Interaction of Vegetation and Atmospheric Dynamical Mechanisms in the Mid-Holocene African Monsoon*

2006 ◽  
Vol 19 (16) ◽  
pp. 4105-4120 ◽  
Author(s):  
Katrina Hales ◽  
J. David Neelin ◽  
Ning Zeng
Keyword(s):  
Land Surface ◽  
Climate Modeling ◽  
Atmospheric Model ◽  
Moist Static Energy ◽  
North African ◽  
Orbital Parameters ◽  
Model Studies ◽  
Vegetation Feedbacks ◽  
Highly Sensitive ◽  
Static Energy

Abstract Paleoevidence indicates that generally wetter conditions existed in the Sahara during the mid-Holocene. Climate modeling studies addressing this issue generally agree that mid-Holocene values of the earth’s orbital parameters favored an enhanced North African summer monsoon but also suggest that land surface and vegetation feedbacks must have been important factors. Attempts to reproduce the “green” mid-Holocene Sahara in model studies with interactive vegetation may be interpreted to indicate that the problem is highly sensitive to the atmospheric dynamics of each model employed. In other work, dynamical mechanisms have been hypothesized to affect monsoon poleward extent, particularly ventilation, by import of low-moist static energy air to the continent. Here, interactive vegetation and the ventilation mechanism are studied in an intermediate complexity atmospheric model coupled to simple land and vegetation components. Interactive vegetation is found to be effective at enhancing the precipitation and vegetation amount in regions where the monsoon has advanced because of changes in orbital parameters or ventilation yet not very effective in moving the monsoon boundary if ventilation is strong. The poleward extent of the mid-Holocene monsoon and the steppe boundary are primarily controlled by the strength of ventilation in the atmospheric model. Within this boundary, the largest changes in monsoon precipitation and vegetation occur when interactive vegetation and reduced ventilation act simultaneously, as these greatly reinforce each other.

scholarly journals Moist Static Energy Budget of MJO-like Disturbances in the Atmosphere of a Zonally Symmetric Aquaplanet

2012 ◽  
Vol 25 (8) ◽  
pp. 2782-2804 ◽  
Author(s):  
Joseph Allan Andersen ◽  
Zhiming Kuang
Keyword(s):  
Spectral Feature ◽  
Low Frequency ◽  
Atmospheric Model ◽  
Moist Static Energy ◽  
Eddy Activity ◽  
Vertical Advection ◽  
Time Space ◽  
Synoptic Eddies ◽  
Moist Static Energy Budget ◽  
Static Energy

Abstract A Madden–Julian oscillation (MJO)-like spectral feature is observed in the time–space spectra of precipitation and column-integrated moist static energy (MSE) for a zonally symmetric aquaplanet simulated with Superparameterized Community Atmospheric Model (SPCAM). This disturbance possesses the basic structural and propagation features of the observed MJO. To explore the processes involved in propagation and maintenance of this disturbance, this study analyzes the MSE budget of the disturbance. The authors observe that the disturbances propagate both eastward and poleward. The column-integrated longwave heating is the only significant source of column-integrated MSE acting to maintain the MJO-like anomaly balanced against the combination of column-integrated horizontal and vertical advection of MSE and latent heat flux. Eastward propagation of the MJO-like disturbance is associated with MSE generated by both column integrated horizontal and vertical advection of MSE, with the column longwave heating generating MSE that retards the propagation. The contribution to the eastward propagation by the column-integrated horizontal advection of MSE is dominated by synoptic eddies. Further decomposition indicates that the advection contribution to the eastward propagation is dominated by meridional advection of MSE by anomalous synoptic eddies caused by the suppression of eddy activity ahead of the MJO convection. This suppression is linked to the barotropic conversion mechanism, with the gradients of the low-frequency wind experienced by the synoptic eddies within the MJO envelope acting to modulate the eddy kinetic energy. The meridional eddy advection’s contribution to poleward propagation is dominated by the mean state’s (meridionally varying) eddy activity acting on the anomalous MSE gradients associated with the MJO.

scholarly journals Trends in continental temperature and humidity directly linked to ocean warming

2018 ◽  
Vol 115 (19) ◽  
pp. 4863-4868 ◽  
Author(s):  
Michael P. Byrne ◽  
Paul A. O’Gorman
Keyword(s):  
Relative Humidity ◽  
Land Surface ◽  
Moisture Transport ◽  
Climate Change Impacts ◽  
Atmospheric Dynamics ◽  
Specific Humidity ◽  
Climate Projections ◽  
Moist Static Energy ◽  
Temperature And Humidity ◽  
Static Energy

In recent decades, the land surface has warmed substantially more than the ocean surface, and relative humidity has fallen over land. Amplified warming and declining relative humidity over land are also dominant features of future climate projections, with implications for climate-change impacts. An emerging body of research has shown how constraints from atmospheric dynamics and moisture budgets are important for projected future land–ocean contrasts, but these ideas have not been used to investigate temperature and humidity records over recent decades. Here we show how both the temperature and humidity changes observed over land between 1979 and 2016 are linked to warming over neighboring oceans. A simple analytical theory, based on atmospheric dynamics and moisture transport, predicts equal changes in moist static energy over land and ocean and equal fractional changes in specific humidity over land and ocean. The theory is shown to be consistent with the observed trends in land temperature and humidity given the warming over ocean. Amplified land warming is needed for the increase in moist static energy over drier land to match that over ocean, and land relative humidity decreases because land specific humidity is linked via moisture transport to the weaker warming over ocean. However, there is considerable variability about the best-fit trend in land relative humidity that requires further investigation and which may be related to factors such as changes in atmospheric circulations and land-surface properties.

Characterizing Drying in the South American Monsoon Onset Season with the Moist Static Energy Budget

2020 ◽  
Vol 33 (22) ◽  
pp. 9735-9748
Author(s):  
Jane E. Smyth ◽  
Yi Ming
Keyword(s):  
Atmospheric Model ◽  
Dry Season ◽  
Monsoon Onset ◽  
Geophysical Fluid Dynamics Laboratory ◽  
South American ◽  
Moist Static Energy ◽  
The South ◽  
South American Monsoon ◽  
Sst Warming ◽  
Static Energy

AbstractThe tropical atmospheric circulation and attendant rainfall exhibit seasonally dependent responses to increasing temperatures. Understanding changes in the South American monsoon system is of particular interest given the sensitivity of the southern Amazon rainforest to changes in dry season length. We utilize the latest Geophysical Fluid Dynamics Laboratory Atmospheric Model (GFDL AM4) to analyze the response of the South American monsoon to uniform sea surface temperature (SST) warming. SST warming is a poorly understood yet impactful component of greenhouse gas–induced climate change. Region-mean rainfall declines by 11%, and net precipitation (precipitation minus evaporation) declines by 40%, during the monsoon onset season (September–November), producing a more severe dry season. The column-integrated moist static energy (MSE) budget helps elucidate the physical mechanisms of the simulated drying. Based on the seasonal analysis, precipitation reductions tend to occur when 1) a convecting region’s climatological MSE export is dominated by horizontal rather than vertical advection, and 2) the horizontal MSE advection increases in the perturbed climate, impeding ascent. On a synoptic scale, the South American low-level jet strengthens and exports more moisture from the monsoon sector, exacerbating spring drying.

scholarly journals Mechanisms of Climate Change in the Semiarid African Sahel: The Local View

2010 ◽  
Vol 23 (3) ◽  
pp. 743-756 ◽  
Author(s):  
Alessandra Giannini
Keyword(s):  
Climate Change ◽  
Land Surface ◽  
Southern Oscillation ◽  
Net Energy ◽  
Moist Static Energy ◽  
Near Surface ◽  
Vertical Stability ◽  
First Case ◽  
Static Energy ◽  
The Impact

Abstract Application of the moist static energy framework to analyses of vertical stability and net energy in the Sahel sheds light on the divergence of projections of climate change. Two distinct mechanisms are sketched. In one, anthropogenic warming changes continental climate indirectly: warming of the oceans increases moist static energy at upper levels, affecting vertical stability globally, from the top down, and driving drying over the Sahel, in a way analogous to the impact of El Niño–Southern Oscillation on the global tropical atmosphere. In the other, the increase in anthropogenic greenhouse gases drives a direct continental change: the increase in net terrestrial radiation at the surface increases evaporation, favoring vertical instability and near-surface convergence from the bottom up. In both cases the surface warms, but in the first precipitation and evaporation decrease, while in the second they increase. In the first case, land surface warming is brought about by the remotely forced decrease in precipitation and consequent decrease in evaporation and increase in net solar radiation at the surface. In the second, it is brought about by the increase in net terrestrial radiation at the surface, amplified by the water vapor feedback associated with an increase in near-surface humidity.

scholarly journals Improving Energy-Based Estimates of Monsoon Location in the Presence of Proximal Deserts

2016 ◽  
Vol 29 (13) ◽  
pp. 4741-4761 ◽  
Author(s):  
Ravi Shekhar ◽  
William R. Boos
Keyword(s):  
Energy Flux ◽  
Energy Budget ◽  
Land Surface ◽  
Correct Diagnosis ◽  
Lower Troposphere ◽  
Equatorial Position ◽  
Quasi Equilibrium ◽  
Moist Static Energy ◽  
Dry Air ◽  
Static Energy

Abstract Two theoretical frameworks have been widely used to understand the response of monsoons to local and remote forcings: the vertically integrated atmospheric energy budget and convective quasi-equilibrium (CQE). Existing forms of these frameworks neglect some of the complexities of monsoons, such as the shallow meridional circulations that advect dry air from adjacent deserts into the middle and lower troposphere of monsoon regions. Here the fidelity of energy budget and CQE theories for monsoon location is assessed in a three-dimensional beta-plane model with boundary conditions representative of an off-equatorial continent with a tropical grassland and an adjacent subtropical desert. Energy budget theories show mixed success for various SST and land surface albedo forcings, with the ITCZ being collocated with the energy flux equator but a nonmonotonic relationship existing between ITCZ latitude and cross-equatorial energy transport. Accounting for the off-equatorial position of the unperturbed energy flux equator is shown to be important when a linearization of meridional energy transports is used to quantitatively diagnose ITCZ location. CQE theories that diagnose ITCZ location based on the subcloud moist static energy maximum are shown to have large biases; accounting for convective entrainment of dry air by using a lower-tropospheric mean moist static energy provides a more correct diagnosis of ITCZ location. Finally, it is shown that although ITCZ shifts can be diagnosed by modified CQE and energy budget frameworks, neither can be used in a quantitatively prognostic capacity because of unpredictable feedbacks that are often larger than the imposed forcing.

scholarly journals Land surface albedo and vegetation feedbacks enhanced the millennium drought in south-east Australia

2017 ◽  
Vol 21 (1) ◽  
pp. 409-422 ◽  
Author(s):  
Jason P. Evans ◽  
Xianhong Meng ◽  
Matthew F. McCabe
Keyword(s):  
Land Surface ◽  
Climate Model ◽  
Southern Oscillation ◽  
Regional Climate ◽  
Surface Characteristics ◽  
Surface Precipitation ◽  
Vegetation Fraction ◽  
Vegetation Feedbacks ◽  
Millennium Drought ◽  
Precipitation Anomalies

Abstract. In this study, we have examined the ability of a regional climate model (RCM) to simulate the extended drought that occurred throughout the period of 2002 through 2007 in south-east Australia. In particular, the ability to reproduce the two drought peaks in 2002 and 2006 was investigated. Overall, the RCM was found to reproduce both the temporal and the spatial structure of the drought-related precipitation anomalies quite well, despite using climatological seasonal surface characteristics such as vegetation fraction and albedo. This result concurs with previous studies that found that about two-thirds of the precipitation decline can be attributed to the El Niño–Southern Oscillation (ENSO). Simulation experiments that allowed the vegetation fraction and albedo to vary as observed illustrated that the intensity of the drought was underestimated by about 10 % when using climatological surface characteristics. These results suggest that in terms of drought development, capturing the feedbacks related to vegetation and albedo changes may be as important as capturing the soil moisture–precipitation feedback. In order to improve our modelling of multi-year droughts, the challenge is to capture all these related surface changes simultaneously, and provide a comprehensive description of land surface–precipitation feedback during the droughts development.

scholarly journals Impacts of Shallow Convection on MJO Simulation: A Moist Static Energy and Moisture Budget Analysis

2013 ◽  
Vol 26 (8) ◽  
pp. 2417-2431 ◽  
Author(s):  
Qiongqiong Cai ◽  
Guang J. Zhang ◽  
Tianjun Zhou
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Lower Troposphere ◽  
Longwave Radiation ◽  
Reanalysis Data ◽  
Specific Humidity ◽  
Moist Static Energy ◽  
Shallow Convection ◽  
Budget Analysis ◽  
Static Energy

Abstract The role of shallow convection in Madden–Julian oscillation (MJO) simulation is examined in terms of the moist static energy (MSE) and moisture budgets. Two experiments are carried out using the NCAR Community Atmosphere Model, version 3.0 (CAM3.0): a “CTL” run and an “NSC” run that is the same as the CTL except with shallow convection disabled below 700 hPa between 20°S and 20°N. Although the major features in the mean state of outgoing longwave radiation, 850-hPa winds, and vertical structure of specific humidity are reasonably reproduced in both simulations, moisture and clouds are more confined to the planetary boundary layer in the NSC run. While the CTL run gives a better simulation of the MJO life cycle when compared with the reanalysis data, the NSC shows a substantially weaker MJO signal. Both the reanalysis data and simulations show a recharge–discharge mechanism in the MSE evolution that is dominated by the moisture anomalies. However, in the NSC the development of MSE and moisture anomalies is weaker and confined to a shallow layer at the developing phases, which may prevent further development of deep convection. By conducting the budget analysis on both the MSE and moisture, it is found that the major biases in the NSC run are largely attributed to the vertical and horizontal advection. Without shallow convection, the lack of gradual deepening of upward motion during the developing stage of MJO prevents the lower troposphere above the boundary layer from being preconditioned for deep convection.

scholarly journals Simulating carbon exchange using a regional atmospheric model coupled to an advanced land-surface model

2008 ◽  
Vol 5 (5) ◽  
pp. 4161-4207 ◽  
Author(s):  
H. W. Ter Maat ◽  
R. W. A. Hutjes
Keyword(s):  
Carbon Dioxide ◽  
Land Surface ◽  
Large Scale ◽  
Regional Scale ◽  
Atmospheric Model ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Surface Model ◽  
Carbon Exchange ◽  
Regional Atmospheric Modelling System

Abstract. A large scale mismatch exists between our understanding and quantification of ecosystem atmosphere exchange of carbon dioxide at local scale and continental scales. This paper will focus on the carbon exchange on the regional scale to address the following question: What are the main controlling factors determining atmospheric carbon dioxide content at a regional scale? We use the Regional Atmospheric Modelling System (RAMS), coupled with a land surface scheme simulating carbon, heat and momentum fluxes (SWAPS-C), and including also sub models for urban and marine fluxes, which in principle include the main controlling mechanisms and capture the relevant dynamics of the system. To validate the model, observations are used which were taken during an intensive observational campaign in the central Netherlands in summer 2002. These included flux-site observations, vertical profiles at tall towers and spatial fluxes of various variables taken by aircraft. The coupled regional model (RAMS-SWAPS-C) generally does a good job in simulating results close to reality. The validation of the model demonstrates that surface fluxes of heat, water and CO2 are reasonably well simulated. The comparison against aircraft data shows that the regional meteorology is captured by the model. Comparing spatially explicit simulated and observed fluxes we conclude that in general simulated latent heat fluxes are underestimated by the model to the observations which exhibit large standard deviation for all flights. Sensitivity experiments demonstrated the relevance of the urban emissions of carbon dioxide for the carbon balance in this particular region. The same test also show the relation between uncertainties in surface fluxes and those in atmospheric concentrations.

Quantifying the interplay of clouds and their environment through an energetic lens during EUREC4A

2021 ◽  
Author(s):  
Anna Lea Albright ◽  
Sandrine Bony ◽  
Bjorn Stevens ◽  
Raphaela Vogel
Keyword(s):  
Large Scale ◽  
Hydrological Cycle ◽  
Trade Wind ◽  
Energy Budgets ◽  
Moist Static Energy ◽  
Radiative Effect ◽  
The North ◽  
Cloud Radiative Effect ◽  
Moist Static Energy Budget ◽  
Static Energy

<p>The trades form an important link in the atmospheric energy supply, transporting moisture and momentum to the deep tropics and influencing the global hydrological cycle. Trade-wind cumuli are the most ubiquitous cloud type over tropical oceans, yet models disagree in simulating their response to warming. Our study takes advantage of extensive in-situ soundings performed during the EUREC4A campaign, which took place in the downstream trades of the North Atlantic in winter 2020. We employ 1068 dropsondes made in a ca. 2deg x 2deg area to close the moisture and energy budgets of the subcloud layer and atmospheric column. Our motivation for closing moisture and energy budgets using EUREC4A data is two-fold. First, we try to understand which large-scale environmental factors control variability in subcloud layer moisture and moist static energy, given their influence on setting convective potential. Second, we quantify the interplay between clouds and their environment through an energetic lens. The cloud radiative effect emerges as a residual from the total column moist static energy budget, yielding an energetic estimate of clouds. We quantify how this cloud radiative effect compares with coincident satellite and geometric (i.e. cloud fraction) estimates of cloudiness, varies on different scales, and relates to large-scale environmental conditions.</p>

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