Scandinavian Pattern Research Articles

Dominant modes of interannual variability in spring compound dry and hot events over Northern Asia and the possible mechanisms

In this study, spatial and temporal variations of spring compound dry and hot events (CDHEs) over northern Asia (NA) during 1950–2020 and the related possible mechanisms are investigated on the interannual timescale. The standardized compound event indicator (SCEI) is used to represent compound dry and hot conditions over NA, which is validated by the observed summer case of CDHEs over western Russia in 2010. The first empirical orthogonal function (EOF1) mode of the SCEI over NA presents a monopole pattern, while the EOF2 mode shows an east-west dipole pattern. Possible mechanism analysis indicates that the CDHEs tend to be modulated by local high-pressure anomalies, accompanied by reduced cloud cover and more solar radiation. The high-pressure anomalies can lead to warm temperature and water vapor divergence, which synergistically contributes to the occurrence of CDHEs. Further analysis shows that the EOF1 mode is jointly affected by the Scandinavian (SCAND) pattern and the Arctic Oscillation (AO); while the EOF2 mode is influenced by the Atlantic-Eurasian (AEA) teleconnection. Moreover, the North Atlantic tripolar pattern of sea surface temperature (SST) can influence the EOF1 mode through changing the AO. And the diagonal tripolar SST pattern over the North Atlantic affects the EOF2 mode via altering the AEA pattern. The influence of the SST patterns is further confirmed by numerical experiments using the Community Atmospheric Model version CAM-5.3.

Annual and seasonal mean daily discharge in natural and regulated rivers in Northern Finland: variability, trends, and links to climate teleconnections

ABSTRACT This study aimed to investigate long-term (1969–2023) variability and trends in annual mean daily discharge (AMDD) and seasonal mean daily discharge (SMDD) in 10 rivers throughout Northern Finland in connection to climate teleconnections. The lowest AMDDs were mostly recorded during the first 12 years (1969–1980) of our study period, while the highest ones were during the last 12 years (2012–2023). Significant trends in AMDD were found only in three natural rivers of Tornionjoki, Simojoki, and Kuivajoki during 1969–2023. Such variations and trends in AMDD were most significantly associated with the Scandinavia (SCA) pattern, which is also an influential climate teleconnection for annual precipitation anomalies across Northern Finland. The highest (lowest) SMDDs were experienced in spring (winter). Only increasing trends in SMDD were statistically significant (p < 0.05). Such substantial increases in SMDDs were detected in winter, spring, and summer (autumn) in association with the Arctic Oscillation or AO and the North Atlantic Oscillation or NAO (East Atlantic/West Russia or EA/WR). Generally speaking, recent milder and wetter climatic conditions in association with strong positive AO and NAO (negative EA/WR) phases could increase SMDD for winter, spring, and summer (autumn) seasons. Hence, both AMDD and SMDD in rivers throughout Northern Finland were primarily influenced by precipitation.

Atmospheric Teleconnections Responsible for the Dominant Patterns of Interannual Variability in Summer Drought over Northern Asia

Abstract Summer drought over northern Asia (NA) seriously threatens the local fragile ecological environment and social economy development. In this study, using the standardized precipitation evapotranspiration index (SPEI), we first identify the dominant modes of interannual variability in summer drought condition over NA and then explore the atmospheric patterns responsible for the formation of the modes. The results show that the first empirical orthogonal function (EOF1) mode of summer SPEI over NA exhibits a meridional dipole pattern, which is influenced primarily by the polar–Eurasian teleconnection (POL) and circumglobal teleconnection (CGT) patterns. Under the influence of negative POL and positive CGT patterns, there is an anomalous anticyclone (cyclone) over northwestern Siberia (Lake Baikal to Northeast China). Such atmospheric circulations lead to meridional dipole patterns in air temperature, moisture condition, vertical motion, and cloud cover over NA, favoring decreased (increased) precipitation and increased (decreased) potential evapotranspiration over northern (southern) NA, finally contributing to the formation of EOF1. The second EOF (EOF2) shows an approximate zonal dipole pattern, which is influenced by the British–Baikal Corridor (BBC) and Scandinavia (SCA) teleconnection patterns. The positive BBC and SCA patterns can lead to an anomalous anticyclone over the Ural Mountains and a cyclone over Lake Baikal. Such atmospheric circulations result in a zonal dipole pattern in precipitation and potential evapotranspiration over NA through changing the local moisture condition, air temperature, and radiation, consequently favoring the formation of EOF2. Fitting analysis indicates that the aforementioned atmospheric factors can explain 76% (55%) of the interannual variability of EOF1 (EOF2).

Precipitation-induced abrupt decrease of Siberian wildfire in summer 2022 under continued warming

Wildfires in Northeast (NE) Siberia have become more frequent owing to the warming climate, exerting a profound impact on the global carbon cycle. While an increase in global temperature is recognized as a primary driver of unprecedented wildfires, the role of precipitation during wildfire season is relatively unexplored. Here, we present evidence that an increase in summer precipitation led to a sudden decrease in NE Siberian wildfires, especially in 2022, notwithstanding the persistent warming trend in the northern high latitudes. The interannual variability of summer precipitation, linked to the large-scale atmospheric circulation, known as the Scandinavia (SCAND) pattern, significantly impacts the regulation of wildfires. Climate models project enhanced variability in summer precipitation, potentially amplifying year-to-year fluctuations in wildfire occurrences. The interplay between the temperature and precipitation patterns in NE Siberia under ongoing warming may increase the occurrence of extreme wildfires, leading to a substantial release of carbon and further contributing to climate warming.

Atlantic origin of the increasing Asian westerly jet interannual variability

The summer Eurasian westerly jet is reported to become weaker and wavier, thus promoting the frequent weather extremes. However, the primary driver of the changing jet stream remains in debate, mainly due to the regionality and seasonality of the Eurasian jet. Here we report a sharp increase, by approximately 140%, in the interannual variability of the summertime East Asian jet (EAJ) since the end of twentieth century. Such interdecadal change induces considerable changes in the large-scale circulation pattern across Eurasia, and consequently weather and climate extremes including heatwaves, droughts, and Asian monsoonal rainfall regime shifts. The trigger mainly emerges from preceding February North Atlantic seesaw called Scandinavian pattern (contributing to 81.1 ± 2.9% of the enhanced EAJ variability), which harnesses the “cross-seasonal-coupled oceanic-atmospheric bridge” to exert a delayed impact on EAJ and thus aids relevant predictions five months in advance. However, projections from state-of-the-art models with prescribed anthropogenic forcing exhibit no similar circulation changes. This sheds light on that, at the interannual timescale, a substantial portion of recently increasing variability in the East Asian sector of the Eurasian westerly jet arises from unforced natural variability.

Impact of the Eurasian Zonal Circulation on the Interannual Variability of Winter Surface Air Temperature and Subseasonal Temperature Reversal in North China

AbstractThis study focuses on the interannual variability of winter mean surface air temperature (SAT) and subseasonal SAT reversal in North China, which have profound impacts on national life and economic activities. The analysis explores the temporal and spatial distribution characteristics of these variations and their relationship with the Eurasian Zonal Circulation (EZ). The findings reveal distinct interdecadal changes in North China’s winter mean SAT, along with a general subseasonal reversal of cold and warm conditions. In comparison to the North Atlantic oscillation/Arctic Oscillation, the EZ has a more significant influence on the interannual variability of winter and subseasonal reversals. Strong EZ years are associated with the weakening of the Siberian High and the Ural Blocking High, causing the East Asian Trough to move eastward, which leads to the retention of cold air from the north and increased SAT in North China. Conversely, weak EZ years exhibit opposite circulation patterns. The winter mean EZ is influenced by preceding autumn Arctic sea ice concentrations (SIC). Reduced autumn SIC results in decreased heat flux and reduced eddy energy, leading to substantial attenuation of the winter storm track across Scandinavia and Western Asia. This attenuation, mediated through the interaction between eddies and mean flow, triggers a slowdown of Eurasian zonal circulation and intensification of the Siberian High. These conditions favor a positive phase of the winter Scandinavia pattern, facilitating the transport of cold Arctic air into North China. Likewise, subseasonal reversals in SIC influence corresponding alterations in the EZ.

Energy budget characteristics of the subseasonal Scandinavian pattern

Based on daily Japanese 55-year reanalysis data, this study analysed the daily energy budget characteristics of 94 Scandinavian (SCA) events in the positive phase during 62 winters from 1958 to 2020. Low-frequency circulation anomalies characterize the SCA events. The maintenance of low-frequency anomalies mainly comes from the extraction of both available potential energy (APE) and kinetic energy (KE) from the background field during both the developing stage and the decaying stage. This energy conversion characteristic is related to the “locking” effect of the background field. During the developing stage, the interactions among the low-frequency circulation anomalies continuously generate KE, which is the main source for initiating the primary circulation anomalies around western Europe and the Scandinavian Peninsula. Meanwhile, the horizontal KE advection maintains the KE of the circulation anomalies around Lake Baikal. The decay process of the SCA events is mainly related to the damping effect on the APE due to diabatic heating and the interactions among the low-frequency circulation anomalies. The SCA event is a type of atmospheric internal dynamic pattern.

Regime shift of the leading mode of winter surface wind speed in northwest China in 2003/04 and its possible link with Barents–Kara sea ice

AbstractUsing the ERA‐5 reanalysis datasets, NCEP/NCAR sea ice concentration (SIC) and observed winter surface wind speed (WSWS) in northwest China during 1979–2020, this study examined the regime shift of the leading mode of observed WSWS in northwest China and investigated its possible link with the winter Barents–Kara Sea (BKS) SIC. The leading mode of observed WSWS in northwest China showed negative anomalies in southern and northeastern Xinjiang, Gansu, western and northern Shaanxi Provinces, and positive anomalies in northwestern and middle Xinjiang, Qinghai and Ningxia Provinces. The signs of WSWS in the leading mode is generally opposite to their trends during 1979–2020. Leading principal component (PC1) of observed WSWS in northwest China underwent an obvious decreasing regime shift in 2003/04, as well as the winter BKS SIC. There are significant correlations between PC1 and BKS SIC on both interdecadal and interannual timescales. Decadal decrease of BKS SIC is accompanied with a positive phase of Scandinavia pattern (SP) with positive anomalies of 500‐hPa geopotential height over subtropical North Atlantic, Arctic‐Ural region and southern Asia, and negative anomalies over northern North Atlantic‐West Europe and Siberia regions. BKS SIC and SP significantly influenced the Northern China Wind Index (NCWI) at 200 hPa on interdecadal timescale, which was related to the PC1 on both interdecadal and interannual timescales after removing the signal of Arctic Oscillation. Meridional configuration of Siberian cyclonic anomaly and southern Asian anticyclonic anomaly associated with the winter BKS SIC could enhance the intensity of NCWI, which favoured the increase of observed WSWS in northwest China and thereby may contribute to the regime shift in 2003/04 of the leading mode. However, there were still some stations showing decreasing regime shift, which mainly located at the region of steeping terrain. These decreasing changes of WSWS may be related to circumfluence effect of terrain, urbanization and local microclimate, and these reasons still need further detailed study.

North Atlantic atmospheric circulation indices: Links with summer and winter temperature and precipitation in north‐west Europe, including persistence and variability

AbstractVariability in seasonal weather in north‐west Europe is substantially determined by jet stream variability. The North Atlantic Oscillation (NAO) has been well studied as a key representation of this jet stream variability, but other circulation indices are also important. Here the first three principal component empirical orthogonal functions (EOFs) of 500 hPa geopotential height (GPH), which broadly correspond to the NAO, the East Atlantic pattern (EA) and Scandinavian pattern (SCA), as well as jet speed and latitude, are correlated with temperature and precipitation anomalies over Europe with a focus on north‐west Europe, as well as measures of persistence and variability. In high summer (July and August), all three of the principal EOFs are significantly correlated with extreme temperatures in large areas of northern Europe. In winter, for much of north‐west Europe, both temperatures and precipitation are positively correlated with the jet speed, and precipitation is negatively correlated with EOF3. There is some non‐stationarity in some of the relationships, notably between winter precipitation and EOF1, and between July/August precipitation and EOF2. In addition to single variate correlations, multiple correlation coefficients are also used to determine areas of significant correlation when combining two or three of the circulation indices. The multiple correlation analyses show that combining the three EOFs produces significant correlations with temperature and precipitation over much of Europe. These analyses provide scope for using seasonal forecasts to predict likely temperature and precipitation anomalies based on predicting the atmospheric circulation anomalies and downscaling them. Improved seasonal forecasts of temperature and precipitation, including persistence and variability, will be useful to a number of users, such as agrifood, transport, energy supply and insurance.

Probabilistic seasonal forecasts of North Atlantic atmospheric circulation using complex systems modelling and comparison with dynamical models

AbstractDynamical seasonal forecast models are improving with time but tend to underestimate the amplitude of atmospheric circulation variability and to have lower skill in predicting summer variability than in winter. Here, we construct Nonlinear AutoRegressive Moving Average models with eXogenous inputs (NARMAX) to develop the analysis of drivers of North Atlantic atmospheric circulation and jet‐stream variability, focusing on the East Atlantic (EA) and Scandinavian (SCA) patterns as well as the North Atlantic Oscillation (NAO) index. New time series of these indices are developed from empirical orthogonal function (EOF) analysis. Geopotential height data from the ERA5 reanalysis are used to generate the EOFs. Sets of predictors with known associations with these drivers are developed and used to formulate a sliding‐window NARMAX model. This model demonstrates a high degree of predictive accuracy, as indicated by its average correlation coefficients over the testing period (2006–2021): 0.78 for NAO, 0.83 for EA and 0.68 for SCA. In comparison, the SEAS5 and GloSea5 dynamical forecast models exhibit lower correlations with observed circulation changes: for NAO, the correlation coefficients are 0.51 for SEAS5 and 0.34 for GloSea5, for EA they are 0.15 and 0.09, respectively, and for SCA, they are 0.28 and 0.24, respectively. Comparison of NARMAX predictions with forecasts and hindcasts from the SEAS5 and GloSea5 models highlights areas where NARMAX can be used to help improve seasonal forecast skill and inform the development of dynamical models, especially in the case of summer.

Persistent Model Biases in the Spatial Variability of Winter North Atlantic Atmospheric Circulation

AbstractThe three leading modes of the North Atlantic atmospheric circulation explain about 70% of the winter climate variability. Although climate models generally can capture these modes, biases may induce large uncertainties in regional climate predictions. Here, we evaluate the leading winter modes simulated by CMIP5‐PMIP3 and CMIP6‐PMIP4 models from the last millennium to future scenarios in comparison with historical reanalysis and paleo‐reconstructions. The models generally have a good representation of the average spatial pattern of the North Atlantic Oscillation (NAO) while showing a larger spread in performance for the East Atlantic and Scandinavian patterns. In contrast to historical reanalysis, the simulated NAO pattern tends to be rather stationary under various climate states over the years 861–2100. Such underestimated spatial variability in the simulated NAO is directly related to the biased spatial shifts in NAO‐related regional temperature and precipitation changes, inducing uncertainties in climate projections over the North Atlantic sector.

The Leading Modes of Northern Eurasian Winter Snowfall Variability and the Potential Influencing Factors

Abstract This study investigates the dominant modes of the interannual variability of the northern Eurasian winter snowfall during 1982–2020 and explores their potential influencing factors and the associated physical processes. The first and second empirical orthogonal function (EOF) modes feature coherent snowfall anomalies over the high latitudes of Eurasia and western Siberia, respectively. Further analyses indicate that the anomalous atmospheric circulations play a major role in forming the snowfall variability, which could be further attributed to the influences of the atmospheric teleconnection patterns and Arctic sea ice variations. Specifically, the anomalous circulations related to the first EOF mode are mainly contributed by the effects of the teleconnections of the Polar–Eurasian and Scandinavian patterns. The formation of the second EOF mode has a close connection with the North Atlantic Oscillation and the Eurasian pattern. In addition, the sea ice variations over Baffin Bay exert a considerable influence on the snowfall anomalies related to the second EOF mode by exciting a wave train–like anomalous circulation. This effect is further verified by a numerical simulation. An empirical statistical model based on the above influencing factors can well explain the temporal evolutions of the two dominant modes, verifying the important value of our results to improve the understanding of interannual variability of northern Eurasian winter snowfall.

Characteristics of mean and extreme precipitation in Ny Ålesund, Arctic

The intensification of precipitation in the Arctic can impact the ecosystem by altering the freshwater flux, albedo, sea ice, and permafrost stability. In a location like the Arctic with a harsh environment, precipitation observations are difficult to make. In the present study, precipitation characteristics are investigated using precipitation records from Ny Ålesund, Svalbard, where the longest record of precipitation in the Atlantic sector of the Arctic is available. Precipitation is expected to increase with the warming of the atmosphere; however, significant trends in precipitation are only observed during fall (0.36 mm/decade). Even though a strong temperature increase was noticed in winter, the precipitation trend was not significant. Precipitation is separated into weak and extreme events to gain more insight into its characteristics. Again, fall was characterised by a significant positive trend in weak precipitation (150 × 10−3mm/decade), while extreme winter precipitation displayed a positive trend during 2000–2019 (4.13 mm/decade). Further analysis indicated that winter extreme precipitation followed a 2-CC relationship while the precipitation during fall followed 2-CC/1-CC scaling for mean/extreme events. The atmospheric circulation pattern during the extreme events resembles the Scandinavian pattern, characterised by high MSLP anomalies over northwestern Europe. While widespread evaporation was noticed over the North Atlantic during extreme events, vertically integrated moisture divergence indicated West Spitzbergen Current (WSC) could be a possible moisture source for extreme precipitation events during winter.

Solar, Atmospheric, and Volcanic Impacts on 10Be Depositions in Greenland and Antarctica During the Last 100 Years

AbstractCosmogenic radionuclides (e.g., 10Be) from ice cores are a powerful tool for solar reconstructions back in time. However, superimposed on the solar signal, other factors like weather/climate and volcanic influences on 10Be can complicate the interpretation of 10Be data. A comprehensive study of 10Be records over the recent period, when atmospheric 10Be production and meteorological conditions are relatively well‐known, can improve our interpretation of 10Be records. Here we conduct a systematic study of the production and climate/volcanic signals in Antarctica and Greenland 10Be records, including a new 10Be record from the East GReenland Ice‐core Project site. Greenland and Antarctica records show significant decreasing trends (5%–6.5%/decade) for 1900–1950, which is comparable with the expected production rate inferred from sunspot observations. By comparing 10Be records with reanalysis data and atmospheric circulation patterns, 10Be records from Southern/Southeastern Greenland are significantly correlated with the Scandinavia pattern. Stacking 10Be records from different locations can enhance the production signal. However, this approach is not always straightforward as uncertainties in some records can lead to a weaker solar signal. A strategy can be employed to select records for the bipolar stack by comparing Greenland records with Antarctica records, assuming the shared signal is a production signal. Finally, we observe significant increases (36%–64%) in 10Be depositions in Greenland related to the Agung eruption. This large increase in Greenland 10Be records after the Agung eruption, could be partly explained by the enhanced air mass transport from mid‐latitudes coinciding with the decreased precipitation en‐route.

Interdecadal variation and possible causes of summer extreme precipitation over northern Xinjiang province, northwestern China

Using daily precipitation data from 25 stations, NCEP/NCAR reanalysis data, and 12 climate indices for boreal summers from 1963 to 2017, we investigate the interdecadal variation and causes of extreme precipitation in northern Xinjiang Province (NXJ), northwestern China. The summer frequency of daily precipitation extremes (DPEs), summer precipitation from DPEs, and contribution of precipitation from DPEs to total summer precipitation have increased significantly since the early 1990s, and this increase in extreme precipitation is more intense at stations in the eastern part of NXJ. Comparing the periods 1963–1988 and 1992–2017, the anomalous circulation over the Mongolian Plateau changed from cyclonic to anti-cyclonic, and the center of the anomalous cyclonic circulation over Central Asia shifted northward from the Iranian Plateau to the Aral Sea. These changes in anomalous circulation pattern induced changes in the paths of water vapor transport and the distribution of moisture convergence, which explain well the interdecadal increase in extreme precipitation and its spatial variability. Further analysis confirms evident impacts of the Indian Ocean basin mode (IOBM) and Scandinavia teleconnection pattern on the development of anomalous circulation related to the interdecadal increase in extreme precipitation. A positive IOBM favors the northward transport of moisture from the northern Indian Ocean to Xinjiang Province and the development of anomalous cyclonic circulation over Central Asia in the middle and upper troposphere. The negative phase of the Scandinavia pattern, which has prevailed since the late 1970s, and the enhanced out of phase connection between the anomalous circulation over Scandinavia Peninsula and the West Siberia during 1992–2017 favors the development of anomalous anti-cyclonic circulation over the Mongolian Plateau.

Carry-Over Effects of Climate Variability at Breeding and Non-Breeding Grounds on Spring Migration in the European Wren Troglodytes troglodytes at the Baltic Coast.

Many studies have linked changes in avian phenology in Europe to the North Atlantic Oscillation (NAO), which serves as a proxy for conditions in western Europe. However, the effects of climate variation in other regions of Europe on the phenology of short-distance migrants with large non-breeding grounds remain unclear. We determined the combined influence of large-scale climate indices, NAO, the Mediterranean Oscillation Index (MOI), and the Scandinavian Pattern (SCAND), during the preceding year on spring migration timing of European wren at the southern Baltic coast during 1982-2021. We modelled the effects of these climate variables on the entire passage and subsequent percentiles of the wren's passage at Bukowo-Kopań and Hel ringing stations. Over 1982-2021, the start and median of migration shifted earlier at Hel, but the end of passage shifted later at both stations. In effect, the duration of passage at Hel was extended by 7.6 days. Early passage at Hel was related with high MOI in spring and the preceding autumn. Spring passage at Bukowo-Kopań was delayed after high NAO in the previous breeding season, and high winter and spring NAO. Late spring passage occurred at both stations following a high SCAND in the previous summer. At both locations, an early start or median of passage followed high local temperatures. We conclude that phenology of the wren's spring migration at the Baltic coast was shaped by conditions encountered at wintering quarters in western Europe, where NAO operates, and in the south-eastern Europe, where the MOI operates, in conjunction with conditions in Scandinavia during the previous breeding season. We demonstrated that climate variability in various parts of the migrants' range has combined carry-over effects on in migrants' phenology in Europe.

Snow–vegetation–atmosphere interactions in alpine tundra

Abstract. The interannual variability of snow cover in alpine areas is increasing, which may affect the tightly coupled cycles of carbon and water through snow–vegetation–atmosphere interactions across a range of spatio-temporal scales. To explore the role of snow cover for the land–atmosphere exchange of CO2 and water vapor in alpine tundra ecosystems, we combined 3 years (2019–2021) of continuous eddy covariance flux measurements of the net ecosystem exchange of CO2 (NEE) and evapotranspiration (ET) from the Finse site in alpine Norway (1210 m a.s.l.) with a ground-based ecosystem-type classification and satellite imagery from Sentinel-2, Landsat 8, and MODIS. While the snow conditions in 2019 and 2021 can be described as site typical, 2020 features an extreme snow accumulation associated with a strong negative phase of the Scandinavian pattern of the synoptic atmospheric circulation during spring. This extreme snow accumulation caused a 1-month delay in melt-out date, which falls in the 92nd percentile in the distribution of yearly melt-out dates in the period 2001–2021. The melt-out dates follow a consistent fine-scale spatial relationship with ecosystem types across years. Mountain and lichen heathlands melt out more heterogeneously than fens and flood plains, while late snowbeds melt out up to 1 month later than the other ecosystem types. While the summertime average normalized difference vegetation index (NDVI) was reduced considerably during the extreme-snow year 2020, it reached the same maximum as in the other years for all but one of the ecosystem types (late snowbeds), indicating that the delayed onset of vegetation growth is compensated to the same maximum productivity. Eddy covariance estimates of NEE and ET are gap-filled separately for two wind sectors using a random forest regression model to account for complex and nonlinear ecohydrological interactions. While the two wind sectors differ markedly in vegetation composition and flux magnitudes, their flux response is controlled by the same drivers as estimated by the predictor importance of the random forest model, as well as by the high correlation of flux magnitudes (correlation coefficient r=0.92 for NEE and r=0.89 for ET) between both areas. The 1-month delay of the start of the snow-free season in 2020 reduced the total annual ET by 50 % compared to 2019 and 2021 and reduced the growing-season carbon assimilation to turn the ecosystem from a moderate annual carbon sink (−31 to −6 gC m−2 yr−1) to a source (34 to 20 gC m−2 yr−1). These results underpin the strong dependence of ecosystem structure and functioning on snow dynamics, whose anomalies can result in important ecological extreme events for alpine ecosystems.

Seasonal and interannual variability of Mediterranean Sea overturning circulation

The overturning streamfunction is a widely used metric to monitor ocean circulation changes in the North Atlantic Ocean. Analogously, it is known that an overturning circulation develops in the Mediterranean Sea, although substantially weaker and smaller than in the Atlantic. In this work we use monthly mean fields from a high-resolution ocean reanalysis (1/16°×1/16°) to explore seasonal and interannual variability of Mediterranean Sea overturning circulation in depth and water mass parameter spaces (potential density, potential temperature, salinity) from 1987 to 2018 (both included). Results show a clear single zonal clockwise transport cell in all three spaces that extends from the Strait of Gibraltar to the Levantine Basin, in which water masses tend to densify as they move eastward. This densification is mainly controlled by salinity in the Eastern Mediterranean basin (EMED), while in the Western Mediterranean (WMED) both salinity and potential temperature variations should be taken into account. In contrast, the meridional overturning transport, much more complex, is able to better capture smaller-scale variations since it responds faster to perturbations in the circulation.Seasonal variations in overturning transport are reflected in the size and strength of overturning cells in all spaces, being cells remarkably narrower in winter than in summer. Regarding interannual changes, we show that the meridional overturning transport in density space is significantly correlated with the North Atlantic Oscillation, the Mediterranean Oscillation Index and the Scandinavian pattern during wintertime in the WMED, and with the Eastern Atlantic and the East Atlantic–Western Russian in the EMED. The overturning metric in density space captures well the Eastern Mediterranean Transient, that occurred during years 1992–1996. Finally, we explore the usefulness of overturning metric to detect and track water mass changes associated to shorter scale events, using as example the strong deep convection events of years 2004–2006 in the Gulf of Lion, which marked the beginning of the Western Mediterranean Transition.

Atmospherically Driven Seasonal and Interannual Variability in the Lagrangian Transport Time Scales of a Multiple‐Inlet Coastal System

AbstractIntense short‐term wind events can flush multiple‐inlet systems and even renew the water entirely. Nonetheless, little is known about the effect of wind variations at seasonal and interannual scales on the flushing of such systems. Here, we computed two Lagrangian transport time scales (LTTS), the residence and exposure times, for a multiple‐inlet system (the Dutch Wadden Sea) over 36 years using a realistic numerical model simulation. Our results reveal pronounced seasonal and interannual variability in both system‐wide LTTS. The seasonality of the LTTS is strongly anti‐correlated to the wind energy from the prevailing directions, which are from the southwesterly quadrant and coincidentally aligned with the geographical orientation of the system. This wind energy, which is stronger in autumn‐winter than in spring‐summer, triggers strong flushing (and hence low values of the LTTS) during autumn‐winter. The North Atlantic Oscillation (NAO) and the Scandinavia Pattern (SCAN) are shown to be the main drivers of interannual variability in the local wind and, ultimately, in both LTTS. However, this coupling is much more efficient during autumn‐winter when these patterns show larger values and variations. During these seasons, a positive NAO and a negative SCAN induce stronger winds in the prevailing directions, enhancing the flushing efficiency of the system. The opposite happens during positive SCAN and negative NAO, when weaker flushing during autumn‐winter is observed. Thus, large‐scale atmospheric patterns strongly affect the interannual variability in flushing and are potential drivers of the long‐term ecology and functioning of multiple‐inlet systems.

Precursory atmospheric teleconnection patterns for strong Siberian High events

Based on Japanese 55-year daily reanalysis data, the present study reveals that a strong Siberian high (SH) is preceded by three teleconnection patterns—namely, the Scandinavian (SCA) pattern, western Pacific (WP) pattern, and Polar/Eurasian (POL) pattern. Accordingly, strong SH events are classified into three types. Composite results show that there are evident differences among the three types in their typical circulation characteristics and impacts on surface air temperature (SAT) in East Asia. The SCA type is characterized by a wave-train anomaly over mid- and high-latitude Eurasia, while the WP type is characterized by a western expansion of circulation anomalies from the downstream western North Pacific/Far East. The significant signal of the POL type is a gradual southward movement of anticyclonic anomalies from the North Pole. In terms of the impact on SAT in East Asia, the SCA type and POL type are similar since they can cause a wide range of cold anomalies in East Asia. However, the cold anomalies caused by the POL type in northern China show significant signals earlier. The WP type causes only modest cold anomalies over northern and eastern China, but the persistence of SAT anomalies is obvious.摘要本文利用日本55年逐日再分析资料, 发现在SH异常增强前, 有三种不同的大气遥相关型前兆信号, 即斯堪的纳维亚 (SCA) 型, 西太平洋 (WP) 型, 极地-欧亚 (POL) 型. 据此, 本文将异常增强的SH分为对应的三类事件. 合成结果表明, 三类事件在典型环流特征和对东亚气温的影响上存在着明显的不同. 具体而言, SCA类事件主要表现为在欧亚中高纬度上自西向东的波列异常, WP类事件表现为环流异常自下游太平洋/俄罗斯远东地区向西发展的特征, POL型事件的显著信号则来源于自极区向南移动的反气旋式环流异常. 在对东亚地表气温的影响上, SCA类与POL类事件类似, 它们均可造成大范围的低温异常. 而WP类仅在我国北方和东部地区造成强度较弱的地表气温异常, 但该异常的持续性特征较明显.