Large-scale circulation patterns favourable to tropical cyclogenesis over the western North Pacific and associated barotropic energy conversions

AbstractThe intraseasonal variability of tropical cyclogenesis in the western North Pacific (WNP) basin is explored in this study. The relation of cyclogenesis in each of the five large-scale patterns identified in recent work by Yoshida and Ishikawa is associated with the Madden–Julian oscillation (MJO). Confirming previous results, more events of cyclogenesis are found during the active MJO phase in the WNP. Furthermore, results indicate that most of the tropical cyclogenesis is associated with the monsoon shear line large-scale pattern during the active phase. The genesis potential index (GPI) and its individual components are used to evaluate the environmental factors that most contribute toward cyclogenesis under the different phases of the MJO. GPI exhibits a large positive anomaly during the active phase of the MJO, and such an anomaly is spatially correlated with the events of cyclogenesis. The analysis of each factor indicates that low-level relative vorticity and midlevel relative humidity are the two dominant contributors to the MJO-composited GPI anomalies. The positive GPI anomalies during the active phase are partially offset by the negative contributions from vertical wind shear and potential intensity. This is valid for all five large-scale patterns. It is noteworthy that the easterly wave (EW) large-scale pattern, while exhibiting the same influence of relative vorticity and midlevel humidity contributing toward positive GPI anomalies, presents slightly more cyclogenesis events under the inactive phase of the MJO. This unexpected result suggests that other factors not included in the definition of the GPI and/or changes in environmental flows on other time scales contribute to the tropical cyclogenesis associated with the EW large-scale pattern.

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Human-perceived temperature changes in South Korea and their association with atmospheric circulation patterns

Journal of Climate ◽

10.1175/jcli-d-20-0344.1 ◽

2020 ◽

pp. 1-58

Author(s):

Hyun-Ju Lee ◽

Wonbae Jeon ◽

Woo-Seop Lee ◽

Hwa Woon Lee

Keyword(s):

Relative Humidity ◽

South Korea ◽

Atmospheric Circulation ◽

North Pacific ◽

Western North Pacific ◽

Large Scale ◽

Height Anomaly ◽

Atmospheric Circulation Patterns ◽

Circulation Patterns ◽

Perceived Temperature

AbstractThis study investigates the spatiotemporal characteristics of human-perceived temperature (HPT) data, which describe the joint effects of temperature and humidity on the human body, and examines the related large-scale atmospheric circulation patterns for the summer season (July–August) in South Korea using trend and composite analyses. The increasing trend of HPT was stronger than that of the maximum, mean, and minimum temperatures during 1981–2018. There was an abrupt change in HPT between 1981–2009 and 2010–2018, which is likely caused by the northward upper-level subtropical jet, strengthened downward motion, anomalous anticyclones around South Korea, and increased sea surface temperature over the western North Pacific (WNP), which are related to the enhancement and western expansion of the western North Pacific subtropical high (WNPSH). These results highlight the importance of the activity of the WNPSH in the variability of HPT in South Korea. When the western edge of the WNPSH is located in the northwest, a positive geopotential height anomaly at 500 hPa is centered over Korea, which is associated with high temperatures and low relative humidity. The southwestern extension of the WNPSH modifies the wind circulation pattern and brings warm and moist air from the West Sea along the ridge-line of the WNPSH. Eventually, it leads to extreme HPT, associated with high relative humidity and temperature over South Korea, particularly in the southern part of the country. Therefore, we concluded that monitoring and predicting the location of WNPSH and understanding the mechanism and factors influencing the movement of WNPSH under global warming are necessary for predicting and coping with extreme HPT.

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Effects of Monsoon Trough Intraseasonal Oscillation on Tropical Cyclogenesis over the Western North Pacific*

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-13-0407.1 ◽

2014 ◽

Vol 71 (12) ◽

pp. 4639-4660 ◽

Cited By ~ 29

Author(s):

Xi Cao ◽

Tim Li ◽

Melinda Peng ◽

Wen Chen ◽

Guanghua Chen

Keyword(s):

North Pacific ◽

Moisture Transport ◽

Western North Pacific ◽

Large Scale ◽

Diabatic Heating ◽

Intraseasonal Oscillation ◽

Monsoon Trough ◽

Specific Humidity ◽

Tropical Cyclogenesis ◽

Background Fields

Abstract The effects of intraseasonal oscillation (ISO) of the western North Pacific (WNP) monsoon trough on tropical cyclone (TC) formation were investigated using the Advanced Research Weather Research and Forecasting (ARW) Model. A weak vortex was specified initially and inserted into the background fields containing climatological-mean anomalies associated with active and inactive phases of monsoon trough ISOs. The diagnosis of simulations showed that monsoon trough ISO can modulate TC development through both dynamic and thermodynamic processes. The dynamic impact is attributed to the lower–midtropospheric large-scale vorticity associated with monsoon trough ISO. Interactions between cyclonic vorticity in the lower middle troposphere during the active ISO phase and a vortex lead to the generation of vortex-scale outflow at the midlevel, which promotes the upward penetration of friction-induced ascending motion and thus upward moisture transport. In addition, the low-level convergence associated with active ISO also helps the upward moisture transport. Both processes contribute to stronger diabatic heating and thus promote a positive convection–circulation–moisture feedback. On the other hand, the large-scale flow associated with inactive ISO suppresses upward motion near the core by inducing the midlevel inflow and the divergence forcing within the boundary layer, both inhibiting TC development. The thermodynamic impact comes from greater background specific humidity associated with active ISO that allows a stronger diabatic heating. Experiments that separated the dynamic and thermodynamic impacts of the ISO showed that the thermodynamic anomaly from active ISO contributes more to TC development, while the dynamic anomalies from inactive ISO can inhibit vortex development completely.

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The Analysis of Tropical Cyclone Tracks in the Western North Pacific through Data Mining. Part I: Tropical Cyclone Recurvature

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-12-045.1 ◽

2013 ◽

Vol 52 (6) ◽

pp. 1394-1416 ◽

Cited By ~ 28

Author(s):

Wei Zhang ◽

Yee Leung ◽

Johnny C. L. Chan

Keyword(s):

Data Mining ◽

Tropical Cyclone ◽

North Pacific ◽

Western North Pacific ◽

Large Scale ◽

North Pacific Ocean ◽

Classification Tree ◽

Wind Fields ◽

Large Scale Circulation ◽

Data Mining Approach

AbstractThis paper is the first of a two-part series of papers that employs the data-mining approach to analyze tropical cyclone (TC) movement in the western North Pacific Ocean. Part I unravels conditions under which TCs tend to recurve, and Part II uncovers conditions leading to TCs making landfall. Here in Part I, a detailed study is carried out into TC recurvature over the South China Sea and western North Pacific. The investigation focuses on the unraveling of rules governing TC recurvature hidden in TC data. The historical TC track database comprises recurving TCs and straight movers. Potential parameters affecting TC recurvature are categorized into three groups: large-scale circulation, circulations surrounding TCs, and variables characterizing TCs. The tree construction algorithm, C4.5, is applied to classify recurving and straight-moving TCs. Parameters measuring large-scale circulation patterns and characterizing TCs play significant roles in building the classification tree. Altogether, 18 rules are discovered from the processed database. Most of the 18 rules can be explained by existing theories and are supported by various empirical findings on TC recurvatures. Rules governing TC recurvature discovered by the present study contain quantitative descriptions of factors such as composite wind fields, geopotential heights, and deep-layer mean winds that are essential to the understanding, interpretation, and prediction of TC recurvatures.

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