Large-scale Circulation Patterns Research Articles

Spatiotemporal Variations in Light Precipitation Events in the Yellow River Basin, China, and Relationships with Large-Scale Atmospheric Circulation Patterns

Light precipitation events are an essential feature of rainfall for agricultural production, risk prediction of drought or flood disasters, and natural resource management in a certain area. We investigated the spatiotemporal variations in light precipitation events with intensities of 0.1–10 mm/day, based on daily precipitation data for the Yellow River Basin (YRB), China, during 1960–2018, and explored their relationships with large-scale atmospheric circulation patterns (LSACPs) and altitude. For further analysis of the changes in the light precipitation events, we classified light rain (0.1 ≤ P < 10.0 mm/day) into five grades of intensity by using equal interval division. Results indicate that the mean annual light precipitation amount and days were 182.7 mm and 80 days, respectively, from 1960 to 2018 over the YRB, accounting for 39.2% and 85.2% of the total annual precipitation amount and days, respectively. Both the amount of light rain and the number of light precipitation days declined by −1.3 mm/decade and −1.4 days/decade, respectively, and suggested that most rain events were of low intensity (0.1 ≤ P < 2.0 mm/day). Light precipitation events mainly occurred in the upper and middle reaches of the YRB and decreased from the southwest to the north. Additionally, changes in light rain events appear to be complex and possibly related to LSACPs and altitude. We found that the LSACPs were a possible mechanism for light precipitation events in the YRB over the past decades. With increasing elevation, light precipitation events decreased significantly throughout the study period. Thus, the decrease in precipitation days mainly occurred at lower altitudes in the YRB. The results also reflect the complexity of regional climate change in the YRB because atmospheric circulation related to climate phenomena not only causes the complex variation in precipitation but also changes its altitude dependence.

Destructive Interference of ENSO on North Pacific SST and North American Precipitation Associated with Aleutian Low Variability

Abstract Identifying the origins of wintertime climate variations in the Northern Hemisphere requires careful attribution of the role of El Niño–Southern Oscillation (ENSO). For example, Aleutian low variability arises from internal atmospheric dynamics and is remotely forced mainly via ENSO. How ENSO modifies the local sea surface temperature (SST) and North American precipitation responses to Aleutian low variability remains unclear, as teasing out the ENSO signal is difficult. This study utilizes carefully designed coupled model experiments to address this issue. In the absence of ENSO, a deeper Aleutian low drives a positive Pacific decadal oscillation (PDO)-like SST response. However, unlike the observed PDO pattern, a coherent zonal band of turbulent heat flux–driven warm SST anomalies develops throughout the subtropical North Pacific. Furthermore, non-ENSO Aleutian low variability is associated with a large-scale atmospheric circulation pattern confined over the North Pacific and North America and dry precipitation anomalies across the southeastern United States. When ENSO is included in the forcing of Aleutian low variability in the experiments, the ENSO teleconnection modulates the turbulent heat fluxes and damps the subtropical SST anomalies induced by non-ENSO Aleutian low variability. Inclusion of ENSO forcing results in wet precipitation anomalies across the southeastern United States, unlike when the Aleutian low is driven by non-ENSO sources. Hence, we find that the ENSO teleconnection acts to destructively interfere with the subtropical North Pacific SST and southeastern United States precipitation signals associated with non-ENSO Aleutian low variability.

Changes in rainfall erosivity from combined effects of multiple factors in China’s Loess Plateau

Rainfall erosivity is a key factor leading to soil erosion, and it is thus critical to investigate its spatiotemporal variability and controlling factors. Despite that the effects of a single factor have been largely investigated, the combined effects of multiple factors have rarely been considered. This study first investigated the spatiotemporal variability of rainfall erosivity in the Loess Plateau for 1961–2020, and then examined its relations to rainfall amount, temperature, and large-scale atmospheric circulation patterns by the bivariate and multiple wavelet coherence analysis. Averaged over the Loess Plateau, the annual rainfall erosivity had an insignificant increase at the rate of 1.74 MJ mm·ha−1·h−1·a−1. However, the changes exhibited spatial variability with a downward trend in the southeast and southwest, while an upward trend in the west, northeast and central regions. The regional-scale controlling factor was found to be rainfall amount, rather than temperature. For global atmospheric circulation, ENSO (El Niño-Southern Oscillation) and ENSO–AO (Arctic Oscillation) control rainfall erosivity individually or simultaneously at different timescales. This study provides technical support on how to identify the combined effects of multiple factors, and the results are helpful to project rainfall erosivity and control soil erosion.

Impact of Meteorological Conditions and Human Activities on Air Quality During the COVID-19 Lockdown in Northeast China

During the lockdown implemented to curb the spread of COVID-19, human activities have drastically reduced, providing a valuable opportunity to study and compare the impact of meteorological conditions and human activities on air quality. In this study, large-scale weather circulation, local meteorological conditions, and the impact of human activities are comprehensively considered, and changes in the concentration of major air pollutants in the northeast during this period are systematically studied. The large-scale weather circulation patterns that mainly affect the northeast region are divided into nine types by using the T-mode Principal components analysis objective circulation classification method. It is found that the northeast region is located at the edge of weak high pressure (Types 1, 2, and 7) and at the rear of high pressure (Type 4) and has higher concentrations of PM2.5, NO2, SO2, and CO; in cyclonic weather systems, low vortices (Types 3 and 5) and under the influence of the updraft (Type 6) in front of the trough, the ozone concentration is higher. The changes in the concentrations of PM2.5, NO2, CO, SO2, and O3 in the three cities, namely Shenyang, Changchun, and Harbin, during the lockdown period are compared, and it is found that the concentrations of PM2.5, NO2, CO, and SO2 have a tendency to first decrease and then increase, while the changes of O3 concentration are cyclical and increased significantly during this period. This demonstrates that pollutants such as PM2.5, NO2, CO, and SO2 are more susceptible to human activities and local meteorological conditions, and changes in O3 concentration are more closely related to changes in weather circulation types. Finally, the FLEXPART-WRF model is used to simulate the pollution process of nine circulation types, which confirms that particulate pollution in the northeast is mainly affected by local emissions and local westward sinking airflow.

Insight on Poleward Moisture and Energy Transport into the Arctic from ERA5

With the new-generation reanalysis product (ERA5), the spatiotemporal characteristics of poleward atmospheric moisture and energy transport over the past four decades (1979–2020) were examined. The main channels of atmospheric transport entering the Arctic in the Northern Hemisphere include the Chukchi Sea at 170° W, Baffin Bay at 50° W, North Atlantic at 0° E, and central Siberia at 90° E. Summer (winter) is characterized by high moisture (energy) transport across 70° N. No clear trend in moisture transport was found, whereas the winter and spring energy transport are declining significantly at a rate of −7.31 × 105 W/m/a (99% confidence) and −6.04 × 105 W/m/a (95% confidence), respectively. Meanwhile, an increasing trend was found in summer (4.48 × 105 W/m/a, 95% confidence) and autumn (3.61 × 105 W/m/a, not significant). The relationship between atmospheric moisture and energy transport and different large-scale atmospheric circulation patterns, including the Arctic Oscillation (AO), North Atlantic Oscillation (NAO), and Dipole Anomaly (DA), was explored. Among them, DA was identified as the most favorable pattern in relation to moisture and/or energy intrusion into the Arctic. As a result, the surface air temperature increases are more pronounced over most of the central Arctic under the regulation of DA.

A tree ring-based hydroclimate reconstruction for eastern Europe reveals large-scale teleconnection patterns

We present a new beech (Fagus sylvatica L.) tree-ring width composite chronology from five natural low-elevation forests in eastern Romania, which represent the species’ continental distribution limit. Our regional beech chronology reflects April–June hydroclimate variability in form of the Standardized Precipitation Evapotranspiration Index over large parts of Romania, Ukraine, and the Republic of Moldova, for which high-resolution paleoclimatic evidence is broadly missing. Most of the reconstructed hydroclimatic extremes back to 1768 CE are confirmed by documentary evidences, and a robust association is found with large-scale atmospheric circulation patterns in the Northern Hemisphere and sea surface temperatures over the North Atlantic. Reconstructed pluvials coincide with a high-pressure system over the North Atlantic Ocean and north-western Europe, and with a low-pressure system over south-western, central and eastern Europe, whereas historical droughts coincide with a high-pressure system over Europe and a low-pressure system over the central part of the Atlantic Ocean. Our study demonstrates the potential to produce well-replicated, multi-centennial beech chronologies for eastern Europe to reconstruct regional hydroclimate variation and better understand the causes and consequences of large-scale teleconnection patterns.

Identification of close relationship between large-scale circulation patterns and ozone-precursor sensitivity in the Pearl River Delta, China

To curb the continuous deterioration of ozone (O3) pollution in China, identifying the O3-precursor sensitivity (OPS) and its driving factors is a prerequisite for formulating effective O3 pollution control measures. Traditional OPS identification methods have limitations in terms of spatiotemporal representation and timeliness; therefore, they are not appropriate for making OPS forecasts for O3 contingency control. OPS is not only influenced by local precursor emissions but is also closely related to meteorological conditions governed by large-scale circulation (LSC). In this study, a localized three-dimensional numerical modeling system was used to investigate the relationship between LSC and OPS in the Pearl River Delta (PRD) of China during September 2017, a month with continuous O3 pollution. Our results highlighted that there was a close relationship between LSC and OPS over the PRD, and the four dominant LSC patterns corresponded well to the NOx-limited, NOx-limited, VOC-limited, and transitional regimes, respectively. The clear linkage between LSC and OPS was mainly driven by the spatial heterogeneity of NOx and VOC emissions within and beyond the PRD along the prevailing winds under different LSC patterns. A conceptual model was developed to highlight the intrinsic causality between the LSC and OPS. Because current technology can accurately forecast LSC 48–72 h in advance, the LSC-based OPS forecast method provided us with a novel approach to guide contingency control and management measures to reduce peak O3 at a regional scale.

Dynamic Variability of Wind Erosion Climatic Erosivity and Their Relationships with Large-Scale Atmospheric Circulation in Xinjiang, China

Xinjiang has a serious wind erosion problem due to its fragile ecological condition and sensitivity to climate change. Wind erosion climatic erosivity is a measure of climatic factors influencing wind erosion; evaluating its spatiotemporal variations and relationship with the large-scale circulation pattern can contribute to the understanding of the climate change effect on wind erosion risk. Thus, this study quantified the wind erosion climatic erosivity and examined the connections between climatic erosivity and climate indices using trend analysis, geo-statistical analysis, and cross-wavelet analysis based on the observed daily meteorological data from 64 weather stations in Xinjiang, China during 1969–2019 (50 years). The results indicated that the climatic erosivity showed a significant downward trend at seasonal and annual scales over the past 50 years. Strong seasonality in the C-factor was found, with its highest values in the spring and summer and its lowest values in the winter. The average climatic erosivity was weaker during El Niño events than during La Niña events. The impact of El Niño events on climatic erosivity in Xinjiang continued from the beginning of the event to two months after the end of the events. The La Niña events had a lag effect on the climatic erosivity in Xinjiang, with a lag period of 4 months. From a statistical perspective, the El Niño-Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Arctic Oscillation (AO) indices showed relationships to the climatic erosivity in Xinjiang in terms of their correlation and periodicity. The relationships between the climatic erosivity and ENSO were not clearly positive or negative, with many correlations advanced or delayed in phase. The NAO and AO indices showed a consistent in-phase relationship with climatic erosivity on significant bands, whereas the profound mechanisms involved in this require further study. The results of this study provide a preliminary perspective on the effect of large-scale atmospheric circulation on wind erosion risk in arid and semi-arid regions.

Overview of the MOSAiC expedition: Atmosphere

With the Arctic rapidly changing, the needs to observe, understand, and model the changes are essential. To support these needs, an annual cycle of observations of atmospheric properties, processes, and interactions were made while drifting with the sea ice across the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition from October 2019 to September 2020. An international team designed and implemented the comprehensive program to document and characterize all aspects of the Arctic atmospheric system in unprecedented detail, using a variety of approaches, and across multiple scales. These measurements were coordinated with other observational teams to explore cross-cutting and coupled interactions with the Arctic Ocean, sea ice, and ecosystem through a variety of physical and biogeochemical processes. This overview outlines the breadth and complexity of the atmospheric research program, which was organized into 4 subgroups: atmospheric state, clouds and precipitation, gases and aerosols, and energy budgets. Atmospheric variability over the annual cycle revealed important influences from a persistent large-scale winter circulation pattern, leading to some storms with pressure and winds that were outside the interquartile range of past conditions suggested by long-term reanalysis. Similarly, the MOSAiC location was warmer and wetter in summer than the reanalysis climatology, in part due to its close proximity to the sea ice edge. The comprehensiveness of the observational program for characterizing and analyzing atmospheric phenomena is demonstrated via a winter case study examining air mass transitions and a summer case study examining vertical atmospheric evolution. Overall, the MOSAiC atmospheric program successfully met its objectives and was the most comprehensive atmospheric measurement program to date conducted over the Arctic sea ice. The obtained data will support a broad range of coupled-system scientific research and provide an important foundation for advancing multiscale modeling capabilities in the Arctic.

Sixfold Increase in Historical Northern Hemisphere Concurrent Large Heatwaves Driven by Warming and Changing Atmospheric Circulations

Abstract Simultaneous heatwaves affecting multiple regions (referred to as concurrent heatwaves) pose compounding threats to various natural and societal systems, including global food chains, emergency response systems, and reinsurance industries. While anthropogenic climate change is increasing heatwave risks across most regions, the interactions between warming and circulation changes that yield concurrent heatwaves remain understudied. Here, we quantify historical (1979–2019) trends in concurrent heatwaves during the warm season [May–September (MJJAS)] across the Northern Hemisphere mid- to high latitudes. We find a significant increase of ∼46% in the mean spatial extent of concurrent heatwaves and ∼17% increase in their maximum intensity, and an approximately sixfold increase in their frequency. Using self-organizing maps, we identify large-scale circulation patterns (300 hPa) associated with specific concurrent heatwave configurations across Northern Hemisphere regions. We show that observed changes in the frequency of specific circulation patterns preferentially increase the risk of concurrent heatwaves across particular regions. Patterns linking concurrent heatwaves across eastern North America, eastern and northern Europe, parts of Asia, and the Barents and Kara Seas show the largest increases in frequency (∼5.9 additional days per decade). We also quantify the relative contributions of circulation pattern changes and warming to overall observed concurrent heatwave day frequency trends. While warming has a predominant and positive influence on increasing concurrent heatwave frequency, circulation pattern changes have a varying influence and account for up to 0.8 additional concurrent heatwave days per decade. Identifying regions with an elevated risk of concurrent heatwaves and understanding their drivers is indispensable for evaluating projected climate risks on interconnected societal systems and fostering regional preparedness in a changing climate. Significance Statement Heatwaves pose a major threat to human health, ecosystems, and human systems. Simultaneous heatwaves affecting multiple regions can exacerbate such threats. For example, multiple food-producing regions simultaneously undergoing heat-related crop damage could drive global food shortages. We assess recent changes in the occurrence of simultaneous large heatwaves. Such simultaneous heatwaves are 7 times more likely now than 40 years ago. They are also hotter and affect a larger area. Their increasing occurrence is mainly driven by warming baseline temperatures due to global heating, but changes in weather patterns contribute to disproportionate increases over parts of Europe, the eastern United States, and Asia. Better understanding the drivers of weather pattern changes is therefore important for understanding future concurrent heatwave characteristics and their impacts.

Moisture origin, transport pathways, and driving processes of intense wintertime moisture transport into the Arctic

Abstract. A substantial portion of the moisture transport into the Arctic occurs in episodic, high-amplitude events with strong impacts on the Arctic's climate system components such as sea ice. This study focuses on the origin of such moist-air intrusions during winter and examines the moisture sources, moisture transport pathways, and their linkage to the driving large-scale circulation patterns. For that purpose, 597 moist-air intrusions, defined as daily events of intense (exceeding the 90th anomaly percentile) zonal mean moisture transport into the polar cap (≥70∘ N), are identified. Kinematic backward trajectories combined with a Lagrangian moisture source diagnostic are then used to pinpoint the moisture sources and characterize the airstreams accomplishing the transport. The moisture source analyses show that the bulk of the moisture transported into the polar cap during these moist-air intrusions originates in the eastern North Atlantic with an uptake maximum poleward of 50∘ N. Trajectories further reveal an inverse relationship between moisture uptake latitude and the level at which moisture is injected into the polar cap, consistent with ascent of poleward-flowing air in a baroclinic atmosphere. Focusing on intrusions in the North Atlantic (424 intrusions), we find that lower tropospheric moisture transport is predominantly accomplished by two types of airstreams: (i) cold, polar air warmed and moistened by surface fluxes and (ii) air subsiding from the mid-troposphere into the boundary layer. Both airstreams contribute about 36 % each to the total transport. The former accounts for most of the moisture transport during intrusions associated with an anomalously high frequency of cyclones east of Greenland (218 intrusions), whereas the latter is more important in the presence of atmospheric blocking over Scandinavia and the Ural Mountains (145 events). Long-range moisture transport, accounting for 17 % of the total transport, dominates during intrusions with weak forcing by baroclinic weather systems (64 intrusions). Finally, mid-tropospheric moisture transport is invariably associated with (diabatically) ascending air and moisture origin in the central and western North Atlantic, including the Gulf Stream front, accounting for roughly 10 % of the total transport. In summary, our study shows that moist-air intrusions into the polar atmosphere result from a combination of airstreams with predominantly high-latitude or high-altitude origin, whose relative importance is determined by the underlying driving weather systems (i.e., cyclones and blocks).

Can synoptic patterns influence the track and formation of tropical cyclones in the Mozambique Channel?

<abstract> <p>The influence of large-scale circulation patterns on the track and formation of tropical cyclones (TCs) in the Mozambique Channel is investigated in this paper. The output of the hourly classification of circulation types (CTs), in Africa, south of the equator, using rotated principal component analysis on the T-mode matrix (variable is time series and observation is grid points) of sea level pressure (SLP) from ERA5 reanalysis from 2010 to 2019 was used to investigate the time development of the CTs at a sub-daily scale. The result showed that at specific seasons, certain CTs are dominant so that their features overlap with other CTs. CTs with synoptic features, such as enhanced precipitable water and cyclonic activity in the Mozambique Channel that can be favorable for the development of TC in the Channel were noted. The 2019 TC season in the Mozambique Channel characterized by TC Idai in March and TC Kenneth afterward in April were used in evaluating how the CTs designated to have TC characteristics played role in the formation and track of the TCs towards their maximum intensity. The results were discussed and it generally showed that large-scale circulation patterns can influence the formation and track of the TCs in the Mozambique Channel especially through (ⅰ) variations in the position and strength of the anticyclonic circulation at the western branch of the Mascarene high; (ⅱ) modulation of wind speed and wind direction; hence influencing convergence in the Channel; (ⅲ) and modulation of the intensity of cyclonic activity in the Channel that can influence large-scale convection.</p> </abstract>

Understanding Circulation Changes in Earth’s Oceans and Atmosphere

Large-scale circulation patterns can be found throughout Earth’s oceans and atmosphere, and play a crucial role in maintaining the stability of regional climates. As the climate warms, researchers find that these patterns are experiencing a fundamental transformation. Using a combination of satellite observations and computer models, Dr Hu Yang at the Alfred Wegener Institute, identifies and explains the mechanisms of these changes. His study offers crucial insights into how both human populations and natural ecosystems will be affected by these transformations – and how they will need to adapt to cope with them.

Toward the Detection and Imaging of Ocean Microplastics With a Spaceborne Radar

Ocean microplastic concentrations are known to vary significantly by location, with especially high levels in the North Atlantic and North Pacific gyres. Most direct measurements come from plankton net trawling made in these regions; concentrations in other regions have been estimated by microplastic transport models that depend on large-scale ocean circulation patterns. However, global measurements of microplastic distribution and its temporal variability are lacking. A new method is presented for detecting and imaging the global distribution of ocean microplastics from space. The method uses spaceborne bistatic radar measurements of ocean surface roughness and relies on an assumed reduction in responsiveness to wind-driven roughening caused by surfactants that act as tracers for microplastics near the surface. Annual mean microplastic distributions estimated by the radars are generally consistent with model predictions. The spaceborne observations are also able to detect temporal changes that are not resolved by the models. For example, seasonal dependencies are observed at mid-latitudes in both Northern and Southern Hemispheres, with lower concentrations noted in the winter months. Time lapse images at finer spatial and temporal scales reveal episodic bursts of microplastic tracers in the outflow from major river discharges into the sea. This new method will provide better monitoring of ocean microplastics and will support future model development and validation.

Weak upstream westerly wind attracts western North Pacific typhoon tracks to west

The steering flow of the large-scale circulation patterns over the western North Pacific and North East Asia, constrains typhoon tracks. Westerly winds impinging on the Tibetan Plateau, and the resulting flow uplift along the slope of the mountain, induce atmospheric vortex flow and generate stationary barotropic Rossby waves downstream. The downstream Rossby wave zonal phase is determined by the upstream zonal wind speed impinging on the Tibetan Plateau. Negative anomaly of westerly wind forcing tends to induce a westward shift of the large-scale Rossby wave circulation pattern, forming an anti-cyclonic circulation anomaly over North East Asia. In this study, we show that the Tibetan Plateau dynamically impacts the tracks of western Pacific typhoons via modulation of downstream Rossby waves. Using the topographically forced stationary Rossby wave theory, the dynamical mechanisms for the formation of the North East Asian anti-cyclonic anomaly and its impact on the typhoon tracks are analyzed. The westward shift of typhoon tracks caused by the southeasterly wind anomaly, or track density increase over the western flank of the North East Asian anti-cyclonic circulation anomaly is robust in June, August and September, but it is not statistically significant in July. The physical understanding of the large-scale circulation pattern affecting typhoon trajectories has large implications not only at the seasonal prediction of the high impact weather phenomena, but also at the right understanding of the long-term climate change.

Structure of the Western Tibetan Vortex inconsistent with a thermally-direct circulation

The Western Tibetan Vortex (WTV) is a large-scale circulation pattern identified from year-to-year circulation variability, which was used to understand the causal mechanisms for slowdown of the glacier melting over the western Tibetan Plateau (TP). A recent argument has suggested the WTV is the set of wind field anomalies resulting from variability in near-surface air temperatures over the western TP (above 1500 m), which, in turn, is likely driven by the surface net radiation. This study thereby evaluates the above putative thermal-direct mechanism. By conducting numerical sensitivity experiments using a global atmospheric circulation model, SAMIL, we find a WTV-like structure cannot be generated from a surface thermal forcing imposed on the western TP. A thermally-direct circulation generated by the surface or near surface heating is expect to cause upward motions and a baroclinic structure above it. In contrast, downward motions and a quasi-barotropic are observed in the vertical structure of the WTV. Besides, we find variability of the surface net radiation (sum of the surface shortwave and longwave net radiation) over the western TP can be traced back to the WTV variability based on ERA5 data. The anticyclonic (cyclonic) WTV reduces (increases) the cloudiness through the anomalous downward (upward) motions, causes more (less) input shortwave net radiation and thereby more (less) surface net radiations, resulting in the warmer (cooler) surface and near-surface air temperature over the western TP. The argument is constructive in encouraging examination of the radiative balance processes that complements previous studies.

Spatiotemporal variation in temperature extremes and their association with large scale circulation patterns in the Central Karakorum during 1982–2019

Understanding dynamical changes in climate extremes are vital in predicting extreme climate events and lowering their associated impacts on the environment and society. The objectives of this study is to analyze the spatiotemporal variations in temperature extremes and their association with selected atmospheric circulation indices. We use daily maximum temperature (TX) and minimum daily temperature (TN) data for five glaciers in the Hunza district of central Karakoram for the period 1982–2019. We validate the European Reanalysis (ERA5) data with available observed and in-situ data before further analysis. A total of eight indices based on maximum and minimum temperatures are chosen. They include maximum of TX, minimum of TN, three warm indices, and three cold indices. The Mann-Kendall method and linear trend analysis were used to examine the trends, and Pearson correlation analysis was used to analyze the correlation between the temperature extremes and different atmospheric circulation patterns. The warm indices, including the number of warm nights, warm days, and Warm Spell Duration Index (WSDI), and TN and TX, showed increasing trends in the central Karakoram from 1982 to 2019. On the other hand, cold indices, including the number of cold nights, cold days, and Cold Spell Duration Indicator (CSDI), showed decreasing trends. Warm indices and TX showed significant positive correlations with atmospheric circulations. The Arctic Oscillation index (AO) displayed significant correlations with the warm indices (p < 0.01) and negative correlations with the cold indices. The results also showed that warm indices and extreme indices had no correlations with the ENSO index. These findings will provide useful insights in predicting extreme climate events and taking measures to reduce their associated impacts on the water resources in the Karakoram.

The Connection between Extreme Precipitation Variability over Monsoon Asia and Large-Scale Circulation Patterns

Spatial and temporal variability in precipitation has been dramatically changed due to climate variability and climate change over the global domain. Increasing in extreme precipitation events are pronounced in various regions, including monsoon Asia (MA) in recent decades. The present study evaluated precipitation variability in light of intensity, duration, and frequency with several extreme precipitation climate change indices developed by the Expert Team on Climate Change Detection Indices (ETCCDI) over the MA region. This study uses an improved version (APHRO_V1901) of the Asian Precipitation Highly Resolved Observation Data Integration Towards Evaluation of extreme events (APHRODITE-2) gridded rainfall product. Results showed that the spatial variability of the extreme precipitation climate change indices is reflected in the annual mean rainfall distribution in MA. Maximum one-day precipitation (R × 1) and precipitation contributed from extremes (R95) depict a peak in decadal mean rainfall values over topography regions. A significant positive trend in R × 1 (with a slope of 0.3 mm/yr) and precipitation greater than the 95th percentile (R95: with a slope of 0.5 mm/yr) are predominantly observed in decadal trends in regional average extreme precipitation climate change indices over MA. Maritime continental countries exhibit an inclined trend in R10, whereas central Asian arid regions show a decreasing tendency in continuous dry days (CDD). The positive trend in R95 is observed over central India, the monsoon region in China, countries that reside over the equator and some parts of Japan, and the Philippines. When comparing the influence of surface temperature (T) and total column water vapor (TCW) on precipitation climate change indices, TCW seems to be a crucial attributor to climate change indices meridional variability. The mutual correlation analysis depicts that precipitation contributed from extremes (R95) strongly correlates in terms of temporal variability with all extreme precipitation indices. Among various global circulation patterns, the prevalent conditions of sea surface temperature (SST) over the equatorial Pacific Ocean have a significant influence on decadal variability in extreme precipitation climate change indices. R10 and R95 possess a relatively significant correlation (0.86 and 0.91) with the Southern Oscillation Index. The maximum number of consecutive dry days (CDD) shows an increasing trend with a positive phase of the North Atlantic Oscillation Index.

Forecasting river basin yield using information of large-scale coupled atmospheric–oceanic circulation and local outgoing longwave radiation

ABSTRACT Global and local climate parameters influence the distribution of precipitation over continents. The spatio-temporal distribution of rainfall and the depth of rainfall over a river basin influence the river basin yield. This study deals with the prediction of the river basin scale yield of the ‘Upper Bhima River basin’ from the Maharashtra State of India. The information on large-scale circulation patterns El Nino-Southern Oscillation (ENSO), Equatorial Indian Ocean Oscillation (EQUINOO) index, Multivariate ENSO Index (MEI), and local meteorological input viz. Outgoing longwave radiation (OLR) has been used to predict the river basin scale yield. The Artificial Intelligence (AI) tool – Genetic Programming (GP) – is used for developing prediction models. Ten different combinations of input variables are attempted for the development of monthly river basin yield models to arrive at the best input variable combination for the best predictions with varying lead times. Also, three combinations of input variables were tested for the prediction of ‘Seasonal Yield’. The findings of this research work indicate that GP-derived monthly River Basin Scale Yield forecasting models are successful in the prediction of yield with a correlation coefficient of 0.83. The seasonal yields could be predicted with a correlation coefficient of 0.75.

Data-constrained assessment of ocean circulation changes since the middle Miocene in an Earth system model

Abstract. Since the middle Miocene (15 Ma, million years ago), the Earth's climate has undergone a long-term cooling trend, characterised by a reduction in ocean temperatures of up to 7–8 ∘C. The causes of this cooling are primarily thought to be due to tectonic plate movements driving changes in large-scale ocean circulation patterns, and hence heat redistribution, in conjunction with a drop in atmospheric greenhouse gas forcing (and attendant ice-sheet growth and feedback). In this study, we assess the potential to constrain the evolving patterns of global ocean circulation and cooling over the last 15 Ma by assimilating a variety of marine sediment proxy data in an Earth system model. We do this by first compiling surface and benthic ocean temperature and benthic carbon-13 (δ13C) data in a series of seven time slices spaced at approximately 2.5 Myr intervals. We then pair this with a corresponding series of tectonic and climate boundary condition reconstructions in the cGENIE (“muffin” release) Earth system model, including alternative possibilities for an open vs. closed Central American Seaway (CAS) from 10 Ma onwards. In the cGENIE model, we explore uncertainty in greenhouse gas forcing and the magnitude of North Pacific to North Atlantic salinity flux adjustment required in the model to create an Atlantic Meridional Overturning Circulation (AMOC) of a specific strength, via a series of 12 (one for each tectonic reconstruction) 2D parameter ensembles. Each ensemble member is then tested against the observed global temperature and benthic δ13C patterns. We identify that a relatively high CO2 equivalent forcing of 1120 ppm is required at 15 Ma in cGENIE to reproduce proxy temperature estimates in the model, noting that this CO2 forcing is dependent on the cGENIE model's climate sensitivity and that it incorporates the effects of all greenhouse gases. We find that reproducing the observed long-term cooling trend requires a progressively declining greenhouse gas forcing in the model. In parallel to this, the strength of the AMOC increases with time despite a reduction in the salinity of the surface North Atlantic over the cooling period, attributable to falling intensity of the hydrological cycle and to lowering polar temperatures, both caused by CO2-driven global cooling. We also find that a closed CAS from 10 Ma to present shows better agreement between benthic δ13C patterns and our particular series of model configurations and data. A final outcome of our analysis is a pronounced ca. 1.5 ‰ decline occurring in atmospheric (and ca. 1 ‰ ocean surface) δ13C that could be used to inform future δ13C-based proxy reconstructions.