Predictability of large interannual Arctic sea-ice anomalies

In projections of twenty-first century climate, Arctic sea ice declines and at the same time exhibits strong interannual anomalies. Here, we investigate the potential to predict these strong sea-ice anomalies under a perfect-model assumption, using the Max-Planck-Institute Earth System Model in the same setup as in the Coupled Model Intercomparison Project Phase 5 (CMIP5). We study two cases of strong negative sea-ice anomalies: a 5-year-long anomaly for present-day conditions, and a 10-year-long anomaly for conditions projected for the middle of the twenty-first century. We treat these anomalies in the CMIP5 projections as the truth, and use exactly the same model configuration for predictions of this synthetic truth. We start ensemble predictions at different times during the anomalies, considering lagged-perfect and sea-ice-assimilated initial conditions. We find that the onset and amplitude of the interannual anomalies are not predictable. However, the further deepening of the anomaly can be predicted for typically 1 year lead time if predictions start after the onset but before the maximal amplitude of the anomaly. The magnitude of an extremely low summer sea-ice minimum is hard to predict: the skill of the prediction ensemble is not better than a damped-persistence forecast for lead times of more than a few months, and is not better than a climatology forecast for lead times of two or more years. Predictions of the present-day anomaly are more skillful than predictions of the mid-century anomaly. Predictions using sea-ice-assimilated initial conditions are competitive with those using lagged-perfect initial conditions for lead times of a year or less, but yield degraded skill for longer lead times. The results presented here suggest that there is limited prospect of predicting the large interannual sea-ice anomalies expected to occur throughout the twenty-first century.     

Large sea-ice volume anomalies simulated in a coupled climate model

Climate Dynamics - The processes leading to the formation of a large anomaly of sea-ice volume integrated over the Northern Hemisphere have been investigated in a coarse-resolution...     

Subseasonal transition of Barents-Kara sea-ice anomalies in winter related to the reversed warm Arctic-cold Eurasia pattern

“暖北极-冷欧亚”模态(WACE)的次季节反转会触发中国东部冬季的极端冷/暖转换以及春季沙尘暴.在这一显著的高纬度大气模态反转的驱动下,巴伦支海-喀拉海(BKS)的海冰异常也表现出次季节转换.当北极变暖以及乌拉尔山高压增强时,BKS异常向下的湍流热通量和增强的向下红外辐射有利于海冰融化;近地面的异常偏南风使海冰从薄冰区向多年冰区漂移,进一步扩大了开阔洋面.相反的机制出现在WACE反位相,导致BKS海冰异常偏多.当WACE次季节反转时,上述机制分别发生在前冬和后冬,使得BKS海冰异常出现显著的次季节转换.近10年随着WACE反转频率增多,BKS海冰前后冬的次季节变化增强.本文构建了WACE次季节反转的综合示意图,有助于更好地理解和预测中国东部极端气候.     

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.     

Change of Arctic sea-ice volume and its relationship with sea-ice extent in CMIP5 simulations

由于对未来北极海冰体积的变化研究相对较少,本文利用多模式比较计划模拟的海冰结果,对北极9月海冰体积及其与海冰覆盖范围的关系进行了分析。结果发现,相对于海冰覆盖范围,多模式模拟得到的北极海冰体积的差异跨度更大,但这种差异跨度随着时间的演变迅速减小,表明海冰体积可能是比海冰覆盖范围更为敏感的因子。同时少数几个能够更加合理反映观测的海冰特征与变化的模式的模拟结果显示,在高排放情景下,北极海冰在本世纪70年代时,基本达到了无冰的状态。     

冬季北半球大气对秋冬季巴伦支海海冰异常的敏感性研究

基于ERA-Interim再分析资料,借助大气模式CAM4,分析了北半球冬季不同月份的平均大气对巴伦支海不同振幅及不同季节海冰扰动的敏感性,并考察了中高纬度典型大气模态的分布变化情况.结果表明,冬季巴伦支海海冰的减少,会导致湍流热通量异常向上、局地异常变暖及水汽含量的异常升高,且相关异常的强度和范围随着海冰减少幅度的减...     

Causes of the record-low Antarctic sea-ice in austral summer 2022

2022年夏季(2021年12月至2022年2月)南极海冰面积达到历史新低,西南极减少最显著.2021年8～10月南半球环状模接近历史最强和7～9月海洋性大陆附近海温显著增暖是两个关键因素.由于平流层臭氧破纪录减少使得前者维持历史最强或接近最强,导致阿蒙森低压(ASL)加深并向西南移动,有利于海冰减少.后者持续到夏季,有利于拉尼娜的发展,通过激发罗斯贝波列加深ASL.在热力上,臭氧减少导致向下净短波辐射增加,引起西南极增暖.此外,净短波辐射-海温-云形成正反馈,与埃克曼输送一起,放大表面增暖,从而促进海冰融化.     

Time-dependence of sea-ice concentration and multiyear ice fraction in the Arctic Basin

The time variation of the sea-ice concentration and multiyear ice fraction within the pack ice in the Arctic Basin is examined, using microwave images of sea ice recently acquired by the Nimbus-5 spacecraft and the NASA CV-990 airborne laboratory. The images used for these studies were constructed from data acquired from the Electrically Scanned Microwave Radiometer (ESMR) which records radiation from earth and its atmosphere at a wavelength of 1.55 cm. Data are analyzed for four seasons during 1973–1975 to illustrate some basic differences in the properties of the sea ice during those times. Spacecraft data are compared with corresponding NASA CV-990 airborne laboratory data obtained over wide areas in the Arctic Basin during the Main Arctic Ice Dynamics Joint Experiment (1975) to illustrate the applicability of passive-microwave remote sensing for monitoring the time dependence of sea-ice concentration (divergence). These observations indicate significant variations in the sea-ice concentration in the spring, late fall and early winter. In addition, deep in the interior of the Arctic polar sea-ice pack, heretofore unobserved large areas, several hundred kilometers in extent, of sea-ice concentrations as low as 50% are indicated.     

Implementation of a one-dimensional enthalpy sea-ice model in a simple pycnocline prediction model for sea-ice data assimilation studies

To further explore enthalpy-based sea-ice assimilation, a one-dimensional(1D) enthalpy sea-ice model is implemented into a simple pycnocline prediction model. The 1D enthalpy sea-ice model includes the physical processes such as brine expulsion, flushing, and salt diffusion. After being coupled with the atmosphere and ocean components, the enthalpy sea-ice model can be integrated stably and serves as an important modulator of model variability. Results from a twin experiment show that the sea-ice data assimilation in the enthalpy space can produce smaller root-mean-square errors of model variables than the traditional scheme that assimilates the observations of ice concentration, especially for slow-varying states. This study provides some insights into the improvement of sea-ice data assimilation in a coupled general circulation model.     

On the climatology of persistent circulation anomalies in the atmosphere and in a general circulation model

Abstract

The climatologies of upper‐air persistent circulation anomalies found in observations of the Northern Hemisphere and in a General Circulation Model (GCM) integration are compared with each other and with those found in previous studies. The model simulation is that of the Canadian Climate Centre GCM run at resolution T20. The objective criteria that define the persistent events differ from those of some earlier investigations in that the anomalies are not required to be as nearly stationary. It is found that the GCM generates persistent circulation anomalies downstream of the synoptic‐scale storm tracks, in very nearly the correct geographical locations, but that the frequency of occurrence is too low in the model. A kinetic energy and streamfunction variance analysis is presented for both dataseis to clarify the differences between the observed and simulated distributions of circulation anomalies. It is evident that, apart from the mean annual cycle, the middle‐latitude transient eddies of the model are too weak.     

On the effect of ENSO precipitation anomalies in a global ocean GCM

An ocean general circulation model is used to study the influence of positive precipitation anomalies associated with El Nino and La Nina events. In this idealized model, the precipitation over the appropriate part of the equatorial Indo-Pacific region is doubled for one year. At the surface, salinity anomalies of up to –0.9 parts per thousand result from this anomalous precipitation. Perturbation surface currents ranging from 10–100% of the climatological values are induced in the tropical Indian and Pacific Oceans. A return flow is found beneath the thermocline with upwelling (downwelling) in (outside) the region of enhanced precipitation. The net effect of the precipitation anomalies is to generate a zonal overturning cell which transports fresher surface water away from the forcing region and replaces it with cooler, more saline water from below.     

2022年夏秋季鄱阳湖流域气象要素异常特征及干旱演变

2022年夏秋季鄱阳湖流域发生了历史罕见的气象干旱事件,科学分析此次干旱过程气象要素特征和干旱发展演变对开展干旱预报预警以及防旱抗旱具有重要意义。利用鄱阳湖流域内87个国家站1961—2022年的逐日降水量、气温和蒸发资料,应用本地化的综合气象干旱指数,分析此次干旱过程的主要气象要素异常特征和气象干旱发展演变。结果表明：2022年夏秋季,鄱阳湖流域及其五大支流平均降水量为1961年以来同期最少;平均气温和高温日数为同期新高或次高,多站极端最高气温创新高;各流域蒸发量远远大于降水量,且秋季降水蒸发差大于夏季;流域各地出现了夏秋连旱,且秋旱重于伏旱,其中9—10月全流域出现重旱或特旱。     

A mechanism of decadal variability of the sea-ice volume in the Northern Hemisphere

A long-term simulation performed with a coarse-resolution, global, atmosphere-ocean-sea-ice model displays strong decadal variability of the sea-ice volume in the Northern Hemisphere with a significant peak at about 15-18 years. This model results from the coupling of ECBILT, a spectral T21, 3-level quasi-geostrophic atmospheric model, and CLIO, a sea-ice-ocean general circulation model. First, the mechanism underlying the variability of ice volume in the model was studied by performing correlation analyses between the simulated variables. In a second step, a series of additional sensitivity experiments was performed in order to illustrate the role of specific physical processes. This has allowed us to identify a feedback loop in the ice-ocean system, which proceeds as follows: an increase in Arctic sea-ice volume induces an increase in the salinity there. This salinity anomaly is transported to the Greenland Sea where it promotes convective activity. This warms up the surface oceanic layer and the atmosphere in winter and induces a decrease of the ice volume, completing half a cycle. The changes in ice volume are driven by a geopotential height pattern characterised by centres of action of opposite signs over Greenland and the Barents-Kara-Central Arctic area. Thermodynamic feedback between the ice and the atmosphere appear also to be very important for the persistence of the oscillation. The dynamical response of the atmosphere to sea-ice and temperature anomalies at surface plays a smaller role.     

On Outflow Passages in the South China Sea

Both box and inverse methods are used to study the relative importance of outflow passages of the South China Sea.The physical meaning of the box model is described in detail,and its optimization is based on the most efficient way to remove the warm and fresh water in terms of the heat and salt budgets.The box model results suggest that the Kalimantan Strait is the main outflow passage,because the water near the Kalimantan is the warmest and freshest.The system of the South China Sea advects the warmest and...