Increase In Atmospheric Water Vapor Research Articles

Linkages between atmospheric rivers and humid heat across the United States

Abstract. The global increase in atmospheric water vapor due to climate change tends to heighten the dangers associated with both humid heat and heavy precipitation. Process-linked connections between these two extremes, particularly those which cause them to occur close together in space or time, are of special concern for impacts. Here we investigate how atmospheric rivers relate to the risk of summertime humid heat in the United States. We find that the hazards of atmospheric rivers and humid heat often occur in close proximity, most notably across the northern third of the United States. In this region, high levels of water vapor – resulting from the spatially organized horizontal moisture plumes that characterize atmospheric rivers – act to amplify humid heat, generally during the transition from dry high-pressure ridge conditions to wet low-pressure trough conditions. In contrast, the US Southeast, Southwest, and Northwest tend to experience atmospheric rivers and humid heat separately, representing an important negative correlation of joint risk.

Observed humidity trends in dry regions contradict climate models.

Arid and semi-arid regions of the world are particularly vulnerable to greenhouse gas-driven hydroclimate change. Climate models are our primary tool for projecting the future hydroclimate that society in these regions must adapt to, but here, we present a concerning discrepancy between observed and model-based historical hydroclimate trends. Over the arid/semi-arid regions of the world, the predominant signal in all model simulations is an increase in atmospheric water vapor, on average, over the last four decades, in association with the increased water vapor-holding capacity of a warmer atmosphere. In observations, this increase in atmospheric water vapor has not happened, suggesting that the availability of moisture to satisfy the increased atmospheric demand is lower in reality than in models in arid/semi-arid regions. This discrepancy is most clear in locations that are arid/semi-arid year round, but it is also apparent in more humid regions during the most arid months of the year. It indicates a major gap in our understanding and modeling capabilities which could have severe implications for hydroclimate projections, including fire hazard, moving forward.

Reduced terrestrial evaporation increases atmospheric water vapor by generating cloud feedbacks

Reduced terrestrial evaporation directly warms the surface by reducing latent cooling, but also indirectly modifies surface climate by altering atmospheric processes. We use a global climate model to explore two end cases of terrestrial evaporation, comparing the climate of SwampLand, a world where land is always fully saturated with water, to that of DesertLand, where land is always completely lacking in soil moisture. When we suppress evaporation to create a desert-like planet, we find that temperatures increase and precipitation decreases in the global mean. We find an increase in atmospheric water vapor over both land and ocean in the DesertLand simulation. Suppressing evaporative cooling over the continents reduces continental cloud cover, allowing more energy input to the surface and increasing surface moist static energy over land. The residence time of atmospheric water vapor increases by about 50 percent. Atmospheric feedbacks such as changes in air temperatures and cloud cover contribute larger changes to the terrestrial surface energy budget than the direct effect of suppressed evaporation alone. Without the cloud feedback, the land surface still warms with suppressed land evaporation, but total atmospheric water vapor decreases, and the anomalous atmospheric circulations over the continents are much shallower than in simulations with cloud changes; that is, the cloud feedback changes the sign of the water vapor response. This highlights the importance of accounting for atmospheric feedbacks when exploring land surface change impacts on the climate system.

Temperature and cloud condensation nuclei (CCN) sensitivity of orographic precipitation enhanced by a mixed-phase seeder–feeder mechanism: a case study for the 2015 Cumbria flood

Abstract. The formation of orographic precipitation in mixed-phase clouds depends on a complex interplay of processes. This article investigates the microphysical response of orographic precipitation to perturbations of temperature and cloud condensation nuclei (CCN) concentration. A case study for the 2015 Cumbria flood in northern England is performed with sensitivities using a realization of the “piggybacking” method implemented into a limited-area setup of the Icosahedral Nonhydrostatic (ICON) model. A 6 % K−1 enhancement of precipitation results for the highest altitudes, caused by a “mixed-phase seeder–feeder mechanism”, i.e. the interplay of melting and accretion. Total 24 h precipitation is found to increase by only 2 % K−1, significantly less than the 7 % K−1 increase in atmospheric water vapour. A rain budget analysis reveals that the negative temperature sensitivity of the condensation ratio and the increase in rain evaporation dampen the precipitation enhancement. Decreasing the CCN concentration speeds up the microphysical processing, which leads to an increase in total precipitation. At low CCN concentration the precipitation sensitivity to temperature is systematically smaller. It is shown that the CCN and temperature sensitivities are to a large extent independent (with a ±3 % relative error) and additive.

Increase in Tropospheric Water Vapor Amplifies Global Warming and Climate Change

Among the greenhouse gases (GHGs), atmospheric water vapor is the most abundant, has a large influence on the radiation budget of Earth, and plays a decisive role in regional weather processes. We investigate the long-term (1980–2020) changes in global tropospheric water vapor using satellite, radiosonde, and reanalysis data and assess the impact of changes in water vapor on regional and global climate with respect to its radiative feedback. The annual climatology of global tropospheric water vapor varies from 5 to 60 kg/m 2 across different regions. Except in the tropics, there is a strong seasonal cycle in both the southern and northern hemispheres, with the highest values in summer (25 to 65 kg/m 2 ) and smallest values in winter (5 to 20 kg/m 2 ). Most regions show positive trends in the annual mean tropospheric water vapor, at about 0.025 to 0.1 kg/m 2 /year, for the period of 1980–2020, with a notable increase in the Arctic because of the high rise in temperature there. Throughout the troposphere (except 200 hPa), the annual mean specific humidity shows significant positive trends over both land and oceans, with the highest values of approximately 0.015 g/kg/year at 1000 hPa in the tropics. The associated radiative effects on shortwaves at the surface vary from −5 to −70 W/m 2 , with the highest values at Manaus, Porto, and Hanty–Mawsijsk (tropical stations) and the smallest values of about −5 to −10 W/m 2 in the polar regions. The model projections for future high-emission scenarios show a large increase in atmospheric water vapor, approximately twice the current value in the polar latitudes by the end of the 21st century. This is a great concern for global and regional climate, as the rise in water vapor would further augment global warming and phenomena, such as the Arctic amplification. Therefore, this study cautions that there is a significant rise in tropospheric water vapor across latitudes and altitudes, which could further increase the global temperature and, thus, accelerate global climate change.

Differing responses of precipitation in Northern Hemisphere mid-latitudes to increased black carbon aerosols and carbon dioxide

As the most important contributors to global warming in recent decades, anthropogenic carbon dioxide (CO2) and black carbon (BC) play significant roles in driving the global/regional hydrological cycle. Most of previous studies on the climate effects of CO2 and BC focused on tropics and monsoon regions. The influences and their differences of CO2 and BC on the precipitation in Northern Hemisphere mid-latitudes (NHML) have not been paid enough attention. Here we investigate the NHML precipitation responses to a tenfold increase in BC and a doubling of CO2 by analyzing the multi-model simulation results from the Precipitation Driver Response Model Intercomparison Project (PDRMIP). Our results show that the NHML precipitation changes induced by BC and CO2 distinctly differ in trends and seasons. The increased BC will reduce the NHML precipitation, especially in summer, whereas the doubled CO2 will enhance the regional precipitation, mainly in winter. The differences between the BC and CO2 induced NHML precipitation changes are most distinct in Central Asia and central North America. Further analyses reveal the underlying mechanisms of the distinct responses of precipitation: the decrease in NHML precipitation induced by BC aerosols mainly results from the dynamic effect by reducing the temperature gradient, thereby weakening the zonal wind, while the increased precipitation by CO2 is caused by the increase in atmospheric water vapor through the thermodynamic effect. The results of these simulations are helpful for understanding the mechanism of anthropogenic precipitation changes in mid-latitudes.

Advances in understanding large-scale responses of the water cycle to climate change.

Globally, thermodynamics explains an increase in atmospheric water vapor with warming of around 7%/°C near to the surface. In contrast, global precipitation and evaporation are constrained by the Earth's energy balance to increase at ∼2-3%/°C. However, this rate of increase is suppressed by rapid atmospheric adjustments in response to greenhouse gases and absorbing aerosols that directly alter the atmospheric energy budget. Rapid adjustments to forcings, cooling effects from scattering aerosol, and observational uncertainty can explain why observed global precipitation responses are currently difficult to detect but are expected to emerge and accelerate as warming increases and aerosol forcing diminishes. Precipitation increases with warming are expected to be smaller over land than ocean due to limitations on moisture convergence, exacerbated by feedbacks and affected by rapid adjustments. Thermodynamic increases in atmospheric moisture fluxes amplify wet and dry events, driving an intensification of precipitation extremes. The rate of intensification can deviate from a simple thermodynamic response due to in-storm and larger-scale feedback processes, while changes in large-scale dynamics and catchment characteristics further modulate the frequency of flooding in response to precipitation increases. Changes in atmospheric circulation in response to radiative forcing and evolving surface temperature patterns are capable of dominating water cycle changes in some regions. Moreover, the direct impact of human activities on the water cycle through water abstraction, irrigation, and land use change is already a significant component of regional water cycle change and is expected to further increase in importance as water demand grows with global population.

Clausius-Clapeyron Scaling of Extremely Heavy Precipitations: Case Studies of the July 2017 and July 2018 Heavy Rainfall Events over Japan

The usefulness of Clausius–Clapeyron (CC) scaling in explaining extremely heavy precipitations is explored in the present-day climate and in pseudo-global-warming (PGW) conditions. This is analyzed by conducting regional-scale numerical simulations at 1-km grid resolution for two recent extreme rainfall events that occurred in Japan: the case in northern Kyushu during 5–6 July 2017 and the case in Shikoku Island during 5–8 July 2018. The Weather Research and Forecasting (WRF) model was used for the simulation, and the data samples were collected at each grid point individually for each hour over the two regions. We found that the frequency and intensity of extremely heavy precipitation associated with the two events are increased under PGW conditions. The extremely heavy precipitations (> 50 mm h−1) followed CC scaling for the temperatures up to 22°C in the present-day climate, while those under the PGW conditions followed CC-scaling up to 24°C. The peak intensity of the extremely heavy precipitations in the precipitation-temperature relationship is found as ∼ 140 mm h−1 at 25°C in the present-day climate, while the same with PGW conditions is projected as ∼ 160 mm h−1 at 27°C. The increasing rate of the extremely heavy precipitations in the present-climate condition is noticed as ∼ 3 % °C−1 and that under the PGW conditions is anticipated as ∼ 3.5 % °C−1. The increase in peak precipitation intensity and the rate of precipitation increase against temperature in future warming climate are attributed to the decrease in temperature lapse rate and increase in atmospheric water vapor and convective available potential energy. To our knowledge, this is one of the first quantitative investigations of CC scaling of extremely heavy precipitations based on case studies.

Testing the hypothesis that variations in atmospheric water vapour are the main cause of fluctuations in global temperature

A hypothesis that the increasing application of both surface and ground water for irrigation of crops is a significant source of anthropogenic global warming is tested. In climate models, water is already assigned a major secondary amplifying role in warming, solely as a positive feedback from an atmosphere previously warmed by other GHGs. However, this conclusion ignores the direct anthropogenic forcing from increasing use of water in dry regions to grow crops for the human population. The area irrigated worldwide increased by around 1.5% annually between 1960 and 2000, almost trebling in magnitude. Importantly, though only a small proportion of the Earth’s surface, this additional water vapour is dynamically focussed on dry land, intensifying its potential to elevate the troposphere and reduce the regional OLR. Our modelling analysis suggests that the increase in atmospheric water vapour from irrigation could be significantly more than 1% by 2050 compared to 1950, imposing a global forcing exceeding 1.0 W/m2. Fortunately, this hypothesis can be tested, for example, using the satellite data on OLR acquired since the 1970s, relating this to local trends of increasing irrigation or major floods in arid regions. If found consistent with the data, current proposals to mitigate climate change by limiting combustion of fossil fuels may prove less effective. This prediction regarding the warming effect of increasing irrigation is tested using NCAR reanalysis data made possible by the natural experiments of the periodic flooding of Lake Eyre in Australia's semi-arid interior. It is recommended that this hypothesis be tested using data from local studies in irrigated regions such as changes in outgoing longwave radiation and in increased absorption of incoming shortwave radiation in air.

Martian dust storm impact on atmospheric H2O and D/H observed by ExoMars Trace Gas Orbiter.

Global dust storms on Mars are rare1,2 but can affect the Martian atmosphere for several months. They can cause changes in atmospheric dynamics and inflation of the atmosphere3, primarily owing to solar heating of the dust3. In turn, changes in atmospheric dynamics can affect the distribution of atmospheric water vapour, with potential implications for the atmospheric photochemistry and climate on Mars4. Recent observations of the water vapour abundance in the Martian atmosphere during dust storm conditions revealed a high-altitude increase in atmospheric water vapour that was more pronounced at high northern latitudes5,6, as well as a decrease in the water column at low latitudes7,8. Here we present concurrent, high-resolution measurements of dust, water and semiheavywater (HDO) at the onset of a global dust storm, obtained by the NOMAD and ACS instruments onboard the ExoMars Trace Gas Orbiter. We report the vertical distribution of the HDO/H2O ratio (D/H) from the planetary boundary layer up to an altitude of 80 kilometres. Our findings suggest that before the onset of the dust storm, HDO abundances were reduced to levels below detectability at altitudes above 40 kilometres. This decrease in HDO coincided with the presence of water-ice clouds. During the storm, an increase in the abundance of H2O and HDO was observed at altitudes between 40 and 80 kilometres. We propose that these increased abundances may be the result of warmer temperatures during the dust storm causing stronger atmospheric circulation and preventing ice cloud formation, which may confine water vapour to lower altitudes through gravitational fall and subsequent sublimation of ice crystals3. The observed changes in H2O and HDO abundance occurred within a few days during the development of the dust storm, suggesting a fast impact of dust storms on the Martian atmosphere.

Global Positioning System precipitable water vapour (GPS‐PWV) jumps before intense rain events: A potential application to nowcasting

A rapid increase in atmospheric water vapour is a fundamental ingredient for many intense rainfall events. High‐frequency precipitable water vapour (PWV) estimates (1 min) from a Global Positioning System (GPS) meteorological site are evaluated in this paper for intense rainfall events during the CHUVA Vale field campaign in Brazil (November and December 2011) in which precipitation events of differing intensities and spatial dimensions, as observed by an X‐band radar, were explored. A sharp increase in the GPS‐PWV before the more intense events was found and termed GPS‐PWV “jumps.” These jumps are probably associated with water vapour convergence and the continued formation of cloud condensate and precipitation particles. A wavelet correlation analysis between the high temporal‐resolution GPS‐PWV time series and rainfall events evaluated in this study shows that there are oscillations in the PWV time series correlated with the more intense rainfall events. These oscillations are on scales related to periods from about 32 to 64 min (associated with GPS‐PWV jumps) and from 16 to 34 min (associated with positive pulses of the PWV). The GPS‐PWV time‐derivative histogram for the time window before the rainfall event reveals different distributions influenced by positive pulses of the GPS‐PWV (derivative > 9.5 mm/hr) for higher intensity and extension events. These features are indicative of the occurrence of intense precipitation and, consequently, have the potential for application in nowcasting activities.

Diagnosing Human-Induced Dynamic and Thermodynamic Drivers of Extreme Rainfall

Abstract Factors responsible for extreme monthly rainfall over Texas and Oklahoma during May 2015 are assessed. The event had a return period of at least 400 years, in contrast to the prior record, which was roughly a 100-yr event. The event challenges attribution science to disentangle factors because it occurred during a strong El Niño, a natural pattern of variability that affects the region’s springtime rains, and during the warmest global mean temperatures since 1880. Effects of each factor are diagnosed, as is the interplay between El Niño dynamics and human-induced climate change. Analysis of historical climate simulations reveals that El Niño was a necessary condition for monthly rains to occur having the severity of May 2015. The model results herein further reveal that a 2015 magnitude event, whether conditioned on El Niño or not, was made neither more intense nor more likely to be due to human-induced climate change over the past century. The intensity of extreme May rainfall over Texas and Oklahoma , analogous to the 2015 event, increases by roughly 5% by the latter half of the twenty-first century. No material changes occur in either El Niño–related teleconnections or in overall atmospheric dynamics during extreme May rainfall over the twenty-first century. The increased severity of Texas/Oklahoma May rainfall events in the future is principally due to thermodynamic driving, although much less than implied by simple Clausius–Clapeyron scaling arguments given a projected 23% increase in atmospheric precipitable water vapor. Other thermodynamic factors are identified that act in opposition to the increase in atmospheric water vapor, thereby reducing the effectiveness of overall thermodynamic driving of extreme May rainfall changes over Texas and Oklahoma.

Process-Based Decomposition of the Decadal Climate Difference between 2002–13 and 1984–95

This study examines at the process level the climate difference between 2002–13 and 1984–95 in ERA-Interim. A linearized radiative transfer model is used to calculate the temperature change such that its thermal radiative cooling would balance the energy flux perturbation associated with the change of an individual process, without regard to what causes the change of the process in the first place. The global mean error of the offline radiative transfer model calculations is 0.09 K, which corresponds to the upper limit of the uncertainties from a single term in the decomposition analysis. The process-based decomposition indicates that the direct effect of the increase of CO2 (0.15 K) is the largest contributor to the global warming between the two periods (about 0.27 K). The second and third largest contributors are the cloud feedback (0.14 K) and the combined effect of the oceanic heat storage and evaporation terms (0.11 K), respectively. The largest warming associated with the oceanic heat storage term is found in the tropical Pacific and Indian Oceans, with relatively weaker warming over the tropical Atlantic Ocean. The increase in atmospheric moisture adds another 0.1 K to the global surface warming, but the enhancement in tropical convections acts to reduce the surface warming by 0.17 K. The ice-albedo and atmospheric dynamical feedbacks are the two leading factors responsible for the Arctic polar warming amplification (PWA). The increase of atmospheric water vapor over the Arctic region also contributes substantially to the Arctic PWA pattern.

Changes in atmospheric water content associated with an unusual high snowfall during June 2004 at Maitri station (Schirmacher Oasis, East Antarctica) and the role of South West Indian ridge geodynamics

We study changes in the atmospheric water vapour associated with the unusual snowfall during June 2004 polar night at the Indian Antarctica station Maitri (MAIT), located ~70 km inland of the East Antarctic north coast. The GPS-derived Zenith Total Delay data for the 2004 polar night are used to evaluate the atmospheric water content at four GPS locations along the northern margin of East Antarctica. Stations Maitri and Syowa (SYOG) showed significant increase in atmospheric water vapour from GPS day 162 (10 June 2004) to 176 (24 June 2004) and correlate well with the duration of heavy snowfall at MAIT. The precipitable water vapour values computed for IGS station SYOG confirm high water vapour during this high snowfall period. Such an anomalous water vapour over East Antarctica in this peak winter time, characterized by the complete absence of solar radiations, suggests a link between this phenomenon and the high evaporation rate over South West Indian Ridge triggered by the geothermal heat radiated to the sea bottom through active magma spreading.

Contrasting the eastern Pacific El Niño and the central Pacific El Niño: process-based feedback attribution

This paper examines the roles of radiative and non-radiative air–sea coupled thermodynamic processes in modifying sea surface temperature (SST) anomalies driven by (air–sea coupled) oceanic dynamic processes, focusing on their contributions to the key differences between the eastern Pacific (EP) El Niño and the central Pacific (CP) El Niño. The attribution is achieved by decomposing SST anomalies into partial temperature anomalies due to individual processes using a coupled atmosphere-surface climate feedback-response analysis method. Oceanic processes induce warming from the central to the eastern equatorial Pacific and cooling over the western basin with a maximum warming center in the central Pacific for both types of El Niño. The processes that act to oppose the oceanic process-induced SST anomalies are surface latent heat flux, sensible heat flux, cloud, and atmospheric dynamic feedbacks, referred to as negative-feedback processes. The cooling due to each of the four negative-feedback processes is the strongest in the region where the initial warming due to oceanic processes is the largest. Water–vapor feedback is the sole process that acts to enhance the initial warming induced by oceanic processes. The increase in atmospheric water vapor over the eastern Pacific is much stronger for the EP El Niño than for the CP El Niño. It is the strong water–vapor feedback over the eastern Pacific and the strong negative feedbacks over the central equatorial Pacific that help to relocate the maximum warming center from the central Pacific to the eastern basin for the EP El Niño.

Dynamical and thermodynamical modulations on future changes of landfalling atmospheric rivers over western North America

AbstractThis study examines future changes of landfalling atmospheric rivers (ARs) over western North America using outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The result reveals a strikingly large increase of AR days by the end of the 21st century in the RCP8.5 scenario, with fractional increases between 50% and 600%, depending on the seasons and landfall locations. These increases are predominantly controlled by the super‐Clausius‐Clapeyron rate of increase of atmospheric water vapor with warming, while changes of winds that transport moisture in the ARs, or dynamical effect, mostly counter the thermodynamical effect of increasing water vapor, limiting the increase of AR events in the future. The consistent negative effect of wind changes on AR days during spring and fall can be linked to the robust poleward shift of the subtropical jet in the North Pacific basin.

Investigating vegetation–climate feedbacks during the early Eocene

Abstract. Evidence suggests that the early Eocene was a time of extreme global warmth. However, there are discrepancies between the results of many previous modelling studies and the proxy data at high latitudes, with models struggling to simulate the shallow temperature gradients of this time period to the same extent as the proxies indicate. Vegetation–climate feedbacks play an important role in the present day, but are often neglected in these palaeoclimate modelling studies, and this may be a contributing factor to resolving the model–data discrepancy. Here we investigate these vegetation–climate feedbacks by carrying out simulations of the early Eocene climate at 2 × and 4 × pre-industrial atmospheric CO2 with fixed vegetation (homogeneous shrubs everywhere) and dynamic vegetation. The results show that the simulations with dynamic vegetation are warmer in the global annual mean than the simulations with fixed shrubs by 0.9 °C at 2 × and 1.8 °C at 4 ×. Consequently, the warming when CO2 is doubled from 2 × to 4 × is 1 °C higher (in the global annual mean) with dynamic vegetation than with fixed shrubs. This corresponds to an increase in climate sensitivity of 26%. This difference in warming is enhanced at high latitudes, with temperatures increasing by over 50% in some regions of Antarctica. In the Arctic, ice–albedo feedbacks are responsible for the majority of this warming. On a global scale, energy balance analysis shows that the enhanced warming with dynamic vegetation is mainly associated with an increase in atmospheric water vapour but changes in clouds also contribute to the temperature increase. It is likely that changes in surface albedo due to changes in vegetation cover resulted in an initial warming which triggered these water vapour feedbacks. In conclusion, dynamic vegetation goes some way to resolving the discrepancy, but our modelled temperatures cannot reach the same warmth as the data suggest in the Arctic. This suggests that there are additional mechanisms, not included in this modelling framework, behind the polar warmth or that the proxies have been misinterpreted.

Holocene climate variations retrieved from Gun Nuur lake-sediment core in the northern Mongolian Plateau

Our pollen-based climate reconstruction shows that both temperature and moisture were low during the early Holocene from ~10,000 to ~8000 cal. a BP and gradually climbed from ~8000 to ~6000 cal. a BP. The last ~6000 years have been generally warm and wet with high-amplitude fluctuations. The high-moisture conditions for the past ~6000 years are not only corroborated by high arboreal pollen/non-arboreal (AP/NAP; trees and shrubs over herbs) pollen ratio and high pollen concentration but also by high organic matter content and diatom data. We suggest that lower winter insolation and remaining ice covers in the early Holocene might have slowed down temperature rise in the continental interiors and that the resulted low temperature might have suppressed regional evaporation that was a major water vapor source for precipitation. At the same time, icemelt-water injection into the North Atlantic Ocean might have also suppressed water vapor supplies to the downwind areas, including the northern Mongolian Plateau. The increase in winter insolation and the final melting of ice sheets, together with the increased atmospheric CO2 concentration and the associated increase in atmospheric water vapor, could be responsible for the higher temperature during the past ~6000 years. The icemelting and the associated rise of sea level during the past ~6000 years might be responsible for the weakening of the winter monsoon in the interior of Asian continent, thus alluring farther eastward penetration of the relatively warm and wet Atlantic air masses and consequently bringing more precipitation to the northern Mongolian Plateau.

Assessing Future Changes in the East Asian Summer Monsoon Using CMIP3 Models: Results from the Best Model Ensemble

Abstract Future changes in the East Asian summer monsoon (EASM) have been estimated from the six best-performing models in phase 3 of the Coupled Model Intercomparison Project (CMIP3) included in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). The composite mean rainband over East Asia during the summer season exhibits the characteristic EASM front relatively well with slightly less precipitation over east–central China and the baiu front aligned more steeply in the latitudinal direction. An ensemble from the coupled models that poorly simulated the EASM is shown to degrade the overall results, giving rise to an underestimation of the projected increase rate in precipitation. In a quantitative estimate, the 22-member ensemble-mean precipitation for the period 2079–99 is anticipated to increase by only 5%–10%, which is half the increase in the corresponding mean precipitation from the good models (10%–20%). Moisture budget analysis demonstrates that the good-model ensemble mean is in a much closer correspondence with observations than the poor- or all-model ensemble mean, supporting the reliable selection of the best-performing models. An increase in the vertically integrated moisture flux convergence is attributed to the enhanced precipitation in the future climate. In particular, a significant increase in atmospheric water vapor because of a warmer SST over the western Pacific plays a more critical role in the enhanced precipitation than a change in monsoonal low-level circulation does. In contrast to the apparent meridional shift of the primary location of the future EASM front presented in previous studies, the current analysis shows a negligible amount of meridional movement.

Changing Frequency of Heavy Rainfall over the Central United States

AbstractRecords of daily rainfall accumulations from 447 rain gauge stations over the central United States (Minnesota, Wisconsin, Michigan, Iowa, Illinois, Indiana, Missouri, Kentucky, Tennessee, Arkansas, Louisiana, Alabama, and Mississippi) are used to assess past changes in the frequency of heavy rainfall. Each station has a record of at least 50 yr, and the data cover most of the twentieth century and the first decade of the twenty-first century. Analyses are performed using a peaks-over-threshold approach, and, for each station, the 95th percentile is used as the threshold. Because of the count nature of the data and to account for both abrupt and slowly varying changes in the heavy rainfall distribution, a segmented regression is used to detect changepoints at unknown points in time. The presence of trends is assessed by means of a Poisson regression model to examine whether the rate of occurrence parameter is a linear function of time (by means of a logarithmic link function). The results point to increasing trends in heavy rainfall over the northern part of the study domain. Examination of the surface temperature record suggests that these increasing trends occur over the area with the largest increasing trends in temperature and, consequently, with an increase in atmospheric water vapor.