The Hydroclimate of East Africa: Seasonal cycle, Decadal Variability, and Human induced Climate Change

The hydroclimate of East Africa shows distinctive variabilities on seasonal to decadal time scales and poses a great challenge to climatologists attempting to project its response to anthropogenic emissions of greenhouse gases (GHGs). Increased frequency and intensity of droughts over East Africa in recent decades raise the question of whether the drying trend will continue into the future. To address this question, we first examine the decadal variability of the East African rainfall during March to May (MAM, the major rainy season in East Africa) and assess how well a series of models simulate the observed features. Observational results show that the drying trend during MAM is associated with decadal natural variability of sea surface temperature (SST) variations over the Pacific Ocean. The multimodel mean of the SST forced, Coupled Model Intercomparison Project Phase 5 (CMIP5) AMIP experiment models reproduces both the climatological annual cycle and the drying trend in recent decades. The fully coupled models from the CMIP5 historical experiment, however, have systematic errors in simulating the East African rainfall annual cycle by underestimating the MAM rainfall while overestimating the October to December (OND, the second rainy season in East Africa) rainfall. The multimodel mean of the historical coupled runs of the MAM rainfall anomalies, which is the best estimate of the radiatively forced change, shows a weak wetting trend associated with anthropogenic forcing. However, the SST anomaly pattern associated with the MAM rainfall has large discrepancies with the observations. The errors in simulating the East African hydroclimate with coupled models raise questions about how reliable model projections of future East African climate are. This motivates a fundamental study of why East African climate is the way it is and why coupled models get it wrong. East African hydroclimate is characterized by a dry annual mean climatology compared to other deep tropical land areas and a bimodal annual cycle with the major rainy season during MAM (often called the ``long rains'' by local people) and the second during OND (the ``short rains''). To explore these distinctive features, we use the ERA Interim Re Analysis data to analyze the associated annual cycles of atmospheric convective stability, circulation and moisture budget. The atmosphere over East Africa is found to be convectively stable, in general, year round but with an annual cycle dominated by the surface moist static energy (MSE), which is in phase with the precipitation annual cycle. Throughout the year, the atmospheric circulation is dominated by a pattern of convergence near the surface, divergence in the lower troposphere and convergence again at upper levels. Consistently, the convergence of the vertically integrated moisture flux is mostly negative across the year, but becomes weakly positive in the two rainy seasons. It is suggested the semi-arid/arid climate in East Africa and its bimodal rainfall annual cycle can be explained by the ventilation mechanism, in which the atmospheric convective stability over East Africa is controlled by the import of low MSE air from the relatively cool Indian Ocean off the coast and the cold winter hemisphere. During the rainy seasons, however, the off coast SST increases (and is warmest during the long rains season) and the northerly or southerly weakens, and consequently the air imported into East Africa becomes less stable. The MSE framework is then applied to study the coupling induced bias of the East African rainfall annual cycle often found in CMIP3/5 coupled models that overestimates the OND rainfall and underestimates the MAM rainfall, by comparing the historical (coupled) and the AMIP runs (SST forced) for each model. It is found that a warm north and cold south SST bias over the Indian Ocean induced in coupled models is responsible for the dry MAM rainfall bias over East Africa while the ocean dynamics induced warm west and cold east SST bias over the Indian Ocean contributes to the wet OND rainfall bias in East Africa. Finally, to understand the East African regional climate in the context of the broader tropical climate and circulation, zonal momentum balance of the tropical atmospheric circulation during the global monsoon mature months (January and July) are analyzed in three dimensions based on the ERA-Interim Re-Analysis. It is found that the dominant terms in the balance of the atmospheric boundary layer (ABL) in both months are the pressure gradient force, the Coriolis force and friction. The nonlinear advection term plays a significant role only in the Asian summer monsoon regions including off East Africa. In the upper troposphere, the pressure gradient force, the Coriolis force and nonlinear advection are the dominant terms. The transient eddy force and the residual force (which can be explained as convective momentum transfer over open oceans) are secondary yet can not be neglected near the equator. Zonal mean equatorial upper troposphere easterlies are maintained by the absolute angular momentum advection associated with the cross equatorial Hadley circulation. Equatorial upper troposphere easterlies over the Asian monsoon regions are also controlled by the absolute angular momentum advection but are mainly maintained by the pressure gradient force in January. The equivalent linear Rayleigh friction, which is widely applied in simple tropical models, is calculated and the corresponding spatial distribution of local coefficient and damping time scale are estimated from the linear regression. It is found that the linear momentum model is in general capable of crudely describing the tropical atmospheric circulation dynamics yet the caveat should be kept in mind that the friction coefficient is not uniformly distributed and is even negative in some regions

The East African Long Rains in Observations and Models

Decadal variability of the East African precipitation during the season of March–May (long rains) is examined and the performance of a series of models in simulating the observed features is assessed. Observational results show that the drying trend of the long rains is associated with decadal natural variability associated with sea surface temperature (SST) variations over the Pacific Ocean. Empirical orthogonal function (EOF), linear regression, and composite analyses all show the spatial pattern of the associated SST field to be La Niña like. The SST-forced International Research Institute for Climate and Society (IRI) forecast models are able to capture the East African precipitation climatology, the decadal variability of the long rains, and the associated SST anomaly pattern but are not consistent with observations from the 1970s. The multimodel mean of the SST-forced models from the Coupled Model Intercomparison Project phase 5 (CMIP5) Atmospheric Model Intercomparison Project (AMIP) experiment captures the climatology and the drying trend in recent decades. The fully coupled models from the CMIP5 historical experiment, however, have systematic errors in simulating the East African precipitation climatology by underestimating the long rains while overestimating the short rains. The multimodel mean of the historical simulations of the long rains anomalies, which is the best estimate of the radiatively forced change, shows a weak wetting trend associated with anthropogenic forcing. The SST anomaly pattern associated with the long rains has large discrepancies with the observations. The results herein suggest caution in projections of East African precipitation from CMIP5 or the relationship between the East African precipitation and the SST spatial pattern found in paleoclimate studies with coupled climate models

Nonperturbative photon qqˉq\bar{q} light front wave functions from a contact interaction model

We propose a method to calculate the $q\bar{q}$ light front wave functions (LFWFs) of photon at low-virtuality, i.e., the light front amplitude of $\gamma^*\rightarrow q\bar{q}$ at low $Q^2$, based on a light front projection approach. We exemplify this method using a contact interaction model within Dyson-Schwinger equations formalism and obtain the nonperturbative photon $q\bar{q}$ LFWFs. In this case, we find the nonperturbative effects are encoded in the enhanced quark mass and a dressing function of covariant quark-photon vertex, as compared to the leading order quantum electrodynamics photon $q\bar{q}$ LFWFs. We then use nonperturbative-effect modified photon $q\bar{q}$ LFWFs to study the inclusive deep inelastic scattering HERA data in the framework of the color dipole model. The results demonstrate that the theoretical description of data at low $Q^2$ can be significantly improved once the nonperturbative corrections are included in the photon LFWFs.Comment: 11 pages, 4 figure

Epidemic dynamics of respiratory syncytial virus in current and future climates.

A key question for infectious disease dynamics is the impact of the climate on future burden. Here, we evaluate the climate drivers of respiratory syncytial virus (RSV), an important determinant of disease in young children. We combine a dataset of county-level observations from the US with state-level observations from Mexico, spanning much of the global range of climatological conditions. Using a combination of nonlinear epidemic models with statistical techniques, we find consistent patterns of climate drivers at a continental scale explaining latitudinal differences in the dynamics and timing of local epidemics. Strikingly, estimated effects of precipitation and humidity on transmission mirror prior results for influenza. We couple our model with projections for future climate, to show that temperature-driven increases to humidity may lead to a northward shift in the dynamic patterns observed and that the likelihood of severe outbreaks of RSV hinges on projections for extreme rainfall

The Annual Cycle of East African Precipitation

East African precipitation is characterized by a dry annual mean climatology compared to other deep tropical land areas and a bimodal annual cycle with the major rainy season during March–May (MAM; often called the ‘‘long rains’’) and the second during October–December (OND; often called the ‘‘short rains’’). To explore these distinctive features, ERA-Interim data are used to analyze the associated annual cycles of atmospheric convective stability, circulation, and moisture budget. The atmosphere over East Africa is found to be convectively stable in general year-round but with an annual cycle dominated by the surface moist static energy (MSE), which is in phase with the precipitation annual cycle. Throughout the year, the atmospheric circulation is dominated by a pattern of convergence near the surface, divergence in the lower troposphere, and convergence again at upper levels. Consistently, the convergence of the vertically integrated moisture flux is mostly negative across the year, but becomes weakly positive in the two rainy seasons. It is suggested that the semiarid/arid climate in East Africa and its bimodal precipitation annual cycle can be explained by the ventilation mechanism, in which the atmospheric convective stability over East Africa is controlled by the import of low MSE air from the relatively cool Indian Ocean off the coast. During the rainy seasons, however, the off-coast sea surface temperature (SST) increases (and is warmest during the long rains season) and consequently the air imported into East Africa becomes less stable. This analysis may be used to aid in understanding overestimates of the East African short rains commonly found in coupled models

The Rainfall Annual Cycle Bias over East Africa in CMIP5 Coupled Climate Models

East Africa has two rainy seasons: the long rains [March–May (MAM)] and the short rains [October– December (OND)]. Most CMIP3/5 coupled models overestimate the short rains while underestimating the long rains. In this study, the East African rainfall bias is investigated by comparing the coupled historical simulations from CMIP5 to the corresponding SST-forced AMIP simulations. Much of the investigation is focused on the MRI-CGCM3 model, which successfully reproduces the observed rainfall annual cycle in East Africa in the AMIP experiment but its coupled historical simulation has a similar but stronger bias as the coupled multimodel mean. The historical–AMIP monthly climatology rainfall bias in East Africa can be explained by the bias in the convective instability (CI), which is dominated by the near-surface moisture static energy (MSE) and ultimately by the MSE’s moisture component. The near-surface MSE bias is modulated by the sea surface temperature (SST) over the western Indian Ocean. The warm SST bias in OND can be explained by both insufficient ocean dynamical cooling and latent flux, while the insufficient shortwave ra- diation and excess latent heat flux mainly contribute to the cool SST bias in MAM

Alkali burn induced corneal spontaneous pain and activated neuropathic pain matrix in the central nerve system in mice

Purpose: To explore whether alkali burn causes corneal neuropathic pain and activates neuropathic pain matrix in the central nerve system in mice. Methods: A corneal alkali burn mouse model (grade II) was used. Mechanical threshold in the cauterized area was tested using Von Frey hairs. Spontaneous pain behavior was investigated with conditioned place preference (CPP). Phosphor extracellular signal-regulated kinase (ERK), which is a marker for neuronal activation in chronic pain processing, was investigated in several representative areas of the neuropathic pain matrix: the two regions of the spinal trigeminal nucleus (subnucleus interpolaris/caudalis ,Vi/Vc; subnucleus caudalis/upper cervical cord , Vc/C1), insular cortex, anterior cingulated cortex (ACC), and the rostroventral medulla (RVM). Further, pharmacologically blocking pERK activation in ACC of alkali burn mice was performed in a separate study. Results: Corneal alkali burn caused long lasting damage to the corneal subbasal nerve fibers and mice exhibited spontaneous pain behavior. By testing in several representative areas of neuropathic pain matrix in the higher nerve system, phosphor extracellular signal-regulated kinase (ERK) was significantly activated in Vc/C1, but not in Vi/Vc. Also, ERK was activated in the insular cortex, ACC, and RVM. Furthermore, pharmacologically blocking ERK activation in ACC abolished alkali burn induced corneal spontaneous pain. Conclusion: Alkali burn could cause corneal spontaneous pain and activate neuropathic pain matrix in the central nerve system. Furthermore, activation of ERK in ACC is required for alkali burn induced corneal spontaneous pain