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 Mechanisms of Meridional Teleconnection Observed between a Summer Monsoon System and a Subtropical Anticyclone. Part I: The Pacific–Japan Pattern

2010 ◽  
Vol 23 (19) ◽  
pp. 5085-5108 ◽  
Author(s):  
Yu Kosaka ◽  
Hisashi Nakamura
Keyword(s):  
Energy Conversion ◽  
Diabatic Heating ◽  
Lower Troposphere ◽  
The Philippines ◽  
Convective Activity ◽  
Mean Flow ◽  
Subtropical Anticyclone ◽  
The Pacific ◽  
Monsoon System ◽  
Dynamical Mode

Abstract Summertime atmospheric circulation over the midlatitude western North Pacific (WNP) is influenced by anomalous convective activity near the Philippines. This meridional teleconnection, observed in monthly anomalies and known as the Pacific–Japan (PJ) pattern, is characterized by zonally elongated cyclonic and anticyclonic anomalies around the enhanced convection center and to its northeast, respectively, in the lower troposphere, with an apparent poleward phase tilt with height. The authors’ idealized two-layer linear model, whose basic state consists of a zonal subtropical jet and a pair of a monsoon system and a subtropical anticyclone, can simulate a PJ-like response against diabatic heating located between the pair. Each of the observed and simulated patterns can gain energy through barotropic and baroclinic conversions from the zonally varying baroclinic mean flow, in an efficiency comparable with that of energy generation due to the anomalous diabatic heating, indicating a characteristic of the pattern as a dry dynamical mode. In fact, the conversion efficiency is sensitive to the location of the anomaly pattern relative to the climatological-mean flow. Furthermore, the second-least damped mode identified in the idealized model bears certain resemblance with the observed PJ pattern, indicating its modal characteristics as well as a critical importance of these features in the mean field for the pattern. In addition to the PJ pattern, another meridional teleconnection pattern with high efficiency for its energy conversion is identified observationally in association with anomalous convection near the Bonin Islands. The anomalous circulation of the PJ pattern, in turn, can intensify the anomalous convective activity near the Philippines through enhancing evaporation and moisture convergence and dynamically inducing anomalous ascent. It is thus hypothesized that the PJ pattern can be regarded as a moist dynamical mode that sustains itself both via dry energy conversion and interaction with moist processes.

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 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 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 Distinct Influences of the ENSO-Like and PMM-Like SST Anomalies on the Mean TC Genesis Location in the Western North Pacific: The 2015 Summer as an Extreme Example

2018 ◽  
Vol 31 (8) ◽  
pp. 3049-3059 ◽  
Author(s):  
Chi-Cherng Hong ◽  
Ming-Ying Lee ◽  
Huang-Hsiung Hsu ◽  
Wan-Ling Tseng
Keyword(s):  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Southern Oscillation ◽  
Boreal Summer ◽  
The Pacific ◽  
Sea Surface Temperature Anomalies ◽  
The Mean ◽  
East West

Abstract This study reports the different effects of tropical and subtropical sea surface temperature anomalies (SSTAs) on the mean tropical cyclone (TC) genesis location in the western North Pacific (WNP), a TC–SSTA relationship that has been largely ignored. In the Pacific, the interannual variability of the tropical SSTA in the boreal summer is characterized by an El Niño–Southern Oscillation (ENSO)-like pattern, whereas the subtropical SSTA exhibits a Pacific meridional mode (PMM)-like structure. Partial correlation analysis reveals that the ENSO-like and PMM-like SSTAs dominate the south–north and east–west shift of mean TC genesis location, respectively. The 2015/16 El Niño was a strong event comparable with the 1997/98 event in terms of Niño-3.4 SSTA. However, the mean TC genesis location in the WNP during the summer of 2015 exhibited an unprecedented eastward shift by approximately 10 longitudinal degrees relative to that in 1997. Whereas the ENSO-like SSTAs in 1997 and 2015 were approximately equal, the amplitude of the PMM-like SSTA in 2015 was approximately twice as large as that in 1997. Numerical experiments forced by the ENSO-like and PMM-like SSTAs in June–August 2015 reveal that the positive PMM-like SSTA forces an east–west overturning circulation anomaly in the subtropical North Pacific with anomalously ascending (descending) motion in the subtropical central (western) Pacific. The mean TC genesis location in the WNP therefore shifts eastward when warmer SST occurs in the subtropical eastern Pacific. This finding supports the hypothesis that the extremely positive PMM-like SSTA in the summer of 2015 caused the unprecedented eastward shift of the TC genesis location in the WNP.

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.

scholarly journals Influence of the Monsoon Trough on Westward-Propagating Tropical Waves over the Western North Pacific. Part II: Energetics and Numerical Experiments

2015 ◽  
Vol 28 (23) ◽  
pp. 9332-9349 ◽  
Author(s):  
Liang Wu ◽  
Zhiping Wen ◽  
Renguang Wu
Keyword(s):  
North Pacific ◽  
Numerical Experiments ◽  
Western North Pacific ◽  
Monsoon Trough ◽  
Water Model ◽  
Mean Flow ◽  
Horizontal Structure ◽  
Rotational Flows ◽  
The Mean ◽  
The Waves

Abstract Part I of this study examined the modulation of the monsoon trough (MT) on tropical depression (TD)-type–mixed Rossby–gravity (MRG) and equatorial Rossby (ER) waves over the western North Pacific based on observations. This part investigates the interaction of these waves with the MT through a diagnostics of energy conversion that separates the effect of the MT on TD–MRG and ER waves. It is found that the barotropic conversion associated with the MT is the most important mechanism for the growth of eddy energy in both TD–MRG and ER waves. The large rotational flows help to maintain the rapid growth and tilted horizontal structure of the lower-tropospheric waves through a positive feedback between the wave growth and horizontal structure. The baroclinic conversion process associated with the MT contributes a smaller part for TD–MRG waves, but is of importance comparable to barotropic conversion for ER waves as it can produce the tilted vertical structure. The growth rates of the waves are much larger during strong MT years than during weak MT years. Numerical experiments are conducted for an idealized MRG or ER wave using a linear shallow-water model. The results confirm that the monsoon background flow can lead to an MRG-to-TD transition and the ER wave amplifies along the axis of the MT and is more active in the strong MT state. Those results are consistent with the findings in Part I. This indicates that the mean flow of the MT provides a favorable background condition for the development of the waves and acts as a key energy source.

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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