scholarly journals East-west SST contrast over the tropical oceans and the post El Niño western North Pacific summer monsoon

2005 ◽  
Vol 32 (15) ◽  
Author(s):  
Toru Terao
Keyword(s):  
Summer Monsoon ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Tropical Oceans ◽  
East West

scholarly journals Characteristics of the Onset, Withdrawal, and Breaks of the Western North Pacific Summer Monsoon in the 1949–2014 Period

2020 ◽  
Vol 33 (17) ◽  
pp. 7371-7389
Author(s):  
Inmaculada Vega ◽  
Pedro Ribera ◽  
David Gallego
Keyword(s):  
Interannual Variability ◽  
Summer Monsoon ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
La Niña ◽  
Onset Date ◽  
Central Pacific ◽  
La Nina

ABSTRACTThe western North Pacific summer monsoon (WNPSM) onset and withdrawal dates as well as its breaks have been determined throughout the 1949–2014 period by defining the monsoon daily directional index (MDDI). This index, developed exclusively with wind direction observations, is an upgrade of the monthly western North Pacific directional index. The onset date shows a high interannual variability, varying between early May and early August, whereas the WNPSM withdrawal shows a lower interannual variability, occurring between October and mid-November. The MDDI reflects the multibreak character of the WNPSM. Breaks, which tend to last a few weeks, are more likely to happen from mid-August to early September and from late June to mid-July. This bimodal distribution shows decadal variability. In addition, the monsoon dates determined by the MDDI show very good agreement with relationships previously described in literature, such as the influence of tropical Pacific SST on the monsoon onset/withdrawal and changes in tropical cyclone (TC) tracks related to monsoon breaks. The WNPSM tends to start earlier (later) and finish later (earlier) under eastern Pacific (EP) La Niña (El Niño) conditions, especially from the 1980s on. Central Pacific (CP) ENSO is also associated with the monsoon withdrawal, which is advanced (delayed) under CP El Niño (La Niña). TCs tend to move from the Philippine Sea to the South China Sea during active monsoon days whereas they tend to reach higher latitudes during inactive monsoon days, especially in August and July.

scholarly journals Influences of ENSO on Western North Pacific Tropical Cyclone Kinetic Energy and Its Meridional Transport

2013 ◽  
Vol 26 (1) ◽  
pp. 322-332 ◽  
Author(s):  
Yao Ha ◽  
Zhong Zhong ◽  
Yijia Hu ◽  
Xiuqun Yang
Keyword(s):  
Tropical Cyclone ◽  
Kinetic Energy ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Reanalysis Data ◽  
Meridional Transport ◽  
Dipole Pattern ◽  
East West

Abstract This study investigates the influences of ENSO on tropical cyclone (TC) kinetic energy and its meridional transport in the western North Pacific (WNP) using the TC wind field obtained after a method for removing TC vortices from reanalysis data is applied. Results show that ENSO strongly modulates TC kinetic energy and its meridional transport in the WNP, but their effects and regions differ. The TC kinetic energy is positively correlated with the Niño-3.4 index in the entire WNP, and its poleward transport is positively (negatively) correlated with the Niño-3.4 index in the eastern WNP (the western WNP and the South China Sea); these correlations are statistically significant. The maximum TC kinetic energy is located around 25°N, 135°E (25°N, 125°E) in the warm (cold) year, showing an east–west pattern during different ENSO phases. The meridional transport of TC kinetic energy exhibits a dipole pattern over the WNP, with the poleward (equatorward) transport in the eastern (western) WNP. Both poleward and equatorward transports strengthen (weaken) and shift eastward (westward) in El Niño (La Niña) years. Therefore, El Niño has strong influences on TC kinetic energy and its meridional transport.

scholarly journals The Zonal Oscillation and the Driving Mechanisms of the Extreme Western North Pacific Subtropical High and Its Impacts on East Asian Summer Precipitation

2019 ◽  
Vol 32 (10) ◽  
pp. 3025-3050 ◽  
Author(s):  
Tat Fan Cheng ◽  
Mengqian Lu ◽  
Lun Dai
Keyword(s):  
Summer Monsoon ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
La Niña ◽  
Positive Phase ◽  
La Nina ◽  
Asian Summer ◽  
Zonal Oscillation

Abstract This paper scrutinizes the zonal oscillation of the western North Pacific subtropical high (WNPSH) via diagnosing its two extreme phases, which are defined by the top 10% strongest (positive phase) and the weakest (negative phase) WNPSH index (WNPSHI) days during summers in 1979–2016. Key findings include the following: a tripole pattern consisting of intensified (weakened) precipitation over the Maritime Continent and the East Asian summer monsoon regions, and suppressed (strengthened) precipitation over the western North Pacific summer monsoon region during positive (negative) WNPSH phases; a westward movement of WNPSH-induced precipitation anomalies that subsequently affects eastern China, Japan, and the Korean Peninsula at different time lags; an OLR–vorticity pattern explained by atmospheric responses to thermal sources is suggested to drive the oscillation; and the competitive interaction of local air–sea feedbacks, especially during the positive phase. In addition, moderate-to-strong positive correlations between the WNPSHI and the Niño-3.4 index are found on 1–2-, 2–3-, and 3–6-yr time scales; both exhibit decadal shifts to a higher-frequency mode, suggesting the intensification of both the zonal WNPSH oscillation and the ENSO under the changing climate and their close interdecadal association. A nonlinear quasi-biennial WNPSH–ENSO relationship is identified: the positive (negative) WNPSH phase sometimes occurs during 1) a decaying El Niño (La Niña) in the preceding summer/autumn, and/or 2) a developing La Niña (El Niño) in the current summer/autumn. A full ENSO transition from moderate-to-strong El Niño to La Niña is often seen during the positive phase, offering potential in predicting ENSO events and extreme WNPSH phases and thereby the summer monsoon rainfall in East Asia.

scholarly journals Reconstructing the Western North Pacific Summer Monsoon since the Late Nineteenth Century

2017 ◽  
Vol 31 (1) ◽  
pp. 355-368 ◽  
Author(s):  
Inmaculada Vega ◽  
David Gallego ◽  
Pedro Ribera ◽  
F. de Paula Gómez-Delgado ◽  
Ricardo García-Herrera ◽  
...  
Keyword(s):  
Twentieth Century ◽  
Summer Monsoon ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
La Niña ◽  
El Niño Modoki ◽  
La Nina ◽  
The Relationship

Abstract A new index, the western North Pacific directional index (WNPDI), based on historical wind direction observations taken aboard sailing ships, has been developed to characterize the western North Pacific summer monsoon (WNPSM) since 1898. The WNPDI measures the persistence of the surface westerly winds in the region 5°–15°N, 100°–130°E and easterly winds in the region 20°–30°N, 110°–140°E, exhibiting a consistent relationship with the summer precipitation in the areas affected by the WNPSM throughout the entire twentieth century. Its length doubles that of the previous WNPSM index (1948–2014) based on reanalysis products, which allows uncovering different relevant features of the WNPSM variability. The WNPSM had a significant interdecadal variability throughout the twentieth century. In particular, the period 1918–48 was characterized by less variable and stronger monsoons than in recent decades. Additionally, the relationship between the WNPSM and ENSO or El Niño Modoki has been evaluated during the entire twentieth century for the first time. It is confirmed that the WNPSM tends to be strong (weak) when El Niño (La Niña) develops during the whole record. Nevertheless, the relationship during the ENSO-decaying phase is not stable in time. Thus, the WNPSM tended to be strong (weak) when La Niña (El Niño) decayed only since the late 1950s, with an opposite relationship in the earliest part of the record. El Niño Modoki shows a rather stable and high correlation with the WNPDI during the whole study period throughout the twentieth century.

scholarly journals Extremely Early Summer Monsoon Onset in the South China Sea in 2019 Following an El Niño Event

2020 ◽  
Vol 148 (5) ◽  
pp. 1877-1890 ◽  
Author(s):  
Peng Hu ◽  
Wen Chen ◽  
Shangfeng Chen ◽  
Yuyun Liu ◽  
Ruping Huang
Keyword(s):  
South China Sea ◽  
Summer Monsoon ◽  
South China ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Monsoon Onset ◽  
El Niño Event ◽  
Preceding Winter

Abstract The El Niño–Southern Oscillation (ENSO) is regarded as one of the most important factors for onset of the South China Sea summer monsoon (SCSSM). Previous studies generally indicated that an El Niño event tends to result in a late onset of the SCSSM monsoon. However, this relationship has not been true in recent years, particularly when an extremely early SCSSM onset (1 May 2019) occurred following the 2018/19 El Niño event in the preceding winter. The processes of the second earliest SCSSM onset in the past 41 years were investigated using NCEP–DOE reanalysis, OLR data, and ERSST. A negative sea surface temperature and associated anticyclonic anomalies were absent over the western North Pacific in the late spring of 2019 following an El Niño event in the preceding winter. Thus, the mean circulation in the late spring of 2019 does not prevent SCSSM onset, which is in sharp contrast to the composited spring of the El Niño decaying years. The convective active and westerly phases of a 30–60-day oscillation originating from the Indian Ocean provided a favorable background for the SCSSM onset in 2019. In addition, the monsoon onset vortex over the Bay of Bengal and the cold front associated with a midlatitude trough over East Asia also played important roles in triggering the early onset of the SCSSM in 2019. No tropical cyclone appeared over the western North Pacific during April and May, and the enhancement of quasi-biweekly oscillation mainly occurs after the SCSSM onset; thus, these two factors contribute little to the SCSSM onset in 2019.

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 Interannual Variability of the Western North Pacific Summer Monsoon: Differences between ENSO and Non-ENSO Years

2003 ◽  
Vol 16 (13) ◽  
pp. 2275-2287 ◽  
Author(s):  
Chia Chou ◽  
Jien-Yi Tu ◽  
Jia-Yuh Yu
Keyword(s):  
Interannual Variability ◽  
Summer Monsoon ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Wave Train ◽  
La Niña ◽  
Temperature Anomalies ◽  
La Nina

Abstract The interannual variability of the western North Pacific (WNP) summer monsoon is examined for the non-ENSO, ENSO developing, and ENSO decaying years, respectively. The ENSO developing (decaying) year is defined as the year before (after) the mature phase of ENSO, and the non-ENSO year is defined as the year that is neither the ENSO developing year nor the ENSO decaying year. A strong (weak) WNP summer monsoon tends to occur during the El Niño (La Niña) developing year and a weak (strong) WNP summer monsoon tends to occur during the El Niño (La Niña) decaying year. In all non-ENSO, ENSO developing, and ENSO decaying years, the strong (weak) WNP summer monsoon is associated with the positive (negative) rainfall anomalies, cold (warm) sea surface temperature anomalies, warm (cold) upper-tropospheric temperature anomalies, low (high) surface pressure anomalies, and a low-level cyclonic (anticyclonic) circulation anomaly over the subtropical WNP. The 850-hPa wave train associated with the WNP and east Asian (EA) summer monsoons in the non-ENSO, ENSO developing, and ENSO decaying years extends northward and suggests a possible teleconnection between the WNP summer monsoon and the North American climate. The wave train extended into the Southern Hemisphere in the non-ENSO and ENSO developing years implies a teleconnection between the WNP summer monsoon and the Australian winter climate. The anomalous WNP monsoon in the non-ENSO and ENSO developing years exists only in summer, while the anomalous WNP monsoon in the ENSO decaying year persists from the beginning of the year to the summer season. The anomalous WNP summer monsoon exhibits a strong ocean–atmosphere interaction, especially in the ENSO decaying year. This study suggests that the anomalous WNP summer monsoon in the non-ENSO year is associated with the variation of the meridional temperature gradient in the upper troposphere, while the anomalous WNP summer monsoon in the ENSO developing and decaying years is associated with ENSO-related SST anomalies.

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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