scholarly journals Rapid Weakening of Tropical Cyclones in Monsoon Gyres Over the Western North Pacific: A Revisit

2021 ◽  
Vol 9 ◽  
Author(s):  
Kexin Song ◽  
Li Tao ◽  
Jianyun Gao
Keyword(s):  
Environmental Factors ◽  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Inner Core ◽  
Vertical Wind Shear ◽  
Reanalysis Data ◽  
Monsoon Gyre

The low-level monsoon trough over the western North Pacific (WNP) can evolve into a large cyclonic circulation, which is often termed a monsoon gyre (MG). Previous studies have revealed that tropical cyclones (TCs) embedded in MGs can experience rapid weakening (RW) and such RW might be attributed to the convective activity in the southeastern quadrant of the MG, which could induce asymmetries in a TC’s inner core structure, while the environmental factors, including the sea surface temperature (SST) and vertical wind shear (VWS), were not primary contributors to RW events. In this study, the possible role of large-scale environmental factors in association with the RW of TCs in MGs over the WNP is revisited based on the best-track TC and global reanalysis data during 2000–2018. Results indicate that TCs tend to weaken rapidly when they are embedded in the eastern semicircle of a MG, with the extreme RW events often occurring in the southeastern quadrant of a MG. However, different from previous studies, results from this study demonstrated that lower SST and strong large-scale VWS in the eastern semicircle of a MG are two major environmental factors contributing to the RW of TCs in MGs over the WNP. The different findings in this study from those in previous studies could be partly due to the different methods used to obtain the MG circulations and partly due to the environmental factors being analyzed in different quadrants of MG in this study.

scholarly journals Observational Analysis of Tropical Cyclone Formation Associated with Monsoon Gyres

2013 ◽  
Vol 70 (4) ◽  
pp. 1023-1034 ◽  
Author(s):  
Liguang Wu ◽  
Huijun Zong ◽  
Jia Liang
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Western North Pacific ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Relative Vorticity ◽  
Monsoon Trough ◽  
Cyclonic Circulation ◽  
Tropical Cyclone Formation ◽  
Monsoon Gyre

Abstract Large-scale monsoon gyres and the involved tropical cyclone formation over the western North Pacific have been documented in previous studies. The aim of this study is to understand how monsoon gyres affect tropical cyclone formation. An observational study is conducted on monsoon gyres during the period 2000–10, with a focus on their structures and the associated tropical cyclone formation. A total of 37 monsoon gyres are identified in May–October during 2000–10, among which 31 monsoon gyres are accompanied with the formation of 42 tropical cyclones, accounting for 19.8% of the total tropical cyclone formation. Monsoon gyres are generally located on the poleward side of the composited monsoon trough with a peak occurrence in August–October. Extending about 1000 km outward from the center at lower levels, the cyclonic circulation of the composited monsoon gyre shrinks with height and is replaced with negative relative vorticity above 200 hPa. The maximum winds of the composited monsoon gyre appear 500–800 km away from the gyre center with a magnitude of 6–10 m s−1 at 850 hPa. In agreement with previous studies, the composited monsoon gyre shows enhanced southwesterly flow and convection on the south-southeastern side. Most of the tropical cyclones associated with monsoon gyres are found to form near the centers of monsoon gyres and the northeastern end of the enhanced southwesterly flows, accompanying relatively weak vertical wind shear.

scholarly journals Rapid Weakening of Tropical Cyclones in Monsoon Gyres over the Tropical Western North Pacific

2018 ◽  
Vol 31 (3) ◽  
pp. 1015-1028 ◽  
Author(s):  
Jia Liang ◽  
Liguang Wu ◽  
Guojun Gu
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Tropical Cyclone Intensity ◽  
Cyclone Intensity ◽  
Operational Forecasts ◽  
Tropical Western North Pacific

Abstract As one major source of forecasting errors in tropical cyclone intensity, rapid weakening of tropical cyclones [an intensity reduction of 20 kt (1 kt = 0.51 m s−1) or more over a 24-h period] over the tropical open ocean can result from the interaction between tropical cyclones and monsoon gyres. This study aims to examine rapid weakening events occurring in monsoon gyres in the tropical western North Pacific (WNP) basin during May–October 2000–14. Although less than one-third of rapid weakening events happened in the tropical WNP basin south of 25°N, more than 40% of them were associated with monsoon gyres. About 85% of rapid weakening events in monsoon gyres occurred in September and October. The rapid weakening events associated with monsoon gyres are usually observed near the center of monsoon gyres when tropical cyclone tracks make a sudden northward turn. The gyres can enlarge the outer size of tropical cyclones and tend to induce prolonged rapid weakening events with an average duration of 33.2 h. Large-scale environmental factors, including sea surface temperature changes, vertical wind shear, and midlevel environmental humidity, are not primary contributors to them, suggesting the possible effect of monsoon gyres on these rapid weakening events by modulating the tropical cyclone structure. This conclusion is conducive to improving operational forecasts of tropical cyclone intensity.

scholarly journals What Caused The Increase of Tropical Cyclones In The Western North Pacific During The Period of 2011-2020?

2021 ◽  
Author(s):  
Haili Wang ◽  
Chunzai Wang
Keyword(s):  
Tropical Cyclones ◽  
North Pacific ◽  
High Frequency ◽  
Western North Pacific ◽  
Large Scale ◽  
Low Frequency ◽  
Vertical Wind Shear ◽  
Significant Negative Correlation ◽  
Relative Vorticity ◽  
The Pacific

Abstract Based on satellite era data after 1979, we find that the tropical cyclone (TC) variations in the Western North Pacific (WNP) can be divided into three-periods: a high-frequency period from 1979-1997 (P1), a low-frequency period from 1998-2010 (P2), and a high-frequency period from 2011-2020 (P3). Previous studies have focused on WNP TC activity during P1 and P2. Here we use observational data to study the WNP TC variation and its possible mechanisms during P3. Compared with P2, more TCs during P3 are due to the large-scale atmospheric environmental conditions of positive relative vorticity, negative vertical velocity and weak vertical wind shear. Warmer SST is found during P3, which is favorable for TC genesis. The correlation between the WNP TC frequency and SST shows a significant positive correlation around the equator and a significant negative correlation around 36°N, which is similar to the warm phase of the Pacific Decadal Oscillation (PDO). The correlation coefficient between the PDO and TC frequency is 0.71, significant at 99% confidence level. The results indicate that the increase of the WNP TC frequency during 2011-2020 is associated with the phase transition of the PDO and warmer SST. Therefore, more attention should be given to the warmer SST and PDO phase when predicting WNP TC activity.

scholarly journals The Influence of Large-Scale Environment on the Extremely Active Tropical Cyclone Activity in November 2019 over the Western North Pacific

Atmosphere ◽  
2021 ◽  
Vol 12 (4) ◽  
pp. 501
Author(s):  
Mengying Shi ◽  
Sulei Wang ◽  
Xiaoxu Qi ◽  
Haikun Zhao ◽  
Yu Shu
Keyword(s):  
Tropical Cyclone ◽  
Environmental Factors ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Sea Surface ◽  
Formation Mechanisms

In November 2019, tropical cyclone (TC) frequency over the western North Pacific reached its record high. In this study, the possible causes and formation mechanisms of that record high TC frequency are investigated by analyzing the effect of large-scale environmental factors. A comparison between the extremely active TC years and extremely inactive TC years is performed to show the importance of the large-scale environment. The contributions of several dynamic and thermodynamic environmental factors are examined on the basis of two genesis potential indexes and the box difference index that can measure the relative contributions of large-scale environmental factors to the change in TC genesis frequency. Results indicate that dynamical factors played a more important role in TC genesis in November 2019 than thermodynamic factors. The main contributions were from enhanced low-level vorticity and strong upward motion accompanied by positive anomalies in local sea surface temperature, while the minor contribution was from changes in vertical wind shear. Changes in these large-scale environmental factors are possibly related to sea surface temperature anomalies over the Pacific (e.g., strong Pacific meridional mode).

scholarly journals Rainfall Characteristics of Recurving Tropical Cyclones over the Western North Pacific

2018 ◽  
Vol 31 (2) ◽  
pp. 575-592 ◽  
Author(s):  
Difei Deng ◽  
Elizabeth A. Ritchie
Keyword(s):  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Motion Vector ◽  
Vertical Wind Shear ◽  
Rainfall Characteristics ◽  
Group 2 ◽  
Baroclinic Zone ◽  
Group 1

A dataset of 88 recurving western North Pacific tropical cyclones from 2004 to 2015 is investigated for rainfall characteristics during their period of recurvature. The TCs are categorized into two groups based on different large-scale patterns from empirical orthogonal function analysis. Group 1 is characterized by an intense midlatitude baroclinic zone and close distance between the zone and TC, while Group 2 is characterized by a weaker midlatitude baroclinic zone and more remote distance between the zone and TC at the time of recurvature. The results show the large-scale environment has substantial impact on TC rainfall patterns. In Group 1, as the TC approaches and is embedded into the baroclinic zone, a relatively strong interaction between the TC and midlatitudes occurs, which is reflected by a rapid increase of environmental vertical wind shear and TC translation speed, the alignment of the shear vector and motion vector, and a sharp contrast of temperature and moisture. Higher rainfall and wider coverage of rainfall tends to be produced along the track after recurvature, and the rainfall pattern turns from a right-of-track (ROT) to a left-of-track (LOT) preference. Conversely, in Group 2, a relatively weak interaction between the TC and midlatitude circulation occurs, which is reflected by weaker vertical wind shear and slower TC motion, a separation of the shear vector and motion vector, and a weak gradient of temperature and moisture. The corresponding rainfall swath for Group 2 exhibits a narrower rainfall swath after recurvature. The rain pattern changes from a LOT to ROT preference.

Characteristics of Binary Tropical Cyclones Observed in the Western North Pacific for 62 Years (1951–2012)

2015 ◽  
Vol 143 (5) ◽  
pp. 1749-1761 ◽  
Author(s):  
Wook Jang ◽  
Hye-Yeong Chun
Keyword(s):  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
Vertical Wind Shear ◽  
Temporal Variations ◽  
Reanalysis Data ◽  
Track Density ◽  
The North ◽  
Steering Flow ◽  
Analysis Technique

Abstract The statistical and dynamical characteristics of binary tropical cyclones (TCs) observed in the western North Pacific (WNP) for 62 years (1951–2012) are investigated by using best track and reanalysis data. A total of 98 binary TCs occurred with an annual average of 1.58. The occurrence frequency of binary TCs shows significant year-to-year variations and there are two peaks in the mid-1960s and early 1990s. Three-fourths (76.3%) of the binary TCs occurred between July and September, which is consistent with the high activity season of TCs in general. A relatively higher track density for binary TCs is present to the east of the maximum track density for total TCs. This result is likely due to the differences in the locations of genesis and environmental steering flow between binary and total TCs. The poleward steering flow, weaker vertical wind shear, and warmer sea surface temperature are pronounced for binary TCs, and these result in a longer lifetime of TCs, which can increase the chances that they would be detected as binary TCs. By applying the clustering analysis technique, six representative trajectories of the binary TCs are obtained. The transitional speed and recurving location are significantly different with respect to the clustered types. The trajectories of each type are strongly related to the temporal variations in the environmental steering flow and the location of the North Pacific high.

scholarly journals The Outflow–Rainband Relationship Induced by Environmental Flow around Tropical Cyclones

2019 ◽  
Vol 76 (7) ◽  
pp. 1845-1863 ◽  
Author(s):  
Yi Dai ◽  
Sharanya J. Majumdar ◽  
David S. Nolan
Keyword(s):  
Numerical Simulation ◽  
Tropical Cyclones ◽  
Large Scale ◽  
Inner Core ◽  
Vertical Wind Shear ◽  
Environmental Flow ◽  
Asymmetric Structure ◽  
Upper Level ◽  
The Relationship

Abstract This study investigates the role of the asymmetric interaction between the tropical cyclone (TC) and the environmental flow in governing the TC inner-core asymmetric structure. Motivated by the limitations of bulk measures of vertical wind shear in representing the complete environmental flow, the TC outflow is used as a focus for the asymmetric interaction. By analyzing an idealized numerical simulation, it is demonstrated that parcels can go directly from the asymmetric rainband to the upper-level outflow. The relatively large vertical mass flux in the rainband region also suggests that the asymmetric rainband is an important source of the outflow. In a simulation that suppresses convection by reducing the water vapor within the rainband region, the upper-level outflow is weakened, further supporting the hypothesis that the rainband and outflow are directly connected. Finally, it is demonstrated that the asymmetric outflow and the outer rainband are coupled through the descending inflow below the outflow. Some of the main characteristics of the outflow–rainband relationship are also supported by a real-case numerical simulation of Hurricane Bill (2009). The relationship is potentially useful for understanding and predicting the evolution of the TC inner-core structure during the interaction with the large-scale environmental flow.

scholarly journals Influence of Track Changes on the Poleward Shift of LMI Location of Western North Pacific Tropical Cyclones

2019 ◽  
Vol 32 (23) ◽  
pp. 8437-8445
Author(s):  
Ruifang Wang ◽  
Liguang Wu
Keyword(s):  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
North Pacific Ocean ◽  
Mixed Layer Depth ◽  
Vertical Wind Shear ◽  
Environmental Parameters ◽  
Ocean Basin ◽  
The Mean

Abstract The annual mean latitude at which tropical cyclones (TCs) reach their lifetime maximum intensity (LMI) over the western North Pacific Ocean basin has shifted northward since the early 1980s, and it is suggested that the shift is due to the northward migration of the mean TC formation location. In this study, the TC intensity is simulated with an intensity model to assess the historical records of TC intensity. During the period 1980–2015, the simulated poleward trend in the mean latitude of LMI is 0.44° (10 yr)−1, which agrees well with the one [0.48° (10 yr)−1] derived from the Joint Typhoon Warning Center (JTWC) dataset. This suggests that the observed poleward trend in the mean latitude of LMI is physically consistent with changes in the large-scale ocean–atmosphere environment and TC track. This study also demonstrates that the temporal change in the environmental parameters (sea surface temperature, outflow temperature, vertical wind shear, and ocean mixed layer depth) has little influence on the observed shift of the mean LMI latitude. The poleward migration of the mean LMI latitude is mainly due to the TC track shift, which results primarily from the change in the large-scale steering flow.

scholarly journals Multidecadal Variability of Tropical Cyclone Rapid Intensification in the Western North Pacific

2015 ◽  
Vol 28 (9) ◽  
pp. 3806-3820 ◽  
Author(s):  
Xidong Wang ◽  
Chunzai Wang ◽  
Liping Zhang ◽  
Xin Wang
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Wind Shear ◽  
Western North Pacific ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Trade Wind ◽  
Rapid Intensification ◽  
Multidecadal Variability ◽  
Heat Potential

Abstract This study investigates the variation of tropical cyclone (TC) rapid intensification (RI) in the western North Pacific (WNP) and its relationship with large-scale climate variability. RI events have exhibited strikingly multidecadal variability. During the warm (cold) phase of the Pacific decadal oscillation (PDO), the annual RI number is generally lower (higher) and the average location of RI occurrence tends to shift southeastward (northwestward). The multidecadal variations of RI are associated with the variations of large-scale ocean and atmosphere variables such as sea surface temperature (SST), tropical cyclone heat potential (TCHP), relative humidity (RHUM), and vertical wind shear (VWS). It is shown that their variations on multidecadal time scales depend on the evolution of the PDO phase. The easterly trade wind is strengthened during the cold PDO phase at low levels, which tends to make equatorial warm water spread northward into the main RI region rsulting from meridional ocean advection associated with Ekman transport. Simultaneously, an anticyclonic wind anomaly is formed in the subtropical gyre of the WNP. This therefore may deepen the depth of the 26°C isotherm and directly increase TCHP over the main RI region. These thermodynamic effects associated with the cold PDO phase greatly support RI occurrence. The reverse is true during the warm PDO phase. The results also indicate that the VWS variability in the low wind shear zone along the monsoon trough may not be critical for the multidecadal modulation of RI events.

scholarly journals Strong Modulation of the Pacific Meridional Mode on the Occurrence of Intense Tropical Cyclones over the Western North Pacific

2018 ◽  
Vol 31 (19) ◽  
pp. 7739-7749 ◽  
Author(s):  
Si Gao ◽  
Langfeng Zhu ◽  
Wei Zhang ◽  
Zhifan Chen
Keyword(s):  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
Vertical Wind Shear ◽  
Low Level ◽  
The Pacific ◽  
Main Development ◽  
Development Region ◽  
Meridional Mode ◽  
Pacific Meridional Mode

This study finds a significant positive correlation between the Pacific meridional mode (PMM) index and the frequency of intense tropical cyclones (TCs) over the western North Pacific (WNP) during the peak TC season (June–November). The PMM influences the occurrence of intense TCs mainly by modulating large-scale dynamical conditions over the main development region. During the positive PMM phase, anomalous off-equatorial heating in the eastern Pacific induces anomalous low-level westerlies (and cyclonic flow) and upper-level easterlies (and anticyclonic flow) over a large portion of the main development region through a Matsuno–Gill-type Rossby wave response. The resulting weaker vertical wind shear and larger low-level relative vorticity favor the genesis of intense TCs over the southeastern part of the WNP and their subsequent intensification over the main development region. The PMM index would therefore be a valuable predictor for the frequency of intense TCs over the WNP.

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