Intraseasonal Oscillation Intensity Research Articles

Interannual variability of mid-high-latitude intraseasonal oscillation intensity at the southern hemisphere during austral summer

Interannual variability of mid-high-latitude intraseasonal oscillation intensity at the southern hemisphere during austral summer

Asymmetric Changes in Intraseasonal Oscillation Intensity over the Tropical Western North Pacific in El Niño and La Niña Developing Summers

Abstract The intraseasonal oscillations (ISOs) over the tropical western North Pacific (WNP) modulate atmospheric convection and heating and affect weather and climate in remote regions through atmospheric teleconnection. The present study unravels the fact that the ISO intensity increase over the tropical WNP in El Niño developing summers is larger, with the center located eastward, compared with the decrease in La Niña developing summers. The asymmetric ISO intensity changes are attributed to the eastward shift of regions of anomalous low-level westerly winds, ascent, easterly shear of zonal winds, and large moisture in El Niño developing summers and westward shift of regions of opposite anomalies in La Niña developing summers, respectively. The asymmetric atmospheric mean anomalies, in turn, are due to the westward shift of anomalous cooling in La Niña developing summers compared to anomalous warming in El Niño developing summers. The 10–20- and 30–60-day ISOs show different patterns of intensity variations due to their different source regions and propagation paths. Atmospheric model simulations confirm the asymmetric response of boreal summer ISO intensity over the tropical WNP to El Niño and La Niña events and the role of asymmetric atmospheric background field changes. Sensitivity experiments illustrate that asymmetric changes in the ISO intensity and atmospheric background fields over the tropical WNP are due to their asymmetric response to opposite tropical central-eastern Pacific SST anomalies. The asymmetry in tropical central-eastern Pacific SST anomalies in El Niño and La Niña events has a small impact on asymmetric ISO intensity changes over the tropical WNP.

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.

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.

Impact of the intra‐seasonal oscillation on tropical cyclone genesis over the western North Pacific

This study investigates the impact of the East Asian–western North Pacific (EAWNP) intra‐seasonal oscillation (ISO) on tropical cyclone (TC) genesis over the South China Sea (SCS) and the western North Pacific (WNP). The result indicates that the 20–70‐day oscillation (EAWNP‐ISO1) mainly modulates the frequency of TC genesis, while the 10–20‐day quasi‐biweekly oscillation (EAWNP‐ISO2) usually influences the genesis location. The impacts of ISO are not only on the individual TC genesis but also on the intra‐seasonal variation of TC activity.In order to better monitor and predict TC genesis, we group the ISO intensity into three categories (wet, normal, and dry) using the EAWNP ISO‐1 index and classify the ISO location into three categories (south, middle, and north) using the EAWNP ISO‐2 index. By combining these six categories, nine combinations are defined to further investigate how the ISO impact on TC genesis. It was found that through affecting the strength of monsoon trough and the location of the western Pacific subtropical high, the ISO combinations modulate the thermodynamic conditions and lead to changes in frequency and location of TC genesis.The relative contributions of four variables of the genesis potential index (GPI) tend to be dependent on ISO conditions. When the ISO conditions are favourable (unfavourable) to TC genesis, the mid‐level humidity and low‐level vorticity contribute positively (negatively) to the GPI anomalies over the SCS and WNP region, while the vertical wind shear, except for in the north SCS, and the potential intensity contribute negatively (positively). During the normal ISO conditions, the contributions of these four variables are generally weaker than those under the other two backgrounds.

Characteristic interdecadal change of quasi-biweekly and intraseasonal oscillations of summer convection over the South China Sea and the Western Pacific

Observational analyses indicate that there are significant interdecadal changes of quasi bi-weekly oscillation (QBWO) and lower frequency intra-seasonal oscillation (ISO) of convection in summer (May–October) over the South China Sea and the Western Pacific (SCS-WP) sector since the mid 1990s. For the QBWO modes, the northwestward propagation feature is clearly seen in both the periods, but its intensity is stronger in the later period (1998–2013). For the ISO modes, there is clear northeastward propagation of convection band in the tropical Indian Ocean in the later period and the convection band has a northwest-southeast tilted structure extending from the India Subcontinent to the Maritime Continent. Such feature is not seen in the earlier period. It is argued that the interdecadal change of the mean state is responsible for the QBWO and ISO intensity change. The increase of mean sea surface temperature in both the tropical Indian Ocean and the western Pacific in the later period and associated large-scale circulation changes such as enhanced rainfall and low-level moisture provide a favorable environmental condition for the growth of both the QBWO and ISO perturbations.

Spatiotemporal change of intraseasonal oscillation intensity over the tropical Indo-Pacific Ocean associated with El Niño and La Niña events

The present study analyzes the intraseasonal oscillation (ISO) intensity change over the tropical Indo-Pacific associated with the El Nino-Southern Oscillation (ENSO) and compares the intensity change between El Nino and La Nina years and between the 10–20-day and 30–60-day ISOs. The ISO intensity change tends to be opposite between El Nino and La Nina years in the developing and mature phases. The intensity change features a contrast between the tropical southeastern Indian Ocean and the tropical western North Pacific (WNP) in the developing phases and between the Maritime Continent and the tropical central Pacific in the mature phase. In the decaying phases, the intensity change shows notable differences between El Nino and La Nina events and between fast and slow decaying El Nino events. Large intensity change is observed over the tropical WNP in the developing summer, over the tropical southeastern Indian Ocean in the developing fall, and over the tropical WNP in the fast decaying El Nino summer due to a combined effect of vertical shear, vertical motion, and lower-level moisture. In the ENSO developing summer and in the El Nino decaying summer, the 10–20-day ISO intensity change displays a northwest-southeast tilted distribution over the tropical WNP, whereas the large 30–60-day ISO intensity change is confined to the off-equatorial WNP. In the La Nina decaying summer, the 30–60-day ISO intensity change features a large zonal contrast across the Philippines, whereas the 10–20-day ISO intensity anomaly is characterized by a north–south contrast over the tropical WNP.

Intraseasonal oscillation features of the South China Sea summer monsoon and its response to abnormal Madden and Julian Oscillation in the tropical Indian Ocean

By applying the OLR and wind data, rainfall data and the Madden and Julian Oscillation (MJO) index, the paper deals with intraseasonal oscillation features and inter-annual differences of the South China Sea (SCS) summer monsoon, distribution of its LF circulation and convection fields and rainfall, and path of summer monsoon ISO spreading, as well as impact of tropical Indo-MJO on SCS summer monsoon ISO during 1979–2008. It is found that (1) there are three intraseasonal oscillations of the SCS summer monsoon Intraseasonal Oscillation (ISO) in summer (from May to August) in the climate normal. The SCS summer monsoon ISO goes through six phases (exclusive of weak phase) at every complete fluctuation: developing, the strongest, weakening, restraining, the weakest, and recovering. Due to tropical LC convection spreading to the east and north, the LR convection and circulation fields in the 1st–3rd and 4th–6th phases present the anti-phase in the Arabian Sea-West Pacific latitudinal band. Its corresponding rain bands in the 1st–3rd and 4th–6th phases also present anti-phase roughly. The rain band, mainly in tropical regions in the south of 20°N, moves eastward with LR convection shifting eastward, while the rain band moves northward with LR convection shifting northward in East Asia (EA) subtropical regions in the north of 20°N. (2) The SCS summer monsoon ISO presents significant inter-annual variations in intensity. There are three stronger monsoon intraseasonal oscillations in summer in the strong SCS monsoon ISO year. The first two oscillations from the tropical Indian Ocean ISO spread northward to the Bay of Bengal first, and then to the South China Sea (SCS) along the 10°–20°N latitudinal band. They are strengthened there and stimulate the ISO moving to the north to form the tropical Indo-ISO. Finally they spread to South China (SC) by relay way in the longitude-latitude direction. Moreover, in the weaker SCS summer monsoon ISO, the oscillation weakens greatly and irregularly in intensity with the weaker ISO spreading in the longitude-latitude direction. In average conditions, the tropical Indian ISO spreads to the SCS by about 20 days (one half ISO periods). (3) MJO1 (the first modal of MJO index provided by the CPC) averaged value in the 1st–2nd pentads of April has the negative correlation with the SCS monsoon ISO intensity. The tropical Indo-MJO is slightly stronger in the subsequent May to August when it is more active in the 1st–2nd pentads of April, and the ISO also spreads strongly to the SCS, so that the SCS summer monsoon ISO strengthens. Conversely, the SCS summer monsoon ISO weakens. The abnormal MJO in the 1st–2nd pentads of April contributes to a certain theory basis for us to predict the subsequent SCS summer monsoon ISO intensity and analyze the related regions’ abnormal rainfall.

Impacts of Pacific and Indian Ocean coupling on wintertime tropical intraseasonal oscillation: a basin‐coupling CGCM study

AbstractThis study examines the respective influences of Pacific and Indian Ocean couplings on tropical intraseasonal oscillation (ISO) in boreal winter (November–April). Three basin‐coupling experiments are performed with a coupled atmosphere‐ocean general circulation model (CGCM), in which air‐sea coupling is limited respectively to the Indian Ocean, the Pacific Ocean, and both the oceans. The modelling results show that zonal ISO propagations can be found in the Indo‐Pacific region with or without the ocean coupling; however, the propagation signals are enhanced by the coupling. In this particular model the Pacific Ocean coupling has a stronger influence on the ISO propagation than the Indian Ocean coupling. Without the Pacific coupling, the ISO propagation signal is weakened significantly when it enters the Pacific Ocean sector. Without the Indian Ocean coupling, the simulated ISO intensity is weakened less in the Indain Ocean, and significant zonal propagations of ISO can still be simulated in the sector. The relative importance of the ocean coupling is likely related to the smaller ISO‐related sea surface temperature (SST) anomalies in the Indian Ocean than in the Pacific Ocean. In this particular CGCM, the ocean coupling affects the ISO propagation mainly through a wind‐evaporation‐SST feedback rather than a cloud‐radiation‐SST feedback, while in observations both processes are equally important. To further confirm the importance of the ocean coupling, forced atmospheric GCM experiments are performed with SSTs prescribed from the climatologies produced in the basin‐coupling experiments. It is found that the eastward propagating feature seen in the CGCM experiments is weakened and dominated by the strong standing feature in the forced AGCM runs. The difference demonstrates the contribution of the ocean coupling to the ISO propagation, and also confirm that biases in the mean SST and low‐level wind caused by the ocean coupling are responsible for the spurious standing ISO oscillation feature in the central Pacific and Indian Oceans. Copyright © 2009 Royal Meteorological Society

Impacts of intraseasonal oscillation on the onset and interannual variation of the Indian summer monsoon

The role of the intraseasonal oscillation (ISO) on the seasonal and interannual variations of the Indian summer monsoon is investigated based on the analysis of observational data. It is shown that the ISO significantly contributes to the establishment of low-level westerlies during the monsoon onset and developing periods. The effect of the ISO on the annual cycle of the monsoon is through nonlinear eddy momentum transport. On the interannual timescale, the Indian summer monsoon rainfall exhibits a significant out-of-phase relationship with the ISO intensity over the Indian monsoon region. In strong ISO years it appears the weak monsoon when there is an abnormal high over the India subcontinent in the lower troposphere. In weak ISO years there exists an abnormal low and the strong monsoon appears.

Interactions between the summer mean monsoon and the intraseasonal oscillation in the Indian monsoon region

The relationship between interannual variations of the seasonal mean monsoon and intraseasonal oscillation (ISO) during boreal summer over the Indian monsoon region is investigated. The result shows a negative correlation between all‐India summer monsoon rainfall and the ISO intensity. It is argued that the negative correlation is primarily attributed to the impact of the mean state of Indian summer monsoon on ISO. A strong Indian monsoon leads to the weakening of convection over the equatorial eastern Indian Ocean. The latter may further suppress the eastward and northward propagating ISO variances over the Indian Ocean and lead to a weakened intraseasonal activity over the Indian monsoon region. The ISO may feed back to the Indian summer mean monsoon through nonlinear eddy momentum transport. In addition, the ISO intensity over India increases (decreases) during the ENSO developing (decaying) summers.

The impact of ocean–atmosphere coupling on the predictability of boreal summer intraseasonal oscillation

The impact of ocean–atmosphere coupling on the simulation and prediction of the boreal summer intraseasonal oscillation (ISO) has been investigated by diagnosing 22-year retrospective forecasts using the Seoul National University coupled general circulation model (CGCM) and its atmospheric GCM (AGCM) forced with SSTs derived from the CGCM. Numerous studies have shown that the ocean–atmosphere coupling has a significant effect on the improvement of ISO simulation and prediction. Contrary to previous studies, this study shows similar results between CGCM and AGCM, not only in regard to the ISO simulation characteristics but also the predictability. The similarities between CGCM and AGCM include (1) the ISO intensity over the entire Asian-monsoon region; (2) the spatiotemporal evolution of the northward propagating ISO (NPISO); and (3) the potential and practical predictability. A notable difference between CGCM and AGCM is the phase relationship between precipitation and SST anomalies. The CGCM and observation exhibits a near-quadrature relationship between precipitation and SST, with the former lagging about two pentads. The AGCM shows a less realistic phase relationship. The similar structure and propagation characteristics of ISO between the CGCM and AGCM suggest that the internal atmospheric dynamics could be more essential to the ISO than the ocean–atmosphere interaction over the Indian monsoon region.

Pacific–East Asian Teleconnection. Part II: How the Philippine Sea Anomalous Anticyclone is Established during El Niño Development*

The anomalous Philippine Sea anticyclone (PSAC) conveys impacts of El Niño to east Asian climate during the mature and decay of an El Niño (from the winter to ensuing summer). It is shown that the anomalous PSAC forms in fall about one season prior to the peak El Niño; its strength increases with the El Niño intensity and its sign reverses during a La Niña. The PSAC formation concurs with abnormal deepening of the east Asian trough and with increasing number of northward recurvature of tropical storms in the western Pacific. The PSAC establishment is abrupt, coupling with a swing from a wet to dry phase of an intraseasonal oscillation (ISO) and often concurrent with early retreat of the east Asian summer monsoon. The ISO becomes inactive after PSAC establishment. The development of the PSAC is attributed to combined effects of the remote El Niño forcing, tropical–extratropical interaction, and monsoon–ocean interaction. The developing El Niño induces off-equatorial ascending Rossby wave responses and land surface cooling in northeast Asia; both deepen the east Asian trough in fall and induces vigorous tropical–extratropical exchange of air mass and heat, which enhances the cold air outbreak and initiation of the PSAC. Through exciting descending Rossby waves, the El Niño–induced Indonesian subsidence generates low-level anticyclonic vorticity over south Asia, which is advected by mean monsoon westerly, instigating the anomalous PSAC. The ISO interacting with the underlying ocean plays a critical role in the abrupt establishment of PSAC. The wind–evaporation/entrainment feedback tends to amplify (suppress) ISO before (after) winter northeasterly monsoon commences, suggesting the roles of atmosphere–ocean interaction and the seasonal march of background winds in changing the Philippine Sea ISO intensity and maintaining PSAC.