Tropospheric Temperature Changes Research Articles

Long-term changes in tropospheric temperature in India: Insights from radiosonde measurements, reanalysis data and CMIP6 model projections

The continuous rise in greenhouse gases (GHGs) in the atmosphere is one of the major factors responsible for the global warming and enhanced tropospheric temperature. In this context, we examine the long-term spatiotemporal changes in the tropospheric temperature (1000–100 hPa) in India using radiosonde measurements and the reanalysis data from European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5), Modern-Era Retrospective analysis for Research and Application Version 2 (MERRA2) and National Center for Environmental Prediction (NCEP) for the period 1980–2020. We also investigate the future projections of tropospheric temperature in the Shared Socio-economic Pathways (SSPs) 2–4.5 and 5–8.5 by using the Coupled Model Intercomparison Project 6 (CMIP6) results for the period 2015–2100 over India. Our analysis shows a significant positive trend (0.01–0.04 °C/yr) in the annual mean temperature in the upper troposphere (500–100 hPa) in India during the period 1980–2020. The warming trends are also noticeable in the middle and lower troposphere of India. Among the regions, the northeast, north central and northwest India exhibit significant warming of about 0.01–0.06 °C/yr in the upper troposphere (500–100 hPa). On the other hand, most regions show a decline in temperature at the tropopause (100 hPa), which is stronger in the interior peninsular, north central and east coast of India, at about −0.05 ± 0.01°C/yr. All reanalysis data show consistent warming and cooling trends in the respective regions as observed from the radiosondes, albeit slight differences in their values. The future high emission scenario of SSP5–8.5 shows a warming trend of about 0.04–0.08 °C/yr at 1000–200 hPa by the end of the century, which is highest at 250 hPa. The increasing trend in tropospheric temperature is a serious concern, which calls for adaptation and mitigation measures to alleviate the impacts of this accelerated warming in India.

Spatiotemporal characteristics of the time of emergence for anthropogenic tropospheric temperature changes based on the CMIP6 multi-model results

In the 20th century, with the intensification of human activities, the Earth is experiencing unprecedented warming. However, there are certain differences in the sensitivity of temperature changes to anthropogenic forcings in different regions and at different altitudes of the troposphere. The time of emergence (TOE) is the key point at which the anthropogenic climate change signal exceeds from the internal climate variability serving as a noise. It is a crucial variable for climate change detection, climate prediction and risk assessment. Here, we systematically analyzed the spatiotemporal characteristics of the TOE of temperature changes over the past century by calculating the SNR based on the selected CMIP6 multi-model outputs. The results show that the temperature TOE, particularly in the lower and middle troposphere, shows distinct latitude dependence, displaying an ‘M-type’ distribution from the Antarctic to the Arctic: it first appears in low-latitudes, followed by high-latitudes, and last appears in the two mid latitude bands. For the tropics, the TOE of tropospheric temperatures becomes earlier with increasing altitude: the TOE of air temperatures at the surface, mid-tropospheric 500 hPa and upper-tropospheric 200 hPa occurs in 1980 ± 15, 1965 ± 20, and 1930 ± 30, respectively. The TOEs of tropospheric temperatures in eastern equatorial Pacific are 10–30 years later than those in the western equatorial Pacific. For the regional TOEs of surface air temperature diverse differences exist on land and ocean in various latitudes of two hemispheres.

Enhancing InSAR accuracy: Unveiling more accurate displacement fields through 3-D troposphere tomography

Interferometric Synthetic Aperture Radar (InSAR) measurements are susceptible to tropospheric delay induced by changes in tropospheric temperature, pressure, and water vapor. While pressure and temperature temporal variations have a limited impact on phase gradients, variations in water vapor, which contribute to the wet component of tropospheric delay, play a significant role. Therefore, obtaining a more accurate estimation of the wet refractivity of the tropospheric layer is crucial for improving InSAR displacement field accuracy. This paper presents the utilization of Global Navigation Satellite Systems (GNSS) tropospheric tomography for reconstructing three-dimensional (3-D) wet refractivity. Various data sources are employed, including GNSS measurements, ERA5 reanalysis data, radiosonde observations, and Sentinel-1A radar acquisitions. ERA5 reanalysis data and radiosonde observations are used to calculate tropospheric hydrostatic delay and to evaluate the wet refractivity obtained from tomography, respectively. The effectiveness of tomography techniques in mitigating the tropospheric signal in unwrapped interferograms is compared with the total tropospheric delay obtained from Generic Atmospheric Correction Online Service (GACOS) products, which is one of the most common approach for tropospheric correction of InSAR measurements. To ensure a comprehensive analysis, the effects of the correction methods are investigated in two different weather conditions. Based on the results, GNSS tropospheric tomography reduces the mean Root Mean Square Error (RMSE) by about 1.2 cm compared to GACOS products. This technique proves to be highly effective in improving the accuracy of InSAR displacement field estimation, particularly under challenging weather conditions.

Physically based equation representing the forcing-driven precipitation in climate models

This study aims to improve our understanding of the response of precipitation to forcings by proposing a physically-based equation that resolves simulated precipitation based on the atmospheric energy budget. The equation considers the balance between latent heat release by precipitation and the sum of the slow response by tropospheric temperature changes and the fast response by abrupt radiative forcing (RF) changes. The equation is tuned with three parameters for each climate model and then adequately reproduces time-varying precipitation. By decomposing the equation, we highlight the slow response as the largest contributor to forcing-driven responses and uncertainty sizes in simulations. The second largest one to uncertainty is the fast-RF response from aerosols or greenhouse gases (GHG), depending on the low or highest Coupled Model Intercomparison Projection 6 future scenarios. The likely range of precipitation change at specific warming levels under GHG removal (GGR) and solar radiation management (SRM) mitigation plans is evaluated by a simple model optimizing the relationship between temperature and decomposed contributions from multi-simulations under three scenarios. The results indicate that GGR has more severe effects from aerosols than GHG for a 1.5 K warming, resulting in 0.91%–1.62% increases in precipitation. In contrast, SRM pathways project much drier conditions than GGR results due to the tropospheric cooling and remaining anthropogenic radiative heating. Overall, the proposed physically-based equation, the decomposition analysis, and our simple model provide valuable insights into the uncertainties under different forcings and mitigation pathways, highlighting the importance of slow and fast responses to human-induced forcings in shaping future precipitation changes.

Rise in Mid-Tropospheric Temperature Trend (MSU/AMSU 1978–2022) over the Tibet and Eastern Himalayas

The high-altitude Hindu Kush-Himalayan region (HKH, average ~5 km from msl) and the adjacent Indo-Gangetic plains (IG plains, ~0–250 m msl), due to their geographical location and complex topography, are reported to be highly sensitive to climatic changes. Recent studies show that the impacts of climate change and associated changes in water resources (glacial/snow melt water and rainfall) in this region are multifaceted, thereby affecting ecosystems, agriculture, industries, and inhabitants. In this study, 45 years of Microwave Sounding Unit/Advanced Microwave Sounding Unit (MSU/AMSU)-derived mid-tropospheric temperature (TMT, 3–7 km altitude) and lower tropospheric temperature (TLT, 0–3 km altitude) data from the Remote Sensing Systems (RSS Version 4.0) were utilized to analyze the overall changes in tropospheric temperature in terms of annual/monthly trends and anomalies. The current study shows that the mid-tropospheric temperature (0–3 km altitude over the HKH region) has already alarmingly increased (statistically significant) in Tibet, the western Himalayas, and the eastern Himalayas by 1.49 °K, 1.30 °K, and 1.35 °K, respectively, over the last 45 years (1978–2022). As compared to a previous report (TMT trend for 30 years, 1979–2008), the present study of TMT trends for 45 years (1978–2022) exhibits a rise in percent change in the trend component in the high-altitude regions of Tibet, the western Himalayas, and the eastern Himalayas by approximately 310%, 80%, and 170%, respectively. In contrast, the same for adjacent plains (the western and eastern IG plains) shows a negligible or much lower percent change (0% and 40%, respectively) over the last 14 years. Similarly, dust source regions in Africa, Arabia, the Middle East, Iran, and Pakistan show only a 130% change in warming trends over the past 14 years. In the monthly breakup, the ‘November to March’ period usually shows a higher TMT trend (with peaks in December, February, and March) compared to the rest of the months, except in the western Himalayas, where the peak is observed in May, which can be attributed to the peak dust storm activity (March to May). Snow cover over the HKH region, where the growing season is known to be from September to February, is also reported to show the highest snow cover in February (with the peak in January, February, or March), which coincides with the warmest period in terms of anomaly and trend observed in the long-term mid-tropospheric temperature data (1978–2022). Thus, the current study highlights that the statistically significant and positive TMT warming trend (95% CI) and its observed acceleration over the high-altitude region (since 2008) can be attributed to being one of the major factors causing an acceleration in the rate of melting of snow cover and glaciers, particularly in Tibet and the Eastern Himalayas.

Tropical tropospheric warming pattern explained by shifts in convective heating in the Matsuno–Gill model

AbstractHorizontal temperature gradients in the tropical free troposphere are fairly weak, and tropical tropospheric warming is usually treated as uniform. However, we show here that projected tropospheric warming is spatially inhomogeneous in Coupled Model Intercomparison Project Phase 6 models, as well as in a storm‐resolving climate model. We relate the upper tropospheric warming pattern to sea‐surface temperature changes that reorganise convection and thereby cause spatial shifts in convective heating. Using the classical Gill model for tropical circulation and forcing it with precipitation changes that arise due to greenhouse gas warming, we can understand and reproduce the different warming patterns simulated by a range of global climate models. Forcing the Gill model with precipitation changes from a certain region demonstrates how local tropospheric temperature changes depend on local changes in convective heating. Close to the Equator, anomalous geopotential gradients are balanced by the dissipation term in the Gill model. The optimal dissipation time‐scale to reproduce the warming pattern varies depending on the Coupled Model Intercomparison Project Phase 6 model, and is between 1 and 10 days. We demonstrate that horizontal advection and eddy momentum fluxes have large enough equivalent dissipation time‐scales to balance the gradients in geopotential and thereby shape the warming pattern. Though climate models show a large spread in projections of tropical sea‐surface temperature and precipitation changes, our results imply that, once these predictions improve, our confidence in the predicted upper tropospheric warming pattern should also increase.

The Role of Coupled North Pacific Atmosphere–Ocean Interactions in Impacts of Tibetan Plateau Snow Anomalies on the Northern Hemisphere Winter Atmospheric Circulation

Abstract Previous studies indicate observed influences of autumn and winter Tibetan Plateau (TP) snow-cover anomalies on the winter Pacific–North American (PNA) teleconnection. This study simulates atmospheric and oceanic responses to persistent autumn–winter TP snow forcing using an atmospheric general circulation model (AGCM) and a coupled atmospheric-oceanic general circulation model (AOGCM), and quantifies the role of atmosphere–ocean interactions over North Pacific in TP snow effects. The AOGCM experiment induces a stronger and more realistic remote PNA response to heavy TP snow anomalies, and also a significant winter horseshoe-like North Pacific sea surface temperature (SST) pattern resulting from an anomalous equivalent barotropic cyclone, or a strengthened Aleutian low, with associated cyclonic wind stress anomalies. The horseshoe-like SST anomaly pattern is used as boundary forcing (without prescribed heavy TP snow) in another AOGCM experiment, which simulates an enhanced winter Aleutian low and a PNA-like response similar to the original AOGCM responses, indicating that that the direct Pacific–North American atmospheric response to persistent TP snow forcing in the AGCM is amplified in the AOGCM by the North Pacific midlatitude atmosphere–ocean interactions. This suggests that the mechanisms of the winter PNA responses to TP snow forcing involve dynamical atmospheric processes such as horizontal propagation of Rossby wave energy and transient eddy feedbacks, and also North Pacific atmosphere–ocean interactions, which provide a positive feedback on the development of the remote PNA teleconnection. Significance Statement The Tibetan Plateau has a 2.5 million km2 area and a 4000 m average elevation, and is often called “the third pole” because of its polar-like climate in midlatitude Eurasia. A heavy Tibetan Plateau snow cover increases the albedo effect and causes tropospheric cold temperature and low height changes that can be carried or advected to the North Pacific by the prevailing westerlies. The purpose of this study is to better understand how Tibetan Plateau snow anomalies influence the North Pacific and the remote atmospheric circulation through numerical simulations. Our results find that those North Pacific responses amply the direct atmospheric response to Tibetan Plateau snow-cover anomalies. This greatly improves our understanding of physical mechanisms of Tibetan Plateau snow impacts.

Global Changes in Water Vapor 1979–2020

AbstractGlobal‐scale changes in water vapor and responses to surface temperature variability since 1979 are evaluated across a range of satellite and ground‐based observations, a reanalysis (ERA5) and coupled and atmosphere‐only CMIP6 climate model simulations. Global‐mean column integrated water vapor increased by 1%/decade during 1988–2014 in observations and atmosphere‐only simulations. However, coupled simulations overestimate water vapor trends and this is partly explained by past studies showing that internal climate variability suppressed observed warming in this period. Decreases in low‐altitude tropical water vapor in ERA5 and ground‐based observations before around 1993 are considered suspect based on inconsistency with simulations and increased column integrated water vapor in microwave satellite data since 1979. Atmospheric Infra‐red Sounder satellite data does not capture the increased tropospheric water vapor since 2002 shown by other satellite, reanalysis, and model products. However, global water vapor responses to interannual surface temperature variability are consistent across data sets with increases of ∼4%–5% near the surface and 10%–15% at 300 hPa for each 1°C increase in global surface temperature. Global water vapor responses are explained by thermodynamic amplification of upper tropospheric temperature changes and the Clausius Clapeyron temperature dependence of saturation vapor pressure that are dominated by the tropical ocean responses. Upper tropospheric moistening is larger in climate model simulations with greater upper tropospheric warming.

Upper Vertical Thermal Contrast Over Western North Pacific and its Impact on the East Side of Tibetan Plateau During ENSO Years

ABSTRACT The present study reveals significantly out-of-phase changes at 100 and 250 hPa over the western North Pacific (WNP) during ENSO years. We found that the tropospheric temperature exhibits an increase and decrease in the 100 and 250 hPa respectively over the WNP during the La Niña episodes. The tropospheric temperature pattern over the 100 and 250 hPa during La Niña episodes are found to be described by an anti-cyclonic and cyclonic circulation in the wind shear respectively. Concomitant with the tropospheric temperature change an increase/decrease of convection over the WNP is observed during the La Niña/El Niño years. The further finding shows the robust dynamical changes in the subtropical jet over the WNP during ENSO episodes act as pathways leading to moisture transport towards the east side of the Tibetan Plateau and thereby affect the convection pattern over there. To affirm the findings, a novel causal inference technique is used to identify a dominant causality between the WNP vertical thermal contrast (VTC) and the longwave fluxes over the east side of the Tibetan plateau.

Future summer warming pattern under climate change is affected by lapse-rate changes

Abstract. Greenhouse-gas-driven global temperature change projections exhibit spatial variations, meaning that certain land areas will experience substantially enhanced or reduced surface warming. It is vital to understand enhanced regional warming anomalies as they locally increase heat-related risks to human health and ecosystems. We argue that tropospheric lapse-rate changes play a key role in shaping the future summer warming pattern around the globe in mid-latitudes and the tropics. We present multiple lines of evidence supporting this finding based on idealized simulations over Europe, as well as regional and global climate model ensembles. All simulations consistently show that the vertical distribution of tropospheric summer warming is different in regions characterized by enhanced or reduced surface warming. Enhanced warming is projected where lapse-rate changes are small, implying that the surface and the upper troposphere experience similar warming. On the other hand, strong lapse-rate changes cause a concentration of warming in the upper troposphere and reduced warming near the surface. The varying magnitude of lapse-rate changes is governed by the temperature dependence of the moist-adiabatic lapse rate and the available tropospheric humidity. We conclude that tropospheric temperature changes should be considered along with surface processes when assessing the causes of surface warming patterns.

Snow-darkening versus direct radiative effects of mineral dust aerosol on the Indian summer monsoon onset: role of temperature change over dust sources

Abstract. Atmospheric absorptive aerosols exert complicated effects on the climate system, two of which are through their direct radiative forcing and snow-darkening forcing. Compared to black carbon, the snow-darkening effect of dust on climate has been scarcely explored till now. When depositing in snow, dust can reduce the albedo of snow by darkening it and increasing the snowmelt. In this study, the snow-darkening effect of dust, as well as the direct radiative effect, on the Indian summer monsoon are evaluated by atmospheric general circulation model experiments. The results show that the snow-darkening and direct radiative forcing of dust both have significant impacts on the onset of the Indian monsoon, but they are distinctly opposite. The snow-darkening effect of dust weakens the Indian monsoon precipitation during May and June, opposite to black carbon. The surface temperature over central Asia and the western Tibetan Plateau becomes warmer due to the dust-induced decrease in snow cover, which leads to a local low-level cyclonic anomaly as well as an anticyclonic anomaly over the Indian subcontinent and Arabian Sea. This circulation pattern allows air currents penetrating into the Indian subcontinent more from central Asia but less from the Indian Ocean. In contrast, the direct radiative forcing of dust warms the low troposphere over the Arabian Peninsula, which intensifies moisture convergence and precipitation over the Indian monsoon region. The upper tropospheric atmospheric circulation over Asia is also sensitive to both effects. Compared to previous studies which emphasized the temperature over the Tibetan Plateau, our results further highlight an important role of surface/low tropospheric temperature changes over dust source areas, which can also significantly modify the response of summer monsoon. Thus, links between the climatic impact of dust and complicated thermal conditions over Asia are of importance and need to be clarified accurately.

Aerosol versus greenhouse gas impacts on Southern Hemisphere general circulation changes

This study re-examines the possible impacts of anthropogenic aerosols (AERs), which are primarily concentrated in the Northern Hemisphere extratropics, on atmospheric general circulation changes in the Southern Hemisphere (SH). The long-term trends of the SH Hadley Cell edge and Southern Annular Mode (SAM) are evaluated for the historical and single forcing experiments of 12 models archived for the Fifth Coupled Model Inter-comparison Project (CMIP5). The AER-induced temperature changes are typically opposite in sign to that of the greenhouse gas (GHG)-induced changes. However, the zonal-mean circulation changes driven by the AERs are not simply opposite to those due to the GHGs. Depending on the analysis period, they can be either opposite or the same as the GHG-induced ones. It is also found that the inter-model spread of AER-driven circulation changes is primarily dependent on the uncertainty of tropospheric temperature changes, particularly static stability changes in the subtropics, whereas that of GHG-induced changes in the austral summer is influenced by the uncertainty of both the tropospheric and polar lower-stratospheric temperature changes. This accentuates that the AER-induced SH circulation changes are not simply mirroring the GHG-induced ones.

Nonlocality of Tropical Cyclone Activity in Idealized Climate Simulations

AbstractThe response of tropical cyclone activity in the predicted warmer future climate is still a topic of scientific research. However, by the nonlocality hypothesis, tropical cyclone activity depends rather on relative than absolute sea surface temperature (SST). This hypothesis is investigated through idealized experiments performed with the global atmospheric climate model PlaSim. The model includes a prescribed SST and adopts a spectral resolution of T170 for resolving tropical cyclones. An idealized land‐sea configuration with two oceans and two continents has been used to study the nonlocality mechanism. The sensitivity experiments WARMBASIN and COLDBASIN include a positive and negative SST anomaly of 2.5 K in the northeastern basin. The tropical cyclone activity in the control run is similar in all four ocean basins while experiment WARMBASIN simulates a striking local increase of tropical cyclones. However, they nearly vanish in the other three ocean basins. The response is weaker and in the opposite way in COLDBASIN with a local reduction and a nonlocal increase. Analysis of well‐known cyclogenesis indices shows that nonlocality could be explained by the vorticity, relative humidity, and upper tropospheric temperature changes in experiment WARMBASIN while only vorticity is in agreement with nonlocality in experiment COLDBASIN. The vorticity anomalies determine the presence or absence of a steady state large‐scale low at the location where tropical cyclones favorably form. This low is modified by an SST‐induced change of a Walker‐like planetary circulation.

Postmillennium changes in stratospheric temperature consistently resolved by GPS radio occultation and AMSU observations

AbstractTemperature changes in the lower and middle stratosphere during 2001–2016 are evaluated using measurements from GPS Radio Occultation (RO) and Advanced Microwave Sounding Unit (AMSU) aboard the Aqua satellite. After downsampling of GPS‐RO profiles according to the AMSU weighting functions, the spatially and seasonally resolved trends from the two data sets are in excellent agreement. The observations indicate that the middle stratosphere has cooled in the time period 2002–2016 at an average rate of −0.14 ± 0.12 to −0.36 ± 0.14 K/decade, while no significant change was found in the lower stratosphere. The meridionally and vertically resolved trends from high‐resolution GPS‐RO data exhibit a marked interhemispheric asymmetry and highlight a distinct boundary between tropospheric and stratospheric temperature change regimes matching the tropical thermal tropopause. The seasonal pattern of trend reveals significant opposite‐sign structures at high and low latitudes, providing indication of seasonally varying change in stratospheric circulation.

Long-Term Rainfall Variability in the Eastern Gangetic Plain in Relation to Global Temperature Change

ABSTRACTIndia’s annual weather cycle consists mainly of wet and dry periods with monsoonal rains being one of the significant wet periods that shows strong spatiotemporal variability. This study includes the climatological characteristics, fluctuation features, and periodic cycles of annual, seasonal, and monthly rainfall of seven river basins across the eastern Gangetic Plain (EGP) using the longest possible instrumental area-averaged monthly rainfall series (1829–2012). Understanding the relationships between these parameters and global tropospheric temperature changes and El Niño and La Niña climatic signals is also attempted.Climatologically, mean annual rainfall in the EGP varies from 1070.5 mm in the Tons River basin to 1508.6 mm in the Subarnarekha River basin. The highest rainfall in the EGP occurs during monsoon (1188 mm). The annual rainfall in all river basins and monsoon rainfall in four river basins is normally distributed. Annual and monsoonal rainfall in the Brahmani and Son River basins show a significant decreasing long-term trend. Over the last 20 years, annual rainfall in all river basins and monsoonal rainfall in five river basins show a decreasing trend. The power spectra for all rainfall series are characterized by consistent significant wavelength peaks at 3–5 years, 10–20 years, 40 years, and more than 80 years. Short-term fluctuations with a period less than 10 years is the major contributor to total variance in annual and/or monsoon rainfall (77.6%), followed by decadal variations with a period of 10–30 years (13.1%) and a long-term trend with a period greater than 30 years (9.3%).Temperature and thickness gradients from the Tibet–Himalaya–Karakoram–Hindu Kush highlands to eight strong highs show a significant correlation with rainfall during the onset and withdrawal phases of summer monsoon in the EGP.

Isotopic ordering in atmospheric O2 as a tracer of ozone photochemistry and the tropical atmosphere

AbstractThe distribution of isotopes within O2 molecules can be rapidly altered when they react with atomic oxygen. This mechanism is globally important: while other contributions to the global budget of O2 impart isotopic signatures, the O(3P) + O2 reaction resets all such signatures in the atmosphere on subdecadal timescales. Consequently, the isotopic distribution within O2 is determined by O3 photochemistry and the circulation patterns that control where that photochemistry occurs. The variability of isotopic ordering in O2 has not been established, however. We present new measurements of 18O18O in air (reported as Δ36 values) from the surface to 33 km altitude. They confirm the basic features of the clumped‐isotope budget of O2: Stratospheric air has higher Δ36 values than tropospheric air (i.e., more 18O18O), reflecting colder temperatures and fast photochemical cycling of O3. Lower Δ36 values in the troposphere arise from photochemistry at warmer temperatures balanced by the influx of high‐Δ36 air from the stratosphere. These observations agree with predictions derived from the GEOS‐Chem chemical transport model, which provides additional insight. We find a link between tropical circulation patterns and regions where Δ36 values are reset in the troposphere. The dynamics of these regions influences lapse rates, vertical and horizontal patterns of O2 reordering, and thus the isotopic distribution toward which O2 is driven in the troposphere. Temporal variations in Δ36 values at the surface should therefore reflect changes in tropospheric temperatures, photochemistry, and circulation. Our results suggest that the tropospheric O3 burden has remained within a ±10% range since 1978.

Understanding the Intermodel Spread in Global-Mean Hydrological Sensitivity*

Abstract This paper assesses intermodel spread in the slope of global-mean precipitation change ΔP with respect to surface temperature change. The ambiguous estimates in the literature for this slope are reconciled by analyzing four experiments from phase 5 of CMIP (CMIP5) and considering different definitions of the slope. The smallest intermodel spread (a factor of 1.5 between the highest and lowest estimate) is found when using a definition that disentangles temperature-independent precipitation changes (the adjustments) from the slope of the temperature-dependent precipitation response; here this slope is referred to as the hydrological sensitivity parameter η. The estimates herein show that η is more robust than stated in most previous work. The authors demonstrate that adjustments and η estimated from a steplike quadrupling CO2 experiment serve well to predict ΔP in a transient CO2 experiment. The magnitude of η is smaller in the coupled ocean–atmosphere quadrupling CO2 experiment than in the noncoupled atmosphere-only experiment. The offset in magnitude due to coupling suggests that intermodel spread may undersample uncertainty. Also assessed are the relative contribution of η, the surface warming, and the adjustment on the spread in ΔP on different time scales. Intermodel variation of both η and the adjustment govern the spread in ΔP in the years immediately after the abrupt forcing change. In equilibrium, the uncertainty in ΔP is dominated by uncertainty in the equilibrium surface temperature response. A kernel analysis reveals that intermodel spread in η is dominated by intermodel spread in tropical lower tropospheric temperature and humidity changes and cloud changes.

On the decadal scale correlation between African dust and Sahel rainfall: The role of Saharan heat low-forced winds.

A large body of work has shown that year-to-year variations in North African dust emission are inversely proportional to previous-year monsoon rainfall in the Sahel, implying that African dust emission is highly sensitive to vegetation changes in this narrow transitional zone. However, such a theory is not supported by field observations or modeling studies, as both suggest that interannual variability in dust is due to changes in wind speeds over the major emitting regions, which lie to the north of the Sahelian vegetated zone. We reconcile this contradiction showing that interannual variability in Sahelian rainfall and surface wind speeds over the Sahara are the result of changes in lower tropospheric air temperatures over the Saharan heat low (SHL). As the SHL warms, an anomalous tropospheric circulation develops that reduces wind speeds over the Sahara and displaces the monsoonal rainfall northward, thus simultaneously increasing Sahelian rainfall and reducing dust emission from the major dust "hotspots" in the Sahara. Our results shed light on why climate models are, to date, unable to reproduce observed historical variability in dust emission and transport from this region.

Tropospheric temperature gradient and its relation to the South and East Asian precipitation variability

Using the NCEP–DOE AMIP-2 daily reanalysis data sets, the tropospheric temperature (TT) changes over East Asia for the period 1988–2010 are analyzed. It is found that on the layer-averaged TT between 1000 and 400 mb, there exist two centers, one sitting over Mongolia, another over Tibet. An index, called TT index, is defined as the difference between the TT over these centers. The TT index is observed to reflect the circulation anomaly through thermal wind relation. A significant increase in magnitude is identified after 1999; the trend, however, reveals a much milder slope in comparison to that prior to 1999. It is found that the TT index is highly correlated to the South and East Asian precipitation variability. It is related to other monsoon indices in that it takes a lead of approximately 15 days; computation with a newly developed rigorous causality analysis reveals unambiguously a one-way causality from the TT index to the latter. That is to say, we could have identified something that may help better predict the precipitation variability.

Drivers of the tropospheric ozone budget throughout the 21st century under the medium-high climate scenario RCP 6.0

Abstract. Because tropospheric ozone is both a greenhouse gas and harmful air pollutant, it is important to understand how anthropogenic activities may influence its abundance and distribution through the 21st century. Here, we present model simulations performed with the chemistry–climate model SOCOL, in which spatially disaggregated chemistry and transport tracers have been implemented in order to better understand the distribution and projected changes in tropospheric ozone. We examine the influences of ozone precursor emissions (nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs)), climate change (including methane effects) and stratospheric ozone recovery on the tropospheric ozone budget, in a simulation following the climate scenario Representative Concentration Pathway (RCP) 6.0 (a medium-high, and reasonably realistic climate scenario). Changes in ozone precursor emissions have the largest effect, leading to a global-mean increase in tropospheric ozone which maximizes in the early 21st century at 23% compared to 1960. The increase is most pronounced at northern midlatitudes, due to regional emission patterns: between 1990 and 2060, northern midlatitude tropospheric ozone remains at constantly large abundances: 31% larger than in 1960. Over this 70-year period, attempts to reduce emissions in Europe and North America do not have an effect on zonally averaged northern midlatitude ozone because of increasing emissions from Asia, together with the long lifetime of ozone in the troposphere. A simulation with fixed anthropogenic ozone precursor emissions of NOx, CO and non-methane VOCs at 1960 conditions shows a 6% increase in global-mean tropospheric ozone by the end of the 21st century, with an 11 % increase at northern midlatitudes. This increase maximizes in the 2080s and is mostly caused by methane, which maximizes in the 2080s following RCP 6.0, and plays an important role in controlling ozone directly, and indirectly through its influence on other VOCs and CO. Enhanced flux of ozone from the stratosphere to the troposphere as well as climate change-induced enhancements in lightning NOx emissions also increase the tropospheric ozone burden, although their impacts are relatively small. Overall, the results show that under this climate scenario, ozone in the future is governed largely by changes in methane and NOx; methane induces an increase in tropospheric ozone that is approximately one-third of that caused by NOx. Climate impacts on ozone through changes in tropospheric temperature, humidity and lightning NOx remain secondary compared with emission strategies relating to anthropogenic emissions of NOx, such as fossil fuel burning. Therefore, emission policies globally have a critical role to play in determining tropospheric ozone evolution through the 21st century.