Large-scale Atmospheric Dynamics Research Articles

European Summer Wet-Bulb Temperature: Spatiotemporal Variations and Potential Drivers

Abstract Heat stress is projected to intensify with global warming, causing significant socioeconomic impacts and threatening human health. Wet-bulb temperature (WBT), which combines temperature and humidity effects, is a useful indicator for assessing regional and global heat stress variability and trends. However, the variations of European WBT and their underlying mechanisms remain unclear. Using observations and reanalysis datasets, we demonstrate a remarkable warming of summer WBT during the period 1958–2021 over Europe. Specifically, the European summer WBT has increased by over 1.0°C in the past 64 years. We find that the increase in European summer WBT is driven by both near-surface warming temperatures and increasing atmospheric moisture content. We identify four dominant modes of European summer WBT variability and investigate their linkage with the large-scale atmospheric circulation and sea surface temperature anomalies. The first two leading modes of the European WBT variability exhibit prominent interdecadal to long-term variations, mainly driven by a circumglobal wave train and concurrent sea surface temperature variations. The last two leading modes of European WBT variability mainly show interannual variations, indicating a direct and rapid response to large-scale atmospheric dynamics and nearby sea surface temperature variations. Further analysis shows the role of global warming and changes in midlatitude circulations in the variations of summer WBT. Our findings can enhance the understanding of plausible drivers of heat stress in Europe and provide valuable insights for regional decision-makers and climate adaptation planning. Significance Statement Wet-bulb temperature, which takes into account the combined effect of temperature and humidity, is a good indicator for assessing heat stress. In the context of global warming, heat stress is anticipated to escalate, posing significant risks to human health and causing socioeconomic losses. However, variations in wet-bulb temperature and the associated physical mechanisms have received limited attention. This study aims to improve our understanding of the temporal and spatial variations and the potential driving mechanisms of summer wet-bulb temperature across Europe in recent decades. We have observed a noteworthy increase in summer wet-bulb temperature, indicating a regional intensification of heat stress, particularly within the last 10 years. We further investigate the connections between variations in summer wet-bulb temperature, large-scale atmospheric circulation, and sea surface temperature. Additionally, we explore their associations with global warming and changes in midlatitude atmospheric circulation. The outcomes of this study not only contribute to establishing a scientific basis for evaluating heat-related risks in Europe but also facilitate preparedness for future climate adaptation and mitigation at both regional and local scales.

The existence and asymptotic behavior of solutions to 3D viscous primitive equations with Caputo fractional time derivatives

On the one hand, the primitive three-dimensional viscous equations for large-scale ocean and atmosphere dynamics are commonly used in weather and climate predictions. On the other hand, ever since the middle of the last century, it has been widely recognized that the climate variability exhibits long-time memory. In this paper, we first prove the global existence of weak solutions to the primitive equations of large-scale ocean and atmosphere dynamics with Caputo fractional time derivatives. Then we establish the existence of an absorbing set, which is positively invariant. Finally, an attractor (strictly speaking, the minimal attracting set containing all the limiting dynamics) is constructed for the time fractional primitive equations, which means that the present state of a system may have long-time influences on the states in far future. However, there was no work on the long-time behavior of the time fractional primitive equations and we fill this gap in this paper.

Weathering pulses during glacial-interglacial transitions: Insights from well-dated paleosols in the Azores volcanic province (Central North Atlantic)

Volcanic islands evolve through complex interactions between volcano growth and surface processes. Climate changes impact the physical and chemical processes that drive weathering and denudation. While global paleoclimate has been extensively studied for the late Quaternary, elucidating the local climatic response to global forcing on such islands remains challenging. São Miguel is a volcanic island in the Eastern Azores, located in the Central North Atlantic, a region susceptible to changes in large-scale atmospheric and oceanic dynamics. It comprises numerous paleosols (PSs), whose geochemistry results from volcanic rocks' weathering and can serve as a proxy to reconstruct paleoclimatic conditions. New K–Ar and 40Ar/39Ar ages of volcanic units “bracketing” (under and overlying) PSs reveal four periods of enhanced soil formation (∼820–765 ka, ∼425–430 ka, ∼170–75 ka, <10 ka), coinciding with rapid glacial-interglacial transitions (Terminations I, II, V, IX). Our reconstructed mean annual precipitation (MAP; 500–1200 mm yr−1) and air temperature (MAAT; 12–18 °C) are higher during interglacial peaks. This, in addition to the coherence of MAAT with previous Sea Surface Temperature reconstructions, shows consistency between local and global climate dynamics. The texture of parental rocks played a significant role in weathering, with PSs predominantly restricted to the brecciated upper part of lava flows and to pyroclastic deposits, which exhibited distinctive precipitation thresholds for their formation (∼800 mm yr−1 and ∼500 mm yr−1, respectively). PSs developed on basaltic lava flows exhibit greater elemental loss, due to a high glass proportion and low permeability, which prevents fluids percolation out of the soils. As present-day precipitation and temperature exceed those of the past, enhanced weathering is expected in São Miguel and other volcanic islands, with local to global impacts (e.g., carbon cycling), especially in the context of ongoing global warming.

Northeast Pacific marine heatwaves associated with high-latitude atmospheric blocking

The Northeast Pacific Ocean (NEP) is one of the hotspots of marine heatwaves (MHWs) occurring in the global ocean. The causes of MHWs in this region have been widely investigated, but the physical processes underlying heatwaves and regional climate variability remain under debate. By analyzing interannual large-scale high-latitude atmospheric dynamics and oceanic physical conditions over the NEP, we show that winter-spring sea surface temperature (SST) anomalies are strongly correlated with winter-spring atmospheric blocking events over Alaska. The occurrence of weaker westerly wind over the subarctic region over the NEP during the period of the blocking, accompanies a shallower vertical mixed layer, less southward horizontal Ekman transport, and higher SST in the upper NEP. These findings establish a linkage between high-latitude atmospheric dynamics and subarctic oceanic conditions and reveal the physical mechanisms of this connection, providing new insight into the possible causes of MHW in the NEP.

The effects of diachronous surface uplift of the European Alps on regional climate and the oxygen isotopic composition of precipitation

Abstract. This study presents the simulated response of regional climate and the oxygen isotopic composition of precipitation (δ18Op) to different along-strike topographic evolution scenarios. These simulations are conducted to determine if the previously hypothesized diachronous surface uplift in the Western and Eastern Alps would produce δ18Op signals in the geologic record that are sufficiently large and distinct to be detected using stable isotope paleoaltimetry. We present a series of topographic sensitivity experiments conducted with the water-isotope-tracking atmospheric general circulation model (GCM) ECHAM5-wiso. The topographic scenarios are created from the variation of two free parameters, (1) the elevation of the Western–Central Alps and (2) the elevation of the Eastern Alps. The results indicate Δδ18Op values (i.e., the difference between δ18Op values at the low- and high-elevation sites) of up to −8 ‰ along the strike of the Alps for the diachronous uplift scenarios, primarily due to changes in orographic precipitation and adiabatic lapse rate driven localized changes in near-surface variables. These simulated magnitudes of Δδ18Op values suggest that the expected isotopic signal would be significant enough to be preserved and measured in geologic archives. Moreover, the simulated slight δ18Op differences of 1 ‰–2 ‰ across the low-elevation sites support the use of the δ–δ paleoaltimetry approach and highlight the importance of sampling far-field low-elevation sites to differentiate between the different surface uplift scenarios. The elevation-dependent rate of change in δ18Op (“isotopic lapse rate”) varies depending on the topographic configuration and the extent of the surface uplift. Most of the changes are significant (e.g., −1.04 ‰ km−1 change with slope error of ±0.09 ‰ km−1), while others were within the range of the statistical uncertainties (e.g., −0.15 ‰ km−1 change with slope error of ±0.13 ‰ km−1). The results also highlight the plausible changes in atmospheric circulation patterns and associated changes in moisture transport pathways in response to changes in the topography of the Alps. These large-scale atmospheric dynamics changes can complicate the underlying assumption of stable isotope paleoaltimetry and therefore require integration with paleoclimate modeling to ensure accurate reconstruction of the paleoelevation of the Alps.

Necessary and sufficient criteria for existence, regularity, and asymptotic stability of enhanced pullback attractors with applications to 3D primitive equations

We introduce several new concepts called enhanced pullback attractors for nonautonomous dynamical systems by improving the compactness and attraction of the usual pullback attractors in strong topology spaces uniformly over some infinite time intervals. Then we establish several necessary and sufficient criteria for the existence, regularity and asymptotic stability of these enhanced pullback attractors under general theoretical frameworks which are expected to be applied to many kinds of nonautonomous PDEs. These abstract results are applied to the famous 3D primitive equations modeling the large-scale ocean and atmosphere dynamics. We establish the existence, regularity and asymptotic stability of the enhanced pullback attractors for the nonautonomous primitive equations in [Formula: see text] and [Formula: see text] when the forces satisfy some mild conditions. The uniform pullback asymptotic compactness of the solution operators in [Formula: see text] and [Formula: see text] are established by deriving the uniform “flattening effects” of the solutions in [Formula: see text] and the uniform [Formula: see text]-Hölder continuity of the solutions from [Formula: see text] to [Formula: see text] with respect to the initial data, respectively. Our approaches are applicable to prove the existence of global attractors for the autonomous primitive equations in [Formula: see text] provided [Formula: see text].

Dynamically coupled kinetic chemistry in brown dwarf atmospheres – I. Performing global scale kinetic modelling

ABSTRACT The atmospheres of brown dwarfs have been long observed to exhibit a multitude of non-equilibrium chemical signatures and spectral variability across the L, T, and Y spectral types. We aim to investigate the link between the large-scale 3D atmospheric dynamics and time-dependent chemistry in the brown dwarf regime, and to assess its impact on spectral variability. We couple the miniature kinetic chemistry module ‘mini-chem’ to the Exo-FMS general circulation model (GCM). We then perform a series of idealized brown dwarf regime atmospheric models to investigate the dynamical 3D chemical structures produced by our simulations. The GCM output is post-processed using a 3D radiative transfer model to investigate hemisphere-dependent spectral signatures and rotational variability. Our results show the expected strong non-equilibrium chemical behaviour brought on by vertical mixing and global spatial variations due to zonal flows. Chemical species are generally globally homogenized, showing variations of ±10 per cent or less, dependent on pressure level, and follow the dynamical structures present in the atmosphere. However, we find localized storm regions and eddies can show higher contrasts, up to ±100 per cent, in mixing ratio compared to the background global mean. This initial study represents another step in understanding the connection between 3D atmospheric flows in brown dwarfs and their rich chemical inventories.

Precipitation isoscapes in areas with complex topography: Influence of large-scale atmospheric dynamics versus microclimatic phenomena

While studies of the spatial distribution of precipitation isotope contents is well known at large scale and for more or less homogeneous areas, regions with complex topography and aerology are more sparsely or not at all documented. The aim of this research is to understand the factors influencing spatial/temporal changes of isotopes content in rainfall over a heterogeneous region. The region presents strong topographic contrasts, an insular context with varied weather exposure and subsequent microclimates. The approach consists in building 'isotopes landscapes (isoscapes)' for different months taking advantage of the 7 year-long monthly monitoring survey of the Corsica Network for Isotopes in Precipitation (CNIP, Corsica, France). Isoscapes use linear predictions based on multiple regressions of station-based precipitation isotope, climatic and topographic data. These regressions reveal the controlling effect of altitude (with a dichotomous pattern observed between the winter and the summer periods) and slope, while meteorological settings partially influence isotope contents. By explaining ∼88% of the δ18O spatial variability, isoscapes highlight both a controlling effect of local-scale meteorological circulation, and a contribution of micro-scale dynamics (due to topographic forcing), on the precipitation isotopes content. The analysis of residual values revealed that these two effects are not sufficient to explain the depletion in heavy isotopes. The deuterium-excess was used to investigate the influence of large-scale meteorological systems. Taking into account previous meteorological investigation, it was possible for the first time to hypothesize an influence of types of precipitation (convective or stratiform) on the rainwater isotopic composition in the study region. Finally, these results appear as evidence of the climate complexity in areas with steep reliefs and a potential limitation for generating reliable isoscapes in Mediterranean regions. They also demonstrate the role that isoscapes can play in documenting the microclimate processes in the world.

Airflow and teleconnection patterns driving the spatial and temporal variability of high 7Be air concentrations in Europe.

The long-term monitoring of the cosmogenic 7Be activity concentrations has been used to better understand the influence of large-scale atmospheric circulation dynamics in the troposphere. With an aim, this study analyzes weekly 7Be data from 15 sampling stations in Europe over 2005-2014. We first define peak (or extremely high) events in each station as those activity concentrations above the 90th percentile, and then investigate their temporal and spatial variability. These events are most frequent in the spring and summer seasons, with a notable latitudinal variability in their number. Next, we use back-trajectory cluster analysis to identify the main advection pathways associated with these high concentrations. To achieve this, persistent periods, i.e., sampling periods over which at least 60% of the calculated backward trajectories arriving at a given site correspond to the same airflow pattern, are taken as reference. This method reveals large differences in the association between the airflow patterns observed at different stations in connection with the 7Be peaks. A comparison between stations shows no clear spatial pattern, which suggests a further influence of mesoscale/local physical processes on the surface 7Be activity concentrations. Finally, the main airflow pattern at each sampling site and the associated 7Be peaks, are related to the main teleconnection patterns of large scale and regional climate variability in Europe: North Atlantic Oscillation, Arctic Oscillation, East Atlantic, East Atlantic/Western Russia, Scandinavian pattern and Western Mediterranean Oscillation. The results point out the connection between the negative phases of NAO and We-MO, and the positive phase of EA with the spatial and temporal variability and occurrence of the 7Be peak activity concentrations. These results show a latitudinal division between northern and southern sites, with similar influence of teleconnection patterns, while those located in the central part of Europe present larger variability in the impact of teleconnection patterns.

Strong Solvability of a Variational Data Assimilation Problem for the Primitive Equations of Large-Scale Atmosphere and Ocean Dynamics

For the primitive equations of large-scale atmosphere and ocean dynamics, we study the problem of determining by means of a variational data assimilation algorithm initial conditions that generate strong solutions which minimize the distance to a given set of time-distributed observations. We suggest a modification of the adjoint algorithm whose novel elements is to use norms in the variational cost functional that reflects the H^1-regularity of strong solutions of the primitive equations. For such a cost functional, we prove the existence of minima and a first-order adjoint condition for strong solutions that provides the basis for computing these minima. We prove the local convergence of a gradient-based descent algorithm to optimal initial conditions using the second-order adjoint primitive equations. The algorithmic modifications due to the H^1-norms are straightforwardly to implement into a variational algorithm that employs the standard L^2-metrics.

Changes in Frequency and Location of East Coast Low Pressure Systems Affecting Southeast Australia

Low pressure systems off the southeast coast of Australia can generate intense rainfall and associated flooding, destructive winds, and coastal erosion, particularly during the cool season (April–September). Impacts depend on coastal proximity, strength and latitude. Therefore, it is important to investigate changes in frequency, duration, location, and intensity of these systems. First, an existing observation-based database of these low pressure systems, for 1970–2006, is extended to 2019, focusing on April–September and using archived Australian Bureau of Meteorology MSLP charts. Second, data consistency between 1970 and 2006 and 2007 and 2019 is confirmed. Third, permutation testing is performed on differences in means and variances between the two 25-year intervals 1970–1994 and 1995–2019. Additionally, trends in positions, durations and central pressures of the systems are investigated. p-values from permutation tests reveal statistically significant increases in mean low pressure system frequencies. Specifically, a greater frequency of both total days and initial development days only, occurred in the latter period. Statistically significant lower variance for both latitude and longitude in systems that developed in both subtropical easterly and mid-latitude westerly wind regimes indicate a shift south and east in the latter period. Furthermore, statistically significant differences in variance of development location of explosive low pressure systems that develop in a low level easterly wind regime indicate a shift further south and east. These changes are consistent with fewer systems projected to impact the east coast. Finally, important changes are suggested in the large scale atmospheric dynamics of the eastern Australian/Tasman Sea region.

A 5000-Year Sedimentary Record of East Asian Winter Monsoon from the Northern Muddy Area of the East China Sea

The variability of the winter monsoon is one of the key components of the Asian monsoon, significantly influencing paleoenvironmental evolution in East Asia. However, whether the winter or the summer monsoon is the dominated factor controlling sedimentary dynamics of the muddy areas of the continental shelves of the East China Sea is debated, due to lack of consistency between various winter monsoon proxies in previous studies. In this work, the sediments of the upper part of core ECS-DZ1 with several marine surface samples were studied in terms of sediment grain size and radiocarbon dating, and changes in sedimentary dynamics of the northern muddy area of the ECS over the past 5000 years were documented. The main findings are as follows: (1) regional sedimentary dynamics were low and did not significantly change since the middle Holocene; (2) coarse particles are the dominated component in the sediments; (3) a proxy can be derived to indicate changes in winter monsoon. Based on this reconstructed winter monsoon record, we found that this record was generally negatively correlated to the stalagmite-based summer monsoon variability over the past 3500 years, but positively correlated before that. Moreover, this record can be well correlated to changes in the Kuroshio Current and the Bond ice-rafting debris events in the North Atlantic on millennial timescales, inferring large-scale and common atmospheric dynamics across the Asian continent over the past 5000 years. Therefore, we concluded that the winter monsoon is the predominant factor controlling sedimentary dynamics in the northern part of the ECS and proposed that the contribution of coarse particles may be one of potential indices to identify the role of the winter and the summer monsoons in sedimentary evolution.

Optimal distributed control of the three dimensional primitive equations of large-scale ocean and atmosphere dynamics

&lt;p style='text-indent:20px;'&gt;The main objective of this paper is to study the optimal distributed control of the three dimensional non-autonomous primitive equations of large-scale ocean and atmosphere dynamics. We apply the well-posedness and regularity results of solutions for this system as well as some abstract results from the nonlinear functional analysis to establish the existence of optimal controls as well as the first-order necessary optimality condition for an associated optimal control problem in which a distributed control is applied to the temperature.&lt;/p&gt;

Existence of a strong solution to moist atmospheric equations with the effects of topography

In this paper, we consider the primitive three-dimensional viscous equations for large-scale atmosphere dynamics with topography effects and water vapor phase transition process. This modified climate model is commonly used in weather and climate predictions, and few theoretical analyses have been performed on them. The existence and uniqueness of a global strong solution to this climate model is established based on the initial data assumptions.

The Influence of Stratospheric Wave Reflection on North American Cold Spells

AbstractUnderstanding and predicting midlatitude cold spells is of scientific and public interest, given often associated severe impacts. However, large-scale atmospheric dynamics related to these events are not fully understood. The winter of 2017/18 was characterized by several cold spells affecting large parts of North America and Eurasia. Here, the role of stratosphere–troposphere coupling for the occurrence of cold spells in this winter is investigated using different wave propagation diagnostics. While the European cold spell in late February 2018 was influenced by a major sudden stratospheric warming (SSW) associated with wave absorption, the cold spells over North America at the end of December 2017 and early February 2018 were related to downward reflected waves over the North Pacific. Previously proposed wave reflection indices, however, either miss these reflection events or are not able to distinguish them from the major SSW related to wave absorption. To overcome this, a novel simple index based on eddy heat flux is proposed here, capturing regional wave reflection over the North Pacific. Reflection events detected with this index are shown to be followed by North Pacific blocking and negative temperature anomalies over North America. An improved understanding of the contribution of wave reflection for cold spells is crucial to better predict such events in the future.

A multi-scalar climatological analysis in preparation for extreme heat at the Tokyo 2020 Olympic and Paralympic Games

ABSTRACT Extreme heat can be harmful to human health and negatively affect athletic performance. The Tokyo Olympic and Paralympic Games are predicted to be the most oppressively hot Olympics on record. An interdisciplinary multi-scale perspective is provided concerning extreme heat in Tokyo—from planetary atmospheric dynamics, including El Niño Southern Oscillation (ENSO), to fine-scale urban temperatures—as relevant for heat preparedness efforts by sport, time of day, and venue. We utilize stochastic methods to link daytime average wet bulb globe temperature (WBGT) levels in Tokyo in August (from meteorological reanalysis data) with large-scale atmospheric dynamics and regional flows from 1981 to 2016. Further, we employ a mesonet of Tokyo weather stations (2009–2018) to interpolate the spatiotemporal variability in near-surface air temperatures at outdoor venues. Using principal component analysis, two planetary (ENSO) regions in the Pacific Ocean explain 70% of the variance in Tokyo’s August daytime WBGT across 35 years, varying by 3.95°C WGBT from the coolest to warmest quartile. The 10-year average daytime and maximum intra-urban air temperatures vary minimally across Tokyo (<1.2°C and 1.7°C, respectively), and less between venues (0.6–0.7°C), with numerous events planned for the hottest daytime period (1200–1500 hr). For instance, 45% and 38% of the Olympic and Paralympic road cycling events (long duration and intense) occur midday. Climatologically, Tokyo will present oppressive weather conditions, and March–May 2020 is the critical observation period to predict potential anomalous late-summer WBGT in Tokyo. Proactive climate assessment of expected conditions can be leveraged for heat preparedness across the Game’s period.

Reduced European aerosol emissions suppress winter extremes over northern Eurasia

Winter extreme weather events receive major public attention due to their serious impacts1, but the dominant factors regulating their interdecadal trends have not been clearly established2,3. Here, we show that the radiative forcing due to geospatially redistributed anthropogenic aerosols mainly determined the spatial variations of winter extreme weather in the Northern Hemisphere during 1970–2005, a unique transition period for global aerosol forcing4. Over this period, the local Rossby wave activity and extreme events (top 10% in wave amplitude) exhibited marked declining trends at high latitudes, mainly in northern Eurasia. The combination of long-term observational data and a state-of-the-art climate model revealed the unambiguous signature of anthropogenic aerosols on the wintertime jet stream, planetary wave activity and surface temperature variability on interdecadal timescales. In particular, warming due to aerosol reductions in Europe enhanced the meridional temperature gradient on the jet’s poleward flank and strengthened the zonal wind, resulting in significant suppression in extreme events over northern Eurasia. These results exemplify how aerosol forcing can impact large-scale extratropical atmospheric dynamics, and illustrate the importance of anthropogenic aerosols and their spatiotemporal variability in assessing the drivers of extreme weather in historical and future climate. Anthropogenic aerosol emissions decreased over North America and Europe but increased over Asia since the 1970s. This caused jet stream winds to shift poleward over the Atlantic, decreasing planetary wave activity and partially inhibiting extreme winter weather over northern Eurasia.

Distinct seasonal climate drivers revealed in a network of tree-ring records from Labrador, Canada

Traditionally, high-latitude dendroclimatic studies have focused on measurements of total ring width (RW), with the maximum density of the latewood (MXD) serving as a complementary variable. Whilst MXD has typically improved the strength of the growing season climate connection over that of RW, its measurements are costly and time-consuming. Recently, a less costly and more time-efficient technique to extract density measurements has emerged, based on lignin’s propensity to absorb blue light. This Blue Intensity (BI) methodology is based on image analyses of finely-sanded core samples, and the relative ease with which density measurements can be extracted allows for significant increases in spatio-temporal sample depth. While some studies have attempted to combine RW and MXD as predictors for summer temperature reconstructions, here we evaluate a systematic comparison of the climate signal for RW and latewood BI (LWBI) separately, using a recently updated and expanded tree ring database for Labrador, Canada. We demonstrate that while RW responds primarily to climatic drivers earlier in the growing season (January–April), LWBI is more responsive to climate conditions during late spring and summer (May–August). Furthermore, RW appears to be driven primarily by large-scale atmospheric dynamics associated with the Pacific North American pattern, whilst LWBI is more closely associated with local climate conditions, themselves linked to the behaviour of the Atlantic Multidecadal Oscillation. Lastly, we demonstrate that anomalously wide or narrow growth rings consistently respond to the same climate drivers as average growth years, whereas the sensitivity of LWBI to extreme climate conditions appears to be enhanced.

A balanced state consistent with planetary-scale motion for quasi-geostrophic dynamics

The role of planetary-scale zonally-asymmetric thermal forcing on large-scale atmospheric dynamics is crucial for understanding low-frequency phenomena in the atmosphere. Despite its paramount importance, good theoretical foundation for the understanding is still lacking. Here, we address this issue by providing a general framework for including planetary-scale thermal forcing in large-scale atmospheric dynamics studies. This is accomplished by identifying two distinct geostrophic motions of horizontal length scale L in terms of the external Rossby deformation length scale LD : i) and ii) where ϵ is the Rossby number. In addition, via multi-scale analysis, we show that the large-scale atmospheric dynamics can be described by mutual interaction between the two scales. The analysis results in planetary geostrophic equations with large-scale thermal forcing that provide the basic balanced states for processes such as the growth of synoptic waves. In the long-time limit, the continuous growth and decay of synoptic waves provide the convergence of horizontal heat and vorticity fluxes, which contributes to the energy flux balance in the planetary geostrophic scale with planetary-scale advection and thermal forcing.

Investigation of seasonal droughts and related large-scale atmospheric dynamics over the Potwar Plateau of Pakistan

As a result of climate change and unsustainable land use management in the recent past, droughts have become one of the most devastating climatic hazards whose impacts may prolong from months to years. This study presents analysis of droughts for two major cropping seasons, i.e., Kharif (May–September) and Rabi (October–April), over the Potwar Plateau of Pakistan. The analysis is performed using various datasets viz. observational, reanalysis, and Regional Climate Models (RCMs), for the past (1981–2010) and future (2011–2100) time periods. The following two methods for the identification of dry and wet years, also referred to as drought and wetness, are applied: (1) the percentile rank approach and (2) the drought indices, Standardized Precipitation Index (SPI) and Reconnaissance Drought Index (RDI). Future projections of droughts are investigated using RCM (RegCM4.4 and RCA4) outputs from CORDEX South Asia domain under two Representative Concentration Pathway (RCP) scenarios, RCP4.5 and RCP8.5. Generally, the indices show non-significant decreasing trends of drought severity in the recent past for all cases; however, significant increasing trends are observed for annual (0.006) and Kharif (0.007) cases under RCP4.5 scenario. The analysis of large-scale atmospheric dynamics suggests the significant role of low-level geopotential height anomalies over Tibetan Plateau (northwest of Pakistan) during Kharif (Rabi) season in controlling drought occurrence by transporting moisture from the Bay of Bengal (Arabian Sea). Moreover, composites of vertically integrated moisture transport, moisture flux convergence/divergence, and precipitable water anomalies show their marked contribution in maintaining the drought/wetness conditions over the Potwar region.