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Qiuyun Wang, Jianping Li, Yanjie Li, Jingwen Zhang, and Jiayu Zheng

Abstract

The influence of the intraseasonal Indo–western Pacific convection oscillation (IPCO) on tropical cyclone (TC) genesis location and frequency over the Indo–western North Pacific (WNP) during the boreal extended summer (May–October) is explored. Observational analysis shows that the impacts of the intraseasonal IPCO on TCs over the Indo–WNP include an evident “phase lock of TC genesis location” and distinct differences in TC frequency. In the WNP, in the positive intraseasonal IPCO phase, the atmosphere gains heat through the release of latent heat in cumulus convective condensation, and the anomalous cyclonic circulation weakens the western Pacific subtropical high (WPSH) and enhances TC genesis, thereby tending to produce many more TCs. Moreover, the diminished WPSH and the westward shift of the centers of anomalous cyclonic circulations lock TC genesis locations to the west WNP and lower latitudes (around 5°–20°N), especially in the South China Sea. The almost opposite situation occurs in a negative phase. In the north Indian Ocean, the total TC genesis frequencies in the two intraseasonal IPCO phases are approximate. However, in the positive intraseasonal IPCO phase, the environmental conditions to the north of 13°N are similar to those in the WNP except without the WPSH control, whereas south of 13°N the situation is reversed, leading to a northward shift of the TC genesis location (around 13°–20°N). The negative phase reflects an opposite situation.

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Yipeng Guo, Jianping Li, Juan Feng, Fei Xie, Cheng Sun, and Jiayu Zheng

Abstract

Previous studies show that the first principal mode of the variability of the seasonal mean Hadley circulation (HC) is an equatorial asymmetric mode (AM) with long-term trend. This study demonstrates that the variability of the boreal autumn [September–November (SON)] HC is also dominated by an AM, but with multidecadal variability. The SON AM has ascending and descending branches located at approximately 20°N and 20°S, respectively, and explains about 40% of the total variance. Further analysis reveals that the AM is closely linked to the Atlantic multidecadal oscillation (AMO), which is associated with a large cross-equatorial sea surface temperature (SST) gradient and sea level pressure (SLP) gradient. The cross-equatorial thermal contrast further induces an equatorial asymmetric HC anomaly. Numerical simulations conducted on an atmospheric general circulation model also suggest that AMO-associated SST anomalies can also induce a cross-equatorial SLP gradient and anomalous vertical shear of the meridional wind at the equator, both of which indicate asymmetric HC anomaly. Therefore, the AM of the variability of the boreal autumn HC has close links to the AMO. Further analysis demonstrates that the AMO in SON has a closer relationship with AM than those in the other seasons. A possible reason is that the AMO-associated zonal mean SST anomaly in the tropics has differences among the four seasons, which leads to different atmospheric circulation responses.

The AM in SON has inversed impacts on the tropical precipitation, suggesting that the precipitation difference between the northern and southern tropics has multidecadal variability.

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Jia-Lin Lin, Toshiaki Shinoda, Brant Liebmann, Taotao Qian, Weiqing Han, Paul Roundy, Jiayu Zhou, and Yangxing Zheng

Abstract

This study evaluates the intraseasonal variability associated with summer precipitation over South America in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of each model’s twentieth-century climate simulation are analyzed. Two dominant intraseasonal bands associated with summer precipitation over South America are focused on: the 40- and the 22-day band. The results show that in the southern summer (November–April), most of the models underestimate seasonal mean precipitation over central-east Brazil, northeast Brazil, and the South Atlantic convergence zone (SACZ), while the Atlantic intertropical convergence zone (ITCZ) is shifted southward of its observed position. Most of the models capture both the 40- and 22-day band around Uruguay, but with less frequent active episodes than observed. The models also tend to underestimate the total intraseasonal (10–90 day), the 40-, and the 22-day band variances. For the 40-day band, 10 of the 14 models simulate to some extent the 3-cell pattern around South America, and 6 models reproduce its teleconnection with precipitation in the south-central Pacific, but only 1 model simulates the teleconnection with the MJO in the equatorial Pacific, and only 3 models capture its northward propagation from 50° to 32°S. For the 7 models with three-dimensional data available, only 1 model reproduces well the deep baroclinic vertical structure of the 40-day band. For the 22-day band, only 6 of the 14 models capture its northward propagation from the SACZ to the Atlantic ITCZ. It is found that models with some form of moisture convective trigger tend to produce large variances for the intraseasonal bands.

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