Arctic Oscillation Pattern Research Articles

Structural fluctuations of the Arctic Oscillation tied to the Atlantic Multidecadal Oscillation

The Arctic Oscillation (AO) has been observed to undergo distinct decadal structural fluctuations that significantly influence regional weather and climate. Understanding the drivers and mechanisms behind the AO’s spatial nonstationarity is critical for improving climate predictions related to the AO. We present evidence that the Atlantic Multidecadal Oscillation (AMO) plays a pivotal role in modulating AO’s Pacific center in recent decades. The poleward amplified cooling associated with negative AMO enhances the north-south temperature gradient and results the strengthened westerly winds and stratospheric polar vortex (SPV) responses, which reflects more planetary waves from the North Pacific to the North Atlantic. This enhances the atmospheric coupling between these regions and leads to a more pronounced Pacific center within the AO pattern. Numerical simulations from ECHAM5 and 35 CMIP6 models further corroborate the essential role of the AMO. These findings advance our understanding of the mechanisms driving the variability of the AO pattern.

Impact of Polar Vortex Modes on Winter Weather Patterns in the Northern Hemisphere

This study is an additional investigation of stratosphere–troposphere coupling based on the recent stratospheric winter descriptions in five distinct modes: January, February, Double, Dynamical, and Dadiative. These modes, established in a previous study, categorize the main stratospheric winter typologies modulated by the timing of important sudden stratospheric warmings (SSWs) and final stratospheric warmings (FSWs). The novelty of this research is to investigate the Northern Annular Mode, mean sea level pressure (MSLP) anomalies in the Ural and Aleutian regions, and the decomposition of Eliassen–Palm flux into wavenumbers 1 and 2 within each mode. The results show that the January and Double modes exhibit similar pre-warming surface signals, characterized by Ural blocking and Aleutian trough events preceding weak polar vortex events. The January mode displays a positive MSLP anomaly of +395 hPa (−191 hPa) in the Ural (Aleutian) region in December, while the Double mode shows +311 hPa (−89 hPa) in November. These modes are primarily wave-1 driven, generating tropospheric responses via negative Arctic Oscillation patterns. Conversely, the February and Dynamical modes show opposite signals, with Aleutian blocking and Ural trough events preceding strong polar vortex events. In December, the February mode exhibits MSLP anomalies of +119 hPa (Aleutian) and −180 hPa (Ural), while the Dynamical mode shows +77 hPa and −184 hPa, respectively. These modes, along with important SSWs in February and dynamical FSWs, are driven by both wave-1 and wave-2 and do not significantly impact the troposphere. The Radiative mode’s occurrence is strongly related to the Aleutian blocking presence. These findings confirm that SSW timing is influenced by specific dynamical forcing related to surface precursors and underscore its importance in subsequent tropospheric responses. This study establishes a connection between early winter tropospheric conditions and upcoming stratospheric states, potentially improving seasonal forecasts in the northern hemisphere.

Variability Assessment of Global Extreme Coastal Sea Levels Using Altimetry Data

This study assesses the variability of coastal extreme sea levels globally by utilizing nearly three decades of along-track, multi-mission satellite altimetry data. An altimetry-based global coastal database of the non-tidal residual sea level component has been produced. The climate variability of extremes is modeled through a parametric, non-stationary statistical model. This model captures intra-annual, inter-annual and long-term variations in non-tidal residual return levels. Comparisons with tide gauge data demonstrate the ability of altimetry data to capture the variability of coastal extreme sea levels. Our findings reveal a greater complexity in the monthly variability patterns of non-tidal residual extremes in tropical latitudes, often exhibiting multiple storm periods, contrasting with coasts in extratropical latitudes, which are mostly controlled by a winter–summer pattern. This study also highlights the significant influence of established climate circulation patterns on sea level extremes. The positive phase of the Arctic Oscillation pattern leads to increases of over 25% in non-tidal residual return levels in Northwestern Europe with respect to a neutral phase. Furthermore, return levels in the western coast of Central America could be 50% higher during El Niño compared to La Niña. Our results show a robust increasing trend in non-tidal residual return levels along most global coastlines. A comparative analysis shows that variations during the 1995–2020 period were primarily driven by intra-annual variations.

Changes of Water Vapor Budget over East Asia in Response to 4xCO2 Concentration Forcing

Water resources are essential for the economic development and social security in East Asia, especially under global warming. Based on newly released CMIP6 149-year simulation data from a pre-industrial control experiment (piControl) and a forced experiment on the abrupt quadrupling of CO2 concentration (abrupt-4xCO2), changes of water vapor budget over East Asia due to 4xCO2 concentration forcing and their possible mechanisms are investigated. Change of precipitation (P) demonstrates a spatial pattern of “Southern Flood and Northern Drought” (SFND) in eastern China, which can also be seen in the change of evaporation (E), though at a much smaller amplitude. The change of water vapor budget represented by E–P is dominated by P, which is primarily induced by changes of water vapor divergence associated with both moisture-related thermodynamic contribution and atmospheric circulation-related dynamic contribution. Specifically, under global warming, tropical El Nino-like SST warming causes weakened Walker circulation through decreased zonal temperature gradient, while amplified Arctic warming induces a negative Arctic Oscillation pattern via reduced meridional temperature gradient. The combined signals from tropical and mid-high latitudes result in significant long-term changes of water vapor convergence as well as much more precipitation in the Yangtze River Valley, forming the SFND. Furthermore, the intensity of the SFND change pattern could also have notable interdecadal variation, which is mainly attributed to the modulation of interdecadal signals of the Indian Ocean basin mode (IOBM) and Pacific Decadal Oscillation (PDO). Results of this study could provide an important scientific basis for the future planning and management of water resources over East Asia, specifically in eastern China.

Relations of Enhanced High‐Latitude Concurrent Blockings With Recent Warm Arctic‐Cold Continent Patterns

AbstractA warm Arctic‐cold continent (WACC) pattern has been one of the main characteristics of the Northern Hemisphere (NH) climate variability in winter during the last two decades. However, the factors contributing this pattern remain unclear. We compared the two leading modes of surface temperature in the NH winter on an interdecadal timescale and explored the role of high‐latitude concurrent blockings (HCBs) and the Arctic oscillation (AO) in these two modes. Results show that the first mode resembling the WACC pattern is more related to enhanced HCBs over the Ural Mountains and the North Pacific than the AO. The HCBs induce a warmer Arctic by simultaneously transporting large amounts of moisture and heat. The HCBs excite a tropospheric anticyclonic anomaly near the Arctic Circle, which, in turn, triggers anomalies in the propagation of planetary waves and the meridional circulation, with a subsequent redistribution of momentum and heat. This is accompanied by a weak polar night jet and a poleward shift in the subtropical westerly jet, resulting in mild cold mid‐latitude continents. The second mode represents the AO pattern, which has a different mechanism from WACC. The high‐pressure anomaly over the North Pacific has played an increasingly important part in the WACC mode in recent decades and the WACC pattern with HCBs is more easily reproduced in simulations with human activity. HCBs therefore require further consideration under the current and future conditions of global warming.

Pacific Decadal Oscillation modulates the Arctic sea-ice loss influence on the midlatitude atmospheric circulation in winter

Abstract. The modulation of the winter impacts of Arctic sea-ice loss by the Pacific Decadal Oscillation (PDO) is investigated in the IPSL-CM6A-LR ocean–atmosphere general circulation model. Ensembles of simulations are performed with constrained sea-ice concentration following the Polar Amplification Model Intercomparison Project (PAMIP) and initial conditions sampling warm and cold phases of the PDO. Using a general linear model, we estimate the simulated winter impact of sea-ice loss, PDO and their combined effects. On the one hand, a negative North Atlantic Oscillation (NAO)-like pattern appears in response to sea-ice loss together with a modest deepening of the Aleutian Low. On the other hand, a warm PDO phase induces a large positive Pacific–North America pattern, as well as a small negative Arctic Oscillation pattern. Both sea-ice loss and warm PDO responses are associated with a weakening of the poleward flank of the eddy-driven jet, an intensification of the subtropical jet and a weakening of the stratospheric polar vortex. These effects are partly additive; the warm PDO phase therefore enhances the response to sea-ice loss, while the cold PDO phase reduces it. However, the effects of PDO and sea-ice loss are also partly non-additive, with the interaction between both signals being slightly destructive. This results in small damping of the PDO teleconnections under sea-ice loss conditions, especially in the stratosphere. The sea-ice loss responses are compared to those obtained with the same model in atmosphere-only simulations, where sea-ice loss does not significantly alter the stratospheric polar vortex.

Combined impact of the cold vortex and atmospheric blocking on cold outbreaks over East Asia and the potential for short-range prediction of such occurrences

This study explores the consequences of independent and combined effects of blockings on the northeast Asian cold vortex (NACV), and corresponding cold outbreaks over East Asia (EA) during boreal winters of 1979–2019. The results show that the development of NACV is closely associated with blocking over the Ural Mountains (UB) upstream and eastern Siberia to mid-North Pacific (SPB) downstream. Here we focus on the initial periods before the peak day of NACV events. It is found that the strong NACV events are usually induced by the initial-UB situation, leading to the greatest temperature drop in EA. While the weak NACV events may be associated with the initial-SPB condition, which can bring less dramatic outbreaks but longer duration, owing to the Ural ridge that formed by the westward shift of SPB. Furthermore, an SPB-UB relay effect is discovered against the background of a negative Arctic Oscillation pattern. In such cases, UB is formed by the westward shift of downstream SPB after the occurrence of NACV, forming a relay effect that motivates the second NACV process, hence prolonging the duration of cold anomalies in EA. These findings highlight the importance of the combined effect of blockings and NACV in the intraseasonal time scale. Compared to the ‘Initial-UB’ and ‘Initial-SPB’ situation, this ‘SPB-UB relay’ scenario can produce longer-lasting cold extreme in EA, which may be indicative of the short-term weather forecasting of such extreme cold weather.

Representation of the wintertime Arctic Oscillation in a multi‐model ensemble

AbstractThe Arctic Oscillation (AO) is a well‐known mode that affects climate variability in the Northern Hemisphere. The equal‐weighed multi‐model ensemble (MME) of six state‐of‐the‐art models from the Copernicus Climate Change Service (C3S) and Pusan National University (PNU) was analysed to understand the wintertime AO performance for the hindcast period December–February 1993/1994–2016/2017. The hindcasts were chosen with lead times of 1 and 4 months with respect to the initialization date (August and November, respectively). The spread of the AO prediction skills of the individual models was significant. In general, the MME demonstrates superior skill compared to the average of single‐model skills in representing the AO pattern at lead times of 1 and 4 months. The AO‐related vertical structure predicted by MME is similar to the observation, but the upper‐level structure is relatively poor compared to the structure of the hindcasted lower‐ or mid‐level atmosphere. Both observation and MME indicate that since the mid‐1990s, the relationship between the AO and East Asian winter monsoon (EAWM) has been weak compared to the connection between the AO and El Niño–Southern Oscillation (ENSO). Simultaneously, the North Pacific centre of the AO moved eastward during the observational period. The MME showed an AO pattern similar to that observed. The eastward shift of the North Pacific centre of the AO may contribute to deepening the Aleutian low and its effect on the tight AO–ENSO relation demonstrated in observations and MME. Strong AO–ENSO relations and weak AO–EAWM connections are found in both observations and MME model. The observation and MME represent the wave activity flux from 60°N to the equator in the troposphere; consequently, the wave activity flux may contribute to the AO and ENSO connection in both observation and MME.

Analysis of Recent Mean Temperature Trends and Relationships with Teleconnection Patterns in California (U.S.)

The global mean surface temperature has risen since the late 19th century. However, temperatures do not increase uniformly in space or time and few studies have focused on that peculiarity in the State of California. The aim of this research is to deepen our knowledge of the evolution of mean temperatures in the State of California on monthly, seasonal and annual time scales. The period under study comprises 40 years (from 1980 to 2019) and data from 170 meteorological stations were analysed. Statistical techniques, including Sen’s slope and Mann-Kendall, were applied to each of the stations to establish the sign and slopes of trends and their statistical significance. The spatial distribution of monthly, seasonal and annual trends was analysed using the Empirical Bayesian Kriging (EBK) geostatistical technique. The trend analysis was also carried out for the State as a whole. This research also studies the relationships between mean temperatures and nine teleconnection patterns with influence on the Californian climate. To find out these links, a correlation analysis was performed using the partial non-parametric Spearman Test at a 95% confidence level. The study reveals a positive trend of +0.01 °C year−1 for the whole state and that Southern California is getting warmer than Northern California for the study period. On a seasonal scale, the local temperature increased significantly both in autumn and summer (+0.06 °C and +0.035 °C year−1 respectively) from 1980 to 2019. On a monthly scale, the largest increases are found in November at +0.04 °C year−1. Temperatures in February, March, April and May are highly correlated with most of the teleconnection patterns studied in the State of California. West Pacific Oscillation (WPO) teleconnection pattern has shown the highest negative correlation. However, The Pacific Decadal Oscillation (PDO) has a positive correlation with mean temperatures in coastal areas such as Los Angeles, San Francisco and Monterey. Moreover, Antarctic Oscillation (AAO) and Arctic Oscillation patterns (AO) are unlikely to show great influence on average temperature trends in California.

Orbital and Millennial Variations in Sea Ice in the Southwestern Okhotsk Sea Since the Last Interglacial Period and Their Implications

Sea ice in the Okhotsk Sea plays a significant role in global climate change. However, the history and mechanism of changes in sea ice spanning the last glacial cycle remain controversial. In this study, an 8.8 m core (LV55-40-1) was recovered from the southwestern Okhotsk Sea that contains a continuous sea ice record over the past ∼110 kyr. The sand fraction and dropstones were used as ice-rafted debris proxies to reconstruct the history of sea ice variations over the last ∼110 kyr and to determine the underlying causes on orbital and millennial timescales. Sea ice expansions occurred during MIS 5b, MIS 4, mid-MIS 3, and early MIS 1, which were controlled mainly by decreased autumn insolation on an orbital timescale. Superimposed on the orbital-scale changes, millennial-scale variations in sea ice were also observed, with 19 expansion events that coincided with cold Dansgaard-Oeschger stadials. Millennial scale sea ice variations were most likely controlled by both the Arctic oscillation and the East Asian summer monsoon. During periods of negative Arctic oscillation patterns, decreased air temperatures over the Okhotsk Sea caused more active sea ice formation. Such conditions could have been reinforced, by a reduced influence of warm advection at the surface of the Okhotsk Sea caused by decreased discharge from the Amur River that resulted from a weakened East Asian summer monsoon during cold stadials.

How to Recognize a True Mode of Atmospheric Circulation Variability

AbstractIt has been demonstrated several times that when principal component analysis (PCA) is used for detection of modes of atmospheric circulation variability (teleconnections), principal components must be rotated. Despite it, unrotated PCA is still often used. Here we demonstrate on the examples of North Atlantic Oscillation (NAO), Arctic Oscillation (AO), Barents Oscillation (BO), and the summer East Atlantic (SEA) pattern that unrotated PCA results in patterns that are artifacts of the analysis method rather than true modes of variability. This claim is based on the comparison of the spatial patterns of the modes with spatial autocorrelations, on the sensitivity of the patterns to spatial and temporal subsampling, and, for the SEA pattern, on correlations with tropical sea surface temperature. Unlike NAO, which is defined by rotated PCA, the other modes, that is, AO, BO, and SEA pattern, defined by unrotated PCA, do not correspond well to underlying autocorrelation structures and are more sensitive to choices of spatial domain and time interval over which they are defined. We reiterate that a great care must be taken when interpreting outputs of PCA when applied to the detection of modes of circulation variability: a comparison with spatial autocorrelations and check for their spatial and temporal stability are necessary to distinguish true modes from statistical artifacts, which we call “ghost patterns.”

Decoupling of the Arctic Oscillation and North Atlantic Oscillation in a warmer climate

The North Atlantic Oscillation and the Arctic Oscillation are modes of climate variability affecting temperature and precipitation in the mid-latitudes. Here we use reanalysis data and climate model simulations of historical and warm climates to show that the relationship between the two oscillations changes with climate warming. The two modes are currently highly correlated, as both are strongly influenced by the downward propagation of stratospheric polar vortex anomalies into the troposphere. When considering a very warm climate scenario, the hemispherically defined Arctic Oscillation pattern shifts to reflect variability of the North Pacific storm track, while the regionally defined North Atlantic Oscillation pattern remains stable. The stratosphere remains an important precursor for North Atlantic Oscillation, and surface Eurasian and Aleutian pressure anomalies precede stratospheric anomalies. Idealized general circulation model simulations suggest that these modifications are linked to the stronger warming of the Pacific compared with the slower warming of the Atlantic Ocean. The Arctic Oscillation and North Atlantic Oscillation are modes of Northern Hemisphere climate variability with high temporal and spatial correlation. With strong warming, climate models suggest their link breaks down due to a divergent response to the Pacific and Atlantic oceans and stratosphere.

An Extraordinary Winter in the Polar North

An exceptionally strong stratospheric polar vortex coincided with a record-breaking Arctic Oscillation pattern and ozone destruction during the 2019–2020 winter season.

Enhanced Northern Hemisphere Correlation Skill of Subseasonal Predictions in the Strong Negative Phase of the Arctic Oscillation

AbstractThe Arctic Oscillation (AO) is the most dominant atmospheric variability in the Northern Hemisphere in boreal winter. Its negative phases sometimes bring extreme cold conditions over Eurasia and North America in boreal winter, impinging on various socioeconomic sectors. Thus, accurate prediction of the AO‐related conditions with a long lead time is greatly anticipated. This study investigates conditional prediction skill in the northern extratropics relative to AO phases using retrospective forecast data of multiple models provided by the Subseasonal to Seasonal Prediction Project, which is jointly conducted by the World Weather Research Programme and the World Climate Research Programme. We found that predictions starting from the strong negative AO phase tend to have enhanced prediction skill in terms of the anomaly correlation coefficient of 500‐hPa geopotential height, which measures the similarity of spatial patterns. The skill enhancement is not apparent in terms of the root mean square error score. We also discuss dynamical mechanisms behind the enhanced prediction skill. The Eliassen‐Palm flux diagnosis indicated that the strong negative AO phase induces the stronger eddy‐zonal flow feedback to sustain the anomalous zonal flow condition than the strong positive AO phase. Moreover, the anomalous zonal flow is associated with an anomalous zonally asymmetric pattern. As a result, the anomalous AO pattern is better predicted in the strong negative AO phase, contributing to the enhancement of the Northern Hemisphere correlation skill. Results highlight that dynamics inherent in the extratropical atmosphere can provide the subseasonal predictability in a certain atmospheric condition.

The Relationship between the Wintertime Cold Extremes over East Asia with Large-Scale Atmospheric and Oceanic Teleconnections

The influence of large-scale teleconnection patterns, Western Pacific (WP), Arctic Oscillation (AO) and El Niño-Southern Oscillation (ENSO), on the minimum surface air temperature (Tmin) anomalies and extremes over East Asia during the boreal winter from 1979 to 2017 were investigated by the composite analysis in terms of atmospheric and oceanic processes. The relationship between the Tmin and the geopotential height at 500 hPa (Z500) as well as sea surface temperature (SST) were first examined. Then we explored and estimated the contribution of the teleconnection patterns to the occurrence of extremely cold days and months quantitatively, and discussed other key factors in relation to the cold extremes. The WP and AO patterns play an important part in the prevalence of significant Tmin variability, whereas the effect of ENSO is relatively weak. Most of the cold extremes tend to appear in the negative phase of teleconnections, while there some extremes that occur in the opposite phase. In addition, the extreme months are more related to the preferred phase of the dominant pattern when compared to days. We conclude that the daily extremes are primarily triggered by the local-synoptic atmospheric circulations embedded in the large-scale teleconnection patterns, while the monthly extremes have a closer relationship with these low-frequency patterns.

Potential contribution of winter dominant atmospheric mode over the mid‐latitude Eurasia to the prediction of subsequent spring Arctic Oscillation

AbstractSpring Arctic Oscillation (AO) has been revealed to exert a great impact on the Northern Hemispheric climate; however, its seasonal prediction skill has been limited so far. In this paper, we find that the dominant mode of the winter 500‐hPa geopotential height anomalies over the mid‐latitude Eurasia has a close connection with the succeeding spring AO. The analysis on the physical processes indicates that the troposphere–stratosphere interaction plays an important role in the connection between the two. This predominant atmospheric mode in the mid‐latitude Eurasia during winter can change the vertical wave activity and lead to an anomalous stratospheric polar vortex. Thereafter, the signal of the anomalous stratospheric polar vortex propagates downward to troposphere in the subsequent spring, consequently inducing the anomalous AO pattern in the troposphere. The results in this study imply that this previous winter leading atmospheric mode over the mid‐latitude Eurasia may supply a potential source for a spring AO's prediction.

Associations of environmental variables with different types of stroke

Abstract Background Air temperature (T) and atmospheric pressure (AP) are among the most closely studied weather variables; increases, decreases, and fluctuations in both have been significantly linked to numerous stroke subtypes. We decided to detect the association between daily numbers of ischaemic stroke (IS) and haemorrhagic stroke (HS) and daily North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) indices and monthly indices of Quasi-Biennal Oscillation (QBO). Methods The study was conducted in Kaunas city from 2000 to 2010. Kaunas stroke register presented daily numbers of IS, subarachnoid haemorrhages (SAH), and intracerebral haemorrhages (ICH). We evaluated the association between these types of stroke and NAO, AO, and QBO indices (NAOI, AOI, and QBOI) by applying Poisson regression, adjusting for month and other weather variables. Results During the study period, we analysed 3,992 cases (2,205 men and 1,787 women) with stroke. IS composed 3,199 (80.1%), ICH 533 (13.4%), and SAH - 260 (6.5%). A change in mean daily atmospheric pressure (AP) of > 3.9 hPa and QBOI <-27 were associated with the risk of SAH (RRs with 95% CI were, respectively, 1.54 (1.18-2.03), and 1.68 (1.06-2.66)). The risk of HS was associated with daily increases in AP and QBOI <8.37 (p < 0.05). The risk of IS was negatively associated with AOI (RR = 0.97 (0.94-0.99). During November-March, NAOI >0 was associated with HS (RR = 1.29 (1.03-1.62)), and a negative association between NAOI and IS (RR = 0.91 (0.84-0.98)) was found. Conclusions The results of our study provided new evidence that the NAO, AO, and QBO pattern may affect the risk of stroke. The impact of these teleconnection indices is not identical for different types of stroke. Key messages This abstract is part of publication, which will be published later on. In some cases, environmental impact is not essential, but quite often determines the course of various diseases, especially of the circulatory system.

Projections of climate changes over mid-high latitudes of Eurasia during boreal spring: uncertainty due to internal variability

This study examines uncertainties of projected spring surface air temperature (SAT) and precipitation trends during 2006–2060 at regional scales over the mid-high latitudes of Eurasia due to internal variability based on 40 ensemble members projections of CCSM3. The 40 ensemble members are initiated at a slightly different atmospheric conditions but with the same external forcing. Thus, the differences of the projected spring SAT and precipitation trends among the 40 ensemble members are attributed to the internal variability. Results suggest that superposition of internal variability and external forcing leads to a large spread of projected spring SAT and precipitation trends over Eurasia. In comparison, the projected spring precipitation trend has a larger uncertainty than the spring SAT trend. In particular, the signal-to-noise ratios of spring SAT (precipitation) trends are larger than two (one) over most regions of the mid-high latitudes of Eurasia. The internal atmospheric circulation variability is an important source of the uncertainties of the projected spring SAT and precipitation trends. The first mode of the internal atmospheric circulation variability resembles the Arctic Oscillation pattern. The second mode displays feature similar to the North Atlantic Oscillation and anomalous Siberian High patterns. A dynamical adjustment technique is employed to reduce internal atmospheric circulation generated variability in spring SAT and precipitation trends. Result indicates that projected trends of the dynamically adjusted spring SAT and precipitation over the next 55 years over Eurasia are similar across the 40 ensemble members both in the spatial structure and amplitude.

Numerical simulation of climate response to ultraviolet irradiation forcing

Using the Whole Atmosphere Community Climate Model, sensitivity experiments are performed to identify the climate response to ultraviolet (UV) irradiation forcing. In the experiment, total solar irradiance (TSI) and solar spectral irradiation (SSI) are adopted as solar input from the dataset (1610–2009) reconstructed by Lean. Results show that UV variability has a strong impact on the middle atmosphere. Ozone (O3) distribution in the atmosphere is sensitive to UV irradiation variability. When UV irradiation is enhanced, the O3 content increases at altitudes from 30 km to 60 km and decreases at altitudes from 15 km to 30 km. The atmospheric temperature appears positive anomaly below the mesosphere. Anomalous warmth develops above 35 km with maximum warming anomaly apparent at the stratopause, consistent with the atmospheric distribution of O3, which indicates that the ozone heating mechanism plays an important role in warming. The O3 and temperature respond in an opposite manner when the UV irradiation is reduced. Furthermore, the stronger UV forcing leads to a change in the temperature distribution because of the O3 heating mechanism. This condition then modulates the zonal wind in the stratosphere and upper troposphere. Although the tropospheric response is weak on a global scale, a notable local response occurs over mid–high latitudes in the Northern Hemisphere (NH) during winter. Along with the enhanced zonal wind in the lower stratosphere near the North Pole, the positive phase of the Arctic Oscillation pattern appears over the NH, which causes a temperature increase over mid-latitude Asia. Finally, the behavior evident in the reanalysis data is compared with the results of the numerical experiments. The similar structure of the response fields means the results of simulation experiment is credible to some extent.

Diversity of the Wintertime Arctic Oscillation Pattern among CMIP5 Models: Role of the Stratospheric Polar Vortex

AbstractThe wintertime Arctic Oscillation (AO) pattern in phase 5 of the Coupled Model Intercomparison Project (CMIP5) climate models displays notable differences from the reanalysis. The North Pacific center of the AO pattern is larger in the ensemble mean of 27 models than in the reanalysis, and the magnitude of the North Pacific center of the AO pattern varies largely among the models. This study investigates the plausible sources of the diversity of the AO pattern in the models. Analysis indicates that the amplitude of the North Pacific center is associated with the coupling between the North Pacific and North Atlantic, which in turn is primarily modulated by the strength of the stratospheric polar vortex. A comparative analysis is conducted for the strong polar vortex (SPV) and weak polar vortex (WPV) models. It reveals that a stronger stratospheric polar vortex induces more planetary waves to reflect from the North Pacific to the North Atlantic and more wave activity fluxes to propagate from the North Pacific to the North Atlantic in the SPV models than in the WPV models. Thus, the coupling of atmospheric circulation between the North Pacific and North Atlantic is stronger in the SPV models, which facilitates more North Pacific variability to be involved in the AO variability and induces a stronger North Pacific center in the AO pattern. The increase in vertical resolution may improve the simulation of the stratospheric polar vortex and thereby reduces the model biases in the North Pacific–North Atlantic coupling and thereby the amplitude of the North Pacific center of the AO pattern in models.