Preface to the Special Topic on Ocean, Sea Ice and Northern Hemisphere Climate:In Remembrance of Professor Yongqi GAO's Key Contributions

This special issue commemorates the life work of Prof. Yongqi GAO who passed away in July 2021, his time cut short by illness. He had many great achievements, but still much more to contribute. The seven articles in this special issue are from research areas where he contributed, and they illustrate how his close colleagues are continuing his work.

Pathways of Influence of the Northern Hemisphere Mid–high Latitudes on East Asian Climate: A Review

This paper reviews recent progress made by Chinese scientists on the pathways of influence of the Northern Hemisphere mid-high latitudes on East Asian climate within the framework of a“coupled oceanic-atmospheric(land-atmospheric or seaice-atmospheric)bridge”and“chain coupled bridge”.Four major categories of pathways are concentrated upon,as follows:Pathway A—from North Atlantic to East Asia;Pathway B—from the North Pacific to East Asia;Pathway C—from the Arctic to East Asia;and Pathway D—the synergistic effects of the mid-high latitudes and tropics.In addition,definitions of the terms“combined effect”,“synergistic effect”and“antagonistic effect”of two or more factors of influence or processes and their criteria are introduced,so as to objectively investigate those effects in future research.

Therapeutic effect of nerve growth factor on cerebral infarction in dogs using the hemisphere anomalous volume ratio of diffusion-weighted magnetic resonance imaging

A model of focal cerebral ischemic infarction was established in dogs through middle cerebral artery occlusion of the right side.Thirty minutes after occlusion,models were injected with nerve growth factor adjacent to the infarct locus.The therapeutic effect of nerve growth factor against cerebral infarction was assessed using the hemisphere anomalous volume ratio,a quantitative index of diffusion-weighted MRI.At 6 hours,24 hours,7 days and 3 months after modeling,the hemisphere anomalous volume ratio was significantly reduced after treatment with nerve growth factor. Hematoxylin-eosin staining,immunohistochemistry,electron microscopy and neurological function scores showed that infarct defects were slightly reduced and neurological function significantly improved after nerve growth factor treatment.This result was consistent with diffusion-weighted MRI measurements.Experimental findings indicate that nerve growth factor can protect against cerebral infarction,and that the hemisphere anomalous volume ratio of diffusion-weighted MRI can be used to evaluate the therapeutic effect.

A 38-Year Climatology of Explosive Cyclones over the Northern Hemisphere

Explosive cyclones(ECs)over two basins in the Northern Hemisphere(20°-90°N)from January 1979 to December2016 are investigated using ERA-Interim and Optimum Interpolation Sea Surface Temperature(OISST)data.The classical definition of an EC is modified considering not only the rapid drop of the central sea level pressure of the cyclone,but also the strong wind speed at the height of 10 m in which maximum wind speeds greater than 17.2 m s^-1are included.According to the locations of the northern Atlantic and northern Pacific,the whole Northern Hemisphere is divided into the"A region"(20°-90°N,90°W-90°E)and"P region"(20°-90°N,90°E-90°W).Over both the A and P regions,the climatological features of ECs,such as their spatial distribution,intensity,seasonal variation,interannual variation,and moving tracks,are documented.

Linkage Between the Northeast Mongolian Precipitation and the Northern Hemisphere Zonal Circulation

The long-term relationship between the tree-ring-reconstructed annual precipitation in northeastern Mongolia （PRM） and the Northern Hemisphere Zonal Circulation （NHZC）, defined as the normalized zonal mean sea-level pressure at 60°N in May-June-July, is examined in this study. A significant correlation coefficient （0.31） was found between the NHZC indices and PRM based on the dataset for the period of 1872-1995. The mechanisms responsible for the relationship are discussed through analyses of the atmospheric general circulation variability associated with NHZC. It follows that NHZC-related atmospheric circulation variability provides an anomalous southeast flow from the ocean to Northeast Mongolia （northwest flow from Northeast Mongolia to the ocean） in the middle and low troposphere in positive （negative） phase of NHZC, resulting in more （less） water vapor transport to the target region and more （less） precipitation in Northeast Mongolia.

Single‑Layer ZnO Hollow Hemispheres Enable High‑Performance Self‑Powered Perovskite Photodetector for Optical Communication

The carrier transport layer with reflection reduction morphology has attracted extensive attention for improving the utilization of light.Herein,we introduced single-layer hollow ZnO hemisphere arrays(ZHAs)behaving light trapping effect as the electron transport layer in perovskite photodetectors(PDs).The singlelayer hollow ZHAs can not only reduce the reflection,but also widen the angle of the effective incident light and especially transfer the distribution of the optical field from the ZnO/FTO interface to the perovskite active layer confirmed by the 3D finitedifference time-domain simulation.These merits benefit for the generation,transport and separation of carriers,improving the light utilization efficiency.Finally,our optimized FTO/ZHA/CsPbBr3/carbon structure PDs showed high self-powered performance with a linear dynamic range of 120.3 dB,a detectivity of 4.2×10^(12) Jones,rise/fall time of 13/28μs and the f_(−3) dB of up to 28 kHz.Benefiting from the high device performance,the PD was demonstrated to the application in the directional transmission of encrypted files as the signal receiving port with super high accuracy.This work uniquely utilizes the features of high-performance self-powered perovskite PDs in optical communication,paving the path to wide applications of all-inorganic perovskite PDs.

A Preliminary Study on the Relationship Between Arctic Oscillation and Daily SLP Variance in the Northern Hemisphere During Wintertime

In the present study, the authors investigated the relationship between the Arctic Oscillation (AO) and the high-frequency variability of daily sea level pressures in the Northern Hemisphere in winter (November through March), using NCEP/NCAR reanalysis datasets for the time period of 1948/49-2000/01. High-frequency signals are defined as those with timescales shorter than three weeks and measured in terms of variance, for each winter for each grid. The correlations between monthly mean AO index and high-frequency variance are conducted. A predominant feature is that several regional centers with high correlation show up in the middle to high latitudes. Significant areas include mid- to high-latitude Asia centered at Siberia, northern Europe and the middle-latitude North Atlantic east of northern Africa. Their strong correlations can also be confirmed by the singular value decomposition analysis of covariance between mean SLP and high-frequency variance. This indicates that the relationship of AO with daily Sea Level Pressure (SLP) is confined to some specific regions in association with the inherent atmospheric dynamics. In middle-latitude Asia, there is a significant (at the 95% level) trend of variance of-2.26% (10 yr)-1. Another region that displays a strong trend is the northwestern Pacific with a significant rate of change of 0.80% (10 yr)-1. If the winter of 1948/49, an apparent outlier, is excluded, a steady linear trend of +1.51% (10 yr)-1 shows up in northern Europe. The variance probability density functions (PDFs) are found to change in association with different AO phases. The changes corresponding to high and low AO phases, however, are asymmetric in these regions. Some regions such as northern Europe display much stronger changes in high AO years, whereas some other regions such as Siberia show a stronger connection to low AO conditions. These features are supported by ECMWF reanalysis data. However, the dynamical mechanisms involved in the AO-high frequency SLP variance connection have not been well understood, and this needs further study.

Linkage between the Atlantic Tropical Hurricane Frequency and the Antarctic Oscillation in the Western Hemisphere

To examine the zonal asymmetry of the Antarctic oscillation (AAO), different portions of the AAO from June to October (JJASO) in the interannual variability of the Atlantic tropical hurricanes number (ATHN) are documented in this research. It follows that the AAO in the Western Hemisphere (AAOWH) is positively correlated with the ATHN, at 0.36 during the period of 1871-1998 and 0.42 during the period of 1949-98. After removing the linear regressions on the Southern Oscillation Index (SOI) in all time series, the above correlation coefficients are 0.25 and 0.30, respectively. The underlying mechanisms are studied through analyses of the atmospheric general circulation variability associated with the AAOWH. It turns out that the positive (negative) phase of JJASO AAOWH corresponds with several factors: decreased (increased) vertical zonal wind shear magnitude, low-level anomalous convergence (divergence), high-level anomalous divergence (convergence), and warmed (cooled) sea surface temperature in the tropical Atlantic. Therefore, the positive (negative) phase of JJASO AAOWH is favorable (unfavorable) to the tropical hurricane genesis.

Influence of the Preceding Austral Summer Southern Hemisphere Annular Mode on the Amplitude of ENSO Decay

There is increasing evidence of the possible role of extratropical forcing in the evolution of ENSO. The Southern Hemi- sphere Annular Mode （SAM） is the dominant mode of atmospheric circulation in the Southern Hemisphere extratropics. This study shows that the austral summer （December-January-February; DJF） SAM may also influence the amplitude of ENSO decay during austral autumn （March-April-May; MAM）. The mechanisms associated with this SAM-ENSO relationship can be briefly summarized as follows： The SAM is positively （negatively） correlated with SST in the Southern Hemisphere middle （high） latitudes. This dipole-like SST anomaly pattern is referred to as the Southern Ocean Dipole （SOD）. The DJF SOD, caused by the DJF SAM, could persist until MAM and then influence atmospheric circulation, including trade winds, over the Nifio3.4 area. Anomalous trade winds and SST anomalies over the Nifio3.4 area related to the DJF SAM are further developed through the Bjerkness feedback, which eventually results in a cooling （warming） over the Nifio3.4 area followed by the positive （negative） DJF SAM.

Evaluation of Atmospheric Circulation in the Southern Hemisphere in 20CRv2

Evaluation of the mean climate and climate variability in the Southern Hemisphere （SH） in the Twentieth Century Reanalysis data version 2 （20CRv2） is conducted and the results are compared with the NCEP/NCAR version 2 Reanalysis data （NCEPv2） and the Hadley Center sea-level pressure data （HadSLPv2）.The results show that SH polar High,SH subtropical High,upper level split jet,cross-equatorial flow,Antarctic Oscillation （AAO）,and the pattem of Pacific-South-America （PSA） has been effectively captured by 20CRv2 during 1979-2010,with an apparent zonal asymmetry of AAO in the austral winter （June-July-August,JJA）.The notable upward linear trend of AAO in the entire period of 1871-2010 is represented in both 20CRv2 and HadS1Pv2.The most remarkable discrepancy of the SH climate variability between 20CRv2 and HadSLPv2 occurred in 1897-1920 and was partly caused by such factors as the paucity of meteorological and oceanographic data in the SH to be assimilated,the handling of the specified sea-ice concentration in South Pole,and imperfect climate models.The consistency of these reanalysis data is increased with the use of a large amount of satellite observation and radiosonde data,particularly after 1979.

Simulated effect of soil freeze-thaw process on surface hydrologic and thermal fluxes in frozen ground region of the Northern Hemisphere

Soil freeze-thaw process is closely related to surface energy budget,hydrological activity,and terrestrial ecosystems.In this study,two numerical experiments(including and excluding soil freeze-thaw process)were designed to examine the effect of soil freeze-thaw process on surface hydrologic and thermal fluxes in frozen ground region in the Northern Hemisphere based on the state-of-the-art Community Earth System Model version 1.0.5.Results show that in response to soil freeze-thaw process,the area averaged soil temperature in the shallow layer(0.0175−0.0451 m)decreases by 0.35℃in the TP(Tibetan Plateau),0.69℃in CES(Central and Eastern Siberia),and 0.6℃in NA(North America)during summer,and increases by 1.93℃in the TP,2.28℃in CES and 1.61℃in NA during winter,respectively.Meanwhile,in response to soil freeze-thaw process,the area averaged soil liquid water content increases in summer and decrease in winter.For surface heat flux components,the ground heat flux is most significantly affected by the freeze-thaw process in both summer and winter,followed by sensible heat flux and latent heat flux in summer.In the TP area,the ground heat flux increases by 2.82 W/m2(28.5%)in summer and decreases by 3.63 W/m2(40%)in winter.Meanwhile,in CES,the ground heat flux increases by 1.89 W/m2(11.3%)in summer and decreases by 1.41 W/m2(18.6%)in winter.The heat fluxes in the Tibetan Plateau are more susceptible to the freeze-thaw process compared with the high-latitude frozen soil regions.Soil freeze-thaw process can induce significant warming in the Tibetan Plateau in winter.Also,this process induces significant cooling in high-latitude regions in summer.The frozen ground can prevent soil liquid water from infiltrating to deep soil layers at the beginning of thawing;however,as the frozen ground thaws continuously,the infiltration of the liquid water increases and the deep soil can store water like a sponge,accompanied by decreasing surface runoff.The influence of the soil freeze-thaw process on surface hydrologic and thermal fluxes varies seasonally and spatially.

Communicating with the non-dominant hemisphere:Implications for neurological rehabilitation

Aphasic syndromes usually result from injuries to the dominant hemisphere of the brain. Despite the fact that localization of language functions shows little interindividual variability, several brain areas are simultaneously activated when language tasks are undertaken. Mechanisms of language recovery after brain injury to the dominant hemisphere seem to be relatively stereotyped, including activations of perilesional areas in the acute phase and of homologues of language areas in the non-dominant hemisphere in the subacute phase, later returning to dominant hemisphere activation in the chronic phase. Plasticity mechanisms reopen the critical period of language development, more specifically in what leads to disinhibition of the non-dominant hemisphere when brain lesions affect the dominant hemisphere. The non-dominant hemisphere plays an important role during recovery from aphasia, but currently available rehabilitation therapies have shown limited results for efficient language improvement. Large-scale randomized controlled trials that evaluate well-defined interventions in patients with aphasia are needed for stimulation of neuroplasticity mechanisms that enhance the role of the non-dominant hemisphere for language recovery. Ineffective treatment approaches should be replaced by more promising ones and the latter should be evaluated for proper application. The data generated by such studies could substantiate evidence-based rehabilitation strategies for patients with aphasia.

NUMERICAL SIMULATION OF SSTA IMPACTS UPON THE INTERDECADAL VARIATION OF THE CROSS-EQUATORIAL FLOWS IN EASTERN HEMISPHERE

Impacts of regional sea surface temperature(SST)anomalies on the interdecadal variation of the cross-equatorial flows(CEFs)in Eastern Hemisphere are studied using numerical simulations with a global atmospheric circulation model(NCAR CAM3)driven with 1950-2000 monthly SSTs in different marine areas(the globe,extratropics,tropics,tropical Indian Ocean-Pacific,and tropical Pacific)and ERA-40reanalysis data.Results show that all simulations,except the one driven with extratropical SSTs,can simulate the interdecadal strengthening of CEFs around Somali,120oE,and 150oE that occurred in the midand late-1970s.Among those simulated CEFs,the interdecadal variability in Somali and its interdecadal relationship with the East Asian summer monsoon are in better agreement with the observations,suggesting that changes in the SSTs of tropical oceans,especially the tropical Pacific,play a crucial role in the interdecadal variability of CEFs in Somali.The interdecadal change of CEFs in Somali is highly associated with the interdecadal variation of tropical Pacific SST.As the interdecadal warmer(colder)SST happens in the tropical Pacific,a"sandwich"pattern of SST anomalies,i.e."+,-,+"("-,+,-"),will occur in the eastern tropical Pacific from north to south with a pair of anomalous anticyclone(cyclone)at the lower troposphere;the pair links to another pair of anomalous cyclone(anticyclone)in the tropical Indian Ocean through an atmospheric bridge,and thus strengthens(weakens)the CEFs in Somali.

The water mass variability and southward shift of the Southern Hemisphere mid-depth supergyre

The Southern Hemisphere subtropical supergyre at intermediate depths connects all three ocean basins and plays a significant role in responding and conveying the climate-change-related variations in the glob- al ocean. On the basis of the Simple Ocean Data Assimilation/SODA） ocean reanalysis, the thermohaline variability and southward shift of the mid-depth supergyre are demonstrated. The steric height of the sub- surface relative to 1 500 m （400-1 500 m） from the SODA depicts exactly the flow patterns and variability of the oceanic supergyre. During 1958-2007 the water masses in the gyre interiors become cooler/fresher, with the significant exceptions of the Agulhas Current system and Agulhas leakage. The results also exhibit a pronounced strengthening of the inter-basin connection of the supergyre, and the strongest southward shift, by about 2.5° over the whole period, occurs in the central-south Pacific, which is associated with the changes in the basin-scale wind forcing.

Boreal Winter Rainfall Anomaly over the Tropical Indo-Pacific and Its Effect on Northern Hemisphere Atmospheric Circulation in CMIP5 Models

Experimental outputs of 11 Atmospheric Model Intercomparison Project （AMIP） models from phase 5 of the Coupled Model Intercomparison Project （CMIP5） are analyzed to assess the atmospheric circulation anomaly over Northern Hemisphere induced by the anomalous rainfall over tropical Pacific and Indian Ocean during boreal winter.The analysis shows that the main features of the interannual variation of tropical rainfall anomalies,especially over the Central Pacific （CP） （5°S-5°N,175°E-135°W） and Indo-western Pacific （IWP） （20°S-20°N,110°-150°E） are well captured in all the CMIP5/AMIP models.For the IWP and western Indian Ocean （WIO） （10°S-10°N,45°-75°E）,the anomalous rainfall is weaker in the 11 CMIP5/AMIP models than in the observation.During El Ni（n）o/La Ni（n）a mature phases in boreal winter,consistent with observations,there are geopotential height anomalies known as the Pacific North American （PNA） pattern and Indo-western Pacific and East Asia （IWPEA） pattern in the upper troposphere,and the northwestern Pacific anticyclone （cyclone） （NWPA） in the lower troposphere in the models.Comparison between the models and observations shows that the ability to simulate the PNA and NWPA pattern depends on the ability to simulate the anomalous rainfall over the CP,while the ability to simulate the IWPEA pattern is related to the ability to simulate the rainfall anomaly in the IWP and WIO,as the SST anomaly is same in AMIP experiments.It is found that the tropical rainfall anomaly is important in modeling the impact of the tropical Indo-Pacific Ocean on the extratropical atmospheric circulation anomaly.

Seasonality of the Interaction between Convection over the Western Pacific and General Circulation in the Northern Hemisphere

The seasonality of the interaction between convection over the western Pacific and general circulation in the Northern Hemisphere (NH) is analyzed in the present paper with singular value decomposition (SVD) and empirical orthogonal function (EOF) analysis approaches, based on 500 hPa monthly mean geopotential height data and high-cloud amount data. The analyses demonstrate that coupled dominant patterns in the interaction between the convection over the western Pacific and the general circulation in NH are different in various seasons. In spring, the convection over the western Pacific is closely related with the western Atlantic (WA) and North Pacific (NP) like patterns of the general circulation in NH, and some associations between the WA and NP like patterns and the El Ni o /Southern Oscillation (ENSO) cycle are also existed. The Pacific Japan (PJ) pattern is the dominant pattern in the interaction between the interannual variabilities of the convection over the western Pacific and the general circulation in NH summer. The WA like pattern and 3-4 year period oscillation are also relatively obvious for the summer case. In autumn, the convection over the western Pacific is closely linked with the Eurasian (EU) like pattern and the Atlantic oscillation in the general circulation in NH, it is suggested that in autumn the variation of convective activity over the western Pacific is largely affected by the general circulation anomaly (cold air from high latitudes ) through EU like teleconnection pattern. Abrupt change happened by the end of 1980′s in the autumn interaction. The strong interaction between the western Pacific (WP) and EU like patterns in the general circulation in NH and the convection over the western Pacific and a linear trend of increasing of this interaction are also suggested in winter. It is also demonstrated that the interaction in summer and winter is stronger than in the transition seasons (spring and autumn).

Seasonal linkage of the Southern Hemisphere extratropical climate variability to two types of ENSO

This study uses the Climate Forecast System Reanalysis(CFSR) to investigate the responses of the Southern Hemisphere(SH) extratropical climate to two types of El Nino–Southern Oscillation(ENSO)-the eastern Pacific(EP) type and the central Pacific(CP) type in different seasons. The responses are denoted by the anomalies of climate variables associated with one-standard-deviation increase in the Nino3 or Nino4 index. The results show that in austral spring the differences in the ENSO-related anomaly(ERA) patterns of atmospheric circulation between the EP ENSO period(1979–1998) and CP ENSO period(1999–2010) are mainly associated with the change in the ENSO-PSA2 relationship. Such differences affect the ERA fields of surface air temperature and mixed layer temperature, and finally result in significant differences in sea-ice concentration anomalies in the Atlantic sector. In austral summer, significant correlation exists between the variations of SAM and both of the variations of Nino3 and Nino4 in 1979–1998, while the correlation between SAM and Nino4 disappears in 1999–2010. For all seasons, the strength of the climate ERAs depend on if there are close relationship between ENSO and the major climate variation modes of the SH extratropics. For the climate variables, the ERA patterns of surface air temperature are generally controlled by surface wind anomalies and mirrored by the mixed layer temperature anomalies. The mixed layer depth anomalies are primarily modulated by surface heat flux anomalies and occasionally by anomalous wind. There are strikingly strong anomalies of surface heat flux in the autumn of 1979–1998 related to the Nino3 variation, the period when there is only significant correlation between ENSO and PSA2. There are no evidence that the SH extratropical climate variability induced by Nino3 variations are stronger in the EP-ENSO period, and that variability induced by Nino4 variations are stronger in the CP-ENSO period.

Impacts of Two Types of El Nio on Atmospheric Circulation in the Southern Hemisphere

Based on NCEP/NCAR （National Centers for Environmental Prediction/National Center for Atmo- spheric Research） reanalysis data from 1979 to 2010, the impacts of two types of E1 Nino on atmospheric circulation in the Southern Hemisphere （SH） are analyzed. It is shown thaL when a warming event occurs in the equatorial eastern Pacific （EP E1 Nino）, there is a negative sea level pressure （SLP） anomaly in the east- ern Pacific and a positive one in the western Pacific. Besides, there exists a negative anomaly between 40°S and 60°S and a positive anomaly to the south of 60°S. When a warming event in the central Pacific （CP E1 Nino） occurs, there appears a negative SLP anomaly in the central Pacific and a positive SLP anomaly in the eastern and western Pacific, but the SLP anomalies are not so evident in the SH extratropics. In particular, the Pacific-South America （PSA） pattern induced by the CP E1 Nino is located more northwestward, with a weaker anomaly compared with the EP E1 Nino. This difference is directly related with the different position of heating centers associated with the two types of E1 Nino events. Because the SST anomaly associated with CP E1 Nino is located more westward than that associated with EP El Nino, the related heating center tends to move westward and the response of SH atmospheric circulation to the tropical heating changes accordingly, thus exciting a different position of the PSA pattern. It is also noted that the local meridional cell plays a role in the SH high latitudes during EP E1 Nino. The anomalous ascending motion due to the enhancement of convection over the eastern Pacific leads to an enhancement of the local Hadley cell and the meridional cell in the middle and high latitudes, which in turn induces an anomalous descending motion and the related positive anomaly of geopotential height over the Amundsen-Bellingshausen Sea.

Metabolite changes in the ipsilateral and contralateral cerebral hemispheres in rats with middle cerebral artery occlusion

Cerebral ischemia not only causes pathological changes in the ischemic areas but also induces a series of secondary changes in more distal brain regions（such as the contralateral cerebral hemisphere）. The impact of supratentorial lesions, which are the most common type of lesion, on the contralateral cerebellum has been studied in patients by positron emission tomography, single photon emission computed tomography, magnetic resonance imaging and diffusion tensor imaging. In the present study, we investigated metabolite changes in the contralateral cerebral hemisphere after supratentorial unilateral ischemia using nuclear magnetic resonance spectroscopy-based metabonomics. The permanent middle cerebral artery occlusion model of ischemic stroke was established in rats. Rats were randomly divided into the middle cerebral artery occlusion 1-, 3-, 9-and 24-hour groups and the sham group. ~1H nuclear magnetic resonance spectroscopy was used to detect metabolites in the left and right cerebral hemispheres. Compared with the sham group, the concentrations of lactate, alanine, γ-aminobutyric acid, choline and glycine in the ischemic cerebral hemisphere were increased in the acute stage, while the concentrations of N-acetyl aspartate, creatinine, glutamate and aspartate were decreased. This demonstrates that there is an upregulation of anaerobic glycolysis（shown by the increase in lactate）, a perturbation of choline metabolism（suggested by the increase in choline）, neuronal cell damage（shown by the decrease in N-acetyl aspartate） and neurotransmitter imbalance（evidenced by the increase in γ-aminobutyric acid and glycine and by the decrease in glutamate and aspartate） in the acute stage of cerebral ischemia. In the contralateral hemisphere, the concentrations of lactate, alanine, glycine, choline and aspartate were increased, while the concentrations of γ-aminobutyric acid, glutamate and creatinine were decreased. This suggests that there is a difference in the metabolite changes induced by ischemic injury in the contralateral and ipsilateral cerebral hemispheres. Our findings demonstrate the presence of characteristic changes in metabolites in the contralateral hemisphere and suggest that they are most likely caused by metabolic changes in the ischemic hemisphere.

Responses and changes in the permafrost and snow water equivalent in the Northern Hemisphere under a scenario of 1.5℃ warming

In this study, the period that corresponds to the threshold of a 1.5℃ rise (relative to 1861e1880) in surface temperature is validated using a multi-model ensemble mean from 17 global climate models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). On this basis, the changes in permafrost and snow cover in the Northern Hemisphere are investigated under a scenario in which the global surface temperature has risen by 1.5℃, and the uncertainties of the results are further discussed. The results show that the threshold of 1.5℃ warming will be reached in 2027, 2026, and 2023 under RCP2.6, RCP4.5, RCP8.5, respectively. When the global average surface temperature rises by 1.5℃, the southern boundary of the permafrost will move 1e3.5
northward (relative to 1986e2005), particularly in the southern Central Siberian Plateau. The permafrost area will be reduced by 3.43x106 km2 (21.12%), 3.91x106 km2 (24.1%) and 4.15x106 km2 (25.55%) relative to 1986e2005 in RCP2.6, RCP4.5 and RCP8.5, respectively. The snow water equivalent will decrease in over half of the regions in the Northern Hemisphere but increase only slightly in the Central Siberian Plateau. The snow water equivalent will decrease significantly (more than 40% relative to 1986e2005) in central North America, western Europe, and northwestern Russia. The permafrost area in the QinghaieTibet Plateau will decrease by 0.15x106 km2 (7.28%), 0.18x 106 km2 (8.74%), and 0.17x106 km2 (8.25%), respectively, in RCP2.6, RCP4.5, RCP8.5. The snow water equivalent in winter (DJF) and spring (MAM) over the QinghaieTibet Plateau will decrease by 14.9% and 13.8%, respectively.