Large-scale Circulation Research Articles

On warm bias and mesoscale dynamics setting the Southern Ocean large-scale circulation mean state

A realistic representation of the Southern Ocean (SO) in climate models is critical for reliable global climate projections. However, many models are still facing severe biases in this region. Using a fully coupled global climate model at non-eddying (1/2∘) and strongly eddying (1/10∘) grid resolution in the SO, we investigate the effect of a 0.5 °C, 1.0 °C and 1.6 °C warmer than observed SO on i) the spin-up behaviour of the high-resolution simulation, and ii) the representation of main dynamical features, i.e., the Antarctic circumpolar current (ACC), the subpolar gyres, the overturning circulation and the Agulhas regime in a quasi-equilibrium state. The adjustment of SO dynamics and hydrography critically depends on the initial state and grid resolution. When initialised with an observed ocean state, only the non-eddying configuration quickly builds up a strong warm bias in the SO. The high-resolution configuration initialised with the biased non-eddying model state results in immense spurious open ocean deep convection, as the biased ocean state is not stable at eddying resolution, and thus causes an undesirable imprint on global circulation. The SO heat content also affects the large-scale dynamics in both low- and high-resolution configurations. A warmer SO is associated with a stronger Agulhas current and a temperature-driven reduction of the meridional density gradient at 45∘S to 65∘S and thus a weaker ACC. The eddying simulations have stronger subpolar gyres under warmer conditions while the response in the non-eddying simulations is inconsistent. In general, SO dynamics are more realistically represented in a mesoscale-resolving model at the cost of requiring an own spin-up.

Increasing heat waves frequencies over India during post-El Niño spring and early summer seasons

The increasing frequency of extreme Heat Waves (HWs) has generated significant societal impacts in recent years. This study used different observational datasets to investigate the HW characteristics over India during the post-El Niño spring and early summer seasons (April to June; AMJ). Analysis suggests that HW days are more prevalent over India, predominantly increased in south-central and northwest India, during the decaying phase of strong El Niño years. It is found that anomalous anticyclone circulation accompanied by high pressure extending from the Western North Pacific region towards the Bay of Bengal and India is responsible for enhanced HW days and intensity during the AMJ of strong El Niño decay years. This anomalous anticyclone-induced downdraft reduces the specific humidity in the lower troposphere, leading to decreased cloud cover over India. As a result, shortwave radiation is enhanced at the surface, which causes abnormal HWs over India. During the decaying phase of strong El Niño years, the HW days over India contributed to an increase in the frequency of Discomfort Index hours (above 28 °C), maximum temperatures exceeding 40 °C (hours per day), and Universal Thermal Climate Index days above 38 °C and 46 °C during the spring and early summer months, especially in the East Coast, central, and northwestern parts of India. Thus, proper prediction of large-scale atmospheric circulation over the Indo-western Pacific region during El Niño can help to predict the HW conditions three months in advance. This would help to implement suitable adaptation measures and put into practice strong mitigation policies to limit the increased risk of such events during AMJ of El Niño decay years.

Synoptic controls on warm-season O3 pollution in eastern China: A focus on O3-NOx-VOC chemistry

The design of emission abatement measures to effectively reduce ozone (O3) pollution is highly complex due to non-linear chemistry, multi-source precursor emissions, and regional-scale transport. All these processes also depend on the synoptic circulation conditions. This study classified large-scale circulations to explore the synoptic controls on surface O3 pollution over eastern China (EC) during the warm seasons (May to September) from 2013 to 2022. Four prevailing circulation patterns (CPs) were identified, distinguishable by distinct differences in surface O3 anomaly distribution over the EC region. Regional meteorological variables, satellite-based precursors (NO2 and CH2O), and CH2O/NO2 ratio-based O3 formation sensitivity (OFS) were projected onto each CP to explore the underlying physiochemical mechanism of synoptic midualtion to surface O3 pollution. The results indicated that circulation-driven regional transport governed the spatial variability of surface O3 anomaly distribution on a synoptic timescale. Additionally, circulation-driven temperature changes caused unsynchronized changes in NO2 and CH2O across different CPs, leading to circulation-dependent OFS distribution. Geographical O3 anomaly changes generally followed the O3-VOC-NOx chemical principle in most synoptic circulation situations, thereby providing a framework for formulating spatiotemporal sensitivity-oriented mitigation strategies for forthcoming O3 pollution episodes with the assistance of synoptic circulation forecasts.

Grand dipole response of Asian summer monsoon to orbital forcing

The coherent continental-wide speleothem water isotope records are accompanied by regionally inconsistent moisture patterns within the Asian summer monsoon (ASM) region, leaving a disputed explanation of monsoonal hydroclimate variability at the orbital timescale. Here, the transient simulation forced by orbital parameters during the past 300,000 years in an isotope-enabled fully coupled model shows that the complex ASM response can be understood as a grand dipole pattern of oxygen isotope and rainfall between South Asia and Japan, with South China lying in a hybrid transition region. While the dipole monsoon rainfall change between South Asia and Japan is related to the large-scale circulation adjustment in response to Northern Hemisphere summer insolation, the dipole oxygen isotope pattern is associated with the changing contribution of moisture sources. Our results provide new clues to reconcile ASM hydroclimate patterns at orbital timescale as revealed by the water isotope records and other hydrological proxies.

Prediction of streamflow in Chalakudy River Basin, Kerala, by integrating teleconnection patterns of large-scale atmospheric circulations

ABSTRACT In this study, the relationship between large-scale climatic drivers and streamflow of the Chalakudy River Basin, Kerala, was analysed using methods such as bivariate wavelet coherence (BWC), multiple wavelet coherence (MWC), and partial wavelet coherence (PWC) analysis. The four prominent global climate indices chosen are the Indian Ocean Dipole (IOD), North Atlantic Oscillation (NAO), El Niño Southern Oscillation (ENSO), and Pacific Decadal Oscillation (PDO), along with other local climate drivers. The BWC analysis showed that streamflow and rainfall had a very strong in-phase relationship, whereas the maximum temperature and average temperature showed an anti-phase relationship. In the case of global climate drivers, ENSO has a significant impact on the streamflow of the Chalakudy River Basin. The average wavelet coherence (AWC), which quantifies the teleconnections, confirms the observation with a high coherency of 0.75 between streamflow and rainfall and 0.51 for streamflow and ENSO. Streamflow prediction models were developed using random forest (RF) and artificial neural network (ANN) techniques by considering the influence of significant global and local climatic drivers. It was observed that the RF model performed slightly better than the ANN model, with R = 0.875, NSE = 0.766, RMSE = 23.468, and RSR = 0.524.

Interacting internal waves explain global patterns of interior ocean mixing

Across the stable density stratification of the abyssal ocean, deep dense water is slowly propelled upward by sustained, though irregular, turbulent mixing. The resulting mean upwelling determines large-scale oceanic circulation properties like heat and carbon transport. In the ocean interior, this turbulent mixing is caused mainly by breaking internal waves: generated predominantly by winds and tides, these waves interact nonlinearly, transferring energy downscale, and finally become unstable, break and mix the water column. This paradigm, long parameterized heuristically, still lacks full theoretical explanation. Here, we close this gap using wave-wave interaction theory with input from both localized and global observations. We find near-ubiquitous agreement between first-principle predictions and observed mixing patterns in the global ocean interior. Our findings lay the foundations for a wave-driven mixing parameterization for ocean general circulation models that is entirely physics-based, which is key to reliably represent future climate states that could differ substantially from today’s.

Synchronous spring precipitation in Southeastern China and Bengal: a potential indicator for the Indian summer monsoon?

Abstract Spring precipitation in southeastern China and Bengal, occurring during the transitional phase from winter to summer monsoons, serves as a critical window into the dynamics of large-scale circulations and the subsequent summer monsoon. While many studies have analyzed spring precipitation in southeastern China and Bengal, their interconnections and implications for the summer monsoon have remained relatively under explored. We utilized the Empirical Orthogonal Function of spring precipitation to reveal Synchronous Spring Precipitation (SSP) in southeastern China and Bengal. This synchronicity is bridged by the East Asian Subtropical Jet (EASJ) that extends from Bengal to southeastern China. The EASJ was predominantly correlated with precipitation in southeastern China prior to the 1990s, while it developed a more profound connection with precipitation in Bengal after the 1990s. Notably, SSP anomalies occurred during the developing phase of the El Niño-Southern Oscillation (ENSO). The predictive capacity of SSP for the Indian summer monsoon (ISM) amplifies during periods of the intensified SSP-ENSO correlations and positive phase of the North Pacific Meridional Mode. Tree-ring based reconstructions spanning the past two centuries further corroborate the persistent linkages among the SSP, ISM, and ENSO. Our research sheds light on the intricate interplay of these factors and their significance in understanding and predicting the monsoon dynamics in the region.

Large-scale circulation reversals explained by pendulum correspondence

Large-scale circulation reversals explained by pendulum correspondence

Translation speed slowdown and poleward migration of western North Pacific tropical cyclones

Detecting and interpreting long-term changes in typhoon translation speed in observations remains challenging, contrasting with increased confidence in the poleward migration of typhoons. Here, I show a significant relationship between the basin-wide translation speed and the latitudinal position of tropical cyclones in the western North Pacific over 1980–2023. First, because tropical cyclones move faster at higher latitudes, the significant poleward migration (80 km/decade) increases the yearly basin-wide translation speed by 5% over the period. This effect reduces the detectability of a slowing trend. Second, the basin-wide translation speed solely contributed by regional translation speed has slowed by 18%, mostly in the late stage of the cyclone lifecycle. The translation speed slowdown and the poleward migration are likely caused by the same climate drivers through the interconnected large-scale atmospheric circulation between the tropics and subtropics. My findings suggest exacerbated tropical cyclone-related risk in the subtropical regions in a changing climate.

Bathymetry-constrained ocean geostrophic currents play a key role in shaping the sea ice circulation in the Canada Basin, Arctic Ocean

Abstract Satellite-based observations and a pan-Arctic coupled sea ice-ocean model are utilized to study the effect of ocean geostrophic currents on large-scale sea ice circulation in the Canada Basin, Arctic Ocean. We find that surface winds primarily drive sea ice drifts in the west-east direction, while the geostrophic currents in the Beaufort Gyre promote north-south ice drifts. Wind fluctuations can create variable ice drifts, yet geostrophic currents respond more slowly due to their larger vertical scale, serving as a slowly-evolving conveyor belt for maintaining the anticyclonic ice circulation. It is further demonstrated that the bathymetry can regulate the movement of sea ice via constraining the expansion of ocean circulation. This mechanism is indirect in the sense that the ice is far from the seafloor. Our research underscores the necessity of considering the bathymetry-constrained geostrophic currents in understanding Arctic sea ice dynamics. With the rapid retreat of Arctic sea ice, the multi-scale interactions between ice drifts and ocean currents may have significant implications for the Arctic ecosystem, climate, and shipping corridors.

A Laboratory Model of the Large-Scale Atmospheric Circulation of Tidally Locked Exoplanets

A Laboratory Model of the Large-Scale Atmospheric Circulation of Tidally Locked Exoplanets

Characterizing the dynamics of multi-scale global high impact weather events

The quantitative characterization and prediction of localized severe weather events that emerge as coherences generated by the highly non-linear interacting multivariate dynamics of global weather systems poses a significant challenge whose solution is increasingly important in the face of climate change where weather extremes are on the rise. As weather measurement systems (multiband satellite, radar, etc) continue to dramatically improve, increasingly complex time-dependent multivariate 3D datasets offer the potential to inform such problems but pose an increasingly daunting computational challenge. Here we describe the application to global weather systems of a novel computational method called the Entropy Field Decomposition (EFD) capable of efficiently characterizing coherent spatiotemporal structures in non-linear multivariate interacting physical systems. Using the EFD derived system configurations, we demonstrate the application of a second novel computational method called Space-Time Information Trajectories (STITs) that reveal how spatiotemporal coherences are dynamically connected. The method is demonstrated on the specific phenomenon known as atmospheric rivers (ARs) which are a prime example of a highly coherent, in both space and time, severe weather phenomenon whose generation and persistence are influenced by weather dynamics on a wide range of spatial and temporal scales. The EFD reveals how the interacting wind vector field and humidity scalar field couple to produce ARs, while the resulting STITS reveal the linkage between ARs and large-scale planetary circulations. The focus on ARs is also motivated by their devastating social and economic effects that have made them the subject of increasing scientific investigation to which the EFD may offer new insights. The application of EFD and STITs to the broader range of severe weather events is discussed.

Contribution of internal variability to the Mongolian Plateau summer precipitation trends in MPI-ESM large-ensemble model

Summer precipitation over the Mongolian Plateau (MP) has experienced a consistent decline in recent decades. While the influence of atmospheric wave train on this reduction in precipitation has been recognized in prior studies, this study delves deeper into the physical mechanisms and quantifies the contributions of the internal atmosphere and oceanic variations to the diminishing precipitation utilizing a comprehensive 100-member ensemble simulations from the Max Planck Institute Earth System Model (MPI-ESM). Results show that the ensemble-mean precipitation in MP exhibits a positive trend and cannot explain the observed results. The precipitation trends vary significantly among individual ensemble members, highlighting the pivotal role of internal variability. The leading EOF mode of precipitation trends among ensemble members exhibits uniform variations. Further investigations reveal that the internal summer precipitation in MP is affected by the internal atmospheric circulation, the remote influence of the North Atlantic Dipole sea surface temperature (SST) anomalies, and Pacific Decadal Oscillation-like SST patterns. An eastward-propagating Rossby wave originating from the North Atlantic dipole SST anomalies provides the anomalous large-scale circulation that influences summer precipitation. The PDO contributes to reinforcing the anticyclonic anomaly over the MP. Additionally, the uncertainty of precipitation trends in MPI-ESM can be reduced by 13% through removing the internal atmospheric wave train-related precipitation variation, while oceanic factors only contribute about 7% uncertainty of precipitation variations. Our insights enhance the understanding of the physical drivers behind summer precipitation variability in the MP and effectively quantify the uncertainties stemming from internal variability.

Interannual relationship of the extreme precipitation dipole mode over South China and Indochina Peninsula with the tropical Pacific-Indian Ocean sea surface temperature anomaly in May

This study investigates the interannual relationship of extreme precipitation days (EPD) over South China (SC) and Indochina Peninsula (ICP) with tropical Pacific-Indian Ocean Sea surface temperature (SST) anomalies during May, utilizing observations, reanalysis, and Coupled-Model Intercomparison Project 6 (CMIP6) model outputs. The results uncover a dipole mode in the interannual variations of May EPD over SC and ICP, characterized by an east–west inverse pattern. This dipole mode is significantly attributed to large-scale circulation anomalies induced by tropical Pacific–Indian Ocean mode (PIM). During the positive phase of PIM, SST anomalies in the tropical east-central Pacific and western Indian Oceans are anomalously warm, contrasted with negative SST anomalies in the western Pacific, leading to anomalous Walker circulations. Accordingly, two robust anticyclones appear in the lower levels of the Northwest Pacific and the Bay of Bengal, respectively. The southwesterlies on the western flank of the anticyclone in the Northwest Pacific cause moisture convergence over SC, favoring the occurrence of extreme precipitation in SC. Conversely, the anticyclonic circulation centered at the Bay of Bengal is not conducive to the transport of warm water vapor from oceans to ICP, resulting in reduced occurrence of extreme precipitation there. The results derived from CMIP6 models unequivocally support the notion that the dipole mode is predominantly shaped by large-scale circulation anomalies induced by PIM. Our results underscore the importance of accounting for the collective impact of oceanic drivers in the tropics, laying a scientific foundation for enhanced precision in simulating extreme precipitation across SC and ICP.

Seasonal predictions of summer compound humid heat extremes in the southeastern United States driven by sea surface temperatures

Humid heat extreme (HHE) is a type of compound extreme weather event that poses severe risks to human health. Skillful forecasts of HHE months in advance are crucial for developing strategies to enhance community resilience to extreme events1,2. This study demonstrates that the frequency of summertime HHE in the southeastern United States (SEUS) can be skillfully predicted 0–1 months in advance using the SPEAR (Seamless system for Prediction and EArth system Research) seasonal forecast system. Sea surface temperatures (SSTs) in the tropical North Atlantic (TNA) basin are identified as the primary driver of this prediction skill. The responses of large-scale atmospheric circulation and winds to anomalous warm SSTs in the TNA favor the transport of heat and moisture from the Gulf of Mexico to the SEUS. This research underscores the role of slowly varying sea surface conditions in modifying large-scale environments, thereby contributing to the skillful prediction of HHE in the SEUS. The results of this study have potential applications in the development of early warning systems for HHE.

Westward displacement of atmospheric East West Circulation ameliorated drought-induced conditions in Australia and India during the major 2023-2024 and 1997-1998 El Niño events.

Abstract The results presented in this Essay were revealed through quasi-regular interrogations of various online climate diagnostics bulletins and publications. These are compiled by climate scientists in various institutions around the world for use and reference by the wider climate community. Thus, we are looking to bring our findings to the attention of interested parties using the above material as the basis for discussion and further interrogation, not to conduct a full original analysis. We show evidence for a significant variation in the spatial pattern of the large-scale vertical zonal atmospheric circulation patterns across the equatorial Indo-Australasian domain of the eastern hemisphere during the major 2023-2024 El Niño event. The region of large-scale subsidence, and its associated teleconnection patterns, that are usually centered across Indonesia (the ‘Maritime Continent’) were displaced to the west over the equatorial Indian Ocean. In the record of El Niño events with readily available online dynamical tropospheric fields (one source from 1947 and the other from 1979), this has only been seen two other times, during the strong 1997-1998 El Niño and the weaker 1977-1978 event. These three events may well be consistent with internal variability, but at present the reason for such occurrences has not been established.

Synergic influence of lower and upper troposphere large-scale circulations on tropical cyclone genesis over the western North Pacific

Synergic influence of lower and upper troposphere large-scale circulations on tropical cyclone genesis over the western North Pacific

Near- and Superinertial Internal Wave Responses and the Associated Energy Transfer after the Passage of Tropical Cyclone Fitow at a Midlatitude Shelf Slope

Abstract Observations from a mooring station at the East China Sea (ECS) shelf slope revealed both near- and superinertial dynamic responses to tropical cyclone (TC) Fitow. Different from the typical near-inertial response, near-inertial internal waves (NIWs) after TC Fitow showed a complicated phase pattern due to its superposition with parametric subharmonic instability (PSI)-generated M1 subharmonic waves. The wind-injected near-inertial kinetic energy (NIKE) was largely restrained to the upper 250 m. Wave packet analysis revealed the co-occurrence of enhanced NIKE, circularly polarized near-inertial currents, veering NIW propagation direction, and shrinking NIW vertical wavenumber at the base of the Kuroshio (∼180 m). This indicated the trapping and stalling of the TC-generated NIWs. Intense high-frequency internal waves (HFIWs) appeared immediately after TC Fitow which had an average period of ∼24 min and lasted ∼8 h. HFIWs also existed before the arrival of TC Fitow with a regular semidiurnal cycle. However, the HFIW after TC did not follow the semidiurnal cycle and had much larger amplitudes and longer-lasting periods. Local generation of supercritical flow over a slope or evolution from propagating internal tide as modified by TC may have induced these HFIWs. Along with the occurrence of intense HFIWs after TC Fitow, intense energy transfers from NIWs to HFIWs were identified. Due to the limited vertical propagation of TC-induced NIWs, it was the PSI-generated M1 subharmonic wave rather than the wind-induced NIW that contributed most to the energy transfer. Significance Statement Winds blowing over the ocean generate not only surface waves but also another type, so-called internal waves, which can travel down into the ocean. These internal waves carry a large amount of wind-injected energy into the ocean interior and are thought to play an important role in sustaining ocean turbulent mixing and circulation. We provided evidence that the passage of a tropical cyclone (TC) can induce different types of internal wave responses: near-inertial and high-frequency internal waves. Interactions and energy transfer between different internal waves occurred. With the TC-induced near-inertial internal waves trapped in the upper ocean, the background M1 subharmonic waves contributed most to the interactions with high-frequency internal waves. This fact is crucial in understanding the propagation and dissipation of wind-injected energy in the ocean, which plays an important role in controlling the ocean interior mixing and large-scale circulation.

Direct numerical simulation of open-channel flow over a heterogeneous particle bed at low relative submergence

This study investigates turbulent open-channel flows over beds of irregularly arranged particles, using direct numerical simulations at a friction Reynolds number of Reτ=300. Two distinct cases are examined: a polydisperse bed (P800) composed of multiple layers of randomly distributed spheres of varying sizes, and a monodisperse bed (M1015) formed by a random distribution of uniform sized spheres, with a bottommost single layer of varied-sized particles to introduce realistic randomness. Our investigation unveils a rich network of low- and high-speed streaks within the flow field, exhibiting distinctive behaviors in different bed configurations. The P800 case presents a poorly organized flow pattern induced by the varied particle sizes and arrangements, while the M1015 case shows a more regular flow pattern, marked by larger streaks. We also observe that total wall shear stress is substantially influenced by surface roughness-induced drag, extending beyond the effects documented in existing studies of open-channel flows. The present study reveals intricate secondary flow patterns over irregular particle beds. Large-scale circulations are discerned around particle crests in the P800 case and localized circulations with increased turbulence in the M1015 case. Furthermore, analysis of Reynolds stress tensor components indicates that roughness disrupts coherent turbulent eddies, consequently mitigating peak stress. We quantify correlations between drag force and local fluid velocity fluctuations. Notably, a larger deviation in drag is observed in the P800 case compared to M1015, accentuating the influence of particle size and distribution on fluid–particle interactions.

Deciphering Mean Flow–Eddy Interaction in Pacific–North American Teleconnection Linked to Storm Tracks at Subseasonal Time Scales

Abstract The features of large-scale atmospheric circulations, storm tracks, and the mean flow–eddy interaction during winter Pacific–North American (PNA) events are investigated using National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data at subseasonal time scales from 1979 to 2022. The day-to-day variations of storm-track activity and streamfunction reveal that storm-track activity varies along the evolution of mean flow. To better understand storm-track variability with the mean flow–eddy interaction, further exploration is made by analyzing local energy energetics. The changes in horizontal and vertical baroclinic energy conversions from background flow correspond to the storm-track anomalies over the North Pacific, indicating that the anomalies in storm tracks are due to the anomalous mean flow associated with PNA patterns impacting energy conversion through mean flow–eddy interaction. Eddy feedback driven by vorticity and heat fluxes is analyzed. This provides a concrete illustration of how eddy feedback serves as a positive factor for the upper-tropospheric circulation anomalies associated with the PNA pattern. Significance Statement The background flow plays a crucial role in governing storm-track dynamics. Our emphasis is on the Pacific storm tracks (PST) and their relation to Pacific–North American (PNA) patterns at subseasonal time scales. We unveil the relationship between anomalies of PST and PNA patterns using local energetics and eddy feedback on a day-by-day basis. It is noteworthy that the evolution of anomalous storm tracks during PNA events is the manifestation of mean flow–eddy interaction. Additionally, we provide detailed confirmation of the impact of anomalous storm tracks on large-scale anomalies associated with the PNA pattern.