Linking the northern hemisphere sea-ice reduction trend and the quasi-decadal arctic sea-ice oscillation

The nature of the reduction trend and quasi-decadal oscillation in Northern Hemisphere sea-ice extent is investigated. The trend and oscillation that seem to be two separate phenomena have been found in data. This study examines a hypothesis that the Arctic sea-ice reduction trend in the last three decades amplified the quasi-decadal Arctic sea-ice oscillation (ASIO) due to a positive ice/ocean-albedo feedback, based on data analysis and a conceptual model proposed by Ikeda et al. The theoretical, conceptual model predicts that the quasi-decadal oscillation is amplified by the thinning sea-ice, leading to the ASIO, which is driven by the strong positive feedback between the atmosphere and ice-ocean systems. Such oscillation is predicted to be out-of-phase between the Arctic Basin and the Nordic Seas with a phase difference of 3/4, with the Nordic Seas leading the Arctic. The wavelet analysis of the sea ice data reveals that the quasi-decadal ASIO occurred actively since the 1970s following the trend starting in the 1960s (i.e., as sea-ice became thinner and thinner), as the atmosphere experienced quasi-decadal oscillations during the last century. The wavelet analysis also confirms the prediction of such out-of-phase feature between these two basins, which varied from 0.62 in 1960 to 0.25 in 1995. Furthermore, a coupled ice-ocean general circulation model (GCM) was used to simulate two scenarios, one without the greenhouse gas warming and the other having realistic atmospheric forcing along with the warming that leads to sea-ice reduction trend. The quasi-decadal ASIO is excited in the latter case compared to the no-warming case. The wavelet analyses of the simulated ice volume were also conducted to derive decadal ASIO and similar phase relationship between the Arctic Ocean and the Nordic Seas. An independent data source was used to confirm such decadal oscillation in the upper layer (or freshwater) thickness, which is consistent with the model simulation. A modified feedback loop for the sea-ice trend and ASIO was proposed based on the previous one by Mysak and Venegas and the ice/albedo and cloud/albedo feedabcks, which are responsible for the sea ice reduction trend.     

Displacements and transformations of nitrate-rich and nitrate-poor water masses in the tropical Pacific during the 1997 El Niño

A Lagrangian analysis was applied to the outputs of a coupled physical-biogeochemical model to describe the redistribution of nitrate-rich and nitrate-poor surface water masses in the tropical Pacific throughout the major 1997 El Niño. The same tool was used to analyze the causes of nitrate changes along trajectories and to investigate the consequences of the slow nitrate uptake in the high nutrient low chlorophyll (HNLC) region during the growth phase of the event. Three patterns were identified during the drift of water masses. The first mechanism is well known along the equator: oligotrophic waters from the western Pacific are advected eastward and retain their oligotrophic properties along their drift. The second concerns the persistent upwelling in the eastern basin. Water parcels have complex trajectories within this retention zone and remain mesotrophic. This study draws attention to the third process which is very specific to the HNLC region and to the El Niño period. During the 1997 El Niño, horizontal and vertical inputs of nitrate decreased so dramatically that nitrate uptake by phytoplankton became the only mechanism driving nitrate changes along pathways. The study shows that because of the slow nitrate uptake characteristic of the tropical Pacific HNLC system, nitrate in the pre-El Niño photic layer can support biological production for a period of several months. As a consequence, the slow nitrate uptake delays the gradual onset of oligotrophic conditions over nearly all the area usually occupied by upwelled waters. Owing to this process, mesotrophic conditions persist in the tropical Pacific during El Niño events.     

Continued post-bleaching decline and changed benthic community of a Kenyan coral reef

During the global coral bleaching event of 1997/1998 Kenyan reefs experienced between 50% and 90% coral mortality, with coral cover at Malindi being reduced from 35–45% (pre-bleaching) to 10–20%. Even before this event there was concern that these reefs were being impacted by increased sediment loads from the nearby Sabaki River. Here we report that since 1998 coral cover has declined yet further with, in 2004, means of 5.1% being recorded at North Reef (within the non-fished Malindi Marine National Park) and 2.3% on Leopard Reef (within the fished Marine Reserve). Prior to bleaching 55 coral genera were recorded from the area, currently we find only 23. Meanwhile algal cover, especially the calcareous green alga Halimeda, has increased, and on Leopard Reef is twice that on North Reef. Taken with the evidence of previous studies, these data suggest a combined impact of coral bleaching with sedimentation and fishing.     

北大西洋涛动位相转换的数值研究

借助一个非线性准地转正压模型,模拟了基本西风气流的强弱、不同空间分布的天气尺度扰动涡对北大西洋涛动(NAO)位相转换的作用,以及大尺度双波地形对其的影响.通过一系列的对比试验发现,适当弱的基本西风气流有利于下游系统西退使NAO由正(负)位相转换为负(正)位相;振幅强、活动范围大的天气尺度扰动波是NAO由负位相转换为正位...     

一个观察北极涛动与北大西洋涛动关系的典型个例

北极涛动与大西洋涛动是否属同一气候变率模态一直是北极涛动动力学研究方面的一个颇具争议的话题。文中通过对"0801南方雪灾"期间及其前后北极涛动与北大西洋涛动异常及产生原因进行个例分析,对两者之间的关系进行了讨论。首先使用交叉子波变换与子波相关方法分析了两者的相位关系。发现在30—60天时间尺度上北极涛动与北大西洋涛动相位相差90°或-90°。而在10 20天这一尺度上北极涛动与北大西洋涛动具有大致相同的相位。对北极涛动及北大西洋涛动形成的动力过程及其在拉尼娜背景下各自特点的分析表明,这种不同尺度上位相关系的差异来自于波-流相互作用动力学的局域性。众所周知,北极涛动的3个活动中心的形成与分别位于北大西洋、北太平洋和北极平流层的3个波流相互作用中心有关。而北大西洋涛动则主要与位于北大西洋的波-流相互作用中心有关。拉尼娜事件的出现通过影响太平洋急流及行星尺度的准定常波从而进一步强化了30-60天时间尺度上北极涛动与北大西洋涛动的这种差异。这主要是因为太平洋急流或准定常行星波在对流层中直接影响了位于该区域的北极涛动的活动中心。同时准定常行星波冬季向上传播至平流层并与平流层极涡相互作用从而也影响了北极涛动在北极的活动中心。而在10—20天时间尺度上的北极涛动与北大西洋涛动同步关系则说明它们都是北极涛动的另一活动中心即大西洋上同一波-流相互作用现象——天气波破碎的反映。基于上述分析.文中倾向于认同将北极涛动和北大西洋涛动区别考虑的观点。     

Increased Tibetan Plateau Snow Depth：An Indicator of the Connection between Enhanced Winter NAO and Late-Spring Tropospheric Cooling over East Asia

The authors present evidence to suggest that variations in the snow depth over the Tibetan Plateau (TP) are connected with changes of North Atlantic Oscillation (NAO) in winter (JFM). During the positive phase of NAO, the Asian subtropical westerly jet intensifies and the India-Myanmar trough deepens. Both of these processes enhance ascending motion over the TP. The intensified upward motion, together with strengthened southerlies upstream of the India-Myanmar trough, favors stronger snowfall over the TP, which is associated with East Asian tropospheric cooling in the subsequent late spring (April--May). Hence, the decadal increase of winter snow depth over the TP after the late 1970s is proposed to be an indicator of the connection between the enhanced winter NAO and late spring tropospheric cooling over East Asia.     

Warming in the northwestern Indian Ocean associated with the El Niño event

This paper investigates possible warming effects of an E1 Nifio event on the sea surface temperature anomaly （SSTA） in the northwestern Indian Ocean. Most pure positive Indian Ocean dipole （IOD） events （without an E1 Nifio event co-occurring） have a maximum positive SSTA mainly in the central Indian Ocean south of the equator, while most co-occurrences with an E1 Nifio event exhibit a northwest-southeast typical dipole mode. It is therefore inferred that warming in the northwestern Indian Ocean is closely related to the E1 Nifio event. Based on the atmospheric bridge theory, warming in the northwestern Indian Ocean during co-occurring cases may be primarily caused by relatively less latent heat loss from the ocean due to reduced wind speed. The deepened thermocline also contributes to the warming along the east coast of Africa through the suppressed upwelling of the cold water. Therefore, the E1 Nifio event is suggested to have a modulating effect on the structure of the dipole mode in the tropical Indian Ocean.     

The Antarctic oscillation-East Asian summer monsoon connections in NCEP-1 and ERA-40

Connections between the spring Antarctic Oscillation(AAO)and the East Asian summer monsoon (EASM)in two reanalysis datasets—NCEP-1(NCEP/NCAR Reanalysis 1)and ERA-40(ECMWF 40- year Reanalysis)—are investigated in this study.Both show significant correlation between AAO and EASM rainfall over the Yangtze River valley,especially after about 1985.Though ERA-40 shows weaker anomalous signals connecting AAO and EASM over southern high latitudes than NCEP-1,both datasets reveal similar connecting patterns between ...     

平流层准两年周期振荡对CH4双峰的影响

分析了1992～2001年1月HALOE(卤素掩星试验装置)的CH4资料,研究了CH4的多年平均分布,结果表明:CH4混合比在平流层下层较大,向上迅速减小.同时,中纬度与热带CH4混合比的等压面梯度也随着高度逐渐减小,甚至在平流层顶附近和平流层上层形成双峰,这在北半球的春夏季特别明显.作者用NCAR的SOCRTAES二维模式的模拟结果来分析CH4混合比在平流层顶和平流层上层形成双峰的机理.模式作了以下两个模拟,其一为没有热带纬向风场QBO(准两年振荡)的状况,其二为加入QBO强迫后的模拟.结果发现QBO引起的余差环流对CH4混合比在热带和副热带平流层顶和平流层上层的输送是影响CH4双峰的一个主要原因.     

对ENSO循环机理的一些新认识

近几年国内学者通过资料诊断、理论分析和数值试验,对ENSO循环的机理提出了一些新的看法,其中包括El Nino和La Nina的起源,与ENSO循环相联系的暖、冷水的传播过程,热带西太平洋纬向风应力异常,以及热带东太平洋经向风应力异常在ENSO循环中的动力作用等.作者介绍了我国在这些方面的一些研究成果.