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.

Interactions between Boreal Summer Intraseasonal Oscillations and Synoptic-Scale Disturbances over the Western North Pacific. Part II: Apparent Heat and Moisture Sources and Eddy Momentum Transport*

2011 ◽  
Vol 24 (3) ◽  
pp. 942-961 ◽  
Author(s):  
Pang-Chi Hsu ◽  
Tim Li
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Wave Train ◽  
Intraseasonal Oscillation ◽  
Momentum Transport ◽  
Boreal Summer ◽  
Synoptic Scale ◽  
Mean Flow ◽  
The North ◽  
Intraseasonal Oscillations

Abstract The interactions between the boreal summer intraseasonal oscillation (ISO) and synoptic-scale variability (SSV) are investigated by diagnosing the atmospheric apparent heat source (Q1), apparent moisture sink (Q2), and eddy momentum transport. It is found that the synoptic Q1 and Q2 heating (cooling) anomalies are in phase with cyclonic (anticyclonic) vorticity disturbances, aligned in a southeast–northwest-oriented wave train pattern over the western North Pacific (WNP). The wave train is well organized and strengthened (loosely organized and weakened) during the ISO active (suppressed) phase. The nonlinearly rectified Q1 and Q2 fields due to the eddy–mean flow interaction account for 10%–30% of the total intraseasonal Q1 and Q2 variabilities over the WNP. During the ISO active (suppressed) phase, the nonlinearly rectified intraseasonal Q1 and Q2 heating (cooling) appear to the northwest of the ISO enhanced (suppressed) convection center, favoring the northwestward propagation of the ISO. A diagnosis of the zonal momentum budget shows that the eddy momentum flux convergence forces an intraseasonal westerly (easterly) tendency to the north of the ISO westerly (easterly) center during the ISO active (suppressed) phase. As a result, the eddy momentum transport may contribute to the northward propagation of the boreal summer ISO over the WNP.

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.

scholarly journals Reintensification of the Anomalous Western North Pacific Anticyclone during the El Niño Modoki Decaying Summer: Relative Importance of Tropical Atlantic and Pacific SST Anomalies

2020 ◽  
Vol 33 (8) ◽  
pp. 3271-3288
Author(s):  
Juan Feng ◽  
Wen Chen ◽  
Xiaocong Wang
Keyword(s):  
El Niño ◽  
North Pacific ◽  
General Circulation ◽  
Western North Pacific ◽  
El Nino ◽  
Central Pacific ◽  
El Niño Modoki ◽  
Sst Cooling ◽  
Sst Warming ◽  
Sst Anomalies

AbstractThe El Niño Modoki–induced anomalous western North Pacific anticyclone (WNPAC) undergoes an interesting reintensification process in the El Niño Modoki decaying summer, the period when El Niño Modoki decays but warm sea surface temperature (SST) anomalies over the tropical North Atlantic (TNA) and cold SST anomalies over the central-eastern Pacific (CEP) dominate. In this study, the region (TNA or CEP) in which the SST anomalies exert a relatively important influence on reintensification of the WNPAC is investigated. Observational analysis demonstrates that when only anomalous CEP SST cooling occurs, the WNPAC experiences a weak reintensification. In contrast, when only anomalous TNA SST warming emerges, the WNPAC experiences a remarkable reintensification. Numerical simulation analysis demonstrates that even though the same magnitude of CEP SST cooling and TNA warming is respectively set to force the atmospheric general circulation model, the response of the WNPAC is still much stronger in the TNA warming experiment than in the CEP cooling experiment. Further analysis demonstrates that this difference is caused by the distinct location of the effective tropical forcing between the CEP SST cooling and TNA SST warming for producing a WNPAC. The CEP cooling-induced effective anomalous diabatic cooling is located in the central Pacific, by which the forced anticyclone becomes gradually weak from the central Pacific to the western North Pacific. Thus, a weak WNPAC is produced. In contrast, as the TNA SST warming–induced effective anomalous diabatic cooling is just located in the western North Pacific via a Kelvin wave–induced Ekman divergence process, the forced anticyclone is significant and powerful in the western North Pacific.

scholarly journals Boreal Summer Synoptic-Scale Waves over the Western North Pacific in Multimodel Simulations

2016 ◽  
Vol 29 (12) ◽  
pp. 4487-4508 ◽  
Author(s):  
Haikun Zhao ◽  
Xianan Jiang ◽  
Liguang Wu
Keyword(s):  
Spatial Pattern ◽  
North Pacific ◽  
General Circulation ◽  
Western North Pacific ◽  
Climate Model ◽  
Climate Modeling ◽  
Active Role ◽  
General Circulation Models ◽  
Boreal Summer ◽  
Synoptic Scale

During boreal summer, vigorous synoptic-scale wave (SSW) activity, often evident as southeast–northwest-oriented wave trains, prevails over the western North Pacific (WNP). In spite of their active role for regional weather and climate, modeling studies on SSWs are rather limited. In this study, a comprehensive survey on climate model capability in representing the WNP SSWs is conducted by analyzing simulations from 27 recent general circulation models (GCMs). Results suggest that it is challenging for GCMs to realistically represent the observed SSWs. Only 2 models out of the 27 GCMs generally well simulate both the intensity and spatial pattern of the observed SSW mode. Plausible key processes for realistic simulations of SSW activity are further explored. It is illustrated that GCM skill in representing the spatial pattern of the SSW is highly correlated to its skill in simulating the summer mean patterns of the low-level convergence associated with the WNP monsoon trough and conversion from eddy available potential energy (EAPE) to eddy kinetic energy (EKE). Meanwhile, simulated SSW intensity is found to be significantly correlated to the amplitude of 850-hPa vorticity, divergence, and conversion from EAPE to EKE over the WNP. The observed modulations of SSW activity by the Madden–Julian oscillation are able to be captured in several model simulations.

scholarly journals Synoptic-Scale Waves in Sheared Background Flow over the Western North Pacific

2016 ◽  
Vol 73 (11) ◽  
pp. 4583-4603 ◽  
Author(s):  
Tao Feng ◽  
Xiu-Qun Yang ◽  
Wen Zhou ◽  
Ronghui Huang ◽  
Liang Wu ◽  
...  
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Lower Troposphere ◽  
Synoptic Scale ◽  
Mean Flow ◽  
Wave Growth ◽  
Lower Upper ◽  
Sheared Flow ◽  
Upper Level ◽  
Level Disturbance

Abstract Tropical depression (TD)-type waves are the dominant mode of synoptic-scale fluctuations over the western North Pacific. By applying spatiotemporal filters to the observed OLR data and the NCEP–DOE AMIP-II reanalysis data for 1979–2013, this study reveals the characteristics and energetics of convectively coupled TD-type waves under the effects of different circulation patterns in association with vertical wind shear. Results exhibit that different ambient sheared flows significantly affect the vertical structure of westward-propagating TD-type waves, with a lower-tropospheric mode in an easterly sheared background and an upper-tropospheric mode in a westerly sheared background. Energetic diagnoses demonstrate that when the disturbance is trapped in the lower (upper) level by easterly (westerly) shear, the horizontal mean flow in the lower (upper) level favors wave growth by converting energy from the shear of the zonal mean flow (from the convergence of the meridional mean flow). During the penetration of a westward-propagating synoptic-scale disturbance from a westerly sheared flow into an easterly sheared flow, the upper-level disturbance decays, and the lower-level disturbance intensifies. Meanwhile, the upper-level kinetic energy is transferred downward, but the effect induces the wave growth only confined to the midlevels. Consequently, the low-level growth of the westward-propagating upper-level synoptic-scale disturbance is mainly attributed to the barotropic conversion of horizontal mean flow in the lower troposphere.

scholarly journals Decadal Transition of the Leading Mode of Interannual Moisture Circulation over East Asia–Western North Pacific: Bonding to Different Evolution of ENSO

2018 ◽  
Vol 32 (2) ◽  
pp. 289-308 ◽  
Author(s):  
Xiuzhen Li ◽  
Zhiping Wen ◽  
Deliang Chen ◽  
Zesheng Chen
Keyword(s):  
Indian Ocean ◽  
East Asia ◽  
North Pacific ◽  
Western North Pacific ◽  
Eastern Pacific ◽  
Enso Cycle ◽  
Central Pacific ◽  
Huai River ◽  
Leading Mode ◽  
The Impact

Abstract The El Niño–Southern Oscillation (ENSO) cycle has a great impact on the summer moisture circulation over East Asia (EA) and the western North Pacific [WNP (EA-WNP)] on an interannual time scale, and its modulation is mainly embedded in the leading mode. In contrast to the stable influence of the mature phase of ENSO, the impact of synchronous eastern Pacific sea surface temperature anomalies (SSTAs) on summer moisture circulation is negligible during the 1970s–80s, while it intensifies after 1991. In response, the interannual variation of moisture circulation exhibits a much more widespread anticyclonic/cyclonic pattern over the subtropical WNP and a weaker counterpart to the north after 1991. Abnormal moisture moves farther northward with the enhanced moisture convergence, and thus precipitation shifts from the Yangtze River to the Huai River valley. The decadal shift in the modulation of ENSO on moisture circulation arises from a more rapid evolution of the bonding ENSO cycle and its stronger coupling with circulation over the Indian Ocean after 1991. The rapid development of cooling SSTAs over the central-eastern Pacific, and warming SSTAs to the west over the eastern Indian Ocean–Maritime Continent (EIO-MC) in summer, stimulates abnormal descending motion over the western-central Pacific and ascending motion over the EIO-MC. The former excites an anticyclone over the WNP as a Rossby wave response, sustaining and intensifying the WNP anticyclone; the latter helps anchor the anticyclone over the tropical–subtropical WNP via an abnormal southwest–northeast vertical circulation between EIO-MC and WNP.

scholarly journals Role of climate variability in the potential predictability of tropical cyclone formation in tropical and subtropical western North Pacific Ocean

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Yu-Lin K. Chang ◽  
Yasumasa Miyazawa ◽  
Swadhin Behera
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Western North Pacific ◽  
Southern Oscillation ◽  
North Pacific Ocean ◽  
Eastern Pacific ◽  
Central Pacific ◽  
Potential Predictability ◽  
Vorticity Anomaly ◽  
Tropical Western North Pacific

AbstractThe out of phase tropical cyclone (TC) formation in the subtropical and tropical western North Pacific associated with local low-level wind vorticity anomaly, driven by the remote central and eastern equatorial Pacific warming/cooling, is investigated based on the reanalysis and observational data in the period of 1979−2017. TC frequencies in the subtropical and tropical western North Pacific appear to be connected to different remote heating/cooling sources and are linked to eastern and central Pacific warming/cooling, which are in turn related to canonical El Niño/Southern Oscillation (ENSO) and ENSO Modoki, respectively. TCs formed in subtropics (SfTC) are generally found to be associated with a dipole in wind vorticity anomaly, which is driven by the tropical eastern Pacific warming/cooling. Tropically formed TCs (TfTC) are seen to be triggered by the single-core of wind vorticity anomaly locally associated with the warming/cooling of central and eastern Pacific. The predicted ENSOs and ENSO Modokis, therefore, provide a potential source of seasonal predictability for SfTC and TfTC frequencies.

scholarly journals Accumulated Effect of Intra-Seasonal Oscillation Convections over the Tropical Western North Pacific on the Meridional Location of Western Pacific Subtropical High

2020 ◽  
Vol 8 ◽  
Author(s):  
Zongci Huang ◽  
Wenjun Zhang ◽  
Xin Geng ◽  
Pang-Chi Hsu
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Late Summer ◽  
Western Pacific Subtropical High ◽  
Western Pacific ◽  
Subtropical High ◽  
Boreal Summer ◽  
Seasonal Oscillation ◽  
Tropical Western North Pacific ◽  
Sst Anomalies

An extreme northward displacement of the western Pacific subtropical high (WPSH) was detected during the boreal mid-late summer (July-August) of 2018, bringing record-breaking heat waves over northern East Asia. Negative sea surface temperature (SST) anomalies in the northern India Ocean (NIO) are usually accompanied with a northward shift of the WPSH. However, no prominent NIO SST anomalies were observed during the 2018 boreal summer. It is found that this extreme northward-shifted WPSH event is largely attributed to the accumulated effect of intra-seasonal oscillation (ISO) convection anomalies over the tropical western North Pacific (WNP). The accumulated effect on the WPSH meridional location is further supported by their significant correlation based on the data since 1979. While the relationship between the NIO SST anomalies and WPSH meridional location has substantially weakened since the late 1990s, the accumulated effect of the tropical WNP ISO convections keeps playing a crucial role in modulating the WPSH meridional displacement. The active WNP ISO activities can stimulates a poleward propagating Rossby wave train, which favors a northward shift of the WPSH. Our results suggest that the accumulated effect of the tropical WNP ISO convections should be considered when predicting the WPSH during the boreal mid-late summer season.

scholarly journals Mechanisms of Meridional Teleconnection Observed between a Summer Monsoon System and a Subtropical Anticyclone. Part II: A Global Survey

2010 ◽  
Vol 23 (19) ◽  
pp. 5109-5125 ◽  
Author(s):  
Yu Kosaka ◽  
Hisashi Nakamura
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Tropical Convection ◽  
Boreal Summer ◽  
Mean Flow ◽  
Subtropical Anticyclone ◽  
Global Survey ◽  
South Indian ◽  
The Pacific ◽  
Monsoon System

Abstract A global survey is conducted for atmospheric anomaly patterns of meridional teleconnection over the summer hemisphere associated with anomalous tropical convection. The patterns may be akin to the Pacific–Japan (PJ) teleconnection pattern analyzed in detail in the companion paper. From the survey, meridional teleconnections are identified over five regions, namely, the western North Pacific and Central/North America in boreal summer, as well as the western South Indian Ocean, central South Pacific, and western South Atlantic in austral summer. All of the patterns are observed in the western peripheries of the summertime surface subtropical anticyclones over the individual ocean basins. Although all of the patterns can convert available potential energy (APE) efficiently from the vertically sheared subtropical westerly jets, the efficiencies of barotropic energy conversion from the mean flow and diabatic APE generation differ from one pattern to another. Still, all of the patterns gain energy as the net, to maintain themselves against dissipative processes. Both the anomalous moisture convergence near the surface and the midtropospheric anomalous ascent required for the vorticity and thermal balance act to sustain the anomalous tropical convection, while the wind-evaporation feedback contributes positively only to the PJ pattern over the western North Pacific. Examination of common features and discrepancies among the five teleconnection patterns with respect to their structures and energetics reveals that climatological background features, including the largest horizontal extent of the Asian monsoon system and the North Pacific subtropical anticyclone, in addition to particularly high SST over the Pacific warm pool, render the PJ pattern an outstanding mode of variability.

scholarly journals Cluster Analysis of Tropical Cyclone Tracks over the Western North Pacific Using a Self-Organizing Map

2016 ◽  
Vol 29 (10) ◽  
pp. 3731-3751 ◽  
Author(s):  
Han-Kyoung Kim ◽  
Kyong-Hwan Seo
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Western North Pacific ◽  
The Philippines ◽  
Self Organizing Map ◽  
Central Pacific ◽  
Philippine Sea ◽  
Genesis Frequency ◽  
Sst Anomalies ◽  
Self Organizing

Abstract Tropical cyclone (TC) tracks over the western North Pacific (WNP) in 1979–2013 are classified by a self-organizing map technique. A false detection rate method identifies five optimal TC clusters. Physical mechanisms of the intraseasonal and interannual variations in the TC genesis frequency are investigated for each cluster. The five clusters are separated by genesis location, from the westernmost area (east of the Philippines, C1) to the easternmost (~150°E, C5) onset area over the WNP. The intraseasonal Madden–Julian oscillation (MJO) significantly affects the genesis frequency for all clusters except for C5. In particular, MJO phases 5 and 6 (1 and 2) provide significantly favorable (unfavorable) large-scale conditions for TC genesis. Two types of El Niño–Southern Oscillation influence the interannual variation of the genesis frequency for only C2 (generated over the western Philippine Sea and East China Sea) and C4 (formed near the eastern Philippine Sea). Enhanced eastern Pacific sea surface temperature (SST) anomalies lead to a ~40% decrease in the C2 TC frequency through a reversed Walker circulation with downward motion over the WNP. Conversely, increased central Pacific SST anomalies generate a cyclonic Rossby wave northwest of the forcing, inducing a significant increase (~50%) in the C4 TC frequency. The interannual variability for the C5 TCs is strongly controlled by the variation of the western Pacific subtropical high (WPSH). A positive WPSH variation reduces the C5 TC genesis frequency by 66%, while negative WPSH anomalies enhance the frequency by 50%. A prediction scheme using information from the first four 6-h TC locations demonstrates a skillful determination of TC clusters.

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