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Jie Song and Chongyin Li

Abstract

Using daily data, this study compares the subseasonal seesaw relationship between anomalous tropical western North Pacific (WNP) convection and anomalous rainfall over subtropical East Asia during boreal summers (June–August) in which the Indian Ocean (IO) sea surface temperature is either warmer or colder than normal. It is found that the precipitation anomalies over central-eastern China (25°–35°N, 110°–120°E) associated with the anomalous tropical WNP convection activities during the IO cold summers are weaker and less evident compared to that in the IO warm summers, indicating the seesaw relationship in the IO cold summers becomes obscure. This contrasting seesaw relationship between the IO warm and cold summers is attributed to different patterns of anomalous moisture transportation and vertical motion over central-eastern China. The anomalous circulations associated with the anomalous tropical WNP convection [the Pacific–Japan (PJ) pattern] during the IO warm and cold summers show that, relative to the IO warm summers, the Japan action center of the PJ pattern has an evident northwestward displacement in the IO cold summers. It is argued that this northwestward displacement of the Japan action center plays a key role in the formation of the distinct seesaw relationship through modifying the anomalous moisture transportation and vertical motion.

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Xiong Chen, Jian Ling, and Chongyin Li

Abstract

Evolution characteristics of the Madden–Julian oscillation (MJO) during the eastern Pacific (EP) and central Pacific (CP) types of El Niño have been investigated. MJO activities are strengthened over the western Pacific during the predeveloping and developing phases of EP El Niño, but suppressed during the mature and decaying phases. In contrast, MJO activities do not show a clear relationship with CP El Niño before their occurrence over the western Pacific, but they increase over the central Pacific during the mature and decaying phases of CP El Niño. Lag correlation analyses further confirm that MJO activities over the western Pacific in boreal spring and early summer are closely related to EP El Niño up to 2–11 months later, but not for CP El Niño. EP El Niño tends to weaken the MJO and lead to a much shorter range of its eastward propagation. Anomalous descending motions over the Maritime Continent and western Pacific related to El Niño can suppress convection and moisture flux convergence there and weaken MJO activities over these regions during the mature phase of both types of El Niño. MJO activities over the western Pacific are much weaker in EP El Niño due to the stronger anomalous descending motions. Furthermore, the MJO propagates more continuously and farther eastward during CP El Niño because of robust moisture convergence over the central Pacific, which provides adequate moisture for the development of MJO convection.

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Xiuzhen Li, Wen Zhou, Chongyin Li, and Jie Song

Abstract

The variation in regional precipitation over southeast and southwest China depends strongly on externally imported moisture rather than local evaporation. Associated with the different climate over the two regions, great discrepancies appear in the annual cycles of the moisture supply. Stationary moisture transport dominates externally imported moisture to a large extent, with transient transport being much weaker. The stationary moisture sink over southeast China is strong during spring and summer due to strong moisture input via the southern boundary and weak during fall and winter due to the offset between the output via the southern boundary and the net zonal boundary atmospheric flux. Zonal stationary moisture transport dominates the variation in moisture supply over southwest China. Negative net zonal boundary atmospheric flux countervails (collaborates) with positive meridional transport during the dry (wet) season.

Stationary moisture circulations dominate regional atmospheric moisture convergence anomalies over both southeast and southwest China. Weak cold air activity is favorable for a strong moisture sink over southeast China, while the reverse appears over southwest China in spring. The east-to-west location of the abnormal anticyclone determines whether strong moisture converges over southeast China or southwest China in fall. The anticyclonic circulation anomaly over the Philippine Sea, remotely forced by El Niño, is crucial to the strong moisture sink over southeast China from winter to spring, while it does not play a role in the abnormal moisture sink over southwest China.

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Gang Li, Chongyin Li, Yanke Tan, and Xin Wang

Abstract

The present study investigates the relationships between the December–February (DJF) South Pacific tripole (SPT) sea surface temperature anomaly (SSTA) pattern and the following March–May (MAM) rainfall over eastern China based on multiple datasets. It is found that the relationships between the DJF SPT and the following MAM rainfall over eastern China are modulated by the El Niño–Southern Oscillation (ENSO). When the ENSO signal is removed, the positive DJF SPT is significantly associated with more rainfall over eastern China during the following boreal spring. However, such significant relationships disappear if ENSO is considered. After removing ENSO impacts, the possible mechanisms through which the DJF SPT impacts the following MAM rainfall over eastern China are investigated. The positive DJF SPT is associated with the significantly positive SSTA in the tropical western Pacific, which can persist to the following MAM. In response to the positive SSTA in the tropical western Pacific, a wave-like train in the low-level troposphere extends from the tropical western Pacific (an anomalous cyclone) to the western North Pacific (an anomalous anticyclone) during the following MAM. The anomalous anticyclone over the western North Pacific enhances the anomalous southwesterly over eastern China, which can bring more moisture and favor anomalous increased rainfall. It should be pointed out that La Niña (El Niño) could induce an anomalous cyclone (anticyclone) over the western North Pacific, which offsets the MAM anomalous anticyclone (cyclone) caused by the positive (negative) SPT in the preceding DJF and thus weakens the relationship between the SPT and the rainfall over eastern China.

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Xin Li, Chongyin Li, Jian Ling, and Yanke Tan

Abstract

This study introduces a new methodology for identifying El Niño and La Niña events. Sea surface temperature (SST) anomaly patterns for El Niño and La Niña onset, peak, and end phases are classified by self-organizing maps (SOM) analysis. Both onset and end phases for El Niño and La Niña exhibit eastern Pacific (EP) and central Pacific (CP) types. The SST anomaly patterns in peak phase can be classified into EP, EP-like, and CP types for El Niño, and EP, mixed (MIX), and CP types for La Niña.

The general type of each El Niño or La Niña event is then defined according to the SST type for each of the three phases. There is no robust connection between the general types of the contiguous El Niño and La Niña except that the MIX La Niña rarely induces a subsequent CP El Niño. However, there are strong relationships between the end-phase type of El Niño and the onset-phase type of the subsequent La Niña. The EP-end-type El Niño favors transition to the CP-onset-type La Niña, while the CP-end-type El Niño favors transition to the EP-onset-type La Niña. On the other hand, the CP-end-type La Niña favors transition to EP-onset-type El Niño. Furthermore, an El Niño that occurs after the decay of La Niña favors initiating as an EP-onset type. These relationships are driven by different atmosphere–ocean dynamics, such as coupled air–sea feedback, thermocline feedback, slow SST mode, and Bjerknes feedbacks.

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Xiuzhen Li, Wen Zhou, Deliang Chen, Chongyin Li, and Jie Song

Abstract

The water vapor transport and moisture budget over eastern China remotely forced by the cold-tongue (CT) and warm-pool (WP) El Niño show striking differences throughout their lifetime. The water vapor transport response is weak in the developing summer but strong in the remaining phases of CT El Niño, whereas the opposite occurs during WP El Niño. WP El Niño causes a moisture deficit over the Yangtze River valley (YZ) in the developing summer and over southeastern China (SE) in the developing fall, whereas CT El Niño induces a moisture surplus first over SE during the developing fall with the influential area expanding in the decaying spring and shifting northward in the decaying summer. It is the divergence of meridional water vapor transport that dominates the total water vapor divergence anomaly, with the divergence of zonal transport showing an opposite pattern with smaller magnitude.

Investigation of the vertical profile of moisture budget shows a great baroclinicity, with the strongest abnormal moisture budget occurring in different levels. The moisture transport via the southern boundary plays a crucial role in the regional moisture budget anomalies and is located near the surface over SE, in the lower troposphere over the YZ, and at the lower-middle troposphere over the eastern part of northern China. The enhanced moisture surplus near the surface forced by WP El Niño over SE in the mature winter and decaying spring is offset by a moisture deficit within the lower-middle troposphere due to a diverse response circulation at different vertical levels.

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Jie Song, Chongyin Li, Jing Pan, and Wen Zhou

Abstract

The characteristics of the climatological distribution of the anticyclonic (LC1) and cyclonic (LC2) Rossby wave breaking (RWB) in the Southern Hemisphere (SH) are investigated by calculating the occurrence frequency of the LC1- and LC2-like stratospheric potential vorticity (PV) streamers in the SH during the austral summer [December–February (DJF)] and wintertime [June–August (JJA)] on several isentropic surfaces by using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) daily dataset. The results show that 1) on the equatorward flank of the climatological midlatitude jet (MLJ), the LC1-like PV streamers are frequently found over the central oceanic regions, whereas the LC2-like PV streamers are almost absent. On the poleward flank of the climatological MLJ, both types of PV streamers are frequently observed and the LC2-like PV streamers predominate; 2) the regions where the occurrences of the PV streamers are frequent overlap the weak zonal wind regions; and 3) in austral winter, a “double-jet” setting is evident in two regions of the SH [the double-jet upstream (DU) and the spilt jet region]. In the double-jet setting regions, the LC1-like PV streamers are frequently found both in the DU and the split-jet regions, while the occurrence of the LC2-like PV streamers is frequent in the split-jet region but is rather infrequent in the DU region.

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Guiwan Chen, Jian Ling, Chongyin Li, Yuanwen Zhang, and Chidong Zhang

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This study explores possible mechanisms for the barrier effect of the Indo-Pacific Maritime Continent (MC) on MJO propagation. In particular, this study examines whether similar mechanisms can be found in both observations and CMIP5 simulations. All models simulate individual MJO events but underestimate the percentage of MJO events propagating into the MC. The simulations are grouped into the top and bottom 50% based on their capability of reproducing the MJO spectral signal. When compared with the observations, the bottom 50% of the simulations significantly underestimate the MJO strength and exaggerate the barrier effect intensity, whereas these discrepancies are not significant in the top 50% of the simulations. From the top 50% of the simulations, the MJO strength, moisture processes, and surface evaporation in the MC all play important roles in constituting the barrier effect. No such evidence is found in observations. The discrepancies may come from small observed sample size and/or misrepresentations of key physical processes in the models. A consistent result is found in the observations and simulations: Whether MJO events can cross the MC depends on the degree to which dominant precipitation over land shifts to over water in the MC as MJO convection centers approach the MC and cross it. This result emphasizes the critical role of precipitation over water in carrying convective signals of the MJO through the MC. The results suggest that diagnosing the model alone on mechanisms for the barrier effect could be misleading; further investigations using a combination of observations, global gridded data, and high-resolution models are needed.

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Jie Song, Wen Zhou, Xin Wang, and Chongyin Li

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This study investigates linkages between the zonal asymmetry of the annular mode (AM) zonal pattern and the subtropical jet (STJ) over its downstream regions of the storm track by using an idealized model. Observational analyses show that the AM zonal patterns are more zonally asymmetric during days when the STJ downstream of the storm track is unusually strong, and vice versa. In the idealized model, the STJ downstream of the storm track is varied by introducing an additional zonally localized tropical heating. The model’s AM variability exhibits a nearly zonally uniform structure when there is no or only weak tropical heating. However, the signatures of the AM are locally strengthened in the heating sector; thus, the AM zonal pattern is zonally asymmetric when the tropical heating is large enough to create a strong STJ. The model results also show that the percentage of the variance explained by the AM, the persistence of the AM index, and the intensity of eddy feedback are also increased when the tropical heating becomes stronger. It is argued herein that the zonal asymmetry of the AM pattern is caused by the zonal asymmetry of the anomalous synoptic eddy forcing projecting on the AM, which is primarily due to the zonal asymmetry of the variations of the storm track between the nonheating and heating sectors.

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Minghao Yang, Chongyin Li, Xin Li, Yanke Tan, Xiong Chen, and Chao Zhang

Abstract

Based on the daily NCEP reanalysis, the present study investigates the interdecadal change in the relationship between the winter North Pacific storm track (WNPST) and the East Asian winter monsoon (EAWM), and evaluates the WNPST-EAWM relationship in 17 CMIP6 models. The results show that the out-of-phase WNPST-EAWM relationship underwent an interdecadal change in the mid-1980s. The WNPST-EAWM relationship became less significant during P2 (1990-2015). The atmospheric circulation anomaly related to the EAWM during P1 (1955-1980) is more robust than that during P2. The interdecadal weakening WNPST-EAWM relationship may be attributed to the interdecadal damping WNPST-EAWM interaction. The EAWM-related anomalous baroclinic energy conversion and moisture effect, including meridional and vertical eddy moisture fluxes, contribute to the significant attenuation of the WNPST during P1. The transient eddy-induced dynamic forcing and thermal forcing anomalies, as well as the barotropic process represented by the local Eliassen-Palm flux divergence associated with WNPST, can also significantly manipulate the upper-tropospheric jet during P1. However, the atmospheric circulation and interaction between the WNPST and EAWM during P2 are not as significant as those during P1. The effect of ENSO on the WNPST is significantly different before and after the mid-1980s. After the mid-1980s, the WNPST shows the characteristic of moving equatorward during El Niño events. It seems that ENSO takes over the WNPST from the EAWM after the mid-1980s. In addition, except for BCC-ESM1, CanESM5 and SAM0-UNICON, most of the CMIP6 models cannot reproduce the significant out-of-phase WNPST-EAWM relationship.

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