Positive Phase Of Arctic Oscillation Research Articles

Interdecadal change and projection of the relationship between spring Arctic Oscillation and summer precipitation in the Yangtze River valley in CMIP6 models

This paper assesses the performance of 20 CMIP6 models in simulating the relationship between spring Arctic Oscillation (AO) and summer precipitation in the Yangtze River valley (YRP) over the period 1980–2014. Their relationship during 2015–2100 under SSP2-4.5 is also projected. The assessment indicates that four models (ACCESS-ESM1-5, CMC-CM2-SR5, MRI-ESM2-0, and NorESM2-LM) can reasonably simulate the observed interdecadal weakening of the AO–YRP connection in the late 1990s. During 1980–1998, corresponding to the positive phase of spring AO, the East Asian jet (EAJ) shifts northward in summer, favoring descending anomalies over the Yangtze River valley. Meanwhile, the western Pacific subtropical high is weaker than normal and anomalous northeasterlies prevail in the lower troposphere of the Yangtze River valley, reducing the water vapor transport to the target region. These situations are unfavorable for the occurrence of precipitation, consequently resulting in a decrease in summer YRP. During 1999–2014, however, the association of the above atmospheric circulations with spring AO becomes insignificant, thus diluting the AO–YRP connection. The ensemble of the four models projects that the significant out-of-phase relationship between spring AO and summer YRP will recover in the near term (2015–2040) and weaken again afterwards. Such projected relationships are supported by the changes in the linkage of summer atmospheric circulations to spring AO.摘要评估了20个CMIP6模式对春季北极涛动 (AO) 与长江流域夏季降水 (YRP) 关系的模拟能力. 结果表明, 4个模式 (ACCESS-ESM1-5, CMC-CM2-SR5, MRI-ESM2-0, NorESM2-LM)) 能合理模拟出1990年代后期AO–YRP关系的减弱. 1980–1998年, 当春季AO位于正位相时, 夏季东亚急流北移, 长江流域为异常下沉运动, 同时西太平洋副热带高压减弱, 减少向长江流域的水汽输送, 结果导致降水减少. 1999–2014年, 上述大气环流与春季AO的联系不显著, 从而减弱AO–YRP的关系. 利用这四个模式进一步预估了RCP4.5情景下2015–2100年期间AO–YRP的关系. 两者在2015–2040年为显著负相关关系, 随后再次减弱.

What Determines the Leading Mode of Wintertime Northern Hemisphere Snow-Cover Interannual Variability?

Abstract The Northern Hemisphere snow-cover (NHSC) interannual variability can serve as a crucial source of climate predictability. However, the spatiotemporal characteristics of the NHSC interannual variability and associated formation mechanism lack a thorough investigation using the latest reliable datasets. In this observationally based study, we identify the leading mode of the wintertime NHSC interannual variability and reveal the major determinants and mechanisms for forming the mode. Results show that the leading mode exhibits a hemispheric dipole pattern characterized by snow-cover anomalies in the central-eastern United States and Europe, contrary to those in the Tibetan Plateau (TP). Importantly, the leading mode of TP-only snow-cover variability is essentially a regional manifestation of the dipole NHSC anomaly pattern. The wintertime hemispheric atmospheric circulation anomaly mode, that is, the Arctic Oscillation (AO), primarily determines the dipole NHSC anomaly pattern via altering snowfall and snowmelt. During a positive phase of AO, an anomalous atmospheric anticyclone over the midlatitude North Atlantic favors increased warm-air advection over the central-eastern United States and Europe, reducing the snowfall and the snow cover there. Meanwhile, the anomalously warm atmospheric thermal condition in Europe accompanied by enhanced westerly moisture flux convergence tends to heat the underlying land surface via reducing upward sensible heat flux and increasing downward longwave radiation, thereby causing increased snowmelt and decreased snow cover in northeastern Europe. Associated with the aforementioned atmospheric circulation anomalies, an anomalous atmospheric cyclone prevailing over the TP tends to cause increased southeasterly moisture flux convergence, favoring snowfall and snow increase in the TP.

Wintertime sea surface temperature variability modulated by Arctic Oscillation in the northwestern part of the East/Japan Sea and its relationship with marine heatwaves

The northwestern part of the East/Japan Sea (EJS) is a region with large sea surface temperature (SST) variability and is known as a hotspot of marine heatwaves (MHW) stress for marine environments that peaked in boreal winter (January-February-March). This could have profound impacts on the marine ecosystems over the EJS. Here, we used a set of high-resolution satellite and reanalysis products to systematically analyze the spatiotemporal SST variations and examine their linkage to a large-scale mode of climate variability, such as the Arctic Oscillation (AO). The results show that AO-related wind forcing modulates the SST variability over the EJS via the oceanic dynamic adjustment processes. In particular, the abnormally warm SSTs in the northwestern part of the EJS are driven by the anomalous anticyclonic eddy-like circulation and Ekman downwelling during a positive AO phase. This physical linkage between a positive AO and the abnormally warm SST could be conducive to MHW occurrences in the EJS as in the extremely positive AO event during the winter of 2020. These results have implications that the MHW occurrences in the EJS could be amplified by natural climate variability along with long-term SST warming.

The bimodality of the East Siberian fast ice extent: mechanisms and changes

Abstract Using operational sea-ice maps, we provide first insight into the seasonal evolution of fast ice in the East Siberian Sea for the period between 1999 and 2021. The fast ice season tends to start later by 4.7 d per decade and to end earlier by 9.7 d per decade. As a result, there is a trend towards a shorter length of fast ice season by 2 weeks per decade. The analysis of air temperatures indicates that onset and end of the fast ice season are largely driven by thermodynamic processes. Two spatial modes (large, L-mode and small, S-mode) of East Siberian fast ice cover which have significant areal differences were distinguished. The occurrence of L- and S-modes was linked to the polarity of the Arctic Oscillation (AO) index. Negative AO phase leads to increased sea-ice convergence in the region, which in turn favours sea-ice grounding and promotes the development of large fast ice extent (L-mode). Lower deformation rates in the region during positive AO phase does not allow the formation of grounded features which results in small fast ice extent (S-mode). An analysis of sea-ice divergence confirms that L-mode seasons are characterised by higher on-shore convergence compared with S-mode seasons.

Two types of cold waves affecting northeast China and the corresponding different key regions of precedent sea ice and sea surface temperature

AbstractBased on the daily minimum temperature data from 199 meteorological stations in northeast China (NEC), the cold wave (CW) processes in winter from 1980 to 2020 are identified. Then, by using the reanalysis 500‐hPa geopotential height data provided by the European Centre for Medium‐Range Weather Forecasts (ECMWF), the CWs are objectively classified into two types (abbreviated as Type 1 and Type 2) according to the corresponding atmospheric circulation through the hierarchical cluster algorithm (HCA). The relationships of the CWs with the Arctic sea sic (SIC) and sea surface temperature (SST) are revealed. For Type 1, the SIC in the north of the Barents Sea–Kara Sea is relatively less in the previous autumn, and a wave train propagating from the polar regions to Northeast Asia is stimulated under the ice–atmosphere interaction. This wave train further cooperates with the positive SST anomaly in the southern Indian Ocean to excite the low‐vortex circulation under the negative phase of Arctic Oscillation (AO) in winter, which is conducive to the occurrence of Type 1. However, the relatively low SST in the western Pacific warm pool in previous autumn stimulates a northward propagating wave train along the meridian, which increases the degree of meridionality of the atmospheric circulation over East Asia and thus indirectly causes the polar‐vortex‐splitting circulation under the positive AO phase, which favours the formation of Type 2. The numerical simulations can well reproduce the observational facts, further confirming the possible impact of the SIC and SST in the previous autumn on the two types of CW over the NEC.

On the Relationship of Arctic Oscillation with Atmospheric Rivers and Snowpack in the Western United States Using Long-Term Multi-Platform Dataset

Atmospheric rivers (ARs) are narrow bands of enhanced integrated water vapor transport, modulated by large-scale and synoptic-scale variability. Here, we investigate how ARs and snowpack are shaped by large-scale variability such as arctic oscillation (AO) by examining the synoptic conditions and characteristics of ARs and snowpack in the different phases of AO. Using Integrated Multi-Satellite Retrievals for Global Precipitation Measurement (IMERG) data, Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA2) reanalysis data, and in-situ observation data over the eastern Pacific and western United States. we found that more precipitation is observed in lower latitudes (35° N–45° N) during negative AO months and farther north (north of 45° N) in latitude during positive AO months. These are associated with wavelike synoptic patterns in negative AO months and more straight-line type synoptic patterns in positive AO months. The different phases of AO also modulate the AR characteristics: 2.6% less intense (5.3% more intense) integrated water vapor transport and total precipitation, and 16.0% shorter (21.1% longer) duration of ARs than the climatological mean (1980–2019) for positive AO (negative AO) phase. AR frequency is also higher (~50.4%) than the climatological mean for negative AO phase, but there is no statistically significant difference between either negative AO or positive AO phase, especially in southern California. In addition, the snow water equivalent (SWE) tends to be reduced in the positive AO phase and under high-temperature conditions, especially in recent years (2010s). The similar relationships are found in the early 1990s and 2000s, but their statistical significances are low. Considering that lower atmospheric temperature keeps increasing over the eastern Pacific and the western U.S., and SWE tends to be reduced in the positive AO phase in recent years, SWE may decrease over northern California if the warming condition persists. These findings highlight how the characteristics of local extreme weather can be shaped by large-scale climate variability.

Subseasonal Reversal of Winter Temperature Over Northeast China in 2014/2015: Role of Arctic Sea Ice

This study investigates the temperature reversal over Northeast China (NEC) in winter 2014/2015, focusing on the variations of related general circulations and the affecting mechanisms of the Arctic sea ice on daily scale. It turns out to be the coupled impacts of the eastward propagations of tropospheric wave trains from the North Atlantic Ocean and the downward reflections of planetary wavenumber-1 from the stratosphere that resulted in the subseasonal reversal of winter temperature over NEC in winter 2014/2015. Also, such anomalous atmospheric circulations can be attributed to sea-ice anomalies over the Davis Strait–Baffin Bay (SIC-DSBB) and the Barents–Kara Sea (SIC-BKS) in November 2014. SIC-DSBB anomalies in November 2014 excited the eastward propagation of Rossby waves via the tripole pattern of sea surface temperature over the North Atlantic Sea, leading to the colder condition over NEC in the first and middle 10 days of December 2014. Anomalously heavy SIC-BKS also triggered wave trains from the polar region to Eurasia, strengthening the Rossby wave induced by SIC-DSBB. Moreover, the wave trains suppressed the upward propagation of planetary wavenumber-1 over the Siberia region, strengthening the stratospheric polar vortex. However, the sea-ice anomalies over these two domains only existed in November 2014. Thus, the tropospheric mechanisms by which the Arctic sea ice affected the temperature over NEC only lasted to late December 2014. Meanwhile, the stratospheric anomalies propagated downwards with the planetary wavenumber-1, favoring the positive phase of Arctic Oscillation in the troposphere and the warm condition over NEC since the last week of December 2014. Consequently, the temperature over NEC reversed in winter 2014/2015.

Arctic sea ice anomalies during the MOSAiC winter 2019/20

Abstract. During the winter of 2019/2020, as the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) project started its work, the Arctic Oscillation (AO) experienced some of its largest shifts, ranging from a highly negative index in November 2019 to an extremely positive index during January–February–March (JFM) 2020. The permanent positive AO phase for the 3 months of JFM 2020 was accompanied by a prevailing positive phase of the Arctic Dipole (AD) pattern. Here we analyze the sea ice thickness (SIT) distribution based on CryoSat-2/SMOS satellite-derived data augmented with results from the hindcast simulation by the fully coupled Regional Arctic System Model (RASM) from November 2019 through March 2020. A notable result of the positive AO phase during JFM 2020 was large SIT anomalies of up to 1.3 m that emerged in the Barents Sea (BS), along the northeastern Canadian coast and in parts of the central Arctic Ocean. These anomalies appear to be driven by nonlinear interactions between thermodynamic and dynamic processes. In particular, in the Barents and Kara seas (BKS), they are a result of enhanced ice growth connected with low-temperature anomalies and the consequence of intensified atmospherically driven sea ice transport and deformations (i.e., ice divergence and shear) in this area. The Davies Strait, the east coast of Greenland and the BS regions are characterized by convergence and divergence changes connected with thinner sea ice at the ice borders along with an enhanced impact of atmospheric wind forcing. Low-pressure anomalies that developed over the eastern Arctic during JFM 2020 increased northerly winds from the cold Arctic Ocean to the BS and accelerated the southward drift of the MOSAiC ice floe. The satellite-derived and simulated sea ice velocity anomalies, which compared well during JFM 2020, indicate a strong acceleration of the Transpolar Drift relative to the mean for the past decade, with intensified speeds of up to 6 km d−1. As a consequence, sea ice transport and deformations driven by atmospheric surface wind forcing accounted for the bulk of the SIT anomalies, especially in January 2020 and February 2020. RASM intra-annual ensemble forecast simulations with 30 ensemble members forced with different atmospheric boundary conditions from 1 November 2019 through 30 April 2020 show a pronounced internal variability in the sea ice volume, driven by thermodynamic ice-growth and ice-melt processes and the impact of dynamic surface winds on sea ice formation and deformation. A comparison of the respective SIT distributions and turbulent heat fluxes during the positive AO phase in JFM 2020 and the negative AO phase in JFM 2010 corroborates the conclusion that winter sea ice conditions in the Arctic Ocean can be significantly altered by AO variability.

Asymmetrical Trends of Burned Area Between Eastern and Western Siberia Regulated by Atmospheric Oscillation

AbstractWildfires in Siberian cause organic carbon released from Arctic ecosystems to the atmosphere, which further enhances greenhouse effects. However, the trends of Siberian wildfires and their mechanisms are not well understood. We show that the burned fraction trends were opposite in the eastern and western Siberia. The contrasting changes are explained by vapor pressure deficit and soil moisture inferring atmospheric and ground aridity conditions, respectively. The difference in aridity change in the eastern and western Siberia is regulated by increased positive phase of Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO), which leads to reduced moisture transport to the east but increased transport to the west from the North Atlantic and Arctic Ocean mainly during May–August. The growing trend of positive phase of AO and NAO will continuously lead to the asymmetrical trends of aridity condition, which could enhance wildfire in the eastern Siberia.

Controlling Factors of Historical Variation of Winter Tibetan Plateau Snow Cover Revealed by Large‐Ensemble Experiments

AbstractThis study investigated the factors controlling the interannual variability of Tibetan Plateau snow cover (TPSC) in winter using large‐ensemble simulations. Composite analysis reveals that years with high TPSC are associated with a positive Arctic Oscillation (AO)‐like pattern in which a strengthened subtropical westerly enhances zonal water vapor flux, resulting in greater precipitation over the Tibetan Plateau. Observations and simulations commonly show that TPSC tends to be high during the positive phase of AO, and vice versa. Meanwhile, the magnitude of the contribution of tropical sea surface temperature (SST) to TPSC is different based on observations and simulations. Observations indicate that TPSC is insensitive to El Niño/La Niña events, whereas the simulated ensemble mean TPSC is correlated strongly with Niño 3.4 SST index. In El Niño years, higher specific humidity and more numerous synoptic disturbances are found around the Tibetan Plateau. As several ensemble members show features close to those observed, the observed insensitivity of TPSC to El Niño–Southern Oscillation (ENSO) events is considered attributable to the limited sample size of the observations. This is corroborated in the analysis for a relatively short period, in which the observed TPSC is sensitive to ENSO. Comparison of the results of the historical simulations with those of the simulations without warming suggests that global warming has reduced the snow‐to‐rain ratio over the Tibetan Plateau, which is compensated by the overall increase in precipitation. Consequently, the impact of global warming on TPSC was negligibly weak until the 2000s.

Contrast Relationships Between Arctic Oscillation and Ozone in the Stratosphere Over the Arctic in Early and Mid‐to‐Late Winter

AbstractPrevious studies paid more attention on the winter‐mean relationship between the surface Arctic Oscillation (AO) and stratospheric ozone over the Arctic. However, few studies focused on the subseasonal relationship between the stratospheric ozone and surface AO in boreal winter. We investigated the subseasonal relationship between the AO and stratospheric ozone over the Arctic in each boreal winter month during 1980–2017 that is independent on the El Niño–Southern Oscillation and Quasi‐Biennial Oscillation signals using Modern‐Era Retrospective Analysis for Research and Applications version 2 reanalysis data sets. Results showed that the positive AO phases correspond to two negative anomalous ozone centers located in the middle stratosphere (∼30 hPa) and upper troposphere–lower stratosphere over the Arctic in each month. There is an in‐phase relationship between the AO and the Arctic ozone at 70–100 hPa in December, which is opposite to the out‐of‐phase relationship between these two metrics in mid‐to‐late winter. In December, the subtropical jet in the Pacific under the positive AO phase shifts poleward with strengthened planetary wavenumber‐2 waves in the lower stratosphere. The strengthened wavenumber‐2 wave flux contributes to the positive ozone anomalies at 70–100 hPa via modifying the asymmetric component of polar vortex. However, in January and February, the subtropical jet weakens during the positive AO years, along with less planetary wavenumber‐1 waves propagating into the Arctic lower stratosphere. This weakened wavenumber‐1 wave tends to result in the negative ozone anomalies via weakening the downwelling branch of Brewer–Dobson circulation and strengthening the stratospheric Arctic vortex.

Characteristics and causes of surface wind speed variations in Northwest China from 1979 to 2019

New characteristics of surface wind speed variations in Northwest China and the possible explanations for them have remained undetermined over the recent decades. This study presents the characteristics and possible mechanisms of surface wind speed variations in Northwest China from 1979 to 2019. The results show that the surface wind speed in Northwest China underwent a two-stage change rather than a persistent decline from 1979 to 2019; specifically, it declined significantly before the early 2000s and then increased gradually thereafter. The meridional pressure gradient decrease caused by the meridional temperature gradient reduction corresponded to a decrease in the winter surface wind speed in Northwest China during 1979–2002. The meridional pressure gradient increase caused by the meridional temperature gradient increase was consistent with an increase in the winter surface wind speed in Northwest China during 2003–2019. This suggests that the spatially inhomogeneous variations in surface air temperature in Eurasia's mid-high latitudes may be the main reason for the surface wind speed variations in Northwest China. Furthermore, the decadal strengthening of the East Asian winter monsoon (EAWM) may have resulted in an upward trend of the surface wind speed in Northwest China after the early 2000s by strengthening the Siberian High (SH). Although the variations in the surface wind speed in Northwest China are inconsistent with those in the winter Arctic Oscillation (AO) index, there is a significant negative correlation between them on the interannual timescale. With a negative (positive) AO phase, the surface wind speed in Northwest China increases (decreases) in winter.

Different Relationships between Arctic Oscillation and Ozone in the Stratosphere over the Arctic in January and February

We compare the relationship between the Arctic Oscillation (AO) and ozone concentration in the lower stratosphere over the Arctic during 1980–1994 (P1) and 2007–2019 (P2) in January and February using reanalysis datasets. The out-of-phase relationship between the AO and ozone in the lower stratosphere is significant in January during P1 and February during P2, but it is insignificant in January during P2 and February during P1. The variable links between the AO and ozone in the lower stratosphere over the Arctic in January and February are not caused by changes in the spatial pattern of AO but are related to the anomalies in the planetary wave propagation between the troposphere and stratosphere. The upward propagation of the planetary wave in the stratosphere related to the positive phase of AO significantly weakens in January during P1 and in February during P2, which may be related to negative buoyancy frequency anomalies over the Arctic. When the AO is in the positive phase, the anomalies of planetary wave further contribute to the negative ozone anomalies via weakening the Brewer–Dobson circulation and decreasing the temperature in the lower stratosphere over the Arctic in January during P1 and in February during P2.

Global teleconnections in droughts caused by oceanic and atmospheric circulation patterns

Long-duration droughts are usually tied to persistent local or remote forcings; for example, persistent droughts over California are frequently observed along with the ‘ridiculously resilient ridge’ over the West Coast. It is now evident that some oceanic forcings (e.g. El Niño–Southern Oscillation) have global reaches and affect multiple regions concurrently during their progression. Here, we show robust significant temporal concordancy of persistent droughts in many regions, revealing multiple teleconnections (distant regions experiencing droughts concurrently), such as the ‘Western North America–Mediterranean (WNA–MED)’ and the ‘Southeast Asia–Southern Africa (SEA–SAF)’ teleconnections. Composite pressure and sea surface temperature anomalies during concurrent droughts in WNA and the MED reveal a persistent weather regime that resembles the positive phase of Arctic Oscillation and negative phase of Pacific Decadal Oscillation. During concordant droughts of SEA and SAF, composite pressure anomalies remarkably resemble the El Niño pattern, which we infer as the leading cause of the teleconnection. The insights gained here offer a new dimension to understanding droughts and improving their long-term predictability.

Spatial-temporal evolution and driving forces of rainfall erosivity in a climatic transitional zone: A case in Huaihe River Basin, eastern China

Rainfall erosivity (R-factor) is a crucial natural factor in assessing soil loss by water erosion. Understanding the spatial–temporal variation of R-factor and its driving forces is essential to reduce soil and water loss as well as formulate a regional ecological restoration plan. In this study, the Huaihe River Basin (HRB), a typical climatic transitional zone of eastern China, was selected as the study region. Using daily precipitation data of 36 national standard meteorological stations in the HRB during 1960–2014, the temporal variation and spatial pattern of R-factor were examined. To explore the influence of large-scale atmospheric circulations, three climatic indicators – the tropical Pacific sea surface temperature (SST), the East Asian summer monsoon (EASM), and the Arctic Oscillation (AO) – were analyzed with R-factor by the cross-wavelet transform. Results indicated that: (1) Average annual R-factor was 5339.64 MJ∙mm∙hm−2∙h−1 with an insignificantly increasing trend. Summer accounted the most of seasonal R-factor, followed by autumn and spring. Only the winter R-factor presented a significantly increasing trend. (2) The spatial gradient of R-factor decreased from south to north in the HRB. The common high value of R-factor appeared in the Dabie Mountain. In summer, another high value of R-factor mainly concentrated in the coastal of HRB. (3) Climatic indicators had a strong influence on R-factor in this region. The non-stationary feature was reflected by the resonant period between R-factor and climatic indicators. It should be noticed that the winter R-factor is increasing when the positive phase of AO in the long-term period. This study is helpful to understand the variation of rainfall erosivity in the climatic transitional zone of eastern China.

Diverse influences of spring Arctic Oscillation on the following winter El Niño–Southern Oscillation in CMIP5 models

This study evaluates the ability of 35 climate models, which participate in the Coupled Model Intercomparison Project Phase 5 (CMIP5) historical climate simulations, in reproducing the connection between boreal spring Arctic Oscillation (AO) and its following winter El Nino–Southern Oscillation (ENSO). The spring AO–winter ENSO correlations range from − 0.41 to 0.44 among the 35 models for the period of 1958–2005. Ensemble means of the models with positive and negative AO–ENSO correlations both show strong spring sea surface temperature (SST) cooling in the subtropical North Pacific during a positive phase of spring AO, which is conducive to occurrence of a La Nina event in the following winter. However, the models with positive AO–ENSO relations produce a pronounced spring cyclonic anomaly over the subtropical northwestern Pacific and westerly anomalies over the tropical western Pacific (TWP). These westerly wind anomalies bring SST warming and positive precipitation anomalies in the tropical central-eastern Pacific (TCEP) during the following summer, which would maintain and develop into the following winter that support an El Nino-like pattern in the TCEP via a positive air-sea feedback mechanism. By contrast, the models with negative AO–ENSO connections fail to reproduce the spring AO-related cyclonic anomaly over the subtropical northwestern Pacific and westerly wind anomalies in the TWP. Thus, these models would produce a La Nina-like pattern in the subsequent winter. Difference in the spring AO-associated atmospheric anomalies over the subtropical North Pacific among the CMIP5 models may be attributed to biases of the models in simulating the spring climatological storm track.

Modulation of ENSO Teleconnection on the Relationship between Arctic Oscillation and Wintertime Temperature Variation in South Korea

Despite progressing global warming, extreme cold events in East Asia are still occurring frequently with temperature variability enhanced. To understand this situation, it is necessary to determine external and internal climatic factors and their modulation effects that influence regional temperature variability. We found that the positive correlation between Arctic Oscillation (AO) and surface air temperature (SAT) in South Korea during winter is modulated strongly by tropical influences associated with El Niño Southern Oscillation (ENSO). In the case of a negative (positive) SAT anomaly in South Korea during the positive (negative) AO phase, a state that is opposite to the typical relationship between AO and SAT, the tropical sea surface temperature shows a typical negative (positive) ENSO-like pattern. The atmospheric teleconnection associated with the negative (positive) ENSO conditions contributes to a deepening (flattening) of the climatological East Asian trough and an enhancing (weakening) of the East Asian jet, which leads to negative (positive) SAT anomalies in South Korea. This modulation effect is robustly observed in the historical simulations of three different models of CMIP5.

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.

Variability in observed snow depth over China from 1960 to 2014

AbstractSnow depth is a critical variable that reflects snow variability and has an important impact on the energy and hydrological cycles. However, snow depth changes in response to regional climate warming have not been quantified in detail across China. Here, we investigated the variability in observed snow depth based on a recently released dataset collected at ~1,500 stations from 1960 to 2014. To address the non‐normal distribution of daily snow depth data, the annual cumulative amount of snow depth (CASD) is employed, and the trend is calculated and tested for the change point and field significance. The results showed that CASD experienced a slight increase of 1.75%·decade−1in China. An increasing trend of CASD was observed in Northeastern and Northwestern China, a decreasing trend was observed in the eastern Tibetan Plateau, Loess Plateau, and Yangtze River Basin, and these changes were more remarkable after the 2000s. Since the 2000s, the increased CASD in Northwestern and Northeastern China has been accompanied by a significant decrease in winter temperature and a sharply increasing trend in snowfall events in the northeastern region. The decreased CASD in the Eastern Tibetan Plateau and Loess Plateau after the 2000s was accompanied by a decrease in snowfall events and an increase in winter temperature. Large‐scale atmospheric circulation, such as the Arctic Oscillation (AO), has a remarkable regional effect on the variability in snow depth. Statistically significant negative correlations were found in the northern part of China (i.e., positive AO phase with reduced CASD), while significant positive correlations were shown in the middle part of China (i.e., positive AO phase with enhanced CASD). Furthermore, China experienced an obviously shorter snow season during the whole period (decreased by 3.87%·decade−1), jointly resulting from later starting dates and earlier ending dates.

Modulation of the Westerly and Easterly Quasi-Biennial Oscillation Phases on the Connection between the Madden–Julian Oscillation and the Arctic Oscillation

Previous studies have revealed the relationship between the Madden–Julian oscillation (MJO) and the Arctic Oscillation (AO). The MJO phase 2/3 is followed by the positive AO phase, and the MJO phase 6/7 is followed by the negative AO phase. This study reveals that the MJO phase 6/7–AO connection is modulated by the Quasi-Biennial Oscillation (QBO) through both tropospheric and stratospheric pathways during boreal winter. The MJO 2/3 phase and AO relationship is favored in both QBO easterly (QBOE) and westerly (QBOW) years because of the MJO-triggered tropospheric Rossby wave train from the tropics toward the polar region. The AO following the MJO 6/7 phase shifts to negative in QBOW years, but the MJO–AO connection diminishes in QBOE years. In QBOW years, the Asian-Pacific jet is enhanced, leading to more evident poleward propagation of tropospheric Rossby wave train, which contributes to the tropospheric pathway of the AO–MJO 6/7 connection. Besides, the enhanced Asian-Pacific jet in QBOW years is favorable for vertical propagation of planetary waves into the stratosphere in MJO phase 6/7, leading to negative AO, which indicates the stratospheric pathway of the AO–MJO 6/7 connection.