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Lu Wang
and
Tim Li

Abstract

A normalization method is applied to MJO-scale precipitation and column integrated moist static energy (MSE) anomalies to clearly illustrate the phase evolution of MJO. It is found that the MJO peak phases do not move smoothly, rather they jump from the original convective region to a new location to its east. Such a discontinuous phase evolution is related to the emerging and developing of new congestus convection to the east of the preexisting deep convection. While the characteristic length scale of the phase jump depends on a Kelvin wave response, the associated time scale represents the establishment of an unstable stratification in the front due to boundary layer moistening. The combined effect of the aforementioned characteristic length and time scales determines the observed slow eastward phase speed. Such a phase evolution characteristic seems to support the moisture mode theory of the second type that emphasizes the boundary layer moisture asymmetry, because the moisture mode theory of the first type, which emphasizes the moisture or MSE tendency asymmetry, might favor more “smooth” phase propagation. A longitudinal-location-dependent premoistening mechanism is found based on moisture budget analysis. For the MJO in the eastern Indian Ocean, the premoistening in front of the MJO convection arises from vertical advection, whereas for the MJO over the western Pacific Ocean, it is attributed to the surface evaporating process.

Free access
Jiahao Lu
,
Tim Li
, and
Lu Wang

Abstract

The modulation of the diurnal cycle (DC) of precipitation over the Maritime Continent (MC) by the background annual cycle mean state was studied for the period of 1998–2014 through observational analyses and high-resolution simulations using the Weather Research and Forecasting (WRF) Model. The observational analyses reveal that there are statistically significant differences in the DC amplitude between boreal winter and summer. The amplitude of precipitation DC reduces by about 35% during boreal summer compared to boreal winter, especially over the MC major islands and adjacent oceans. A precipitation budget analysis indicates that the DC amplitude difference is primarily attributed to vertically integrated convergence of the mean moisture by diurnal winds. The relative roles of the background dynamic and thermodynamic states in causing the enhanced diurnal wind activity in boreal winter are further investigated through idealized WRF simulations. The results show that the seasonal mean background moisture condition is most critical in inducing the winter–summer difference of the precipitation DC over the MC, followed by atmospheric static stability (i.e., vertical temperature gradient) and circulation conditions.

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Xinyu Li
and
Riyu Lu

Abstract

The meridional teleconnection over the western North Pacific and East Asia (WNP–EA) plays a predominant role in affecting the interannual variability of East Asian climate in summer. This study identified a breakdown of the meridional teleconnection since the early 2000s. Before the early 2000s, there are close tropical–extratropical relationships in light of both circulation and rainfall anomalies. For instance, the westward extension of the western North Pacific subtropical high (WNPSH) is closely associated with the southward shift of the East Asian westerly jet (EAJ), and more rainfall in the tropical WNP closely corresponds to less rainfall in the subtropical WNP–EA. However, after the early 2000s, the tropical–extratropical relationships are absent. Particularly, the tropical WNP precipitation anomalies can induce WNPSH anomalies, but the WNPSH anomalies cannot induce subtropical precipitation in the latter period, due to the absence of EAJ-related extratropical circulation anomalies. Further results indicate that in the latter period, the westward extension of the WNPSH is associated with the decay of central Pacific-like El Niño, and simultaneous summer sea surface temperature (SST) anomalies in the central eastern Pacific favor the northward shift of the EAJ, resulting in the disruption of the WNPSH–EAJ relationship. This evolution of tropical SSTs is sharply different from the decay of canonical El Niño and simultaneous summer tropical Indian Ocean warming, which favor the WNPSH–EAJ correspondence in the former period.

Free access
Xinyu Li
and
Riyu Lu

Abstract

The Yangtze River basin (YRB), a typical East Asian monsoon region, experiences a large year-to-year variability in summer precipitation and is subject to both floods and droughts. There is a well-known seesaw relationship in precipitation between the tropical western North Pacific and the YRB, but more than half of the variance in precipitation in the YRB cannot be explained by this seesaw pattern. The authors therefore investigated other physical factors that might affect precipitation in the YRB. The results indicate that the northeasterly anomaly in the lower troposphere to the north of the YRB plays an important role in the variability in precipitation. This northeasterly anomaly is paired with the southwesterly anomaly to the south of the YRB. They both play an important role in water vapor accumulation over the YRB and intensify the meridional gradient of the equivalent potential temperature θ e over the YRB by bringing dry and cool air from the north and wet air from the south. This intensified θ e gradient favors convective instability and heavier rainfall in the YRB, as previous studies on mei-yu weather have indicated. Furthermore, it is found that the zonally oriented teleconnection along the Asian westerly jet and the meridional displacement of the jet can affect circulation in the lower troposphere and precipitation in the YRB. These results highlight the role of extratropical circulation anomalies and thus contribute to a more comprehensive understanding of the variability of precipitation in the YRB.

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Yifeng Cheng
,
Lu Wang
, and
Tim Li

Abstract

Large-scale circulation anomalies associated with 10–30-day-filtered persistent heavy rainfall events (PHREs) over the middle and lower reaches of the Yangtze River Valley (MLYV) in boreal summer for the period of 1961–2017 were investigated. Two distinct types of PHREs were identified based on configurations of anomalies in the western Pacific subtropical high (WPSH) and South Asian high (SAH) during the peak wet phase. One type, referred to as PSAH, is characterized by eastward extension of the SAH, and the other, referred to as NSAH, featured a westward retreat of the SAH; they both exhibit westward extension of the WPSH. Both types of PHREs are dominated by mei-yu frontal systems. The lower-level circulation anomalies play a crucial role in initiating rainfall but through different processes. Prior to rainfall occurrence, a strong anticyclonic circulation anomaly is over the western North Pacific Ocean (WNP) for the PSAH events and the related southwesterly wind anomaly prevails over southeastern China, which advects moisture into the MLYV, moistens the boundary layer, and induces atmospheric convective instability. For the NSAH events, the WNP anticyclonic circulation is weak while a strong northerly wind is observed north of the MLYV. It brings the cold air mass southward, favoring initiating frontal rainfall over the MLYV. The formation of upper-level circulation anomalies over the MLYV is primarily due to the shift of anomalous circulations from the mid-high latitudes. After the rainfall generation, the precipitation would influence the lower- and upper-level circulation anomalies.

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Yifeng Cheng
,
Lu Wang
, and
Tim Li

Abstract

The southern China (SC) summer rainfall exhibits prominent intraseasonal variability, which exhibits a significant increase in the early 1990s with the turning point at 1993. The SC intraseasonal rainfall events could be divided into three categories according to different propagations, including the southward-propagating (SP) events, the northwestward-propagating (NWP) events, and the northward-propagating (NP) events. This study explores the causes of the observed interdecadal increase in the intraseasonal rainfall variability over SC by comparing the SC intraseasonal rainfall events of each category between the former decadal period (P1) and the later decadal period (P2). The result indicates that such interdecadal change is due to the more frequent NP events coming from the South China Sea (SCS). Based on the moisture and vorticity budget analysis, it is revealed that the summer mean southerly wind in the middle to lower troposphere is the dominant factor of the northward propagation over the SCS, as it could induce positive meridional moisture and vorticity advection anomalies ahead of the convection. A marked interdecadal enhancement of the summer mean southerly wind over the SCS is the cause of more frequent occurrence of NP events over SC, as it provides more favorable conditions for the northward propagation. The change of the atmospheric instability over the SCS where the NP convection perturbation originates was also investigated, but no significant change was found.

Free access
Lu Wang
,
Tim Li
, and
Tomoe Nasuno

Abstract

There are contrasting views concerning the impact of Rossby wave component of MJO flow on its eastward propagation. One view (called “drag effect”) argues that because Rossby waves propagate westward, a stronger Rossby wave component slows down the eastward propagation. The other view (called “acceleration effect”) argues that a stronger Rossby wave enhances east–west asymmetry of moist static energy (MSE) tendency and thus favors the eastward propagation. This study aims to resolve this issue through diagnosis of both idealized aquaplanet simulations and 26 models from the MJO Task Force/GEWEX Atmospheric System Studies (MJOTF/GASS). In the aquaplanet experiments, three sets of zonally uniform, equatorially symmetric SST distributions are specified. The MJO phase speed is faster in the presence of a narrower SST meridional profile, in which both the Rossby and the Kelvin wave components are stronger and the east–west asymmetry of MSE tendency is larger. A further analysis of the 26 general circulation models reveals that the MJO propagation skill and phase speed are positively correlated to both the Rossby wave and the Kelvin wave strength in the lower free atmosphere (above 800 hPa). Models that have a stronger Rossby and Kelvin wave component tend to simulate realistic and faster eastward propagation. Therefore, both the aquaplanet and the multimodel simulations support the Rossby wave acceleration effect hypothesis.

Open access
Tim Li
,
Bin Wang
, and
Lu Wang

Abstract

In a recent paper, Stuecker et al. applied a “combination mode” (C-mode) theory to explain the formation of the anomalous western North Pacific anticyclone (WNPAC) during El Niño events. The C-mode, arising from interaction between the annual cycle and ENSO, is an Indo-Pacific basin mode with two “near annual” time scales (roughly 10 and 15 months, respectively). This comment discusses to what extent the C-mode can explain the WNPAC dynamics. The major findings are the following: 1) spectral analysis of the Indo-Pacific circulation anomaly fields indicates that the 10-month mode is not observed and the 15-month mode is only seen in the western North Pacific (WNP), where its spectral peak is statistically insignificant; 2) the 15-month mode (with a period of 13–19 months) accounts for only a small portion (13%) of the observed sea level pressure anomaly in the WNP; and 3) the C-mode evolution does not capture the observed timing of the WNPAC onset in the northern fall of El Niño developing year. In addition it is shown, based on observational analyses and numerical experiments, that local atmosphere–ocean interaction plays an important role in formation of the anomalous anticyclonic center over the Philippine Sea.

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Lu Wang
,
Tim Li
, and
Tianjun Zhou

Abstract

The structure and evolution characteristics of intraseasonal (20–100 day) variations of sea surface temperature (SST) and associated atmospheric and oceanic circulations over the Kuroshio Extension (KE) region during boreal summer are investigated, using satellite-based daily SST, observed precipitation data, and reanalysis data. The intraseasonal SST warming in the KE region is associated with an anomalous anticyclone in the overlying atmosphere, reduced precipitation, and northward and downward currents in the upper ocean. The corresponding atmospheric and oceanic fields during the SST cooling phase exhibit a mirror image with an opposite sign. A mixed layer heat budget analysis shows that the intraseasonal SST warming is primarily attributed to anomalous shortwave radiation and latent heat fluxes at the surface. The anomalous sensible heat flux and oceanic advection also have contributions, but with a much smaller magnitude.

The SST warming caused by the atmospheric forcing further exerts a significant feedback to the atmosphere through triggering the atmospheric convective instability and precipitation anomalies. The so-induced heating leads to quick setup of a baroclinic response, followed by a baroclinic-to-barotropic transition. As a result, the atmospheric circulation changes from an anomalous anticyclone to an anomalous cyclone. This two-way interaction scenario suggests that the origin of the atmospheric intraseasonal oscillation over the KE region may partly arise from the local atmosphere–ocean interaction.

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Jiazheng Lu
,
Li Li
,
Xunjian Xu
, and
Tao Feng

Abstract

Based on ERA-Interim data, gauge observations, transmission line icing observational data, and hindcasted predictors from a numerical forecast system of transmission line icing, a new transmission line icing thickness (TLIT) dataset was constructed to solve the problem of limited historical data. The reliability of the dataset was analyzed using case studies and climate data. The results showed that the descriptions of three icing events in southern China by the TLIT were consistent with the actual observational data, and the icing thickness differences were less than 2 mm. The spatial distribution of annual icing days and icing thickness calculated using meteorological observation station icing data (OIT) and the TLIT data had a similar pattern, with small differences in the numerical values. A rotated empirical orthogonal function (REOF) decomposition was conducted for 67 transmission line icing events. It was found that the spatial distributions of the first three characteristic vectors of the TLIT and OIT data were similar, and the correlation coefficients for the time coefficients of the first three characteristic vectors were 0.801, −0.443, and 0.576, respectively. Three key areas were identified based on the first three patterns of REOF, and the average icing thickness of 67 events in southern China and the three key areas was calculated. The correlation coefficients of icing thickness calculated by the TLIT and OIT data for these areas were 0.648, 0.384, 0.565, and 0.599, respectively. The results illustrate that the TLIT data can reflect the temporal and spatial variations of ice thickness in southern China.

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