North Atlantic Oscillation Pattern Research Articles

Understanding the Intermittency of the Wintertime North Atlantic Oscillation and East Atlantic Pattern Seasonal Forecast Skill in the Copernicus C3S Multi‐Model Ensemble

AbstractThe wintertime North Atlantic Oscillation (NAO) and East Atlantic Pattern (EA) are the two leading modes of North Atlantic pressure variability and have a substantial impact on winter weather in Europe. The year‐to‐year contributions to multi‐model seasonal forecast skill in the Copernicus C3S ensemble of seven prediction systems are assessed for the wintertime NAO and EA, and well‐forecast and poorly‐forecast years are identified. Years with high NAO predictability are associated with substantial tropical forcing, generally from the El Niño Southern Oscillation (ENSO), while poor forecasts of the NAO occur when ENSO forcing is weak. Well‐forecast EA winters also generally occurred when there was substantial tropical forcing, although the relationship was less robust than for the NAO. These results support previous findings of the impacts of tropical forcing on the North Atlantic and show this is important from a multi‐model seasonal forecasting perspective.

Climate change and human impacts on aquatic communities at Etoliko Lagoon in western Greece

The Etoliko Lagoon in western Greece has experienced extensive human modification since the 20th century, both on the surrounding land and in the aquatic environment. To examine human impacts and disentangle climatic from anthropogenic changes, we present a suite of biomarker records that span the past two centuries (∼1790–2011). Specifically, we use terrigenous (n-alkanes, polycyclic aromatic hydrocarbons (PAHs), and phytosterols) and aquatic (dinosterol, brassicasterol, cholesterol, and stigmasterol) biomarkers to document changes in nutrient inputs, combustion, and algal productivity. During most of the 19th and 20th centuries, aquatic communities respond to temperature, forced mainly by solar irradiance and volcanic activity, and precipitation, controlled largely by summer and winter North Atlantic Oscillation (NAO) patterns that determine freshwater runoff. PAHs illustrate the acceleration of coal combustion during the 1800s, and declining concentrations since the 1950s correspond to the implementation of emission controls and reductions in rainfall that likely inhibited PAH transport. As human pressures increased in the late 1900s and water column anoxia grew, the absence of a clear human waste and eutrophication signal suggests that other factors also contributed to limited oxygen availability. Overall, environmental degradation of the late 20th and early 21st centuries is clear and can be attributed to a combination of especially arid conditions and human interferences that altered lagoon hydrography, trophic state, and aquatic community composition.

Predictability of European winter 2021/2022: Influence of La Niña and stratospheric polar vortex

AbstractThe Northern Hemisphere winter of 2021/2022 exhibited a positive North Atlantic Oscillation (NAO) which led to largely mild and wet conditions for Northern Europe. A moderate La Niña in the tropical Pacific and a stronger than average stratospheric polar vortex together explained the observed anomalies over the winter. Winter 2021/2022 was well predicted in general by seasonal forecast systems. The ensemble mean indicated a positive winter NAO and the forecast spread of forecasts from the Met Office GloSea6 seasonal prediction system spanned the observed mean sea level pressure anomaly for the whole winter and the individual months. However, December showed the largest departure from the mean of the forecast which is consistent with evidence from previous work that early winter ENSO teleconnections are too weak in model predictions. Nevertheless, around one in four members captured the negative NAO pattern in December. The strong pressure gradient and positive NAO predicted for the latter part of the winter allowed successful warning of the possibility of above average storminess and strong winds which occurred in February 2022. This is potentially useful information for the energy sector who increasingly rely on wind power and the insurance industry for warning of storm damage.

Coherency and time lag analyses between MODIS vegetation indices and climate across forests and grasslands in the European temperate zone

Abstract. Identifying the climate-induced variability in the condition of vegetation is particularly important in the context of recent climate change and plants' impact on the mitigation of climate change. In this paper, we present the coherence and time lags in the spectral response of three individual vegetation types in the European temperate zone to the influencing meteorological factors in the period 2002–2022. Vegetation condition in broadleaved forest, coniferous forest and pastures was measured with monthly anomalies of two spectral indices – normalised difference vegetation index (NDVI) and enhanced vegetation index (EVI). As meteorological elements we used monthly anomalies of temperature (T), precipitation (P), vapour pressure deficit (VPD), evapotranspiration (ETo), and the teleconnection indices North Atlantic Oscillation (NAO) and North Sea Caspian Pattern (NCP). Periodicity in the time series was assessed using the wavelet transform, but no significant intra- or interannual cycles were detected in both vegetation (NDVI and EVI) and meteorological variables. In turn, coherence between NDVI and EVI and meteorological elements was described using the methods of wavelet coherence and Pearson's linear correlation with time lag. In the European temperate zone analysed in this study, NAO produces strong coherence mostly for forests in a circa 1-year band and a weaker coherence in a circa 3-year band. For pastures these interannual patterns are hardly recognisable. The strongest relationships occur between conditions of the vegetation and T and ETo – they show high coherence in both forests and pastures. There is a significant cohesion with the 8–16-month (ca. 1-year) and 20–32-month (ca. 2-year) bands. More time-lagged significant correlations between vegetation indices and T occur for forests than for pastures, suggesting a significant lag in the forests' response to the changes in T.

Analysing the Effects of Atmospheric Teleconnections on Streamflow Regime in the Eastern Black Sea Basin in Türkiye

Analysing the variations in hydrological cycle components is essential for water resources planning and management. In this study, the relationship between the streamflow data belonging to five discharge gauging stations in the Eastern Black Sea Basin in Türkiye and the Arctic Oscillation (AO), East Atlantic-Western Russia (EAWR), North Atlantic Oscillation (NAO) and North Sea Caspian Pattern (NCP) was investigated. For this purpose, Spearman’s correlation test, ensemble empirical mode decomposition (EEMD) and relative importance analysis were used. Accordingly, Spearman’s correlation coefficients were calculated between raw streamflow data, decomposed streamflow data via EEMD and atmospheric teleconnections. Then, the relative importance analysis was applied to determine the atmospheric teleconnections’ influences on streamflow data. The findings showed that the relationship between raw streamflow data and atmospheric teleconnections is generally more significant and negative in the winter and spring. Furthermore, it was observed that the linkage between the decomposed streamflow data and atmospheric teleconnections could differentiate. Although no significant correlation between atmospheric teleconnections and raw streamflow data was detected in some months, significant correlations were detected between atmospheric teleconnections and decomposed streamflow data. This reveals the importance of examining the relationship between atmospheric teleconnections and streamflow data for different periods. The relative importance analysis revealed that the influence of atmospheric teleconnections on streamflow data could change from station to station and from component to component. This study showed that investigating the effects of atmospheric teleconnections on streamflow data for different components and periods is important.

Two distinct declining trend of autumn Arctic sea ice concentration before and after 2002

This study investigates the Arctic sea ice concentration trend during 1979–2021 and explores why the autumn Arctic sea ice loss is accelerated after 2002 and its trend declining center shifts from the Chukchi Sea to the Barents-Kara-Laptev Seas. Attribution analysis reveals that the enhanced summer sea ice concentration negative trend in large part explains the autumn sea ice concentration accelerating reduction, whereas it is the trend center shift of increased downward longwave radiation that accounts for mostly of the autumn sea ice concentration decline center shift. Further analysis suggests the downward longwave radiation trend is closely related to large-scale atmospheric circulation changes. A tendency towards a dipole structure with an anticyclonic circulation over Greenland and the Arctic Ocean and a cyclonic circulation over Barents-Kara Seas enhances (suppresses) the downward longwave radiation over Western (Eastern) Arctic by warming and moistening (cooling and drying) the lower troposphere during 1979–2001. In comparison, a tendency towards a stronger Ural anticyclone combined with positive phase of the North Atlantic Oscillation pattern significantly promotes the increase of downward longwave radiation over Barents-Kara-Laptev Seas during 2002–2021. Our results set new insights into the Arctic sea ice variability and deepen our understanding of the climate change.

Important Influence of North Atlantic Oscillation on Interannual Variability of the Boreal Autumn East Siberian–Beaufort Sea Ice

Abstract The East Siberia–Beaufort Sea (EsCB) sea ice during boreal autumn has recently been reported to play important roles in the climate over Eurasia and North America. The EsCB sea ice exhibits remarkable year-to-year fluctuations in autumn (August–October), the season in which its minimum extent usually occurs. However, the physical driver of the autumn EsCB sea ice interannual variability remains unclear, impeding the seasonal prediction of local sea ice. Here we find that the autumn EsCB sea ice variability is largely driven by the preceding summer (May–July) dipolar atmospheric anomalies over the North Atlantic, resembling the North Atlantic Oscillation (NAO) pattern. During the negative NAO-like phase, the circumpolar anticyclonic anomalies tend to transport the warm air from Greenland toward the EsCB region, which triggers rapid sea ice melt there. The associated EsCB sea ice anomalies can be maintained or even intensified by the local sea ice–albedo positive feedback until autumn. Therefore, the abnormal signals of Arctic sea ice tend to show significant persistence in summer and autumn. The influence of the summer NAO-like atmospheric circulation on the ensuing autumn EsCB sea ice can be realistically reproduced in the historical simulation of the E3SM-1-0 model, supporting our findings based on the observation. This lagged relationship provides a promising pathway for skillful seasonal prediction of the EsCB sea ice and its related climatic impacts.

A regional attentive empirical orthogonal function analysis: An application to North Atlantic Oscillation

AbstractEmpirical orthogonal function (EOF) analysis is widely adopted for identifying spatial patterns in regular latitude‐longitude gridded geographical data. However, a standard EOF analysis could underestimate variances from low latitudes due to the increasing grid cell areas when moving toward the equator. A broadly adopted compensating approach is the area‐weighted EOF, where the grid data are multiplied by a factor proportional to the grid area, thereby forcing each datum to represent identical areas. In this article, we revisit the area‐weighting scheme and discuss its potential drawbacks. In particular, we show that along with compensating the unequal areas, another unaddressed issue is which region of the data is more relevant to our focused problem. We propose a regional attentive EOF (RA‐EOF) method, which resolves area‐compensation and region selection simultaneously. We conduct case studies of the North Atlantic Oscillation (NAO) analysis and perform RA‐EOF to the sea level pressure variability in the North Atlantic sector. Experiments show that our method evidently detects the NAO pattern in the leading mode of sea level pressure variability and suggests the southwestward movement of the southern action centre of NAO during the summers. Our findings clarify the ambiguity about the summer NAO in previous studies. The proposed methodology provides a potent instrument for discerning the spatial distribution and analysing the temporal variability of atmospheric teleconnections and oscillations, particularly in cases characterized by weak signal strength.

An interdecadal change in the relationship between May North Atlantic oscillation and early summer precipitation over the Eastern Tibetan Plateau around the late 1990s

Based on the observational and reanalysis datasets, a dipole pattern of the early summer (June) precipitation over the eastern Tibetan Plateau (TP) is identified through empirical orthogonal function. In contrast to previous researches, this paper introduces the May North Atlantic Oscillation (NAO) as a preceding signal and investigates its connection with subsequent early summer precipitation over the eastern TP during 1980–2019, focusing on predictive aspects. Results reveal that the correlation between NAO and primary eastern TP precipitation pattern (Precip_EOF1) has weakened since 2000. During 1980–2000, an equivalent barotropic Silk Road Pattern (SRP) observed across Eurasia triggers southerly wind anomalies over the TP. This, in turn, leads to the northward shift of the water vapor, which favors the ascending motion over the northern TP (NTP) due to water vapor convergence. Additionally, the SRP-related sensible heating also plays a critical role in vertical motion over the eastern TP by enhancing the TP thermal effect. Consequently, these factors contribute to shaping the dipole pattern of eastern TP precipitation. However, the weakened relationship between NAO and Precip_EOF1 during 2001–2019 is attributed to the absence of NAO-related atmospheric circulation over Eurasia. Further analysis suggests that these circulation anomalies arise from the interdecadal variability of the NAO and its phase shift. Moreover, when the influence of sea surface temperature anomalies over the northern North Atlantic is removed, the correlation of Precip_EOF1 with NAO and Atlantic Multidecadal Oscillation (AMO) both become insignificant. This suggests that the NAO-related dipole pattern of eastern TP precipitation is modulated by preceding northern North Atlantic sea surface temperature rather than AMO on the interdecadal timescale.

Teleconnections link to summer heat extremes in the south-central U.S.: Insights from CMIP5 and CMIP6 simulations

This study examines the relationship between teleconnections, which represent quasiperiodic modes of large-scale atmospheric and oceanic variability, and extreme heatwaves in the south-central (SC) U.S. region, aiming to understand multiyear variability in the heat extremes. Observational data and model simulations reveal that soil water has a stronger influence on surface air temperature in the SC region than individual teleconnections, though not as strong as their combined influence. Particularly, the Pacific North America (PNA), North Atlantic Oscillation (NAO), and Pacific Decadal Oscillation (PDO) teleconnections have a substantial impact on surface air temperature in the region through anomalous anticyclonic circulation, during their negative phases. The wave-activity flux analysis shows that the wave train associated with the PNA teleconnection originates from the extratropical North Pacific Ocean, likely influenced by sea surface temperature (SST) forcing. Similarly, the wave packet linked to the PDO pattern originates from the central Pacific Ocean in the middle latitudes. In contrast, the wave train related to the NAO pattern exhibits little connection to SST forcing, originating from Greenland and propagating across Europe, Russia, the North Pacific, and North America. While climate models generally capture the PNA and PDO wave trains, they struggle to accurately simulate the NAO wave train across the North Pacific Ocean towards North America. This indicates the current generation of climate models face challenges in representing the complex dynamics underlying the NAO wave train.

Persistent Model Biases in the Spatial Variability of Winter North Atlantic Atmospheric Circulation

AbstractThe three leading modes of the North Atlantic atmospheric circulation explain about 70% of the winter climate variability. Although climate models generally can capture these modes, biases may induce large uncertainties in regional climate predictions. Here, we evaluate the leading winter modes simulated by CMIP5‐PMIP3 and CMIP6‐PMIP4 models from the last millennium to future scenarios in comparison with historical reanalysis and paleo‐reconstructions. The models generally have a good representation of the average spatial pattern of the North Atlantic Oscillation (NAO) while showing a larger spread in performance for the East Atlantic and Scandinavian patterns. In contrast to historical reanalysis, the simulated NAO pattern tends to be rather stationary under various climate states over the years 861–2100. Such underestimated spatial variability in the simulated NAO is directly related to the biased spatial shifts in NAO‐related regional temperature and precipitation changes, inducing uncertainties in climate projections over the North Atlantic sector.

Snow Cover on the Tibetan Plateau and Lake Baikal Intensifies the Winter North Atlantic Oscillation

AbstractThis paper revealed a physical connection between the antiphase variation in the preceding autumn Tibetan Plateau (TP) and Lake Baikal snow cover anomalies (TBSA) and the following winter North Atlantic Oscillation (NAO) on interannual time scales during 1979–2021. The antiphase variation in TBSA, accounting for 44% of the total years, has a dipole structure in autumn, which prolonged into the following winter. The persistent antiphase TBSA associated diabatic forcing, disturbances and transient eddies favor a double wave train structure spanning the TP (east of Baikal) and North Atlantic from autumn to winter. Amid the wave train, the circulation anomalies over the North Atlantic extract more energy from the basic flow due to the seasonal increase in the westerly jet, which further evolves into the winter NAO pattern. Our results provide new insights into the formation and projection of winter NAO from the perspective of subtropical and extratropical Eurasia snow.

Comparative Analysis for Atmospheric Oscillations

In recent decades, studies on atmospheric circulations indicate that those patterns have influences on meteorological variables. This paper investigates the comparative statistical analysis of atmospheric oscillations with climatological elements. Based on analysis of the climate data obtained from observed values of meteorological station in Antalya, it was pointed that atmospheric elements such as meteorological variables were associated with atmospheric oscillations such as North Atlantic Oscillation, Arctic Oscillation, Antarctic Oscillation and Pacific-North American pattern. Spearman’s rho and Kendall’s tau statistics were employed to reveal the relations between atmospheric variables and atmospheric oscillations as statistically significant. Both coefficients were compared in interpreting the direction and strength of the relationships. It was seen that Spearman’s rho coefficients presented more suitable values generally.

11-year solar cycle influences on the late-wintertime South Asian jet variability

The South Asian jet leading wave train (SAJLWT) is a zonally elongated and equivalently barotropic stationary Rossby wave train along the wintertime South Asian subtropical jet, which stands out as the leading empirical orthogonal function mode of monthly meridional winds at the upper troposphere over southern Eurasia. The SAJLWT is closely related to weather and climate extremes over southern Eurasia, but up to now, the mechanisms of SAJLWT variability are still not fully understood. The present study reveals a significant 11-year solar cycle (SC) influence on the SAJLWT variability in late winter (January–March). The in-phase correlation between the SC and the SAJLWT is not only statistically significant but also consistent over time. Associated with the solar-induced SAJLWT anomalies, significant surface cooling exists over northeastern Africa and Middle East, while significant positive precipitation anomalies appear over southern China. Two routes of mechanisms are presented to clarify the SC–SAJLWT linkage. One is due to the solar-induced northwest–southeast tilted North Atlantic Oscillation (TNAO) pattern, which may reflect a “top–down” solar forcing and the ocean–atmosphere couplings in the North Atlantic Ocean. This TNAO pattern would trigger southeastward wave activity fluxes toward the SAJ, thus significantly impacting the upstream portions of the SAJLWT pattern. The other mechanism involves weakened convection over the Maritime Continent (MC) region, reflecting solar-induced weakening of Pacific Walker Circulation via “bottom–up” processes. The MC convection anomaly would also excite a SAJLWT-like circulation pattern. Our analysis highlights that the solar “top–down” and “bottom–up” mechanisms may act in concert to exert a significant impact on the SAJLWT and indicates that the SC forcing is an appreciable source of decadal predictability in southern Eurasia.

The signature of NAO and EA climate patterns on the vertical structure of the Canary Current upwelling system

Abstract. The current study aims to analyse the vertical structure of the ocean during upwelling events using in situ and modelled data. Additionally, the influence of climate patterns, namely the North Atlantic Oscillation (NAO) and the East Atlantic (EA) pattern, on the vertical structure and their impact on the upwelling activity are assessed for a period of 25 years (1993–2017). The study focuses on the central part of the Canary Current (25–35∘ N) with persistent upwelling throughout the year, with an annual cycle and the strongest events from June to September. Upwelling is determined using two different approaches: one index is calculated based on temperature differences between the coastal and the offshore area, and the other is calculated based on wind data and the resulting Ekman transport. Different datasets were chosen according to the indices. Stable coastal upwelling can be observed in the study area for the analysed time span, with differences throughout the latitudes. A deepening of the isothermal layer depth and a cooling of temperatures are observed in the vertical structure of coastal waters, representing a deeper mixing of the ocean and the rise of cooler, denser water towards the surface. During years of a positive NAO, corresponding to a strengthening of the Azores High and the Icelandic Low, stronger winds lead to an intensification of the upwelling activity, an enhanced mixing of the upper ocean, and a deeper (shallower) isothermal layer along the coast (offshore). The opposite is observed in years of negative NAO. Both effects are enhanced in years with a coupled, opposite phase of the EA pattern and are mainly visible during winter months, where the effect of both indices is the greatest. The study therefore suggests that upwelling activities are stronger in winters of positive North Atlantic Oscillation coupled with a negative East Atlantic pattern and emphasizes the importance of interactions between the climate patterns and upwelling.

ПОЯВИ ЛЬОДОВИХ ЯВИЩ НА ВОДОСХОВИЩАХ ДНІПРОВСЬКОГО КАСКАДУ ТА МОЖЛИВОСТІ ЇХ ДОВГОСТРОКОВОГО ПРОГНОЗУВАННЯ

Long-term forecasting of the appearance dates of ice phenomena on the Dnipro Cascade reservoirs, especially with the forecast lead time at least 1 month, has important practical and scientific significance. So, it is on the basis of this characteristic of the ice regime that the terms and depths of autumn operation of each of the six Dnipro reservoirs are determined. Along with this, there are no methods for long-term forecasting of the appearance date of ice phenomena on the Dnipro Cascade reservoirs. Obviously, this is due to the fact that the problem of developing reliable long-term forecasts of the ice regime of water bodies is one of the most difficult in hydrometeorology, which has not yet received a fully satisfactory solution. The objective of this of the research is to find connections between quantitative indicators of atmospheric processes in the North Atlantic and the appearance dates of ice phenomena on the Dnipro Cascade reservoirs, and to evaluate the possibilities of using the obtained results for operational long-term forecasting. In long-term forecasting of ice regime is used the statistical, correlational, regression analysis as well as the atmospheric indicators as predictors. Therefore, the minimum atmospheric pressure in the center of the Icelandic minimum, the North Atlantic Oscillation (NAO) pattern and the Scandinavian Oscillation (SCAND) pattern were used in the research. These indicators characterize the atmospheric processes of the North Atlantic. Materials of the Department of Meteorological Forecasts of the Ukrainian Hydrometeorological Center (Kyiv) and the National Oceanic & Atmospheric Administration USA (archival synoptic maps, the values of NAO and SCAND patterns) were used as initial data. The obtained results show that such indicators of the atmospheric processes of the North Atlantic as the minimum atmospheric pressure in the center of the Icelandic minimum, NAO and SCAND patterns undoubtedly have a fairly significant influence on the formation of the appearance dates of ice phenomena on the Dnipro Cascade reservoirs, but such an influence is not predominant and such indicators can only be partially used as predictors for long-term forecasting. Therefore, to obtain more acceptable results, it is necessary to expand the search for predictors of atmospheric processes both in space and time.

A possible relationship between Changma and tropical cyclone genesis frequency in the western North Pacific linked to the North Atlantic Oscillation

AbstractThis study analysed the relationship between Changma and tropical cyclone (TC) genesis in the western North Pacific (WNP). The correlations between Changma onset, withdrawal, and duration in Jeju and the southern and central regions of Korea and TC genesis frequencies in June, July, and June–July in the WNP were analysed. The results showed that Changma duration in the southern region and June–July TC genesis frequency exhibited the highest negative correlation. To investigate the mechanism underlying the correlations, the differences in large‐scale environments between the 14 years with the longest Changma duration and the 14 years with the shortest Changma duration were analysed. For 850‐hPa stream flows, anomalous anticyclones are found in most WNP regions, while anomalous cyclones are located in the East Asian mid‐latitude region. In the long Changma duration years, the WNP subtropical high was strengthened in the north–south and east–west directions, the monsoon trough was weakened, and anomalous downward flows are formed at the latitudinal (longitudinal) band of 10°–27°N (100°–180°E) where TCs mainly occur. All these environments provide unfavourable conditions for TCs to occur. The difference in sea surface temperature revealed triple anomaly patterns, such as warm, cold, and warm, in the north–south direction in the North Atlantic. This spatial distribution was similar to the negative North Atlantic Oscillation (NAO) pattern. From the analysis of 500‐hPa stream flow, we confirmed that the NAO acts as a bridge connecting the TC and Changma in the WNP.

Long-term changes of water flow, water temperature and heat flux of two largest arctic rivers of European Russia, Northern Dvina and Pechora

The phases of long-lasting (more than 10–15 years) increased and decreased water flow, water temperature and heat flux values in the Northern Dvina River and the Pechora River were studied for the observation period from the 1930s to 2020. To distinguish between different phases, statistical homogeneity tests and normalized cumulative deviation curves were used. Generally, the identified phases displayed statistically significant differences between average values of the measured characteristics. During contrasting phases, the general pattern of water temperature during the warm season, water runoff and heat flux in the Northern Dvina and Pechora River Basins differed considerably. The number of the identified phases varied between the studied rivers and ranged from two to four contrasting phases in the Northern Dvina River exceeded those of the Pechora River. Consequently, the duration of the phases also varied quite significantly. The difference in mean values of the hydrological characteristics during the contrasting phases in the Northern Dvina River exceeded those of the Pechora River. The longest phases of increased and decreased heat flux nearly coincide with contrasting periods of water runoff and water temperature. The phases of simultaneous increased or decreased values of all hydrological characteristics were associated with corresponding periods of increased or decreased air temperature (on average for a year and for the open water period) and annual precipitation values. Those long-lasting phases of simultaneously increased or decreased values of river flow, heat flux, and water temperature were associated with changes of the global thermal regime, regional cryosphere variations, and long-term periods of intensification or weakening of the atmospheric circulation over the North Atlantic, characterised by variability in macrocirculation indices such as the North Atlantic Oscillation and Scandinavian circulation pattern.

The relationship between surface weather over North America and the Mid-Latitude Seasonal Oscillation

The influence of the 120-day Mid-Latitude Seasonal Oscillation (MLSO) on near surface circulation, temperature and precipitation over North America is investigated as an independent mode and in combination with the El Niño-Southern Oscillation (ENSO), as well as the North Atlantic Oscillation (NAO) and Pacific North American (PNA) teleconnection patterns. Empirical evidence based on frequency ratios such as anomalous warm over cold days and wet over dry days and composite analysis reveal an influence of the MLSO during both boreal winter and summer. During winter, above (below) normal MLSO conditions can cause below (above) average temperatures across the US. During summer, the below normal MLSO conditions can cause strong above average temperatures across the US, whereas the below normal MLSO conditions can cause weak temperature anomalies of both signs. The influence of MLSO on the surface air temperature anomalies is more significant and stronger than the effect on precipitation. It is also found that the MLSO plays a role in exciting variability when interacting with other modes, exerting influence over the same region. Expected patterns of temperature and precipitation associated with well-known climate modes show alterations when they are further decomposed to account for the MLSO influence.

Regionalization of climate teleconnections across Central Asian mountains improves the predictability of seasonal precipitation

Mountains play a critical role in water cycles in semiarid regions by providing for the majority of the total runoff. However, hydroclimatic conditions in mountainous regions vary considerably in space and time, with high interannual fluctuations driven by large-scale climate oscillations. Here, we investigated teleconnections between global climate oscillations and the peak precipitation season from February to June in the Tian-Shan and Pamir Mountains of Central Asia. Using hierarchical climate regionalization, we identified seven subregions with distinct precipitation patterns, and assessed correlations with selected climate oscillations at different time lags. We then simulated the seasonal precipitation in each subregion from 1979 to 2020 using the most prevalent teleconnections as predictors with support vector regression (SVR). Our findings indicate that the El Niño–Southern Oscillation, the Pacific Decadal Oscillation, and the Eastern Atlantic/West Russia pattern are among the major determinants of the seasonal precipitation. The dominant lead-lag times of these oscillations make them reliable predictors ahead of the season. We detected notable teleconnections with the North Atlantic Oscillation and Scandinavian Pattern, with their strongest associations emerging after onset of the season. While the SVR-based models exhibit robust prediction skills, they tend to underestimate precipitation in extremely wet seasons. Overall, our study highlights the value of appropriate spatial and temporal aggregations for exploring the impacts of climate teleconnections on precipitation in complex terrains.