Intraseasonal Oscillation Research Articles

Data-driven global subseasonal forecast for intraseasonal oscillation components

Data-driven global subseasonal forecast for intraseasonal oscillation components

Multiscale Influences on Persistent Extreme Precipitation Events in North China

This study classifies regional persistent extreme precipitation events (PEPEs) in North China into two types in accordance with variance contributions and significance of different timescale rainfall variability in boreal summer. For type 1, PEPEs are dominated by a 10–20-day periodicity, and for type 2, PEPEs are mainly influenced by a 30–60-day mode. Atmospheric circulation anomalies associated with the two types of PEPEs are characterized by a zonal wave train (the EU pattern) in the mid–high latitudes in type 1 but a meridional wave train (the EAP pattern) in East Asia in type 2. The common feature of the two types is anomalous southerly on the west edge of the West Pacific Subtropical High (WPSH), which favors anomalous moisture transport into the key region. Additional moisture source for type 2 is linked to anomalous cross-equatorial flow. Both types of PEPEs result from the combined effect of intraseasonal oscillations in both the mid–high latitudes and the tropics. The impact of ENSO on the two types of PEPEs is investigated. While a La Niña SST condition in the preceding winter favor the occurrence of PEPEs, their subsequent transition in central and eastern equatorial Pacific will determine which of the two types of PEPEs is pronounced.

Factors determining the subseasonal prediction skill of summer extreme rainfall over southern China

The occurrence of summer extreme rainfall over southern China (SCER) is closely related to the boreal summer intraseasonal oscillation (BSISO), and whether operational models can reasonably predict the BSISO evolution and its modulation on SCER probability is crucial for disaster prevention and mitigation. Here, we find that the skill of subseasonal-to-seasonal (S2S) operational models in predicting the first component of BSISO (BSISO1) might determine the forecast skill of SCER. A systematic assessment is conducted on the reforecast data from two operational models that participated in the S2S project, i.e., the model of European Centre for Medium-Range Weather Forecasts (ECMWF) and the model of China Meteorological Administration (CMA). The results show that the ECMWF model can yield skillful prediction of the BSISO1 index up to 24 days in advance, while the skill of the CMA model is about 10 days. Accordingly, the SCER occurrence is correctly predicted by ECMWF (CMA) model at a forecast lead time of ~ 14 (7) days. The diagnostic results of modeled moisture processes further suggest that the anomalous moisture convergence (advection) induced by the BSISO1 activity serves as the primary (secondary) source of subseasonal predictability of SCER. With better prediction of the moisture convergence anomaly in the specific phases of BSISO1, higher skills can be obtained in the probability prediction of SCER. The present study implies that a further improvement in predicting the BSISO and its related moisture processes is crucial to promoting the subseasonal prediction skill of SCER probability.

Modulation of the Atmospheric Heat Source Over the Tibetan Plateau on the Intra-seasonal Oscillation of Summer Precipitation in the Yangtze-Huaihe River Basin

ABSTRACT Based on the daily precipitation from China Meteorological Administration and the daily atmospheric circulation data from the Japanese 55-year Reanalysis (JRA-55) from 1979 to 2018, this paper analyzes the evolution of 10-30-day intra-seasonal oscillation (ISO) of the precipitation in the Yangtze-Huaihe River Basin (YHRB) and the modulation of the atmospheric heat source over the Tibetan Plateau (TP) with different intensity on the peak and trough values of the precipitation in the YHRB. When the atmospheric heating is strong on the southern flank of TP (STP), the lower-level anomalous low strengthens on the STP, which leads to convergence of airflow in the lower troposphere, ascent and divergence in the upper troposphere. Thus, the intense pumping action on the STP results in the convergence of water vapour from the Bay of Bengal to the STP. Due to the high altitude of TP, the water vapour turns eastward and increases the convergence and ascent of water vapour in the YHRB, which is conductive to enhancement of the peak values or weakening of trough values of the ISO of precipitation in the YHRB. When the STP heating is weak, the lower-level anomalous low decreases on the STP, leading to divergence in the lower troposphere, descent and convergence in the upper troposphere. Then the convergence of water vapour from the Bay of Bengal to the STP is suppressed and the transport of water vapour to the YHRB is interrupted, thus the peak values of precipitation are weakened or the trough are enhanced.

Intra-seasonal oscillations of South Asian summer monsoon in coupled climate model cohort CMIP6

Intra-seasonal oscillations of South Asian summer monsoon in coupled climate model cohort CMIP6

Diurnal to Intraseasonal Precipitation Variation Around the Northwestern Coast of Luzon Island: Results From YMC‐BSM 2018 Field Campaign

AbstractThis study investigates multiscale variability of summer‐time precipitation around the northwestern coast of Luzon Island, the Philippines, by analyzing in situ observation data of a field campaign in summer 2018 conducted under the Years of the Maritime Continent‐Boreal Summer Monsoon study in 2018 (YMC‐BSM 2018) project. It is found that boreal summer intraseasonal oscillation modulates activity of precipitation diurnal cycle, with active diurnal cycle observed only under easterly (offshore) wind conditions in the free troposphere. On the other hand, in the period of westerly winds, quasi‐biweekly fluctuation is observed, with two heavy precipitation events. These events are associated with further intensified westerly winds caused by westward‐propagating quasi‐biweekly oscillations. Day‐to‐day precipitation variability over land during the two events is associated with the strength of monsoon winds blowing toward the mountain range, which is modulated cooperatively by the quasi‐biweekly oscillations and tropical depressions over the South China Sea.

Different Impacts of Intraseasonal Oscillations on Precipitation in Southeast China between Early and Late Summers

Different Impacts of Intraseasonal Oscillations on Precipitation in Southeast China between Early and Late Summers

Reduction of Equatorial Obduction by Atmospheric Intraseasonal Oscillations in the Western and Central Pacific Ocean

AbstractObduction is the large‐scale, irreversible upward water transport from the permanent pycnocline to the surface mixed layer. Using Hybrid Coordinate Ocean Model (HYCOM), this study explores the impact of atmospheric intraseasonal oscillations (ISOs) on the Pacific equatorial obduction that plays a fundamental role in regulating the tropical Pacific climate. Parallel HYCOM experiments forced by atmospheric fields with and without ISO variability are compared to assess the effect of ISOs on the ocean. The results suggest that the ISOs can reduce the annual obduction rate of the western and central equatorial Pacific Ocean (WCEPO; 150°E−160°W, 5°S–5°N) by ∼12%, and their impact on the eastern Pacific is rather limited. The ISOs cause prominent intraseasonal variability of mixed layer depth (MLD) in the WCEPO primarily through surface wind forcing, which in turn affects the two key processes controlling the annual obduction rate. First, the intraseasonal MLD deepening events narrow down the total time windows for obduction by ∼30% and thereby give rise to the decrease in obduction rate; second, it leads to high‐frequency entrainment events and enhances the entrainment rate by ∼18%. The increased entrainment rate is overwhelmed by the reduced time windows for effective entrainment, and the net effect of ISOs is to reduce the obduction rate and modify its seasonal cycle. This work highlights the importance of atmospheric ISOs in the equatorial ocean dynamics, with implications for tropical surface temperature and climate variabilities and biogeochemical cycle.

Decadal Change of Meiyu Onset over Yangtze River and Its Causes

Meiyu onset marks the beginning of the rainfall season in the densely populated Yangtze River Basin, whether the Meiyu initiates early or late in June, and thus has a profound effect on the several hundred million people living there. Applying a Bayesian change-point analysis to data from 1960–2014, we objectively detected an abrupt change of Meiyu onset around 2002. The Meiyu onset date averaged over 2002–2014 was 19 June, delayed by about two weeks compared to that of 1989–2001 (6 June). This decadal change is attributable to the distinct amplitude of moisture transport toward the Yangtze River Basin induced by the changes in climatological intraseasonal oscillation (CISO). The CISO emerges as the annual cycle interacts with the transient intraseasonal perturbations. The wet/dry phases of the CISO are consistent with the climatological active/break stages of the East Asian summer monsoon. In early June, the northwestward-propagating CISO convective/cyclonic anomalies over the western North Pacific (WNP) show weaker amplitude during the earlier-onset epoch compared to the delayed-onset epoch. Thus, relative to the delayed onset epoch, a quasi-barotropic anticyclonic CISO anomaly appears over the WNP in early June during the earlier-onset years. This anticyclonic anomaly was conducive to the westward extension of the WNP subtropical high, conveying warm, moist air from the tropics toward the Yangtze River Basin for the rainy season onset. Model experiments suggest that the decadal changes in WNP CISO intensity were associated with the epochal changes in large-scale background circulation and sea surface temperature over the WNP.

Features of climatological intraseasonal oscillation during Asian summer monsoon onset and their simulations in CMIP6 models

Features of climatological intraseasonal oscillation during Asian summer monsoon onset and their simulations in CMIP6 models

Prediction of the Central Indian Ocean Mode in S2S Models

Prediction of precipitation during the Indian summer monsoon (ISM) is a persistent scientific challenge. The central Indian Ocean (CIO) mode was proposed as a subseasonal climate mode over the tropical Indian Ocean, and it has a close relation with monsoon intraseasonal oscillations (MISO) during the ISM both in observations and simulations. In this study, the prediction skill of the CIO mode in the subseasonal-to-seasonal (S2S) air–sea coupled models is examined. The ECMWF and UKMO models display significantly higher skills for up to about 2 and 3 weeks, respectively, which are longer than other S2S models. The decline of the CIO mode prediction skill is due to the reduced signal of subseasonal zonal winds at 850 hPa over the tropical central Indian Ocean (especially along the equator; 5°S–5°N, 70°E–85°E). Therefore, a better simulation of tropical subseasonal zonal winds is required to improve the CIO mode prediction in models, and the improvement will benefit a better MISO simulation and a higher prediction skill during the ISM.

Characteristics of rapidly intensifying tropical cyclones in the South China Sea, 1980–2016

The differences in the characteristics of the rapid intensification (RI) during the TCs that form in the SCS (referred as local TCs) and that enter the SCS from the western North Pacific (WNP; referred as entering TCs) have not been well studied, which could contribute the inaccuracy of current TC intensity forecast in the SCS. In this study, we used TC observations, reanalysis data and model experiments to analyze the RI occurrences during local TCs and entering TCs in 1980–2016. We found that the significant interannual and interdecadal variations in RI occurrences during local eastward-moving TCs were related to the strong intraseasonal oscillation (ISO) over the SCS and the WNP under La Niña conditions. RI during local westward-moving TCs showed insignificant variations as a result of the complex interactions among the monsoon trough, ISO and the large-scale circulation. RI during entering TCs showed strong interdecadal variations, with increased RI after 1997, even though the total number of entering TCs has decreased since 1997, which is a result of a higher number of entering TCs in the northwestern quadrant of the WNP, a stronger ISO and weak vertical windshear over the SCS and east of the Philippines under negative phase of Pacific Decadal Oscillation. The different variations and related mechanisms of RI indicates that distinct forecasting factors should be considered for intensity prediction during local eastward- and westward-moving TCs and entering TCs.

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.

A Statistical Intraseasonal Prediction Model of Extended Boreal Summer Western North Pacific Tropical Cyclone Genesis

Abstract An L2 regularized logistic regression model is developed in this study to predict weekly tropical cyclone (TC) genesis over the western North Pacific (WNP) Ocean and subregions of the WNP including the South China Sea (SCS), the western WNP (WWNP), and the eastern WNP (EWNP). The potential predictors for the TC genesis model include a time-varying TC genesis climatology, the Madden–Julian oscillation (MJO), the quasi-biweekly oscillation (QBWO), and ENSO. The relative importance of the predictors in a constructed L2 regression model is justified by a forward stepwise selection procedure for each region from a 0-week to a 7-week lead. Cross-validated hindcasts are then generated for the corresponding prediction schemes out to a 7-week lead. The TC genesis climatology generally improves the regional model skill, and the importance of intraseasonal oscillations and ENSO is regionally dependent. Over the WNP, there is increased model skill over the time-varying climatology in predicting weekly TC genesis out to a 4-week lead by including the MJO and QBWO, whereas ENSO has a limited impact. On a regional scale, ENSO and then either the MJO or QBWO are the two most important predictors over the EWNP and WWNP after the TC genesis climatology. The MJO is found to be the most important predictor over the SCS. The logistic regression model is shown to have comparable reliability and forecast skill scores to the ECMWF dynamical model on intraseasonal time scales. Significance Statement Skillful forecasts of tropical cyclone activity on time scales from short-range to seasonal are now issued operationally. Although there has been great progress in the understanding of physical mechanisms driving tropical cyclone (TC) activity, intraseasonal prediction of TCs remains a significant scientific challenge. This study develops a statistically based intraseasonal model to predict weekly TC genesis over the western North Pacific Ocean basin. The intraseasonal prediction model developed here for TC genesis over the western North Pacific basin shows skill extending out to four weeks. We discuss the regional dependence of the model skill on ENSO and other subseasonal climate oscillations. This approach provides skillful intraseasonal forecasting of TCs over the western North Pacific basin.

Relationship between the Intraseasonal Oscillation over Mid-High-Latitude Eurasia and the Stratospheric Sudden Warming Event in February 2018

Taking the stratospheric sudden warming (SSW) event in February 2018 as an example, the relationship between the SSW event and the intraseasonal oscillation (ISO) mode over mid-high-latitude Eurasia is investigated by daily reanalysis data. First, the 2018 SSW event and mid-high-latitude ISO are reviewed. The 2018 SSW event is a typical vortex-split event defined by the NCEP-DOE dataset, and the ISO mode features a southeastward propagation. Along with the ISO propagation, temperature anomalies developed from troposphere to stratosphere in the 2018 wintertime. It is also found that a strong ISO event occurred before the onset of SSW in this wintertime. Our analysis reveals that the correlation is significant when mid-high-latitude ISO leads the 2018 SSW event by 9–13 days. Occurrence of strong ISO 9–13 days before the SSW event is found to be conducive to its onset. The diagnosis of planetary wave activity indicates that stronger ISO leads to strengthened wavenumber-2 geopotential height anomalies; then, its in-phase superposition of climatological geopotential height makes up for the upward-propagating tropospheric planetary waves, which is in favor of the onset of the 2018 SSW event. The outcome of energy conversion equations also reveals that ISO perturbation contributes to the onset of SSW. Lastly, the contribution of the vertical component of Plumb wave activity flux propagated upward from the region of mid-high-latitude ISO is estimated at approximately 69%.

Changes to tropical eastern North Pacific intraseasonal variability under global warming, implications for tropical cyclogenesis

Changes to the tropical eastern North Pacific Intraseasonal Oscillation (ISO) at the end of the 21st century and implications for tropical cyclone (TC) genesis are examined in the Shared Socioeconomic Pathways (SSP585) scenario of the Coupled Model Intercomparison Project phase 6 (CMIP6) data set. Multimodel mean composite low-level wind and precipitation anomalies associated with the leading intraseasonal mode indicate that precipitation amplitude increases while wind amplitude weakens under global warming, consistent with previous studies for the Indo-Pacific warm pool. The eastern North Pacific intraseasonal precipitation/wind pattern also tends to shift southwestward in a warmer climate, associated with weaker positive precipitation anomalies near the coast of Mexico and Central America during the enhanced convection/westerly wind phase. Implications for the modulation of TC genesis by the leading intraseasonal mode are then explored using an empirical genesis potential index (GPI). In the historical simulation, GPI shows positive anomalies in the eastern North Pacific in the convectively enhanced phase of the ISO. The ISO’s modulation of GPI weakens near the coast of Mexico and Central America with warming, associated with a southward shift of GPI anomalies. Further examination of the contribution from individual environmental variables that enter the GPI shows that relative humidity and vorticity changes during ISO events weaken positive GPI anomalies near the Mexican coast with warming and make genesis more favorable to the southwest. The impact of vertical shear anomaly changes is also to favor genesis away from the coast. These results suggest a weaker modulation of TCs near the Mexican Coast by the ISO in a warmer climate.

Dynamic and Thermal Effects of the Tibetan and Iranian Plateaus on the Northward-Propagating Intraseasonal Oscillation during Boreal Summer

Abstract This study investigates the dynamic and thermal effects of the Tibetan and Iranian Plateaus (TIP) on the characteristics of the northward-propagating boreal summer intraseasonal oscillation (BSISO) and reveals associated physical mechanisms. The relative importance of the Tibetan Plateau (TP) and Iranian Plateau (IP) to the northward propagation of BSISO is also detected. The findings are based on comparative analyses of the control run and six sensitivity experiments using the atmospheric model component of CAS FGOALS-f3-L (the low-resolution version of the Chinese Academy of Sciences Flexible Global Ocean–Atmosphere–Land System model, finite-volume version 3). It is demonstrated that the absence of TIP surface sensible heat or terrain restricts the occurrence of upper-level easterly and lower-level westerly anomalies, as well as moisture convergence over the Asian summer monsoon (ASM) region, which leads to a weakened easterly vertical wind shear with a narrowed range and the generation of a negative meridional specific humidity gradient in the northern Bay of Bengal. These mean-state changes are not conducive to the formation of positive barotropic vorticity together with planetary boundary layer moisture convergence to the north of the convective center, thus impeding the northward propagation of the BSISO. Results indicate that the combined TIP area, via its dynamic and thermal effects, modulates the northward-propagating BSISO by directly affecting the vertical wind shear and meridional moisture gradient in the northern Indian Ocean. By contrast, the TP plays a decisive role in the BSISO northward propagation due to the limited dynamic and thermal effects of IP on the background circulation and water vapor convergence over the ASM region.

Katabatic and convective processes drive two preferred peaks in the precipitation diurnal cycle over the Central Himalaya

AbstractThe diurnal cycle of precipitation over the Central Himalaya is governed by a complex interaction between the diurnal cycle of tropical convection and local orographic flow. Understanding this interaction is crucial for model evaluation, where the simulation of such processes is highly sensitive to model resolution and choice of parametrisation schemes. In this study, the mean diurnal cycle is computed using gridded satellite data–Integrated Multi‐satellitE Retrievals for Global Precipitation Measurement (GPM) (IMERG) data and is shown to be bimodal, with one peak in the late afternoon (1700 India Standard Time (IST)) and a stronger one in the early morning (0200 IST). This structure is an artefact of compositing, as individual days are associated with single peaks. The late‐afternoon “convective” peak is shown to be linked to the diurnal cycle of tropical convection, whereas the nocturnal “katabatic” peak is shown to be triggered by nocturnal downslope flow converging with the background monsoon circulation. As such, the katabatic peak is strongly favoured by an active monsoon trough, which provides greater southeasterly moisture flux to the foothills, resulting in increased low‐level moisture flux convergence upon interaction with the katabatic northerlies. When the trough is less active, precipitation is brought to the region by mesoscale convective systems, ranging in scale from tens to thousands of kilometres, resulting in convective peaks. We hypothesise that these convective afternoon peaks are enhanced by anabatic flow. It is shown that the Boreal Summer Intraseasonal Oscillation does not play a significant role in modulating either the timing or the amplitude of the diurnal cycle; however, low‐pressure systems do: either by intensifying the trough (and hence the katabatic peak), or, when further north, by providing deep convection (hence supporting the convective peak). Two reanalyses and a separate 17‐km model with parametrised convection capture both peaks, but overestimate the magnitude of the convective peak and underestimate the magnitude of the katabatic peak.

Maintenance of Cyclonic Vortex During Monsoon Intraseasonal Oscillation: A View From Kinetic Energy Budget

AbstractThe Monsoon intraseasonal oscillation (MISO) is the predominant variability over the tropical Indian Ocean during the Indian summer monsoon. A cyclonic vortex plays a critical role in the northward propagation of MISO. Here, we find that the pronounced intraseasonal kinetic energy accompanying the vortex is attributable to the barotropic energy gain from the background circulation. In particular, due to the phase shift between vortex and low‐level horizontal wind convergence, the covariance of the intraseasonal easterly and downdraft reinforces the intraseasonal kinetic energy before vortex. In contrast, the interaction between westerly and updraft nourishes the kinetic energy after vortex. Overall, the nonlinear processes across different scales are vital for the energy exchange during MISO, and this is dynamically different from that of Madden‐Julian oscillations. It implies that although these two major intraseasonal oscillations share the same spatiotemporal scales over the Indian Ocean, they are controlled by fundamentally different dynamical processes.

Key process diagnostics for monsoon intraseasonal oscillation over the Indian Ocean in coupled CMIP6 models

The simulations of the monsoon intraseasonal oscillation (MISO) during the Indian summer monsoon (ISM) are evaluated with 19 atmosphere–ocean coupled general circulation models (CGCMs) from phase 6 of the Coupled Model Inter-comparison Project (CMIP6). The focus is on the northward propagation of MISO. The CMIP6 models have great improvement in simulating the mean rainfall, as 17 out of 19 models can reasonably simulate the mean rainfall. However, many models fail to reproduce the realistic patterns of the mean rainfall and the MISO amplitude, particularly over land in the monsoon region. The underestimation of the MISO amplitude is still a notable model bias in CMIP6. Moreover, 9 out of 19 models cannot generate realistic northward propagation features, and some even reproduce a stationary MISO pattern. Process diagnostics based on the seasonal mean vertical zonal wind shear, low-level mean moisture, and vortex tilting are also examined. It is found that the accuracy of model simulations of vortex tilting is strongly associated with the northward propagation of MISO. In contrast, the model fidelity in MISO is not dependent on the simulation skill for the seasonal mean state. In addition, decomposition analysis of vortex tilting illustrates that the meridional shear of the intraseasonal vertical velocity is crucial to the tilting simulation. The poor model fidelity in vortex tiling is caused by the weak convection, particularly the absence of downdraft to the north of the convection center. The coupling between the moisture in the boundary layer and the tilting in the free troposphere may be responsible for capturing the vertical motions. In summary, vortex tilting can be a useful metric for evaluating the northward propagation of MISO.