactive phase, moisture supplied by meridional moisture convergence and boundary layer diffusion generates shallow and congestus clouds that moisten the lower troposphere while horizontal mixing tends to dry it. As the lower troposphere is moistened, it becomes a source of moisture for the subsequent deep convection during the MJO active phase. As the active phase ends, the lower troposphere dries out primarily by condensation and horizontal divergence that dominates over the moisture supply by vertical transport. In the simulation, the characteristic time scales of convective vertical transport, mixing, and condensation of moisture in the midtroposphere are estimated to be about 2 days, 4 days, and 20 h respectively. The small differences among these time scales result in an effective time scale of MJO moistening of about 25 days, half the period of the simulated MJO. Furthermore, various cloud types have a destabilizing or damping effect on the amplitude of MJO temperature signals, depending on their characteristic latent heating profile and its temporal covariance with the temperature. The results are used to identify possible sources of the difficulties in simulating MJO in low-resolution models that rely on cumulus parameterizations.

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Samson Hagos, L. Ruby Leung, and Jimy Dudhia

Abstract

To identify the main thermodynamic processes that sustain the Madden–Julian oscillation (MJO), an eddy available potential energy budget analysis is performed on a regional model simulation with moisture constrained by observations. The model realistically simulates the two MJO episodes observed during the winter of 2007/08. Analysis of these two cases shows that instabilities and damping associated with variations in diabatic heating and energy transport work in concert to provide the MJO with its observed characteristics. The results are used to construct a simplified paradigm of MJO thermodynamics.

Furthermore, the effect of moisture nudging on the simulation is analyzed to identify the limitations of the model cumulus parameterization. Without moisture nudging, the parameterization fails to provide adequate low-level (upper level) moistening during the early (late) stage of the MJO active phase. The moistening plays a critical role in providing stratiform heating variability that is an important source of eddy available potential energy for the model MJO.

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Samson M. Hagos and Kerry H. Cook

Abstract

Previous studies show that the climatological precipitation over South America, particularly the Nordeste region, is influenced by the presence of the African continent. Here the influence of African topography and surface wetness on the Atlantic marine ITCZ (AMI) and South American precipitation are investigated.

Cross-equatorial flow over the Atlantic Ocean introduced by north–south asymmetry in surface conditions over Africa shifts the AMI in the direction of the flow. African topography, for example, introduces an anomalous high over the southern Atlantic Ocean and a low to the north. This results in a northward migration of the AMI and dry conditions over the Nordeste region.

The implications of this process on variability are then studied by analyzing the response of the AMI to soil moisture anomalies over tropical Africa. Northerly flow induced by equatorially asymmetric perturbations in soil moisture over northern tropical Africa shifts the AMI southward, increasing the climatological precipitation over northeastern South America. Flow associated with an equatorially symmetric perturbation in soil moisture, however, has a very weak cross-equatorial component and very weak influence on the AMI and South American precipitation. The sensitivity of the AMI to soil moisture perturbations over certain regions of Africa can possibly improve the skill of prediction.

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Samson M. Hagos and Kerry H. Cook

Abstract

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.

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Samson M. Hagos and Kerry H. Cook

Abstract

A regional ocean–atmosphere coupled model is developed for climate variability and change studies. The model allows dynamic and thermodynamic interactions between the atmospheric boundary layer and an ocean mixed layer with spatially and seasonally varying depth prescribed from observations. The model reproduces the West African monsoon circulation as well as aspects of observed seasonal SST variations in the tropical Atlantic. The model is used to identify various mechanisms that couple the West African monsoon circulation with eastern Atlantic SSTs. By reducing wind speeds and suppressing evaporation, the northward migration of the ITCZ off the west coast of Africa contributes to the modeled spring SST increases. During this period, the westerly monsoon flow is expanded farther westward and moisture transport on to the continent is enhanced. Near the end of the summer, upwelling associated with this enhanced westerly flow as well as the solar cycle lead to the seasonal cooling of the SSTs. Over the Gulf of Guinea, the acceleration of the southerly West African monsoon surface winds contributes to cooling of the Gulf of Guinea between April and July by increasing the entrainment of cool underlying water and enhancing evaporation.

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Gregory R. Foltz, Karthik Balaguru, and Samson Hagos

ABSTRACT

Sea surface temperature (SST) is one of the most important parameters for tropical cyclone (TC) intensification. Here, it is shown that the relationship between SST and TC intensification varies considerably from basin to basin, with SST explaining less than 4% of the variance in TC intensification rates in the Atlantic, 12% in the western North Pacific, and 23% in the eastern Pacific. Several factors are shown to be responsible for these interbasin differences. First, variability of SST along TCs’ tracks is lower in the Atlantic. This is due to smaller horizontal SST gradients in the Atlantic, compared to the Pacific, and stronger damping of prestorm SST’s contribution to TC intensification by the storm-induced cold SST wake in the Atlantic. The damping occurs because SST tends to vary in phase with TC-induced SST cooling: in the Gulf of Mexico and northwestern Atlantic, where SSTs are highest, TCs tend to be strongest and their translations slowest, resulting in the strongest storm-induced cooling. The tendency for TCs to be more intense over the warmest SST in the Atlantic also limits the usefulness of SST as a predictor since stronger storms are less likely to experience intensification. Finally, SST tends to vary out of phase with vertical wind shear and outflow temperature in the western Pacific. This strengthens the relationship between SST and TC intensification more in the western Pacific than in the eastern Pacific or Atlantic. Combined, these factors explain why prestorm SST is such a poor predictor of TC intensification in the Atlantic, compared to the eastern and western North Pacific.

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Samson M. Hagos and Kerry H. Cook

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. In general, the influence of SSTs on Sahel precipitation is related to their modulation of the easterly flow and the associated moisture transport. Precipitation anomalies are further enhanced by the circulation patterns associated with local convergence anomalies. These convergence anomalies and circulation patterns are sensitive to the scale and distribution of the SST anomalies and the moisture.

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