Physical Mechanism of Formation of the Bimodal Structure in the Meiyu Front System

The bimodal structure of the Meiyu front system is readdressed after Zhou et al.（2005）. The physical mechanism of the formation of the bimodal distribution is discussed. The bimodal structure of the Melyu front system considerably results from atmospheric moisture gradients, though atmospheric temperature gradients are also not negligible. According to the definition of equivalent potential temperature, and by scale analysis, we find that atmospheric equivalent potential temperature gradients, which could be regarded as an indicator of the Meiyu front system, could be mainly attributed to the variations of atmospheric potential temperature gradients with a scaling factor of 1 and moisture gradients multiplied by a scaling factor of an order of about 2.5 × 10^3, which means that small variations of atmospheric moisture gradients could lead to large variations of equivalent potential temperature gradients, and thus large variations of the Meiyu front system. Quantitative diagnostics with a mesoscale simulation data in the vicinity of the Meiyu front system show that moisture gradients contribute to equivalent potential temperature gradients more than potential temperature gradients.

Spring Indian Ocean–Western Pacific SST Contrast and the East Asian Summer Rainfall Anomaly

studying the relationship between SST in the tropical Indian Ocean （TIO）, tropical western Pacific （TWP）, and tropical eastern Pacific （TEP） and East Asian summer rainfall （EASR）, using data provided by NOAA/OAR/ESRL PSD and the National Climate Center of China for the period 1979-2008, an index, SSTDI, was defined to describe the SST difference between the TIO and TWP. In comparison with the winter ENSO, the spring SST contrast between the TIO and TWP was found to be more significantly associated with summer rainfall in East Asia, especially along the EASR band and in Northeast China. This spring SST contrast can persist into summer, resulting in a more significant meridional teleconnection pattern of lower-tropospheric circulation anomalies over the western North Pacific and East Asia. These circulation anomalies are dynamically consistent with the summer rainfall anomaly along the EASR band. When the SSTDI is higher （lower） than normal, the EASR over the Yangtze River valley, Korea, and central and southern Japan is heavier （less） than normal. The present results suggest that this spring SST contrast can be used as a new and better predictor of EASR anomalies.

Combined impact of in-phase and out-of-phase variation between the northern East Asian low and western North Pacific subtropical high on East Asian summer rainfall

East Asian summer rainfall is affected by both the continental northern East Asian low （NEAL） and the western North Pacific subtropical high （WNPSH） in the lower troposphere. This study investigates the joint effect of the two circulation factors on East Asian summer rainfall. It is found that the rainfall in East Asia behaves differently in the years with in-phase and out-of-phase variation between the NEAL and WNPSH. When the NEAL and WNPSH vary in phase, i.e. when they are both stronger, the rainfall anomaly shows a dipole pattern in East Asia and displays opposite changes between north and south of 30°N. When the two circulation factors vary out of phase, the rainfall anomaly is concentrated in the Yangtze River valley.

Two anomalous convective systems in the tropical western Pacific and their influences on the East Asian summer monsoon

By decomposing outgoing Iongwave radiation through empirical orthogonal function （EOF） analysis, the authors identify two anomalous convective systems in the tropical western Pacific. Besides the classical convective system near the Philippines （PC）, there is another convective system near the Federated States of Micronesia （MC）. As the first EOF component in this region, the variance explained by MC is higher than that by PC. Both MC and PC are regulated by the tropical sea surface temperature （SST） anomalies. While PC is associated with an El Ni^o event, MC is correlated with SST anomalies in the central and eastern Pacific during summer. It is also found that the East Asian summer monsoon （EASM） is influenced by these two convective systems. In general, enhanced （suppressed） convection corresponds to an eastwards （westwards） western Pacific subtropical high with weak （strong） intensity. Besides, the summer monsoon rainfall from the Yangtze River basin to Japan tends to increase （decrease） when PC is suppressed （enhanced）. By comparison, the influence of MC is generally weak, with vague signals in the East Asian continent. Moreover, the influence of suppressed convection on the EASM is more significant than that of enhanced convection.

Modulation by the Atlantic Multidecadal Oscillation of the intensity of the interannual seesaw between the Somali and Australian cross-equatorial flows

The decadal variation of the intensity of the interannual opposite connection （seesaw） between the Somali and Australian cross-equatorial flows （CEFs） is investigated. During the period prior to the mid-1960s, and after the early-2000s, the intensity of the interannual seesaw connection is significantly weaker relative to the period between. Such interdecadal shifts in the interannual seesaw intensity bear a resemblance to the decadal shift in ENSO＇s strength, and can be further attributed to the phase transition of the Atlantic Multidecadal Oscillation （AMO）. When the AMO is in a positive phase, the ENSO amplitude reduces and the seesaw strength becomes weakened, and vice versa.The historical simulation outputs of the CMIP5 models are used to verify the connection, and a similar result was obtained. Thus, the notion that the intensity of the interannual opposite connection （seesaw） between the CEFs is modulated by the AMO is robust.