Negative Phase Of Arctic Oscillation Research Articles

Comparison of the mass circulation and AO indices as indicators of cold air outbreaks in northern winter

AbstractUsing the daily ERA‐Interim reanalysis data set for the 32 winters in 1979–2011, we find that midlatitude cold air outbreaks (CAOs) tend to preferentially occur within a week after simultaneously stronger mass circulations into the Arctic region in upper levels and out of the Arctic region below. The relationship of CAOs with Arctic Oscillation (AO) is less robust because temporal changes of AO are resulted from a small imbalance between the poleward and equatorward branches of the mass circulation. Results indicate that only when the poleward branch leads the equatorward branch (44% of all cases), CAOs tend to take place within a week after a negative phase of AO, whereas when the equatorward branch leads (19%), CAOs tend to occur after a positive phase of AO. In the remaining cases when the two branches are almost in phase, CAOs can be observed during either negative (24%) or positive (13%) phase of AO.

Impact of Stratospheric Sudden Warming on East Asian Winter Monsoons

Fifty-two Stratospheric sudden warming (SSW) events that occurred from 1957 to 2002 were analyzed based on the 40-year European Centre for Medium-Range Weather Forecasts Reanalysis dataset. Those that could descent to the troposphere were composited to investigate their impacts on the East Asian winter monsoon (EAWM). It reveals that when the SSW occurs, the Arctic Oscillation (AO) and the North Pacific Oscillation (NPO) are both in the negative phase and that the tropospheric circulation is quite wave-like. The Siberian high and the Aleutian low are both strengthened, leading to an increased gradient between the Asian continent and the North Pacific. Hence, a strong EAWM is observed with widespread cooling over inland and coastal East Asia. After the peak of the SSW, in contrast, the tropospheric circulation is quite zonally symmetric with negative phases of AO and NPO. The mid-tropospheric East Asian trough deepens and shifts eastward. This configuration facilitates warming over the East Asian inland and cooling over the coastal East Asia centered over Japan. The activities of planetary waves during the lifecycle of the SSW were analyzed. The anomalous propagation and the attendant altered amplitude of the planetary waves can well explain the observed circulation and the EAWM.

PERIODIC ANALYSIS OF SOLAR ACTIVITY AND ITS LINK WITH THE ARCTIC OSCILLATION PHENOMENON

Based on spectrum analysis, we provide the arithmetic expressions of the quasi 11 yr cycle, 110 yr century cycle of relative sunspot numbers, and quasi 22 yr cycle of solar magnetic field polarity. Based on a comparative analysis of the monthly average geopotential height, geopotential height anomaly, and temperature anomaly of the northern hemisphere at locations with an air pressure of 500 hPa during the positive and negative phases of AO (Arctic Oscillation), one can see that the abnormal warming period in the Arctic region corresponds to the negative phase of AO, while the anomalous cold period corresponds to its positive phase. This shows that the abnormal change in the Arctic region is an important factor in determining the anomalies of AO. In accordance with the analysis performed using the successive filtering method, one can see that the AO phenomenon occurring in January shows a clear quasi 88 yr century cycle and quasi 22 yr decadal cycle, which are closely related to solar activities. The results of our comparative analysis show that there is a close inverse relationship between the solar activities (especially the solar magnetic field index changes) and the changes in the 22 yr cycle of the AO occurring in January, and that the two trends are basically opposite of each other. That is to say, in most cases after the solar magnetic index MI rises from the lowest value, the solar magnetic field turns from north to south, and the high-energy particle flow entering the Earth's magnetosphere increases to heat the polar atmosphere, thus causing the AO to drop from the highest value; after the solar magnetic index MI drops from the highest value, the solar magnetic field turns from south to north, and the solar high-energy particle flow passes through the top of the Earth's magnetosphere rather than entering it to heat the polar atmosphere. Thus the polar temperature drops, causing the AO to rise from the lowest value. In summary, the variance contribution rate of the changes in the quasi 110 yr century cycle and quasi 22 yr decadal cycle for the AO reaches 62.9%, indicating that solar activity is an important driving factor of the AO.

Weakening of the stratospheric polar vortex by Arctic sea-ice loss.

Successive cold winters of severely low temperatures in recent years have had critical social and economic impacts on the mid-latitude continents in the Northern Hemisphere. Although these cold winters are thought to be partly driven by dramatic losses of Arctic sea-ice, the mechanism that links sea-ice loss to cold winters remains a subject of debate. Here, by conducting observational analyses and model experiments, we show how Arctic sea-ice loss and cold winters in extra-polar regions are dynamically connected through the polar stratosphere. We find that decreased sea-ice cover during early winter months (November-December), especially over the Barents-Kara seas, enhances the upward propagation of planetary-scale waves with wavenumbers of 1 and 2, subsequently weakening the stratospheric polar vortex in mid-winter (January-February). The weakened polar vortex preferentially induces a negative phase of Arctic Oscillation at the surface, resulting in low temperatures in mid-latitudes.

The effects of ENSO under negative AO phase on spring dust activity over northern China: an observational investigation

ABSTRACTThe effects of El Niño/Southern Oscillation (ENSO) under negative Arctic Oscillation (AO) phase on the Asian dust activity are investigated for springs of the period 1961–2002. The spring dust index (DI) describing the monthly frequencies of three types of dust events (e.g. dust storm, blowing dust, and floating dust) exhibits a significant increase in the years of negative AO phase (hereafter AO−) and El Niño, compared with that in the years of AO− and La Niña. Averaged over all observation stations, the spring DI (49.7) during the El Niño/AO− years is higher by 11.4% or 29.8% than that (38.3) during the La Niña/AO− years. We suggest possible physical mechanism that the anomalous large‐scale environments associated with AO− and El Niño are more effective to provide favourable conditions to enhance Asian dust activity. During the El Niño/AO− years, meridional gradients of pressure and temperature over the dust source regions are significantly enhanced by decreasing the geopotential height and warming air temperature that originated from the north and south of source regions, respectively, under the influence of AO− and El Niño. These also intensify the zonal wind shear and atmospheric baroclinicity, thereby producing enhanced cyclogenesis and dust occurrences over the major source regions. At the same time, dust transport paths with the stronger westerly winds are developed by the combined constraints of anomalous cyclone over the Siberia and the Mongolia and anomalous anticyclone over the western North Pacific, and thus strengthen dust transport to the downwind regions.

Low Altitude Atmospheric Circulation Characteristics of Arctic Oscillation and Its Relation to Solar Activity

AbstractA comparative analysis is made to the monthly average geopotential height, geopotential height anomaly, and temperature anomaly of the Northern Hemisphere at the place with an air pressure of 1000 hPa during the positive and negative phases of arctic oscillation (AO). The result shows that the abnormal warming period in the Arctic region corresponds to the negative phase of AO, while the anomalous cold period corresponds to its positive phase. It means that the abnormal change in the Arctic region is an important factor deciding the anomalies of AO. The AO phenomena occurring in January have a clear quasi‐110‐year centurial cycle and quasi‐22‐year decadal cycle, which are closely related to solar activities. The specific expressions are as follows: (1) The AO phenomena occurring in January have a clear quasi‐110‐year centurial cycle closely related to solar activities. The variance contribution rate of this cycle for AO indexes reaches 44.4%, which is the most significant centurial change characteristic of the AO phenomena in January. (2) The spectral analysis results show that the AO indexes in January after filtering out the 110‐year centurial variation have a significant 22‐year cycle, and their variance contribution rates reach 18.5%, which is the second important characteristics of the interdecadal changes of AO indexes after the quasi‐110‐year centurial cycle. The comparative analysis results show that there is a close inverse relationship between the solar activities (especially the solar magnetic field index changes) and the changes in the 22‐year cycle of the AO in January, and that the two trends are basically opposite to each other. That is to say, in most cases after the solar magnetic index MI rises from the lowest value, the solar magnetic field turns from north to south, and the high‐energy particle flow entering the Earth's magnetosphere increases to heat the polar atmosphere, thus causing the AO to drop from the highest value. After the solar magnetic index MI drops from the highest value, the solar magnetic field turns from south to north, and the solar high‐energy particle flow passes through the top of the Earth's magnetosphere rather than entering it to heat the polar atmosphere. And thus the polar temperature drops, causing the AO to rise from the lowest value. In summary, the variance contribution rate of the changes in the Quasi‐110‐year centurial cycle and Quasi‐22‐year decadal cycle to the AO is up to 62.9%, indicating that solar activity is an important driving factor of the AO.

Changes in the frequencies of record‐breaking temperature events in China and its association with East Asian Winter Monsoon variability

AbstractThe daily maximum and minimum temperatures observed at the 1897 meteorological stations of China over the past 60 years (1951–2010) are analyzed in this study to examine the interdecadal variation of frequency for record‐breaking event (RBE) of temperature in the context of global warming. The results indicate that the frequency of record‐breaking high temperature in the first decade of the 21st century is the highest in the three decades from the 1980s to the 2000s, implying a distinct warming trend. Meanwhile, frequencies of record‐breaking low temperature in the 1990s and the beginning of the 21st century are also significant. In particular, the RBEs of low temperature occurred over most of China in the 1990s but concentrated in northern China during the 2000s. To understand why the record low temperatures in northern China are repeatedly broken in the 2000s, the related East Asian Winter Monsoon (EAWM) variability is investigated. The empirical orthogonal function analysis of surface air temperature reveals that the northern mode of the EAWM variability, which is highly associated with the Arctic Oscillation (AO) activities at both interdecadal and interannual timescales, has been intensifying since late 1990s. Corresponding to the intensification of the northern mode of the EAWM variability and the negative phase of AO in the 2000s, the Siberian High and East Asian trough intensify while the polar‐front jet stream strengthens and the subtropical westerly jet stream abnormally shifts northward. As a result, anomalously strong cold air masses, originated from Siberia, intrude into East Asia but are blocked by the enhanced northward subtropical westerly jet and cannot reach low‐latitude area. Therefore, the extremely strong cold air masses are amassed in mid‐high latitudes of East Asia, resulting in RBEs of low temperature in this area.

The 2010/2011 snow season in California's Sierra Nevada: Role of atmospheric rivers and modes of large-scale variability

[1] The anomalously snowy winter season of 2010/2011 in the Sierra Nevada is analyzed in terms of snow water equivalent (SWE) anomalies and the role of atmospheric rivers (ARs)—narrow channels of enhanced meridional water vapor transport between the tropics and extratropics. Mean April 1 SWE was 0.44 m (56%) above normal averaged over 100 snow sensors. AR occurrence was anomalously high during the period, with 20 AR dates during the season and 14 in the month of December 2010, compared to the mean occurrence of nine dates per season. Fifteen out of the 20 AR dates were associated with the negative phases of the Arctic Oscillation (AO) and the Pacific-North American (PNA) teleconnection pattern. Analysis of all winter ARs in California during water years 1998–2011 indicates more ARs occur during the negative phase of AO and PNA, with the increase between positive and negative phases being ∼90% for AO, and ∼50% for PNA. The circulation pattern associated with concurrent negative phases of AO and PNA, characterized by cyclonic anomalies centered northwest of California, provides a favorable dynamical condition for ARs. The analysis suggests that the massive Sierra Nevada snowpack during the 2010/2011 winter season is primarily related to anomalously high frequency of ARs favored by the joint phasing of −AO and −PNA, and that a secondary contribution is from increased snow accumulation during these ARs favored by colder air temperatures associated with −AO, −PNA, and La Niña.

Influence of the Arctic Oscillation on the vertical distribution of clouds as observed by the A-Train constellation of satellites

Abstract. The main purpose of this study is to investigate the influence of the Arctic Oscillation (AO), the dominant mode of natural variability over the northerly high latitudes, on the spatial (horizontal and vertical) distribution of clouds in the Arctic. To that end, we use a suite of sensors onboard NASA's A-Train satellites that provide accurate observations of the distribution of clouds along with information on atmospheric thermodynamics. Data from three independent sensors are used (AQUA-AIRS, CALIOP-CALIPSO and CPR-CloudSat) covering two time periods (winter half years, November through March, of 2002–2011 and 2006–2011, respectively) along with data from the ERA-Interim reanalysis. We show that the zonal vertical distribution of cloud fraction anomalies averaged over 67–82° N to a first approximation follows a dipole structure (referred to as "Greenland cloud dipole anomaly", GCDA), such that during the positive phase of the AO, positive and negative cloud anomalies are observed eastwards and westward of Greenland respectively, while the opposite is true for the negative phase of AO. By investigating the concurrent meteorological conditions (temperature, humidity and winds), we show that differences in the meridional energy and moisture transport during the positive and negative phases of the AO and the associated thermodynamics are responsible for the conditions that are conducive for the formation of this dipole structure. All three satellite sensors broadly observe this large-scale GCDA despite differences in their sensitivities, spatio-temporal and vertical resolutions, and the available lengths of data records, indicating the robustness of the results. The present study also provides a compelling case to carry out process-based evaluation of global and regional climate models.

Decadal changes in surface air temperature variability and cold surge characteristics over northeast Asia and their relation with the Arctic Oscillation for the past three decades (1979–2011)

Decadal changes in surface air temperature (SAT) variability and cold surge characteristics over Northeast Asia during late winter (January–March) are analyzed for the past three decades. Power spectrum analysis of SAT reveals that the low‐frequency variabilities with a period longer than 10 days are significantly enhanced, while the high‐frequency variabilities with a period shorter than 10 days are weakened in the 1980s and 2000s. Moreover, cold surges were stronger and lasted longer during the 1980s and 2000s compared to those that occurred in the 1990s. Here, we propose that large‐scale atmospheric conditions manifested by a different phase of the Arctic Oscillation (AO) provide preconditioning for a cold surge event, which showed a prominent decadal fluctuation. The more (less) frequent strong and long‐lasting cold surge occurrences in the 1980s and 2000s (1990s) are preceded by the more dominant negative (positive) phase of the AO. Lag‐composite analyses for cold surge events categorized by the AO phases indicate that stronger and longer‐lasting cold air advection dominates at the lower‐level, when upper‐level wave train and coastal trough are developed over East Asia under the strong negative AO phase. These results suggest that the decadal changes in SAT variability and cold surge characteristics are strongly associated with the decadal changes in the phase distribution of the AO.

Anomalous precipitation signatures of the Arctic Oscillation in the time-variable gravity field by GRACE

The Arctic Oscillation (AO) controls winter climates in the Northern Hemisphere to a large extent. Positive AO brings lower/higher surface temperature and higher/lower precipitation in high/middle latitude regions, and negative AO reverses the situation. In this study, we investigate signals of anomalous precipitations caused by AO using the data of the Gravity Recovery and Climate Experiment (GRACE) satellites. Wintertime mass deviations inferred from GRACE in the high and middle (boundary ∼55N) latitude regions in Eurasia showed highly positive and negative correlations with the AO indices. This possibly reflects the northward and southward shift of the centre of winter precipitation during the positive and negative phases of AO, respectively. Wintertime mass deviations also showed positive and negative correlations with the AO indices in the northern and southern parts of Greenland, respectively, but the boundary was further to the north, say ∼75N. AO redistributes the water mass as much as ∼1000 Gt between high and middle latitude regions in the Northern Hemisphere. Such mass redistribution causes significant surface deformation by loading large enough to be observed by Global Positioning System. This also causes the shift of the Earth's rotation axis especially towards the Greenwich Meridian large enough to be detected with space geodetic techniques. AO signatures are also derived from the empirical orthogonal function (EOF) analysis, as the first leading mode of GRACE data after excluding seasonal, linear and quadratic components. The EOF analysis also demonstrates that though AO is a main contributor to bring anomalous winter precipitation in the Northern Hemisphere as a whole, in North America, however, influence of the El Niño and Southern Oscillation on the winter precipitation anomaly is larger than AO.

우리나라 겨울철 기온과 북반구 대기 순환과의 상관성 분석

This study investigates the relationship between the Korean temperature and the atmospheric circulation such as Arctic Oscillation, Siberian High and Aleutian Low during the winter (December-January) for the period of 1970-2011. It is found that all indices to represent aforementioned circulations are significantly correlated with Korean winter temperature for the period of 1970 - 2011. There are marked contrasts in such relationship, however, before and after the mid-1980s when a significant regime shift of Korean winter temperature occurred. While Korean winter temperature has a close relationship with Arctic Oscillation after the mid-1980s, its relationship with Siberian High and Aleutian Low is weakened. The composite analysis between a positive and negative phase of Arctic Oscillation before and after the mid-1980s is conducted to examine a recent strengthening of Arctic Oscillation-Korean winter temperature relationship. It is found that the structural changes of low-level wind and the geopotential height at 500 hPa between the two phases of Arctic Oscillation are more effective to influence Korean winter temperature after the mid-1980s. This may induce a close relationship between the Korean winter temperature variability and Arctic Oscillation after the mid-1980s compared to before the mid-1980s.

Possible connection between summer tropical cyclone frequency and spring Arctic Oscillation over East Asia

This study shows that the frequency of summer tropical cyclones (TCs) in the areas of Japan, Korea, and Taiwan (JKT), which are located in the middle latitudes of East Asia, has a positive correlation with the Arctic Oscillation (AO) occurring during the preceding spring, while summer TC frequency in the Philippines (PH), located in the low latitudes, has a negative correlation with the AO of the preceding spring. During a positive AO phase, when the anomalous anticyclone forms over the mid-latitudes of East Asia, other anomalous cyclones develop not only in the high latitudes but also in the low latitudes from the preceding spring to the summer months. With this change, while southeasterlies in the JKT area derived from the mid-latitude anticyclone plays a role in steering TCs toward this area, northwesterlies strengthened in the PH area by the low-latitude cyclone plays a role in preventing TC movement toward this area. In addition, because of this pressure systems developed during this AO phase, TCs occur, move, and recurve in further northeastern part of the western North Pacific than they do during a negative AO phase.

Possible influence of Arctic Oscillation on dust storm frequency in North China

This study has investigated the influence of Arctic Oscillation (AO) on dust storm frequency in North China in spring seasons during 1961–2007. There is a significant linkage between dust storm frequency and AO; a negative (positive) AO phase is related to an increased (decreased) dust storm frequency in North China. This relationship is closely related to changes in the cold air activity in Mongolia. The cold air activity exerts large impacts on the dust storm frequency; the frequency of cold air activity over Mongolia not only positively correlates with the dust storm frequency in North China, but also shows a long-term decreasing trend that is an important reason for the long-term decreasing of dust storm frequency in North China. The AO has large influence on the frequency of cold air activity over Mongolia; a negative (positive) AO phase is highly related to an increased (decreased) frequency of cold air activity over Mongolia, which results in an increased (decreased) dust storm frequency in North China.

Influence of Arctic Oscillation on dust activity over northeast Asia

Abstract The northeast Asian dust process during the spring seasons in the years 1982–2006 was simulated by the Integrated Wind Erosion Modeling System (IWEMS). The influence of Arctic Oscillation (AO) on dust activities was investigated by analyzing surface observations and model simulations. There is a significant relationship between AO and dust activity; a positive AO phase is associated with decreased (increased) dust storm frequency in Mongolia (Taklimakan Desert) and enhanced anticyclonic (southeastward) dust transport over northwestern China (North China). The AO-dust relation is mainly due to changes in the westerly jet and geopotential height in the middle troposphere; a positive AO phase induces a northward shift of the polar jet, an intensified westerly jet over northern Tibetan Plateau, and a positive geopotential height anomaly over Mongolia. The northern shift of the polar jet reduces the frequency of intense cyclones in Mongolia, thereby causing a decrease in the dust storm frequency. The intensified westerly jet stream over the northern Tibetan Plateau increases the dust storm frequency in the Taklimakan Desert. The positive geopotential height anomaly over Mongolia initiates an anticyclonic dust transport anomaly in the middle troposphere over northwestern China. It also induces a southeastward dust transport anomaly over North China. The reverse situations are true for a negative AO phase.

Influence of the Arctic oscillation on central United States summer rainfall

Effects of the Arctic oscillation (AO) on summer rainfall variability in the central United States are examined in order to improve understanding and prediction of the interannual variation in the summer rainfall. Major results show persistent AO effects that resulted in less summer rainfall in the central United States during the positive phase of the AO and more rainfall during the negative AO phase. These effects are most prominent at the interannual time scale. The key physical processes connecting the AO with the rainfall variations are shown in changes in latitudinal location of the midlatitude upper tropospheric westerly jet over North America, the transverse circulation around the jet, and the low‐level moisture flux divergence in the central United States. Diagnostic analyses show that the change of the jet stream location may have resulted from the AO‐induced eddy heat and momentum forcing on the mean zonal flow in the upper troposphere. The eddy‐mean flow interactions caused a northward shift of the jet in the positive phase of the AO. The associated anomalies of downward motion and moisture divergence in the lower troposphere over the central United States suppressed rainfall development. A set of reversed anomalies developed in the negative phase of the AO, encouraging summer rainfall.

Statistical analysis and comparison of external factor effects on the total ozone field over the Russian territory in 1973–2007

The analysis of external factors, which are most significant for the formation of the monthly mean total ozone (TO) field and ozone transport over the Russian Federation, based on observation data obtained from about 30 ground-based stations of the ozonometric network averaged over a year, December through March and June through August, over five climatic regions, is considered. Performed spectrum and discriminant analysis allowed obtaining quantitative estimates of the impact of the Arctic Oscillation, deviation of the winter temperature of the lower polar stratosphere, quasi-biennial oscillations (QBO), 11-year solar cycle, El Nino-Southern Oscillation (ENSO) on the TO and to assess the regional differences in the effects of these factors. In December–March, in the years with a negative Arctic Oscillation phase, warm stratosphere, and the easterly QBO phase (QBO-E), the ozone content increases significantly relative to the opposite phases of oscillations on average by 35, 28, and 26 Dobson units (DU), respectively. The spectra, similar to the discriminant function, demonstrate strong influence of the 11-year solar cycle and QBO on the TO even in the summer months, while the QBO is more pronounced in the eastern part of the Russian Federation. The ENSO effect was not singled out against the general “noisy” background of the cold six-month period, when many atmospheric processes become active: however, during the summer months, in warm periods of the ENSO, the TO, at the 97% significance level, increases over most of the Russian area. The rest of the obtained results are significant at the 95–99.9% level.

The Leading Mode of Wintertime Cold Wave Frequency in Northern China during the Last 42 Years and its Association with Arctic Oscillation

This study examined wintertime (Novem- ber−April) cold wave frequency (CWF) in northern China during the last 42 years and its association with Arctic Oscillation (AO) through analysis of daily mean surface temperature from 280 stations across northern China and European Centre for Medium-Range Weather Forecasts (ECMWF) 40-Year Re-analysis ERA-40 data. The lead- ing empirical orthogonal function EOF mode of winter- time CWF (CWF-EOF1) indicates an identical signal over most northern China, with the characteristic trend of linear decline for the leading principal component (CWF-PC1). After the linear trend is removed, remarkable inter-annual variability is found to be the dominant feature of the CWF-PC1. The regression map for sea level pressure based on CWF-PC1 corresponds to the negative phase of AO. Correlation analysis further proves that CWF-PC1 has a significant negative correlation with AO at the in- ter-annual time scale. The relationship between AO and global surface air temperature is also investigated in order to understand its association with cold air activity over East Asia, and it is suggested that the anomalies of at- mospheric circulation in Siberia may serve as a bridge for interaction between AO and CWF in northern China dur- ing wintertime. Keywords: cold wave frequency, Arctic Oscillation Citation: Wei, J.-H., and Z.-H. Lin, 2009: The leading mode of wintertime cold wave frequency in northern China during the last 42 years and its association with Arctic Oscillation, Atmos. Oceanic Sci. Lett., 2, 130−134.

Relationship between Korean Peninsula Landfalling Tropical Cyclones and Interannual Climate Variabilities

The relationship between two interannual climate variabilities and the frequency of tropical cyclone (TC) that landed over the Korean Peninsula (KP) has investigated for the period of 1951-2004. In the analysis of the relationship between KP-landfall TC frequency and the ENSO phase, most TCs of C-14 (TCs that do not pass through mainland China before landing the KP) and C-23 (TCs that pass through mainland China before landing the KP) tended to more land in the warm phase than normal and cold phases. However, TC intensity at landfall was stronger in the cold and normal phases. In the analysis of the relationship between KP-landfall TC frequency and Arctic Oscillation (AO) phase, the TCs of C-14 tended to more land in the positive (POS) phase of AO and the negative (NEG) phase of AO for C-23. It was found that AO index was negatively correlated with the Nio-3.4 index. And then the TCs of C-14 landed more frequently over the KP in the AO POS - Nio-3.4 NEG phases and in the AO NEG - Nio-3.4 POS phases for the TCs of C-23.

A Hidden Markov Model Perspective on Regimes and Metastability in Atmospheric Flows

Abstract In this study, data from three atmospheric models are analyzed to investigate the existence of atmospheric flow regimes despite nearly Gaussian statistics of the planetary waves in these models. A hierarchy of models is used, which describes the atmospheric circulation with increasing complexity. To systematically identify atmospheric regimes, the presence of metastable states in the data is searched for by fitting so-called hidden Markov models (HMMs) to the time series. A hidden Markov model is designed to describe the situation in which part of the information of the system is unknown or hidden and another part is observed. Within the context of this study, some representative variable of planetary-scale flow (e.g., mean zonal flow or leading principal component) is known (“observed”), but its dynamics may depend crucially on the overall flow configuration, which is unknown. The behavior of this latter, “hidden” variable is described by a Markov chain. If the Markov chain possesses metastable (or quasi persistent) states, they are identified as regimes. In this perspective, regimes can be present even though the observed data have a nearly Gaussian probability distribution. The parameters of the HMMs are fit to the time series using a maximum-likelihood approach; well-established and robust numerical methods are available to do this. Possible metastability of the Markov chain is assessed by inspecting the eigenspectrum of the associated transition probability matrix. The HMM procedure is first applied to data from a simplified model of barotropic flow over topography with a large-scale mean flow. This model exhibits regime behavior of its large-scale mean flow for sufficiently high topography. In the case of high topography, the authors find three regimes, two of which correspond to zonal flow and the third to blocking. Next, a three-layer quasigeostrophic model is used as a prototype atmospheric general circulation model (GCM). Its first empirical orthogonal function (EOF) is similar to the Arctic Oscillation (AO) and exhibits metastability. For this model, two regime states are found: one corresponding to the positive phase of the AO with large amplitude and decreased variability of the streamfunction field, and another corresponding to the negative AO phase with small amplitude and increased variability. Finally, the authors investigate a comprehensive GCM. The leading four EOFs of this model show no signs of metastability. The results of the barotropic flow over topography and of the quasigeostrophic model suggest that the observed small skewness of planetary wave probability density functions (PDFs) is an imprint of blocked circulation states.