Persistent Drought Research Articles

Comparison of soil moisture at different depths for drought monitoring based on improved soil moisture anomaly percentage index

To better understand the characteristics and mechanisms of droughts at different drought stages, this study selected the Xiangjiang River Basin in China as the study area, and evaluated soil moisture (SM) at different depths for drought monitoring, through SM data simulated with the variable infiltration capacity (VIC) model. To solve the problem of unreasonable drought/wetness classifications based on the soil moisture anomaly percentage index (SMAPI), an improved soil moisture anomaly percentage index (ISMAPI) was developed by introducing the Box–Cox transformation. The drought/wetness frequency generated by ISMAPI demonstrated preferable spatial comparability in comparison with those from SMAPI. The lag time of ISMAPI relative to the standardized precipitation evapotranspiration index was closely related to soil depth, and was characterized by a fast response in shallow soil layers and a relatively slow response in deep soil layers. SM in shallow soil layers provided a measure for monitoring short-term droughts, whereas SM in deep soil layers provided a better measure for long-term persistent drought events. Furthermore, the occurrence and mitigation time of drought events identified by SM in deep soil layers usually lagged behind that identified by SM in shallow soil layers. Compared with deep SM, SM in shallow soil layers responded faster to meteorological anomalies, thereby resulting in shorter periods of SM persistence in shallow soil layers than in deep soil layers. This can explain the differences of SM at different depths in drought monitoring.

EatingChapattiswith Salt and Water: Bundelkhand’s Chronic Food Insecurity

Food insecurity prevails when the availability of sufficient nutritious and safe food is threatened. This article examines conditions in Bundelkhand’s Banda district and analyses the reasons behind the conditions of food insecurity prevalent there. On the basis of household-level primary data, body mass index (BMI) and 24-hour dietary recall (24-HDR) have been used to measure the chronic and transitory dimensions of food insecurity. The overall sex specific prevalence of chronic energy deficiency (CED) shows that both males and females are passing through a critical situation with respect to their nutritional status, with females being more underprivileged. Nearly two-fifths of the households remain hungry and are consuming calories less than the recommended dietary allowance (RDA) of 1,890 calories, with wasted children being the biggest sufferers of CED. Persistent drought conditions combined with the unavailability of assured means of irrigation and faulty implementation of government systems and services, have ensured that Banda district remains in a state of chronic food insecurity.

Agricultural Drought Prediction Based on Conditional Distributions of Vine Copulas

AbstractMonitoring and prediction of agricultural drought are paramount to food security at the global and regional scales, particularly under the influence of climate change and anthropogenic activities. Soil moisture is an effective indicator for monitoring and characterizing agricultural drought. Soil moisture (agricultural drought) is mainly affected by precipitation (meteorological drought) and temperature (hot conditions). Owing to the flexibility of vine copulas in handling multidimensional variables by decomposing them into pair copula constructions (PCCs), we propose a novel drought prediction method using three predictors, namely antecedent meteorological drought, previous hot conditions, and persistent agricultural drought, based on the conditional distributions of C‐vine copulas in a four‐dimensional scenario. The proposed model was applied to agricultural drought (characterized by the standardized soil moisture index (SSI)) prediction with 1–2‐months lead time for the summer season (i.e., August at a 6‐months timescale) in China. Taking two severe agricultural drought events that occurred in many regions across China in August of 2006 and 2014 as validation cases, the SSI predictions with 1–2‐months lead time using the conditional C‐vine copulas model were found to be generally consistent with the corresponding historical SSI observations in most parts of China. Performance evaluation using the Nash‐Sutcliffe efficiency (NSE), coefficient of determination (R2), and F1 score (F1S) for different climate regions also indicated that this model provided a reliable prediction of agricultural drought for most areas of China. The outcome of this study can serve as a guidance for drought prediction, early warning, and drought mitigation.

Women and Economic Production: Towards Sustainable Livelihoods in Zimbabwe

Persistent drought and economic collapse in Zimbabwe have seen most, if not all, rural women shifting from the receiving end to the giving end. Rural women have since initiated several livelihood activities to make ends meet, as they are the most vulnerable whenever they are left to look after children at home. The paper aims to examine rural livelihoods and how they contribute to economic production in Ward 5 of Bikita district. A mixed design in the form of a case study was employed in this study. Systematic random sampling was used to select 40 households, which provided data for the study out of 409 households in Ward 5. The study used questionnaires and semi-structured interviews as data collecting instruments. Several livelihood activities were noted in the ward including seasonal farming, gardening, community-based and money lending and saving schemes (fushai), informal trading, and petty trading as selling thatch grass and firewood, among others. However, climate change and drought, economic crisis, lack of capital and poor soils and poor farming methods were some of the constraints faced in rural livelihoods. The paper concludes with several recommendations for eradicating rural livelihood challenges.

Cape Town's “Day Zero” water crisis: A manufactured media event?

Day Zero was a purposefully designed narrative in political communication to change middle-class water consumption behaviour in a highly visible metropolitan context of persistent drought. As an “affective fact”, however it didn't so much elicit panic, but elicited a sense of fun and social solidarity in many.The unfeasibly precise prediction of water ‘running out’ the campaign obscured scientific uncertainties. In this context the contribution considers the role of ‘public’ scientists as highly visible authorities reinforcing or nuancing the Day Zero narrative. While the crisis narrative inevitably showed up rifts in South Africa's social fabric, and triggered protests against an underlying everyday crisis of water penury for marginalised urbanites.Our perspective is informed by documentary and press analysis, as well as a Focus Group Discussion with the South African National Press Club held on October 31, 2019.

Quantitative Response of Maize Vcmax25 to Persistent Drought Stress at Different Growth Stages

Drought stress has adverse effects on crop growth and yield, and its identification and monitoring play vital roles in precision crop water management. Accurately evaluating the effect of drought stress on crop photosynthetic capacity can provide a basis for decisions related to crop drought stress identification and monitoring as well as drought stress resistance and avoidance. In this study, the effects of different degrees of persistent drought in different growth stages (3rd leaf stage, 7th leaf stage and jointing stage) on the maximum carboxylation rate at a reference temperature of 25 °C (Vcmax25) of the first fully expanded leaf and its relationship to the leaf water content (LWC) were studied in a field experiment from 2013 to 2015. The results indicated that the LWC decreased continuously as drought stress continued and that the LWC decreased faster in the treatment with more irrigation. Vcmax25 showed a decreasing trend as the drought progressed but had no clear relationship to the growth stage in which the persistent drought occurred. Vcmax25 showed a significantly parabolic relationship (R2 = 0.701, p < 0.001) with the LWC, but the different degrees of persistent drought stress occurring in different growth stages had no distinct effect on the LWC values when Vcmax25 reached its maximum value or zero. The findings of this study also suggested that the LWC was 82.5 ± 0.5% when Vcmax25 reached its maximum value (42.6 ± 3.6 μmol m−2 s−1) and 67.6 ± 1.2% (extreme drought) when Vcmax25 reached zero. These findings will help to improve crop drought management and will be an important reference for crop drought identification, classification and monitoring as well as for the development of drought monitoring and early warning systems for other crops or maize varieties.

Assessment of carbon fluxes to coastal area during persistent drought conditions

This study assesses the seasonal and hydrological factors controlling carbon fluxes and quality of organic matter through the maximum turbidity zone (MTZ) of a tropical semiarid estuary under extreme drought, the Jaguaribe River estuary located in northeast Brazil. Dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations decreased in the Jaguaribe River estuary during the past ten years, reflecting drought intensification and river damming. DOC and POC behavior were strongly correlated to the freshwater residence time and freshwater fraction. Correlations with Chl-a indicated that the biological activity did not regulate DOC but did POC. Stable isotope ratios (δ13C and δ15N) of POC showed that it was predominantly terrestrial-derived, suggesting mangrove as an important POC source. Dissolved inorganic carbon concentrations reflected the seasonal variability of the semiarid climate, being higher in the dry season due to the negative water balance during this period. The Jaguaribe River estuary’s MTZ behaved as a retainer of POC (-1.5 ± 0.2 × 104 tons.year −1), DOC (-1.0 × 105 tons.year −1) and DIC (-4.8 ± 3.3 × 105 tons.year −1) during the dry season and as an exporter of DOC (+ 2 x 104 tons.year −1) and DIC (+1.4 × 105 tons.year −1) in the rainy season. However, the reduced freshwater supply to the estuary resulted in carbon concentrations 50% lower than those reported in 2004 and carbon fluxes smaller than expected for a tropical region. This study indicates that, under a scenario of climatic changes, the intensification of droughts in NE Brazil can further increase carbon retention in the estuary, reducing its exportation to the ocean.

Meteorological Drought Events and Their Evolution from 1960 to 2015 Using the Daily SWAP Index in Chongqing, China

Meteorological droughts are natural disasters that have been linked to economic losses and casualties. Decision-makers need to understand the temporal and spatial variation of meteorological drought events at a daily to weekly scale to develop a more elaborate framework for drought risk management. The present study used the standardized weighted average of precipitation index (SWAP) as an indicator of meteorological droughts, computed from the daily precipitation dataset (1960–2015) of 34 meteorological stations in Chongqing, China. The multi-threshold run theory was applied to identify drought events. Variation of drought characteristics was estimated by the modified Sen’s trend test. The results suggested the following findings: (1) the onset, duration, and severity of drought events identified by the SWAP index are in good agreement with the real local records; (2) there was no significant linear trend and abrupt change in annual duration and severity of drought events, but the decadal variation was obvious. From a decadal perspective, the annual frequency, duration, and severity of drought events showed a steady decreasing trend before the 1990s, and then fluctuated upward; (3) the spatial variation of the duration and severity of ordinary drought events was quite inconsistent at different periods. The annual drought days and severity increased from 1960 to 2015 but decreased after 1990. From 1960 to 2015, the duration days and severity of persistent, long persistent, severe, and extreme drought events declined insignificantly in most parts of the middle and southeast regions but increased in the western and northeast regions. The drought situation in Chongqing shows a large range of variation and obvious spatial heterogeneity. The SWAP index is an effective tool to identify the evolution of daily scale meteorological drought events.

Leaf nitrogen have a better relationship with photosynthesis performance across wheat species under elevated CO2 and drought

Elevated CO2 concentrations and persistent drought are predicted to inhibit the photosynthetic performance and yield of crops. Such effects vary considerably between species groups. Modern cultivated (Triticum aestivum L., AABBDD) and wild (Triticum dicoccoides Korn L., AABB and Triticum monococcum L., AA) species of wheat were subjected to elevated CO2 concentration (ambient concentration +200 μmol mol−1) and drought (well watered: 75–85% of the field water capacity; drought: 50–60% of the field water capacity) at open-top chamber experimental facilities. Elevated CO2 concentration decreased the limitation of stomatal morphology traits on stomatal conductance regulation. This could compensate the disadvantage of plants who has low stomatal density and large single stomatal area as well as low leaf water use efficiency such as modern wheat Z9023 in drought acclimation in the future CO2 rising world. Moreover, elevated CO2 concentration largely increased the dependence of light harvesting and electron transportation performance per photosynthesis system II reaction center, maximum rubisco carboxylation rate, and maximum Ribulose-1,5-bisphosphate regeneration rate on leaf nitrogen concentration across the selected wheat species and water regimes. Modern cultivated cultivars Z9023 and CH58 have higher photosynthetic performance per unit of leaf nitrogen than wild species under elevated CO2 concentrations. The increasing CO2 may present opportunities to breeders and possibly allow them to select for cultivars with better photosynthetic nitrogen use efficiency response to future CO2 rising climate.

Persistent Droughts and Water Scarcity: Households’ Perceptions and Practices in Makhanda, South Africa

Households in many cities worldwide consume substantial amounts of water, but increasing aridity will result in serious water supply challenges in the future. In South Africa, droughts are now a common phenomenon, with severe implications on water supply for urban households. Developing interventions to minimise the impacts of drought requires understanding of users’ perceptions of water scarcity, water use practices, and participation in water conservation practices. Using household surveys across different income groups (low, medium, and high) in Makhanda, South Africa, this study investigates households’ perceptions of water scarcity, water use, and conservation practices as a basis for designing pathways for sustainable water use practices. Results indicate that a substantial proportion of households were aware of water scarcity and attributed it to poor municipal planning rather than drought and wasteful use practices. Households reported good water use behaviour, but wasteful practices (e.g., regular flushing of toilets) were evident. Gender, age, education, and environmental awareness influenced water use practices, but the relationships were generally weak. Households participated in water conservation measures but felt the local municipal authority lagged in addressing water supply challenges. The implications of the study are discussed.

Water allocation under climate change

Chile is positioned in the 20th rank of water availability per capita. Nonetheless, water security levels vary across the territory. Around 70% of the national population lives in arid and semiarid regions, where a persistent drought has been experienced over the last decade. This has led to water security problems including water shortages. The water allocation and trading system in Chile is based on a water use rights (WURs) market, with limited regulatory and supervisory mechanisms, where the volume to be granted as permanent and eventual WURs is calculated from statistical analyses of historical streamflow records if available, or from empirical estimations if they are not. This computation of WURs does not consider the nonstationarity of hydrological processes nor climatic projections. This study presents the first large sample diagnosis of water allocation system in Chile under climate change scenarios. This is based on novel anthropic intervention indices (IAI), which were computed as the ratio between the total granted water volume to the water availability within 87 basins in north-central and southern Chile (30°S–42°S). The IAI were evaluated for the historical period (1979–2019) and under modeled-based climatic projections (2055–2080). According to these IAI levels, to date, there are 20 out of 87 overallocated basins, which under the assumption that no further WURs will be granted in the future, increases up to 25 basins for the 2055–2080 period. The results show that, to date most of north-central Chilean catchments already have a large anthropic intervention degree, and the increases for the future period occurs mostly in the southern region of the country (approximately 38°S), which has been considered as possible source of water for large water transfer projects (i.e., water roads). These indices and diagnosis are proposed as a tool to help policy makers to address water scarcity under climate change.

Drought resistance and resilience: The role of soil moisture–plant interactions and legacies in a dryland ecosystem

Abstract In many regions of the world, climate change is projected to reduce water availability through changes in the hydrological cycle, including more frequent and intense droughts, as well as seasonal shifts in precipitation. In water‐limited ecosystems, such as drylands, lower soil water availability may exceed the adaptive capacity of many organisms, leading to cascading ecological effects during (concurrent effects) and after drought (legacy effects). The magnitude and duration of concurrent and legacy effects depends on drought intensity, duration and timing as well as the resistance and resilience of the ecosystem. Here, we investigated the effects of drought seasonality and plant community composition on two dominant perennial grasses, Achnatherum hymenoides (C3 photosynthesis) and Pleuraphis jamesii (C4 photosynthesis), in a dryland ecosystem. The experiment consisted of three precipitation treatments: control (ambient precipitation), cool‐season drought (−66% ambient precipitation November–April) and warm‐season drought (−66% ambient precipitation May–October), applied in two plant communities (perennial grasses with or without a large shrub, Ephedra viridis) over a 3‐year period. We examined the concurrent and legacy effects of seasonal drought on soil moisture, phenology and biomass. Drought treatments had strong concurrent and legacy effects on soil moisture, which impacted the phenology and biomass of the two grasses. Drought reduced growing season length by delaying green‐up (cool‐season drought) or advancing senescence (warm‐season drought) and reduced biomass for both species. Biomass and phenology legacy effects from drought emerged in the second and third years of the experiment. While we observed differential sensitivity to drought legacies between the two grasses, we found limited evidence that shrub presence had interactive effects with the drought treatment. Synthesis. The results from this study highlight how abiotic and biotic legacies can develop and influence a community's resistance and resilience to subsequent droughts. When the frequency of repeated extreme events, such as recurring seasonal droughts, exceeds the capacity of organisms or ecosystems to recover (i.e. resilience), persistent drought legacies can reduce the resistance to subsequent drought events. Overall, these results highlight how drought legacies are a product of ecological resistance and resilience to past drought and can influence ecosystem vulnerability to future droughts.

Assessment of Meteorological Drought with Application of Standardized Precipitation Evapotranspiration Index (SPEI) for Tripura, Northeast India

Drought is one of the major water-related natural hazards. Understanding the spatial and temporal variation of rainfall is of great importance in water resources planning and management as it is related with food security and management of scarce water resource, which becomes critical in case of drought events. The advent of GIS to produce spatially interpolated drought map helps the water managers to undertake appropriate measures in drought relief and prioritization of drought mitigation works. Limitation of literature on Tripura suggests that study of drought over Tripura could help in strengthening of mitigation planes and rationalization of disaster management policies. Hence, the present study is focused to investigate the drought persistence and severity in the Tripura state of India during the period 1980-2013, using Standardized Precipitation Evapotranspiration Index (SPEI). Three time scale i.e., 3, 6 and 12 month time scales were opted for the study. Gridded monthly precipitation data distributed over the four districts of Tripura was used for drought analysis. Significant drought events were detected over the study area during the selected period. Annual analysis of SPI time series showed that the study area received the intense drought during the year 1985. Geospatial technique was used to generate the SPEI drought map for the year 1985.

Causes of the long-term variability of southwestern South America precipitation in the IPSL-CM6A-LR model

Southwestern South America (SWSA) has undergone frequent and persistent droughts in recent decades with severe impacts on water resources, and consequently, on socio-economic activities at a sub-continental scale. The local drying trend in this region has been associated with the expansion of the subtropical drylands over the last decades. It has been shown that SWSA precipitation is linked to large-scale dynamics modulated by internal climate variability and external forcing. This work aims at unravelling the causes of this long-term trend toward dryness in the context of the emerging climate change relying on a large set simulations of the state-of-the-art IPSL-CM6A-LR climate model from the 6th phase of the Coupled Model Intercomparison Project. Our results identify the leading role of dynamical changes induced by external forcings, over the local thermodynamical effects and teleconnections with internal global modes of sea surface temperature. Our findings show that the simulated long-term changes of SWSA precipitation are dominated by externally forced anomalous expansion of the Southern Hemisphere Hadley Cell (HC) and a persistent positive Southern Annular Mode (SAM) trend since the late 1970s. Long-term changes in the HC extent and the SAM show strong co-linearity. They are attributable to stratospheric ozone depletion in austral spring-summer and increased atmospheric greenhouse gases all year round. Future ssp585 and ssp126 scenarios project a dominant role of anthropogenic forcings on the HC expansion and the subsequent SWSA drying, exceeding the threshold of extreme drought due to internal variability as soon as the 2040s, and suggest that these effects will persist until the end of the twenty-first century.

Persistence and spatial-temporal variability of drought severity in Iran.

Drought is a natural hazard that can inflict significant damage to agriculture, society, economy, and ecosystems. The assessment of the persistence of drought severity (PDS) assists in understanding the characteristics of droughts better and enables the development of associated prediction tools and models. This work explores the persistence and spatial-temporal variability of drought severity (DS) in the diverse dryland of Iran. Using monthly precipitation and temperature data of 44 synoptic stations from 1989 to 2018, relationships between DS coefficient of precipitation variation, aridity, and the persistence percentage are determined by the application of the standardized precipitation index (SPI), the dryland index, and the Hurst exponent (H). The results confirm the persistence of droughts in Iran as H exceeded the 0.5 threshold for all stations. The PDS average in Iran is 0.78 with high regional variability reflective of different climatic conditions and geographical locations. An inverse relationship exists between the long-term coefficient of variation of monthly precipitation and PDS in the hyper-arid and arid regions of watersheds. Higher PDS values and increasing trend in the DS are detected in dry-subhumid areas. Also, the effect of the El Niño-Southern Oscillation (ENSO), a teleconnection metric, on the DS displays high spatial and temporal variability in Iran. The results show that the PDS is consistent with the spatial variation of DS changes during the period of 2009-2018.

Dynamics of the Mediterranean droughts from 850 to 2099 CE in the Community Earth System Model

Abstract. In this study, we analyze the dynamics of multi-year droughts over the western and central Mediterranean for the period of 850–2099 CE using the Community Earth System Model version 1.0.1. Overall, the model is able to realistically represent droughts over this region, although it shows some biases in representing El Niño–Southern Oscillation (ENSO) variability and mesoscale phenomena that are relevant in the context of droughts over the region. The analysis of the simulations shows that there is a discrepancy among diverse drought metrics in representing duration and frequencies of past droughts in the western and central Mediterranean. The self-calibrated Palmer drought severity index identifies droughts with significantly longer duration than other drought indices during 850–1849 CE. This re-affirms the necessity of assessing a variety of drought indices in drought studies in the paleoclimate context as well. Independent of the choice of the drought index, the analysis of the period 850–1849 CE suggests that Mediterranean droughts are mainly driven by internal variability of the climate system rather than external forcing. Strong volcanic eruptions show no connection to dry conditions but instead are connected to wet conditions over the Mediterranean. The analysis further shows that Mediterranean droughts are characterized by a barotropic high-pressure system together with a positive temperature anomaly over central Europe. This pattern occurs in all seasons of drought years, with stronger amplitudes during winter and spring. The North Atlantic Oscillation (NAO) and ENSO are also involved during Mediterranean multi-year droughts, showing that droughts occur more frequently with positive NAO and La Niña-like conditions. These modes of variability play a more important role during the initial stage of droughts. As a result, the persistence of multi-year droughts is determined by the interaction between the regional atmospheric and soil moisture variables, i.e., the land–atmosphere feedbacks, during the transition years of droughts. These feedbacks are intensified during the period 1850–2099 CE due to the anthropogenic influence, thus reducing the role of modes of variability on droughts in this period. Eventually, the land–atmosphere feedbacks induce a constant dryness over the Mediterranean region for the late 21st century relative to the period 1000–1849 CE.

Agricultural perspectives on the 2015-2018 Western Cape drought, South Africa: Characteristics and spatial variability in the core wheat growing regions

Between 2015 and 2018, the Western Cape Province of South Africa experienced a multi-year severe drought, which negatively impacted major economic sectors. The province serves as an important producer of wheat in South Africa among other agricultural commodities. This study aims to analyze the 2015-2018 drought and its impacts on wheat production in the winter rainfall region of the Western Cape, South Africa. The central focus is to place the drought in both a historical and future context while emphasising the differences between the two core wheat growing regions. We present an analysis of the drought, as well as climate trends computed from weather data sets (1988–2018) from six weather stations across the two core wheat-growing. We first present a meteorological drought analysis of mean annual and seasonal rainfall and temperatures, subsequently providing an agricultural lens by computing Standardised Precipitation and Evapotranspiration Index (SPEI) accumulated over 12- and 36-month timescales, self-calibrated Palmer Drought Severity Index (sc-PDSI), changes to the start and end of the rainfall season, and their effects on wheat yields. Trend analysis was conducted to determine if drought observations form part of the longer-term trends in the region. Finally, we show how the drought varied spatially across the two regions. Results show that between 1988 and 2018, the wheat growing areas of the Western Cape experienced persistent drought with high spatial-temporal variability. The 2015-2018 drought, however, was the most severe experienced in the 30-year study period at five of the six stations. These results are consistent with conditions that can be expected under future climate change. Moreover, results can be useful for the development of early warning systems since they place the drought in the context of past drought conditions.

Shrub‐rings used to reconstruct drought history of the central Alxa desert, northwest China

AbstractThe climate history of the Alxa desert of northwestern China must be sought both in recent instrumental records (dating from 1960) and in high‐resolution proxy data. Climate proxies are scarce in desert areas; however, long‐lived desert shrubs can offer one such proxy. We have used shrub‐ring data from Zygophyllum xanthoxylum, collected in the central Alxa desert, to reconstruct the Self‐calibrating Palmer Drought Severity Index (scPDSI) for the period 1880–2015. The data covers the May–July growth season. When we compared the history constructed from the shrub‐ring data with the instrumental data collected 1960–2015, we found that the reconstructed history explained 41% of the variance in scPDSI data (40% after adjusting for loss of degrees of freedom). The reconstructed history shows that this region has experienced six persistent droughts and four seasons wetter than usual during the period under study. Of note is a severe drought event in 1920s that affected much of northern China (as noted in contemporary documents). The droughts and wet seasons identified in our reconstruction are consistent with reports from other researchers who have studied the climate history or the region. Our study suggests that it is feasible to reconstruct climate history using shrub‐ring data from long‐lived desert shrubs.

Atmosphere-ocean dynamics of persistent cold states of the tropical Pacific Ocean

AbstractPersistent multiyear cold states of the tropical Pacific Ocean drive hydroclimate anomalies worldwide, including persistent droughts in the extratropical Americas. Here, the atmosphere and ocean dynamics and thermodynamics of multiyear cold states of the tropical Pacific Ocean are investigated using European Centre for Medium-Range Weather Forecasts reanalyses and simplified models of the ocean and atmosphere. The cold states are maintained by anomalous ocean heat flux divergence and damped by increased surface heat flux from the atmosphere to ocean. The anomalous ocean heat flux divergence is contributed to by both changes in the ocean circulation and thermal structure. The keys are an anomalously shallow thermocline that enhances cooling by upwelling and anomalous westward equatorial currents that enhance cold advection. The thermocline depth anomalies are shown to be a response to equatorial wind stress anomalies. The wind stress anomalies are shown to be a simple dynamical response to equatorial SST anomalies as mediated by precipitation anomalies. The cold states are fundamentally maintained by atmosphere-ocean coupling in the equatorial Pacific. The physical processes that maintain the cold states are well approximated by linear dynamics for ocean and atmosphere and simple thermodynamics.

Modeling the Influence of Upstream Land-atmosphere Coupling on the 2017 Persistent Drought over Northeast China

AbstractPersistent drought events that cause serious damages to economy and environment are usually intensified by the feedback between land surface and atmosphere. Therefore, reasonably modeling land-atmosphere coupling is critical for skillful prediction of persistent droughts. However, most high-resolution regional climate modeling focused on the amplification effect of land-atmosphere coupling on local anticyclonic circulation anomaly, while less attention was paid to the non-local influence through altering large-scale atmospheric circulation. Here we investigate how the antecedent land-atmosphere coupling over the area south to Lake Baikal (ASLB) influences the drought events occurred over its downstream region (Northeast China; NEC) by using Weather Research and Forecasting (WRF) model and linear baroclinic model (LBM). When the ASLB is artificially forced to be wet in the WRF simulations during March-May, the surface sensible heating is weakened and results in a cooling anomaly in low level atmosphere during May-July. Consequently, the anticyclonic circulation anomalies over ASLB and NEC are weakened, and the severity of NEC drought during May-July cannot be captured due to the upstream wetting in March-May. In the LBM experiments, idealized atmospheric heating anomaly that mimics the diabatic heating associated with surface wetness is imposed over ASLB, and the quasi-steady response pattern of 500-hPa geopotential height to the upstream wetting is highly consistent with that in the WRF simulation. In addition, the lower level heating instead of the upper level cooling makes a major contribution to the high pressure anomaly over NEC. This study implies the critical role of modeling upstream land-atmosphere coupling in capturing downstream persistent droughts.