在大约6～7亿年前的新元古代时期，地球是否曾经被冰雪完全覆盖而成为了一个“冰雪地球”?如果是，什么诱发了这种全球性的冰川期?又是什么导致了它的融化?新元古代时期的极端气候变化对其后的寒武纪生命大爆发有何影响?围绕这些问题，古地质、古生物和古气候学界在最近几年展开了广泛的研究和激烈争论。根据现有的研究结果，地球在新元古代时期确实经历了数次地球历史上最为严重的全球性冰川期，但地球是否被完全冰封还需要更充分的古地质和古生物方面的证据来证明；利用气候模式对各种可能的外部强迫的模拟试验表明“冰雪地球”是很难形成的。并且，如果地球进入完全被冰封的状态，它将是难以被融化的；关于新元古代时期剧烈的气候变化对寒武纪生命大爆发所起的作用存在2种观点，一种认为气候变化导致了原始生命的基因突变并诱发了寒武纪生命爆发，另一种认为这种影响主要是生态方面的。

人们传统上认为大气平流层很少能对对流层产生重要影响，但是，最近几年的观测研究表明这种自上而下的影响是显著的和非常重要的，特别地，近几年关于北极涛动的研究极大推动了这一问题的研究进展。这方面的研究发现平流层异常可以对对流层天气系统产生重要影响，也就是冬季平流层北极涛动(AO)的负异常可以诱发中高纬度地区寒潮天气，而AO的正异常则导致中高纬度地区的温暖晴朗天气。由于观测分析表明平流层AO的异常信号总是领先对流层AO异常，一些学者甚至建议冬季北半球平流层的异常信号可以作为预报对流层天气变化的先行指标，并可以把对流层天气预报的时限提高到3个星期以上。综述这一领域在最近几年的研究进展、阐述平流层异常影响对流层天气系统的物理机制和总结各种不同的学术观点，并对将来研究中应注意的问题提出了建议。

大气平流层臭氧的严重损耗将导致到达地面的太阳紫外辐射增加已经是公共社会普遍关切的问题。最近几年，平流层臭氧损耗能否对对流层大气环流和气候产生重要影响也引起了广泛的重视，并且已成为大气科学的一个热点问题。观测研究表明近二三十年来在两极臭氧严重损耗的季节，地表气温有明显的变暖趋势，也就是欧亚大陆1—3月份和南极半岛12—5月份的地面温度呈现显著增温。这些增温趋势很可能是平流层两极的严重臭氧损耗造成的。当然，这些增温并非是由臭氧损耗的辐射效应直接造成的，而是通过波一流相互作用以及平流层一对流层相互作用等动力学过程实现的。大量的数值模拟工作也对此问题进行了研究，一些模拟结果与观测结果基本一致，另一些则发现平流层臭氧损耗的强迫作用较弱，也有模拟工作认为其他外界强迫如温室气体增加和热带海面温度升高也非常重要。作者综述了这一研究领域在近几年的最新研究进展，总结有关的观测分析和数值模拟结果，阐述平流层极地臭氧损耗可能影响地面温度的物理机制，以及澄清各种争论要点。

Decadal trends are compared in various fields between Northern Hemisphere early winter, November–December (ND), and late-winter, February–March (FM), months using reanalysis data. It is found that in the extratropics and polar region the decadal trends display nearly opposite tendencies between ND and FM during the period from 1979 to 2003. Dynamical trends in late winter (FM) reveal that the polar vortex has become stronger and much colder and wave fluxes from the troposphere to the stratosphere are weaker, consistent with the positive trend of the Arctic Oscillation (AO) as found in earlier studies, while trends in ND appear to resemble a trend toward the low-index polarity of the AO. In the Tropics, the Hadley circulation shows significant intensification in both ND and FM, with stronger intensification in FM. Unlike the Hadley cell, the Ferrel cell shows opposite trends between ND and FM, with weakening in ND and strengthening in FM. Comparison of the observational results with general circulation model simulations is also discussed.

The probability distribution function (PDF) of a passive tracer, forced by a \mean gradient", is studied. First, we take two theoretical approaches, the Lagrangian and the conditional closure formalisms, to study the PDFs of such an externally forced passive tracer. Then, we carry out numerical simulations for an idealized random flow on a sphere and for European Center for Medium-Range Weather Forecasts (ECMWF) stratospheric winds to test whether the mean-gradient model can be applied to studying stratospheric tracer mixing in midlatitude surf zones, in which a weak and poleward zonal-mean gradient is maintained by tracer leakage through polar and tropical mixing barriers, and whether the PDFs of tracer fluctuations in midlatitudes are consistent with the theoretical predictions. The numerical simulations show that when diffusive dissipation is balanced by the mean-gradient forcing, the PDF in the random flow and the Southern-Hemisphere PDFs in ECMWF winds show time-invariant exponential tails, consistent with theoretical predictions. In the Northern Hemisphere, the PDFs exhibit non-Gaussian tails. However, the PDF tails are not consistent with theoretical expectations. The long-term behavior of the PDF tails of the forced tracer is compared to that of a decaying tracer. It is found that the PDF tails of the decaying tracer are time-dependent, and evolve toward flatter than exponential.