Atmospheric Intraseasonal Oscillations Research Articles

Ocean feedback on 50–80-day boreal winter intraseasonal oscillation in the Indian Ocean: comparison of atmospheric and coupled CESM2 simulations

Abstract The role of sea surface temperature (SST) variability in the boreal winter intraseasonal oscillation (ISO) over the tropical Indian Ocean (TIO) is investigated in this study using the Community Earth System Model Version 2 (CESM2). An atmospheric general coupled model (AGCM) run forced by daily SSTs derived from a parallel coupled circulation model (CGCM) run is compared with reanalysis and the original coupled simulation. The comparison of atmospheric-coupled GCMs reveals the frequency and spatial tendency of the SST effect on the atmospheric ISO. The lower-frequency component of the ISO over the TIO is significantly intensified in the AGCM run. The SST-to-atmosphere effect is most prominent over the southeastern TIO. In the AGCM run, low-level wind anomalies related to SST appear west of the ISO convection center, contrasting with the previous findings that the SST effect appears east of the convection center.

Tropical Atmospheric Intraseasonal Oscillations Leading to Sea Level Extremes in Coastal Indonesia during Recent Decades

Abstract Coastal flooding induced by sea surface high extreme (HEX) events is an increasing risk to human society and infrastructure as both urban growth in coastal areas and anthropogenic sea level rise continue, especially for island nations like Indonesia. This paper investigates the role of atmospheric intraseasonal oscillations (ISOs), which are dominated by the Madden–Julian oscillation (MJO), in forcing HEXs on the coasts of Indonesia bordering the Indian Ocean. We use satellite altimetry data from 1993 to 2021 and tide gauge observations to detect HEXs, and modeling experiments using both the Regional Ocean Modeling System and a Bayesian dynamic linear model to understand the forcing and processes. We find that HEXs exhibit strong seasonality, with most events occurring during boreal winter (December–February) and spring (March–May) that are dominated by seasonal-to-decadal and intraseasonal variability respectively. In 32% of the 56 HEX events detected, the amplitude of ISO-induced sea level anomalies (SLAs) exceeds that of seasonal-to-decadal SLAs. Surface wind stress associated with atmospheric ISOs is the major forcing for intraseasonal SLAs, and both the remote westerly wind stress from the Indian Ocean equator and northwesterly longshore wind stress at the Indonesian coasts play important roles in driving the HEXs. The MJO is the dominant cause of ISO-dominated HEXs and its impact shows strong seasonal differences. Spring MJOs are associated with stronger convective anomalies over the eastern Indian Ocean equator that drive stronger zonal winds across the equatorial basin that lead to more HEX events compared to winter MJOs when the convection is shifted southward.

Increase in optimal configuration of 25–60-day atmospheric circulations for Yangtze heavy rainfall under global warming background

In summer, the middle and lower reaches of the Yangtze River (MLYR) often suffers from severe floods as a result of a dominant intraseasonal oscillation (ISO) of rainfall with the quasi-period of 25–60 days. 75% of flooding events during summers of 1961–2022 are found to occur following wet phases of the 25−60-day ISO in rainfall. This study objectively extracts the optimal configuration (OC) of 25−60-day atmospheric ISOs for the wet phase of MLYR rainfall. The OC is characterized by the quadruple pattern of tropical convection configuring with the 25−60-day Silk Road Pattern (SRP) in the mid−high latitudes. The 25−60-day SRP in cooperation with tropical ISO over the western Pacific induces dipolar vertical cells over eastern China, with ascending branch between the vertical cells providing the vitally dynamical condition for MLYR rainfall. With topographic blocking of the Tibetan Plateau, the northward-propagating tropical ISO over the Indian Ocean enhances maintenance of the 25−60-day SRP via upper-tropospheric divergent flows, thus anchoring anomalous ascents to the MLYR. Numerical experiments further verify the remarkable tropical-extratropical interaction around the Tibetan Plateau in this OC. Under the global warming background, the tropical 25−60-day ISO tends to strengthen, leading to an increase in the occurrence frequency of strong OC through tropical-extratropical interaction, and thus the severe flood events doubled in summers of 1991–2020.

Data-Driven Global Subseasonal Forecast for Intraseasonal Oscillation Components

As a challenge in the construction of a “seamless forecast” system, improving the prediction skills of subseasonal forecasts is a key issue for meteorologists. In view of the evolution characteristics of numerical models and deep-learning models for subseasonal forecasts, as forecast times increase, the prediction skill for high-frequency components will decrease, as the lead time is already far beyond the predictability. Meanwhile, intraseasonal low-frequency components are essential to the change in general circulation on subseasonal timescales. In this paper, the Global Subseasonal Forecast Model (GSFM v1.0) first extracted the intraseasonal oscillation (ISO) components of atmospheric signals and used an improved deep-learning model (SE-ResNet) to train and predict the ISO components of geopotential height at 500 hPa (Z500) and temperature at 850 hPa (T850). The results show that the 10–30 day prediction performance of the SE-ResNet model is better than that of the model trained directly with original data. Compared with other models/methods, this model has a good ability to depict the subseasonal evolution of the ISO components of Z500 and T850. In particular, although the prediction results from the Climate Forecast System Version 2 have better performance through 10 days, the SE-ResNet model is substantially superior to CFSv2 through 10–30 days, especially in the middle and high latitudes. The SE-ResNet model also has a better effect in predicting planetary waves with wavenumbers of 3–8. Thus, the application of data-driven subseasonal forecasts of atmospheric ISO components may shed light on improving the skill of seasonal forecasts.

Two Types of Mid-High-Latitude Intraseasonal Oscillations in the Southern Hemisphere during Austral Summer

Abstract Using NCEP–NCAR reanalysis data, the atmospheric intraseasonal oscillation (ISO) over the mid-high latitudes in the Southern Hemisphere during austral summer is studied. Two types of 10–30-day eastward- and westward-propagating ISO at mid-high latitudes are extracted by extended empirical orthogonal functions. The analysis of wave activity flux reveals that the energy sources of the eastward-propagating wave train are in the southwest Pacific and southern Indian Ocean, and the westward-propagating wave train is over the east coast of South America. The diagnosis of geopotential height tendency shows that the zonal gradient of ISO relative vorticity guided by mean zonal wind plays a major role in the eastward propagation of the wave train, while the meridional gradient of mean relative vorticity and geostrophic vorticity guided by ISO meridional wind plays a major role in the westward propagation of the wave train. Energy analysis shows that the amplitude enhancement over the South Pacific is due to the ISO disturbance gaining energy from the mean flow. The eastward-propagating ISO obtains energy from the mean flow through kinetic and potential energy conversion, while the westward-propagating ISO cannot obtain energy through potential energy conversion. The surface air temperature and precipitation in South America are affected by the circulation and propagation corresponding to both types of ISO. The useful forecasting skills by the subseasonal-to-seasonal forecasting project for eastward- and westward-propagating types can reach to 17 days, while the potential forecasting skills improve to 20 and 19 days, respectively. Significance Statement The intraseasonal oscillation (ISO) has important effects on regional weather and climate, and we focus on the ISO over mid-high latitudes in the Southern Hemisphere, which has been paid little attention. We reveal two leading types of the mid-high-latitude ISO during austral summer, namely, the eastward- and westward-propagating types. Also, the characteristics and effects of the two types are explored. To understand their dynamical process, the geopotential height tendency and energy conversion are diagnosed. It is also found that the two ISO types can be predicted up to 17 days in advance by the subseasonal-to-seasonal project, which may provide guidance for the extended-range forecasts.

Influences of atmospheric intraseasonal oscillation in mid–high latitudes on winter haze pollution over the Beijing‐Tianjin‐Hebei region

AbstractThis paper analyses the intraseasonal evolution of the winter haze pollution over the Beijing‐Tianjin‐Hebei (BTH) region in China between 1979 and 2013, and the influences of the atmospheric intraseasonal oscillations (ISOs) in mid–high latitudes. Two significant ISO signals of 10–20‐day and 20–40‐day are identified from the winter haze pollution over the BTH region, and their superposition notably contributes to the occurrence of haze pollution. On the 10–20‐day timescale, the haze pollution is intimately associated with a mid–high‐level southeastward propagating wave train that originates from the North Atlantic. With the wave evolution, an anomalous anticyclone reaches the Korean Peninsula–Japan, leading to strong descents and anomalous southerly winds over the BTH region, favourable for haze pollution. On the 20–40‐day timescale, the atmospheric wave train originates from the eastern North America, propagates eastward to the northwestern Lake Baikal, and then turns southward to the northeastern China–Japan. The peak stage of the haze pollution is related to the resultant zonal dipole, with the anomalous cyclone over the northwestern Lake Baikal and the anomalous anticyclone over the northeastern China–Japan. The dominance of the mid–upper tropospheric anticyclone over the BTH region suppresses the development of convection, thus hindering the vertical diffusion and wet deposition of pollutants. Moreover, the anomalous westerly winds between the dipole weaken the incursion of cold mass from higher latitudes, thus further contributing to the peak of haze pollution.

Relation of Mid-High-Latitude Eurasian ISO to Ural Blocking Frequency and Their Co-Effect on Extreme Hot Events during Boreal Summer

Based on NCEP reanalysis daily data during 1979–2018, the spatiotemporal evolution of the 10–30-day atmospheric intraseasonal oscillations (ISO) at mid-high-latitude Eurasia and its effect on the Ural blocking frequency are investigated. The co-effect of the blocking and ISO on extreme hot event frequency is also investigated. The ISO exhibits two modes of eastward and westward propagation. During the eastward (westward) propagating mode, the northwest–southeast tilted quadrupole (east–west dipole) quasi-barotropic geopotential height anomaly coupled with the air temperature anomaly at the troposphere propagates southeastward (westward). The phase composite shows that, during both modes, the mid-high-latitude low-frequency Rossby wave trains significantly affect the frequency of the European blocking during the propagating journey. The most frequent European blocking appears in phase 2 during both the eastward- and westward- propagating mode. Compared with the situation without the Ural blocking, the blocking activity results in the positive geopotential height anomalies throughout Europe and north of 60° N in the Ural Mountains and the negative geopotential height anomalies in the south of 60° N in the Ural Mountains and north of the Japan Sea. The occurrence of Ural blocking is conducive to the occurrence of extreme high-temperature events in Europe and the high latitudes of the Ural Mountains, and a reduced frequency of extreme high-temperature events in the mid-latitudes of the Ural Mountains and north of the Japan Sea. Therefore, the Ural blocking activities significantly regulate the effect of the two propagating ISO modes on the extreme hot events over the middle and high latitudes of Eurasia.

Probabilistic subseasonal precipitation forecasts using preceding atmospheric intraseasonal signals in a Bayesian perspective

Abstract. Accurate and reliable subseasonal precipitation forecasts are of great socioeconomic value for various aspects. The atmospheric intraseasonal oscillation (ISO), which is one of the leading sources of subseasonal predictability, can potentially be used as predictor for subseasonal precipitation forecasts. However, the relationship between atmospheric intraseasonal signals and subseasonal precipitation is of high uncertainty. In this study, we develop a spatiotemporal-projection-based Bayesian hierarchical model (STP-BHM) for subseasonal precipitation forecasts. The coupled covariance patterns between the preceding atmospheric intraseasonal signals and precipitation are extracted, and the corresponding projection coefficients are defined as predictors. A Bayesian hierarchical model (BHM) is then built to address the uncertainty in the relationship between atmospheric intraseasonal signals and precipitation. The STP-BHM model is applied to predict both the pentad mean precipitation amount and pentad mean precipitation anomalies for each hydroclimatic region over China during the boreal summer monsoon season. The model performance is evaluated through a leave-1-year-out cross-validation strategy. Our results suggest that the STP-BHM model can provide skillful and reliable probabilistic forecasts for both the pentad mean precipitation amount and pentad mean precipitation anomalies at leads of 20–25 d over most hydroclimatic regions in China. The results also indicate that the STP-BHM model outperforms the National Centers for Environmental Prediction (NCEP) subseasonal to seasonal (S2S) model when the lead time is beyond 5 d for pentad mean precipitation amount forecasts. The intraseasonal signals of 850 and 200 hPa zonal wind (U850 and U200) and 850 and 500 hPa geopotential height (H850 and H500) contribute more to the overall forecast skill of the pentad mean precipitation amount predictions. In comparison, the outgoing longwave radiation anomalies (OLRAs) contribute most to the forecast skill of the pentad mean precipitation anomaly predictions. Other sources of subseasonal predictability, such as soil moisture, snow cover, and stratosphere–troposphere interaction, will be included in the future to further improve the subseasonal precipitation forecast skill.

Distinctive impacts of atmospheric intraseasonal oscillations on the net ecosystem exchange of the southeastern China forest between spring and summer

The southeastern China (SEC) forest is an important terrestrial biospheric carbon sink in the global carbon cycle, with its total net ecosystem exchange (NEE) accounting for about 3.2% of the global forest NEE. The prevailing atmospheric intraseasonal oscillations (ISOs) over East Asia strongly modulate climatic conditions over the SEC during spring and summer, thus leading to significant ISOs in the NEE of the SEC forest. As atmospheric ISOs show strong seasonality, this study examined the distinctive impacts of atmospheric ISOs on the NEE of the SEC forest between spring and summer.During spring, the vertical coupling of 10–30-d atmospheric ISOs in the lower and upper troposphere leads to strong 10‒30-d ISOs of solar radiation and temperature over the SEC. The 10‒30-d ISOs of solar radiation and temperature further result in the 10‒30-d ISOs of gross primary productivity (GPP) and terrestrial ecosystem respiration (TER). With the covariation in GPP and TER, the NEE of the SEC forest exhibits significant 10‒30-d ISOs. In contrast, the intraseasonal variations in climatic conditions over the SEC are associated with the 15‒60-d tropical atmospheric ISO during summer. While the induced 15‒60-d ISO of solar radiation leads to that of GPP, the induced 15‒60-d ISO of temperature is small and less effective; thus, the 15‒60-d ISO of NEE mainly originates from that of GPP.

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.

Cumulative positive contributions of propagating ISO to the quick low-level atmospheric response during El Niño developing years

Based on the Australian Bureau of Meteorology (BoM) El Niño alert system, this study investigates the atmospheric and oceanic conditions during El Niño developing years between 1982 and 2016. It is found that there is a 2–5-month (subseasonal-to-seasonal) lag to establish the steady low-level atmospheric (or the Southern Oscillation Index, SOI) response than the steady El Niño-pattern Sea Surface Temperature Anomaly (SSTA), which is defined as the critical period in this research. According to the duration of this critical period, the quick and slow steady atmospheric response years can be identified among all El Niño–Southern Oscillation (ENSO) developing events. The quick establishments of the Sea Level Pressure Anomaly (SLPA) in the tropical atmosphere are proved to be closely related to the tropical atmospheric Intra-seasonal Oscillation (ISO) events. In the quick response years, the ISO events can even propagate to the eastern tropical Pacific, which lead to the cumulative negative Outgoing Longwave Radiation (OLR) and SLP anomalies there, and make a positive contribution to the quick atmospheric response at the end of the critical period. However, the eastward-propagation of ISO events is mainly restricted in the tropical Western Pacific in the slow response years, causing slow steady atmospheric response with almost no contributions from ISO. Furthermore, observations and several simulations are used to understand this propagation differences of the ISO between quick and slow response years.

Intraseasonal Variability of Sea Level in the Western North Pacific

AbstractSea levels in the Western North Pacific (WNP) are presented with anomalous intraseasonal variations. This study examines the response of sea level in the WNP to the atmospheric Intraseasonal Oscillation modes, namely the Madden–Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation (BSISO), using the 25 years (1993–2017) of satellite altimetry and barotropic model output. In winter, the MJO has significant effects on the component of sea level due to the instant wind and atmospheric pressure effects (high‐frequency), showing an eastward propagation pattern in most regions, with the strongest effects in the western marginal seas. The MJO‐associated pattern of dynamical (low‐frequency) component of sea level propagates southward, with the significant effects mostly in the tropics. In summer, the BSISO‐associated pattern of the high‐frequency component of sea level moves from southwest to northeast, with the largest anomalies in the middle of WNP (20°N‐30°N), while the strongest BSISO effects on the low‐frequency component are detectable mostly in the coasts of China and east of the Philippines. The MJO and BSISO can also modulate the probability of extreme sea level events. In winter, during phases 2–5, MJO increases the chance of extreme high events in the high‐frequency component of sea level by 100%–200% in the western coasts and the tropics. In summer, in BSISO phases 6–7, the chance of extreme high events in the high‐frequency component of sea level is enhanced by >300% in the South China Sea and east of the Philippines.

The influence of atmospheric intraseasonal oscillations on terrestrial biospheric CO2 fluxes in Southeast China Forest

With its strong carbon sequestration capacity and significantly expanding trend, forest over southeast China (SEC) is an important biospheric carbon sink. As intraseasonal oscillation (ISO) is a remarkable variability of the East Asian summer monsoon—the major climate system influencing the carbon exchange of SEC forest, the objective of this study is to examine the impact of atmospheric intraseasonal oscillation on CO2 fluxes in SEC forest based on daily CO2 fluxes and meteorological data. Significant 15‒60-day ISO of CO2 fluxes is identified in SEC forest during May‒August of 1980‒2013. The 15‒60-day ISO of net ecosystem exchange between atmosphere and biosphere mainly results from that of gross primary productivity (GPP), with the explained variances greater than 50% over most SEC forest areas. Due to the high drought-resistance of SEC forest, the 15‒60-day ISO of its GPP is controlled primarily by local solar radiation, which is in turn associated with the northward-propagating atmospheric ISO. Suppressed (active) convection anomaly and the thermally induced anomalous anticyclone (cyclone) on its northwestern side initiate from the southern South China Sea and then propagate northward to SEC coherently in the form of a Rossby wave-like pattern. Decreased (increased) cloud cover associated with the suppressed (active) convection anomaly leads to more (less) solar radiation reaching the SEC forest canopy, thus promoting (impairing) forest productivity.

Upper ocean response to the atmospheric intraseasonal oscillations in the summer of tropical South China Sea

Upper ocean response to the atmospheric intraseasonal oscillations in the summer of tropical South China Sea

Signature of a 120-day oscillation in the MLT winds and tides over São João do Cariri (7.4oS, 36.5oW)

A conspicuous and intermittent signature of a 120-day oscillation is found in the winds and tides in the mesosphere and lower thermosphere (MLT) over a Southern hemispheric equatorial station, São João do Cariri (7.4oS, 36.5oW) for the first time using meteor radar observations during the interval 2004–2008. The intermittent 120-day modulation can be as high as 10 m/s in the MLT winds. However, the seasonal component of the concerned periodicity, i.e. terannual oscillation is relatively smaller in amplitude (<4 m/s) than the instantaneous component. The oscillation feature is also investigated for the dominant tides. The 120-day modulation amplitude is higher (maximum ~ 5 m/s) in the diurnal and semidiurnal tides and relatively smaller in the terdiurnal tide (peak ~ 2 m/s). The intermittent behavior of the particular oscillation in wind and absence in temperature may imply it to be a feature of the MLT wind circulation. Proximity of period may also indicate a possible contribution from the lower atmospheric intraseasonal oscillation to some extent at times. Furthermore, existence of the same periodicity in the solar UV flux may indicate a plausible indirect solar connection with the MLT circulation although direct solar influence cannot be ascertained. The evident imprint of the oscillation in the MLT wind and tides accentuates the need of further extensive studies to lend valuable insights of the underlying physical processes/mechanisms associated to their relationship hitherto remained unexplored.

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.

On the Westward Turning of Hurricane Sandy (2012): Effect of Atmospheric Intraseasonal Oscillations

Abstract Hurricane Sandy (2012) experienced an unusual westward turning and made landfall in New Jersey after its northward movement over the Atlantic Ocean. The landfall caused severe casualties and great economic losses. The westward turning took place in the midlatitude Atlantic where the climatological mean wind is eastward. The cause of this unusual westward track is investigated through both observational analysis and model simulations. The observational analysis indicates that the hurricane steering flow was primarily controlled by atmospheric intraseasonal oscillation (ISO), which was characterized by a pair of anticyclonic and cyclonic circulation systems. The anticyclone to the north was part of a global wave train forced by convection over the tropical Indian Ocean through Rossby wave energy dispersion, and the cyclone to the south originated from the tropical Atlantic through northward propagation. Hindcast experiments using a global coupled model show that the model is able to predict the observed circulation pattern as well as the westward steering flow 6 days prior to Sandy’s landfall. Sensitivity experiments with different initial dates confirm the important role of the ISO in establishing the westward steering flow in the midlatitude Atlantic. Thus the successful numerical model experiments suggest a potential for extended-range dynamical tropical cyclone track predictions.

Intraseasonal responses of sea surface and deep oceanic temperature anomalies in the northern Indian Ocean–western Pacific to the 30–60-day boreal summer atmospheric intraseasonal oscillation

The 30–60-day boreal summer atmospheric intraseasonal oscillation (BSISO) is the predominant intraseasonal variability of the Asian summer monsoon, and has a strong dynamical and thermodynamical impact on the underlying oceans in the northern Indian Ocean–western Pacific. This study uses composite analysis of satellite-observed sea surface temperature (SST) data, as well as atmospheric and oceanic reanalysis datasets for the period 1998–2013, to identify the intraseasonal behavior of SST and deep ocean temperatures during the life cycle of the BSISO. With the BSISO as an organized entity evolving both eastward and northward from the equatorial Indian Ocean, the BSISO-induced SST anomalies propagate coherently into several basins, including the Arabian Sea, Bay of Bengal, South China Sea, and western Pacific, characterized by the alternate occurrence of basin-wide positive and negative SST anomalies, which primarily respond to the active and suppressed convection phases of the BSISO due to anomalous southwesterlies and northeasterlies together with the resultant surface heating anomalies. Significant BSISO-induced oceanic temperature anomalies occur in the equatorial Indian Ocean and western Pacific, with the largest-amplitude anomalous signals arising around the thermocline and extending downward into the deep ocean to below 1000 m or more, which are associated with the equatorial eastward-propagating oceanic Kelvin waves forced by anomalous surface wind stress. However, the local and remote effects of such equatorial oceanic Kelvin waves on the SST and deep oceanic temperature anomalies, especially north of the equator, differ from different basins through the consequential coastal Kelvin waves and reflected Rossby waves.

Intraseasonal Variability of Rainfall and Its Effect on Interannual Variability across the Indian Subcontinent and the Tibetan Plateau

AbstractIntraseasonal variability of rainfall over the Indian subcontinent (IS) and the Tibetan Plateau (TP) has been discussed widely but often separately. In this study, we investigate the covariability of rainfall across the IS and the TP on intraseasonal time scales and its impact on interannual variability of regional rainfall. The most dominant mode of rainfall intraseasonal variability across the region features a dipole pattern with significant out-of-phase rainfall anomalies between the southeastern TP and the central and northern IS. This dipole rainfall pattern is associated with intraseasonal oscillations (ISOs) of 10–20 days and 30–60 days, especially the latter. An active spell of rainfall in the central and northern IS (southeastern TP) is associated with the strengthening (northward shift) of water vapor transport of the Indian summer monsoon, resulting in more water vapor entering into the central and northern IS (southeastern TP) and thus more rainfall. The 10–20-day ISO of the dipole rainfall pattern is caused by the 10–20-day atmospheric ISO in both the tropics and the extratropics, whereas the 30–60-day ISO of the dipole rainfall pattern is only associated with atmospheric ISO in the tropics. The dipole rainfall pattern resembles the most dominant mode of interannual variability of July–August mean rainfall. The 30–60-day ISO of the dipole rainfall pattern has an important contribution to the dipole pattern of July–August mean rainfall anomalies on an interannual time scale due to the different frequencies of occurrence of the active and break phases.

Intraseasonal Variability of the Equatorial Undercurrent in the Indian Ocean

AbstractBy analyzing in situ observations and conducting a series of ocean general circulation model experiments, this study investigates the physical processes controlling intraseasonal variability (ISV) of the Equatorial Undercurrent (EUC) of the Indian Ocean. ISV of the EUC leads to time-varying water exchanges between the western and eastern equatorial Indian Ocean. For the 2001–14 period, standard deviations of the EUC transport variability are 1.92 and 1.77 Sv (1 Sv ≡ 106 m3 s−1) in the eastern and western basins, respectively. The ISV of the EUC is predominantly caused by the wind forcing effect of atmospheric intraseasonal oscillations (ISOs) but through dramatically different ocean dynamical processes in the eastern and western basins. The stronger ISV in the eastern basin is dominated by the reflected Rossby waves associated with intraseasonal equatorial zonal wind forcing. It takes 20–30 days to set up an intraseasonal EUC anomaly through the Kelvin and Rossby waves associated with the first and second baroclinic modes. In the western basin, the peak intraseasonal EUC anomaly is generated by the zonal pressure gradient force, which is set up by radiating equatorial Kelvin and Rossby waves induced by the equatorial wind stress. Directly forced and reflected Rossby waves from the eastern basin propagate westward, contributing to intraseasonal zonal current near the surface but having weak impact on the peak ISV of the EUC.