Meridional Temperature Gradient Research Articles

Increases in the Local Eddy Energetics of the Extratropical Atmosphere over the Last Four Decades

Abstract Changes in the vertical and meridional temperature gradients of the atmosphere drive competing influences on storm-track activity. We apply local eddy energetics to the ERA5, JRA-55, MERRA-2, and NCEP-2 reanalyses during 1980–2020 to determine the locations, magnitudes, and trends of the energy transfer mechanisms for synoptic-scale eddies. Eddy kinetic energy (EKE) increases more rapidly in the Southern Hemisphere at all altitudes and seasons, with larger increases during austral winter and spring. In the Northern Hemisphere, increases occur within the Atlantic and Pacific storm tracks at pressures below 300 hPa but only during boreal winter and spring and confined within a narrow zonal band; EKE decreases during boreal summer and fall. Most EKE changes correspond with trends in baroclinic energy conversion upstream of storm tracks and appear to align with increases in the growth rate of the most unstable baroclinic mode. Barotropic energy conversion of EKE to the mean flow becomes locally more intense downstream of the storm tracks. Conversion of EKE to long-period eddies plays a minor role averaged over a hemisphere but can be important locally. The primary strengthening pathway for removal of EKE is a combination of surface friction and viscous dissipation. The increased baroclinic conversion in the Southern Hemisphere appears related to upper-level tropical temperature increases. In the Northern Hemisphere, increased baroclinic conversion is enabled by a combination of increased vertical heat fluxes and a region of temperature increases within 30°–60°N. Significance Statement Traveling atmospheric disturbances arrange into storm tracks that determine the weather in the midlatitudes. Storm tracks are evolving in time due to anthropogenic warming; however, the location and strength of temperature changes compete for influence on the storm tracks. A framework to quantify the mechanisms of generation of kinetic energy contained by eddies pinpoints the extent of storm-track evolution. Storm tracks generally strengthen across the planet but have increased the most in the Southern Hemisphere. Strengthening in the Northern Hemisphere is limited to the winter in a narrow latitudinal band, because of warming in the Arctic that reduces the primary instability that drives eddies. The locations of northern warming and storm-track strengthening suggest a role for tropical dynamics.

Potential impacts of reduced winter Kara Sea ice on the dipole pattern of cold surge frequency over the tropical western Pacific

The impact of Arctic Sea ice melting on weather and climate extremes in the Northern Hemisphere has garnered widespread attention. Existing research has convincingly demonstrated the importance of this impact in mid-high latitudes, while its influence in areas beyond remains controversial. This study reveals the indirect influence of Kara Sea ice reduction on cold surge (CS) over the tropical western Pacific (TWP), with the East Asian jet stream serving as the connecting link. The leading mode of CSs over the TWP exhibits a zonal dipole characteristic, which is associated with cyclonic anomaly over the Philippine Sea. The enhanced cyclonic anomaly is caused by strengthened and northward-moved subtropical East Asian jet stream and weakened polar jet stream, which can lead to more CSs over the South China Sea and fewer CSs over the Philippine Sea. Such variations in the jet stream are contributed by the facilitated atmospheric blockings west of the Ural Mountains, which suppressed the circumpolar westerly winds and increased meridional temperature gradient in Northeast Asia. The connection between atmospheric blockings and Kara Sea ice can be confirmed through local vertical energy exchange. Simulations of the atmospheric response to the forcing of decreased Kara Sea ice support the proposed connection. Although there is no statistically significant correlation between tropical CSs and Kara Sea ice, this study highlights the potential impacts of Arctic climate change signal on weather and climate extremes over tropical regions.

West African Monsoon Dynamics and Its Control on the Stable Oxygen Isotopic Composition of Precipitation in the Late Cenozoic

AbstractThis study presents an overview of the Late Cenozoic evolution of the West African Monsoon (WAM), and the associated changes in atmospheric dynamics and oxygen isotopic composition of precipitation (δ18Op). This evolution is established by using the high‐resolution isotope‐enabled GCM ECHAM5‐wiso to simulate the climatic responses to paleoenvironmental changes during the Mid‐Holocene (MH), Last Glacial Maximum (LGM), and Mid‐Pliocene (MP). The simulated responses are compared to a set of GCM outputs from Paleoclimate Model Intercomparison Project Phase 4 (PMIP4) to assess the added value of a high resolution and model consistency across different time periods. Results show WAM magnitudes and pattern changes that are consistent with PMIP4 models and proxy reconstructions. ECHAM5‐wiso estimates the highest WAM intensification in the MH, with a precipitation increase of up to 150 mm/month reaching 25°N during the monsoon season. The WAM intensification in the MP estimated by ECHAM5‐wiso (up to 80 mm/month) aligns with the mid‐range of the PMIP4 estimates, while the LGM dryness magnitude matches most of the models. Despite an enhanced hydrological cycle in MP, MH simulations indicate a ∼50% precipitation increase and a greater northward extent of WAM than the MP simulations. Strengthened conditions of the WAM in the MH and MP result from a pronounced meridional temperature gradient driving low‐level westerly, Sahel‐Sahara vegetation expansion, and a northward shift of the Africa Easterly Jet. The simulated δ18Op values patterns and their relationship with temperature and precipitation are non‐stationarity over time, emphasizing the implications of assuming stationarity in proxy reconstruction transfer functions.

Impacts of Greenland Ice Sheet on Blocking in Idealized Simulations

Abstract As the only remaining ice sheet in the Northern Hemisphere, the Greenland ice sheet (GrIS) plays a crucial role in influencing atmospheric circulations, particularly with its rapid melting under global warming. In this paper, the influences of GrIS topography and surface thermal conditions are investigated by a series of aquaplanet experiments. The results show that the GrIS topography induces stationary waves and favors more blocking events through the generation of negative potential vorticity (PV) anomalies, while it tends to suppress local storm activities through the induced stationary waves. The surface cooling center of the GrIS is found to strengthen the jet streams by enhancing the meridional temperature gradient and thermal wind, while it causes the PV and static stability to increase during near-Greenland blocking days, thereby disfavoring blocking onset. Altogether, the topography and surface thermal effects of GrIS appear to compete with each other so that the net effect would determine the final response. Nevertheless, nonlinearity is found in both GrIS-topography alone and GrIS-surface temperature alone experiments, where nonlinear responses of atmospheric circulation are detected when the GrIS topography height or surface temperature exceeds their critical values, respectively. Hence, through this study, the response of the blocking in the vicinity of Greenland to the combined effects of topography and surface thermal conditions may shed light on comprehending the underlying mechanism of blocking aleration in a changing climate.

Arctic amplification‐induced intensification of planetary wave modulational instability: A simplified theory of enhanced large‐scale waviness

AbstractIn the mid–high latitude atmosphere, the instability of planetary waves characterizes enhanced planetary wave activity or amplified large‐scale waviness leading to increased regional weather extremes. In this paper, a nonlinear Schrödinger equation is derived to describe the evolution of planetary waves. Then the consequences of Arctic amplification (AA)‐induced meridional background potential vorticity (PVy) changes on the modulational instability of planetary waves are examined. It is found that the modulational instability of uniform planetary wave trains mainly results from the presence of high‐order dispersion and nonlinearity, even though such an instability depends on the amplitude, vertical structure and zonal wavenumber of uniform planetary waves and the atmospheric stratification. Because the nonlinearity and high‐order dispersion depend on the magnitude of PVy, the modulational instability of planetary waves is significantly influenced by the variation of PVy associated with AA. It is also revealed that stronger modulational instability of planetary waves tends to occur in the smaller PVy region or in higher latitudes due to both stronger nonlinearity and weaker high‐order dispersion for fixed background and planetary wave parameters, which is conducive to more intense large‐scale waviness. However, because AA can reduce PVy in the mid–high latitudes mainly in the lower troposphere via reductions of winter zonal winds and meridional temperature gradients, the reduced PVy under AA can significantly enhance the modulational instability. Thus, the role of AA is to amplify planetary wave activity in mid–high latitudes through strengthening the modulational instability of planetary waves due to reduced PVy, which further enhances large‐scale waviness.

El Niño–Southern Oscillation-Independent Regulation of Western North Pacific Tropical Cyclone Genesis

El Niño–Southern Oscillation-Independent Regulation of Western North Pacific Tropical Cyclone Genesis

A moist‐thermal quasi‐geostrophic model for monsoon depressions

AbstractMonsoon depressions (MDs) are synoptic‐scale storms that occur during the summer phase of the global monsoon cycle and whose dynamical mechanisms remain incompletely understood. To gain insight into the dynamics governing the large‐scale structure of MDs, we formulate an idealised moist‐thermal quasi‐geostrophic model that includes distinct thermal and moisture fields in simple forms. A linear‐stability analysis of the model, with basic states corresponding to typical monsoon conditions, shows three distinct mode classifications: thermal‐Rossby modes, heavy precipitating modes, and a moist‐thermal mode. In the linearised model, the presence of a background precipitation gradient strengthens thermal‐Rossby modes by coupling the dynamics to latent heating. The separation of heavy precipitating modes from fast‐propagating thermal‐Rossby modes is further examined with numerical experiments of large‐amplitude MDs. Wind‐induced evaporation is found to amplify large‐amplitude MDs in conditions analogous to those over the northern Bay of Bengal. An energetic analysis shows the pathways by which the MDs derive energy from the background state. A further series of experiments through a continuum of meridional temperature gradients demonstrates the sensitivity of large‐scale MD dynamics to the background state and suggests a possible mechanism to explain variations in the propagation direction of MDs.

Unraveling weak and short South Asian wet season in the Early Eocene warmth

AbstractThe timing and causative factors underlying the Asian summer monsoon initiation remain contentious as recent proxy data and modeling studies suggested the existence of a wet-dry monsoon-like climate from the Cretaceous period. Leveraging an ensemble of deep-time climate simulations focused on the early Eocene (DeepMIP-Eocene), we show that the early Eocene Asian wet season was notably weaker and shorter than present-day in the absence of an elevated heat source like the Tibetan Plateau. The deficient upper-tropospheric meridional temperature gradient was insufficient to propel the seasonal northward migration of the rainband over South Asia. The weaker cross-equatorial moisture flow encountered obstruction of Gangdese mountain along the southern edge of Asia, leading to significant South Asian aridity. The enhanced greenhouse effect was inadequate to augment the seasonal circulation and rainfall variability to current levels. The altered wet and dry seasonality across South Asia during the early Eocene does not meet the criteria to be classified as a monsoon, suggesting that South Asian monsoon initiation is likely contingent upon a specific level of Tibetan Plateau upliftment.

A Potential Seasonal Predictor for Summer Rainfall over Eastern China: Spring Eurasian Snowmelt

Abstract The hydrological effect of snow over Eurasia is important for regulating regional and global climate through affecting land–atmosphere energy exchange. Based on observational and reanalysis datasets, this study investigates the effect of spring Eurasian snowmelt on the following summer rainfall over eastern China during the period of 1979–2018. The results show that a substantial meridional dipole pattern of summer rainfall anomalies over eastern China is closely associated with the preceding spring snowmelt anomalies over Eurasia, especially over remote Siberia. Excessive snowmelt anomalies over Siberia in spring could result in a wetter local soil condition from spring until the following summer, thereby increasing latent heat fluxes and reducing local surface temperature, and vice versa. Then, the anomalous summer surface cooling over Siberia increases the meridional gradient of temperature between the Eurasian midlatitudes and high latitudes, which intensifies the Eurasian atmospheric baroclinicity and motivates the eddy‐induced geopotential height responses along with the significant wave propagations spreading from the Eurasian high latitudes to Lake Baikal. As a result, excessive spring snowmelt anomalies over Siberia tend to be accompanied with an anomalous anticyclone circulation to the east of Lake Baikal and an anomalous cyclonic circulation over southeastern China in the following summer. This could lead to a meridional dipole pattern of summer rainfall anomalies over eastern China, with deficient rainfall over northern China and slightly excessive rainfall over southern China. The present findings highlight the lagged effect of spring Eurasian snowmelt on summer climate over eastern China, with implications for the regional seasonal climate prediction. Significance Statement Frequent summer droughts and floods over eastern China cause serious damage to the regional economy and society. This study reveals an important effect of spring Eurasian snowmelt anomalies on the following summer rainfall anomalies over eastern China. Through affecting the local land–atmosphere energy exchange and regulating the remote atmospheric circulation, excessive spring Eurasian snowmelt anomalies could induce a substantial meridional dipole pattern of summer rainfall anomalies over eastern China, with deficient rainfall over northern China and excessive rainfall over southern China, and vice versa. Therefore, this work identifies spring Eurasian snowmelt as a potential seasonal predictor for summer rainfall over eastern China, which is home to more than a billion people. This is important for the local livelihood, including agriculture, water resources, ecosystems, human health, and economies.

Changes in Northern Hemisphere extra-tropical cyclone frequency following volcanic eruptions

Explosive volcanic eruptions are well known to influence Earth’s temperature. Changes in Earth’s temperature can affect temperature gradients which in turn could affect the isentropic slope and hence Northern Hemisphere high and mid-latitude weather. Yet, the possible influence of volcanic eruptions on these atmospheric circulation patterns and the potential spatial extent are not well understood. To address this issue, we pursue two independent lines of evidence. Firstly, we simulate volcanic eruptions with the MPI-ESM1.2 Earth System Model and use the TRACK algorithm to explore how extra-tropical cyclone (ETC) frequency is affected in the model experiments. Secondly, we query the Greenland ice core NEEM-2011-S1 for indications of increased Northern Hemisphere ETC frequency correlating with evidence for explosive volcanism by comparing the storm proxies sodium and calcium; with the eruption proxy sulphur. Both the model and proxy evidence suggest that large explosive volcanic eruptions increase storminess around the location of the ice core. Furthermore, the simulations indicate that the number of ETCs increases in the subtropics and at high latitudes, while they decrease in the mid-latitudes. A detailed interrogation of the simulated eruptions reveals that increases in cyclone frequency are linked to steepening of the isentropic slope due to a larger meridional temperature gradient and to a lower tropopause. The steepening is driven by a combination of warming of the tropical stratosphere from absorption of longwave radiation by volcanic aerosols and surface cooling due to the scattering of sunlight by the same aerosols, whereas the lower tropopause may be attributed to a warmer stratosphere.

The Pacific-North American teleconnection in the Cenozoic

The Pacific-North American teleconnection (PNA) is one of the most important atmospheric modes in the Northern Hemisphere in modern climate. Through the PNA teleconnection, climate variations in the tropical Pacific have significant impacts on North America. However, whether there existed the PNA-like mode in the deep-time climate and how it evolved remain unknown. Here, we study the evolution of the PNA in the past 70 million years, using simulation results with the fully coupled Community Earth System Model version 1.2.2 (CESM1.2.2). Our simulation results show that there existed a PNA-like mode (referred to as PNA in this study) in the Cenozoic, but with different strengths and pathways. The strengths of the PNA were weaker over 70–40 million years ago (Ma) than that of the present, while it is comparable to the present over 30–10 Ma. Diagnosis indicates that the weaker strengths and distorted pathways of the PNA before ∼40 Ma are due to weaker meridional temperature gradients in warmer climates, which causes weakened extratropical westerly jet and altered extratropical waveguide of planetary-scale stationary waves. After ∼40 Ma, Earth climate became much colder, entering into the icehouse climate. Thus, the PNA pattern is close to the present state.

Decadal co‐variation of the aerosols over East Asia and the East Asian jet streams in the boreal winter

AbstractThe changes in the East Asian polar‐front jet (EAPJ) and the East Asian subtropical jet (EASJ) profoundly impact the weather and climate in East Asia. However, the link between aerosols and the jet streams is still unclear. Here, we investigated the decadal co‐variation of aerosols and the East Asian jet streams in the boreal winter during the period of 1980–2019. In synch with a positive change in aerosol optical depth over East Asia, 300‐hPa winds show an equatorward shift of the land branch of the EASJ, and weakened EAPJ and oceanic branch of the EASJ. This can be linked to the enhanced meridional temperature gradient along 30°–50° N but weakened in northern regions and the decreasing synoptic‐scale transient eddy kinetic energy over subtropical Pacific. Relative importance estimation of aerosols and ocean signals emphasized the contributions of aerosols in jet variations. In turn, meteorological conditions related to jet streams also contribute to variations in aerosols, the decadal co‐variations are a result of their interactions, particularly for the oceanic branch of EASJ. The findings would be helpful in providing potential indicators of climate change.

Summer Russian Heat Waves Linked to Arctic Sea Ice Anomalies in 2010 and 2016

Abstract This study investigates dominant features of the atmospheric circulation evolution associated with extreme heat waves (HWs) in Russia during the summers of 2010 and 2016, respectively, and their possible association with Arctic sea ice loss. Results show that a region of Russia (20°–70°E, 45°–65°N) experienced a lasting 44-day HW event from 4 July to 16 August 2010 and a 26-day HW event from 2 to 27 August 2016. The associated atmospheric circulation anomalies are characterized by the summer Arctic cold anomaly in the mid- to low troposphere and an anticyclonic circulation anomaly over the Ural Mountains. Simulation experiments forced by summer Arctic sea ice anomalies reproduce the major characteristics of observational associations. Observations and numerical simulations indicate that summer Arctic sea ice anomaly is conducive to the formation of the summer Arctic cold anomaly, which is often accompanied by the enhanced baroclinicity in most of the Arctic troposphere and increased and decreased meridional temperature gradient in the high and midlatitudes, respectively. Such a configuration strengthens westerly winds over most of the Arctic and weakens zonal westerly over the southern Ural Mountains. This anomalous zonal wind pattern establishes the background conditions for the sustained positive geopotential height anomaly in the mid- to low troposphere that dynamically facilitates the prevalence of Russian HW events. Moreover, when compared with 2016, the weaker meridional potential vorticity gradient anomaly in the summer of 2010 prolonged the persistence of Ural blocking, which may lead to longer HW events in Russia.

Roles of Thermal Forced and Eddy‐Driven Effects in the Northward Shifting of the Subtropical Westerly Jet Under Recent Climate Change

AbstractThe zonal‐mean subtropical westerly jet (SWJ) in boreal winter shows a significant northward shift trend under recent climate change. Previous studies proposed thermal forcing—represented by the thermal wind associated with the temperature gradient—and the driving effect of the eddy momentum flux (EMF) convergence that leads to the eddy‐driven jet as explanations for this process; however, their relative importance in influencing the SWJ shift requires further investigation. In this study, we examined the roles of thermal forced and eddy‐driven jet components in the zonal‐mean northward SWJ shift. We also investigated the role of Hadley circulation because its poleward boundary is related to the zonal‐mean SWJ. Results suggest that thermal forced component plays a major role, while EMF‐driven component plays a secondary role. Specifically, the subtropical warming, which is primarily influenced by enhanced adiabatic downward motion of the Hadley circulation, increases the meridional temperature gradient and the associated thermal wind poleward of the SWJ. It also reduces atmospheric static stability aloft and converges the EMF on the poleward side of the climatological SWJ. Enhanced meridional temperature gradient and EMF convergence on the poleward side push the SWJ northward. Results from further mathematical analysis indicate that thermal forced and eddy‐driven zonal wind components account for 72% and 28% of the shift distance, respectively. In addition to elucidating the relative importance of thermal and EMF forcings, this study emphasizes the critical role of the subtropical warming driven by the intensified local descending branch of the Hadley circulation in shifting the zonal‐mean SWJ.

Westward shift of tropical cyclogenesis over the southern Indian Ocean

Tropical cyclones (TCs), commonly called cyclones in the southern Indian Ocean (SIO), represent one of the most devastating disasters in the oceanfront regions of Africa. The present study explores the long-term tendency of annual mean TC genesis location in the SIO. A notable westward shift is detected in the SIO TC genesis longitude since 1979, which is linked to an increase in the TC genesis frequency in the southwestern SIO and a decrease in the TC genesis frequency in the northeastern SIO. The dipole trend pattern of the TC genesis frequency in the SIO is intimately linked to the weakening of the westerly vertical wind shear over the western SIO and the strengthening of the easterly vertical wind shear over the eastern SIO, resulting from a reduced meridional temperature gradient. The weakened meridional temperature gradient is attributed to the enhanced warming of the subtropical troposphere that is a response of atmospheric temperature to global warming. Our study implies a potential increase in the risks faced by coastal and island countries in eastern Africa.

Underlying physical mechanisms of winter precipitation extremes over India's high mountain region

AbstractExtreme precipitation events (EPEs) are among the most pervasive weather hazards in the western Himalayan region (WHR), posing widespread damage to life, infrastructure, and agriculture. This study investigates the synoptic and large‐scale characteristics linked to winter precipitation extremes over the WHR. EPEs are identified as events surpassing the 95th percentile threshold. A composite analysis is employed using two reanalyses—the fifth‐generation European Centre for Medium‐Range Weather Forecasts Reanalysis (ERA5) and the Indian Monsoon Data Assimilation and Analysis (IMDAA)—to elucidate the synoptic conditions conducive to EPEs. Our findings suggest that EPEs in the WHR are linked to an intensified subtropical westerly jet, characteristically shifted to south than normal. Enhanced kinetic energy in the upper troposphere, attributed to increased baroclinic instability, reinforces moisture convergence and strengthens synoptic‐scale circulation, triggering deep convection and supporting EPEs. Notably, the interplay of pronounced Rossby waves sinking over the WHR and regional orography significantly modulates the intensity of western disturbances (WDs). Employing clustering analysis, we observed that the strongest EPEs are linked to anomalous vorticity in the upper to middle troposphere, together with deep convection via strengthened WDs, suggesting the potential role of large‐scale influences. Using Lagrangian method, we identify that the Arabian Sea is the primary moisture source for EPEs in the WHR. We further delved into the role of large‐scale connections and EPEs through quasi‐resonant amplification (QRA) analysis. The findings unveil distinct QRA fingerprints in meridional temperature gradients along with notably magnified, quasi‐stationary midlatitude planetary waves characterized by zonal wave numbers 6/7/8 (baroclinic waves) contributing to EPEs. Overall, our results highlight the underlying physical mechanisms for winter precipitation extremes, emphasizing QRA's role in amplifying planetary waves and promoting EPEs, underscoring the WHR's vulnerability to evolving climatic conditions.

Orbital Controls on North Pacific Dust Flux During the Late Quaternary

AbstractAirborne mineral dust is sensitive to climatic changes, but its response to orbital forcing is still not fully understood. Here, we present a reconstruction of dust input to the Subarctic Pacific Ocean covering the past 190 kyr. The dust composition record is indicative of source moisture conditions, which were dominated by precessional variations. In contrast, the dust flux record is dominated by obliquity variations and displays an out‐of‐phase relationship with a dust record from the mid‐latitude North Pacific Ocean. Climate model simulations suggest precession likely drove changes in the aridity and extent of dust source regions. Additionally, the obliquity variations in dust flux can be explained by meridional shifts in the North Pacific westerly jet, driven by changes in the meridional atmospheric temperature gradient. Overall, our findings suggest that North Pacific dust input was primarily modulated by orbital‐controlled source aridity and the strength and position of the westerly winds.

Atmospheric responses to the interannual variability of sea surface temperature front in the summertime Southern Ocean

Air–sea interactions in mid-latitudes and their climatic effects have long been a research focus. However, the influence of the variability of the Southern Oceanic Front (SOF) on atmospheric processes at interannual timescales remains somewhat ambiguous from existing studies. Using reanalysis data, our findings reveal that the SOF reaches its maximum intensity during the austral summer, characterized by pronounced interannual variability and an insignificant trend. On the one hand, an enhanced SOF intensifies the meridional temperature gradient and atmospheric baroclinicity, accompanied by increased local and downstream baroclinic energy conversion. This amplifies storm track activities in both the lower and upper troposphere. On the other hand, the atmospheric circulation in mid- and high-latitudes exhibits an equivalent barotropic response. This is attributed to the feedback of storm tracks on the mean flow, dominated by the transient eddy vorticity forcing. Moreover, we compare the relative contributions of the South Indian Oceanic Front (SIOF) and South Atlantic Oceanic Front (SAOF) variability to storm track and atmospheric circulation. Results indicate that the SIOF variability dominates the downstream development of storm track response and modulates the anomalous atmospheric circulation around the Antarctic, while the SAOF variability produces only a limited local atmospheric response.

A Theory for Self-Sustained Multicentennial Oscillation of the Atlantic Meridional Overturning Circulation. Part II: Role of Temperature

Abstract In the first part of our research on self-sustained multicentennial oscillation of the Atlantic meridional overturning circulation (AMOC), we utilized a hemispheric box model considering only the salinity equations. In this study, we consider both thermal and saline processes in the box model to investigate the AMOC multicentennial oscillation and the role of temperature. The thermal processes have mainly two effects, shortening the oscillation period and stabilizing the system, which are caused by the fast surface temperature restoration and negative feedback between temperature advection and AMOC, respectively. Introducing nonlinearity into the system can lead to self-sustained AMOC oscillation that is controlled by ocean internal dynamics, whose mechanism is generalized as a growing oscillation restrained by nonlinearity. The nonlinearity can arise from subpolar vertical mixing, or a nonlinear relation between the AMOC anomaly and the meridional difference of density anomaly. Linear stability analyses reveal that the eigenmode of the system is sensitive to model parameters, including model geometry, mean strength of the AMOC, and the AMOC’s sensitivity to density perturbation, surface virtual salt flux, and meridional temperature gradient. A larger surface virtual salt flux enhances the positive salinity advection feedback, and a smaller meridional temperature gradient weakens the negative temperature advection feedback. Both processes destabilize the AMOC multicentennial oscillation. Such situations may be expected in the future due to more intense warming and freshwater hosing at the high latitudes of the Northern Hemisphere.

Asian Anthropogenic Aerosol Forcing Played a Key Role in the Multidecadal Increase in Australian Summer Monsoon Rainfall

Abstract Observations show a significant increase in Australian summer monsoon (AUSM) rainfall since the mid-twentieth century. Yet the drivers of this trend, including the role of anthropogenic aerosols, remain uncertain. We addressed this knowledge gap using historical simulations from a suite of Coupled Model Intercomparison Project phase 6 (CMIP6) models, the CESM2 Large Ensemble, and idealized single-forcing simulations from the Precipitation Driver Response Model Intercomparison Project (PDRMIP). Our results suggest that Asian anthropogenic aerosol emissions played a key role in the observed increase in AUSM rainfall from 1930 to 2014, alongside the influence of internal variability. Sulfate aerosol emissions over Asia led to regional surface cooling and strengthening of the climatological Siberian high over eastern China, which altered the meridional temperature and sea level pressure gradients across the Indian Ocean. This caused an intensification and southward shift of the Australian monsoonal westerlies (and the local Hadley cell) and resulted in a precipitation increase over northern Australia. Conversely, the influence of increased greenhouse gas concentrations on AUSM rainfall was minimal due to the compensation between thermodynamically induced wettening and transient eddy-induced drying trends. At a larger scale, aerosol and greenhouse gas forcing played a key role in the climate response over the Indo-Pacific sector and eastern equatorial Pacific, respectively (coined the “tropical Pacific east–west divide”). These findings contribute to an improved understanding of the drivers of the multidecadal trend in AUSM rainfall and highlight the need to reduce uncertainties in future projections under different aerosol emission trajectories, which is particularly important for northern Australia’s agriculture. Significance Statement Australian summer monsoon (AUSM) rainfall plays a vital role in sustaining northern Australia’s unique biodiversity and extensive agricultural industry. While observations show a significant increase in AUSM rainfall since the mid-twentieth century, the causes remain uncertain. We find that anthropogenic aerosol emissions from Asia played a key role in driving this multidecadal AUSM rainfall trend by inducing dynamic adjustments over the Indo-Pacific sector. These findings highlight the need to consider different aerosol emission trajectories when assessing future projections of AUSM rainfall.