Changes In Precipitable Water Research Articles

Projected changes in extreme daily precipitation linked to changes in precipitable water and vertical velocity in CMIP6 models

Understanding the drivers of precipitation and their changes in a non-stationary climate is crucial for effective climate adaptation and water resource management, as it helps us anticipate and respond to shifting precipitation patterns and their impacts. Here, analysing simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) we show that the conditional probability of extreme daily precipitation given joint extremes of two drivers (precipitable water and vertical velocity) will be stable in a 3 °C warmer future. Consistent with earlier work, we find that the near-global increase in precipitable water (thermodynamic influence) is the baseline for changes in extreme precipitation, which are modulated by changes in vertical velocity (dynamic influence). Thus, in regions where vertical velocity increases, the effect of the two drivers is additive and their changes contribute to an increase in extreme precipitation. The changes of the two drivers are opposite where vertical velocity decreases, resulting in only small increases in extreme precipitation or even a decrease. Furthermore, we reveal that there are moderate changes in the dependence between the drivers, which are larger over the ocean than over landmasses, but they contribute only little to the overall changes in extreme precipitation. We conclude that the use of two very simple drivers that are readily available from climate models can be of great utility for evaluating precipitation extremes in models and understanding their projected changes.

Climatological features of future mesoscale convective systems in convection‐permitting climate models using CMIP6 and ERA5 in the central United States

AbstractMotivated by the limited understanding of future changes in mesoscale convective systems (MCSs), we investigated characteristics of warm‐season (June–August) MCSs in the central United States based on high‐resolution convection‐permitting Weather Research and Forecasting simulations. We examined two 15‐year simulations, which include current simulations (2004–2018) forced by European Centre for Medium‐Range Weather Forecasts Reanalysis version 5 (ERA5) and future simulations (2086–2100) forced by perturbed ERA5 (i.e., ERA5 plus climate change signal derived from 28 Coupled Intercomparison Projected Phase 6 models under the Shared Socioeconomic Pathway–Representative Concentration Pathway 8.5 emission scenario). The initiations and longevities of MCSs were determined using the object‐tracking algorithm MODE‐Time Domain (MTD) from observation, current simulations (ERA), and future simulations (pseudo‐global warming, PGW). Objects identified by MODE‐Time Domain were divided into short‐/long‐lived (based on 75th percentiles of longevity) and daytime (initiated during 0000–1100 UTC)/nighttime (initiated during 1200–2300 UTC). We found that ERA and observation have comparable occurrences of MCSs. MCSs in PGW are associated with intensified rain rates in New Mexico, Colorado, and Kansas and lower rain rates in Texas, Louisiana, and Arkansas than in ERA. Moreover, the statistical analysis based on 15 parameters before MCSs initiation indicates that short‐lived MCSs in PGW are characterized by prominent changes in precipitable water (PW) and the most unstable convective available potential energy. We also found that long‐lived MCSs in PGW are associated with prominent changes in PW, unstable convective available potential energy, and isentropic potential vorticity at 345 K. According to the statistical results, PW is the most important variable in determining the longevity of MCSs and in understanding future changes.

Detection and Attribution of Atmospheric Precipitable Water Changes since the 1970s over China

Atmospheric water vapor increases as air temperature rises, which causes further warming. Thus, understanding the underlying causes of atmospheric water vapor change is vital in climate change research. Here, we conducted detection and attribution analyses of atmospheric precipitable water (PW) changes from 1973–2012 over China using an optimal fingerprinting method by comparing the homogenized radiosonde humidity data with CMIP5 model simulations. Results show that the increase in water vapor can be largely attributed to human activities. The effect of anthropogenic forcing (ANT) can be robustly detected and separated from the response to the natural external forcing (NAT) in the two-signal analysis. The moistening attributable to the ANT forcing explains most of the observed PW increase, while the NAT forcing leads to small moistening. GHGs are the primary moistening contributor responsible for the anthropogenic climate change, and the effect of GHGs can be also clearly detected and successfully attributed to the observed PW increases in a three-signal analysis. The scaling factor is used to adjust the CMIP5 model-projected PW changes over China and the observation-constrained future projections suggest that atmospheric water vapor may increase faster (slower) than that revealed by the raw simulations over whole (eastern) China.

Historical and future changes of atmospheric precipitable water over China simulated by CMIP5 models

Atmospheric water vapor has not only been reported to increase in the real world, but is also expected to increase in climate models. Correctly simulating water vapor and its relationship with temperature is thus vital for climate models. In this study, atmospheric precipitable water (PW) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) is evaluated over China during 1970–2005, and then the future changes of PW and their correlation with near-surface air temperature (Ts) are further investigated. The results show that the PW biases in the model simulations are within ~ 20% for most of eastern China, but have larger root-mean-square error over southeastern China. Most models are able to simulate the observed PW upward trends since 1970, but with large inter-model spread. CanESM2, GFDL-CM3, HadGEM2-ES and MIROC5 are relatively better at reproducing the observed long-term variations and trends for the whole of China and its subregions. An empirical orthogonal function (EOF) analysis shows that the model simulations reproduce the first observed leading modes well, but barely capture the temporal variation revealed by the second observed EOF. The model simulations project ubiquitous increases in PW in the twenty-first century, resulting from an increase in the mean and a flattening of the probability distribution functions of the PW anomalies. In particular, the increases in PW under the most severe future emissions scenario (RCP8.5) are nearly twice as much as those under the low–mid emissions scenario (RCP4.5) for most of China by the late twenty-first century. The model simulations suggest that the changes in PW are largely consistent with the changes in Ts during 1970–2005, with a dPW/dTs slope of ~ 4.6–6.0% K−1, which roughly agrees with the observed results but is slightly lower than the 7% K−1 implied by the Clausius–Clapeyron equation with a constant relative humidity. In the twenty-first century, the dPW/dTs is projected to be about 5.5–7.0% K−1 under the RCP4.5 scenario, but may reach ~ 7.4% K−1 or higher under the RCP8.5 scenario.

Changes in Temperature and Precipitation Extremes in Superparameterized CAM in Response to Warmer SSTs

Subdaily temperature and precipitation extremes in response to warmer SSTs are investigated on a global scale using the superparameterized (SP) Community Atmosphere Model (CAM), in which a cloud-resolving model is embedded in each CAM grid column to simulate convection explicitly. Two 10-yr simulations have been performed using present climatological sea surface temperature (SST) and perturbed SST climatology derived from the representative concentration pathway 8.5 (RCP8.5) scenario. Compared with the conventional CAM, SP-CAM simulates colder temperatures and more realistic intensity distribution of precipitation, especially for heavy precipitation. The temperature and precipitation extremes have been defined by the 99th percentile of the 3-hourly data. For temperature, the changes in the warm and cold extremes are generally consistent between CAM and SP-CAM, with larger changes in warm extremes at low latitudes and larger changes in cold extremes at mid-to-high latitudes. For precipitation, CAM predicts a uniform increase of frequency of precipitation extremes regardless of the rain rate, while SP-CAM predicts a monotonic increase of frequency with increasing rain rate and larger change of intensity for heavier precipitation. The changes in 3-hourly and daily temperature extremes are found to be similar; however, the 3-hourly precipitation extremes have a significantly larger change than daily extremes. The Clausius–Clapeyron scaling is found to be a relatively good predictor of zonally averaged changes in precipitation extremes over midlatitudes but not as good over the tropics and subtropics. The changes in precipitable water and large-scale vertical velocity are equally important to explain the changes in precipitation extremes.

Global Water Vapor Trend from 1988 to 2011 and Its Diurnal Asymmetry Based on GPS, Radiosonde, and Microwave Satellite Measurements

Abstract This study analyzes trends in precipitable water (PW) over land and ocean from 1988 to 2011, the PW–surface temperature Ts relationship, and their diurnal asymmetry using homogenized radiosonde data, microwave satellite observations, and ground-based global positioning system (GPS) measurements. It is found that positive PW trends predominate over the globe, with larger magnitudes over ocean than over land. The PW trend is correlated with surface warming spatially over ocean with a pattern correlation coefficient of 0.51. The PW percentage increase rate normalized by Ts expressed as is larger and closer to the rate implied by the Clausius–Clapeyron (C–C) equation over ocean than over land. The 2-hourly GPS PW data show that the PW trend from 1995 to 2011 is larger at night than during daytime. Nighttime PW monthly anomalies correlate positively and significantly with nighttime minimum temperature Tmin at all stations, but this is not true for daytime PW and maximum temperature Tmax. The ratio of relative PW changes with Tmin () is larger and closer to the C–C equation’s implied value of ~7% K−1 than . This suggests that the relationship between PW and Ts at night is a better indicator of the water vapor feedback than that during daytime, when clouds and other factors also influence Ts.

Trends of regional precipitation and their control mechanisms during 1979–2013

Trends in precipitation are critical to water resources. Considerable uncertainty remains concerning the trends of regional precipitation in response to global warming and their controlling mechanisms. Here, we use an interannual difference method to derive trends of regional precipitation from GPCP (Global Precipitation Climatology Project) data and MERRA (Modern- Era Retrospective Analysis for Research and Applications) reanalysis in the near-global domain of 60°S–60°N during a major global warming period of 1979–2013. We find that trends of regional annual precipitation are primarily driven by changes in the top 30% heavy precipitation events, which in turn are controlled by changes in precipitable water in response to global warming, i.e., by thermodynamic processes. Significant drying trends are found in most parts of the U.S. and eastern Canada, the Middle East, and eastern South America, while significant increases in precipitation occur in northern Australia, southern Africa, western India and western China. In addition, as the climate warms there are extensive enhancements and expansions of the three major tropical precipitation centers–the Maritime Continent, Central America, and tropical Africa–leading to the observed widening of Hadley cells and a significant strengthening of the global hydrological cycle.

Evaluation of atmospheric precipitable water from reanalysis products using homogenized radiosonde observations over China

Many multidecadal atmospheric reanalysis products are available now, but their consistencies and reliability are far from perfect. In this study, atmospheric precipitable water (PW) from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), NCEP/Department of Energy (DOE), Modern Era Retrospective-Analysis for Research and Applications (MERRA), Japanese 55 year Reanalysis (JRA-55), JRA-25, ERA-Interim, ERA-40, Climate Forecast System Reanalysis (CFSR), and 20th Century Reanalysis version 2 is evaluated against homogenized radiosonde observations over China during 1979–2012 (1979–2001 for ERA-40). Results suggest that the PW biases in the reanalyses are within ∼20% for most of northern and eastern China, but the reanalyses underestimate the observed PW by 20%–40% over western China and by ∼60% over the southwestern Tibetan Plateau. The newer-generation reanalyses (e.g., JRA25, JRA55, CFSR, and ERA-Interim) have smaller root-mean-square error than the older-generation ones (NCEP/NCAR, NCEP/DOE, and ERA-40). Most of the reanalyses reproduce well the observed PW climatology and interannual variations over China. However, few reanalyses capture the observed long-term PW changes, primarily because they show spurious wet biases before about 2002. This deficiency results mainly from the discontinuities contained in reanalysis relative humidity fields in the middle-lower troposphere due to the wet bias in older radiosonde records that are assimilated into the reanalyses. An empirical orthogonal function (EOF) analysis revealed two leading modes that represent the long-term PW changes and El Nino–Southern Oscillation-related interannual variations with robust spatial patterns. The reanalysis products, especially the MERRA and JRA-25, roughly capture these EOF modes, which account for over 50% of the total variance. The results show that even during the post-1979 satellite era, discontinuities in radiosonde data can still induce large spurious long-term changes in reanalysis PW and other related fields. Thus, more efforts are needed to remove spurious changes in input data for future long-term reanalyses.

Correlation between Atmospheric Water Vapor and Diurnal Temperature Range over China

AbstractDiurnal temperature range (DTR) is an important measure in studies of climate change and variability. The changes of DTR in different regions are affected by many different factors. In this study, the degree of correlation between the DTR and atmospheric precipitable water (PW) over China is explored using newly homogenized surface weather and sounding observations. The results show that PW changes broadly reflect the geographic patterns of DTR long-term trends over most of China during the period 1970–2012, with significant anticorrelations of trend patterns between the DTR and PW, especially over those regions with higher magnitude DTR trends. PW can largely explain about 40% or more (r2 ≥ 0.40) of the DTR changes, with a d(PW)/d(DTR) slope of −2% to −10% K–1 over most of northwestern and southeastern China, despite certain seasonal dependencies. For China as whole, the significant anticorrelations between the DTR and PW anomalies range from −0.42 to −0.75, with a d(PW)/d(DTR) slope of −6% to −1...

Correlation between Atmospheric Water Vapor and Diurnal Temperature Range over China

Diurnal temperature range(DTR) is an important measure in studies of climate change and variability. The changes of DTR in different regions are affected by many different factors. In this study, the degree of correlation between the DTR and atmospheric precipitable water(PW) over China is explored using newly homogenized surface weather and sounding observations. The results show that PW changes broadly reflect the geographic patterns of DTR long-term trends over most of China during the period 1970–2012, with significant anticorrelations of trend patterns between the DTR and PW, especially over those regions with higher magnitude DTR trends. PW can largely explain about 40% or more(r2 ≥0.40) of the DTR changes, with a d(PW)/d(DTR) slope of-2% to-10% K-1 over most of northwestern and southeastern China, despite certain seasonal dependencies. For China as whole, the significant anticorrelations between the DTR and PW anomalies range from-0.42 to-0.75, with a d(PW)/d(DTR) slope of-6% to-11% K-1. This implies that long-term DTR changes are likely to be associated with opposite PW changes, approximately following the Clausius-Clapeyron equation. Furthermore, the relationship is more significant in the warm season than in the cold season. Thus, it is possible that PW can be considered as one potential factor when exploring long-term DTR changes over China. It should be noted that the present study has a largely statistical focus and that the underlying physical processes should therefore be examined in future work.

Implications for Arctic amplification of changes in the strength of the water vapor feedback

One of the major climatic changes apparent over the Arctic Ocean has been the amplified rate at which air temperature has been increasing relative to the global mean. There are multiple factors which play roles in this amplification, including changes in sea ice/albedo, atmospheric circulation, clouds, and water vapor. We investigate the positive feedback on temperature caused by increasing downward longwave radiation flux (DLF) associated with increasing atmospheric precipitable water (PW). The Japanese 25‐year Reanalysis and ERA‐Interim reanalysis are used to examine the role of the DLF/PW component of the water vapor feedback loop on the enhanced warming in the Arctic between 1979 and 2011. We find a nonlinear relationship between DLF and PW, which suggests that the sensitivity of DLF to changes in PW varies by season, with the highest in winter and the lowest in summer. The positive trends in DLF and PW are widespread over the Arctic during autumn and spring but are centered mainly over the Atlantic sector in winter. The strength of the PW feedback loop depends on both the sensitivity of DLF to changes in PW and the change in PW during 1979–2011. If, in the future, PW were to increase significantly during winter in the central and Pacific sectors of the Arctic, there could be an expansion of Arctic amplification during winter. We also examine the effect of changes in cloud cover and find that such changes account for a much smaller proportion of the changes in DLF than does PW.

Soil Moisture–Precipitation Feedback Processes in the Indian Summer Monsoon Season

Abstract Soil moisture can influence precipitation through a feedback loop with land surface evapotranspiration. A series of numerical simulations, including soil moisture sensitivity experiments, have been performed for the Indian summer monsoon season (ISM). The simulations were carried out with the nonhydrostatic regional climate model Consortium for Small-Scale Modeling (COSMO) in climate mode (COSMO-CLM), driven by lateral boundary conditions derived from the ECMWF Interim reanalysis (ERA-Interim). Positive as well as negative feedback processes through local and remote effects are shown to be important. The regional moisture budget studies have exposed that changes in precipitable water and changes in precipitation efficiency vary in importance, in time, and in space in the simulations for India. Overall, the results show that the premonsoonal soil moisture has a significant influence on the monsoonal precipitation, and thus confirmed that modeling of soil moisture is essential for reliable simulation and forecasting of the ISM.

Trends in Precipitable Water and Relative Humidity in China: 1979–2005

AbstractAnnual and seasonal trends of precipitable water (PW) and relative humidity (RH) at 850, 700, and 500 hPa are studied using the data from 106 radiosonde stations over China during the period 1979–2005. Analysis shows evidence of an increase in PW associated with the slight warming observed in the lower to midtroposphere over China. The northern part of China shows a significant upward trend of PW in summer, and drying of the atmosphere in winter is found in most regions over China. Annual and seasonal trends in RH at the 850-, 700-, and 500-hPa levels show no significant trends in most regions in China except for Xinjiang, which shows an upward trend, and central China, where there was a downward trend in RH at 500 hPa. It is found that changes in PW are coincident with the warming of the surface and the lower to midtroposphere. The RH in the lower to midtroposphere in most regions over China has remained steady during the most recent 30 years, as might be expected given the increasing of PW and the warming above the surface. The long-term trend of precipitation over China may be linked to the trends of PW and RH at the lower level and midlevel.

Trend Analysis of GPS Precipitable Water Vapor Above South Korea Over the Last 10 Years

We analyzed global positioning system (GPS)-derived precipitable water vapor (PWV) trends of the Korea Astronomy and Space Science Institute 5 stations (Seoul, Daejeon, Mokpo, Milyang, Sokcho) where Korea Meteorological Administration meteorological data can be obtained at the same place. In the least squares analysis, the GPS PWV time series showed consistent positive trends (0.11 mm/year) over South Korea from 2000 to 2009. The annual increase of GPS PWV was comparable with the 0.17 mm/year and 0.02 mm/year from the National Center for Atmospheric Research Earth Observing Laboratory and Atmospheric InfraRed Sounder, respectively. For seasonal analysis, the increasing tendency was found by 0.05 mm/year, 0.16 mm/year, 0.04 mm/year in spring (March-May), summer (June-August) and winter (December-February), respectively. However, a negative trend (-0.14 mm/year) was seen in autumn (September-November). We examined the relationship between GPS PWV and temperature which is the one of the climatic elements. Two elements trends increased during the same period and the correlation coefficient was about 0.8. Also, we found the temperature rise has increased more GPS PWV and observed a stronger positive trend in summer than in winter. This is characterized by hot humid summer and cold dry winter of Korea climate and depending on the amount of water vapor the air contains at a certain temperature. In addition, it is assumed that GPS PWV positive trend is caused by increasing amount of saturated water vapor due to temperature rise in the Korean Peninsula. In the future, we plan to verify GPS PWV effectiveness as a tool to monitor changes in precipitable water through cause analysis of seasonal trends and indepth/long-term comparative analysis between GPS PWV and other climatic elements.

Summer Severe-Rainfall Frequency Trend and Variability over Ontario, Canada

Abstract During the last two decades (1979–2002), there has been an ever-increasing frequency of summer severe-rainfall events over Ontario, Canada. This observed upward trend is robust as demonstrated through the Mann–Kendall test with consideration of removing a lag-1 autoregressive process. It is shown through composite analyses using the NCEP reanalysis data that in the presence of warming conditions the summer severe-rainfall events occur more frequently over Ontario, especially under atmospheric conditions with stronger low-level cyclonic circulations and more precipitable water. Further analyses indicate that over north and central Ontario the summer severe-rainfall frequency is linked with a positive trend of precipitable water whereas over central and south Ontario there is a strong interannual response of summer severe-rainfall frequency to the changes in precipitable water through the variations of air temperature.

Variability in sun photometer derived summertime total column ozone over the Indian station Maitri in the Antarctic region

Extensive observations of total column ozone (TCO) made over the Antarctic region at the Indian station ‘Maitri’ (70.76oS, 11.74oE) during two summer periods of 2004–2005 and 2006–2007 using a ground-based optical remote sensing instrument (ozonometer) have been examined to study the short-term variations. Mean TCO during 2004–05 summer was 288.7 DU with a variability of 8% and the corresponding values during 2006–2007 summer were 280.4 DU and 11%. Both ozonometer and Dobson spectrophotometer measurements at this Antarctic station during January 2005 showed a short-term decrease in TCO by about 70 DU before regaining original value. Simultaneous changes in precipitable water and surface temperature points to a possible interplay of chemistry and meteorological conditions. Occurrence of an intense solar proton event in January 2005 also suggests the possible role of precipitating charged particles in causing short-lived ozone decrease through production of OH radical.

On the homogeneity and interpretation of precipitable water time series derived from global GPS observations

Observations of the Global Positioning System (GPS) were reanalyzed over the period from 1994 to 2004 in a joint project of the technical universities in Dresden and Munich. The estimated tropospheric parameters were converted into precipitable water (PW) using surface pressure observations from the World Meteorological Organization and atmospheric mean temperature fields from the European Centre for Medium‐Range Weather Forecasts. For the first time a systematic study of the homogeneity of global GPS‐derived precipitable water time series was carried out regarding the influence of changes in the GPS antennas and radomes as well as changes in the number of recorded observations. The focus of this study is on interannual changes in precipitable water. Over Europe, large parts of North America, and Iceland and in the region south of 30°S, these changes are very small. The range of the PW variations on interannual time scales is less than 2 mm in these areas. However, in the southeastern part of North America and north Australia, these anomalies in precipitable water show a range of up to 6 mm. In the tropics, PW anomalies with a range of up to 10 mm were found. GPS PW was compared with a modeled PW assuming water vapor saturation. This shows that GPS PW of stations located in the middle and high northern and southern latitudes is consistent with the temperature‐related saturation values of water vapor. In the tropics and subtropics the annual temperature variations are low. In these regions the variations in the PW can be dominated by other factors, including water vapor transport. At seasonal time scales the water vapor transport can be associated with atmospheric circulation such as monsoonal flow.

Relationship between temperature and precipitable water changes over tropical oceans

We use observations, climate models and reanalysis output to examine the relationship between changes in temperature and changes in precipitable water. In climate models these variables are highly correlated over the tropical oceans, with a similar scaling ratio for interannual and decadal time scales. This result is consistent with the most recently developed satellite datasets. In contrast, scaling rations based either on an earlier version of the satellite measurements or reanalysis show scaling ratios that are inconsistent with models, and are dependent on time scale. These results demonstrate that climate model output is useful for evaluating differences between divergent observational datasets.

Recent climate changes in precipitable water in the global tropics as revealed in National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis

For the first time, long‐term climate changes in the atmospheric moisture over the global tropics are investigated using precipitable water (PW) from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data sets for two different periods: 1979–1998 and 1948–1998. Dominant modes of PW variability estimated for the shorter period (1979–1998) are in an overall agreement with those of satellite‐estimated precipitation in terms of both empirical orthogonal functions (EOF) spatial patterns and time‐dependent coefficients. Encouraged by this result, the entire time series of PW for the period of 1948–1998 is subsequently examined. Climatology of annual mean PW agrees well with the radiosonde‐based and satellite‐based climatologies of atmospheric moisture. EOF analysis applied to the entire time series indicates that the two leading modes explain 37.3% of the total variance of PW. The first EOF mode reflects that the trend‐like change is associated with evolutions in the observing systems of NCEP/NCAR reanalysis product. The second EOF has a spatial pattern, which is characterized by the zonal dipole structure. This mode is associated with the major climatic signal in the region, El Niño‐Southern Oscillation. Analysis of regional time series of annual mean PW has shown that, coincident with introduction of satellite data, step‐like changes in the mid to late 1970s are present over Malaysia, central tropical Pacific, and central Brazil. Abrupt changes of PW over North Africa are noticed in the late 1960s, and the large positive PW anomalies observed during the 1950s and 1960s are suspected to have quality deficiencies.

Impact of greenhouse warming on the West African summer monsoon

The Meteo-France climate model has been used to run a time-dependent climate change experiment in order to study the impact of increasing amounts of greenhouse gases and aerosols on the West African monsoon system. A transient climate simulation of 150 years has been performed with a coupled ocean/sea-ice/atmosphere model including stratospheric ozone with the greenhouse gas concentrations changing annually according to one of the new scenarios of the Intergovernmental Panel on Climate Change. The simulated climate change has been analyzed as the difference between two 30-year time-slices (1980–2010 and 2070–2100 respectively) with particular attention paid to the hydrological cycle. The climate of the recent period, validated in detail by comparison to reanalyses, shows that the model is able to reproduce a realistic seasonal cycle of the African monsoon. The precipitation changes simulated at the end of the twentyfirst century show an enhanced monsoon precipitation over West Africa. The analysis of the atmospheric branch of the water cycle highlights some of the parameters that control these precipitation anomalies: changes in precipitable water, water vapour recycling, moisture convergence and precipitation efficiency. These changes of the hydrological processes also cause corresponding changes in the thermal state of the atmosphere such as a smaller warming of the surface in the Sahel linked to stronger evaporation in this region. This results in changes of the mean zonal circulation through the thermal wind relationship and of position and strength of key elements of the general circulation over the West Africa (Hadley and Walker circulations, African Easterly Jet).