Representation of the Madden–Julian Oscillation in CAMS-CSM

The Madden–Julian Oscillation(MJO) has a significant impact on global weather and climate and can be used as a predictability resource in extended-term forecasting. We evaluate the ability of the Chinese Academy of Meteorological Sciences Climate System Model(CAMS-CSM) to represent the MJO by using the diagnostic method proposed by the US Climate Variability and Predictability Program(CLIVAR) MJO Working Group(MJOWG). In general,the model simulates some major characteristics of MJO well, such as the seasonality characteristics and geographical dependence, the intensity of intraseasonal variability(ISV), dominant periodicity, propagation characteristics, coherence between outgoing longwave radiation(OLR) and wind, and life cycle of MJO signals. However, there are a few biases in the model when compared with observational/reanalyzed data. These include an overestimate of precipitation in the convergence zone of the North and South Pacific, a slightly weaker eastward propagation, and a shift in the dominant periodicity toward lower frequencies with slower speeds of eastward propagation. The model gives a poor simulation of the northward propagation of MJO in summer and shows less coherence between the MJO convection and wind. The role of moistening in the planetary boundary layer(PBL) in the eastward/northward propagation of MJO was also explored. An accurate representation of the vertical titling structure of moisture anomalies in CAMS-CSM leads to moistening of the PBL ahead of convection, which accounts for the eastward/northward propagation of MJO. Poor simulation of the vertical structure of the wind and moisture anomalies in the western Pacific leads to a poor simulation of the northward propagation of MJO in this area. Budget analysis of the PBL integral moisture anomalies shows that the model gives a good simulation of the moisture charging process ahead of MJO convection and that the zonal advection of moisture convergence term has a primary role in the detour of MJO over the Maritime Continent.     

Intraseasonal variability of sea level and circulation in the Gulf of Thailand: the role of the Madden–Julian Oscillation

Intraseasonal variability of the tropical Indo-Pacific ocean is strongly related to the Madden–Julian Oscillation (MJO). Shallow seas in this region, such as the Gulf of Thailand, act as amplifiers of the direct ocean response to surface wind forcing by efficient setup of sea level. Intraseasonal ocean variability in the Gulf of Thailand region is examined using statistical analysis of local tide gauge observations and surface winds. The tide gauges detect variability on intraseasonal time scales that is related to the MJO through its effect on local wind. The relationship between the MJO and the surface wind is strongly seasonal, being most vigorous during the monsoon, and direction-dependent. The observations are then supplemented with simulations of sea level and circulation from a fully nonlinear barotropic numerical ocean model (Princeton Ocean Model). The numerical model reproduces well the intraseasonal sea level variability in the Gulf of Thailand and its seasonal modulations. The model is then used to map the wind-driven response of sea level and circulation in the entire Gulf of Thailand. Finally, the predictability of the setup and setdown signal is discussed by relating it to the, potentially predictable, MJO index.     

Madden–Julian Oscillation simulated in BCC climate models

This study evaluates the ability of four versions BCC (Beijing Climate Center or National Climate Center) models (BCC_AGCM2.1, BCC_AGCM2.2, BCC_CSM1.1 and BCC_CSM1.1m) in simulating the MJO phenomenon using the outputs of the AMIP (Atmospheric Model Intercomparison Project) and historical runs. In general, the models can simulate some major characteristics of the MJO, such as the intensity, the periodicity, the propagation, and the temporal/spatial evolution of the MJO signals in the tropics. There are still some biases between the models and the observation/reanalysis data, such as the overestimated total intraseasonal variability, but underestimated MJO intensity, shorter significant periodicity, and excessive westward propagation. The differences in the ability of simulating the MJO between AMIP and historical experiments are also significant. Compared to the AMIP runs, the total intraseasonal variability is reduced and more realistic, however the ratio between the MJO and its westward counterpart decreases in the historical runs. This unrealistic simulation of the zonal propagation might have been associated with the greater mean precipitation over the Pacific and corresponded to the exaggeration of the South Pacific Convergence Zone structure in precipitation mean state. In contrast to the T42 versions, the improvement of model resolution demonstrate more elaborate topography, but the enhanced westward propagation signals over the Arabia Sea followed. The underestimated (overestimated) MJO variability over eastern Indian Ocean (Pacific) was assumed to be associated with the mean state. Three sets of sensitive experiments using BCC_CSM1.1m turn out to support this argument.     

Modulation of tropical ocean surface chlorophyll by the Madden–Julian Oscillation

The MJO modulation of sea surface chlorophyll-a (Chl) examined initially by Waliser et al. in Geophys Res Lett, (2005) is revisited with a significantly longer time-series of observations and a more systematic approach to characterizing the possible mechanisms underlying the MJO-Chl relationships. The MJO composite analysis of Chl and lead-lag correlations between Chl and other physical variables reveal regional variability of Chl and corresponding indicative temporal relationships among variables. Along the path of the MJO convection, wind speed—a proxy for oceanic vertical turbulent mixing and corresponding entrainment—is most strongly correlated with Chl when wind leads Chl by a few days. Composite Chl also displays MJO influences away from the path of the MJO convection. The role of wind speed in those regions is generally the same for Chl variability as that along the path of the MJO convection, although Ekman pumping also plays a role in generating Chl variability in limited regions. However, the wind forcing away from the MJO convection path is less coherent, rendering the temporal link relatively weak. Lastly, the potential for bio-physical feedbacks at the MJO time-scale is examined. The correlation analysis provides tantalizing evidence for local bio-feedbacks to the physical MJO system. Plausible hypothesis for Chl to amplify the MJO phase transition is presented though it cannot be affirmed in this study and will be examined and reported in a future modeling study.     

The skill of atmospheric linear inverse models in hindcasting the Madden–Julian Oscillation

A suite of statistical atmosphere-only linear inverse models of varying complexity are used to hindcast recent MJO events from the Year of Tropical Convection and the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the Madden–Julian Oscillation mission periods, as well as over the 2000–2009 time period. Skill exists for over two weeks, competitive with the skill of some numerical models in both bivariate correlation and root-mean-squared-error scores during both observational mission periods. Skill is higher during mature Madden–Julian Oscillation conditions, as opposed to during growth phases, suggesting that growth dynamics may be more complex or non-linear since they are not as well captured by a linear model. There is little prediction skill gained by including non-leading modes of variability.     

Influence of Springtime Atlantic SST on ENSO: Role of the Madden–Julian Oscillation

Increased evidence has shown the important role of Atlantic sea surface temperature (SST) in modulating the El Niño–Southern Oscillation (ENSO). Persistent anomalies of summer Madden–Julian Oscillation (MJO) act to link the Atlantic SST anomalies (SSTAs) to ENSO. The Atlantic SSTAs are strongly correlated with the persistent anomalies of summer MJO, and possibly affect MJO in two major ways. One is that an anomalous cyclonic (anticyclonic) circulation appears over the tropical Atlantic Ocean associated with positive (negative) SSTA in spring, and it intensifies (weakens) the Walker circulation. Equatorial updraft anomaly then appears over the Indian Ocean and the eastern Pacific Ocean, intensifying MJO activity over these regions. The other involves a high pressure (low pressure) anomaly associated with the North Atlantic SSTA tripole pattern that is transmitted to the mid- and low-latitudes by a circumglobal teleconnection pattern, leading to strong (weak) convective activity of MJO over the Indian Ocean. The above results offer new viewpoints about the process from springtime Atlantic SSTA signals to summertime atmospheric oscillation, and then to the MJO of tropical atmosphere affecting wintertime Pacific ENSO events, which connects different oceans.     

The interannual variability of the Madden–Julian Oscillation in an ensemble of GCM simulations

The interannual variability of the Madden– Julian Oscillation (MJO) is investigated in an ensemble of 15 experiments performed with the ECHAM4 T30 general circulation model (GCM). The model experiments have been performed with AMIP conditions from January 1979 to December 1993. The MJO signal has been identified applying a principal oscillation pattern (POP) analysis to the 200-mb tropical velocity potential. The results obtained from the model ensemble are compared with 15?y of ECMWF re-analysis and OLR observations. The results suggest that the warm and cold phases of El Niño have some influence on the spatial propagation of the oscillation. Both in the re-analysis and in the model ensemble, the results indicate that during La Niña conditions the MJO is mostly confined west of the date line, with the largest activity located over the Indian Ocean and the western Pacific. In warm El Niño conditions, the convective anomalies associated with the oscillation appear to penetrate farther into the central Pacific. These changes in the MJO convective forcing seem to affect the zonal mean of the rotational component of the flow anomaly, which tends to weaken during warm El Niño periods. Some weak reproducibility of the interannual variability of the MJO activity is found. The results obtained from four-member and eight-member subsamples of the ensemble indicate that the reproducibility of the interannual behaviour of the MJO can be detected by choosing an ensemble of a larger size. Corresponding to the emergence of reproducibility with the increasing size of the sample, the correlation between the MJO activity and the Niño-3 SST anomaly appears to in-tensify.     

Satellite Measurements of the Madden–Julian Oscillation in Wintertime Stratospheric Ozone over the Tibetan Plateau and East Asia

We investigate the Madden–Julian Oscillation(MJO) signal in wintertime stratospheric ozone over the Tibetan Plateau and East Asia using the harmonized dataset of satellite ozone profiles. Two different MJO indices — the all-season Real-Time multivariate MJO index(RMM) and outgoing longwave radiation-based MJO index(OMI) — are used to compare the MJOrelated ozone anomalies. The results show that there are pronounced eastward-propagating MJO-related stratospheric ozone anomalies(mainly within 20–200 h Pa) over the subtropics. The negative stratospheric ozone anomalies are over the Tibetan Plateau and East Asia in MJO phases 4–7, when MJO-related tropical deep convective anomalies move from the equatorial Indian Ocean towards the western Pacific Ocean. Compared with the results based on RMM, the MJO-related stratospheric column ozone anomalies based on OMI are stronger and one phase ahead. Further analysis suggests that different sampling errors, observation principles and retrieval algorithms may be responsible for the discrepancies among different satellite measurements. The MJO-related stratospheric ozone anomalies can be attributed to the MJO-related circulation anomalies,i.e., the uplifted tropopause and the northward shifted westerly jet in the upper troposphere. Compared to the result based on RMM, the upper tropospheric westerly jet may play a less important role in generating the stratospheric column ozone anomalies based on OMI. Our study indicates that the circulation-based MJO index(RMM) can better characterize the MJOrelated anomalies in tropopause pressure and thus the MJO influence on atmospheric trace gases in the upper troposphere and lower stratosphere, especially over subtropical East Asia.     

Evaluation of the Madden–Julian oscillation in HiRAM

The aim of this study was to understand the cause of Madden–Julian oscillation (MJO) bias in the High Resolution Atmospheric Model (HiRAM) driven by observed SST through process-oriented diagnosis. Wavenumber-frequency power spectrum and composite analyses indicate that HiRAM underestimates the spectral amplitude over the MJO band and mainly produces non-propagating rather than eastward-propagating intraseasonal rainfall anomalies, as observed. Column-integrated moist static energy (MSE) budget analysis is conducted to understand the MJO propagation bias in the simulation. It is found that the bias is due to the lack of a zonally asymmetric distribution of the MSE tendency anomaly in respect to the MJO convective center, which is mainly attributable to the bias in vertical MSE advection and surface turbulent flux. Further analysis suggests that it is the unrealistic simulation of MJO vertical circulation anomalies in the upper troposphere as well as overestimation of the Rossby wave response that results in the bias.摘要本研究评估了高分辨率大气环流模式HiRAM模拟的MJO. 结果表明, HiRAM模拟的MJO东传很弱. 我们通过计算整层积分的湿静力能 (MSE) 收支来诊断MJO东传模拟偏差的原因. 结果发现, MSE倾向相对于MJO对流中心的纬向非对称分布很弱是导致东传模拟偏弱的原因, 这主要是由MSE垂直平流和地表湍流通量的模拟偏差造成的. 进一步研究表明, 对流层上层MJO垂直环流结构的模拟偏差和MJO对流西侧的Rossby波环流偏强共同导致了模式的偏差. 本研究中指出的MJO传播模拟偏差的原因与之前基于多模式结果的结论不同, 这意味着要想了解特定模式的模拟偏差, 有必要对该模式进行具体分析.     

Different Impacts of Madden–Julian Oscillation on Winter Rainfall over Southern China

Winter rainfall over southern China is usually enhanced when Madden–Julian oscillation(MJO) is active over the Indian Ocean, but it can be weakened under certain conditions. Here, the diversity of MJO impacts on winter rainfall and its mechanisms are explored by using scenarios of enhanced and suppressed rainfall anomalies over southern China when MJO is active over the Indian Ocean. The combined effects of low-frequency background moisture and intraseasonal winds are the major contributors to the different rainfall anomalies. Anomalous circulation in mid–high latitudes, especially on intraseasonal timescales, is almost opposite in the two scenarios, which can modulate the response of extratropical atmosphere to MJO heating and then induces the different circulations over southern China. In the enhanced scenario, mid–high latitudes of Eurasia and southern China are dominated by positive and negative sea level pressure anomalies, respectively. The southerly over southern China and the South China Sea induced by MJO heating promotes the anomalous moisture convergence and ascending motion over southern China, resulting in the enhanced rainfall. In the suppressed scenario, however, the circulation in mid–high latitudes does not favor rainfall over southern China and leads to the northerly response to MJO heating over southern China, which enhances moisture divergence and weakens rainfall over southern China.     

The influence of Madden–Julian Oscillation in the genesis of North Indian Ocean tropical cyclones

Cyclonic storms having maximum winds of 34 knots and above that had genesis in north Indian Ocean have been studied with respect to the eastward passage of Madden–Julian Oscillation (MJO). In the three decades (1979–2008), there were a total of 118 cyclones reported in which 96 formed in the region chosen (0–15^oN, 60^oE–100^oE) for the study. Although the percentage of MJO days inducing cyclogenesis is small, it is found that tropical cyclone genesis preferentially occurred during the convective phase of MJO. This accounted for 44 cyclones of the total 54 cyclones (i.e., 81.5%) formed under MJO amplitude 1 and above. The study has shown that, when the enhanced convection of MJO is over the maritime continent and the adjoining eastern Indian Ocean, it creates the highest favorable environment for cyclogenesis in the Bay of Bengal. During this phase, westerlies at 850 hPa are strong in the equatorial region south of Bay of Bengal creating strong cyclonic vorticity in the lower troposphere along with the low vertical wind shear.     

Climate tendencies of changes in multiyear sea ice thickness in the Arctic Basin (1970–2005)

The area integral of the sea ice thickness in the Arctic Basin is estimated from the measurements of sea ice surface fluctuations at drift-ice stations. The 1970–1990 linear trend is indicative of an approximately 10-cm reduction in the average sea ice thickness over the entire Arctic Basin, which makes 3% of the average ice thickness (about 3 m). Seasonal changes made 40 cm. The amplitude of variations of the average ice thickness in that period is 20 cm with a period of changes of approximately 6–8 years. The observations were interrupted during 1991–2003 and then resumed in 2004. During 1990–2005, the old ice thickness over the entire Arctic Basin decreased, on average, by 110 cm.     

Delving into the relationship between autumn Arctic sea ice and central–eastern Eurasian winter climate

北极海冰的快速减少是否已经显著地影响了最近中纬度大陆冬季极端天气气候事件引起了气候学家的广泛争论。问题的争论是来源于观测数据的年限很短以及中高纬度复杂的内部变率。在本研究中,采用气候突变检测的方法,我们将秋季海冰覆盖面积的变化分为三个阶段:1979–1986(高海冰阶段),1987–2006(海冰缓慢减少阶段)和2007–2014(海冰快速减少阶段)。然后,我们分析了与每一个阶段秋季海冰变化相联系的中-东欧亚地区冬季气候(尤其极端天气事件)是什么。结果表明北极海冰减少对西伯利亚西部和东亚极端天气事件影响的信号是稳健可测的。伴随着海冰的快速减少,高低空急流速度的减弱和急流位置的南移;波动振幅的加强、乌拉尔山阻塞频率的增多。这些导致了寒潮事件从亚洲中部到中国东北部地区显著增多。并且,与北极海冰的快速减少相关的环流异常与观测到的环流异常基本一致。相反地,在高海冰阶段,与海冰相关的环流异常和观测的异常并不一致。这个阶段的环流异常是与北极涛动处于持续的负位相有关的。     

Influences of Madden–Julian Oscillations on the eastern Indian Ocean and the maritime continent

Oceanic response to Madden–Julian Oscillations (MJOs) is studied with satellite data, mooring observations, and reanalysis products to demonstrate that oceanic intraseasonal variabilities are a direct response to the atmospheric intraseasonal forcing. They propagate eastward to the Sumatran coast and southward along the coast to the southeastern Indian Ocean (SEIO) and the maritime continent, as coastal Kelvin waves. MJOs contribute to about 20% of the intraseasonal variabilities in the SEIO and the maritime continent. In addition, MJOs reduce the annual mean Indonesian Throughflow (ITF) and the associated westward temperature advection. However, MJOs only have slight influences on the peak ITF in boreal summer. The importance of INSTANT data is obvious not only for understanding of ITF but also for improving ocean reanalysis and should eventually lead to improved predictive understanding of phenomena such as MJOs.     

Will the summer sea ice in the Arctic reach a tipping point?

The Arctic sea-ice cover has decreased in extent, area, and thickness over the last six decades. Most global climate models project that the summer sea-ice extent (SIE) will decline to less than 1 million (mill.) km² in this century, ranging from 2030 to the end of the century, indicating large uncertainty. However, some models, using the same emission scenarios as required by the Paris Agreement to keep the global temperature below 2°C, indicate that the SIE could be about 2 mill. km² in 2100 but with a large uncertainty of ±1.5 mill. km². Here, the authors take another approach by exploring the direct relationship between the SIE and atmospheric CO₂ concentration for the summer–fall months. The authors correlate the SIE and ln(CO₂/CO₂r) during the period 1979–2022, where CO₂r is the reference value in 1979. Using these transient regression equations with an R² between 0.78 and 0.87, the authors calculate the value that the CO₂ concentration needs to reach for zero SIE. The results are that, for July, the CO₂ concentration needs to reach 691 ± 16.5 ppm, for August 604 ± 16.5 ppm, for September 563 ± 17.5 ppm, and for October 620 ± 21 ppm. These values of CO₂ for an ice-free Arctic are much higher than the targets of the Paris Agreement, which are 450 ppm in 2060 and 425 ppm in 2100, under the IPCC SSP1-2.6 scenario. If these targets can be reached or even almost reached, the “no tipping point” hypothesis for the summer SIE may be valid.