scholarly journals Recent Increased Covariability of Tropical Cyclogenesis Latitude and Longitude over the Western North Pacific during the Extended Boreal Summer

2019 ◽  
Vol 32 (23) ◽  
pp. 8167-8179 ◽  
Author(s):  
Haikun Zhao ◽  
Jie Zhang ◽  
Philip J. Klotzbach ◽  
Shaohua Chen
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Regime Shift ◽  
Hadley Circulation ◽  
Walker Circulation ◽  
Boreal Summer ◽  
Tropical Cyclogenesis ◽  
Central Pacific ◽  
Enso Events ◽  
The Walker Circulation

Abstract This study examines interdecadal changes in the interannual relationship between the extended boreal summer (May–November) tropical cyclogenesis (TCG) latitude and longitude over the western North Pacific Ocean (WNP) during 1979–2016. Increasing covariability of WNP TCG latitude and longitude is observed since 1998, which is found to be closely linked to shifting ENSO conditions and a tropical Pacific climate regime shift. Accompanied by an increasing occurrence in central Pacific (CP) ENSO events during recent decades, there has been a more consistent northwestward or southeastward shift of WNP TCG location since 1998. These coherent latitude and longitude shifts were generally not evident during 1979–97, a period characterized by a more conventional eastern Pacific (EP) ENSO pattern. Our statistical results show a robust relationship between TCG latitude and the Hadley circulation and between longitude and the Walker circulation during the period prior to and since the regime shift, and a possible physical explanation for the recent increased covariability of TCG latitude and longitude is given. During 1998–2016, there is a significant association of CP ENSO events with the intensity of both the Hadley and Walker circulations that likely caused the recent increase in the covariability of TCG latitude and longitude. However, the strong association of EP ENSO events with the intensity of the Hadley circulation but not with the Walker circulation during 1979–97 weakened the covariability of TCG latitude and longitude. In addition, changes in tropical Indian Ocean sea surface temperatures appear to also importantly contribute to the recent increased covariability of WNP TCG location.

scholarly journals Recent Strengthening of the Relationship between the Western North Pacific Monsoon and Western North Pacific Tropical Cyclone Activity during the Boreal Summer

2019 ◽  
Vol 32 (23) ◽  
pp. 8283-8299 ◽  
Author(s):  
Haikun Zhao ◽  
Shaohua Chen ◽  
Philip J. Klotzbach
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Western North Pacific ◽  
Southern Oscillation ◽  
Phase Relationship ◽  
Boreal Summer ◽  
Central Pacific ◽  
Enso Events ◽  
The Pacific ◽  
Phase Out

Abstract This study examines the association between the western North Pacific (WNP) summer monsoon (WNPSM) and WNP tropical cyclone (TC) frequency during June–August from 1979 to 2016. The interannual relationship between the WNPSM and the total number of WNP TCs has strengthened since 1998. There has also been a significant reduction in the number of TCs forming within the WNP monsoon trough (WNPMT)—hereafter called ITCs, for internal or inside TCs—since 1998. These two important features are found to be closely associated with the climate regime shift that occurred around 1998. During 1998–2016, the Pacific decadal oscillation (PDO) tended to be in a cold phase, with an increasing occurrence of central Pacific–type El Niño–Southern Oscillation (ENSO) events, whereas the 1979–97 period tended to be characterized by a warm phase of the PDO and east Pacific–type ENSO events. During 1998–2016, the tropical Pacific was characterized by enhanced easterlies, which led to a westward-retreated WNPMT that caused a significant decrease in ITCs over the WNP basin. However, there was little change in TCs outside of the WNPMT region (hereafter called OTCs) compared to that before 1998. A significant in-phase (out-of-phase) relationship between the WNPSM and the number of ITCs (OTCs) is observed before 1998, thus greatly weakening the WNPSM–TC relationship. The recent enhanced relationship between the WNPSM and TCs is mainly due to a strong in-phase relationship between the WNPSM and ITCs. The interannual change in ITCs is mainly controlled by WNPSM changes since 1998, while OTC changes are mainly modulated by changes in the tropical upper-tropospheric trough.

scholarly journals Characterizing the highest tropical cyclone frequency in the Western North Pacific since 1984

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Joseph Basconcillo ◽  
Eun-Jeong Cha ◽  
Il-Ju Moon
Keyword(s):  
Tropical Cyclone ◽  
Summer Monsoon ◽  
North Pacific ◽  
Western North Pacific ◽  
Tropical Indian Ocean ◽  
Boreal Summer ◽  
Central Pacific ◽  
Impact Reduction ◽  
Cyclone Frequency ◽  
Sst Anomalies

AbstractThe 2018 boreal summer in the Western North Pacific (WNP) is highlighted by 17 tropical cyclones (TC)—the highest record during the reported reliable years of TC observations. We contribute to the existing knowledge pool on this extreme TC frequency record by showing that the simultaneous highest recorded intensity of the WNP summer monsoon prompted the eastward extension of the monsoon trough and enhancement of tropical convective activities, which are both favorable for TC development. Such changes in the WNP summer monsoon environment led to the extreme TC frequency record during the 2018 boreal summer. Meanwhile, the highest record in TC frequency and the intensity of the WNP summer monsoon are both attributed with the combined increase in the anomalous westerlies originating from the cold tropical Indian Ocean sea surface temperature (SST) anomalies drawn towards the convective heat source that is associated with the warm central Pacific SST anomalies. Our results provide additional insights in characterizing above normal tropical cyclone and summer monsoon activities in the WNP in understanding seasonal predictable horizons in the WNP, and in support of disaster risk and impact reduction.

Decadal Variability of the Anticyclone in the Western North Pacific

2020 ◽  
Vol 33 (20) ◽  
pp. 9031-9043
Author(s):  
Mingmei Xie ◽  
Chunzai Wang
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Decadal Variability ◽  
East Asian ◽  
Hadley Circulation ◽  
Walker Circulation ◽  
Decadal Time Scale ◽  
Anomalous Anticyclone ◽  
East Asian Climate ◽  
Decadal Mode

AbstractThe western North Pacific anomalous anticyclone (WNPAC) significantly affects East Asian climate. Previous studies have elucidated interannual variability of the WNPAC associated with El Niño, but decadal variability of the WNPAC remains unknown. The present paper investigates the dominant modes of decadal variability of the WNPAC by using observational data. The first decadal mode, characterized by an anomalous anticyclone centered over the western North Pacific, is associated with the Pacific decadal oscillation (PDO). The relationship between the first mode and the PDO shifted from in phase to out of phase around 1966. From 1900 to 1966 when the PDO and the first mode are in phase, the anticyclone is maintained by the effects of both the strengthened Aleutian low through meridional atmospheric forcing and Indian Ocean warming through enhanced zonal Walker circulation. From 1967 to 2012, the anticyclone is induced by cold SST anomalies over the central equatorial Pacific when the PDO and the first mode are out of phase. The second decadal mode is characterized by an anomalous anticyclone extending from southeastern China to the Philippine Sea and is associated with the Maritime Continent (MC). This anticyclone resides in the sinking branch of the local Hadley circulation, triggered by enhanced convection associated with the MC warming from 1900 to 2012. The finding of the decadal WNPAC in this paper may provide a new way to explain East Asian climate on a decadal time scale.

scholarly journals What modulates the intensity of synoptic scale variability over the western North Pacific during boreal summer and fall?

2021 ◽  
pp. 1
Author(s):  
Renguang Wu ◽  
Yuqi Wang ◽  
Xi Cao
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Eastern Pacific ◽  
Propagation Path ◽  
Boreal Summer ◽  
Synoptic Scale ◽  
Mean Flow ◽  
Central Pacific ◽  
Background Fields ◽  
Sst Anomalies

AbstractThe present study investigates the factors that affect the year-to-year change in the intensity of synoptic scale variability (SSV) over the tropical western North Pacific (TWNP) during boreal summer and fall. It is found that the intensity of the TWNP SSV in summer is associated with the equatorial central-eastern Pacific sea surface temperature (SST) anomalies that modulates the background fields through a Rossby wave response both in the source region and along the propagation path of the synoptic scale disturbances. In fall, the intensity of the TWNP SSV is related to an SST anomaly pattern with opposite anomalies in the equatorial central Pacific and TWNP that modulates the background fields from the equatorial central Pacific to TWNP. However, the equatorial central Pacific SST anomalies alone fail to change the intensity of the TWNP SSV as the induced background field changes are limited to the equatorial central Pacific. It is shown that tropical western Pacific SST anomalies may induce notable changes in the intensity of the TWNP SSV. The relation of the TWNP SSV to the equatorial eastern Pacific SST is weak due to opposite SST anomalies in different types of years. Both seasonal mean and intraseasonal flows provide source of barotropic energy for the change in the intensity of the TWNP synoptic scale disturbances in summer. Seasonal mean flow has a main contribution to the barotropic energy conversion for the change in the intensity of the TWNP synoptic scale disturbances in fall.

scholarly journals Factors of boreal summer latent heat flux variations over the tropical western North Pacific

2021 ◽  
Author(s):  
Yuqi Wang ◽  
Renguang Wu
Keyword(s):  
Heat Flux ◽  
Wind Speed ◽  
North Pacific ◽  
High Frequency ◽  
Western North Pacific ◽  
Surface Wind ◽  
Low Frequency ◽  
Boreal Summer ◽  
Wind Intensity ◽  
Tropical Western North Pacific

AbstractSurface latent heat flux (LHF) is an important component in the heat exchange between the ocean and atmosphere over the tropical western North Pacific (WNP). The present study investigates the factors of seasonal mean LHF variations in boreal summer over the tropical WNP. Seasonal mean LHF is separated into two parts that are associated with low-frequency (> 90-day) and high-frequency (≤ 90-day) atmospheric variability, respectively. It is shown that low-frequency LHF variations are attributed to low-frequency surface wind and sea-air humidity difference, whereas high-frequency LHF variations are associated with both low-frequency surface wind speed and high-frequency wind intensity. A series of conceptual cases are constructed using different combinations of low- and high-frequency winds to inspect the respective effects of low-frequency wind and high-frequency wind amplitude to seasonal mean LHF variations. It is illustrated that high-frequency wind fluctuations contribute to seasonal high-frequency LHF only when their intensity exceeds the low-frequency wind speed under which there is seasonal accumulation of high-frequency LHF. When high-frequency wind intensity is smaller than the low-frequency wind speed, seasonal mean high-frequency LHF is negligible. Total seasonal mean LHF anomalies depend on relative contributions of low- and high-frequency atmospheric variations and have weak interannual variance over the tropical WNP due to cancellation of low- and high-frequency LHF anomalies.

Interactions between Boreal Summer Intraseasonal Oscillations and Synoptic-Scale Disturbances over the Western North Pacific. Part I: Energetics Diagnosis*

2011 ◽  
Vol 24 (3) ◽  
pp. 927-941 ◽  
Author(s):  
Pang-chi Hsu ◽  
Tim Li ◽  
Chih-Hua Tsou
Keyword(s):  
Kinetic Energy ◽  
Energy Conversion ◽  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
Intraseasonal Oscillation ◽  
Active Phase ◽  
Boreal Summer ◽  
Synoptic Eddies ◽  
Barotropic Energy Conversion

Abstract The role of scale interactions in the maintenance of eddy kinetic energy (EKE) during the extreme phases of the intraseasonal oscillation (ISO) is examined through the construction of a new eddy energetics diagnostic tool that separates the effects of ISO and a low-frequency background state (LFBS; with periods longer than 90 days). The LFBS always contributes positively toward the EKE in the boreal summer, regardless of the ISO phases. The synoptic eddies extract energy from the ISO during the ISO active phase. This positive barotropic energy conversion occurs when the synoptic eddies interact with low-level cyclonic and convergent–confluent ISO flows. This contrasts with the ISO suppressed phase during which the synoptic eddies lose kinetic energy to the ISO flow. The anticyclonic and divergent–diffluent ISO flows during the suppressed phase are responsible for the negative barotropic energy conversion. A positive (negative) EKE tendency occurs during the ISO suppressed-to-active (active-to-suppressed) transitional phase. The cause of this asymmetric EKE tendency is attributed to the spatial phase relation among the ISO vorticity, eddy structure, and EKE. The southwest–northeast-tilted synoptic disturbances interacting with cyclonic (anticyclonic) vorticity of ISO lead to a positive (negative) EKE tendency in the northwest region of the maximum EKE center. The genesis number and location and intensification rate of tropical cyclones in the western North Pacific are closely related to the barotropic energy conversion. The enhanced barotropic energy conversion favors the generation and development of synoptic seed disturbances, some of which eventually grow into tropical cyclones.

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