Summer Atmospheric Circulation 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.

Westward extension of summer atmospheric circulation over the North Pacific after the 1990s

This paper documents the westward extension of the summer atmospheric circulation over the North Pacific (NP circulation) around the 1990s and investigates the possible reason, based on the North Pacific index. The NP circulation was centered in the Northeast Pacific during 1961–1983 and in the central North Pacific during 1994–2016. Further results show that the westward movement of the NP circulation was closely linked with an intensification of the relationship between the NP circulation and the previous spring sea surface temperature of the midlatitude North Pacific (SST_NP) after the 1990s. The spring SST_NP anomalies exerted considerable impacts on surface heat flux (i.e., sensible heat flux and latent heat flux) and vertical motion anomalies over the central North Pacific during the following summer, facilitating a westward shift in the summer NP circulation. Additionally, the interannual variability of the sea level pressure over the central North Pacific increased after the 1990s, which was likely a contributing factor to the westward extension of the NP circulation.摘要本文研究了20世纪90年代前后夏季北太平洋大气环流 (NP circulation) 向西扩展, 并探讨了其可能的原因. 结果表明, 在1961–1983年期间NP中心主要位于东北太平洋, 而在1994–2016年期间NP中心发生西移, 至北太平洋中部. 进一步分析指出, 90年代以后NP活动中心的西移和春季中纬度北太平洋海温(SST_NP) 与NP关系的加强有关. 春季SST_NP异常通过引起后期夏季北太平洋中部地表热通量(即感热通量和潜热通量)和垂直运动异常, 有利于NP活动中心向西移动. 此外, 20世纪90年代以后, 北太平洋中部海平面气压的年际变率增加, 这可能是NP向西扩展的另一原因.

Impacts of the extratropical North Pacific on boreal summer Arctic circulation

A distinct characteristic of the Arctic summer atmospheric circulation is the anomalous anticyclonic circulation centered over the Arctic Ocean associated with significant Arctic warming. Previous studies have related the underlying mechanisms to the earlier spring Eurasian snowmelt and the tropical Pacific forcing. Here, the authors show that the Arctic summer anomalous anticyclonic circulation is significantly related to the extratropical North Pacific sea surface temperature anomalies (SSTAs) in spring, indicating a teleconnection from the extratropical North Pacific to the Arctic. The SSTA pattern is characterized by warm anomalies in the midlatitudes of the extratropical North Pacific surrounded by significant cold anomalies, resembling the negative phase of the Pacific Decadal Oscillation (PDO) but without significant signals in the tropics (referred to as the negative PDO-like pattern). This negative PDO-like pattern in May can stimulate a Rossby wave propagating from the Bering Sea to the Arctic and lead to an anomalous Arctic anticyclone which can be sustained during summer. Meanwhile, the negative PDO-like pattern can persist to summer and induce an anomalous surface low pressure over the Bering Sea in summer. This anomalous surface low causes anomalous rising motions and induces upper-level divergence anomalies which further intensify the summer Arctic anticyclone. The upper-level tropospheric Arctic anticyclone can force anomalous adiabatic descent over the Arctic and sub-Arctic, leading to significant adiabatic heating in the Arctic. As a result, a significant warming emerges over the Arctic with the center located in the middle troposphere. The connection between the negative PDO-like SSTAs and the summer Arctic anticyclone has been confirmed by numerical experiments.

Summer atmospheric circulation over Greenland in response to Arctic amplification and diminished spring snow cover

The exceptional atmospheric conditions that have accelerated Greenland Ice Sheet mass loss in recent decades have been repeatedly recognized as a possible dynamical response to Arctic amplification. Here, we present evidence of two potentially synergistic mechanisms linking high-latitude warming to the observed increase in Greenland blocking. Consistent with a prominent hypothesis associating Arctic amplification and persistent weather extremes, we show that the summer atmospheric circulation over the North Atlantic has become wavier and link this wavier flow to more prevalent Greenland blocking. While a concomitant decline in terrestrial snow cover has likely contributed to this mechanism by further amplifying warming at high latitudes, we also show that there is a direct stationary Rossby wave response to low spring North American snow cover that enforces an anomalous anticyclone over Greenland, thus helping to anchor the ridge over Greenland in this wavier atmospheric state.

Has Arctic sea ice loss affected summer precipitation in North China?

AbstractArctic sea ice loss is one of the most striking features in changing of the climate system and affects weather and climate in the midlatitudes through complex interactions and feedback processes. However, it is not clear whether Arctic sea ice loss has affected summer precipitation in East Asia. This study shows that summer regional precipitation in North China exhibits obvious features of phasic variations. Before 1998, positive and negative precipitation anomalies appeared alternately. While from 1999 to 2011, negative precipitation anomalies were dominant. Since 2012, increased summer precipitation has been observed frequently. A direct cause of increases of summer precipitation in North China is an enhancement of water vapour transport from South China and China's marginal seas, which can be attributed to the strengthening and northward shifting of the northwestern Pacific subtropical high and the spatial configuration with west low–east high and north low–south high in East Asian height fields. Although summer atmospheric circulation anomalies in high latitudes are dominant during the sea ice melting phase, their contributions to summer precipitation variations in North China are little. Simulation experiments show that Arctic sea ice loss can significantly affect the atmospheric circulation in high latitudes; however, it cannot be regarded as a major cause of observed recent increases in precipitation in North China. Additionally, Arctic sea ice loss has little impact on summer precipitation in the Yangtze River valley and South China.

Modeling the effects of global cooling and the Tethyan Seaway closure on North African and South Asian climates during the Middle Miocene Climate Transition

The Middle Miocene was a period of prominent climatic change, marked by the Mid-Miocene Climatic Optimum (MMCO) and the subsequent global cooling due to a decline of the atmospheric CO2 concentrations (pCO2). In addition to this, the closure of the Tethyan Seaway driven by the Arab-Eurasia collision also had an important effect on the paleoclimatic changes during this period. In this study, we use the Community Earth System Model 1.2.2 (CESM 1.2.2) to simulate the effects of global cooling (i.e. pCO2 decline) and the closure of the Tethyan Seaway on the North African and South Asian climates. Our results show that the global cooling led to a precipitation decrease over both North Africa and South Asia, whereas the closure of the Tethyan Seaway resulted in a precipitation decrease over North Africa but an increase over South Asia. The opposite effects over North Africa and South Asia are due to an increased moisture transport from North Africa to South Asia induced by stronger summer atmospheric circulation when the Tethyan Seaway is closed. We further show that the reconstructed records of drying conditions over North Africa during the warming period from the late Early Miocene to the early Middle Miocene from previous studies can be partly explained by the narrowing of the Tethyan Seaway and its climatic continuing deterioration due to the subsequent final closure and global cooling. Both are precursory conditions for the formation of the Sahara desert. The stronger South Asian monsoon during the Middle Miocene transient cooling period found in previous studies can be partially attributed to the final closure of the Tethyan Seaway.

Influence of Seasonal Air–Sea Interaction on the Interannual Variation of the NPP of Terrestrial Natural Vegetation in China

Based on Moderate Resolution Imaging Spectoradiometer (MODIS) remote sensing data, meteorological observation data, multisource atmospheric circulation, and sea surface temperature (SST) data from NCEP/NCAR reanalysis, we estimated the net primary productivity (NPP) of terrestrial natural vegetation in China according to the CASA model and analyzed the linear trend and interannual fluctuation of NPP, as well as the spatial distribution characteristics of the annual NPP response to climatic factors. The obtained results revealed the impact of air–sea interaction on interannual NPP variability in key climatic areas. In China, the annual NPP of natural vegetation, linear NPP trend, and interannual NPP fluctuation showed significant regional characteristics. The annual NPP exhibited a significant increasing trend and interannual fluctuation in North China and Northeast China, with spatially consistent responses from NPP to precipitation and temperature. On the seasonal time scale, NPP in the key climatic area (105~135° E, 35~55° N) exhibited a strong response to both summer precipitation and mean temperature. In the summer atmospheric circulation, the circulation anomaly area is mainly distributed in the northeast cold vortex area in the middle- and high-latitude westerlies in East Asia and in the Sea of Okhotsk with dipole circulation. In the SST of the preceding winter and spring, the key SST anomaly area was the Kuroshio region, with an impact of the Kuroshio SST anomaly on the interannual variation in annual NPP in the key climatic area. The cold vortex in Northeast China played a pivotal role in the influence of the SST anomaly in the Kuroshio region on atmospheric circulation anomalies, resulting in abnormal summer precipitation in the key climatic region and affecting the annual accumulation of NPP of natural vegetation.

Dynamic boreal summer atmospheric circulation response as negative feedback to Greenland melt during the MIS-11 interglacial

Abstract. The unique alignment of orbital precession and obliquity during the Marine Isotope Stage 11 (MIS-11) interglacial produced perhaps the longest period of planetary warmth above preindustrial conditions in the past 800 kyr. Reconstructions point to a significantly reduced Greenland ice sheet volume during this period as a result, although the remaining extent and volume of the ice sheet are poorly constrained. A series of time slice simulations across MIS-11 using a coupled climate model indicates that boreal summer was particularly warm around Greenland and the high latitudes of the Atlantic sector for a period of at least 20 kyr. This state of reduced atmospheric baroclinicity, coupled with an enhanced and poleward-shifted intertropical convergence zone and North African monsoon, favored weakened high-latitude winds and the emergence of a single, unified midlatitude jet stream across the North Atlantic sector during boreal summer. Consequent reductions in the lower-tropospheric meridional eddy heat flux over the North Atlantic therefore emerge as negative feedback to additional warming over Greenland. The relationship between Greenland precipitation and the state of the North Atlantic jet is less apparent, but slight changes in summer precipitation appear to be dominated by increases during the remainder of the year. Such a dynamic state is surprising, as it bears stronger resemblance to the unified-jet state postulated as typical for glacial states than to the modern-day interglacial state.

The influence of the summer tropospheric circulation on the observed ozone mixing ratios at a coastal site in the Eastern Mediterranean

This study combines surface ozone (O3) measurements at the Finokalia site (Eastern Mediterranean) with backward trajectory modeling results, reanalysis, and satellite data to investigate how the summer tropospheric circulation affects the O3 mixing ratios in the Eastern Mediterranean. The whole dataset covers the summer months (June, July, August) of 2011–2018. Our findings show that north-northeastern flow, known as the Etesians, is a high frequent wind system over the Eastern Mediterranean during the summer period. Under this flow regime, polluted air masses from the continental central and Eastern Europe reach the monitoring site, increasing the O3 mixing ratios of the area. Contrary, western flows carrying maritime air masses from the Mediterranean Sea, though less frequent, have a central role in controlling the annual O3 mixing ratios in the Eastern Mediterranean. Furthermore, the vertical O3 profile analysis over the Finokalia site indicates enhanced O3 mixing ratios in the middle troposphere when northern flows prevail, confirming the significant relationship between the low-level summer atmospheric circulation and the downward transport of O3-rich air masses to the lower troposphere over Finokalia station, Eastern Mediterranean.

Exploring the influence of the North Pacific Rossby wave sources on the variability of summer atmospheric circulation and precipitation over the Northern Hemisphere

The influence of Rossby waves emitted in the northeastern Pacific Ocean on the Northern Hemisphere’s atmosphere during summer is analysed using ERA5 reanalysis and a new large ensemble performed with the EC-Earth3 model. The Rossby Wave Sources (RWS) trigger wave-like patterns arising from the upper troposphere of the north-eastern Pacific region, causing a response around the Northern Hemisphere with alternating regions of positive and negative correlation values between RWS and geopotential height at 500 hPa. Increased RWS intensity during summer is related to negative temperature anomalies over western North America, and positive temperature anomalies over eastern North America, concurrently with increased precipitation over the western subtropical Atlantic and Northern Europe during summer. Colder than normal conditions on the North Pacific Ocean intensify the RWS and its impact on the global atmospheric circulation. Different warm or cold states in the Pacific and Atlantic Oceans modify the atmospheric response to RWS, showing a change in the middle troposphere (500 hPa) towards a more-wavy structure with cold Pacific conditions, and towards a less-wavy structure with a warm Pacific Ocean. Furthermore, the North Atlantic plays a very important role in hindering (in the case of warm water) or permitting (cold water) that Rossby waves generated in the Pacific modulate the atmospheric conditions over Europe.

The Southeast Asian monsoon and El Niño–Southern Oscillation impact on the summer atmospheric circulation of East Mediterranean during 20th century based on ERA‐20C and CMIP5 simulations

AbstractEl Niño–Southern Oscillation (ENSO) and the Indian summer monsoon (ISM) are the dominant modes of climate variability affecting the tropical and subtropical regions. In this study, we investigate the impact of ENSO episodes, through the modulation of the ISM configuration, on the Eastern Mediterranean (EMed) summer atmospheric circulation over the whole 20th century using state of the art model simulations from the 5th phase of the Climate Model Intercomparison Project (CMIP5). Results are validated against the European Centre for Medium‐Range Weather Forecasts Re‐Analysis, ERA‐20C. Our findings indicate that the variability of ENSO has a significant impact on the low and middle tropospheric circulation of EMed. La Niña (El Niño) events are related to stronger (weaker) ISM activity, which leads to a deepening (weakening) of the Persian low that affects the EMed tropospheric circulation with stronger (weaker) northerly surface winds over the area. Further, La Niña (El Niño) events are related with stronger (weaker) 500 hPa subsidence over the EMed and an intensification (weakening) of the Subtropical Jet Stream between 35°N to 50°N. This work improves our understanding of the teleconnections between ENSO variability and the EMed summer tropospheric circulation via the modulation of the ISM activity and stresses the importance of ENSO on the EMed climate in summer.

Role of Oceanic Memory Effects in the Barents Sea in the Seasonal Linkage Between the Winter and Summer Arctic Oscillation

AbstractWe investigated the mechanism of inter‐seasonal linkage between the winter Arctic Oscillation (AO) and the subsequent summer AO. Our focus was the collapse of the linkage in the late 1980s, associated with substantial sea ice loss in the Arctic. Because atmospheric circulation itself does not have long‐lasting memory effects from winter to summer, the seasonal persistency of sea ice and sea surface temperature (SST) anomalies are likely to contribute to the inter‐seasonal linkage. In the early significant‐linkage period prior to 1990, the winter AO signal was maintained in the sea ice and SST anomalies over the Barents Sea during inter‐seasonal periods. The surface anomalies then influenced the summer atmospheric circulation by exciting the Rossby wave propagation. However, in the recent poor‐linkage period after 1990, sea ice loss in the Barents Sea and Northern Hemisphere was significant. In addition, the seasonal persistency of sea ice and SST anomalies over the Barents Sea were less significant, and the winter AO signal was not maintained beyond a given season. Thus, the oceanic memory effects of the Barents Sea were reduced and might not have been associated with the recent inter‐seasonal linkage.

Differences Between Present‐Day and Cretaceous Hydrological Cycle Responses to Rising CO2 Concentration

AbstractClimate model simulations have suggested that, in East Asia, the mean precipitation increases with global warming, leading to more humid conditions. In contrast, Cretaceous proxy data show an aridification in the low latitudes of East Asia during the mid‐Cretaceous when CO2 levels were higher and the climate was relatively warmer compared to the early and late Cretaceous. We conducted simulations of the Present‐day and Cretaceous climates with different CO2 levels using an atmosphere‐ocean general circulation model, MIROC4m, to investigate the hydrological cycle changes as CO2 is increased. The results show that the responses of the hydrological cycle in East Asia for the Present‐day and Cretaceous experiments are the exact opposite; an increase in the Present‐day, and a decrease in the Cretaceous, as geological evidence shows. Overall, the opposite response in the Cretaceous is attributed to a weaker summer atmospheric circulation, associated with the absence of the Tibetan Plateau.

Possible impacts of anomalous Arctic sea ice melting on summer atmosphere

AbstractArctic sea ice melting and its possible impacts on climate variability and extreme weather events in the mid‐latitudes have been of wide concern over the past two decades. Compared with the impacts of sea ice melting on winter, its impacts on summer atmospheric circulation variability and extreme weather events have received less attention, and it is not clear whether the rapid sea ice melting can affect summer temperature and precipitation in the mid‐latitudes. Through observational analyses and simulation experiments forced by observed Arctic sea ice concentration, this study investigates possible impacts of anomalous summer sea ice melting on air temperature, frequency of heatwaves, precipitation, and East Asian summer monsoon variability. It is found that the enhanced summer sea ice melting has occurred since 2007 and dynamically corresponded to the positive phase of the summer Arctic dipole anomaly. Simulation experiments showed that the ensemble means of anomalous summer sea ice melting cases significantly increase surface air temperatures (SATs) and frequencies of heatwaves in the mid‐ and high‐latitudes of both Asia and North America, and decreases SATs in Europe and parts of the Asian continent. Anomalous summer sea ice melting increases summer precipitation in the circumpolar region, mid‐latitudes of East Asia, and the Northwestern Pacific and significantly decreases precipitation in mid‐ and low‐latitudes of North America. In addition, anomalous sea ice melting in simulations deepens the thermal depression in East Asia and strengthens East Asian summer monsoon and its variability. These model results suggest that anomalous sea ice melting may exacerbate impacts of global warming on summer atmosphere over Asia and North America though its impact may not be dominant due to atmospheric internal variability and impacts of other factors.

Changes of Extreme Precipitation and its Associated Mechanisms in Northwest China

Characterized by scarce water resources and fragile ecosystems, Northwest China (NWC) has experienced a climate shift from warm-dry to warm-wet conditions since the 1980s that has garnered extensive concern in recent years. In this study, the variability in extreme precipitation (EP) in different climate zones of NWC and the possible mechanisms for this variation were investigated during 1961-2016. The results show that the EP trends significantly increased in most of the westerly zone (WZ) and plateau zone (PZ), while the EP trends did not significantly decrease in the monsoon zone (MZ). The start dates of extreme precipitation (SDEP) and end dates of extreme precipitation (EDEP) advanced and were postponed, respectively, in the WZ and PZ, while the opposite occurred in the MZ. Summer atmospheric circulation, water vapor transport and atmospheric instability over NWC varied greatly with the interdecadal shift in EP before and after 1986. During 1986-2016, upper-level divergence and lower-level convergence occurred in the MZ and PZ, which strengthened ascending flow. In addition, the summer water vapor and atmospheric instability increased in the WZ and PZ. These characteristics created favorable conditions for increased occurrences of EP in the WZ and PZ in summer. Conversely, the upper-level convergence and lower-level divergence in the MZ strengthened descending flow. Decreases in summer water vapor and atmospheric instability occurred in the MZ after 1986. Hence, the environmental conditions in the MZ may have prevented the occurrence and development of EP in summer during 1986-2016.

Recent trends in summer atmospheric circulation in the North Atlantic/European region: is there a role for anthropogenic aerosols?

AbstractThe variability of the westerly jet stream and storm track is crucial for summer weather and climate in the North Atlantic/European region. Observations for recent decades show notable trends in the summer jet from 1970s to 2010s, characterized by an equatorward migration over the North Atlantic accompanied by a poleward migration and weakening of the Mediterranean jet over Europe. These changes in atmospheric circulation were associated with more cyclonic storms traveling across the UK into northern Europe, and fewer over the Mediterranean, leading to wet summers in northern Europe and dry summers in southern Europe.In this study we investigate the potential drivers and processes that may have been responsible for the observed changes in summer atmospheric circulation, with a particular focus on the role of anthropogenic aerosols (AA). We conduct attribution experiments with an atmospheric general circulation model (AGCM) forced with observed changes in sea surface temperatures/sea ice extent (SST/SIE), greenhouse gas concentrations and AA precursor emissions. Comparison between the model results and observations strongly suggests that fast responses to AA changes were likely the primary driver of the observed poleward migration and weakening of the Mediterranean jet, with changes in SST/SIE playing a secondary role. The simulated response shows good agreement with the observed changes in both magnitude and vertical structure, which suggests that common mechanisms - involving aerosol-radiation and aerosol-cloud interactions - are responsible. By contrast, changes in the North Atlantic jet are influenced in the model experiments by changes in both Atlantic SST/SIE (which may themselves have been influenced by changes in AA) and fast responses to AA. In this case, however, there are significant differences between the model response and the observed changes; we argue these differences may be explained by biases in the model climatology.

Large-scale climate signals of a European oxygen isotope network from tree rings

Abstract. We investigate the climate signature of δ18O tree-ring records from sites distributed all over Europe covering the last 400 years. An empirical orthogonal function (EOF) analysis reveals two distinct modes of variability on the basis of the existing δ18O tree-ring records. The first mode is associated with anomaly patterns projecting onto the El Niño–Southern Oscillation (ENSO) and reflects a multi-seasonal climatic signal. The ENSO link is pronounced for the last 130 years, but it is found to be weak over the period from 1600 to 1850, suggesting that the relationship between ENSO and the European climate may not be stable over time. The second mode of δ18O variability, which captures a north–south dipole in the European δ18O tree-ring records, is related to a regional summer atmospheric circulation pattern, revealing a pronounced centre over the North Sea. Locally, the δ18O anomalies associated with this mode show the same (opposite) sign with temperature (precipitation). Based on the oxygen isotopic signature derived from tree rings, we argue that the prevailing large-scale atmospheric circulation patterns and the related teleconnections can be analysed beyond instrumental records.

Recent Climate Change Feedbacks to Greenland Ice Sheet Mass Changes from GRACE

Although a significant effort has been dedicated to studying changes in the mass budget of the Greenland Ice Sheet (GrIS), mechanisms behind these changes are not yet fully understood. In this study, we address this issue by investigating the link between climate controls and mass changes of the GrIS between August 2002 and June 2017. We estimate the GrIS mass changes based on averaging the Gravity Recovery and Climate Experiment (GRACE) monthly gravity field solutions from four processing data centers. We then investigate the possible impact of different climate variables on the GrIS mass changes using the North Atlantic Oscillation (NAO), temperature, precipitation, and the 700 hPa wind retrieved from the ERA-5 reanalysis. Results indicate a decrease of −267.77 ± 32.67 Gt/yr in the total mass of the GrIS over the 16-year period. By quantifying the relationship between climate controls and mass changes, we observe that mass changes in different parts of Greenland have varying sensitivity to climate controls. The NAO mainly controls mass changes in west Greenland, where the summertime NAO modulations have a greater impact on the summer mass loss than the wintertime NAO modulations have on the winter mass gain. The GrIS mass changes are correlated spatially with summer temperature, especially in southwest Greenland. Mass balance changes in northwest Greenland are mostly affected by wind anomalies. These new findings based on wind anomalies indicate that the summer atmospheric circulation anomalies control surface temperature and snow precipitation and consequently affect mass changes in different parts of Greenland.

The North Atlantic as a Driver of Summer Atmospheric Circulation

AbstractSkill in seasonal forecasts in the Northern Hemisphere extratropics is mostly limited to winter. Drivers of summer circulation anomalies over the North Atlantic–European (NAE) sector are poorly understood. Here, we investigate the role of North Atlantic sea surface temperatures (SSTs) in driving summer atmospheric circulation changes. The summer North Atlantic Oscillation (SNAO), the leading mode of observed summer atmospheric circulation variability in the NAE sector, is correlated with a distinct SST tripole pattern in the North Atlantic. An atmospheric general circulation model is used to test whether there are robust atmospheric circulation responses over the NAE sector to concurrent SSTs related to the SNAO. The most robust responses project onto the summer east Atlantic (SEA) pattern, the second dominant mode of observed summer atmospheric circulation variability in the NAE sector, and are most evident at the surface in response to tropical SSTs and at altitude in response to extratropical SSTs. The tropical-to-extratropical teleconnection appears to be due to Rossby wave propagation from SST anomalies, and in turn precipitation anomalies, in the Caribbean region. We identify key biases in the model, which may be responsible for the overly dominant SEA pattern variability, compared to the SNAO, and may also explain why the responses resemble the SEA pattern, rather than the SNAO. Efforts to eradicate these biases, perhaps achieved by higher-resolution simulations or with improved model physics, would allow for an improved understanding of the true response to North Atlantic SST patterns.

Unstable relationship between spring NAO and summer tropical cyclone genesis frequency over the western North Pacific

The present study reveals the fact that the relationship between the spring (April–May) North Atlantic Oscillation (NAO) and the following summer (June–September) tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) during the period of 1950–2018 was not stationary. It is shown that the relationship between the two has experienced a pronounced interdecadal shift, being weak and insignificant before yet strong and statistically significant after the early 1980s. Next we compare the spring NAO associated dynamic and thermodynamic conditions, sea surface temperature (SST) anomalies, and atmospheric circulation processes between the two subperiods of 1954–1976 and 1996–2018, so as to illucidate the possible mechanism for this interdecadal variation in the NAO-TC connection. During the latter epoch, when the spring NAO was positive, enhanced low-level vorticity, reduced vertical zonal wind shear, intensified vertical velocity and increased middle-level relative humidity were present over the WNP in the summer, which is conducive to the genesis of WNP TCs. When the spring NAO is negative, the dynamic and thermodynamic factors are disadvantageous for the summertime TC formation and development over the WNP. The results of further analysis indicate that the persistence of North Atlantic tri-pole SST anomalies from spring to the subsequent summer induced by the spring NAO plays a fundamental role in the linkage between the spring NAO and summer atmospheric circulation. During the period of 1996–2018, a remarkable eastward propagating wave-train occurred across the northern Eurasian continent, forced by the anomalous SST tri-pole in the North Atlantic. The East Asian jet flow became greatly intensified, and the deep convection in the tropics was further enhanced via the changes of the local Hadley circulation, corresponding to a positive spring NAO. During the former epoch, the spring NAO-induced tri-pole SST anomalies in the North Atlantic were non-existent, and the related atmospheric circulation anomalies were extremely weak, thereby leading to the linkage between spring NAO and WNP TC genesis frequency in the following summer being insignificant.