General Circulation Model Experiments Research Articles

Midlatitude Oceanic Fronts Strengthen the Hydrological Cycle Between Cyclones and Anticyclones

AbstractThe Kuroshio‐Oyashio Extension and Gulf Stream oceanic frontal zones are characterized by enhanced activity of synoptic‐scale cyclones and anticyclones and vigorous air‐sea heat and moisture exchange in the cold season. However, the time‐mean air‐sea exchange attributed separately to cyclones and anticyclones has not been assessed. Here we quantify cyclonic and anticyclonic contributions around the frontal zones to surface turbulent heat fluxes, precipitation, and the associated hydrological cycle using atmospheric general circulation model experiments with observed and artificially smoothed sea‐surface temperature gradients. The evaluation reveals that precipitation exceeds evaporation climatologically within cyclonic domains while evaporation dominates within anticyclonic domains. These features as well as the net moisture transport from anticyclonic to cyclonic domains are all enhanced by the sharpness of the frontal zones. Oceanic frontal zones thus climatologically act to strengthen the hydrological cycle. These findings aid our understanding of the relationship between midlatitude air‐sea interactions on synoptic‐ and longer‐time scales.

Zonal Structure of Tropical Pacific Surface Salinity Anomalies Affects the Eastern and Central Pacific El Niños Differently

AbstractMaximum sea surface salinity (SSS) anomalies are found in the central Pacific during the eastern Pacific El Niño (EPEN) and located further westward during the central Pacific El Niño (CPEN), but whether these differences affect event strengths is unclear. By performing ocean general circulation model experiments via modifying freshwater flux anomalies, we find salinity effects on surface warming during both types are highly sensitive to zonal locations of SSS anomalies, with the strongest warming induced by the SSS anomalies near the international dateline. Further analysis reveals that vertical mixing and entrainment dominate this temperature sensitivity, with the strongest response to SSS anomalies occurring in the central Pacific. The central‐Pacific SSS anomalies increase EPEN warming by 0.15°C while the westward‐located SSS anomalies make little contribution to CPEN warming. Therefore, the distinct zonal structures of SSS anomalies facilitate stronger EPEN than the CPEN, increasing their difference in intensity by about 10%.

Impacts of Oceanic Mesoscale Structures on Sea Surface Temperature in the Arabian Sea and Indian Summer Monsoon Revealed by Climate Model Simulations

Abstract Climate models typically have a cold sea surface temperature (SST) bias for the Arabian Sea, which regulates the Indian monsoon system as a water vapor source. Since the SST for the Arabian Sea is critical for the Indian monsoon, a better understanding of the processes that affect the SST is required. In this study, the effects of mesoscale oceanic variability on simulations of the Arabian Sea SST and Indian summer monsoon precipitation were investigated based on a comparison of climate model experiments in which non-eddying and eddy-permitting ocean components are coupled. In the eddy-permitting model, warm water advection driven by mesoscale variability near the Gulf of Aden and the Gulf of Oman increases the SST in the western Arabian Sea. Lateral eddy heat transport enhances warm water outflows to the Arabian Sea and suppresses surface water cooling by coastal upwelling during the southwestern monsoon season. Furthermore, a sensitivity experiment shows the primary importance of resolving oceanic mesoscale eddies and the secondary importance of resolving the Persian Gulf and Red Sea for the Arabian Sea SST. Also, the summer monsoonal precipitation decreases (increases) over the southeastern Arabian Sea (western and northern India) in the eddy-permitting model due to enhancement of wind convergence in the lower troposphere. Atmospheric general circulation model experiments indicate that the precipitation difference is partly caused by SST changes over the western Arabian Sea. The findings imply that the ocean resolution of climate models is a key factor in efficiently simulating the Indian monsoon.

Topographic Trapping of the Leeuwin Current and Its Impact on the 2010/11 Ningaloo Niño

Abstract Previous theoretical studies suggest that the topography along the west coast of Australia plays an important role in strengthening and trapping the Leeuwin Current (LC) at the coast. To isolate and quantify the effect of the continental shelf and slope on the LC and Ningaloo Niño, high-resolution (1/12°) ocean general circulation model experiments with different bottom topographies are performed. The “control” experiment uses a realistic bottom topography along the west coast of Australia, whereas the sensitivity (“no-shelf”) experiment uses a modified topography with no continental shelf and slope near the coast. The mean and variability of LC are realistically simulated in the control experiment. Compared to the control experiment, the strength of LC in the no-shelf experiment decreased by about 28%. The continental shelf influences the development of the 2010/11 Ningaloo Niño through modulating the LC variability: in August–October 2010 and January–February 2011, the LC in the control experiment is enhanced much more than that in the no-shelf experiment. As a result, the upper-50-m ocean temperature in the control experiment is about 26% warmer than the no-shelf experiment from September 2010 to March 2011. Different evolution of SST warming is also found in the two experiments. Comparisons of oceanic processes in the two experiments show that the shelf-slope topography can effectively trap the positive sea level anomaly at the coast in the control experiment while more Rossby waves radiate from the coast in the no-shelf experiment, resulting in a weaker LC.

Intermodel uncertainty in response of the Pacific Walker circulation to global warming

The Pacific Walker circulation (PWC) is one of the major atmospheric circulations that plays an essential role in ocean-atmosphere interactions and global climate. The response of the PWC to greenhouse warming remains a mystery and model results are inconsistent. Based on multimodel simulations from the Coupled Model Intercomparison Project phase 6 (CMIP6), this study explores the intermodel uncertainty of the change in the PWC under global warming. The combination of the El Niño-like warming pattern and the interbasin warming contrast between the Indian and Pacific Oceans strengthens (weakens) the west (east) branch of the Pacific trade winds, resulting in a structural shifting of the PWC. By conducting a set of Atmosphere General Circulation Model (AGCM) experiments, we demonstrate that the western Pacific warming plays a critical role in driving the PWC shift. An intensified western Pacific warming counteracts the effect of Indian Ocean warming on the PWC, leading to a uniformly weakened PWC in the tropical Pacific due to the SST gradient. In contrast, a decreased warming in the western Pacific strengthens the west branch of the PWC, shifting the turning longitude of zonal wind changes eastward. Our finding highlights that the relative warming pattern in the Indian and Pacific Oceans is coupled with the PWC change in a warmer climate.

Combined Effects of Tropical Indo-Pacific–Atlantic SST Anomalies on Record-Breaking Floods over Central-North China in September 2021

Abstract The rainy season typically terminates over the area of central northern China (CNC) in late August with the rapid retreat of the East Asian summer monsoon (EASM). However, the CNC suffered continuous heavy rainfall in September 2021. The monthly accumulative precipitation amount broke its historical record since 1961 and caused severe floods. The present study ascribes this extreme rainfall event to the westward inland intrusion of the western North Pacific subtropical high (WNPSH). Both case study and statistical analysis indicate that westward extension of the WNPSH is significantly positively correlated with convection over the Maritime Continent (MC) and northern Indian Peninsula (NIP) under the influence of sea surface temperature anomalies (SSTAs). Atmospheric general circulation model (AGCM) experiments validate the individual and combined effects of SSTAs in the tropical Indo-Pacific–Atlantic Oceans on WNPSH and CNC rainfall. In particular, the warm SSTAs in the Indo-Pacific warm pool increase the probability of extremely westward extension of the WNPSH and above-normal rainfall over the CNC. The cold SSTAs in the equatorial central-eastern Pacific enlarge the zonal SSTA gradient, strengthen the anomalous convection over the MC and NIP, and intensify the WNPSH and CNC rainfall anomalies. Meanwhile, the warmer tropical Atlantic anchors the anomalous WNPSH and CNC rainfall in their observed positions. A statistical model based on the tropical SSTAs in August can skillfully predict the westward extension of the WNPSH in September 2021, suggesting that the combined effects of tropical Indo-Pacific–Atlantic SSTAs in late summer possibly tend to prolong rainy season of the EASM until early autumn.

Critical Role of Tropical North Atlantic SSTA in Boreal Summer in Affecting Subsequent ENSO Evolution

AbstractThe season‐dependent impacts of the tropical North Atlantic (TNA) sea surface temperature anomaly (SSTA) on subsequent El Niño‐Southern Oscillation (ENSO) evolution were investigated through observational and modeling studies. The results indicate that, although the maximum amplitude of the TNA SSTA occurs during boreal spring, the TNA SSTA in boreal summer generates a stronger rainfall response in situ, which can further induce a significantly stronger zonal wind anomaly over the equatorial western Pacific via Kelvin and Rossby wave processes. The cause of a stronger precipitation response in boreal summer is attributed to the northward migration of the climatological Atlantic warm pool and the Inter‐Tropical Convergence Zone. Idealized Coupled General Circulation Model experiments further demonstrate that a persisting TNA SSTA forcing up to boreal summer is critical in conveying the TNA impact to subsequent ENSO evolutions in the Pacific.

Seasonality in Arctic Warming Driven by Sea Ice Effective Heat Capacity

Abstract Arctic surface warming under greenhouse gas forcing peaks in winter and reaches its minimum during summer in both observations and model projections. Many mechanisms have been proposed to explain this seasonal asymmetry, but disentangling these processes remains a challenge in the interpretation of general circulation model (GCM) experiments. To isolate these mechanisms, we use an idealized single-column sea ice model (SCM) that captures the seasonal pattern of Arctic warming. SCM experiments demonstrate that as sea ice melts and exposes open ocean, the accompanying increase in effective surface heat capacity alone can produce the observed pattern of peak warming in early winter (shifting to late winter under increased forcing) by slowing the seasonal heating rate, thus delaying the phase and reducing the amplitude of the seasonal cycle of surface temperature. To investigate warming seasonality in more complex models, we perform GCM experiments that individually isolate sea ice albedo and thermodynamic effects under CO2 forcing. These also show a key role for the effective heat capacity of sea ice in promoting seasonal asymmetry through suppressing summer warming, in addition to precluding summer climatological inversions and a positive summer lapse-rate feedback. Peak winter warming in GCM experiments is further supported by a positive winter lapse-rate feedback, due to cold initial surface temperatures and strong surface-trapped warming that are enabled by the albedo effects of sea ice alone. While many factors contribute to the seasonal pattern of Arctic warming, these results highlight changes in effective surface heat capacity as a central mechanism supporting this seasonality. Significance Statement Under increasing concentrations of atmospheric greenhouse gases, the strongest Arctic warming has occurred during early winter, but the reasons for this seasonal pattern of warming are not well understood. We use experiments in both simple and complex models with certain sea ice processes turned on and off to disentangle potential drivers of seasonality in Arctic warming. When sea ice melts and open ocean is exposed, surface temperatures are slower to reach the warm-season maximum and slower to cool back down below freezing in early winter. We find that this process alone can produce the observed pattern of maximum Arctic warming in early winter, highlighting a fundamental mechanism for the seasonality of Arctic warming.

Remote influence of the interannual variability of the Australian summer monsoon on wintertime climate in East Asia and the western North Pacific

AbstractAnomalous convective activity in the Tropics forced by sea surface temperature (SST) variability exerts significant remote influence that provides a basis for seasonal prediction in the extratropics. In austral summer convective activity exhibits pronounced interannual variability over northern Australia (NAUS), which is, however, unlikely forced by SST anomalies but essentially a manifestation of internal variability of the Australian summer monsoon (AUSM) system. Based on observational data, the present study reveals its significant remote impacts on the wintertime climate in East Asia and the western North Pacific. The anomalous AUSM excites the Western Pacific (WP) pattern, as confirmed through an atmospheric general circulation model experiment. Through this cross-equatorial teleconnection, the enhanced AUSM leads to the strengthening of the East Asian winter monsoon with a colder winter over the Korean Peninsula and western Japan and reduced precipitation over southern China. The Okhotsk sea-ice extent decreases under warm anomalies and weakened offshore winds. The weakened AUSM leads to the same anomalies but with the opposite polarities. Our observational data analysis and numerical experiments reveal that the WP-like anomalies are excited by the propagation of stationary Rossby waves generated by anomalous upper-level divergent wind from NAUS that extends into the Northern Hemisphere subtropical jet. The climatological Hadley circulation is essential in this process. The concomitant anomalous diabatic heating over East Asia and feedback forcing by transient eddies along the Pacific stormtrack act to further amplify the WP-like response.

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.

Mechanism for Southward Shift of Zonal Wind Anomalies During the Mature Phase of ENSO

AbstractThe cause of southward shift of anomalous zonal wind in the central equatorial Pacific (CEP) during ENSO mature winter was investigated through observational analyses and numerical model experiments. Based on an antisymmetric zonal momentum budget diagnosis using daily ERA-Interim data, a two-step physical mechanism is proposed. The first step involves advection of the zonal wind anomaly by the climatological mean meridional wind. The second step involves the development of an antisymmetric mode in the CEP, which promotes a positive contribution to the observed zonal wind tendency by the pressure gradient and Coriolis forces. Two positive feedbacks are responsible for the growth of the antisymmetric mode. The first involves the moisture–convection–circulation feedback, and the second involves the wind–evaporation–SST feedback. General circulation model experiments further demonstrated that the boreal winter background state is critical in generating the southward shift, and a northward shift of the zonal wind anomaly is found when the same SST anomaly is specified in boreal summer background state.

Sensitivity of East Asian summer monsoon precipitation to the location of the Tibetan Plateau

AbstractRecent studies have highlighted that a primary mechanism of the East Asian summer monsoon (EASM) is the fluid dynamical response to the Tibetan Plateau (TP), that is, orographically forced Rossby waves. With this mechanism in mind, this study explores how changes in the location of the TP affect the EASM precipitation. Specifically, the TP is moved in the four cardinal directions using idealized general circulation model experiments. The results show that the monsoon aspects are entirely determined by the location of the TP. Interestingly, the strongest EASM precipitation occurs when the TP is situated near its current location, a situation in which downstream southerlies are well developed from the surface to aloft. However, southerlies into the EASM region weaken as the TP moves, which in turn reduces the precipitation. Nevertheless, as long as it moves in the east–west direction, the TP is likely to force the stationary waves that induce precipitation over the mid-latitudes (not necessarily over East Asia). In contrast, moving the TP well north of its original location does not induce strong monsoon flows over the EASM region, resulting in the driest case. Meanwhile, although the southward movement of the TP triggers downstream southerlies to some extent, it does not lead to an increase in the precipitation. Overall, these results show that the location of the TP is crucial in determining the EASM precipitation, and the latter is much more sensitive to the displacement of the TP in the meridional direction than in the zonal direction.

Revision of “Dependence of Climate States of Gray Atmosphere on Solar Constant: From the Runaway Greenhouse to the Snowball States” by Ishiwatari et al. (2007)

AbstractIshiwatari et al. (2007) (https://doi.org/10.1029/2006JD007368) investigated multiple equilibrium solutions of a gray atmosphere for various values of solar constant, utilizing two types of models, namely, a one‐dimensional energy balance model and a general circulation model (GCM) with simplified physical processes, both of which permit existence of the runaway greenhouse state. The study was retracted, since there was a bug in their GCM that affected quantitative aspects of the study. Here, we revise the study with re‐performing all of the GCM experiments using an appropriately corrected model. The results of re‐experiments show that the main features of the climate regime diagram drawn in the solar constant ‐ ice line latitude plane are mostly unchanged, except that the ice‐free equilibrium state, which existed in Ishiwatari et al. (2007) (https://doi.org/10.1029/2006JD007368) now disappears. It is confirmed that there are the partially ice‐covered state, the globally ice‐covered state, and the runaway greenhouse state. These three states coexist for intermediate values of solar constant. The existence of the large ice cap instability is also confirmed, although the critical latitude of the partially ice‐covered state extends equatorward. Also same as before, the small ice cap instability does not seem to appear.

The Dynamics of Quasi-Stationary Atmospheric Rivers and Their Implications for Monsoon Onset

AbstractA global Hovmöller diagram of column water vapor (CWV) at 30°N from daily ERA-Interim reanalysis data shows seasonally migrating North Pacific/Atlantic quasi-stationary atmospheric rivers (QSARs) located in the Eastern Pacific/Atlantic in winter and propagate to the Western Pacific/Atlantic in summer. Simplified general circulation model (GCM) experiments produce QSAR-like features if the boundary conditions include (1) the sea surface temperature contrast from the tropical warm pool-cold tongue and (2) topographic contrast similar to the Tibetan plateau. Simulated QSARs form downstream of topographic contrast during winter and coincide with it in summer. Two models of baroclinic instability demonstrate that QSARs coincide with the location where the most unstable mode phase speed equals that of the upper-level zonal winds. A consistent interpretation is that the waves become quasi-stationary at this location and break. The location of quasistationarity migrates from the Eastern Pacific/Atlantic in the winter, when upper-level winds are strong and extended over the basin, to the Western Pacific/Atlantic when winds are weak and contracted. Low-level wind convergence and moist static energy coincide with QSARs, and since the former two are essential ingredients to monsoon formation, this implies an important role for QSARs in monsoon onset. This connection opens a new window into the dynamics of subtropical monsoon extensions.

Opposite Responses of the Dry and Moist Eddy Heat Transport Into the Arctic in the PAMIP Experiments

AbstractGiven uncertainty in the processes involved in polar amplification, elucidating the role of poleward heat and moisture transport is crucial. The Polar Amplification Model Intercomparison Project (PAMIP) permits robust separation of the effects of sea ice loss from sea surface warming under climate change. We utilize a moist isentropic circulation framework that accounts for moisture transport, condensation, and eddy transport, in order to analyze the circulation connecting the mid‐latitudes and the Arctic. In PAMIP's atmospheric general circulation model experiments, prescribed sea ice loss reduces poleward heat transport (PHT) by warming the returning moist isentropic circulation at high latitudes, while prescribed warming of the ocean surface increases PHT by strengthening the moist isentropic circulation. Inter‐model spread of PHT into the Arctic reflects the tug‐of‐war between sea‐ice and surface‐warming effects.

Assessing ENSO Summer Teleconnections, Impacts, and Predictability in North America

AbstractDuring the summer when an El Niño event is transitioning to a La Niña event, the extratropical teleconnections exert robust warming anomalies over the U.S. Midwest threatening agricultural production. This study assesses the performance of current climate models in capturing the prominent observed extratropical responses over North America during the transitioning La Niña summer, based on atmospheric general circulation model experiments and coupled models from the North American Multimodel Ensemble (NMME). The ensemble mean of the SST-forced experiments across the transitioning La Niña summers does not capture the robust warming in the Midwest. The SST-forced experiments do not produce consistent subtropical western Pacific (WP) negative precipitation anomalies and this leads to the poor simulations of extratropical teleconnections over North America. In the NMME models, with active air–sea interaction, the negative WP precipitation anomalies show better agreement across the models and with observations. However, the downstream wave train pattern and the resulting extratropical responses over North America exhibit large disagreement across the models and are consistently weaker than in observations. Furthermore, in these climate models, an anomalous anticyclone does not robustly translate into a warm anomaly over the Midwest, in disagreement with observations. This work suggests that, during the El Niño to La Niña transitioning summer, active air–sea interaction is important in simulating tropical precipitation over the WP. Nevertheless, skillful representations of the Rossby wave propagation and land–atmosphere processes in climate models are also essential for skillful simulations of extratropical responses over North America.

The Influence of South Pacific Convergence Zone Heating on the South Pacific Subtropical Anticyclone

AbstractSubtropical anticyclones and midlatitude storm tracks are key components of the large-scale atmospheric circulation. Focusing on the Southern Hemisphere, the seasonality of the three dominant subtropical anticyclones, situated over the South Pacific, South Atlantic, and south Indian Ocean basins, has a large influence on local weather and climate within South America, southern Africa, and Australia, respectively. Generally speaking, sea level pressure within the Southern Hemisphere subtropics reaches its seasonal maximum during the winter season when the Southern Hemisphere Hadley cell is at its strongest. One exception to this is the seasonal evolution of the South Pacific subtropical anticyclone. While winter maxima are seen in the South Atlantic and south Indian subtropical anticyclones, the South Pacific subtropical anticyclone reaches its seasonal maximum during local spring with elevated values extending into summer. In this study, we investigate the hypothesis that the strength of the austral summer South Pacific subtropical anticyclone is largely due to heating over the South Pacific convergence zone. Using added-cooling and added-heating atmospheric general circulation model experiments to artificially change the strength of austral summer diabatic heating over the South Pacific convergence zone, our results show that increased heating, through increased upper-level divergence, triggers a Rossby wave train that extends into the Southern Hemisphere midlatitudes. This propagating Rossby wave train creates a high and low sea level pressure pattern that projects onto the center of the South Pacific subtropical anticyclone to intensify its area and strength.

Seasonal Variation of the Westerly Jet over Asia in the Last Glacial Maximum: Role of the Tibetan Plateau Heating

AbstractThe westerly jet (WJ) is an important component of atmospheric circulation, which is characterized by prominent seasonal variations in intensity and position. However, the response of the WJ over Asia during the Last Glacial Maximum (LGM) is still not clear. Using general circulation model experiments, the seasonal behaviors of the WJ over central Asia and Japan are analyzed in this paper. The results show that, compared to the present day (PD), the WJ presents a complicated response during the LGM, both in intensity and position. Over central Asia, it becomes weaker in both summer and winter. But over Japan, it is enhanced in summer but becomes diminished in winter. In terms of position, the WJ over central Asia shifts southward in both summer and winter, whereas the WJ over Japan moves southward in summer but does not change obviously relative to PD in winter. Such WJ changes are well explained by meridional temperature gradients in high troposphere, which is closely linked to seasonal thermal anomalies over the Tibetan Plateau (TP). Despite cooler LGM conditions, the anomalous warming center over the TP becomes stronger in summer. Derived from the heat budget equation, the stronger heating center is mainly caused by the weaker adiabatic cooling generated from ascending motion over the area south of the TP. In winter, the cooling over the TP is also strengthened, mostly owing to the subsidence-induced weaker adiabatic heating. Due to the importance of the WJ, the potential role of TP thermal effects should be a focus when explaining the East Asian climate change during the LGM.

The Observed Relationship between Pacific SST Variability and Hadley Cell Extent Trends in Reanalyses

AbstractReanalysis and other observationally based estimates suggest that the tropics have expanded more than simulated by coupled climate models with historical radiative forcing. Previous research has attempted to reconcile this discrepancy by using climate model simulations with constrained tropical Pacific sea surface temperatures (SSTs) to account for the role of internal variability. Here the relationships between Hadley cell extent and internal SST variability and long-term warming are analyzed using purely observational techniques. Using linearly independent components of SST variability with reanalysis datasets, the statistical relationship between Pacific variability and Hadley cell extent is quantified by time scale. There is a strong correlation between North Pacific decadal SST variability and Southern Hemisphere Hadley cell extent. Conversely, there is a weaker observed relation between El Niño–Southern Oscillation (ENSO) and Hadley cell extent when low-frequency variability is filtered out of the ENSO signal. The observed linear sensitivity of Hadley cell width to long-term warming agrees with coupled general circulation model experiments when accounting for uncertainties, and there is a statistically significant relationship between Northern Hemisphere Hadley cell extent and long-term warming during boreal autumn.

Characterizing Regimes of Atmospheric Circulation in Terms of Their Global Superrotation

AbstractThe global superrotation index S compares the integrated axial angular momentum of the atmosphere to that of a state of solid-body corotation with the underlying planet. The index S is similar to a zonal Rossby number, which suggests it may be a useful indicator of the circulation regime occupied by a planetary atmosphere. We investigate the utility of S for characterizing regimes of atmospheric circulation by running idealized Earthlike general circulation model experiments over a wide range of rotation rates Ω, 8ΩE to ΩE/512, where ΩE is Earth’s rotation rate, in both an axisymmetric and three-dimensional configuration. We compute S for each simulated circulation, and study the dependence of S on Ω. For all rotation rates considered, S is on the same order of magnitude in the 3D and axisymmetric experiments. For high rotation rates, S ≪ 1 and S ∝ Ω−2, while at low rotation rates S ≈ 1/2 = constant. By considering the limiting behavior of theoretical models for S, we show how the value of S and its local dependence on Ω can be related to the circulation regime occupied by a planetary atmosphere. Indices of S ≪ 1 and S ∝ Ω−2 define a regime dominated by geostrophic thermal wind balance, and S ≈ 1/2 = constant defines a regime where the dynamics are characterized by conservation of angular momentum within a planetary-scale Hadley circulation. Indices of S ≫ 1 and S ∝ Ω−2 define an additional regime dominated by cyclostrophic balance and strong equatorial superrotation that is not realized in our simulations.