Revisiting effect of ocean diapycnal mixing on Atlantic meridional overturning circulation recovery in a freshwater perturbation simulation

The effects of ocean density vertical stratification and related ocean mixing on the transient response of the Atlantic meridional overturning circulation （AMOC） are examined in a freshwater perturbation simulation using the Bergen Climate Model （BCM）. The results presented here are based on the model outputs of a previous freshwater experiment： a 300-year control integration （CTRL）, a freshwater integration （FW1） which started after 100 years of running the CTRL with an artificially and continuously threefold increase in the freshwater flux to the Greenland-Iceland-Norwegian （GIN） Seas and the Arctic Ocean throughout the following 150-year simulation. In FW1, the transient response of the AMOC exhibits an initial decreasing of about 6 Sv （1 Sv=106 m3 s^-1） over the first 50-year integration and followed a gradual recovery during the last 100-year integration. Our results show that the vertical density stratification as the crucial property of the interior ocean plays an important role for the transient responses of AMOC by regulating the convective and diapycnal mixings under the enhanced freshwater input to northern high latitudes in BCM in which the ocean diapycnal mixing is stratification-dependent. The possible mechanism is also investigated in this paper.     

Simulated Spatiotemporal Response of Ocean Heat Transport to Freshwater Enhancement in North Atlantic and Associated Mechanisms

The Atlantic Meridional Overturning Circulation(AMOC)transports a large amount of heat to northern high latitudes,playing an important role in the global climate change.Investigation of the freshwater perturbation in North Atlantic(NA)has become one of the hot topics in the recent years.In this study,the mechanism and pathway of meridional ocean heat transport(OHT)under the enhanced freshwater input to the northern high latitudes in the Atlantic are investigated by an ocean-sea ice-atmosphere coupled model.The results show that the anomalous OHT in the freshwater experiment(FW)is dominated by the meridional circulation kinetic and ocean thermal processes.In the FW,OHT drops down during the period of weakened AMOC while the upper tropical ocean turns warmer due to the retained NA warm currents.Conversely,OHT recovers as the AMOC recovers,and the mechanism can be generalized as:1)increased ocean heat content in the tropical Southern Ocean during the early integration provides the thermal condition for the recovery of OHT in NA;2)the OHT from the Southern Ocean enters the NA through the equator alongthe deep Ekman layer;3)in NA,the recovery of OHT appears mainly along the isopycnic layers of 24.70-25.77 below the mixing layer.It is then transported into the mixing layer from the "outcropping points"innorthern high latitudes,and finally released to the atmosphere by the ocean-atmosphere heat exchange.     

Assessing the Stability of the Atlantic Meridional Overturning Circulation of the Past, Present, and Future

This paper is a review of the recent development of researches on the stability of the Atlantic meridional overturning circulation （AMOC）. In particular, we will review recent studies that attempt to best assess the stability of the AMOC in the past, present, and future by using a stability indicator related to the freshwater transport by the AMOC. These studies further illustrate a potentially systematic bias in the state-of-the-art atmosphere-ocean generM circulation models （AOCCMs）, in which the AMOCs seem to be over-stabilized relative to that in the real world. This common model bias in the AMOC stability is contributed, partly, to a common tropical bias associated with the double intertropical convergence zone （ITCZ） in most state-of-the- art AOGCMs, casting doubts on future projection of abrupt climate changes in these climate models.     

IMPROVEMENT OF NUMERICAL SIMULATION OF TYPHOON RANANIM (0414) BY USING DOPPLER RADAR DATA

Typhoon Rananim (0414) has been simulated by using the non-hydrostatic Advanced Regional Prediction System (ARPS) from Center of Analysis and Prediction of Storms (CAPS). The prediction of Rananim has generally been improved with ARPS using the new generation CINRAD Doppler radar data. Numerical experiments with or without using the radar data have shown that model initial fields with the assimilated radar radial velocity data in ARPS can change the wind field at the middle and high levels of the troposphere; fine characteristics of the tropical cyclone (TC) are introduced into the initial wind, the x component of wind speed south of the TC is increased and so is the y component west of it. They lead to improved forecasting of TC tracks for the time after landfall. The field of water vapor mixing ratio, temperature, cloud water mixing ratio and rainwater mixing ratio have also been improved by using radar reflectivity data. The model’s initial response to the introduction of hydrometeors has been increased. It is shown that horizontal model resolution has a significant impact on intensity forecasts, by greatly improving the forecasting of TC rainfall, and heavy rainstorm of the TC specially, as well as its distribution and variation with time.     

Mechanisms of Atlantic Meridional Overturning Circulation (AMOC) Variability in a Coupled Ocean–Atmosphere GCM简

The mechanisms involved in the variability of Atlantic Meridional Overturning Circulation （AMOC） are studied using a 2000-yr control simulation of the coupled Fast Ocean-Atmosphere Model （FOAM）.This study identifies a coupled mode between SST and surface heat flux in the North Atlantic at the decadal timescale,as well as a forcing mode of surface heat flux at the interannual timescale.The coupled mode is regulated by AMOC through meridional heat transport.The increase in surface heating in the North Atlantic weakens the AMOC approximately 10 yr later,and the weakened AMOC in turn decreases SST and sea surface salinity.The decreased SST results in an increase in surface heating in the North Atlantic,thus forming a positive feedback loop.Meanwhile,the weakened AMOC weakens northward heat transport and therefore lowers subsurface temperature approximately 19 yr later,which prevents the AMOC from weakening.In the forcing mode,the surface heat flux leads AMOC by approximately 4 yr.     

Sensitivity Experiments on Summer Monsoon Circulation Cell in East Asia

The East-Asian summer monsoon meridional circulation (SMMC) cell is simulated together with two vigorous rainbands in terms of a primitive-equation model including in itself a variety of diabatic heating, frictional dissipation and moist processes under the condition of mountains available. Results are comparable to observations. Also, performed are experiments with the reduction of water content, and exclusion of the cumulus convective process and mountain effect. Contrast analyses indicate that the cell is strongly sensitive to the condition of the humidity field in the atmosphere, more intensely at 120°than at 100°E, and the presence (absence) of the cumulus convection has considerable effect on the intensification(weakening) of the cell, with the mountain ranges exhibiting more influence upon the cell at 100° than 120°E. This may suggest that a great difference lies in the cause of the cell for the two meridions.     

A Two-Moment Bulk Microphysics Coupled with a Mesoscale Model WRF: Model Description and First Results

The Chinese Academy of Meteorological Sciences (CAMS) two-moment bulk microphysics scheme was adopted in this study to investigate the representation of cloud and precipitation processes under different environmental conditions.The scheme predicts the mixing ratio of water vapor as well as the mixing ratios and number concentrations of cloud droplets,rain,ice,snow,and graupel.A new parameterization approach to simulate heterogeneous droplet activation was developed in this scheme.Furthermore,the improved CAMS scheme was coupled with the Weather Research and Forecasting model (WRF v3.1),which made it possible to simulate the microphysics of clouds and precipitation as well as the cloud-aerosol interactions in selected atmospheric condition.The rain event occurring on 27-28 December 2008 in eastern China was simulated using the CAMS scheme and three sophisticated microphysics schemes in the WRF model.Results showed that the simulated 36-h accumulated precipitations were generally agreed with observation data,and the CAMS scheme performed well in the southern area of the nested domain.The radar reflectivity,the averaged precipitation intensity,and the hydrometeor mixing ratios simulated by the CAMS scheme were generally consistent with those from other microphysics schemes.The hydrometeor number concentrations simulated by the CAMS scheme were also close to the experiential values in stratus clouds.The model results suggest that the CAMS scheme performs reasonably well in describing the microphysics of clouds and precipitation in the mesoscale WRF model.     

高纬度淡水强迫增强背景下大西洋经向翻转环流的响应及其机制

利用卑尔根海洋－大气－海冰耦合气候模式(Bergen Climate Model, 简称BCM), 研究在北冰洋及北欧海淡水强迫增强的背景下, 大西洋经向翻转环流(Atlantic Meridional Overturning Circulation, 简称AMOC)的响应及其机制, 着重讨论了海表热力性质、北大西洋深层水 (North Atlantic Deep Water, 简称NADW) 的生成率、 海洋内部等密度层间的垂直混合 (Diapycnal Mixing, 简称DM) 以及大气风场等物理过程随AMOC的响应所发生的时间演变特征。结果显示, 在持续150年增强 (强度为0.4 Sv) 的淡水强迫下 (淡水试验, FW1), AMOC的强度表现为前50年的快速减弱和在接下来100年中的逐渐恢复。同时, 在淡水试验的前50年北大西洋高纬度海表盐度 (Sea Surface Salinity, 简称SSS) 减小, 海水密度降低, 冬季对流混合减弱, 导致NADW生成率快速减弱; 在接下来的100年中, 尽管增强的淡水强迫依然维持, 由于海洋内部自身的调节和海气相互作用, 导致了AMOC的逐渐恢复。恢复机制可以概括为: (1) 随着向南的NADW的减少, 大西洋中低纬度海水垂直层结逐渐减弱, DM随之逐渐增强, 有利于中低纬度海盆内深层水的上升; (2) 南半球西风应力增强与东风应力的减弱及北半球东风的增强使得大西洋向北的埃克曼体积通量净传输恢复; (3) 大西洋向北的盐度传输逐渐恢复及次极地回旋区降水的减弱, 导致SSS和NADW生成率的恢复, 与之对应, AMOC逐渐恢复。研究还发现, 淡水试验中, NADW的恢复主要以厄尔明格海 (Irminger Sea) 为主, 冬季北大西洋海平面气压场 (SLP) 呈现类似正北大西洋涛动 (NAO+) 的模态, 热带降水中心移到赤道以南, 大西洋热带SSS增强。     

耦合模式FGOALS-g2中大西洋经向翻转流对3个典型浓度路径的响应

利用2个关于大西洋经向翻转流(Atlantic Meridional Overturning Circulation,AMOC)的指数:AMOC指数(15oN~65oN、深度为500 m以下的AMOC的最大值)和AMOC扩展指数(15oN~65oN、深度为2000~2500m的AMOC的最大值),研究了耦合模式FGOALS-g2(Grid-point Version 2 of Flexible Global Ocean-AtmosphereLand System Model)中的AMOC在CMIP5(Coupled Model Intercomparison Project Phase 5)的3个典型浓度路径(Representation Concentration Pathways,RCP)(RCP2.6、RCP4.5和RCP8.5分别对应于2100年时490、650和1370 ppm的CO2浓度水平)下的响应问题,发现:在RCP2.6和RCP4.5浓度路径下,2006~2040年时间段内AMOC指数和AMOC扩展指数都呈现快速下降的趋势,2041~2100年时间段内AMOC指数逐渐恢复,AMOC扩展指数基本维持不变;在RCP8.5浓度路径下,2006~2100年时间段内AMOC指数和AMOC扩展指数都表现出快速下降的趋势。通过分析FGOALS-g2中北大西洋深水的成因发现:3个典型浓度路径下AMOC的长期变化趋势主要受到GIN(Greenland–Iceland–Norwegian)海域的深水形成率的调控,而AMOC的年代际尺度的变化则主要受到Labrador海域深水形成率的控制。同时揭示了:由于北大西洋2000 m深度附近的层结稳定性在RCP2.6和RCP4.5下(相比于1980~2005年)提高了30%~40%,使得由AMOC指数恢复产生的深水无法继续下沉,从而导致AMOC扩展指数没有出现恢复的现象。     

Sensitivity of North Atlantic subpolar gyre and overturning to stratification-dependent mixing: response to global warming

We use a reduced complexity climate model with a three-dimensional ocean component and realistic topography to investigate the effect of stratification-dependent mixing on the sensitivity of the North Atlantic subpolar gyre (SPG), and the Atlantic meridional overturning circulation (AMOC), to idealized CO₂ increase and peaking scenarios. The vertical diffusivity of the ocean interior is parameterized as κ ∼ N ^−α, where N is the local buoyancy frequency. For all parameter values 0 ≤ α ≤ 3, we find the SPG, and subsequently the AMOC, to weaken in response to increasing CO₂ concentrations. The weakening is significantly stronger for α ≥ α_cr ≈ 1.5. Depending on the value of α, two separate model states develop. These states remain different after the CO₂ concentration is stabilized, and in some cases even after the CO₂ concentration has been decreased again to the pre-industrial level. This behaviour is explained by a positive feedback between stratification and mixing anomalies in the Nordic Seas, causing a persistent weakening of the SPG.     

Simulated variability of the Atlantic meridional overturning circulation

To examine the multi-annual to decadal scale variability of the Atlantic Meridional Overturning Circulation (AMOC) we conducted a four-member ensemble with a daily reanalysis forced, medium-resolution global version of the isopycnic coordinate ocean model MICOM, and a 300-years integration with the fully coupled Bergen Climate Model (BCM). The simulations of the AMOC with both model systems yield a long-term mean value of 18 Sv and decadal variability with an amplitude of 1–3 Sv. The power spectrum of the inter-annual to decadal scale variability of the AMOC in BCM generally follows the theoretical red noise spectrum, with indications of increased power near the 20-years period. Comparison with observational proxy indices for the AMOC, e.g. the thickness of the Labrador Sea Water, the strength of the baroclinic gyre circulation in the North Atlantic Ocean, and the surface temperature anomalies along the mean path of the Gulf Stream, shows similar trends and phasing of the variability, indicating that the simulated AMOC variability is robust and real. Mixing indices have been constructed for the Labrador, the Irminger and the Greenland-Iceland-Norwegian (GIN) seas. While convective mixing in the Labrador and the GIN seas are in opposite phase, and linked to the NAO as observations suggest, the convective mixing in the Irminger Sea is in phase with or leads the Labrador Sea. Newly formed deep water is seen as a slow, anomalous cold and fresh, plume flowing southward along the western continental slope of the Atlantic Ocean, with a return flow of warm and saline water on the surface. In addition, fast-travelling topographically trapped waves propagate southward along the continental slope towards equator, where they go east and continue along the eastern rim of the Atlantic. For both types of experiments, the Northern Hemisphere sea level pressure and 2 m temperature anomaly patterns computed based on the difference between climate states with strong and weak AMOC yields a NAO-like pattern with intensified Icelandic low and Azores high, and a warming of 0.25–0.5 °C of the central North Atlantic sea-surface temperature (SST). The reanalysis forced simulations indicate a coupling between the Labrador Sea Water production rate and an equatorial Atlantic SST index in accordance with observations. This coupling is not identified in the coupled simulation.     

潮汐混合对大西洋经圈翻转环流(AMOC)模拟影响的数值模拟研究

海洋中的潮汐混合对大西洋经圈翻转环流AMOC(Atlantic Meridional Overturning Circulation)模拟的影响是海洋环流模式研究的热点问题之一。本文采用IAP/LASG发展的气候系统海洋模式LICOM(LASG/IAP Climate system Ocean Model)及与海冰耦合模式进行了有无潮汐混合方案的试验,重点探讨了潮汐混合对AMOC强度模拟的影响。结果显示,引入潮汐混合后模拟的AMOC强度极大值比对照试验增加约1倍,更接近RAPID(Rapid Climate Change Programme)观测。而且,潮汐混合试验中模拟的AMOC上层环流深度(3200 m)比对照试验加深1000 m左右,同样更接近RAPID观测。海洋底部的垂直混合增强,使海洋层结变得更加不稳定,加强了北大西洋高纬地区,特别是拉布拉多海等地区的深对流,这是AMOC加强的直接原因。同时,潮汐混合试验中上层海洋环流也加强,增加了中低纬副热带高盐海水向高纬输送,使表层增密,海洋层结更加不稳定,也可以进一步增强AMOC。     

耦合模式FGOALS-s2模拟的大西洋经圈翻转环流（AMOC）的年代际变率

利用中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室（LASG/IAP）发展的耦合的气候系统模式FGOALS-s2工业革命前控制试验结果研究了大西洋经向翻转流（Atlantic Meridional Overturning Circulation,AMOC）的年代际变率及其物理机制。传统AMOC是利用深度坐标下的质量流函数来表征,本文通过对密度坐标下49.5°N的AMOC指数与其余纬度的AMOC指数作相关分析,发现AMOC的变化有从深水形成区向南传播的过程,且密度坐标下的AMOC变率在北大西洋高纬度明显大于低纬度。分析进一步表明,模式模拟的AMOC具有年代际振荡,周期约为70年。这个低频振荡主要是由与AMOC变化相关的温度和盐度的变化与海表风场之间的相互作用引起,具体机制如下:格陵兰-冰岛-挪威海有异常强的海表风场,导致蒸发增强,继而使海表盐度增加,深水形成增多,从而使AMOC增强。AMOC加强后,会使得向北的热量和盐度输送增加,减弱此处的经向温度梯度,风场随之减弱,从而完成位相的反转。     

Naturally forced multidecadal variability of the Atlantic meridional overturning circulation

The mechanisms by which natural forcing factors alone could drive simulated multidecadal variability in the Atlantic meridional overturning circulation (AMOC) are assessed in an ensemble of climate model simulations. It is shown for a new state-of-the-art general circulation model, HadGEM2-ES, that the most important of these natural forcings, in terms of the multidecadal response of the AMOC, is solar rather than volcanic forcing. AMOC strengthening occurs through a densification of the North Atlantic, driven by anomalous surface freshwater fluxes due to increased evaporation. These are related to persistent North Atlantic atmospheric circulation anomalies, driven by forced changes in the stratosphere, associated with anomalously weak solar irradiance during the late nineteenth and early twentieth centuries. Within a period of approximately 100 years the 11-year smoothed ensemble mean AMOC strengthens by 1.5 Sv and subsequently weakens by 1.9 Sv, representing respectively approximately 3 and 4 standard deviations of the 11-year smoothed control simulation. The solar-induced variability of the AMOC has various relevant climate impacts, such as a northward shift of the intertropical convergence zone, anomalous Amazonian rainfall, and a sustained increase in European temperatures. While this model has only a partial representation of the atmospheric response to solar variability, these results demonstrate the potential for solar variability to have a multidecadal impact on North Atlantic climate.