scholarly journals Contributions of Different Time-Scale Variations to Tropical Cyclogenesis over the Western North Pacific

2018 ◽  
Vol 31 (8) ◽  
pp. 3137-3153 ◽  
Author(s):  
Xi Cao ◽  
Renguang Wu ◽  
Mingyu Bi
Keyword(s):  
El Niño ◽  
North Pacific ◽  
Time Scale ◽  
Western North Pacific ◽  
Large Scale ◽  
El Nino ◽  
Vertical Wind Shear ◽  
Specific Humidity ◽  
Interannual Variations ◽  
Intraseasonal Variations

Abstract The present study investigates relative contributions of different time-scale variations of environmental factors to the tropical cyclone (TC) genesis over the western North Pacific (WNP) during July–August–September–October (JASO). Distinct from previous studies that are concerned with large-scale spatial patterns during a certain period, the present study focuses on local and instantaneous conditions of the TC genesis. Analysis shows that the contribution of convection and lower-level vorticity to the TC genesis is mainly due to intraseasonal and synoptic variations. The contribution of vertical wind shear is largely related to synoptic variations. The contribution of midlevel specific humidity is almost 2 times more from intraseasonal variations than from synoptic variations. The contribution of sea surface temperature (SST) to the TC genesis is mainly due to interannual and intraseasonal variations. The barotropic energy for synoptic-scale disturbances during the TC genesis comes mainly from climatological mean flows over the southwest quadrant and from intraseasonal wind variations over the northeast quadrant of the WNP, respectively. The contribution of interannual variations to the TC genesis is enhanced over the southeast quadrant of the WNP. More TCs form under weak easterly and westerly vertical shears, respectively, during El Niño developing and decaying JASO. The contribution of interannual variations of SST tends to be larger during El Niño decaying than during developing JASO.

scholarly journals Global Warming and Western North Pacific Typhoon Activity from an Observational Perspective

2004 ◽  
Vol 17 (23) ◽  
pp. 4590-4602 ◽  
Author(s):  
Johnny C. L. Chan ◽  
Kin Sik Liu
Keyword(s):  
Global Warming ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Climate Model ◽  
El Nino ◽  
Interannual Variations ◽  
Model Simulations ◽  
Climate Model Simulations

Abstract Based on results from climate model simulations, many researchers have suggested that because of global warming, the sea surface temperature (SST) will likely increase, which will then lead to an increase in the intensity of tropical cyclones (TCs). This paper reports results of a study of the relationship between SST and observed typhoon activity (which is used as a proxy for the intensity of TCs averaged over a season) over the western North Pacific (WNP) for the past 40 yr. The average typhoon activity over a season is found to have no significant relationship with SST in the WNP but increases when the SST over the equatorial eastern Pacific Ocean is above normal. The mean annual typhoon activity is generally higher (lower) during an El Niño (La Niña) year. Such interannual variations of typhoon activity appear to be largely constrained by the large-scale atmospheric factors that are closely related to the El Niño–Southern Oscillation (ENSO) phenomenon. These large-scale dynamic and thermodynamic factors include low-level relative vorticity, vertical wind shear, and moist static energy. Such results are shown to be physically consistent with one another and with those from previous studies on the interannual variations of TC activity. The results emphasize the danger of drawing conclusions about future TC intensity based on current climate model simulations that are not designed to make such predictions.

scholarly journals Interannual Variability of Summer Tropical Cyclone Rainfall in the Western North Pacific Depicted by CFSR and Associated Large-Scale Processes and ISO Modulations

2018 ◽  
Vol 31 (5) ◽  
pp. 1771-1787 ◽  
Author(s):  
Jau-Ming Chen ◽  
Pei-Hua Tan ◽  
Liang Wu ◽  
Hui-Shan Chen ◽  
Jin-Shuen Liu ◽  
...  
Keyword(s):  
Tropical Cyclone ◽  
Interannual Variability ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
El Nino ◽  
Southern Oscillation ◽  
Monsoon Trough ◽  
Tropical Eastern Pacific

This study examines the interannual variability of summer tropical cyclone (TC) rainfall (TCR) in the western North Pacific (WNP) depicted by the Climate Forecast System Reanalysis (CFSR). This interannual variability exhibits a maximum region near Taiwan (19°–28°N, 120°–128°E). Significantly increased TCR in this region is modulated by El Niño–Southern Oscillation (ENSO)-related large-scale processes. They feature elongated sea surface temperature warming in the tropical eastern Pacific and a southeastward-intensified monsoon trough. Increased TC movements are facilitated by interannual southerly/southeasterly flows in the northeastern periphery of the intensified monsoon trough to move from the tropical WNP toward the region near Taiwan, resulting in increased TCR. The coherent dynamic relations between interannual variability of summer TCR and large-scale environmental processes justify CFSR as being able to reasonably depict interannual characteristics of summer TCR in the WNP. For intraseasonal oscillation (ISO) modulations, TCs tend to cluster around the center of a 10–24-day cyclonic anomaly and follow its northwestward propagation from the tropical WNP toward the region near Taiwan. The above TC movements are subject to favorable background conditions provided by a northwest–southeasterly extending 30–60-day cyclonic anomaly. Summer TCR tends to increase (decrease) during El Niño (La Niña) years and strong (weak) ISO years. By comparing composite TCR anomalies and correlations with TCR variability, it is found that ENSO is more influential than ISO in modulating the interannual variability of summer TCR in the WNP.

scholarly journals A Physical Basis for the Probabilistic Prediction of the Accumulated Tropical Cyclone Kinetic Energy in the Western North Pacific

2013 ◽  
Vol 26 (20) ◽  
pp. 7981-7991 ◽  
Author(s):  
Hye-Mi Kim ◽  
Myong-In Lee ◽  
Peter J. Webster ◽  
Dongmin Kim ◽  
Jin Ho Yoo
Keyword(s):  
Tropical Cyclone ◽  
Kinetic Energy ◽  
El Niño ◽  
North Pacific ◽  
Wind Shear ◽  
Western North Pacific ◽  
El Nino ◽  
Vertical Wind Shear ◽  
Tropical Storm ◽  
Probabilistic Prediction

Abstract The relationship between El Niño–Southern Oscillation (ENSO) and tropical storm (TS) activity over the western North Pacific Ocean is examined for the period from 1981 to 2010. In El Niño years, TS genesis locations are generally shifted to the southeast relative to normal years and the passages of TSs tend to recurve to the northeast. TSs of greater duration and more intensity during an El Niño summer induce an increase of the accumulated tropical cyclone kinetic energy (ACE). Based on the strong relationship between the TS properties and ENSO, a probabilistic prediction for seasonal ACE is investigated using a hybrid dynamical–statistical model. A statistical relationship is developed between the observed ACE and large-scale variables taken from the ECMWF seasonal forecast system 4 hindcasts. The ACE correlates positively with the SST anomaly over the central to eastern Pacific and negatively with the vertical wind shear near the date line. The vertical wind shear anomalies over the central and western Pacific are selected as predictors based on sensitivity tests of ACE predictive skill. The hybrid model performs quite well in forecasting seasonal ACE with a correlation coefficient between the observed and predicted ACE at 0.80 over the 30-yr period. A relative operating characteristic analysis also indicates that the ensembles have significant probabilistic skill for both the above-normal and below-normal categories. By comparing the ACE prediction over the period from 2003 to 2011, the hybrid model appears more skillful than the forecast from the Tropical Storm Risk consortium.

scholarly journals Changes in Intensity and Variability of Tropical Cyclones over the Western North Pacific and Their Local Impacts under Different Types of El Niños

Atmosphere ◽  
2020 ◽  
Vol 12 (1) ◽  
pp. 59
Author(s):  
Yuhang Liu ◽  
Sun-Kwon Yoon ◽  
Jong-Suk Kim ◽  
Lihua Xiong ◽  
Joo-Heon Lee
Keyword(s):  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
El Nino ◽  
Pearl River Basin ◽  
Regional Variability ◽  
Central Pacific ◽  
El Niño Events ◽  
El Nino Events

This study investigated the effects of El Niño events on tropical cyclone (TC) characteristics over the western North Pacific (WNP) region. First, TC characteristics associated with large-scale atmospheric phenomena (i.e., genesis position, frequency, track, intensity, and duration) were investigated in the WNP in relation to various types of El Niño events—moderate central Pacific (MCP), moderate eastern Pacific (MEP), and strong basin-wide (SBW). Subsequently, the seasonal and regional variability of TC-induced rainfall across China was analyzed to compare precipitation patterns under the three El Niño types. When extreme El Niño events of varying degrees occurred, the local rainfall varied during the developmental and decaying years. The development of MEP and SBW was associated with a distinct change in TC-induced rainfall. During MEP development, TC-induced rainfall occurred in eastern and northeastern China, whereas in SBW, TC-induced heavy rainfall occurred in southwest China. During SBW development, the southwestern region was affected by TCs over a long period, with the eastern and northeastern regions being affected significantly fewer days. During El Niño decay, coastal areas were relatively more affected by TCs during MCP events, and the Pearl River basin was more affected during SBW events. This study’s results could help mitigate TC-related disasters and improve water-supply management.

scholarly journals Different Influences of Two El Niño Types on Low-Level Atmospheric Circulation over the Subtropical Western North Pacific

2020 ◽  
Vol 33 (3) ◽  
pp. 825-846
Author(s):  
Wei Tan ◽  
Zexun Wei ◽  
Qiang Liu ◽  
Qingjun Fu ◽  
Mengyan Chen ◽  
...  
Keyword(s):  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Eastern Pacific ◽  
Specific Humidity ◽  
Atmospheric Response ◽  
Low Level ◽  
Ep El Niño ◽  
Anomalous Cyclone

ABSTRACTThis study focuses on different evolutions of the low-level atmospheric circulations between eastern Pacific (EP) El Niño and central Pacific-II (CP-II) El Niño. The western North Pacific anomalous anticyclone (WNPAC) originates from the northern South China Sea for EP El Niño, and moves to the western North Pacific (WNP) afterward. Compared with EP El Niño, the origin of the WNPAC is farther west during CP-II El Niño, with the center over the Indochina Peninsula. Moreover, the WNPAC shows a weaker eastward shift. Such discrepancies are attributed to different evolutions of the cyclonic response over the WNP, which can suppress the convection in the western flank of the anomalous cyclone. The eastward retreat of the anomalous cyclone is significant for EP El Niño, but less evident for CP-II El Niño. These discrepancies are related to zonal evolutions of the increased precipitation over the equatorial Pacific. Following the southward migration of the intertropical convergence zone (ITCZ), the deep-convection region extends eastward along the equator, reinforcing the atmospheric response to the eastern Pacific warming in EP El Niño. For CP-II El Niño, the atmospheric response is insignificant over the eastern Pacific without warming. Moreover, the meridional migration of the ITCZ can modulate zonal variations of the easterly trade wind and specific humidity as well. Due to the combined effects of the climatological background and atmospheric anomalies, the specific humidity–induced and wind-induced moist enthalpy advection contribute to different shifts of the precipitation center.

scholarly journals Combined effects of ENSO and PDO on Activity of Major Hurricanes in Eastern North Pacific

2021 ◽  
Author(s):  
Chaoming Huang ◽  
Hailong Liu ◽  
Xidong Wang ◽  
Juncheng Zuo ◽  
Ruyun Wang
Keyword(s):  
El Niño ◽  
North Pacific ◽  
Large Scale ◽  
El Nino ◽  
Southern Oscillation ◽  
Vertical Wind Shear ◽  
Open Ocean ◽  
Hadley Cell ◽  
La Niña ◽  
La Nina

Abstract Major hurricanes (MHs) in the eastern North Pacific (ENP) in 1970-2018 were clustered into 3 categories with different quantity, intensity, lifetime, translation speed, track and large-scale environmental fields. MHs in all three clusters are more active in the Pacific Decadal Oscillation (PDO) warm phase than cold phase period. There are two clusters that their relationship with El Niño Southern Oscillation (ENSO) were modulated by PDO. The first cluster generates and develops in the open ocean and has an increasing trend of annual frequency, which is more active during El Niño years than during La Niña years in the PDO cold phase, but equally active in the PDO warm phase. The second cluster generates in the nearshore and translate rapidly into the ocean, which is more active during La Niña years than during El Niño years in the PDO warm phase, but equally active in the PDO cold phase. The PDO modulation mainly result from that MHs are obviously active during La Niña years in the PDO warm phase, which can be explained by local warming sea surface temperature, lower vertical wind shear, increasing vorticity and weakening sinking branch of circulation like Hadley cell. Therefore, PDO modulation cannot be ignored when predict the activity of tropical cyclone in ENP, especially for MHs that enters the open ocean and threat the islands such as the Hawaiian Islands.

Atmospheric Dynamic and Thermodynamic Processes Driving the Western North Pacific Anomalous Anticyclone during El Niño. Part II: Formation Processes

2017 ◽  
Vol 30 (23) ◽  
pp. 9637-9650 ◽  
Author(s):  
Bo Wu ◽  
Tianjun Zhou ◽  
Tim Li
Keyword(s):  
Rossby Wave ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Specific Humidity ◽  
Central Pacific ◽  
Meridional Gradient ◽  
Sign Change ◽  
Anomalous Anticyclone

In Part I, the authors showed that northerly anomalies associated with the Rossby wave response to El Niño heating anomalies in the equatorial central Pacific lead to the southward advection of low moist enthalpy air forming the western North Pacific anomalous anticyclone (WNPAC). Why does such a remote forcing not cause the formation of the anomalous anticyclone in El Niño–developing summer? The physical mechanism responsible for the timing of the WNPAC formation is investigated in Part II. Through both an observational analysis and idealized numerical model experiments, the authors find that the onset timing of the WNPAC relies on the following three factors. The first is a sign change (from positive to negative) of the meridional gradient of background low-level specific humidity over the key tropical western North Pacific (WNP) region in November. The second is a sign change (from positive to negative) of the meridional gradient of background relative vorticity, which efficiently reduces the westward stretch of the Rossby wave gyre anomalies west of the equatorial heating through equivalent beta effect. As a result, the northern branch of the twin cyclonic anomalies induced by El Niño heating withdraws eastward, leaving space for the onset of the WNPAC. The third factor is attributed to local sea surface temperature anomaly (SSTA) forcing. Pacemaker experiments with a coupled global model indicate that cold SSTAs in the tropical WNP play an important role in starting the anomalous anticyclone over the WNP in late fall. In the absence of the local cold SSTA forcing, the formation of the WNPAC would be delayed to El Niño mature winter.

The 2009 Hurricane Season in the Eastern North Pacific Basin: An Analysis of Environmental Conditions

2011 ◽  
Vol 139 (6) ◽  
pp. 1673-1682 ◽  
Author(s):  
Jennifer M. Collins ◽  
David R. Roache
Keyword(s):  
El Niño ◽  
North Pacific ◽  
Large Scale ◽  
El Nino ◽  
Vertical Wind Shear ◽  
Precipitable Water ◽  
Normal Activity ◽  
El Niño Event ◽  
Hurricane Season ◽  
Development Region

Abstract Despite the presence of an intensifying El Niño event, the 2009 eastern North Pacific hurricane season was near normal when considering overall hurricane activity. This is in contrast to the relative lull in activity observed between 1998 and 2008. Previous research has noted that the eastern North Pacific should be subdivided into two development regions, the western development region (WDR; 10°–20°N, 116°W–180°) and the eastern development region (EDR; 10°–20°N, North American coastline to 115.9°W), when examining interannual hurricane variability. In 2009, the EDR saw below average numbers of tropical cyclones of all intensities, while the WDR saw near-normal activity. However, activity in both regions varied sharply from month to month with periods of high activity in August and October and lower activity in July and September. This monthly variability was also observed in primary environmental forcing factors such as total precipitable water, tropospheric vertical wind shear, and low-level relative vorticity, particularly for the WDR. This variability was obscured by simply examining seasonal means. It is shown that for the 2009 season, large-scale environmental factors forced by the El Niño event and two cycles of the Madden–Julian oscillation contributed strongly to the observed patterns of cyclone activity across the basin.

scholarly journals Weakened Anomalous Western North Pacific Anticyclone during an El Niño–Decaying Summer under a Warmer Climate: Dominant Role of the Weakened Impact of the Tropical Indian Ocean on the Atmosphere

2018 ◽  
Vol 32 (1) ◽  
pp. 213-230 ◽  
Author(s):  
Chao He ◽  
Tianjun Zhou ◽  
Tim Li
Keyword(s):  
Indian Ocean ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Tropical Indian Ocean ◽  
Tropospheric Temperature ◽  
Coupled Models ◽  
Subtropical Anticyclone ◽  
Mean State

Abstract The western North Pacific subtropical anticyclone (WNPAC) is the most prominent atmospheric circulation anomaly over the subtropical Northern Hemisphere during the decaying summer of an El Niño event. Based on a comparison between the RCP8.5 and the historical experiments of 30 coupled models from the CMIP5, we show evidence that the anomalous WNPAC during the El Niño–decaying summer is weaker in a warmer climate although the amplitude of the El Niño remains generally unchanged. The weakened impact of the sea surface temperature anomaly (SSTA) over the tropical Indian Ocean (TIO) on the atmosphere is essential for the weakened anomalous WNPAC. In a warmer climate, the warm tropospheric temperature (TT) anomaly in the tropical free troposphere stimulated by the El Niño–related SSTA is enhanced through stronger moist adiabatic adjustment in a warmer mean state, even if the SSTA of El Niño is unchanged. But the amplitude of the warm SSTA over TIO remains generally unchanged in an El Niño–decaying summer, the static stability of the boundary layer over TIO is increased, and the positive rainfall anomaly over TIO is weakened. As a result, the warm Kelvin wave emanating from TIO is weakened because of a weaker latent heating anomaly over TIO, which is responsible for the weakened WNPAC anomaly. Numerical experiments support the weakened sensitivity of precipitation anomaly over TIO to local SSTA under an increase of mean-state SST and its essential role in the weakened anomalous WNPAC, independent of any change in the SSTA.

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