Negative Northern Annular Mode Research Articles

The onset of stratospheric final warming and record–breaking April surface warming over Central Asia in 2022

In the spring of 2022, a stratospheric final warming (SFW) event occurred on March 19 at 10 hPa in the Northern Hemisphere, ranking the third earliest SFW event during 1979–2022. Meanwhile, Central Asia recorded the highest surface air temperature (SAT) in the ensuing April in recent 44 years. Based on the fifth generation reanalysis data from European Centre for Medium-Range Weather Forecasts, this study reports an intimate linkage between the SFW onset and the extremely high SAT in Central Asia in April 2022. Specifically, after the SFW onset, positive geopotential height anomalies in the stratospheric polar region persist for more than a month, which propagate downward to the troposphere in about one week, resulting in a negative phase of the Northern Annular Mode (NAM) in the upper troposphere during the period from late March to mid-April. Such a negative NAM tends to excite an anomalous Rossby wave train propagating southeastward from the North Atlantic to Central Asia, causing an anomalous high over Central Asia in the middle and upper troposphere. As a result, the associated anomalies in descending motion and adiabatic heating would lead to the in situ unprecedented high SAT in April.

Dynamics and Impacts of the North Pacific Eddy-Driven Jet Response to Sudden Stratospheric Warmings

Abstract In this study, observations and simulations are used to investigate the mechanisms behind the different surface responses over the North Pacific and North Atlantic basins in response to sudden stratospheric warmings associated with a polar-night jet oscillation event (PJO SSWs). In reanalysis and a free-running preindustrial simulation, on average, a negative North Atlantic Oscillation (NAO) response is seen, corresponding to an equatorward shift of the eddy-driven jet. This is considered as the canonical tropospheric response to PJO SSWs. In contrast, the response over the North Pacific is muted. This basin-asymmetric response is shaped by the North Pacific air–sea interactions spun up by the tropospheric precursor to PJO SSWs, which prevent the Pacific eddy-driven jet from responding to the downward influence from the stratosphere. To isolate the downward influence from the sudden warming itself from any preconditioning of the troposphere that may have occurred prior to the warming, a nudging technique is used by which a reference PJO SSW is artificially imposed in a 195-member ensemble spun off from a control simulation. The nudged ensembles show a more basin-symmetric negative Northern Annular Mode (NAM) response, in which the eddy-driven jet shifts equatorward in both the Pacific and Atlantic sectors. Monitoring the atmospheric and oceanic conditions in the North Pacific before and at the onset of PJO SSWs may be useful for forecasting whether a basin-asymmetric negative NAO or basin-symmetric negative NAM response is more likely to emerge. This can be further used to improve subseasonal-to-seasonal predictions of weather and climate. Significance Statement Stratospheric sudden warming events (SSWs) occur when the eastward winds usually found above the Arctic in the winter spontaneously and rapidly reverse. Following their occurrence, the Northern Hemisphere surface westerlies move southward, sometimes over both the North Atlantic and North Pacific and other times over the North Atlantic only. We therefore wanted to understand this uncertainty in the North Pacific surface westerlies response. We find that the North Pacific surface westerlies response to SSWs can be muted by air–sea interactions over the North Pacific. Our results highlight the importance of monitoring the atmospheric and oceanic conditions in the North Pacific before the occurrence of SSWs to forecast whether the Pacific westerlies are likely to respond to SSWs.

Possible influence of sudden stratospheric warmings on the atmospheric environment in the Beijing–Tianjin–Hebei region

Abstract. Using European Centre for Medium-Range Weather Forecasts fifth-generation (ERA5) and second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) reanalysis and surface meteorological observation data, this study explores the possible impact of sudden stratospheric warming (SSW) events on air quality in the Beijing–Tianjin–Hebei (BTH) region. Major SSW events are divided into polar vortex displacement SSW and polar vortex split SSW. As the duration of split SSW events is longer and the stratospheric signal pulses propagate further downward than displacement SSWs, subseasonal variability of the atmospheric particulates in the BTH is larger during split SSWs. The air particulate concentration is light before the SSW onset due to the enhanced perturbation in the troposphere associated with strengthened planetary waves. The air particulate concentration around the SSW onset dates begins to rise due to weakening of the tropospheric disturbance as the enhanced planetary waves enter the stratosphere. In the decaying period of the SSW, the air particulate concentration decreases as the stratospheric negative northern annular mode (NAM) signal propagates downward. Specifically, in the pre-SSW period of displacement (split) SSW events, a wavenumber-1-like (wavenumber-2-like) anomaly pattern is strengthened. The East Asian winter monsoon intensifies as the East Asian trough is deepened, especially before the split SSW event onset, leading to a cleaning period. Around the SSW onset period as the tropospheric perturbation diminishes and the East Asian winter monsoon weakens, a surge of air particulate concentration is observed. After the SSW onset, due to the downward propagation of the stratospheric negative NAM signal, cold anomalies form in northeastern East Asia, especially for split SSWs, corresponding to a cleaning period in the BHT region. The local meteorological conditions during the SSWs are also discussed.

Coupled stratosphere-troposphere-Atlantic multidecadal oscillation and its importance for near-future climate projection

Northern Hemisphere (NH) climate has experienced various coherent wintertime multidecadal climate trends in stratosphere, troposphere, ocean, and cryosphere. However, the overall mechanistic framework linking these trends is not well established. Here we show, using long-term transient forced coupled climate simulation, that large parts of the coherent NH-multidecadal changes can be understood within a damped coupled stratosphere/troposphere/ocean-oscillation framework. Wave-induced downward propagating positive stratosphere/troposphere-coupled Northern Annular Mode (NAM) and associated stratospheric cooling initiate delayed thermohaline strengthening of Atlantic overturning circulation and extratropical Atlantic-gyres. These increase the poleward oceanic heat transport leading to Arctic sea-ice melting, Arctic warming amplification, and large-scale Atlantic warming, which in turn initiates wave-induced downward propagating negative NAM and stratospheric warming and therefore reverse the oscillation phase. This coupled variability improves the performance of statistical models, which project further weakening of North Atlantic Oscillation, North Atlantic cooling and hiatus in wintertime North Atlantic-Arctic sea-ice and global surface temperature just like the 1950s–1970s.

Stratosphere–troposphere coupling enhances subseasonal predictability of Northern Eurasian cold spells

AbstractHere we explore the stratospheric influence on the predictability of Eurasian cold‐spell events using the European Centre for Medium‐Range Weather Forecasts (ECMWF) ensemble hindcasts obtained from the Subseasonal‐to‐Seasonal (S2S) archive. To isolate the stratospheric influence, we subsampled two groups of hindcasts according to the strength of the stratospheric polar vortex preceding the cold spells at the surface. The predicted probability of cold spells, defined as the lowest 10th percentile of weekly mean temperature anomalies over northern Eurasia (10° W–130° E and 50° N–65° N), is systematically higher, by 0.05–0.2, at lags 7–24 days in the forecasts initialized during the weak stratospheric vortex compared to the strong stratospheric vortex group, extending the predictability of cold spells by 3–5 days. Our results suggest that, in the case of the weak polar vortex, stratosphere–troposphere coupling favors the negative Northern Annular Mode (NAM) regime and the cold‐air outbreaks in Eurasia. As a consequence, the long stratospheric predictability extends the predictability of the cold spells. On the other hand, when the polar vortex is strong, the stratospheric anomalies do not favor the observed negative NAM regime, which thus results from the internal tropospheric processes only. In this case the predictability of cold‐air spells is limited. Furthermore, we show that the extended predictability of cold spells arising from the stratosphere–troposphere coupling is captured by a simple statistical model, suggesting that governing large‐scale dynamics behave effectively linearly over some limited periods. Quantified contribution of the stratosphere–troposphere coupling to the enhanced skill of the extended‐range cold‐spell forecasts documented in our paper may prove useful in the development of forecasting tools.

Possible Lagged Impact of the Arctic Sea Ice in Barents–Kara Seas on June Precipitation in Eastern China

Based on the Hadley Centre sea ice concentration, the ERA5 reanalysis, and three precipitation datasets, the possible lagged impact of the Barents–Kara sea ice on June rainfall across China is investigated. Using the singular value decomposition, it is revealed that the state of sea ice concentration in Barents–Kara Seas from November to December is closely related to regional precipitation in June, which is most evident across the Yangtze–Huai Rivers Valley and South China. Possible pathways from preceding Arctic sea ice concentration to June precipitation are examined and discussed. First, the sea ice concentration usually has a long memory, which exerts a long-lasting and lagged impact, although the sea ice anomaly amplitude gradually weakens from early winter to early summer. Second, an increase in Barents–Kara sea ice usually corresponds to a stronger stratospheric polar vortex in midwinter by suppressing extratropical wave activities, which is projected to the positive phase of northern annular mode (NAM). Strong vortex gradually recovers to its normal state and even weakens in spring, which corresponds to the negative NAM response from spring to early summer. Third, the stratospheric anomalies associated with the Barents–Kara sea ice variations propagate downward. Due to the out-of-phase relationship between the lower and upper stratospheric circulation anomalies after midwinter, westerly anomalies in midwinter are followed by easterly anomalies in later months in the circumpolar region, consistent with the positive NAM response in midwinter, negative NAM response in spring, and a wave train-like response in early summer to Barents–Kara sea ice increase (and vice versa). The observed lagged impact of Barents–Kara sea ice on China rainfall in June is limitedly simulated in the ten CMIP6 models used in this study.

Role of the quasi-biennial oscillation in the downward extension of stratospheric northern annular mode anomalies

Using the JRA-55 reanalysis datasets and radiosonde observations, the impact of the quasi-biennial oscillation (QBO) on the downward extension of stratospheric Northern Annular Mode (NAM) signals in its positive and negative phases (positive and negative NAM hereafter) is examined. The results show that the average number of days per month on which stratospheric negative NAM signals extend downward into the troposphere is more than twice as large during the easterly phase of the QBO (EQBO) as those during the westerly phase of the QBO (WQBO). The increasing likelihood of downward extension of stratospheric negative NAM signals during the EQBO is related to three factors: enhanced upward wave-1 fluxes entering the stratosphere, anomalous poleward wave fluxes below 100 hPa and EQBO-induced residual circulation. It is also revealed that the EQBO-induced subtropical tropospheric easterly anomalies lead to positive height anomalies over the subtropical North Atlantic, which are amplified by transient eddies and maintained by convection. These subtropical positive height anomalies further stimulate an anomalous wave train, forcing subpolar positive height anomalies over the Euro-Atlantic sector and enhancing wave 1. Over the Pacific, the EQBO could also induce positive height anomalies in the tropical troposphere, and these anomalies are related to convection and vertical motion. The positive height anomalies can induce anomalous poleward-propagating waves, forcing negative height anomalies over the North Pacific at mid-high latitudes and the enhancing wave 1. Fewer stratospheric positive NAM signals extend downward into the troposphere during the EQBO than during the WQBO, with mechanisms similar to those of negative NAM.

Predicting the Downward and Surface Influence of the February 2018 and January 2019 Sudden Stratospheric Warming Events in Subseasonal to Seasonal (S2S) Models.

Using the real‐time predictions from 11 models, this study analyzes the prediction of the downward propagation and surface impact of the 2018 and 2019 sudden stratospheric warmings (SSWs). These two SSWs differed both in their morphology types (2018: split; 2019: displacement followed by split) and magnitudes (the former being stronger). With a large sample size (>2,200) of multimodel ensemble forecasts, it is revealed that the strength of the SSW is more important than the vortex morphology in determining the magnitude of its downward impact, with strong SSWs more likely to propagate downward than weak SSWs. Therefore, based on the probabilistic forecasts, the observed strong SSW in February 2018 was more likely to have a downward and surface impact than the January 2019 SSW. The relationship between the 10‐hPa dominant wave number and the 100‐hPa polar cap height (or the Northern Annular Mode) is weak, implying that the dominant wave number might not be the primary factor determining the downward propagation of SSWs in the two analyzed cases. Hence, the high polar cap height (or negative Northern Annular Mode) response in the lower stratosphere and troposphere following the February 2018 SSW is mainly attributed to its strong intensity rather than the split morphology. Further, the 2‐m temperature anomaly pattern following the January 2019 SSW is not forecasted due to its weak downward propagation, whereas the 2‐m temperature in North Eurasia, Middle East, south China, and eastern United States could be forecasted for the downward propagating February 2018 SSW. However, regional rainfall anomalies are poorly forecasted (both in a deterministic and probabilistic sense) for both SSWs.

The Corresponding Tropospheric Environments during Downward-Extending and Nondownward-Extending Events of Stratospheric Northern Annular Mode Anomalies

AbstractUsing the NCEP–NCAR reanalysis dataset, this study classifies stratospheric northern annular mode (NAM) anomalies during the negative or positive phase into two categories—anomalies extending into the troposphere [trop event (TE); referred to as negative or positive TEs] and those not extending into the troposphere [nontrop event (NTE); referred to as negative or positive NTEs], and the corresponding tropospheric environments during the TEs and NTEs are identified. Compared with that for the negative NTEs, the upward wave fluxes entering the stratosphere are stronger and more persistent during the negative TEs. Furthermore, the stronger and more persistent upward wave fluxes during the negative TEs are due to more favorable conditions for upward wave propagation, which is manifested by fewer occurrences of negative refractive index squared in the mid- to high-latitude troposphere and stronger wave intensity in the mid- to high-latitude troposphere. However, the tropospheric wave intensity plays a more important role than the tropospheric conditions of planetary wave propagation in modulating the upward wave fluxes into the stratosphere. Stronger and more persistent upward wave fluxes in the negative TEs, particularly wave-1 fluxes, are closely related to the negative geopotential height anomalies over the North Pacific and positive geopotential height anomalies over the Euro-Atlantic sectors. These negative (positive) geopotential height anomalies over the North Pacific (Euro-Atlantic) are related to the positive (negative) diabatic heating anomalies and the decreased (increased) blocking activities in the mid- to high latitudes. The subtropical diabatic heating could also impact the strength of the mid- to high-latitude geopotential height anomalies through modulating horizontal wave fluxes. For positive NAM events, the results are roughly similar to those for negative NAM events, but with opposite signal.

Understanding the variation of stratosphere–troposphere coupling during stratospheric northern annular mode events from a mass circulation perspective

We revisit the various stratosphere–troposphere coupling relation from the perspective of the meridional mass circulation. We constructed 10-hPa northern annular mode (NAM) phase composites to show the typical spatiotemporal evolution of circulation anomalies during the NAM’s life cycle. Our results indicate that there is large case-to-case difference in the temporal evolution and vertical profile of polar temperature anomalies during NAM events, which shows no strong dependence on the intensity and duration of NAM events, but agrees well with the variations of the three branches of mass circulation at 60°N: the stratospheric poleward warm air branch (ST), the poleward warm air branch in the upper troposphere (WB), and the equatorward cold air branch in the lower troposphere (CB). Such correspondence is due to the dynamic heating and cooling anomalies associated with the redistribution of air masses by the anomalous meridional mass circulation in different isentropic layers. The various relationship among the three mass circulation branches is attributed to anomalous wave activities. The amplitude and westward tilt of waves are always stronger (weaker) throughout the stratosphere before (after) the peak time of negative NAM events, leading to a stronger (weaker) ST before (after) the peak time. Variations in WB and CB are mostly dependent on wave variabilities in the mid- to lower troposphere, leading to variations in the timing of in- or out-of-phase coupling of the ST with the WB and CB, and thus various thermostructure during NAM events. At a later stage of the negative NAM events when the polar temperature becomes colder and the polar jet recovers, the weakened baroclinic instability in the lower stratosphere provides favorable conditions for the strengthening of the WB and CB if wave activities strengthen in the troposphere during that period.

Seasonal Winter Forecasts of the Northern Stratosphere and Troposphere: Results from JMA Seasonal Hindcast Experiments

Abstract This study investigates winter forecasts of the northern stratosphere and troposphere using seasonal hindcast experiments of the Japan Meteorological Agency (JMA). A main focus is placed on the seasonal forecasts of the December–February (DJF)-mean northern annular mode (NAM) when the forecasts are initialized in late fall. Results demonstrate that the hindcast data have significant skill for both ensemble-mean and category (probability) forecasts of the NAM but only in the stratosphere. Probability forecasts for DJF major stratospheric sudden warmings (MSSWs) are also suggested to be significant (higher probabilities for actual MSSW years) near the 90% confidence level. The forecast skill of the stratospheric NAM changes with the observed phase of the quasi-biennial oscillation (QBO), although the QBO is not simulated but is only included in initial conditions. The skill is higher for the easterly phase, characterized by hits of negative NAM states (weaker-than-normal polar vortex), whereas it is lower for the westerly phase, reflecting misses of positive NAM states. It is finally shown that a verification score for category forecasts of the stratospheric and tropospheric NAM tends to covary as a whole. The tropospheric forecast skill is significant when the stratosphere has large NAM anomalies in the real world and they are well forecasted. In contrast, the tropospheric forecasts are sometimes poor when the stratospheric forecasts fail to capture observed NAM conditions. It is speculated that stratospheric and tropospheric forecasts could be improved together through the stratosphere–troposphere coupling for such cases, that is, by successfully forecasting anomalous vortex states in the stratosphere.

Predictability of downward propagation of major sudden stratospheric warmings

Major sudden stratospheric warmings (SSWs) are striking phenomena of wintertime stratospheric circulation usually defined as a reversal of zonal mean circulation from westerlies to easterlies. SSWs often have significant impact on tropospheric circulation and cause anomalies in surface climate lasting for up to 2 months. For this reason, dynamics and predictability of SSW receive considerable attention. It is however well‐known that not all SSWs cause significant, long‐lasting impact on the troposphere. In order to explain differences in tropospheric impacts following SSWs, several reasons have been previously proposed, including differences in type of SSW (split or displacement), persistence of stratospheric anomalies, preconditioning of the tropospheric circulation, and whether or not SSW was accompanied by a planetary wave reflection in the stratosphere. Here we address the predictability of tropospheric impacts by SSWs by seeking early precursors of the impacts. We separate midwinter SSWs into two groups: those which are followed by significant, long‐lasting impacts on the tropospheric circulation (defined in terms of anomalous Northern Annular Mode) and those not followed by significant anomalies in the annular mode. We show that SSWs characterised by a more negative Northern Annular Mode index in the lower stratosphere around 150 hPa and enhanced wave activity propagation to the stratosphere during the first few days following the central date have a larger probability of being followed by tropospheric impact, both in reanalyses and in climate model runs. These anomalies play a more important role in the subsequent downward propagation of the signal to the troposphere than the type of SSW: whether it is a split or a displacement, or absorptive or reflective SSW. We propose that using these anomalies as precursors of tropospheric impacts of SSW can enhance climate predictability.

The connection between extreme stratospheric polar vortex events and tropospheric blockings

The variations of tropospheric blockings and their connections with the stratospheric polar vortex during different stages of the life cycle of extreme stratospheric polar vortex events (i.e. strong vortex (SV) events and weak vortex (WV) events) are investigated. The blocking frequency decreases over the Euro‐Atlantic sector and increases over the western North Pacific during the onset and maturation stages of SV events. There are more blocking days over the western North Pacific and weakened upward planetary wave fluxes in the stratosphere during the maturation stage of shorter time‐scale SV events than during longer time‐scale SV events and the weakening of the upward planetary wave fluxes mainly results from the linear wave interference effect. The blocking frequency is increased over the Euro‐Atlantic sector during the decline stage of SV events. This increase is found to be related to the descending stratospheric zonal wind anomalies and the poleward displacement of the eddy‐driven jet stream, which lasts for about 10 days and is associated with the descending positive stratospheric Northern Annular Mode (NAM) anomalies. The increased blocking frequency leads to an enhancement of the planetary wave fluxes in the stratosphere via both the linear wave interference (for shorter time‐scale SV events) and nonlinear wave interference (for longer time‐scale SV events) effect.The changes of the blocking frequency during the growth stage of WV events are almost opposite to those observed during the onset and maturation stages of SV events. The blocking frequency decreases over the Euro‐Atlantic sector after the central dates of WV events, which are related to the descending stratospheric zonal wind anomalies and the equatorward displacement of the eddy‐driven jet stream preceded by the descending negative stratospheric NAM anomalies about 10 days. We found that the nonlinear wave interference begins to make a considerable contribution to the weakening of the upward wave fluxes in the stratosphere after the central date of WV events and this is not emphasized in previous literature.

ENSO Teleconnection to the Isopycnal Depth Fluctuations of the East/Japan Sea Intermediate Water in the Ulleung Basin during 1968–2002

AbstractThis study first detects the decadal variability in the depths of the East/Japan Sea (EJS) Intermediate Water (ESIW) using in situ observations and relates it to strong El Niño–Southern Oscillations (ENSOs). Using multitaper cross-spectrum and cyclostationary empirical orthogonal function analysis, this study found significant coherences at the 99% confidence level and opposite phases between Niño-3 and the ESIW isopycnal depths in the Ulleung Basin (UB) at a period of 14.1 yr during 1968–2002. This suggests a teleconnection between strong ENSOs and the ESIW, the cause of which is explored. When a strong El Niño (EN) develops, the Pacific–North American pattern is intensified by the EN-related Rossby wave interfering constructively with the climatological stationary wave. The amplified wave propagates upward into the stratosphere and breaks, weakening the polar vortex. The EN-related geopotential height increases over the pole with poleward converging air and decreases over the Yakutsk Basin (YB), indicating a negative northern annular mode with the south-to-north gradient balancing the easterly anomaly that responds to vortex weakening. The converged air at the pole warms adiabatically and raises the height as it sinks. This height distribution, including the east-to-west gradient balancing the southward flow, induces the polar vortex split into two with the colder one in the YB where the EJS is closer than from the pole. The EN-related northwesterly wind directing toward the EJS is also strong. Thus, the coldest air with negative wind stress curls reaches the EJS quickly and forms more ESIW, which converges into the UB, causing the observed decadal isopycnal fluctuations.

Dynamics of the Northern Annular Mode at Weekly Time Scales

Abstract Rapid onsets of positive and negative tropospheric northern annular mode (NAM) events during boreal winters are studied using ERA-Interim datasets. The NAM anomalies first appear in the North Pacific from low-frequency Rossby wave propagation initiated by anomalous convection in the western tropical Pacific around 2 weeks before the peak of the events. For negative NAM, the enhanced convection leads to a zonal acceleration of the Pacific jet, while for positive NAM, the reduced convection leads to a poleward-deviated jet in its exit region. The North Atlantic anomalies, which correspond to North Atlantic Oscillation (NAO) anomalies, are formed in close connection with the North Pacific anomalies via downstream propagation of low-frequency planetary-scale and high-frequency synoptic waves, the latter playing a major role during the last onset week. Prior to positive NAM, the generation of synoptic waves in the North Pacific and their downstream propagation is strong. The poleward-deviated Pacific jet favors a southeastward propagation of the waves across North America and anticyclonic breaking in the North Atlantic. The associated strong poleward eddy momentum fluxes push the Atlantic jet poleward and form the positive NAO phase. Conversely, prior to negative NAM, synoptic wave propagation across North America is significantly reduced and more zonal because of the more zonally oriented Pacific jet. This, together with a strong eddy generation in the North Atlantic, leads to equatorward eddy momentum fluxes, cyclonic wave breaking, and the formation of the negative NAO phase. Even though the stratosphere may play a role in some individual cases, it is not the main driver of the composited tropospheric NAM events.

The Dynamical Influence of Separate Teleconnections from the Pacific and Indian Oceans on the Northern Annular Mode

Abstract The northern annular mode (NAM) influences wintertime climate variability in the Northern Hemisphere, and understanding the processes controlling its sign and amplitude is of critical importance. Mounting evidence supports a robust teleconnection between the El Niño–Southern Oscillation (ENSO) and the NAM, while internal variability generated in the tropical Indian Ocean (TIO) may be associated with a NAM response of the opposite sign. This study uses a coupled ocean–atmosphere model to separate the influence on the NAM from teleconnections driven by ENSO and the TIO. In composites constructed using a long preindustrial control integration, increased December–February precipitation in the central/eastern Pacific drives a negative late-winter NAM response. When isolated from ENSO variability, increased precipitation over the western-central TIO drives a strong and persistent positive NAM response throughout the winter. Opposite linear interference of the anomalous wave teleconnections explains most of the opposite-signed planetary wavedriving of the NAM responses. The case with combined ENSO and TIO variability yields cancellation of the wave interference and a weak NAM response. This mechanism is confirmed using experiments where the tropical ocean is nudged separately over the Pacific and TIO to the large-amplitude 1997/98–1998/99 ENSO cycle. The phases of the Rossby wave and NAM responses in these two cases are of opposite sign, providing strong evidence that internal variability over the TIO can induce teleconnections independent of—and with opposite sign to—those associated with ENSO.

Linear interference and the northern annular mode response to El Niño and climate change

The northern annular mode (NAM) characterizes a significant fraction of wintertime climate variability in the Northern Hemisphere. Understanding the processes governing changes in the NAM on interannual and longer timescales is therefore of critical importance. Previous work reveals a consensus around the negative NAM response to El Nino events (ELN), but considerable disagreement among model projections of the NAM response to climate change (CC), despite the tropical oceans warming in both cases. This work presents numerical simulations with two atmospheric GCMs that reveal robust opposite-signed NAM responses to prescribed sea-surface temperature (SST) anomalies representing ELN (negative NAM) and CC (positive NAM). The primary driver of the sign and amplitude of the NAM response is the linear interference between the planetary waves generated in response to the SST perturbations, and the climatological stationary waves. However, the linear interference framework breaks down in the case involving CC, because the total wave driving of the NAM involves a balance between a strong negative contribution from the eddies related to linear interference, and a strong positive contribution from nonlinear and higher frequency eddies. In addition, the response to CC is less robust in the two models, which may be related to the models’ parameterizations causing differences in the sensitivity to imposed SST anomalies.

Summer Arctic Atmospheric Circulation Response to Spring Eurasian Snow Cover and Its Possible Linkage to Accelerated Sea Ice Decrease

Abstract Anticyclonic circulation has intensified over the Arctic Ocean in summer during recent decades. However, the underlying mechanism is, as yet, not well understood. Here, it is shown that earlier spring Eurasian snowmelt leads to anomalously negative sea level pressure (SLP) over Eurasia and positive SLP over the Arctic, which has strong projection on the negative phase of the northern annular mode (NAM) in summer through the wave–mean flow interaction. Specifically, earlier spring snowmelt over Eurasia leads to a warmer land surface, because of reduced surface albedo. The warmed surface amplifies stationary Rossby waves, leading to a deceleration of the subpolar jet. As a consequence, rising motion is enhanced over the land, and compensating subsidence and adiabatic heating occur in the Arctic troposphere, forming the negative NAM. The intensified anticyclonic circulation has played a contributing role in accelerating the sea ice decline observed during the last two decades. The results here provide important information for improving seasonal prediction of summer sea ice cover.

The influence of natural and anthropogenic factors on major stratospheric sudden warmings

Major stratospheric sudden warmings are prominent disturbances of the Northern Hemisphere polar winter stratosphere. Understanding the factors controlling major warmings is required, since the associated circulation changes can propagate down into the troposphere and affect the surface climate, suggesting enhanced prediction skill when these processes are accurately represented in models. In this study we investigate how different natural and anthropogenic factors, namely, the quasi-biennial oscillation (QBO), sea surface temperatures (SSTs), anthropogenic greenhouse gases, and ozone-depleting substances, influence the frequency, variability, and life cycle of major warmings. This is done using sensitivity experiments performed with the National Center for Atmospheric Research's Community Earth System Model (CESM). CESM is able to simulate the life cycle of major warmings realistically. The QBO strengthens the climatological stratospheric polar night jet (PNJ) and significantly reduces the frequency of major warmings through reduction of planetary wave propagation into the PNJ region. Variability in SSTs weakens the PNJ and significantly increases the major warming frequency due to enhanced wave forcing. Even extreme climate change conditions (RCP8.5 scenario) do not influence the total frequency but determine the prewarming phase of major warmings. The amplitude and duration of major warmings seem to be mainly determined by internal stratospheric variability. We also suggest that SST variability, two-way ocean/atmosphere coupling, and hence the memory of the ocean are needed to reproduce the observed tropospheric negative Northern Annular Mode pattern after major warmings.

Planetary‐scale wave activity as a source of varying tropospheric response to stratospheric sudden warming events: A case study

Stratospheric Sudden Warming (SSW) events are typically, but not always, accompanied by negative Northern Annular Mode anomalies in the troposphere. However, large uncertainties remain as to which dynamical processes are responsible for those anomalies. In order to highlight sources of variability in stratosphere‐troposphere coupling among SSW events, we present a case study of three selected events and show detailed Transformed Eulerian Mean diagnostics for momentum changes in the stratosphere and troposphere in the course of those events. Our results suggest that planetary‐scale waves, especially the zonal wave number 2 component, may play an important role not only for the onset of tropospheric anomalies in response to SSW events but also for introducing variability in the vertical coupling, i.e., whether the tropospheric circulation anomalies lag, lead, or occur simultaneous to the weakening of the vortex. Particularly, the meridional propagation of those waves in the upper troposphere could be an important factor that determines whether SSW events lag or lead tropospheric anomalies.