Pacific North American Pattern Research Articles

Interannual Variability of the Mass-Weighted Isentropic Zonal Mean Meridional Circulation in the Northern Hemisphere Winter

Abstract The atmospheric general circulation diagnosed by the mass-weighted isentropic zonal mean exhibits an extratropical direct (ETD) circulation driven by eddy momentum transport. We investigated regional atmospheric modulations associated with the year-to-year variability of ETD circulation in boreal winter using a reanalysis dataset. Composite analyses showed that the interannual variability of ETD circulation accompanies a seesaw-like variation between the Aleutian low (AL) and Icelandic low (IL), which coincides with the Pacific–North American teleconnection pattern and North Atlantic dipole anomaly. A diagnosis using isentropic airmass fluxes representing the geographical Lagrangian mean motion indicated that the significant intensification of upper-tropospheric poleward warm airflow over the eastern North Pacific is responsible for the enhanced ETD circulation, together with the intensified equatorward lower cold airflow over East Asia and eastern North America. The resulting enhanced upward propagation of the stationary planetary waves corresponds to the intensification of the divergence of the Eliassen–Palm flux, which explains the modulated ETD circulation in view of the balance between the Coriolis force and eddy momentum flux convergence. These results suggest that the AL–IL seesaw promotes variations in the global general circulation, and the anomalous ETD circulation interacts with several teleconnection patterns involving the modulation of planetary waves through anomalous meridional heat transport by the basinwide-scale eddies.

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.

Role of the Aleutian Low in Modulating the Connection between the Moderate El Niño and the Following Spring North Tropical Atlantic SST Anomalies

Abstract El Niño events are likely to be followed by significant positive sea surface temperature anomalies (SSTAs) over the north tropical Atlantic (NTA). We find that this is not always true for El Niño with moderate intensity. Nearly half of the moderate El Niño cases are followed by the strong NTA SSTAs, but others are followed by the weak NTA SSTAs, indicating the uncertainty in the connection between the moderate El Niño and the NTA. The differences in the El Niño–related NTA SSTAs are due to the different extratropical teleconnection between El Niño and the NTA, manifesting as a Pacific–North American (PNA) pattern. Further analysis suggests that the deepened Aleutian low (AL) induces the negative North Pacific SSTA, which is associated with a wave train propagating eastward, and, modulates the El Niño–generated PNA pattern. Therefore, El Niños that are accompanied by the strong AL induce a strong PNA pattern that, in turn, leads to the strong NTA SSTAs. The strength of the AL coinciding with El Niño plays a crucial role in modulating the connection between the moderate El Niño and the NTA SSTAs. This observational evidence about the uncertainty in the El Niño–NTA connection and the role of the AL in modulating this connection is further supported by the model simulations that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). These results indicate that the combined effects of El Niño and the AL must be considered to fully understand the NTA SSTA variability. Significance Statement El Niño events often lead to significant warming over the north tropical Atlantic (NTA). We find that the strong El Niño almost always is accompanied by the strong NTA sea surface temperature anomalies (SSTAs), whereas the moderate El Niño can be followed by either the strong or weak NTA SSTAs, indicating the uncertainty in the connection between the moderate El Niño and the NTA SSTAs. Further analysis demonstrates that El Niño, which is accompanied by the strong (weak) Aleutian low leads to the strong (weak) NTA SSTAs, suggesting that the role of the Aleutian low must be considered to fully understand the El Niño–NTA connection.

Revisiting the Influence of ENSO on the Arctic Stratosphere in CMIP5 and CMIP6 Models

The Arctic stratospheric response to El Niño–Southern Oscillation (ENSO) is assessed using the historical simulations provided by the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5 and CMIP6, respectively). CMIP6 models can well reproduce the ENSO signals in the Arctic stratosphere and have an ameliorated performance compared to CMIP5 models. Specifically, El Niño is associated with an intensified Pacific–North American pattern that leads to a considerable enhancement of planetary wavenumber 1 but a small reduction of planetary wavenumber 2, and thus, a warm and weakened stratospheric polar vortex. The case for La Niña is nearly the opposite, with a cool and strengthened stratospheric polar vortex. In CMIP6, the ENSO-related stratospheric signal matures in the February–March–April season and increases with ENSO magnitude, regardless of the ENSO phase. However, the stratospheric response to strong El Niño (La Niña) is weaker (stronger) than that which should be achieved if the response changes linearly with the amplitude of El Niño (La Niña). An asymmetric time evolution of stratospheric signals exists between strong El Niño and La Niña events. The stratospheric response caused by strong El Niño is weaker from late winter to early spring but stronger in middle and late spring compared to that caused by strong La Niña. By contrast, the Arctic stratospheric signal in moderate El Niño events is larger than that in moderate La Niña. Compared to ENSO-neutral winters, stratospheric sudden warming occurs more (less) frequently in El Niño (La Niña), as simulated by CMIP6 high-top models.

Winter cold extremes over the eastern North America: Pacific origins of interannual-to-decadal variability

Winter cold extreme events have been observed to frequently take place over North America mainly over its east side, which show significant interannual and decadal variability and cause huge economic losses in the United States. However, it is unclear what leads to the interannual-decadal variability of winter cold extremes over the eastern North America. In this study, we indicate that the decadal variability of winter cold extremes over the eastern North America, whose period is shortened in the recent decades, is mainly tied to Pacific decadal oscillation (PDO), whereas their interannual variability is mainly regulated by Victoria mode (VM). A positive PDO promotes cold extremes in the lower latitudes of the eastern North America mainly owing to the presence of positive Pacific North American (PNA+) patterns, whereas a positive VM is favorable for intense cold extremes in the higher latitudes of the eastern North America mainly due to the occurrence of negative North Pacific oscillation (NPO−) patterns. Thus, the positive VM and PDO combine to significantly contribute to the interannual-to-decadal variability of winter cold extremes over the eastern North America through changes in the winter NPO− and PNA+ patterns due to the variations of meridional background potential vorticity gradient and basic zonal winds. These new findings can help us understand what are the origins of the interannual-decadal variability of winter cold extremes over the eastern North America.

Effects of Wave-Mean Flow Interaction on the Multi-Time-Scale Variability of the AO Indices: A Case Study of Winters 2007/08 and 2009/10

Wave-mean flow interaction is usually regarded as accounting for the origin of the Arctic Oscillation/Northern Hemisphere Annular Mode (AO/NAM). It is inferred that the combination of the local wave-mean flow interactions at the AO/NAM’s three regional centers of action on three important time scales contributes to the main behavior of the AO/NAM index. To discuss the variations of the AO/NAM indices on the three prominent time scales, we take the 2007/08 and 2009/10 winters as two comparative examples to analyze the local wave-mean flow interactions at the AO/NAM’s three centers. The following three facets are identified: (1) Synoptic-scale wave breakings in the North Atlantic can explain the variances of the AO/NAM index on a time scale of 10–20 days. In the 2007/08 winter, there were both cyclonic and anticyclonic synoptic wave breakings, while in the 2009/10 winter, cyclonic synoptic wave breaking was dominant, and the flow characteristics were strikingly similar to the blocking. (2) In the 2007/08 and 2009/10 winters, the signals of the AO/NAM indices on the time scale of 30–60 days are mainly from the interactions between the upward propagating quasi-stationary waves and the polar vortex in the stratosphere. (3) This work also demonstrates that the AO/NAM is linked to the El Niño–Southern Oscillation (ENSO) by the Pacific–North American pattern (PNA) on the winter mean time scale. In the 2007/08 (2009/10) winter, La Niña (El Niño) forced the Pacific jet to shift poleward (equatorward), in favor of weakening (enhancing) the polar waveguide; thus, the polar vortex became stronger (weaker), corresponding to the positive (negative) winter mean AO/NAM index.

Amplified Asymmetric Impact of ENSO Events on the Wintertime Pacific‐North American Teleconnection Pattern

AbstractEl Niño/Southern Oscillation (ENSO) as the strongest tropical interannual signature has the most prominent impact on wintertime Pacific‐North American (PNA) teleconnection pattern. ENSO exhibits an increasing asymmetry in recent decades. This study quantifies its extratropical asymmetric impact on the amplitude of the PNA pattern, using a normalized asymmetry index defined as a ratio of asymmetric versus symmetric anomalies for El Niño and La Niña. Relative to the ENSO asymmetry, the extratropical asymmetric impact is largely amplified especially downstream by up to 82%. Such an amplification is attributed to noticeable increases, 83.6% (68.9%), in asymmetry of Rossby wave source (RWS) anomalies over two key regions of North Pacific, in which anomalous divergence induced by nonlinear condensational heating feedback and anomalous synoptic eddy forcing are two major contributors. The former contributes 39.3% (47.5%) over the western (eastern) North Pacific through increasing asymmetric RWS anomalies, while the latter contributes 29.5% (21.3%) through decreasing symmetric RWS anomalies.

Spatial changes in two major modes of atmospheric circulation variability during the 20th century

This study presents the time development of two major modes of atmospheric circulation variability in the Northern Hemisphere, the North Atlantic Oscillation (NAO) and the Pacific/North American Pattern (PNA), during the 20th century. We employ 500 hPa winter monthly means of the long-term reanalysis 20CR, which cover the 1871−2011 period. We use a moving Principal Component Analysis calculated for 40-year periods with a one-year step. The spatial structure of NAO is well developed and stable during the whole 20th century using the ensemble mean. However, substantial changes in PNA pattern occur during the late 19th century. Its centres are weaker and smaller, except the centre over the northern Pacific Ocean, which is stronger and larger in the early period. However, these changes are not visible when using ensemble members. Therefore, we suggest that these shifts do not reflect real changes in the atmosphere but are produced by the reanalysis and statistical method itself.

Predictability of extreme streamflow in Brazilian rivers through climate indices

In the present study empirical modeling was used to estimate extreme monthly streamflow in 161 locations of hydroelectric plants in the main Brazilians basins through a climate indices set. Principal components analysis was applied to capture the combined influences of climate indices variability. The main predictors of the models were climate patterns of tropical Atlantic and Pacific (Tropical Southern Atlantic and El Niño Southern Oscillation (ENSO) Modoki), complemented by high latitudes patterns (Antarctic Oscillation and Pacific North American Pattern). The contribution of the ENSO/Pacific Decadal Oscillation mode occurs in preferential months, especially in the transition seasons. There is also a contribution of the lagged streamflow itself, mainly in basins located in the Southeast of Brazil. The accuracy of the model for most of the Brazilian basins is higher than 70%, with higher values than 90% for the estimate of very low streamflow in northeast and north-central Brazil, as well as for very high streamflow in south-central region, which performance decreases with the increase in the lag. The results showed that the climate indices have a highly predictive potential for extreme streamflow, with a higher predictability for more long-term forecasts in case of very low streamflow than very high streamflow.

Dynamics of ENSO-driven stratosphere-to-troposphere transport of ozone over North America

Abstract. The El Niño–Southern Oscillation (ENSO) is known to modulate the strength and frequency of stratosphere-to-troposphere transport (STT) of ozone over the Pacific–North American region during late winter to early summer. Dynamical processes that have been proposed to account for this variability include variations in the amount of ozone in the lowermost stratosphere that is available for STT and tropospheric circulation-related variations in the frequency and geographic distribution of individual STT events. Here we use a large ensemble of Whole Atmosphere Community Climate Model (WACCM) simulations (forced by sea-surface temperature (SST) boundary conditions consistent with each phase of ENSO) to show that variability in lower-stratospheric ozone and shifts in the Pacific tropospheric jet constructively contribute to the amount of STT of ozone in the North American region during both ENSO phases. In terms of stratospheric variability, ENSO drives ozone anomalies resembling the Pacific–North American teleconnection pattern that span much of the lower stratosphere below 50 hPa. These ozone anomalies, which dominate over other ENSO-driven changes in the Brewer–Dobson circulation (including changes due to both the stratospheric residual circulation and quasi-isentropic mixing), strongly modulate the amount of ozone available for STT transport. As a result, during late winter (February–March), the stratospheric ozone response to the teleconnections constructively reinforces anomalous ENSO-jet-driven STT of ozone. However, as ENSO forcing weakens as spring progresses into summer (April–June), the direct effects of the ENSO-jet-driven STT transport weaken. Nevertheless, the residual impacts of the teleconnections on the amount of ozone in the lower stratosphere persist, and these anomalies in turn continue to cause anomalous STT of ozone. These results should prove helpful for interpreting the utility of ENSO as a subseasonal predictor of both free-tropospheric ozone and the probability of stratospheric ozone intrusion events that may cause exceedances in surface air quality standards.

Recently Amplified Interannual Variability of the Great Lakes Ice Cover in Response to Changing Teleconnections

Abstract The interannual variability of the annual maximum ice cover (AMIC) of the Great Lakes is strongly influenced by large-scale atmospheric circulations that drive regional weather patterns. Based on statistical analyses from 1980 to 2020, we identify a reduced number of accumulated freezing degree days across the winter months in recent decades, a step-change decrease of AMIC after the winter of 1997/98, and an increased interannual variability of AMIC since 1993. Our analysis shows that AMIC is significantly correlated with El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Pacific–North American pattern (PNA) before the winter of 1997/98. After that, the AMIC is significantly correlated with the tropical–Northern Hemisphere pattern (TNH) and eastern Pacific oscillation (EPO). Singular value decomposition of the 500-hPa geopotential height and surface air temperature shows a dipole pattern over the northeast Pacific and North America, demonstrating the ridge–trough system. This dipole pattern shifts northward to the northern Rocky Mountains, placing the Great Lakes region in the trough after 1997/98. This shift coincides with the increased interannual variability of the EPO index, as well as the change in the sea surface temperature (SST) over the northeast Pacific, where the second mode of the empirical orthogonal function (EOF) on SST shows a warm blob-like feature manifested over the Gulf of Alaska. The regression of wave activity flux onto the SST EOF shows that the source of upward and eastward propagation of a stationary Rossby wave shifts to the west coast of North America, likely moving the ridge–trough system eastward after the winter of 1997/98. Significance Statement We found that the Great Lakes annual maximum ice cover decreased after one of the strongest El Niño–Southern Oscillation events in 1997/98 and that its year-to-year fluctuations increased in the recent 20 years. After the winter of 1997/98, the Great Lakes annual maximum ice cover started to correlate with the warm sea surface temperature anomaly in the northeast Pacific, which appears to disrupt the polar vortex far up in the stratosphere, and the polar vortex ultimately shifts eastward the ridge–trough system over North America. When the shifted system develops enough, it can encase the Great Lakes region in the Arctic air and thereby cause larger year-to-year fluctuations. This connection was found to be characteristic in the recent few decades, after the winter of 1997/98.

Impacts of ocean-atmosphere teleconnection patterns on the south-central United States

Recent research has linked the climate variability associated with ocean-atmosphere teleconnections to impacts rippling throughout environmental, economic, and social systems. This research reviews recent literature through 2021 in which we identify linkages among the major modes of climate variability, in the form of ocean-atmosphere teleconnections, and the impacts to temperature and precipitation of the South-Central United States (SCUSA), consisting of Arkansas, Louisiana, New Mexico, Oklahoma, and Texas. The SCUSA is an important areal focus for this analysis because it straddles the ecotone between humid and arid climates in the United States and has a growing population, diverse ecosystems, robust agricultural and other economic sectors including the potential for substantial wind and solar energy generation. Whereas a need exists to understand atmospheric variability due to the cascading impacts through ecological and social systems, our understanding is complicated by the positioning of the SCUSA between subtropical and extratropical circulation features and the influence of the Pacific and Atlantic Oceans, and the adjacent Gulf of Mexico. The Southern Oscillation (SO), Pacific-North American (PNA) pattern, North Atlantic Oscillation (NAO) and the related Arctic Oscillation (AO), Atlantic Multidecadal Oscillation/Atlantic Multidecadal Variability (AMO/AMV), and Pacific Decadal Oscillation/Pacific Decadal Variability (PDO/PDV) have been shown to be important modulators of temperature and precipitation variables at the monthly, seasonal, and interannual scales, and the intraseasonal Madden-Julian Oscillation (MJO) in the SCUSA. By reviewing these teleconnection impacts in the region alongside updated seasonal correlation maps, this research provides more accessible and comparable results for interdisciplinary use on climate impacts beyond the atmospheric-environmental sciences.

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.

2021 Texas cold snap: Manifestation of natural variability and a recent warming trend

In this paper, we demonstrate that the extreme cold snap in Texas in February 2021 resulted from a large-scale background circulation anomaly over the North Pacific and North America that was a manifestation of both natural variability (namely, a negative Pacific–North American pattern, PNA) and a warming trend that began in the late 1990s. Numerical experiments revealed that the negative PNA pattern could be driven by negative (La Niña-like) sea surface temperature (SST) anomalies in the equatorial eastern Pacific on an interannual–decadal timescale and the SST warming trend in the subtropical North Atlantic on a longer timescale. We propose that the excitation of the most unstable mode of the Northern Hemisphere winter, which exhibits a combined negative PNA and positive East Atlantic pattern resulting from SST anomalies and trends, leads to the dominance of the large-scale circulation anomaly observed in February 2021 and over the last few decades.Our study demonstrated the key role of extratropical wave activity and the intrinsic mode in causing the extreme cold in North America. The arguments favoring polar or tropical influences may partially explain the source of variability, considering that extratropical weather and climate variability could be modulated by not only polar or tropical perturbations but also the enhanced tropical–extratropical interaction and mid-latitude instability. A systematic approach involving both empirical diagnostics and a suite of carefully designed numerical experiments is required to determine the relative influence of various sources of variability.Regarding future changes, models of the Coupled Model Intercomparison Project Phase 6 project a weakening, rather than an enhancement, of the negative PNA pattern that favors the occurrence of cold snaps despite failing to simulate the observed enhanced trend since the late 1990s.

Arctic Oscillation and Pacific-North American pattern dominated-modulation of fire danger and wildfire occurrence

Based on statistical analyses and Arctic Oscillation (AO) and the Pacific-North American pattern (PNA) induced climate anomalies in the 2001–2020 interval, it has been found that these climate modes drastically influence the fire danger (PFIv2) in differing ways across coastal and inland regions. The AO induces higher fire risk in northern Eurasia and central North America, whereas the PNA increases the fire danger across southern Asia and western North America. Moreover, fires have been predominantly identified, up to 70%, during the positive phases of AO and PNA northward of 50°N, in particular over Alaska, Baltic States and eastern Asia. For coincident positive AO and negative PNA days, a large number of fires have been identified over northwestern North America and northern Eurasia. Spectral analyses demonstrate that weather anomalies related to AO and PNA lead fire danger by 10–20 days, and both modes are significantly correlated to PFIv2 over north America and most of Eurasia. Despite some drawbacks related to the fire danger methods currently applied (PFI and FWI), it is demonstrated that the influence of AO and PNA on potential environmental driven-fires can be anticipated, in some locations on almost 90% of days. Fire danger forecasts are urgently needed and the understanding of factors and conditions, which are able to modify the environmental susceptibility to fire development, are crucial for adequate management to reduce the harmful effects of fire. In this sense, our results reveal that a better prediction of the fire season can be achieved by advanced assessment of the PNA and AO behavior, and shed light on the need to investigate the impact of other modes of climate variability upon wildfire frequency and severity.

The Temperature Anomaly Pattern of the Pacific–North American Teleconnection: Growth and Decay

Abstract Applying composite analysis to ERA-Interim data, the surface air temperature (SAT) anomaly pattern of the Pacific–North American (PNA) teleconnection is shown to include both symmetric and asymmetric SAT anomalies with respect to the PNA phase. The symmetric SAT anomalies, overlying the Russian Far East and western and eastern North America, grow through advection of the climatological temperature by the anomalous meridional wind and vertical mixing. The asymmetric SAT anomalies, overlying Siberia during the positive PNA and the subtropical North Pacific during the negative PNA, grow through vertical mixing only. For all SAT anomalies, vertical mixing relocates the temperature anomalies of the PNA teleconnection pattern from higher in the boundary layer downward to the level of the SAT. Above the level of the SAT, temperature anomaly growth is caused by horizontal temperature advection in all locations except for the subtropical North Pacific, where adiabatic cooling dominates. SAT anomaly decay is caused by longwave radiative heating/cooling, except over Siberia, where SAT anomaly decay is caused by vertical mixing. Additionally, temperature anomaly decay higher in the boundary layer due to nonlocal mixing contributes indirectly to SAT anomaly decay by weakening downgradient diffusion. These results highlight a diverse array of mechanisms by which individual anomalies within the PNA pattern grow and decay. Furthermore, with the exception of Siberia, throughout the growth and decay stages, horizontal temperature advection and/or vertical mixing is nearly balanced by longwave radiative heating/cooling, with the former being slightly stronger during the growth stage and the latter during the decay stage.

Northern Hemisphere Winter Blocking: Differing Onset Mechanisms across Regions

Abstract Atmospheric blocking is a prolific producer of extreme weather with significant socioeconomic impacts. Different physical mechanisms for blocking onset have been proposed and are generally focused on two sectors: the Eurasian and the North Pacific. Here, we objectively separate blocking into four regions and investigate how the blocking onset mechanisms vary from one region to another, focusing on three factors: scale interactions between three frequency bands, Rossby wave breaking (RWB), and diabatic heating. Atlantic blocks are dominated by the low-frequency flow evolution that resembles the negative phase of the North Atlantic Oscillation and are influenced by cyclonic RWB toward the western edge of the anticyclone. Europe blocks are influenced by high-frequency, traveling waves across the Atlantic Ocean and develop rapidly, mainly attributed to strong anticyclonic RWB and interaction between high- and intermediate-frequency flow components. Asian blocks are fixated within a stationary wave train that spans upstream to the western Atlantic Ocean and do not have strong potential vorticity or RWB features. The Pacific blocks are mainly influenced by an intermediate-frequency retrograding wave train, while a low-frequency component resembling the Pacific–North American pattern is evident. The Pacific blocks also contain precursor signals in the stratosphere. Backward trajectory analysis revealed that 35%–45% of parcels initialized within the Atlantic, Europe, and Pacific blocking anticyclones experience heating and ascent, while adiabatic processes dominate Asian blocking. Overall, our analysis demonstrates the importance of decomposing the flow into three frequency bands and illustrates different blocking onset mechanisms over four sectors.

Impacts of Aerosols and Climate Modes on Tropical Cyclone Frequency over the North Indian Ocean: A Statistical Link Approach

Abstract North Indian Ocean (NIO) tropical cyclone activity is strongly influenced by aerosols and climate modes. In this study, we evaluated the impact of aerosols and climate modes on modulating tropical cyclone (TC) frequency over the NIO. A statistical generalized additive model based on Poisson regression was developed to assess their relative impacts. Aerosol optical depth for different compounds simulated by the Goddard Chemistry Aerosol Radiation and Transport model, sunspot number (SN) as solar variability, and eight climate modes—Atlantic meridional mode (AMM), El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Indian Ocean dipole (IOD), Pacific decadal oscillation (PDO), Pacific–North American teleconnection pattern (PNA), Arctic Oscillation (AO), and Antarctic Oscillation (AAO), all based on reanalysis datasets, were analyzed for the 40-yr period 1980–2019. A strong linkage was found between TC activity and the AMM, IOD, and ENSO over the NIO. In addition, black carbon, organic carbon, sea salt, and sulfate aerosols have a significant impact on the cyclone frequency. Among these factors, black carbon, organic carbon, sea salt, and AMM account for the most variance of TCs, and among the other climate modes, IOD contributes more than ENSO. This is the first attempt to have identified this ranked set of aerosols and climate indices according to their relative ability to impact NIO TCs. Possible linkages between the thermodynamic and dynamic effects of aerosols on the Indian monsoon environment and its modifications to the large-scale environmental parameters relevant to TC development, namely, sea surface temperature, vertical wind shear, relative vorticity, and relative humidity during different phases of the climate modes are discussed. Significance Statement Aerosols and climate modes have enormous impact on tropical cyclones (TCs). In this study, we evaluated the impact of aerosols and climate modes that modulate frequency of TCs over the north Indian Ocean. To assess the impact, a statistical generalized additive model based on Poisson regression was developed. A strong linkage was found between TC activity and Atlantic meridional mode, Indian Ocean dipole, and El Niño–Southern Oscillation, whereas other climate modes have no statistical significance. In addition, black carbon, organic carbon, sea salt, and SO4 aerosols have a strong linkage to cyclone frequency. The study postulates that most positive phases of these climate modes are associated with more TCs, while the negative phases are associated with fewer.

Groundwater response to climate variability in Mediterranean type climate zones with comparisons of California (USA) and Portugal

Aquifers are a fundamental source of freshwater, yet they are particularly vulnerable in coastal regions with Mediterranean type climate, due to both climatic and anthropogenic pressures. This comparative study examines the interrelationships between ocean-atmosphere teleconnections, groundwater levels and precipitation in coastal aquifers of California (USA) and Portugal. Piezometric and climate indices (1989–2019) are analyzed using singular spectral analysis and wavelet transform methods. Singular spectral analysis identifies signals consistent with the six dominant climate patterns: the Pacific Decadal Oscillation (PDO), the El Niño-Southern Oscillation (ENSO), and the Pacific/North American Oscillation (PNA) in California, and the North Atlantic Oscillation (NAO), the Eastern Atlantic Oscillation (EA) and the Scandinavian Pattern (SCAND) in Portugal. Lower-frequency oscillations have a greater influence on hydrologic patterns, with PDO (52.75%) and NAO (46.25%) on average accounting for the largest amount of groundwater level variability. Wavelet coherences show nonstationary covariability between climate patterns and groundwater levels in distinct period bands: 4–8 years for PDO, 2–4 years for ENSO, 1–2 years for PNA, 5–8 years for NAO, 2–4 years for EA and 2–8 years for SCAND. Wavelet coherence patterns also show that coupled climate patterns (NAO+ EA– and paired PDO and ENSO phases) are associated with major drought periods in both the Mediterranean climate zones.

Appearance of climatic cycles and oscillations in Carpathian Basin precipitation data

A number of climatic cycles and teleconnections are known on the Earth. By definition, the cycles can have a periodic effect on the global climate, while teleconnections can influence the weather at large distances. At the same time, it is overwhelmingly assumed that the hydrological cycle is permanently intensifying all over the world. In this study, we determine and quantify some connections among these climatic cycles and precipitation data from across Hungary. By using cross-correlation and cross-spectral analysis, the connections of the climatic patterns and oscillations with the precipitation of different Hungarian areas have been defined. We used the 1950–2010 timeframe in order to be able to detect effects of several climatic patterns, such as the El Niño-Southern Oscillation (ENSO), the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), the Pacific/North American teleconnection pattern (PNA) and the Atlantic Multidecadal Oscillation (AMO) on the rainfall events of the Carpathian Basin. Data from four different precipitation measurement sites and oscillation indexes from several databases were used. The results help to understand the patterns and regularities of the precipitation, which is the major source of natural groundwater recharge, and a handy tool for future groundwater management measures. Because of the defined connections, any changes in these teleconnections will probably influence the future utilization of the Hungarian groundwater resources.