Internal Atmospheric Dynamics Research Articles

Asymmetric Efficacies Between Warm and Cold Pacific Meridional Modes in Inducing ENSO

AbstractThis study investigates boreal spring events of Pacific Meridional Mode (PMM) from 1950 to 2022, revealing that cold PMM is more effective in triggering subsequent La Niña compared to warm PMM's induction of following El Niño. This asymmetry stems from the varying origins and sub‐efficacies of PMM groups. The cold PMM is primarily initiated by pre‐existing La Niña, while the warm PMM is comparably activated by pre‐existing El Niño and internal atmospheric dynamics. PMMs initiated by pre‐existing El Niño or La Niña play a crucial role in determining the efficacies of PMMs in triggering subsequent El Niño‐Southern Oscillation (ENSO). The strong discharge of pre‐existing El Niño hampers warm PMM's induction of subsequent El Niño, whereas weak recharge from pre‐existing La Niña enhances the efficacy of cold PMM in inducing subsequent La Niña. Comprehending not only the PMM phase but also its origin is crucial for ENSO research and prediction.

The Impact of Interactive Ocean Dynamics on Atlantic Sea Surface Temperature Variability

Abstract The impact of interactive ocean dynamics on internal variations of Atlantic sea surface temperature (SST) is investigated by comparing preindustrial control simulations of a fully coupled atmosphere–ocean–ice model to the same atmosphere–ice model with the ocean replaced by a motionless slab layer (henceforth slab ocean model). Differences in SST variability between the two models are diagnosed by an optimization technique that finds components whose variance differs as much as possible. This technique reveals that Atlantic SST variability differs significantly between the two models. The two components with the most extreme enhancement of SST variance in the slab ocean model resemble the tripole SST pattern associated with the North Atlantic Oscillation (NAO) and the Atlantic multidecadal variability (AMV) pattern. This result supports previous claims that ocean dynamics are not necessary for the AMV, although ocean dynamics lead to slight increases in the memory of both the AMV and the NAO tripole. The component with the most extreme enhancement of SST variance in the fully coupled model resembles the Atlantic Niño pattern, confirming the ability of our technique to isolate physical modes known to require ocean dynamics. The second component with more variance in the fully coupled model is a mode of subpolar SST variability. Both the reemergence of SST anomalies and changes in ocean heat transport lead to increased SST variance and memory in the subpolar Atlantic. Despite large differences in the mean and variability of SST, atmospheric variability is quite similar between the two models, confirming that most atmospheric variability is generated by internal atmospheric dynamics.

Contributions of Air‐Sea Coupling to Northward‐Propagating Intraseasonal Convection Over the South China Sea

AbstractNorthward‐propagating convection associated with intraseasonal oscillation (ISO) over the South China Sea (SCS) is closely linked to the evolution of the East China rainy season. In this study, we quantitatively examined the contribution of air‐sea coupling to northward propagation of intraseasonal convection over the SCS based on observational diagnosis and model experiments. While oceanic feedback plays a limited role in northward propagation of Indian Ocean and western Pacific ISO, its positive effect on SCS ISO propagation is much larger and nearly comparable with that of atmospheric internal dynamics over the central SCS. Warm intraseasonal sea‐surface temperature (SST) anomalies ahead of the convection induce boundary‐layer convergence and surface turbulent fluxes, largely favoring growth of moist static energy to the north of the ISO convection. Conversely, weak negative SST anomalies induce divergence and reduced fluxes over the convective center. The underlying SST anomalies strongly promote northward propagation of SCS ISO, but destroy the maintenance of deep convection. We then verified these oceanic feedbacks using atmosphere‐only (SPA) and atmosphere–ocean‐mixed‐layer coupled (SPK) configurations of the Super‐Parameterized Community Atmospheric Model. The SPA simulations produced stronger intraseasonal convection but weaker propagation signals than the SPK simulations, confirming the positive contribution of intraseasonal SST to ISO propagation observed over the SCS.

Modulations of Storm-Track Activity Associated with the Baroclinic Annular Mode

Abstract The baroclinic annular mode (BAM) is the leading mode of variability in extratropical eddy activity characterized by its hemispheric-scale pulsing. Based on atmospheric reanalysis data for the Southern Hemisphere, this study reveals BAM-associated systematic modulations not only in fluxes associated with subweekly transient disturbances, as found by earlier studies, but also in their spatial structure involved in the dynamics of the BAM. Specifically, in the positive phase of the BAM characterized by enhanced activity of transient disturbances, their lower-tropospheric baroclinic structure becomes more distinct, and they tend to be more elongated meridionally in both the upper and lower troposphere. These BAM-associated structural modulations of the disturbances favor the more efficient baroclinic development via enhanced poleward heat transport and their downstream development, which can contribute to hemispheric-scale enhancement of kinetic energy associated with the disturbances. In addition, a tendency of the disturbances to exhibit horizontally tilting structure becomes more evident in the positive phase of the BAM, which is favorable for enhanced transport of westerly momentum from the subtropics to the midlatitude polar-front jet, or equivalently enhanced wave-activity propagation from the midlatitude storm track into the subtropics. This modulation lags the peak of anomalous kinetic energy of the disturbances, thus acting to contribute to the decay of the BAM signature. A set of numerical simulations suggests that the BAM-associated pulsing in storm-track activity and structural modulations are manifestations of atmospheric internal dynamics, which can be significantly amplified in the presence of a midlatitude oceanic frontal zone through the formation of more organized and coherent baroclinic wave packets.

The Effect of Interior Heat Flux on the Atmospheric Circulation of Hot and Ultra-hot Jupiters

Many hot and ultra-hot Jupiters have inflated radii, implying that their interiors retain significant entropy from formation. These hot interiors lead to an enhanced internal heat flux that impinges upon the atmosphere from below. In this work, we study the effect of this hot interior on the atmospheric circulation and thermal structure of hot and ultra-hot Jupiters. To do so, we incorporate the population-level predictions from evolutionary models of hot and ultra-hot Jupiters as input for a suite of general circulation models (GCMs) of their atmospheric circulation with varying semimajor axis and surface gravity. We conduct simulations with and without a hot interior, and find that there are significant local differences in temperature of up to hundreds of Kelvin and in wind speeds of hundreds of meters per second or more across the observable atmosphere. These differences persist throughout the parameter regime studied, and are dependent on surface gravity through the impact on photosphere pressure. These results imply that the internal evolution and atmospheric thermal structure and dynamics of hot and ultra-hot Jupiters are coupled. As a result, a joint approach including both evolutionary models and GCMs may be required to make robust predictions for the atmospheric circulation of hot and ultra-hot Jupiters.

Origin of the Warm Arctic–Cold North American Pattern on the Intraseasonal Time Scale

Abstract The warm Arctic–cold continent pattern (WACC) of near-surface air temperature variability has often been associated with the connection between Arctic sea ice reduction and cold weather over the midlatitude continents. Whether the existence of this pattern is due to variability of sea ice or is caused by atmospheric internal dynamics is subject to debate. Based on a long integration of a primitive equation atmospheric model (SGCM), this study examines the origin of the warm Arctic–cold North American pattern (WACNA), which is characterized by a pair of opposite surface air temperature anomalies over the high-latitude Chukchi–Bering Sea region and the North American continent, in boreal winter on the intraseasonal time scale. The model atmosphere is maintained by a time-independent forcing, so that atmospheric internal dynamics is the only source of variability. It is found that the SGCM model simulates well the behavior of the observed WACNA pattern. The WACNA pattern develops by interacting with the time-mean flow and synoptic-scale transient eddies. Two pathways of Rossby wave propagation are associated with WACNA. The northern pathway originates from eastern Siberia moving eastward across the Bering Strait to Canada, and the southern pathway is rooted in the subtropical waveguide propagating across the eastern North Pacific. Our simulation of this pattern implies that tropospheric dynamics alone can generate the WACNA, and the predictability associated with this pattern is likely limited by its internal dynamics nature. Significance Statement The warm Arctic–cold continent pattern of temperature variability has often been associated with the connection between Arctic sea ice reduction and cold weather over the midlatitude continents, which implies possible impacts of polar warming on midlatitude climate. There has been debate on whether the existence of this pattern depends on variability of sea ice or can be caused by processes within the atmosphere. In this study, we use a simple atmospheric model, which has a constant forcing; thus, atmospheric internal dynamics is the only source of variability. We show that atmospheric internal dynamics alone can generate the warm Arctic–cold North American pattern. The result has implications for our understanding of the impact of global warming.

Robustness of BSISO and air-sea interactions in the CMIP (Phase-6) models over the North Indian Ocean

Historical runs of 30 Coupled Model Intercomparison Project Phase-6 (CMIP6) General circulation models (GCMs) are evaluated for the representation of Boreal Summer Intraseasonal Oscillations (BSISO). Several statistical metrics were developed to evaluate the characteristic features of BSISO, such as propagation, phase speed, and exchange of air-sea fluxes at the air-sea interface over the major regions of the Indian Summer Monsoon (ISM). The mean state of the monsoon precipitation in the CMIP6 models is evaluated by using seasonal mean bias, pattern correlation, and root mean square error. The majority of CMIP6 models underestimate the precipitation over central India and overestimate the precipitation over the eastern equatorial region. Multi-model mean (MME) of the models shows good agreement of precipitation pattern with the observations. In the observations, the precipitation anomalies propagate northward from the equatorial latitudes to the northern latitudes over the ISM region (60°E-100°E longitudes). However, the initiation of northward propagating convection shows a significant variation with time in the CMIP6 models. Most of the models well simulated the BSISO propagation over the Bay of Bengal (BoB) and the Indian subcontinent. The majority of the models underestimate the phase speed of BSISO over the Arabian Sea (AS), and easterlies from the western north Pacific, which led to the failure of models in representing the northwest-southeast tilt of convection. Surface turbulent fluxes and zonal winds lag the deep convection over the North Indian Ocean on intraseasonal timescales. However, misrepresentation of air-sea fluxes in the CMIP6 models leads to the significant biases of intraseasonal variances. This study examines the simulation characteristic features of BSISO by CMIP6 models and is mainly attributes them to the atmospheric internal dynamics and air-sea interactions. The present study further suggests that improving atmospheric-oceanic feedback mechanisms, specific humidity, and low-level winds in the CMIP6 models is necessary to accurately predict the ISM intraseasonal variability.

Destructive Interference of ENSO on North Pacific SST and North American Precipitation Associated with Aleutian Low Variability

Abstract Identifying the origins of wintertime climate variations in the Northern Hemisphere requires careful attribution of the role of El Niño–Southern Oscillation (ENSO). For example, Aleutian low variability arises from internal atmospheric dynamics and is remotely forced mainly via ENSO. How ENSO modifies the local sea surface temperature (SST) and North American precipitation responses to Aleutian low variability remains unclear, as teasing out the ENSO signal is difficult. This study utilizes carefully designed coupled model experiments to address this issue. In the absence of ENSO, a deeper Aleutian low drives a positive Pacific decadal oscillation (PDO)-like SST response. However, unlike the observed PDO pattern, a coherent zonal band of turbulent heat flux–driven warm SST anomalies develops throughout the subtropical North Pacific. Furthermore, non-ENSO Aleutian low variability is associated with a large-scale atmospheric circulation pattern confined over the North Pacific and North America and dry precipitation anomalies across the southeastern United States. When ENSO is included in the forcing of Aleutian low variability in the experiments, the ENSO teleconnection modulates the turbulent heat fluxes and damps the subtropical SST anomalies induced by non-ENSO Aleutian low variability. Inclusion of ENSO forcing results in wet precipitation anomalies across the southeastern United States, unlike when the Aleutian low is driven by non-ENSO sources. Hence, we find that the ENSO teleconnection acts to destructively interfere with the subtropical North Pacific SST and southeastern United States precipitation signals associated with non-ENSO Aleutian low variability.

PMODE I: Design and Development of an Observatory for Characterizing Giant Planet Atmospheres and Interiors

The giant planets of our Solar System are exotic laboratories, enshrouding keys which can be used to decipher planetary formation mysteries beneath their cloudy veils. Seismology provides a direct approach to probe beneath the visible cloud decks, and has long been considered a desirable and effective way to reveal the interior structure. To peer beneath the striking belts and zones of Jupiter and to complement previous measurements—both Doppler and gravimetric—we have designed and constructed a novel instrument suite. This set of instruments is called PMODE—the Planetary Multilevel Oscillations and Dynamics Experiment, and includes a Doppler imager to measure small shifts of the Jovian cloud decks; these velocimetric measurements contain information related to Jupiter’s internal global oscillations and atmospheric dynamics. We present a detailed description of this instrument suite, along with data reduction techniques and preliminary results (as instrumental validation) from a 24-day observational campaign using PMODE on the AEOS 3.6 m telescope atop Mount Haleakalā, Maui, HI during the summer of 2020, including a precise Doppler measurement of the Jovian zonal wind profile. Our dataset provides high sensitivity Doppler imaging measurements of Jupiter, and our independent detection of the well-studied zonal wind profile shows structural similarities to cloud-tracking measurements, demonstrating that our dataset may hold the potential to place future constraints on amplitudes and possible excitation mechanisms for the global modes of Jupiter.

Correction to: Intensification and Northward extension of Northwest Pacific anomalous anticyclone in El Niño decaying mid-summer: an energetic perspective

The Northwest Pacific (NWP) anomalous anticyclone (AAC) intensifies and extends northward from El Nino decaying early to mid-summer, despite decaying El Nino-induced sea surface temperature (SST) anomalies in the North Indian Ocean, North Atlantic and subtropical NWP. The present study suggests the intra-seasonal variations of AAC are induced by local mean state changes and investigates the underlying mechanisms from the perspective of energetics. Compared with early summer, the efficiency of dry energy conversion from the mean flow to El Nino-excited AAC pattern increases in El Nino decaying mid-summer. The moist feedback over subtropical NWP is also enhanced in El Nino decaying mid-summer due to the onset of the climatological NWP summer monsoon. Both of them contribute to the intensification of El Nino-excited AAC pattern. Moreover, mean state changes over East Asia–NWP from early to mid-summer are found in favor of the northward shift of the preferred latitude of the circulation anomalies. Thus, the El Nino-excited AAC pattern are more northward-extended in El Nino decaying mid-summer. Empirical orthogonal function analyses further confirm that the northward extension of El Nino-excited AAC pattern stems from changes of local internal dynamic mode. The present study highlights that both the El Nino-induced SST anomalies and local atmospheric internal dynamics are of paramount importance for the intra-seasonal variations of AAC in El Nino decaying summer.

The Effect of the Arctic Oscillation on the Predictability of Mid-High Latitude Circulation in December

The impact of the Arctic Oscillation (AO) on the predictability of mid-high latitude circulation in December is analysed using a full set of hindcasts generated form the Beijing Climate Center Atmospheric General Circulation Model version 2.2 (BCC_AGCM2.2). The results showed that there is a relationship between the predictability of the model on the Eurasian mid-high latitude circulation and the phase of AO, with the highest predictability in the negative AO phase and the lowest predictability in the normal AO phase. Moreover, the difference of predictability exists at different lead times. The potential sources of the high predictability in the negative AO phase in the BCC_AGCM2.2 model were further diagnosed. It was found that the differences of predictability on the Eurasian mid-high latitude circulation also exist in different Arctic sea ice anomalies, and the model performs well in reproducing the response of Arctic sea ice on the AO. The predictability is higher when sudden stratospheric warming (SSW) events occur, and strong SSW events tend to form a negative AO phase distribution in the Eurasian mid-high latitudes both in the observation and model. In addition, the model captured the blocking over the mid-high latitudes well, it may be related to the relatively long duration of the blocking. Changes in the AO will affect the blocking circulations over the mid-high latitudes, which partly explains the high predictability of the model in negative AO phases from the aspect of the internal atmospheric dynamics.

Impacts of a Midlatitude Oceanic Frontal Zone for the Baroclinic Annular Mode in the Southern Hemisphere

AbstractAs a major mode of annular variability in the Southern Hemisphere, the baroclinic annular mode (BAM) represents the pulsing of extratropical eddy activity. Focusing mainly on subweekly disturbances, this study assesses the impacts of a midlatitude oceanic frontal zone on the BAM and its dynamics through a set of “aquaplanet” atmospheric general circulation model experiments with zonally uniform sea surface temperature (SST) profiles prescribed. Though idealized, one experiment with realistic frontal SST gradient reasonably well reproduces observed BAM-associated anomalies as a manifestation of a typical life cycle of migratory baroclinic disturbances. Qualitatively, these BAM features are also simulated in the other experiment where the frontal SST gradient is removed. However, the BAM-associated variability weakens markedly and shifts equatorward, in association with the corresponding modifications in the climatological-mean storm track activity. The midlatitude oceanic frontal zone amplifies and anchors the BAM variability by restoring near-surface baroclinicity through anomalous sensible heat supply from the ocean and moisture supply to cyclones, although the BAM is essentially a manifestation of atmospheric internal dynamics. Those experiments and observations further indicate that the BAM modulates momentum flux associated with transient disturbances to induce a modest but robust meridional shift of the polar-front jet, suggesting that the BAM can help maintain the southern annular mode. They also indicate that the quasi-periodic behavior of the BAM is likely to reflect internal dynamics in which atmospheric disturbances on both subweekly and longer time scales are involved.

Intensification and Northward extension of Northwest Pacific anomalous anticyclone in El Niño decaying mid-summer: an energetic perspective

The Northwest Pacific (NWP) anomalous anticyclone (AAC) intensifies and extends northward from El Niño decaying early to mid-summer, despite decaying El Niño-induced sea surface temperature (SST) anomalies in the North Indian Ocean, North Atlantic and subtropical NWP. The present study suggests the intra-seasonal variations of AAC are induced by local mean state changes and investigates the underlying mechanisms from the perspective of energetics. Compared with early summer, the efficiency of dry energy conversion from the mean flow to El Niño-excited AAC pattern increases in El Niño decaying mid-summer. The moist feedback over subtropical NWP is also enhanced in El Niño decaying mid-summer due to the onset of the climatological NWP summer monsoon. Both of them contribute to the intensification of El Niño-excited AAC pattern. Moreover, mean state changes over East Asia–NWP from early to mid-summer are found in favor of the northward shift of the preferred latitude of the circulation anomalies. Thus, the El Niño-excited AAC pattern are more northward-extended in El Niño decaying mid-summer. Empirical orthogonal function analyses further confirm that the northward extension of El Niño-excited AAC pattern stems from changes of local internal dynamic mode. The present study highlights that both the El Niño-induced SST anomalies and local atmospheric internal dynamics are of paramount importance for the intra-seasonal variations of AAC in El Niño decaying summer.

Potential Predictability of the Tropical Cyclone Frequency Over the Western North Pacific With 50‐km AGCM Ensemble Experiments

AbstractThe potential predictability determined by sea surface temperature (SST) of the frequency distribution of tropical cyclones (TCs) is studied using ensemble simulations with a 50‐km resolution atmospheric general circulation model (AGCM). In this experiment, the interannual variability of the TC frequency over the western North Pacific (WNP) is primarily determined by SST, as the simulated ensemble mean signal is dominant over the ensemble spread and effectively follows the observed TC frequency in June‐July‐August (JJA). In contrast, the correlation between the observed and simulated TC frequency variability is less significant in September‐October‐November (SON). To explore the TC frequency‐SST linkage, the relationship between the TC frequency variability over the WNP and climate modes determined by SST indices is investigated. Through a correlation analysis, we found that El Niño Modoki is the major driver of the TC frequency variability over the WNP in JJA. Partial correlation analysis further reveals that internal atmospheric dynamics are important for the TC frequency variability over the WNP in SON. These results suggest that the tropical climate mode is responsible for the seasonal predictability of the TC frequency variability over the WNP and that the prediction skill is seasonally dependent with a high (low) skill in JJA (SON).

Quantifying atmosphere and ocean origins of North American precipitation variability

How atmospheric and oceanic processes control North American precipitation variability has been extensively investigated, and yet debates remain. Here we address this question in a 50 km-resolution flux-adjusted global climate model. The high spatial resolution and flux adjustment greatly improve the model’s ability to realistically simulate North American precipitation, the relevant tropical and midlatitude variability and their teleconnections. Comparing two millennium-long simulations with and without an interactive ocean, we find that the leading modes of North American precipitation variability on seasonal and longer timescales exhibit nearly identical spatial and spectral characteristics, explained fraction of total variance and associated atmospheric circulation. This finding suggests that these leading modes arise from internal atmospheric dynamics and atmosphere-land coupling. However, in the fully coupled simulation, North American precipitation variability still correlates significantly with tropical ocean variability, consistent with observations and prior literature. We find that tropical ocean variability does not create its own type of atmospheric variability but excites internal atmospheric modes of variability in midlatitudes. This oceanic impact on North American precipitation is secondary to atmospheric impacts based on correlation. However, relative to the simulation without an interactive ocean, the fully coupled simulation amplifies precipitation variance over southwest North America (SWNA) during late spring to summer by up to 90%. The amplification is caused by a stronger variability in atmospheric moisture content that is attributed to tropical Pacific sea surface temperature variability. Enhanced atmospheric moisture variations over the tropical Pacific are transported by seasonal mean southwesterly winds into SWNA, resulting in larger precipitation variance.

Assessment of Ocean-Atmosphere Interactions for the Boreal Summer Intraseasonal Oscillations in CMIP5 Models over the Indian Monsoon Region

The boreal summer intraseasonal oscillations (BSISO) are the prominent features of South Asian summer monsoon and mainly governed by the internal atmospheric dynamics and air-sea interactions. The present study aims to understand and evaluate the relationship between the convection and the associated air-sea interactions during the BSISO over the Indian monsoon region. To accomplish this, the present study utilizes observations and the 22 general circulation model (GCM) simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Representation of Indian monsoon season rainfall, sea surface temperature (SST) and latent heat fluxes in CMIP5 models such as climatological and intraseasonal are assessed using Global Precipitation Climatology Project rainfall by Taylor diagram metric. Results suggest that the majority of CMIP5 models simulated the northward propagation of precipitation and zonal wind at 850 hPa. However, models bias of BSISO variance shows significant spatial heterogeneity over the regions of the Arabian Sea (AS), Sub-Continent of India (SCI) and Bay of Bengal (BoB). The CMIP5 model which shows large biases in the mean state is degrading the northward propagation of BSISO. The phase relationship of ocean (land) and atmospheric interactions are diagnosed with lead-lag regression analysis. On ISO timescales over north Indian Ocean (NIO) convection leads the turbulent fluxes and westerly winds by a week. However, the majority of the models shows large uncertainty to represent this prominent feature over AS and SCI. Further, improper representation of the lead-lag relationship of SST and precipitation on ISO scales over the AS, BoB, and NIO in the CMIP5 models are attributing for significant bias variances. The present study advocates that BSISO propagation in CMIP5 models is mainly attributing from the internal atmospheric dynamics and air-sea interactions. However, for the realistic amplitude simulation of BSISO, proper representation of air-sea feedback mechanisms is crucial in CMIP5 models. The present study further suggests that the oceanic feedback processes of the CMIP5 models need to be improved for the accurate prediction of the intraseasonal variations.

The Interannual Variability of the Tropical Divergence Tilt and Its Connection with the Extratropical Circulation

AbstractPrevious theoretical work has suggested that the strength of the divergent eddy momentum flux in the deep tropics, due to correlations between rotational zonal velocities and divergent meridional velocities, increases with the meridional tilt of the large-scale divergence field. To test that idea, this work investigates the interannual variability of the divergent eddy momentum flux in reanalysis data. Consistent with the theory, it is found that the eddy momentum flux variability is driven by two main parameters: the amplitude of the tropical stationary wave and the tilt of the divergence field. Together, these two parameters account for 80% (90%) of the interannual eddy momentum flux variance during boreal (austral) winter. The interannual variability of these parameters is governed by the internal atmospheric dynamics. During boreal winter, interannual changes in MJO variability explain nearly half of the interannual variance in the stationary wave amplitude, depending on whether on average MJO anomalies interfere constructively or destructively with the stationary wave. The interannual variability of the divergence phase tilt is modulated by tropical–extratropical interactions in the Pacific. The tilt increases during the negative phase of the west Pacific Oscillation associated with a dipole of upper-level divergence (convergence) on the northern (southern) side of the Pacific jet exit region.

Impacts of early/late South China Sea summer monsoon withdrawal on tropical cyclone genesis over the western North Pacific

This study reveals a significant in-phase relationship between the South China Sea summer monsoon (SCSSM) withdrawal date and tropical cyclone (TC) genesis over the western North Pacific (WNP). The number of TCs generated over the WNP from mid-September to mid-October is positively correlated with the SCSSM withdrawal date during the period of 1979–2016. The decreased (increased) number of TCs generated during early (late) SCSSM withdrawal years is attributed to both internal atmospheric dynamics and external sea surface temperature (SST) forcing. Through dynamic (Rossby wave response) and thermodynamic (increased moisture) processes, the warm SST anomalies during late withdrawal years over the tropical WNP contribute to maintaining the monsoon trough (MT) in the boreal autumn and moisturizing the mid-level atmosphere, providing a favorable environment for TC genesis. The remaining MT can facilitate the conversion of mean kinetic energy into eddy kinetic energy (EKE) and enhance synoptic-scale waves. In addition, upper-level baroclinic energy conversion also contributes to EKE development. Both barotropic and baroclinic processes favor TC genesis over the WNP. In contrast, colder SSTs during early withdrawal years induce the early withdrawal of the MT, leading to depressed enhancement of the EKE and weakening the northwestward propagation of synoptic-scale waves. Hence, fewer (more) TCs tend to be generated during early (late) withdrawal years.

Cause for quasi-biweekly oscillation of zonal location of western Pacific subtropical high during boreal summer

The cause for zonal oscillation of the western Pacific subtropical high (WPSH) on a quasi-biweekly (10–20-day) timescale was investigated. Both relative vorticity and geopotential height anomalies exhibit a maximum activity center over the western edge of the climatological WPSH. The quasi-biweekly WPSH oscillation is closely related to a northwestward-propagating quasi-biweekly oscillation (QBWO) mode over the western North Pacific (WNP). A real-time index of the WNP-QBWO is used to analyze the influence of the northwestward-propagating WNP-QBWO mode on the zonal shift of the WPSH. The QBWO convection anomaly propagates northwestward from the tropical WNP to East Asia. It is the alternation of a positive and a negative geopotential height anomaly during the northwestward propagating journey that leads to the zonal oscillation of the WPSH. Once a positive (negative) QBWO geopotential height propagates from tropical WNP into the western edge of the climatological WPSH, it leads to a western extension (eastern retreat) of the WPSH. This provides a monitoring and predicting method of the zonal shift of the WPSH at an extended rang by using the real-time index. Two mechanisms are likely responsible for the northwestward propagation of the WNP-QBWO mode. The first is internal atmospheric dynamics. Both positive vorticity and moisture anomalies appear to the northwest of the QBWO convection, favoring the northwestward propagation. The second is attributed to the atmosphere-ocean interaction. A warm sea surface temperature (SST) anomaly leads the QBWO convection by an approximately 90-degree phase, and both the SST and convection anomalies move northwestward.

Atmospheric Internal Variability in the Summer Indo–Northwestern Pacific: Role of the Intraseasonal Oscillation

AbstractSummer atmospheric interannual variability in the Indo–northwestern Pacific (NWP) is coupled with tropical sea surface temperature (SST) variability. This study investigates the importance and origin of atmospheric internal variability in the Indo-NWP region. Using the reanalysis and the 30-member atmospheric model simulation, two SST-related interannual modes are identified in the Indo-NWP region during boreal summer with the month-reliant empirical orthogonal function analysis. The first mode is related to concurrent El Niño–Southern Oscillation originating from the eastern equatorial Pacific whereas the second mode features an anomalous anticyclone (AAC) in post–El Niño summers over the NWP region, known as the Indo-western Pacific Ocean capacitor. The SST-induced modes show temporal persistence from June to August. The residual variability is the focus of this study. The dominant mode of the residual variability displays an AAC structure over the NWP but little month-to-month persistence, indicative of atmospheric internal dynamics unrelated to SST forcing. Further investigation suggests the monthly internal AAC arises from the summer intraseasonal oscillation (ISO). The broad band of ISO yields nonzero monthly means that project strongly onto the AAC pattern. Finally, the anomalies of rainfall and low-level circulation in summer 2016 are investigated. The reversal of the low-level circulation pattern from an AAC in July to an anomalous cyclone over the NWP in August 2016 is due to the ISO-induced internal variability.