60-day Oscillation Research Articles

Phase determination of persistent heavy rainfall related to 10–30-day intraseasonal oscillation over Southeastern China

Phase determination of persistent heavy rainfall related to 10–30-day intraseasonal oscillation over Southeastern China

Break Events of the Western North Pacific Summer Monsoon during 1979–2018

Abstract The monsoon break is a typical phenomenon representing the monsoon’s subseasonal variability, but our understanding of it is still limited for the western North Pacific (WNP) area. This study identifies all break events of the WNP summer monsoon (WNPSM) from 1979 to 2018. The statistical analysis suggests that break events occur from late June to late October and peak at the end of August. The occurrence frequency of break events decreases as the duration increases, with 74% of events persisting for 3–7 days and merely 26% lasting longer (8–15 days). During the break period, which is characterized by significant suppression of convection, there is an extensive anticyclonic anomaly in the lower troposphere, corresponding to a notable westward retreat of the monsoon trough and a southwestward shift of the subtropical high. Meanwhile, an anomalous cyclone and convergence in the upper troposphere are also conducive to inhibiting convection. The composite results indicate that both 10–25- and 30–60-day oscillations contribute to the break, with their dry phases explaining 49.6% and 37.5% of the original suppression of convection, respectively. Around the break, the phase alternation of the 10–25-day oscillation causes convection fluctuation, while the 30–60-day oscillation maintains a stable dry phase that favors the establishment and maintenance of the break. A further case-by-case diagnosis suggests that 46 (51) out of the 61 break events occur in dry phases of the 10–25-day (30–60-day) oscillation, whereas only 10 (4) events occur in wet phases, indicating that the phase of the two oscillations significantly modulates the occurrence of the monsoon break.

Characterizing the Local Variations of the Northern Australian Rainy Season

AbstractIn this study, we introduce a localized definition of the onset and retreat of the northern Australian rainy season that is solely based on gridded rainfall analysis. Our analysis shows that the local onset/retreat of the rainy season has considerable spatial heterogeneity. Onset is earlier and the length of the rainy season is longer to the west of the Gulf of Carpentaria than to its east. Furthermore, we also find the local onset/retreat is influenced by the wet and dry spells of the 30–60-day intraseasonal oscillation. Much of the retreat of the rainy season occurs in the dry phases of the intraseasonal oscillation. However, intriguingly, a majority of the local onset of the rainy season occurs during dry phases of the intraseasonal oscillation. The ENSO teleconnection with the variable-length northern Australian rainy season also exhibits spatial heterogeneity and significant differences from rainfall anomalies using the fixed-length boreal winter season. The onset, the retreat, the length, and the seasonal rainfall anomalies of the rainy season display a stronger correlation with the ENSO SST anomalies for the region east of 140°E relative to its west. The strong covariability of the local onset date with the corresponding seasonal length and seasonal rainfall anomalies over northern Australia offers the advantage of monitoring the onset of the northern Australian rainy season to provide an outlook for the forthcoming season. The proposed local definition of onset/retreat of the northern Australian rainy season is simple, objective, and unambiguous and is ideally suited for real-time monitoring of the evolution of the season.

The Process of High-Frequency Intraseasonal Oscillation Associated with a Persistent Rainfall Event over South China

AbstractIn the present study, the moist static energy (MSE) budget associated with a persistent rainfall event that occurred over South China in late August 2018 (PR1808) was analyzed using ERA-Interim data. The results revealed that the PR1808 event was closely related to a 12–30-day intraseasonal oscillation (ISO), with a significant westward-propagating mode. The recharge of MSE occurred before ISO deep convection, and the MSE was discharged during and after the peak precipitation. This result indicates that the recharge–discharge cycle of MSE played an important role in regulating the high-frequency ISO (HF-ISO) process during the PR1808 event. However, the midlevel MSE tendency, rather than the low-level tendency, controlled the column-integrated MSE change in this case, and there was no low-level MSE tendency ahead of HF-ISO convection, which is different from previous studies, implying that shallow convection was not a key factor in this case. The recharge of MSE related to the midlevel MSE change mode was mainly attributed to zonal advection, while the forcing related to radiative heating (longwave and shortwave radiation) and heat fluxes (latent and sensible heat fluxes) contributed little to the change in MSE. Furthermore, for the zonal advection of MSE, the main contribution originated from the advection by the low-frequency zonal flow across the low-frequency MSE gradient, and interactions between high- (low-) frequency zonal flow and low- (high-) frequency MSE. In addition, the disturbances from higher latitudes enhanced the persistent rainfall in this case over South China through the southward shift of baroclinic vorticity.

The Quasi-Biweekly Oscillation of Summer Rainfall in Southern China and Its Relationship With the Geopotential Height Anomaly Over the North Atlantic Ocean

Summer precipitation in East Asia has significant quasi-biweekly (10–30-day) oscillation characteristics. By using gauge-based precipitation and ERA-Interim reanalysis data, the basic mode of the quasi-biweekly oscillation of summer precipitation in East Asia and the related circulation from 1979 to 2012 were analyzed. It was found that the middle and lower reaches of the Yangtze River and its south in China were among the key areas for the 10- to 30-day oscillation of summer precipitation. After selecting typical summer precipitation events with 10- to 30-day oscillation characteristics in key areas and conducting composite analysis, it is found that in the dry (wet) phase of quasi-biweekly precipitation in southern China, it is controlled by quasi-biweekly anticyclone (cyclone) at 500 hPa above the key area. During the evolution of quasi-biweekly precipitation, the ridge of the Northwest Pacific Subtropical High is located between 20 and 22°N latitude, and there is no significant variability in the large-scale background circulation. Furthermore, composite analysis of the precursory signal at 500 hPa during quasi-biweekly precipitation in southern China found that there was an obvious quasi-biweekly geopotential height anomaly over the North Atlantic Ocean almost 30 days before the peak day of quasi-biweekly precipitation. While the quasi-biweekly geopotential height anomaly at 500 hPa in the North Atlantic propagates eastward, it also leads the cold air to transport southward. Cold air from high latitudes and warm air from low latitudes converge in southern China, which affects the quasi-biweekly oscillation of precipitation. Hysteresis synthesis of precipitation based on 500 hPa geopotential height’s quasi-biweekly oscillation events over the North Atlantic Ocean comes to almost the same conclusion. Therefore, the 500 hPa geopotential height quasi-biweekly anomaly in the North Atlantic may have important prediction significance for an extended-range forecast of summer rainfall in China.

Potential vorticity analysis of quasi-biweekly rainfall events over the Yangtze Basin in summer 2014

A series of heavy rainfall events occurred over the Yangtze River Valley (YRV) in summer 2014, which were modulated by the 10–20-day quasi-biweekly oscillation (QBWO). Thus, the strongest QBWO cycle for the period 10–24 July was used as a representative case to reveal the dynamical mechanism for the QBWO of the YRV rainfall from the potential vorticity (PV) perspective and based on MERRA-2 reanalysis data. The quasi-biweekly YRV rainfall was found to depend closely on the QBWO of the upper-tropospheric South Asian high (SAH), with the SAH configuration modified by the southward-intruding midlatitude high PV stream along with southwestward-advected high PV, altering the divergent condition over the YRV. Quantitative diagnoses for the anomalous vertical motion demonstrated that, in the wet phase of the QBWO cycle, the upper-tropospheric southward-intruding high PV stream acted as a positive PV advection, while negative PV advection was generated due to the lower-tropospheric southerlies, thereby forming a positive vertical gradient of horizontal PV advection to induce evident isentropic-displacement ascending motion. On the other hand, the southward-intruding high PV stream extended downward to the middle troposphere, causing the isentropic surfaces to become more sloping, thus producing a strong isentropic-gliding ascending component. Subsequently, the stronger diabatic heating-related ascending motion was induced to generate positive rainfall anomalies over the YRV. The opposite situation arose in the dry phase, with weak descending motion in magnitude.摘要2014年夏季长江流域(YRV)发生的多次阶段性强降水事件显著地受10–20天准双周振荡所调控. 代表性振荡过程与合成分析表明, 准双周湿位相主要取决于对流层高层南侵的高位势涡度(PV)与低纬度向西南平流的高PV导致南亚高压形态改变而产生的YRV高空辐散. 高空向南的正PV平流与低层南风输送的负PV平流在YRV上空形成正的PV平流垂直梯度, 激发出等熵面位移的上升运动分量; 高空的高PV同时向中低层伸展, 导致等熵面坡度增大从而增强气块沿等熵面上滑的上升运动分量, 由此产生更强的非绝热有关的上升运动分量. 干位相情况则相反.

Linkage Between the Intraseasonal Oscillation of Atmospheric Heat Sources Over the Tibetan Plateau and Amplified Precipitation to the South of MLYR

ABSTRACT Based on the daily Japanese 55-year Reanalysis (JRA-55) from 1979 to 2015, this study shows that atmospheric heat sources over the Tibetan Plateau (TP) and its surrounding areas (TPSR) in summer have a significant 10- to 30-day intraseasonal oscillation (ISO). The linkage of the ISO with the amplified precipitation to the south of the middle and lower reaches of the Yangtze River (MLYR) is also investigated. The results show that when anomalous positive heat sources over the TPSR strengthen, an anomalous thermal low pressure develops, leading to anomalous positive vorticity over the TPSR. Meanwhile, a mid-high latitude wavetrain with alternating positive and negative vorticity anomaly centres over the Eurasian continent propagates eastward across the TP. The anomalous positive vorticity over the northeastern TP is enhanced when combined with a deep positive vorticity anomaly over the TPSR. The wave-activity flux in the eastern TPSR turns southward or southeastward, with a positive vorticity anomaly propagating to the southeast of China. As a result, positive vorticity advection to the south of the MLYR increases with height and is conducive to convergence, upward motion, and increased precipitation. The situation is the reverse when negative anomalous heat sources develop over the TPSR. Therefore, the 10- to 30-day ISO of the atmospheric heat source over the TPSR in summer could contribute to the amplification of the intraseasonal precipitation anomalies to the south of the MLYR by regulating the range and intensity of the anomalous vorticity of the Eurasian wavetrain.

Intraseasonal oscillation intensity over the western North Pacific: Projected changes under global warming

The 30–60-day intraseasonal oscillation (ISO) and 10–20-day ISO are two dominant oscillation modes over the western North Pacific during boreal summer. With daily data derived from eight CMIP5 models, changes of the ISO intensities are projected under the 1.5 and 2.0 °C global warming levels under the Representative Concentration Pathway (RCP) 4.5 and RCP8.5 scenarios. Most of the models agree that the ISO intensities increase along a belt region from the south Indochina Peninsula (ICP) to the east to the Philippines. The variation pattern shows little difference between different warming levels or scenarios. Results indicate that the spatial distribution of ISO anomalies is related with the variation of background fields. Enriched lower-level humidity and moist static energy favor the intensity increases of ISOs, which are projected to be larger over the whole western North Pacific, with the most conspicuous changes located over the east to the Philippines for humidity but over the south of the ICP for moist static energy. In contrast, the ISOs over the west to Indonesia and northeast to the Philippines decrease, which is consistent with the local descending motions.摘要本文利用8个CMIP5模式的日资料, 预估了RCP4.5和RCP8.5情景下全球增温达1.5 °C和2.0 °C时西北太平洋夏季30~60天和10~20天季节内振荡 (ISO) 强度的变化情况. 大多数模式都认为, 无论增温水平或情景如何, 预估结果均显示从中南半岛南部到菲律宾以东的带状区域内ISO强度增加, 并且关键气象要素背景的变化会对ISO强度异常的空间分布造成影响. 具体表现为, ISO强度增大的区域往往伴随着低层湿度和湿静力能的增加. 其中菲律宾东部的湿度变化最为明显, ICP南部的湿静力能变化最为明显, 上述区域的ISO强度均增强. 相反, 印度尼西亚西部和菲律宾东北部有局地下沉运动增强, 当地的ISO强度减小.

Summer Regional Pentad Extreme Precipitation in Eastern China and Their Possible Causes

Persistent extreme precipitation covering a large area usually causes severe flooding disasters in China, but how to depict it and what are the possible causes are still open questions. With Climate Prediction Center global unified gauge-based analysis of daily precipitation and NCEP/NCAR daily reanalysis dataset from 1979 to 2019, summer regional pentad extreme precipitation (RPEP) is defined according to the threshold of the 95th percentile of pentad precipitation with more than 5% land grids coverage in eastern China. While the definition of RPEP highlights the climate features of both the persistence and the regionality of extreme precipitation, it is distinctly different from the previous definitions that mainly reflect the synoptic aspects with daily data and have strictly temporal-spatial constraints. Four categories of RPEPs are objectively identified by K-means cluster analysis, i.e., South China (SC), South of Yangtze River (SYR), Jianghuai River (JHR), and North China (NC). Along the Yangtze River (SYR and JHR), intensity and area of RPEP are positively correlated with each other, and with the increase of RPEP intensity, its center of gravity tends to move eastward in all the four cluster regions and southward in Jianghuai River and North China, respectively, and vice versa. The RPEPs mostly persist for one pentad but can reach up to two to three pentads at most, and along with the duration of RPEP, its intensity and area are both enhanced accordingly. Furthermore, the frequency of RPEP increased significantly since the late 1990s in SYR, JHR, and SC. Associated with RPEP, strong pentad-mean convergence and ascending motion occur in the middle-lower troposphere, and except for SC that is dominated by the local low-pressure and cyclone anomalies, the other three cluster regions are all forced by the western Pacific subtropical high to the southeast and weak low-pressure trough to the north, and the low-level anticyclone anomaly to the southeast transports abundant water vapors to the RPEP regions accordingly. Besides, all the RPEPs are closely in accordance with obvious subseasonal oscillations, especially the 10–30-day and 30–60-day oscillations, which can be regarded as the potential sources of RPEP predictability in eastern China.

Regulation of the subseasonal variability of winter rainfall in South China by the diversity of El Niño Southern Oscillation

Winter precipitation over South China tends to increase with enhancement of the 10–30-day intraseasonal oscillation (ISO) during El Nino Southern Oscillation (ENSO) events from 1981 to 2017. This study shows that, in contrast with central Pacific El Nino and La Nina events, the 10–30-day ISO of rainfall particularly intensify during eastern Pacific (EP) El Nino events. The seasonal evolution of the anomalous circulation, indicated as the annual cycle (AC), bridges the slow-varying ENSO and the transient 10–30-day ISO of winter rainfall over South China. As to the AC component, the EP El Nino events not only enhance the Philippine Sea anticyclone to provide a wetter low-level background over South China, but develop the mid-latitude cyclone and westerly winds in the upper-level over East Asia by changing tropical convection. The moisture over South China further increases due to the low-level wind convergence on a 10–30-day timescale. In the upper troposphere, the AC component of the anomalous westerly redistributes the 10–30-day relative vorticity by modulating the subseasonal mid-latitude wave train to strengthen the positive vorticity advection and ascending motion over South China. Their collaboration amplifies the subseasonal variance of winter precipitation in situ. By contrast, the AC component of the upper level circulation at mid-latitudes is not well organized in other ENSO subsets, corresponding to their lack of influence on the ISO variability of rainfall over South China.

Impact of potential vorticity anomalies around the eastern Tibetan Plateau on quasi-biweekly oscillations of summer rainfall within and south of the Yangtze Basin in 2016

Six persistent heavy rainfall events that occurred mostly over Hubei–Anhui–Jiangxi (HAJ) provinces within and south of the middle–lower Yangtze Basin during the summer of 2016 were regulated by the 10–20-day quasi-biweekly oscillation (QBWO). The characteristics and mechanisms of the QBWO associated with the HAJ rainfall events were examined using ERA-Interim reanalysis data. Composite analysis shows that during a QBWO cycle, the extreme wet (dry) phase of the HAJ rainfall was characterized by the strongest ascending (descending) motion associated with an anomalous upper-tropospheric saddle-shaped circulation field resulting from an eastward-propagating middle–high latitude wave train and westward-migrating tropical potential vorticity (PV) anomalies. The wet (dry) phase was preceded by the corridor establishment of significant anomalous poleward-directed (equatorward-directed) moisture transport over southern China on the northwestern side of the western North Pacific subtropical high in the lower troposphere. The corridor-related moisture convergence (divergence) over the HAJ tended to intensify as an anomalous anticyclone (cyclone) propagated northwestward to the northern South China Sea, in conjunction with the eastward-propagating mid-latitude wave train. The QBWO of the HAJ rainfall was closely linked with upstream PV anomalies generated over the eastern slope of the Tibetan Plateau (TP) due to topographic lateral friction. The PV budget analysis demonstrates that the horizontal PV advection and subsequent topographic friction with a four-day phase-lag between them dictated the QBWO of PV anomalies around the eastern TP. The TP-generated PV anomalies then migrate downstream to facilitate the development of the anomalous circulation over the HAJ.

A Comparative Study on the Offshore Intensification of Supertyphoon Rammasun (2014) and Typhoon Rumbia (2013): the Role of Summer Monsoon

The offshore intensification mechanisms of Supertyphoon Rammasun (2014) and Typhoon Rumbia (2013) are analyzed and compared based on the best track typhoon data provided by the Shanghai Typhoon Institute of China Meteorological Administration (STI/CMA), ECMWF (CMA and JMA) objective (subjective) predicted typhoon track and intensity, and reanalysis data of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). Results show that the offshore sea surface temperature over 28 °C was conducive to the intensification of the two typhoons in the coastal area prior to their landfall in South China. Rammasun was embedded in an extremely active phase of a 10–20-day oscillation in the summer monsoon over the South China Sea and western North Pacific prior to its landfall. The low-level jet transported warm and moist air to the inner core of Rammasun and the deepening of the southwesterly monsoon flow up to 200 hPa reduced vertical wind shear, both favorable for the offshore rapid intensification of Rammasun. In 2013, the summer monsoon over the South China Sea and western North Pacific experienced quasi-20-day and quasi-10-day oscillations. The out of phase of the two oscillations resulted in relatively weak tropical and subtropical summer monsoon flow prior to the landfall of Rumbia. As a result, the low-level monsoon flow transported relatively less moist air into Rumbia, and the shallow monsoon flow was not conducive to the weakening of vertical wind shear, resulting in a less rapid offshore intensification of Rumbia than Rammasun.

Signature of the 27-day oscillation in the MLT tides and its relation with solar radiation at low latitudes

The modulation of the dominant atmospheric tides (i.e. diurnal, semidiurnal and terdiurnal) in the mesosphere and lower thermosphere (MLT) is investigated using long-term meteor wind database from three Southern hemispheric low-latitude locations, São João do Cariri (7.4° S, 36.5° W), Cachoeira Paulista (22.7° S, 45° W) and Santa Maria (29.7° S, 53.7° W). The spectral analysis reveals an evident and intermittent signature of a 27-day oscillation in the tidal amplitudes. Relationship between the 27-day tidal modulation in the MLT and solar rotation is looked into utilizing solar UV flux (Lyman-α) that indicates a conspicuous linkage of the tides with the solar short-term variability. The strong correlation between the solar variability and tidal modulation in the concerned period with positive lags at certain intervals may indicate predominate solar influence on the MLT tides. Potential involvement of the lower, middle and upper atmospheric dynamics and chemistry to support the observed oscillation feature is deemed plausible.

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

AbstractThe 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.

The Impact of the 20–50-Day Atmospheric Intraseasonal Oscillation on the Gross Primary Productivity between the Yangtze and Yellow Rivers

AbstractGiven their high carbon uptake, the terrestrial ecosystems in the East Asia summer monsoon (EASM) region play an irreplaceable role in the global carbon cycle. Because the rich vegetation growth over East Asia benefits mainly from the sufficient water supply brought by the EASM, which is characterized by a strong intraseasonal oscillation (ISO), the intraseasonal spatiotemporal variations and underlying drivers of photosynthesis activity over East Asia have been comprehensively investigated using the daily gross primary productivity (GPP) and meteorological data. Strong intraseasonal fluctuations of GPP have been identified over the area between the Yangtze and Yellow Rivers (YYR) with a magnitude of 0.4 gC m−2 day−1. The mean power spectrum suggests that 20–50-day variation is the major component of the intraseasonal GPP anomalies over the YYR during the summers of 1980–2013. The 20–50-day ISO of YYR GPP anomalies is modulated by the local 20–50-day precipitation variation via soil moisture, with precipitation (soil moisture) leading GPP by 10 (7) days. The 20–50-day YYR precipitation anomalies are in turn controlled by tropical ISO signals, particularly the convective activity over the western North Pacific. This leading relationship between the 20–50-day atmospheric ISO and GPP suggests a potential for extended-range predictability of vegetation growth.

Dynamics of intraseasonal oscillations in the Bay of Bengal during summer monsoons captured by mooring observations

The intraseasonal variabilities of current and temperature in the southern Bay of Bengal (BoB) were examined using in-situ observations collected in the upper 500 m of the water column for a period of 20 months, between December 2013 and August 2015. Data were sampled by six subsurface moorings deployed by the United States Naval Research Laboratory as part of an international effort focused on the dynamics of the northern Indian Ocean. The moored observations captured motions of multiple time-scales, including seasonal circulations, eddies, and planetary waves that underlie intraseasonal variability. This study focuses on 30–60-day intraseasonal oscillations (ISOs). Spectral analysis of currents shows enhanced energy and momentum fluxes for the period 46–52-days. The ISO velocity fluctuations were coherent in the upper 500 m of the water column, strongest near the surface and nearly uniform below 100 m. The ISO kinetic energy appeared at the beginning of the 2014 summer monsoon and lasted throughout the summer, but the associated potential energy dropped rapidly after it peaked at the beginning of the monsoon season.The correlation between the 30–60 day ISOs of local winds and the oceanic variability below 120 m depth was fund to be as small as 25% suggesting thus that the local winds were not the main forcing mechanism for ISOs. The 30–60-day band-passed sea-surface-height anomalies indicate westward propagating anomalies with phase speeds between 0.17 m s−1 at 8°N and 0.43 m s−1 at 4°N, within the phase-speed ranges of mode-1 and mode-2 linear baroclinic Rossby waves, pointing to the role of Rossby waves in 30–60-day ISOs.The background mean flow acceleration resulting from the meridional divergence of wave momentum flux in the thermocline was about 10−8 m s−2. As a result, within a wave period, ISOs can enhance the eastward flow in the thermocline by about 25%. The negative shear production computed for the same period is consistent with this finding, suggesting that the mean flow has gained kinetic energy at the expense of the ISO band at a rate of about 0.5×10−8 m2 s−2. The meridional heat-flux divergence was -10−7 °C s−1, and has a tendency for cooling the thermocline by about 0.5 °C when the ISOs are active. The ISOs appear to play an important role in the ocean dynamics of the southern BoB during the summer monsoon.

Patterns and factors of interannual variations of boreal summer intraseasonal oscillation intensity over tropical western North Pacific

Present study analyzes spatial patterns and factors of interannual variations of 10–20-day and 30–60-day intraseasonal oscillation (ISO) intensity during boreal summer over tropical western North Pacific (WNP). The three leading modes of 10–20-day ISO intensity variation feature a southeast–northwest distribution, a southeast–northwest contrasting pattern, and large loading over the subtropics, respectively. The two leading modes of 30–60-day ISO intensity variation show a west–east distribution and a west–east dipole pattern, respectively. Three types of sea surface temperature (SST) influences are identified on the ISO intensity variation. One is a direct influence of El Nino–Southern Oscillation (ENSO) on seasonal mean background over tropical WNP via a Rossby wave type response. This type of influence is prominent on the leading mode of 10–20-day ISO intensity and the eastern portion of the second mode of both 10–20-day and 30–60-day ISO intensity. Second is an indirect influence of ENSO on seasonal mean background over subtropical WNP and the Maritime Continent through anomalous Walker circulation. This type of influence includes the western portion of the second mode of both 10–20-day and 30–60-day ISO intensity. Third is an influence of regional SST anomalies. Opposite SST anomalies in the North Indian Ocean and tropical North Pacific work together to modulate seasonal mean background over tropical WNP. This type of influence is prominent on the leading mode of 30–60-day ISO intensity. In addition, internal atmospheric process may play a role in 10–20-day ISO intensity change over subtropical WNP.

On the Study of the Spatio-Temporal Variations in Intensity of 30 - 60-Day Intraseasonal Rainfall Oscillations in Central Africa Using Wavelet-Based Indices

The intraseasonal timescale is attractive in Central Africa (CA) where socio-economic activities are highly based on rainfall. The parameterization of intraseasonal oscillations (ISO) then remains a great challenge for the improvement of sub-seasonal to seasonal (S2S) prediction in this region. In this paper, we applied wavelet analysis on the 2.5° × 2.5° daily Outgoing Longwave Radiation (OLR, used here as rainfall proxy) to study the variations in the intensity of the 30 - 60-day intraseasonal rainfall oscillations over Central Africa within the last three decades (1980-2009). Results showed that the mean ISO intensity (ISOI) strongly fluctuates from day to another. The plots of monthly mean ISOIs revealed that the ISO activity is highly seasonal with above-normal ISO intensity during December-May and below-normal activity during June-November. The analysis of yearly mean ISOI showed that the ISOI exhibits strong interannual variations with the years of very low ISOIs such as 1982, 1994, 2001, 2007, 2009, 2015 and the years of very high ISOIs such as 1981, 1985, 1986, 989/1990, 1997, 2003, 2005. The regression analysis between ISOI and El Ni&#241o Southern Oscillation (ENSO) indices showed that even though the relationship between ISO and ENSO is nonlinear, warm ENSO (El Ni&#241o) events tend to reduce the ISOIs while cold ENSO (La Ni&#241a) events tend to increase it.

Long-lead ENSO control of the boreal summer intraseasonal oscillation in the East Asian-western North Pacific region

The boreal summer 30–60-day intraseasonal oscillation (ISO) in the East Asian-western North Pacific (EAWNP) region strongly influences persistent heavy rainfall in East Asia and tropical cyclone activities in the tropical western Pacific. In this study, we show that there exists a significant interannual south-north shift of the EAWNP-ISO activity between the equatorial eastern Indian Ocean—Maritime continent region and the subtropical South China Sea—western Pacific. This is reflected by the year-to-year changes in the occurrence frequency of individual ISO phases that correspond to different latitudinal locations of the convection anomaly. This interannual shift is largely controlled by the El Niño—Southern Oscillation (ENSO) in the preceding winter and spring. Following an El Niño event, the ISO convective activity tends to occur in the equatorial eastern Indian Ocean—Maritime continents, whereas after a La Niña, frequent ISO-related precipitation occurs in the region of south China and the subtropical South China Sea—western Pacific. This result implies long-lead predictability for the occurrence of the summertime EAWNP-ISO activity and its associated weather.

Structures and Mechanisms of 20–60-Day Intraseasonal Oscillation of the Observed Rainfall in Vietnam

Abstract The present study explores the characteristics of the 20–60-day intraseasonal oscillation (ISO) in the 29-yr observed rainfall in north Vietnam (NVN), central Vietnam (CVN), and south Vietnam (SVN) in rainy seasons. Composite analyses reveal that the 20–60-day ISO in NVN accompanies dual vortices straddling Taiwan, which alternately favor and suppress convection extending from the northern Philippines to NVN. The wet phase in CVN coincides with convergence of northerly and easterly winds over the region. The large-scale pattern governing the 20–60-day ISO in SVN resembles the characteristics of the boreal summer ISO (BSISO). Conditionally unstable anomalies are observed within anomalous anticyclones where the moisture flux diverges out during the dry phase in NVN and SVN, and vice versa. Such anomalies prevent the existence of the anticyclones and finally replace them with anomalous cyclones to start the wet phase. The unstable anomalies could result from descending motion that increases the boundary layer temperature due to adiabatic compression of air. Conversely, boundary layer cooling due to evaporation of rain and interception of solar radiation by clouds produces stable anomalies. The unstable anomalies, moisture flux convergence, and vertical motions shift northward from the convection maximum, leading to the northward propagation of the BSISO convection. The 20–60-day ISO in CVN is not governed by local instability. Vertical cross sections indicate that the ISO in SVN possesses a westward-tilting structure, which is not observed in the NVN and CVN case.