Development of the Western Boundary Undercurrent at Eirik Drift related to changing climate since the early Miocene

Eirik Drift lies on the slope south of Greenland, where it has been formed under the influence of the Western Boundary Undercurrent (WBUC) closely downstream of its origin in the deep-water formation centres of the Nordic Seas. Therefore, the sediment record at Eirik Drift documents modifications in pathways and intensity of the WBUC. These modifications reflect alterations in deep-water formation in the Nordic Seas and are therefore coupled with climate changes. Based on the seismostratigraphic analysis of sedimentary architecture identified in a set of high-resolution seismic reflection data, we have reconstructed the palaeocirculation of the WBUC at Eirik Drift since the early Miocene. We revealed a strong WBUC during warm climate conditions, and in phases of climate cooling with enhanced sea-ice extent we interpreted weak WBUC influence. We suggest a southward shift of the deep-water formation regions along with a shift of the deep current system during the cool phases. This shift implies that the main North Atlantic Deep Water route affected Eirik Drift only during warm phases and that during cool phases weak branches of the circulation system flowed over Eirik Drift.     

月内尺度北半球环状模水平和垂直活动特征分析

采用李建平等定义的更好表征北半球环状模变化的指数,利用NCEP/NCAR逐日再分析资料,从对流层的角度分析了北半球环状模在月内时间尺度上的相关环流的垂直和水平传播特征.结果表明,在水平方向上,温度异常在对流层,尤其是近地层,表现为由极区和高纬度地区向中低纬度地区强的南传特征,而在平流层则为由中低纬度地区向高纬度地区弱的...     

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.     

北半球对流层厚度的时空变化特征

利用1948—2010年NCEP/NCAR逐月位势高度再分析资料、美国国家海洋局提供的1948—2010年逐月海温再分析资料,分别定义了1 000—500 hPa和500—200 hPa厚度,利用EOF、SVD等方法研究了北半球对流层厚度时空演变特征及其与大气环流和海面温度的关系。结果表明,冬季平均厚度EOF第一模态均具有北太平洋及附近高纬度亚洲大陆地区与北美大陆高纬地区反位相变化的特点,而夏季第一模态则是北半球范围内较一致的位相分布;冬、夏季平均厚度EOF第二模态均突出体现了欧亚大陆及附近地区与北半球其他地区反位相变化的特点;冬、夏季厚度场的变化形势与大气环流及海面温度具有密切联系。     

Variability and trends in the annual snow‐cover cycle in Northern Hemisphere land areas, 1972–2000

This study investigated variability and trends in the annual snow‐cover cycle in regions covering high‐latitude and high‐elevation land areas in the Northern Hemisphere. The annual snow‐cover cycle was examined with respect to the week of the last‐observed snow cover in spring (WLS), the week of the first‐observed snow cover in autumn (WFS), and the duration of the snow‐free period (DSF). The analysis used a 29‐year time‐series (1972–2000) of weekly, visible‐band satellite observations of Northern Hemisphere snow cover from NOAA with corrections applied by D. Robinson of Rutgers University Climate Laboratory. Substantial interannual variability was observed in WLS, WFS and DSF (standard deviations of 0·8–1·1, 0·7–0·9 and 1·0–1·4 weeks, respectively), which is related directly to interannual variability in snow‐cover area in the regions and time periods of snow‐cover transition. Over the nearly three‐decade study period, WLS shifted earlier by 3–5 days/decade as determined by linear regression analysis. The observed shifts in the annual snow‐cover cycle underlie a significant trend toward a longer annual snow‐free period. The DSF increased by 5–6 days/decade over the study period, primarily as a result of earlier snow cover disappearance in spring. The observed trends are consistent with reported trends in the timing and length of the active growing season as determined from satellite observations of vegetation greenness and the atmospheric CO₂ record. Copyright © 2002 John Wiley & Sons, Ltd.     

Potential for Southern Hemisphere climate surprises

Climate model results suggest that future climate change in Antarctica will be accompanied by continued strengthening and poleward contraction of the Southern Ocean westerly wind belt. Paleoclimate records suggest past changes in the westerly winds can be abrupt and that healing of the Antarctic ozone hole could lead to poleward contraction of the westerlies and increased meridional atmospheric transport of warm air regionally into Antarctica. An abrupt shift to more meridional circulation could lead to notable changes in moisture availability for extra‐Antarctic regions, increased Antarctic ice sheet disintegration and more rapid sea‐level rise.     

南半球平流层极涡崩溃早晚年环流异常特征

极涡崩溃是平流层大气环流一个重要的变化过程,本文利用31年的再分析资料研究了南半球平流层极涡崩溃早晚年的异常特征.研究结果表明,南半球平流层极涡崩溃偏早年极涡崩溃前后平流层环流场异常表现为整层一致的变化,即都为正温度异常、正位势高度异常和负纬向风异常;而南半球平流层极涡崩溃偏晚年极涡崩溃前后平流层环流场异常的整层一致性的变化不典型,而在符号上与极涡崩溃偏早年的异常相反.与北半球平流层极涡崩溃前后环流异常相反明显不同,南半球平流层极涡崩溃偏早或偏晚年在极涡崩溃前后的环流异常保持相同的性质.进一步分析表明行星波活动在南极极涡的崩溃过程中起到了重要作用,极涡崩溃早年上传行星波比极涡崩溃晚年强,并且持续时间长.通过波流相互作用,行星波的异常使得极涡崩溃早年和晚年10月的平流层高纬地区分别为位势高度正异常和负异常,环流异常持续保持可能最终影响了南半球平流层极涡的崩溃时间.分析显示南半球极涡崩溃偏晚与La Niña事件之间可能存在一定的联系,但在极涡崩溃偏早年与赤道太平洋海表温度异常(SSTA)并无明显关系.     

一次北半球中高纬度地区冬季气象灾害分析

利用NOAA和NCDC原始资料,诊断分析了2007—2008年一次冬季大规模冰冻雨雪灾害天气,重点讨论了各地的阻塞高压(下称阻高)与这次灾害性天气产生原因之间的联系,并通过Rossby波的传播和发展对阻高的影响,分析了阻高产生的原因及在这次天气过程中的作用。结果表明,乌拉尔山阻高和鄂霍次克海阻高的共同作用,阻挡了来自极地冷气流的大股南下,使得来自北方的小股冷空气被阻高分离,南支气流南下与来自副热带高压的暖湿气流交汇,造成降温和持续性降雪,而随着冷平流的输入使阻高原地崩溃,东欧暖平流使阻高西侧新生高压脊发展成新的阻高。北美阻高随着地形作用也导致2007年底美国大部分地区和加拿大东部地区遭受暴风雪的袭击。Rossby波的传播与演变促使了阻高的形成,极涡南移过程中也造成这些地区的大雪和暴雪,阻高的调整(崩溃和重建)导致强冷空气的爆发。     

北半球大冰期(约2.73 Ma)前后北太平洋风尘沉积突变

距今约2.73 Ma,北半球高纬地区冰盖急剧发展,北半球大冰期来临,成为最引人注目的事件。北半球大冰期成因已有诸多解释,其中之一是大陆干旱化加剧,输入到大洋的粉尘增多引起海洋铁肥效应增强和生物量增多,吸收了更多大气CO2,使得地球变冷和冰盖增加。作为全球第二大粉尘释放中心,亚洲内陆干旱区释放的粉尘是北太平洋沉积物中主要陆源组分之一。开展北半球大冰期前后西北太平洋沉积物记录的亚洲粉尘研究,有助于深入认识风尘铁肥效应对北半球大冰期形成的可能贡献。文章依据大洋钻探计划(Ocean Drilling Program,简称ODP)198航次西北太平洋1208钻孔(共钻取沉积物392.3 m;本研究深度121.3 m至130.1 m,时间段为2.62 Ma至2.85 Ma)北半球大冰期前后的高分辨率(约2500年)样品,提取了沉积物中的"风尘组分",计算了风尘通量,研究了风尘通量变化和海洋生产力之间的关联。研究结果显示,约2.73 Ma以来,1208钻孔风尘通量快速增加,已发表的该孔的海洋生产力也表现出快速增加,海水表面温度也快速降低,说明粉尘对海洋的铁肥效应可能是触发北半球大冰期形成的重要因素。文章还测试了"风尘组分"的Nd和Sr同位素,研究了粉尘物源变动。结果显示,塔克拉玛干沙漠是北半球大冰期前后1208钻孔粉尘的主要物源,其中2.73 Ma至2.85 Ma期间Nd和Sr同位素波动较大,推测与火山灰含量较多和戈壁粉尘沉积有关,位置偏北的西风可能是部分戈壁粉尘长距离运输至北太平洋的主要营力。2.73 Ma以来,北极冰盖大量发育,西风位置南移,主要运输塔克拉玛干沙漠粉尘至北太平洋,使得Nd和Sr同位素波动较小,但其中2.72 Ma至2.70 Ma期间出现的Nd同位素偏正、Sr同位素变小很可能是火山灰的突然大量沉积造成的。