Atlantic Multidecadal Oscillation Index Research Articles

Sensitivity of small river channels in the North Carolina Piedmont to change in streamflow conditions, Atlantic Multi-decadal Oscillation, and land use/land cover change

Twenty-four years (1999–2022) of channel change is assessed using specific-gage analysis based on the United States Geological Survey (USGS) rating curve calibration data from thirteen sites (drainage area < 500 km2) to understand the sensitivity of discharge-specific stage (DSS) or its inverse, stage-specific channel capacity (SSCC) to streamflow regimes, hydroclimatological history, and land use in small Piedmont watersheds of North Carolina, USA. We compare observed channel changes to common streamflow and flood metrics, Atlantic Multi-decadal Oscillation Index (AMOI), and land use/land cover data using regression and correlation techniques to suggest drivers of change and infer sensitivity. In addition to interannual variability, our results at most sites reveal three coherent periods of DSS/SSCC variation at the sub-decadal scale as well as multidecadal trend variation. The magnitudes and frequencies of these changes indicative of potential sensitivity vary substantially amongst sites. The trendlines from the DSS plots for the full length of record indicate that six sites showed increased specific stage while the remaining seven sites showed either reduced or a near-zero change in specific stage. Coefficients of determination relating specific stage and the index of AMO are low (R2 = 0.14—0.38) with both positive and negative relationships exhibited. Multiple regression, coefficients of determination relating specific stage and average monthly discharge are more variable, ranging from 0.18 to 0.94. Both positive and negative correlations (r ≤ 0.7) are found between specific stage and basin land use/land cover intensity indices, although few are significant at 95 % confidence interval. Both qualitative observations and correlation analyses support inferences that drought and persistent low mean discharge conditions tend to increase discharge-specific stage values (decreasing SSCC), by aggradation of the bed and/or lower banks. At our sites, this may be responsible for a sub-decadal trend of increasing specific stage between roughly 2013 and 2018, followed by ongoing reductions over the subsequent period.

Prediction of seasonal sea surface temperature based on temperature and salinity of subsurface ocean using machine learning

AbstractThe sea surface temperature (SST) is not only a crucial external factor in the evolution of the atmosphere, but also a primary factor and premonition signal used in climate prediction. It is challenging to obtain a precise SST for generating accurate initial and boundary conditions in numerical models. This study employs a machine learning approach, that is, a convolutional neural network (CNN) algorithm, to predict SST on a seasonal scale. In particular, the subsurface ocean temperature (OT) and ocean salinity (OS) at depths of 5.02, 15.08, 25.16, 35.28, 45.45 and 76.55 m were used as training factors in developing a CNN prediction model. The results indicate that subsurface OT and OS can persist for 6 months or longer, with a maximum persistence of up to 12 months. Using the CNN prediction model, the SST can be reliably predicted 6 months in advance in most cases. The predicted SST has a mean bias of approximately 0–0.8 K on the globe. The bias is small (below 0.5 K) in the open ocean. The root mean square errors (RMSEs) of hindcasting for Interdecadal Pacific Oscillation, North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation indices are all less than 1.0 K. Specifically, the RMSE for El Niño prediction is less than 0.5 K. This study provides a viable method for establishing initial and boundary conditions for climate prediction.

A spatio-temporal analysis of the role of climatic drivers influencing extreme precipitation events in a Costa Rican basin

Regional patterns of rainfall are changing around the world, putting pressure on social, economic, and environmental systems, and changing the outlook for food, water, and health security. This is the case in the Tempisque-Bebedero River Basin (TBRB), located on the Pacific slope of northwest Costa Rica, which relies heavily on water resources for tourism, agriculture, ranching, and fish farming. In this study, we use non-stationary extreme value analysis to characterize the climatic drivers of extreme precipitation in the TBRB. Daily extreme precipitation events at stations across the TBRB are selected using a peak-above-threshold approach, and interannual climate variability is included as time-varying covariates in a non-stationary point process model, using three regional climate indices: the Caribbean Low-Level Jet (CLLJ) index, the Oceanic Niño Index (ONI), representative of the El Niño Southern Oscillation (ENSO), and the Atlantic Multidecadal Oscillation (AMO) index. We find that the magnitude and frequency of precipitation extremes in the basin are driven by the CLLJ and the AMO. The ONI is only a significant driver of extreme precipitation in the northwestern region of the basin. This is likely because ENSO controls the strength of the CLLJ, thus the dominant impact of ENSO on precipitation in the TBRB may be incorporated within the CLLJ component of our model.

Dynamics of codfish stocks in the North Atlantic region

Purpose: to investigate the features of the long-term dynamics of codfish stocks in the North Atlantic region as a basis for predicting their state and sustainable fishing (exploitation). Materials: data on spawning stock biomass, abundance of recruitment at age 0 to 3 years, survival indices at early ontogenesis stage for 11 Atlantic cod stocks, 6 Atlantic haddock stocks, and 4 saithe stocks for the period 1946–2020 Methods: a comparative analysis of the above time series based on their statistical processing. Novelty: For the first time, a 50–60-year cycle which characterizes the long-term variations of the Atlantic Multidecadal Oscillation index, were revealed in the long-term fluctuations of the Northeast Arctic cod recruitment. This assumes a relationship in the long-term variations of two parameters under consideration. Results: The significant interannual variability of all characteristics was identified for each of the 21 commercial fish stocks under consideration. Statistically significant positive correlations were found between changes in recruitment abundance of Atlantic cod in the Northwest Atlantic and Atlantic cod, Atlantic haddock, and saithe in the areas of Greenland, Iceland, North and Baltic Seas. Changes in the recruitment abundance of Labrador cod and Northeast Arctic cod in the Barents Sea are opposite. Practical significance: the results of this work can be used to improve the methods of the medium-range and long-range forecasts of codfish catches in the North Atlantic region.

Spatiotemporal variability in extreme precipitation and associated large-scale climate mechanisms in Central Asia from 1950 to 2019

Increased sea surface temperature, evaporation, air temperature, and moisture holding capacity associated with climate change may the enhance transport of moisture to Central Asia (CA), resulting in increased extreme precipitation events (EPEs). Changes in EPEs may affect the socioeconomic and ecological environment in typical arid and semi-arid regions of CA. Therefore, based on meteorological station data and four reanalyzed precipitation datasets, we used the Taylor diagram, Geo-Detector method, and other methods to examine 10 EPE indices in CA from 1950 to 2019, analyzed their spatial-temporal trends, and quantified the influence of large-scale climate factors on EPEs. The results showed that GSWP3_W5E5 was the optimal option with better performance in terms of monthly and annual precipitation in CA. Overall, EPEs in CA showed an increasing trend, but an abrupt point was found in 1986, when very wet day precipitation (R95p) increased from 36.64 to 47.84 mm; the increasing rate also changed from 0.8 to 5.06 mm/10a. The changes in the Northern Hemisphere Subtropical High Intensity Index (NSI), NINO B Area Sea Surface Temperature Anomaly Index (NINOB), Asian Zonal Circulation Index, and Atlantic Multidecadal Oscillation Index are the main reasons for the increase in EPEs; the NSI and NINOB had the largest contributions to EPEs, with at 44.32 % and 43.80 %, respectively. Additionally, two-factor interactions had a more significant influence. Further analysis revealed that the abnormal warming of sea surface temperature and others led to the strengthen of water vapor transport in CA caused by a series of cyclones and anticyclonic anomalies in the westerly circulation zone, leading to the increase of EPEs. The results obtained in this study can contribute to a more comprehensive understanding of the changes in EPEs and provide a reference for water resource management and disaster prevention in CA.

Distinct Role of a Spring Atmospheric Circulation Mode in the Arctic Sea Ice Decline in Summer

AbstractA distinguishing feature of Arctic sea ice is the rapid declining in the Pacific Arctic Sector (PAS) in recent years. The atmospheric variability during spring (March‐May) plays a critical role in determining the sea ice extent (SIE) during the following summer. The summertime (June‐August) SIE reduction in the eastern part of PAS, including the East Siberian Sea and the Laptev Sea, is strongly related to the first two leading atmospheric circulation modes in spring, the Arctic Oscillation and the Arctic Dipole, retrieved from empirical orthogonal function analysis. Here, we show that the third leading mode of atmospheric circulation in spring, namely the Barents‐Beaufort Oscillation (BBO), can also change the summer SIE, especially on the western regime of the PAS across the Beaufort and Chukchi Seas. We find that a positive phase of springtime BBO (i.e., BBO+), relative to the BBO−, acts to decrease sea ice thickness via advection and generate sea ice deformation through divergence and shear processes, thereby preconditioning for a rapid sea ice loss within the PAS in the following summer. Furthermore, the occurrence of BBO is state‐dependent and potentially modulated by the phase shift of the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO). Since the mid‐1990s, the AMO (PDO) index has changed from a dominant negative (positive) to a positive (negative) phase. These phase shifts in AMO and PDO would have contributed to the subsequently increased appearance of anomalous BBO+ after 2000 and enhanced the summertime sea ice loss in the western regions of the PAS in recent years.

Interactions between large‐scale and mesoscale processes define long‐term rainfall variability and availability of water resources in Western Bahia, Brazil

AbstractWestern Bahia, a region in Northeast Brazil highly dependent on water availability due to the large concentration of irrigated lands, has been suffering from decreasing precipitation in the last decades. The mean climate of Northeast Brazil and its variability have been linked to teleconnections with the Atlantic and Pacific sea surface temperatures, including the Atlantic Multidecadal Oscillation (AMO). This study investigates how large‐scale (AMO) and mesoscale (orographic) interactions are related to precipitation patterns in the region. Time series of precipitation and naturalized flows (corresponding to the flow that would occur in the river section if there were no anthropogenic actions) for the period 1981–2020 are analysed, indicating an average reduction of 11.5% in rainfall since the 1980s. The analyses show a significant (α = 0.05) rainfall reduction in the wettest months, the December and January bimester (DJ). A strong and significant negative correlation between AMO and precipitation (R = −0.62, α = 0.01) indicates a decrease in DJ rainfall when the AMO index is positive. The five rainiest and five driest DJ bimesters between 1981 and 2020 were selected to investigate the changes in atmospheric circulation. In the rainiest years, large‐scale mechanisms interact with the local topography to promote a major convection centre over Western Bahia. The driest years show suppressed convection over the topographic gradient and a change in the convection centre several hundred kilometres to the west. The continued increase in temperatures in the North Atlantic may perpetuate the decline in Western Bahia precipitation, putting pressure on water availability and impacting economic activities in the region.

ТЕНДЕНЦИИ ВРЕМЕННОЙ ИЗМЕНЧИВОСТИ КОЛИЧЕСТВА АТМОСФЕРНЫХ ОСАДКОВ ХОЛОДНОГО ПЕРИОДА НА ТЕРРИТОРИИ РЕСПУБЛИКИ БАШКОРТОСТАН

A regional response to global climate warming is a spatio-temporal change in the humidification regime. This study presents an analysis of changes in the precipitation regime during the cold period conducted for the Republic of Bashkortostan with the use of proven statistical methods. To identify climatic changes in the precipitation regime, different periods were considered: the main one (1966-2020) and the periods recommended by the WMO. Statistical characteristics of precipitation of the cold period are analyzed taking into account the physical and geographical features of the Republic; correlations between precipitation anomalies and atmospheric circulation indices have been found; the distribution of the number of heavy snowfall cases has been obtained. The study revealed that the precipitation of the cold period tended to increase in 1966-2020. A significant increase was found in March. The closest positive relationships were revealed between the amount of precipitation and the Atlantic multidecadal oscillation index, negative relationships were established with the Scandinavian oscillation index. An analysis of heavy snowfall cases showed that there are more of them formed in the Southern Urals.

Long-Term Spatiotemporal Variability of Whitings in Lake Geneva from Multispectral Remote Sensing and Machine Learning

Whiting events are massive calcite precipitation events turning hardwater lake waters to a milky turquoise color. Herein, we use a multispectral remote sensing approach to describe the spatial and temporal occurrences of whitings in Lake Geneva from 2013 to 2021. Landsat-8, Sentinel-2, and Sentinel-3 sensors are combined to derive the AreaBGR index and identify whitings using appropriate filters. 95% of the detected whitings are located in the northeastern part of the lake and occur in a highly reproducible environmental setting. An extended time series of whitings in the last 60 years is reconstructed from a random forest algorithm and analyzed through a Bayesian decomposition for annual and seasonal trends. The annual number of whiting days between 1958 and 2021 does not follow any particular monotonic trend. The inter-annual changes of whiting occurrences significantly correlate to the Western Mediterranean Oscillation Index. Spring whitings have increased since 2000 and significantly follow the Atlantic Multidecadal Oscillation index. Future climate change in the Mediterranean Sea and the Atlantic Ocean could induce more variable and earlier whiting events in Lake Geneva.

Uncertainty in Atlantic Multidecadal Oscillation derived from different observed datasets and their possible causes

As a leading mode of sea surface temperature (SST) variability over the North Atlantic in both observations and model simulations, the Atlantic Multidecadal Oscillation (AMO) can have a substantial influence on regional and global climate. By using Low-Frequency Component Analysis, we explore the uncertainties of the resulting AMO indices and the corresponding spatial patterns derived from three observational SST datasets. We found that the known coherent spatial pattern of the AMO at the basin scale over the North Atlantic appears in two out of the three datasets. Further analysis indicates that both the warming trend and the different techniques used to construct these observed gridded SSTs contribute to the AMO’s spatial coherence over the North Atlantic, especially during periods of sparse data sampling. The SST in the Extended Reconstructed SST dataset version 5 (ERSSTv5), changes from being systematically below the other datasets during the dense sampling periods on either side of the Second World War (WWII), to systematically above the other datasets during WWII, thereby introducing an artificial 10–20-year variability that affects the AMO’s spatial coherence. This coherence in the AMO’s spatial pattern is also affected by bias adjustment in ERSSTv5 at relative cool (i.e., non-summer) seasons, and by the heterogeneous North Atlantic warming pattern. The different AMO patterns can induce the different effects of wind, surface heat fluxes, and then drive ocean circulation and its heat transport convergence, especially for some seasons. For AMO indices, both the different detrending methods and different observational data result in uncertainty for the period 1935–1950. Such SST uncertainty is important to detect the relative role of the atmosphere and ocean in shaping the AMO.

Multidecadal Variations in East Asian Winter Temperature Since 1880: Internal Variability Versus External Forcing

AbstractThis study investigates the relative contributions of internal variability and external forcing to multidecadal variations in the winter (December–February) East Asian averaged surface air temperature (EASAT) since 1880 by analyzing three observational datasets and large ensemble simulations from 12 coupled models. The results indicate that the internal variability plays a dominant role in the EASAT multidecadal variations. Specifically, the Atlantic Multidecadal Oscillation (AMO) affects the EASAT via an atmospheric teleconnection across the Eurasian continent, and the correlation coefficient between internally generated EASAT variation and the AMO index is −0.49. In comparison, the external forcings only modulate the observed EASAT variations due to offsetting effects of the external forcing agents. That is, the multidecadal variations in the greenhouse gas concentrations and anthropogenic/volcanic aerosols make positive and negative contributions, respectively. Our new finding is conducive to EASAT predictions in the following decades due to the predictability of AMO and anthropogenic forcings.

A Place-Based Approach to Drought Forecasting in South-Central Oklahoma.

To assist water managers in south-central Oklahoma prepare for future drought, reliable place-based drought forecasts are produced. Past-, present-, and future-forecasted climate indices (Multivariate ENSO Index, Pacific Decadal Oscillation index, and Atlantic Multidecadal Oscillation index) and past and present Palmer Drought Severity Index (PDSI) are employed as predictor variables to forecast PDSI using a multivariate regression technique. PDSI is forecasted 18months in advance with sufficient skill to provide water managers early warning of drought. Using a training data set obtained from the periodJanuary 1901 to November 2021, a second-order model equation that contains, without any restriction, all the predictors and their interaction terms is built to predict drought intensity. Significant predictors are selected through stepwise regression, with cross-validation producing the simplest restricted model that describes the data well. PDSI values are predicted using 1000 fitted restricted models produced from bootstrapping, then averaged monthly. The technique found the best-fitting model and estimated the model coefficients that minimized the sum of squared deviations between the fitted model and the predictor variables. The adjusted R-squared value of the restricted model is large enough to explain an adequately accurate model, and relatively low values of error measures point to good predictive ability of the model. Although the model slightly overestimates the PDSI forecast maxima and minima, it necessarily captures the timing of the periods of severe to exceptional drought.

Fishery and the state of the Barents and Norwegian Seas cod stock

The assessment of the stock status and prospects for the fishery of the Barents Sea cod is characterized by significant uncertainty caused by the peculiarities of the age structure, which often led to significant differences in the results obtained using different models. As a result, this caused intense discussions both within the framework of the International Council for the Exploration of the Sea (ICES) and at the meetings of the Joint Norwegian-Russian Fisheries Commission. In 2021, at a specialized ICES workshop («benchmark» group), the input data and configuration of the main assessment model - SAM, were refined, related to the inclusion of additional information on older age groups. As a result, two models currently used by the ICES working group to estimate cod stock (SAM and TISVPA), which are very different in their approaches, showed that the number of generations born in 2004–2005 significantly decreased, which led to a decrease in cod biomass. An assessment of the prospects for the state of the stock, performed both on the basis of calculations using the SAM model and on the basis of an alternative approach using the AMO (Atlantic Multidecadal Oscillation Index), showed a high probability of further decline and stabilization of the spawning stock biomass (and catches) in the next 2–3 years, after which restoration of spawning stock biomass is likely due to sufficiently abumdante generations born in 2011 and 2014.

Decadal variability in the austral summer precipitation over the Central Andes: Observations and the empirical‐statistical downscaling model

AbstractThe decadal variability in summer precipitation over the Central Andes (10°–30°S) is investigated from 1921 to 2010 using low‐pass filtered time series of the central and eastern El Niño–Southern Oscillation (ENSO) Pacific (C and E) indices, the South Pacific Convergence Zone (SPCZ) index, the Atlantic SST indices, Atlantic Multidecadal Oscillation (AMO) index, North Atlantic Oscillation (NAO) index, and ERA‐20C reanalysis. Additionally, an empirical‐statistical downscaling (ESD) model was built. A rotated empirical orthogonal function (REOF) analysis shows that the first leading mode of precipitation (RPC1) represents 38.2% of the total decadal variance. RPC2, RCP3, and RPC4 represent 18.8, 12.8, and 9.7% of the total decadal variance, respectively. Furthermore, RPC1 features highest loadings over most of the region. RPC2 features a dipole of highest loadings over the southernmost Bolivian Altiplano and lowest loadings over the northwestern Argentinian Andes. Conversely, RPC3 presents highest loadings over the eastern‐central Bolivian Altiplano and northwestern Argentinian Andes. RPC4 features highest loadings over the southern Bolivian Andes. RPC1 and RPC3 wet summers are associated with moisture transport from the Amazon basin, but RPC1 features the strengthening upper‐level Bolivian high‐Nordeste low system over South America. Conversely, RPC2 and RPC4 wet summers are associated with local processes induced by southward displacement of the South Atlantic Convergence Zone and warm sea surface temperature (SST) anomalies over the Indian Ocean, respectively. According to the ESD model, the decadal variability in the central and eastern Pacific (CP and EP) and Atlantic Ocean reproduces the decadal component of the DJF precipitation over most of the Central Andes.

Interdecadal Variability of Easterly Waves Over the Tropical Northeastern Pacific

AbstractA time‐series analysis of Easterly Wave (EW) activity, based on dynamical and convective variance measures, was carried out over the tropical northeastern Pacific (EPAC). A significant interdecadal change in EW‐activity was identified shifting from reduced activity in 1980–1997, to increased activity during 1998–2015. The changes in EW‐activity are modulated on interdecadal timescales by the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO). EW‐activity is increased when there is a positive AMO index and, to a lesser extent, a negative PDO index. The opposite occurred with a negative AMO index and positive PDO index. This relationship can be understood in terms of how the AMO and PDO impact sea surface temperatures (SSTs) in the EPAC. Warmer SSTs tend to be associated with more EW‐related convection and associated vorticity generation in the EW troughs and vice versa. These results suggest potential predictability of EW‐activity in the EPAC on interdecadal timescales.

Possible Relationships between the Interdecadal Anomalies of Heavy Rainfall under Northeastern China Cold Vortexes and the Sea Surface Temperature (SST)

As an important component of the East Asian monsoon system, the northeast cold vortex (NECV) exerts a significant impact on weather and climate, especially in Northeast China. This study investigated the interdecadal spatiotemporal variability of heavy rainfall under the cold vortex of Northeast China (NECVHR) and its relationship with sea surface temperature (SST) during 1961–2019 over Northeast China. To investigate the dominant factors affecting variability in the heavy rainfall between May and September, an empirical orthogonal function (EOF) analysis was performed. To detect the trends and changes, a Mann-Kendall (MK) test was used. The sliding t-test was used to identify the change points and the significance. Pearson correlation analysis was used to analyze the relationship between SST and NECVHR, and the t-test was used to verify the significance. The results showed that the total amount of cold vortex heavy rainfall during May–September ranged from 153 to 12,509 mm for 1961–2019. An abrupt interdecadal change was seen after 2014 in Northeast China. The EOF analyses revealed that the first, second, and third EOFs explain 76%, 12.1%, and 5.5% of the total variance, respectively. The EOF followed the heavy rainfall pattern, with increases in the south (southeast) and decreases in the north (northwest) over Northeast China. Heavy rainfall over Northeast China positively correlated with the Atlantic multidecadal oscillation (AMO) index. The heavier rainfall under cold vortex (MCVHR) years revealed that the equipotential height was obviously located over the Sea of Japan, west of Northeast China and the Qinghai Tibet plateau. The cyclonic circulation over the East China Sea and north (northeasterly) wind prevails over Northeast China during less heavy rainfall under cold vortex (LCVHR) years. A high anticyclonic circulation over the Qinghai Tibet plateau resulted in stronger cold advection over Northeast China. The anticyclonic circulations over the East China Sea and the Sea of Japan (east), and the western (southwesterly) winds prevail over Northeast China, with a relatively shallow cold trough over the Qinghai Tibet plateau. The findings in this paper provided a better understanding of the interdecadal variability of NECVHR over Northeast China. The findings can be helpful for several stakeholders regarding agricultural production, water resource management, and natural habitat conversation in Northeast China.

Decadal to Multidecadal Variability of the Western North Pacific Subtropical Front and Countercurrent

AbstractThe western North Pacific Subtropical Countercurrent (WSTCC), anchored by the western subtropical front (WSTF), varies from seasonal to multidecadal time scales and affects the climate variability in the North Pacific. However, conflicting views exist for physical mechanisms of the decadal to multidecadal variability of the WSTF/WSTCC. While many previous studies attribute the WSTF/WSTCC decadal variability to the anomalous wind forcing by the Pacific Decadal Oscillation, by synthesizing observations, reanalysis products and the pre‐industrial model simulation experiment, the present study demonstrates that they are controlled by the Atlantic Multidecadal Oscillation (AMO) through the Subtropical Mode Water (STMW) variability. The AMO induced decadal temperature and volume anomalies of the STMW are stored in the subsurface ocean through the subduction process south of the Kuroshio Extension. Then these anomalies propagate southwestward along the thermocline circulation and reaches the northern flank of the WSTCC in about 5 years. During the AMO‐positive phase, the anomalous warm and thin STMW flattens the upper thermocline, reduces the meridional temperature gradient and weakens the WSTF intensity, lagging 5 years behind the AMO index. Subsequently, the WSTCC decreases and moves southward, and vice versa for the AMO‐negative phase. Our results provide a potential predictability for the WSTF and WSTCC decadal to multidecadal variability and are important for the North Pacific climate change and the biogeochemical processes.

The increasing contribution of potential evapotranspiration to severe droughts in the Yellow River basin

Under global warming, droughts have become more severe and more frequent. In this context, it is widely accepted that both water supply (e.g., precipitation, PRE) and demand (e.g., potential evapotranspiration, PET) should be considered in drought assessments due to increasing evapotranspiration. China’s Yellow River basin is located mainly in arid and semi-arid regions that suffer severe droughts. Analyzing the temporal and spatial patterns of droughts in this area and exploring the relative effects of supply and demand on drought occurrence and variations are thus necessary to adapt to increased droughts. Here, we compared three kinds of drought indices, including the standardized precipitation evapotranspiration index (SPEI), the standardized precipitation index (SPI), and the standardized evapotranspiration deficit Index (SEDI), and used SPEI to identify droughts in the Yellow River basin during the six decades from 1956 to 2016. We identified droughts and described drought dynamics based on drought duration, intensity, frequency, and spatial extent. We then evaluated the relative contributions of PRE and PET to droughts of varying severity and long-term trend, as well as the relationships between ocean signals and droughts. Our results show that most of the basin has undergone progressively drier conditions since the 1950 s. This increased drought occurrence has manifested as longer, more frequent droughts affecting a larger spatial extent. PRE was the dominant driver (64%) of drought occurrence in an area of ∼ 86% of the basin, whereas PET was the main driver (55%) in the remaining 14% of the basin. However, changes in PET contributed more than PRE to drought variation, i.e., long-term SPEI trends, in about ∼ 60% of the area. And effects of PET increased during severer droughts due to the intensive interaction between soil moisture and atmospheric water demand in highly arid conditions. Physically, droughts were closely related to several ocean signals, such as the Multivariate ENSO Index (MEI), the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation index (AMO), and the Arctic Oscillation index (AO), at different time scales. Understandings of drought dynamics, their physical mechanisms, and the effects of PRE and PET on droughts of varying severity are essential for water resource management, especially in arid regions.

Do recent meteorological drought events in central Italy result from long‐term trend or increasing variability?

AbstractThe Mediterranean area is one of the most important “hotspot” where the effects of climate change are more evident, with an increase of extreme events such as hot waves and intense droughts. From this perspective, central Italy, which in the recent past suffered from the impact of intense negative anomalies of precipitation on water resources, shows peculiar characteristics, being its meteoclimatic regime determined by global, regional and local factors. In this context, this study aims at exploring the variability in time and space of the precipitation regime in central Italy over the period 1960–2020 analysing global factors, represented by indexes of teleconnection, and local factors such as orography and distance to the sea. Findings indicate an increase of drought events during the last 20 years, in a general pattern characterized by stationary precipitation amounts. This trend is clearer on the Tyrrhenian coast with a number of months under severe drought conditions increasing from 5% (1965–1999) to 24% (2000–2020). The strengthening of the periodic signal of precipitation is related mainly to the Atlantic Multidecadal Oscillation Index and the Western Mediterranean Oscillation Index. Considering the relationship between precipitation and teleconnection patterns over short time scales (few months), correlations are stronger with the North Atlantic Oscillation index and in general with the Atlantic patterns rather than with the Artic patterns, especially for the Adriatic side.

The Increasing Frequency of Tropical Cyclones in the Northeastern Atlantic Sector

North Atlantic Tropical Cyclones (TCs) are major atmospheric hazards that can cause large disruptions to coastal and near-coastal societies. Although most studies focus on those areas with highest impact (e.g., Caribbean Islands, the Gulf and western coast of United States), there is an increasing interest in characterizing changes in occurrence and impacts in areas usually less affected by TCs, particularly in the framework of a changing climate. Here we provide a long-term context evaluating changes in the frequency of TC in the Northeast Atlantic (NEA) basin during the 1978–2019 period. In the last decades, scattered information has shown an impact both from TCs and Post-Tropical Cyclones (PTC) in the NEA. We compute several complementary linear trends and show a significant (p ≤ 0.1) increase in the number of stronger storms in the entire North Atlantic basin, and the amount of TCs and PTCs that reach the NEA, in agreement with previous works. A highly significant relation (p ≤ 0.05) is found between the Atlantic Multidecadal Oscillation (AMO) index and TC activity in both the entire North Atlantic and the NEA basin. Sea surface temperature anomaly maps are produced to better encapsulate the annual variability without the multidecadal oscillation effects and, important cold (warm) pools in cyclogenesis and development zones are found in years with low (high) TC activity. It is also found that the sea surface temperature field plays a minor role in the guiding of storms into the NEA sector. Long-term trends as well as high/low seasonal activity analysis suggest that atmospheric circulation (vertical wind shear, lapse rate, mean sea level pressure and upper-level steering) is more relevant than sea surface temperature in the NEA region.