A latent heating peak in the PBL was detected in a simulation by a global GCM that failed to reproduce Madden Julian Oscillation （MJO）. The latent heating peak in the PBL was generated by very shallow convection, which prevented moisture from being transported to the free troposphere. Large amount of moisture was therefore confined to the PBL, leading to a dry bias in the free atmosphere. Suffering from this dry bias, deep convection became lethargic, and MJO signals failed to occur. When the latent heating peak in the PBL was removed in another simulation, reasonable MJO signals, including the eastward propagation and the structure of its large-scale circulation, appeared. We therefore propose that the excessive latent heating peak in the PBL due to hyperactive shallow convection may be a reason for a lack of MJO signals in some simulations by other GCMs as well.
<正>Severe drought occurred in northern China in winter 2008-09 and the La Nina event might have exerted a majo...
Hui Gao and Song Yang 1 National Climate Center,China Meteorological Administration, Beijing,China. 2 Climate Prediction Center,NOAA,Camp Springs,Maryland,USA.
Impacts of the MJO on winter rainfall and circulation in China are investigated using a real-time multivariate MJO index.Composite results using the daily rainfall anomalies and "rainy day" anomalies according to eight different MJO phases show that the MJO has considerable influence on winter rainfall in China. Rainfall anomalies show systematic and substantial changes(enhanced/suppressed) in the Yangtze River Basin and South China with the eastward propagation of the MJO convective center from the Indian Ocean to the western Pacific.When the MJO is in phase 2 and 3(MJO convective center is located over the Indian Ocean),rainfall probability is significantly enhanced.While in phase 6 and 7(MJO convective center is over the western Pacific),rainfall probability is significantly reduced. MJO in winter influences the rainfall in China mainly through modulating the circulation in the subtropics and mid-high latitudes.For the subtropics,MJO influences the northward moisture transport coming from the Bay of Bengal and the South China Sea by modulating the southern trough of the Bay of Bengal and the western Pacific subtropical high.For the mid-high latitudes,the propagation of the low frequency perturbations associated with the eastward-propagating MJO convection modulate the circulation in the mid-high latitudes,e.g.the East Asian winter monsoon and the low trough over central Asia.
Based on the NCEP/NCAR reanalysis I daily data from 1958 to 2002,climatic characteristics of the 30-60-day intraseasonal oscillations（ISOs） of the zonal wind（u）,meridional wind（v）,and geopotential height（h） over global areas and especially the ISO of v over the subtropical northern Pacific are analyzed using the space-time spectrum analysis and wavelet transform methods.The results show that the ISO of v is very different from those of u and h,with the former representing the meridional low-frequency disturbances,which are the most active in the subtropics and mid-high latitudes,but very weak in the tropics.In the subtropical Northern Hemisphere,the energies of the ISOs of u and h are both concentrated on the waves with wave number of 1 and periods of 30-60 days,while the main energy of the ISO of v is concentrated on the waves with wave numbers of 4-6 and periods of 30-60 and 70-90 days.The westward propagating energies for the 30-60-day oscillations of u,v,and h are all stronger than the eastward propagating energies in the subtropics.In addition,the ISO of v is the strongest（weakest） in summer （winter） over the subtropics of East Asia and northwestern Pacific,while the situation is reversed over the subtropical northeastern Pacific,revealing a ＂seesaw＂ of the ISO intensity with seasons over the subtropics from the northwestern to northeastern Pacific.In the subtropical northwestern Pacific,the interannual and interdecadal changes of the ISO for v at 850 hPa indicate that its activities are significantly strong during 1958-1975,while obviously weak during 1976-1990,and are the strongest during 1991-2000,and its spectral energy is obviously abnormal but ruleless during the ENSO periods.However,in the 2-7-yr bandpass filtering series,the interannual changes of the v ISO over the subtropical northwestern Pacific contain distinct ENSO signals.And in the 9-yr low-pass filtering series,the v ISO changes over the subtropical northwestern Pacific are significantly out of phase with the chang
Possible relationships between MJO and the severe rain-snow weather in Eastern China during November of 2009 are analyzed and results show that a strong MJO process is one of the strong impact factors.MJO is very active over the Indian Ocean in November 2009.Especially,it maintains 9 days in MJO phase 3,just corresponding to the two strongest rain-snow processes.Composites of MJO events show that when the MJO convective center is located over the Indian Ocean,the probability of rainfall is significantly increased and the temperature is lower than normal in eastern China,which is consistent with the situation in November of 2009.Atmospheric circulation anomalies of mid-and higher-latitudes can be influenced by the tropical MJO convection forcing and this influence could be realized by teleconnection.When the MJO is over the Indian Ocean,it is favorable for the maintenance of a circulation pattern of two ridges versus one trough at mid-and higher-latitudes.Meanwhile,the western Pacific subtropical high is stronger and more westward than normal,and a significant convective belt appears over eastern East Asia.All these circulation anomalies shown in the composite result also appeared in the observations in November 2009,which indicates the general features of relationships between the MJO and the circulation anomalies over the extratropics.Besides the zonal circulation anomalies,the MJO convection can also lead to meridional circulation anomalies.When the MJO convection is located over the Indian Ocean,the western Pacific is dominated by anomalous descending motion,and the eastern East Asia is controlled by strong convergence and ascending motion.Therefore,an anomalous meridional circulation is formed between the tropics and middle latitudes,enhancing the northward transportation of low-level moisture.It is potentially helpful to understanding and even forecasting such kind of rain-snow weather anomalies as that in November 2009 using MJO.
