Pacific North American Pattern Research Articles

Spatiotemporally varied extreme precipitation events simultaneously controlled by multiple circulation factors in China's Loess Plateau

AbstractThe intensive and frequent extreme precipitation events (EPE) greatly contribute to the severe soil loss in China's Loess Plateau, and it is thus essential to understand how and why EPE varies with space and time. Particularly, the combined influence of multiple factors on EPE has rarely been explored. This study analysed the spatiotemporal variability of EPE for 1961–2017 after defining six indices for intensity, frequency, and duration of EPE (R5d, SDII, R90t, R90n, CDD, CWD). We employed 12 atmospheric circulation factors to determine their combined effects on EPE on different time scales over time by the multiple wavelet coherence. The EPE indices spatially varied with obvious southeast‐northwest gradients. Averaged across the whole plateau, SDII, R90t, and R90n increased insignificantly, while R5d, CDD, and CWD decreased insignificantly. In addition, the change trend had spatial variations with most wet EPE indices increasing in the north and the southernmost areas and decreasing in the middle‐south areas. SDII, R90t, and R90n at 68%, 66%, and 53% of stations tended to increase, respectively, and R5d, CDD, and CWD decreased at 66%, 70%, and 77% of stations, respectively. The temperature had a greater correlation with the duration of EPE in summer and on the intensity and frequency of EPE in winter. El Niño‐Southern Oscillation (ENSO), Pacific North American teleconnection pattern (PNA), ENSO‐Arctic Oscillation (AO), and ENSO‐AO‐PNA were found to be the main factors influencing EPE individually or simultaneously. This study provides technical support for the identification of combined effects of multiple factors, and the results are helpful for regional environmental management. Specific to the Loess Plateau, the identified individual or multiple factors can be used to project future changes in EPE, and the more frequent and intensive EPE in some regions may increase the difficulty in future soil loss control.

Influence of climate variability on wind‐sea and swell wave height extreme over the Indo‐Pacific Ocean

AbstractIn the Indo‐Pacific Ocean (IPO), extreme significant wave heights (SWHs) can substantially induce coastal erosion, flooding, and devastating impacts on coastal livelihoods. This study examines the seasonal influence of the El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Southern Annular Mode (SAM), and Pacific North American (PNA) pattern on extreme wind‐wave parameters. The climatic extremes are calculated utilizing ERA5 reanalysis datasets from 1979 to 2019 and a nonstationary generalized extreme value distribution. Significant increases in extreme wind‐sea (Hmax) and swell SWH (HmaxSw) response to ENSO occur over the northeast North Pacific (NP) during December–February (DJF) and western NP during June–August (JJA). The PNA influence exhibits a similar pattern to ENSO during DJF yet is inactive in JJA. Hmax and HmaxSw responses to the IOD during September–November (SON) include significant increases over the western Pacific, southern Indian Ocean (IO), and southwest tropical IO, yet decreases over the central tropical IO. The HmaxSw response to the SAM is larger than that for Hmax over the southern IPO during DJF, which extends toward the eastern Pacific in March–May (MAM). Overall, extreme wind‐sea and swell parameter responses are found to be associated with sea level pressure (SLP) and SLP gradients.

The influence of modes of climate variability on the sub-seasonal temporal clustering of extreme precipitation

SummaryTemporally clustered precipitation extremes can have catastrophic impacts. Therefore, understanding their drivers is paramount for risk assessment in current and future climates. Here, we model for each season 3-week extreme precipitation event counts with Poisson Generalized Linear Models and nine major modes of climate variability as covariates. Model goodness-of-fit is highest in the tropics, particularly over the equatorial Pacific, the Maritime Continent, and East Africa, where ENSO, the Indian Ocean Dipole (IOD) and the MJO are the major drivers of sub-seasonal temporal clustering of extreme precipitation. The IOD and MJO also matter over Southwest Asia during boreal fall and winter. In the Northern Hemisphere, the North Atlantic Oscillation impacts clustering west of the Iberian Peninsula and over Scandinavia and Greenland, and the Pacific North American pattern matters over the central/northern Pacific Ocean. Finally, our models show very little skill in the Southern Hemisphere, where temporal clustering is also less frequent.

The Occurrence of Large Floods in the United States in the Modern Hydroclimate Regime: Seasonality, Trends, and Large‐Scale Climate Associations

AbstractMany studies investigate river floods by analyzing annual maximum series that record the largest flow of each year, including many within‐bank events inconsequential for human communities. Fewer focus on larger floods, especially at the continental scale. Using 473 streamgages across the conterminous United States with near‐natural flow from 1966 to 2015, we characterized the seasonality, occurrence, and climatic associations of the 10 largest and 2 largest floods at each site. These are often overbank events that have ecosystem functions and pose risks to humans. We grouped sites into 14 clusters corresponding to climatic and physiographic regions and characterized their flood seasonality at a monthly resolution using a probabilistic method. We then evaluated annual occurrence regionally and nationally, and seasonal occurrence regionally, by complementing a traditional approach to trend analyses with a novel method based on expected occurrence. Relationships between flood occurrence and climate indices were also investigated. Large floods have strong seasonality in some regions, but in areas with numerous flood‐generating mechanisms, seasonality is more complex. There is little evidence nationally that large riverine floods are more or less frequent than expected in recent years and only two regions show significant trends in annual counts; few show seasonal trends. We found some regional relationships between flood counts and climate indices, annually and seasonally; nationally the Pacific North American pattern is related to annual counts of the 2 largest floods. Large‐flood occurrence was generally stable across the United States in the last five decades; this may or may not continue with projected warming.

Characterization of global wildfire burned area spatiotemporal patterns and underlying climatic causes

Wildfires are widespread disasters and are concurrently influenced by global climatic drivers. Due to the widespread and far-reaching influence of climatic drivers, separate regional wildfires may have similar climatic cause mechanisms. Determining a suite of global climatic drivers that explain most of the variations in different homogeneous wildfire regions will be of great significance for wildfire management, wildfire prediction, and global wildfire climatology. Therefore, this study first identified spatiotemporally homogeneous regions of burned area worldwide during 2001–2019 using a distinct empirical orthogonal function. Eight patterns with different spatiotemporal characteristics were identified. Then, the relationships between major burned area patterns and sixteen global climatic drivers were quantified based on wavelet analysis. The most significant global climatic drivers that strongly impacted each of the eight major wildfire patterns were identified. The most significant combinations of hotspots and climatic drivers were Atlantic multidecadal Oscillation-East Pacific/North Pacific Oscillation (EP/NP)-Pacific North American Pattern (PNA) with the pattern around Ukraine and Kazakhstan, El Niño/Southern Oscillation-Arctic Oscillation (AO)-East Atlantic/Western Russia Pattern (EA/WR) with the pattern in Australia, and PNA-AO-Polar/Eurasia Pattern-EA/WR with the pattern in Brazil. Overall, these results provide a reference for predicting wildfire and understanding wildfire homogeneity.

АТМОСФЕРНІ РІКИ: ПОТЕНЦІЙНИЙ ВПЛИВ НА АТМОСФЕРНІ ПРОЦЕСИ ТА МЕТЕОРОЛОГІЧНІ ЯВИЩА НА ТЕРИТОРІЇ УКРАЇНИ

The article presents an overview of publications on the atmospheric rivers (AR) issue. AR events in recent years have got more attention from researchers due to the association with extreme precipitation. Several parameters are used to identify AR event, such as integrated water vapor (IWV) and integrated water transport (IVT). Several approaches to identifying and tracking AR have been developed and mentioned in this article. The foreign experience regarding AR connection with the general circulation of the atmosphere was analyzed. Most of the research on AR are devoted to the Eastern North Pacific and the North Atlantic regions. The key role in the variation of AR occurrence over different areas of those two regions is given to the interaction of the atmospheric circulation patterns, such as Arctic Oscillation and the Pacific North American Oscillation in the North Pacific region, and in the North Atlantic by the Arctic and North Atlantic Oscillation. There also stable connections of the AR phenomenon with the El Niño –Southern Oscillation and the Madden — Julian Oscillation are noted. A significant part of the works is devoted to studying AR influence on pre- cipitation extremes. AR is typically associated with a low-level jet stream and is formed in the warm conveyor belt zone of a cyclone, mainly in extratropical latitudes. An overview of modern research on atmospheric moisture and the heavy precipitation formation conditions over the territory of Ukraine shows that the vast majority of scientific works are based on the series of individual cases, and therefore cannot give an answer to the question of the potential role of AR or moisture advection in the processes of precipitation formation on a regional scale in the climatic aspect. The relevance of studying the AR issue consists of three aspects: the connection of AR events with large-scale circulation; the role of AR in the transport of moisture at the macro- and mesoscale levels; the connection of the AR with extreme and dangerous meteorological phenomena, in particular, heavy precipitation.

Influence of environmental and population factors on Prince William Sound herring spawning phenology

Shifts in spawning phenology may impact the early life stages of small pelagic fishes, affecting their first-year survival and recruitment. In Prince William Sound, Pacific herring is a key forage species that once supported commercial and subsistence fisheries for many decades, but collapsed in 1993 and has yet to recover. Starting in 1980, spawn timing shifted earlier by approximately 2-4 wk over a 27 yr period, then abruptly shifted later by approximately 3 wk over the next 7 yr. We quantified the influence of 15 environmental and population-level covariates on these spawn timing shifts using generalized linear models. Earlier spawn timing was associated with higher biomass in the eastern sound and older mean age in the western sound. Across the entire sound, earlier spawning was associated with weaker downwelling, weaker meridional winds, and the positive phase of the Pacific-North American teleconnection pattern, which is characterized by warmer North Pacific waters. These results are a critical first step towards assessing how changes in spawning phenology impact first-year survival of herring offspring and potentially contribute to persistent poor recruitment that has inhibited the recovery of the Prince William Sound population.

Dynamical mechanisms for the recent ozone depletion in the Arctic stratosphere linked to North Pacific sea surface temperatures

The stratospheric ozone layer, which prevents solar ultraviolet radiation from reaching the surface and thereby protects life on earth, is expected to recover from past depletion during this century due to the impact of the Montreal Protocol. However, how the ozone column over the Arctic will evolve over the next few decades is still under debate. In this study, we found that the ozone level in the Arctic stratosphere at 100–150 hPa during 1998–2018 exhibits a decreasing trend of − 0.12 ± 0.07 ppmv decade–1 from MERRA2, suggesting a continued depletion during this century. About 30% of this ozone depletion is contributed by the second leading mode of sea surface temperature anomalies (SSTAs) over the North Pacific with one month leading and therefore is dynamical in origin. The North Pacific SSTAs associated with this mode tend to result in a weakened Aleutian low, a strengthened Western Pacific pattern and a weakened Pacific–North American pattern, which impede the upward propagation of wavenumber-1 waves into the lower stratosphere. The changes in the stratospheric wave activity may result in decreased ozone in the Arctic lower stratosphere through weakening the Brewer-Dobson circulation. Our findings uniquely linked the recent ozone depletion in the Arctic stratosphere to the North Pacific SSTs and might provide new understanding of how dynamical processes control Arctic stratospheric ozone.

The causal role of South China Sea on the Pacific–North American teleconnection pattern

The causal role of South China Sea on the Pacific–North American teleconnection pattern

Asymmetric impacts of El Niño and La Niña on the Pacific–North American teleconnection pattern: the role of subtropical jet stream

The asymmetric impacts of El Niño and La Niña on the Pacific–North American teleconnection pattern in boreal winter have important implications for the surface air temperature and precipitation anomalies in North America. Previous studies have shown that the varying tropical convective heating contributes to the zonal shift of the teleconnection pattern during different El Niño/Southern Oscillation phases. In this study, using reanalysis, atmospheric general circulation model (AGCM) simulations, and a linear baroclinic model, we further present that the discrepancy of the subtropical jet stream (STJ) during El Niño and La Niña also contributes to the asymmetry. The atmospheric anomalies readily extract kinetic energy and effectively develop at the exit of the STJ. During El Niño (La Niña) years, as the central-eastern tropical Pacific warms up (cools down), the meridional temperature gradient in central subtropical Pacific increases (decreases), leading to the eastward (westward) shift of the STJ. The movement of the STJ leads to the shift of the location where disturbance develops most efficiently, ultimately contributing to the asymmetry of the teleconnection pattern.

Arctic autumn warming since 2002 dominated by changes in moisture modulated by multiple large-scale atmospheric circulations

The clear-sky downward longwave radiation related to water vapor (LW-WV) has been confirmed to be the dominant factor promoting the autumn warming in the Arctic in the boreal autumn. In this study, we reveal the spatiotemporal characteristics of the 2 m air temperature (t2m), clear-sky downward longwave radiation (CDLW), and water vapor in the Arctic and the mechanism affecting the change in the autumn LW-WV. Since 2002, not only have the temporal variations in regional t2m, CDLW, and water vapor been synchronous at the interdecadal scale, but their largest spatial variations have also mainly been located in the Barents-Kara Sea and the Chukchi Sea. A diagnosis of the atmospheric moisture budget shows that the change in the water vapor divergence is dominated by the change in the mean circulation dynamics in the Barents-Kara Sea, while the change in the thermodynamics caused by the changes in the local specific humidity and the mean circulation dynamics are both important to the change in the water vapor divergence in the Chukchi Sea. The positive phase in the Arctic Oscillation (AO) and the Pacific/North American Pattern (PNA) have contributed to the increase in the water vapor in the Barents-Kara Sea, while the positive phase in the North Atlantic Oscillation (NAO) and the PNA are responsible for the increase in the water vapor in the Chukchi Sea. Therefore, synergetic effects of the multiple large-scale circulations play an important role in the recent warming in the Arctic in autumn.

Differential Credibility of Climate Modes in CMIP6

AbstractThis work quantitatively evaluates the fidelity with which the northern annular mode (NAM), southern annular mode (SAM), Pacific–North American pattern (PNA), El Niño–Southern Oscillation (ENSO), Pacific decadal oscillation (PDO), Atlantic multidecadal oscillation (AMO), and the first-order mode interactions are represented in Earth system model (ESM) output from the CMIP6 archive. Several skill metrics are used as part of a differential credibility assessment (DCA) of both spatial and temporal characteristics of the modes across ESMs, ESM families, and specific ESM realizations relative to ERA5. The spatial patterns and probability distributions are generally well represented but skill scores that measure the degree to which the frequencies of maximum variance are captured are consistently lower for most ESMs and climate modes. Substantial variability in skill scores manifests across realizations from individual ESMs for the PNA and oceanic modes. Further, the ESMs consistently overestimate the strength of the NAM–PNA first-order interaction and underestimate the NAM–AMO connection. These results suggest that the choice of ESM and ESM realizations will continue to play a critical role in determining climate projections at the global and regional scale at least in the near term.

Climate response to drastically modified PDO, PNA and NAM in the superinterglacial MIS 31

The Pacific Decadal Oscillation (PDO), the Pacific‐North American Pattern (PNA) and the Northern Annular Mode (NAM) influence the Northern Hemisphere climate over all sorts of time scales, from days to decades. This study evaluates these climate modes under drastically modified conditions. It is found that in Marine Isotope Stage 31 (MIS 31), an interglacial with enhanced seasonal amplitude, the PDO, PNA and NAM are completely different in their temporal and spatial patterns with respect to current conditions. Moreover, the MIS 31 boundary conditions induce an amplification of the interannual variability, but a suppression of the decadal peak. It is found that changes in the air–sea interaction in the NH, in particular due to a weaker Aleutian low, are responsible for the absence of the decadal periodicity. However, no large changes are verified in terms of explained variance of those modes with respect to CTR. However, the amplitude of response related to the PDO, NAM and PNA is weaker in the MIS 31 experiment, very likely due to a reduced meridional thermal gradient. The results presented here are useful for palaeoreconstruction interpretation because proxies may reproduce dominant characteristics of temperature and precipitation related to the persistence of those modes of variability. Thus, their ability to reproduce long‐term environmental conditions in some situations can be related to a preferential phase of the PDO, PNA and NAM.

Remote Sensing of Local Warming Trend in Alberta, Canada during 2001–2020, and Its Relationship with Large-Scale Atmospheric Circulations

Here, the objective was to study the local warming trend and its driving factors in the natural subregions of Alberta using a remote-sensing approach. We applied the Mann–Kendall test and Sen’s slope estimator on the day and nighttime MODIS LST time-series images to map and quantify the extent and magnitude of monthly and annual warming trends in the 21 natural subregions of Alberta. We also performed a correlation analysis of LST anomalies (both day and nighttime) of the subregions with the anomalies of the teleconnection patterns, i.e., Pacific North American (PNA), Pacific decadal oscillation (PDO), Arctic oscillation (AO), and sea surface temperature (SST, Niño 3.4 region) indices, to identify the relationship. May was the month that showed the most significant warming trends for both day and night during 2001–2020 in most of the subregions in the Rocky Mountains and Boreal Forest. Subregions of Grassland and Parkland in southern and southeastern parts of Alberta showed trends of cooling during daytime in July and August and a small magnitude of warming in June and August at night. We also found a significant cooling trend in November for both day and night. We identified from the correlation analysis that the PNA pattern had the most influence in the subregions during February to April and October to December for 2001–2020; however, none of the atmospheric oscillations showed any significant relationship with the significant warming/cooling months.

Influence of Low‐Frequency PNA Variability on MJO Teleconnections to North American Atmospheric River Activity

AbstractThe Madden‐Julian oscillation (MJO) modulates atmospheric river (AR) activity along the West Coast of North America. The forecast skill of ARs based solely on the MJO phase is limited, motivating the identification of other predictive factors. Here we show that low‐frequency variability of the Pacific/North American (PNA) pattern is mostly independent of the MJO, and the monthly PNA pattern influences the MJO‐AR relationship much more robustly than other large‐scale patterns such as the El Niño Southern Oscillation and the quasi‐biennial oscillation. During PNA+ conditions, when the Pacific jet exit region is located further east than average, baroclinic wave packets extend downstream toward North America. The clear contrast in wave packets' zonal scale results in partitioning active and inactive AR phases. The PNA influence is most significant over the Pacific Northwest region of North America, where AR landfall is most frequent, with implied forecasting skill beyond 20 days.

Impacts of large-scale teleconnection indices on chill accumulation for specialty crops in California

Although the impacts of teleconnection indices on climate metrics such as precipitation and temperature in California have been widely studied, less attention has been given to the impact on integrated climate indices such as chill accumulation. This study investigates the linkages between large-scale teleconnections and winter chill accumulation for specialty crops in California, which may enable more effective and dynamic adaptation to in-season climate variability. Three large-scale teleconnection indices were selected: Oceanic Nino Index (ONI), Pacific-North American teleconnection pattern (PNA), and Pacific Decadal Oscillation (PDO) index to assess their effects on chill accumulation. The Chill Hours Model and Dynamic Model are adopted to calculate chill accumulation in Chill Hours (CH) and Chill Portions (CP) from November to January. Three major crop-producing regions, including the Central Coast, Sacramento Valley, and San Joaquin Valley, are used as the focused regions. Our results suggest CP generally has a stronger response to teleconnection patterns than CH in California. The correlations between chill accumulation and teleconnections are generally weaker during the summer than other seasons, and significant correlation can be observed 2–10 months before the start of the chill accumulation period. Among the three teleconnection indices, ONI is most weakly correlated to chill accumulation in focused regions, while PDO shows the strongest positive correlation and explains up to 39% variability of CP. PNA presents the most widespread negative correlation with chill accumulation. When aggregated to different teleconnection modes, +3.6 above-average CP is expected during ONI positive mode; +2.3 above-average CP is expected during PDO positive mode, while +2.1 above-average CP is expected during PNA negative mode. This study provides insights on early-season chill prediction and feasible management and adaptation strategies, and the methodology presented here can be used to develop decision support tools of risk control for agricultural producers and policymakers.

Associations of atmospheric teleconnections with wintertime extratropical cyclones over East Asia and Northwest Pacific

Extratropical cyclones (ETCs) over East Asia and Northwest Pacific are identified and tracked by applying an objective algorithm to the 850-hPa relative vorticity fields from the ERA-Interim reanalysis. A total of 2866 ETCs originating at the western side of the date line have been identified in the extended November–March winters from 1979 to 2018. The ETC tracks are counted and visualized using a hexagonal tessellation rather than the regular longitude–latitude grids. Two generalized linear models (GLMs), Poisson regression model and Gamma regression model, are firstly applied to investigate the associations of wintertime ETCs with three atmospheric teleconnection patterns. The West Pacific (WP) pattern and the Pacific/North American (PNA) pattern are more responsible for the meridional variability of ETC activities in the North Pacific, while the influence of the Polar/Eurasia pattern on ETC activities is negligible. Results of composite analysis are qualitatively consistent with that of regression analysis. Composite maps of differences of jet stream, thermal gradient and mid-tropospheric baroclinicity in the positive and negative phases of teleconnection patterns also support the close associations of ETC activities with WP and PNA teleconnection patterns.

Recent changing characteristics of dry and wet spells in Canada

Under the possible impact of climate warming, recent changes in dry and wet spells have contributed significantly to climate-related hazards around the world. In this work, spatial and temporal variations in dry and wet spells over Canada are investigated using daily precipitation data from 1979 to 2018. The time-varying relationships between precipitation spells and large-scale climate anomalies are modeled using a nonstationary generalized extreme value (GEV) distribution and Bayesian quantile regression. Over the period 1979–2018, significant changes in dry and wet spells have been observed across Canada, particularly in the southern Canadian Prairies (CP), where both the number and duration of dry spells show positive trends. Dry and wet spells over many parts of Canada are nonstationary under the effects of the El Nino–Southern Oscillation (ENSO) and the Pacific–North American pattern (PNA), with PNA having stronger effects on annual maximum dry spells than ENSO, especially in the central CP and eastern Ontario. For western Canada, the influence of ENSO on dry spells tends to be relatively strong, especially for dry spells of high quantiles, as El Nino generally induces atmospheric moisture deficit. For central Canada, ENSO and PNA have a negative (positive) impact on the wet spell duration of low (high) quantiles. For eastern Canada, PNA is negatively correlated with the duration of wet spells, especially for wet spells of high quantiles. Therefore, a better understanding of the spatial and temporal variability in dry and wet spell return periods will be useful for the effective management of water resources, and for developing effective disaster mitigation measures against the possible social and economic impacts of climate-related hazards.

Using avalanche problems to examine the effect of large-scale atmosphere–ocean oscillations on avalanche hazard in western Canada

Abstract. Numerous large-scale atmosphere–ocean oscillations including the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the Pacific North American Teleconnection Pattern (PNA), and the Arctic Oscillation (AO) are known to substantially affect winter weather patterns in western Canada. Several studies have examined the effect of these oscillations on avalanche hazard using long-term avalanche activity records from highway avalanche safety programmes. We present a new approach for gaining additional insight into these relationships that uses avalanche problem information published in public avalanche bulletins during the winters of 2010 to 2019. For each avalanche problem type, we calculate seasonal prevalence values for each forecast area, elevation band, and season, which are then included in a series of beta mixed-effects regression models to explore both the overall and regional effects of the Pacific-centered oscillations (POs; including ENSO, PDO, and PNA) and AO on the nature of avalanche hazard in the study area. We find significant negative effects of PO on the prevalence of storm slab avalanche problems, wind slab avalanche problems, and dry loose avalanche problems, which agree reasonably well with the known impacts of PO on winter weather in western Canada. The analysis also reveals a positive relationship between AO and the prevalence of deep persistent slab avalanche problems, particularly in the Rocky Mountains. In addition, we find several smaller-scale patterns that highlight that the avalanche hazard response to these oscillations varies regionally. Even though our study period is short, our study shows that the forecaster judgement included in avalanche problem assessments can add considerable value for these types of analyses. Since the predictability of the most important atmosphere–ocean oscillations is continuously improving, a better understanding of their effect on avalanche hazard can contribute to the development of informative seasonal avalanche forecasts in a relatively simple way.

Acceleration of western Arctic sea ice loss linked to the Pacific North American pattern

Recent rapid Arctic sea-ice reduction has been well documented in observations, reconstructions and model simulations. However, the rate of sea ice loss is highly variable in both time and space. The western Arctic has seen the fastest sea-ice decline, with substantial interannual and decadal variability, but the underlying mechanism remains unclear. Here we demonstrate, through both observations and model simulations, that the Pacific North American (PNA) pattern is an important driver of western Arctic sea-ice variability, accounting for more than 25% of the interannual variance. Our results suggest that the recent persistent positive PNA pattern has led to increased heat and moisture fluxes from local processes and from advection of North Pacific airmasses into the western Arctic. These changes have increased lower-tropospheric temperature, humidity and downwelling longwave radiation in the western Arctic, accelerating sea-ice decline. Our results indicate that the PNA pattern is important for projections of Arctic climate changes, and that greenhouse warming and the resultant persistent positive PNA trend is likely to increase Arctic sea-ice loss.