Moritz Günther

While extratropical forcing has far‐reaching effects through teleconnections, the Intertropical Convergence Zone (ITCZ) has been shown to partly block the propagation of extratropical perturbations to the opposite hemisphere. However, the influence of the ITCZ location on the tropical response and the blocking effect remain poorly understood. We investigate this using aquaplanet slab‐ocean simulations in MPI‐ESM, by imposing a southern hemispheric extratropical cooling on five control states with different background ITCZ locations. Results show that the cross‐equatorial atmospheric energy transport sensitively depends on the mean ITCZ position, with cloud radiative effects dominating this sensitivity. We also demonstrate that the ITCZ blocks the propagation of sea surface temperature anomalies to the opposite hemisphere through changes in latent heat flux, which originate from the wind response to the imposed extratropical forcing. Our findings suggest that model biases in the ITCZ location may alter the modeled climate response to high‐latitude perturbations.

A robust proportionality between radiative forcing and global-mean temperature has been established for most climate drivers. However, land-surface changes are considered an exception due to their potential to alter surface hydrology, introducing non-radiative effects that may rival those from surface albedo changes. Here, comparing Earth-system-model simulations with large-scale perturbations of surface albedo and water availability, we find that the spatially integrated biophysical climate impact is similarly dominated by radiative processes and scales approximately linearly with the resulting shifts in planetary albedo, implying a ~ 0.4 K surface temperature response per W m ⁻² change in top-of-atmosphere net short-wave radiation. Thus, while non-radiative effects strongly influence local temperatures, their net contribution to the global-mean signal is negligible. The planetary-albedo response is primarily governed by regional cloud-cover characteristics, resulting in a substantially stronger temperature response to land-surface change in mid- and high-latitude regions than in the tropics and subtropics.

The Walker Circulation is commonly understood as driven by oceanic upwelling that cools the eastern Pacific and establishes the equatorial zonal sea-surface temperature gradient. Here we test this canonical view using an atmospheric general circulation model coupled to a slab ocean with prescribed ocean heat uptake. Contrary to this view, removing the zonal asymmetry of ocean heat uptake weakens the Walker Circulation by only 10%. In contrast, removing American land or flattening American orography strongly weakens the Walker circulation. American land and orography seed its eastern, subsiding branch by anchoring subtropical highs, promoting equatorward cold advection, and cooling the eastern Pacific. Equatorial water-vapor and cloud radiative feedbacks amplify this land-forced asymmetry. Our results identify land as a fundamental driver of the Walker Circulation and reframe it as an atmosphere-ocean-land system rather than an air-sea phenomenon alone.

The unresolved recent multidecadal cooling of the eastern Pacific and its uncertain future remain key puzzles in climate dynamics. We propose the land-sea heating contrast as a potential driver of this phenomenon. To test this, we amplify the land-sea contrast by quadrupling carbon dioxide (CO ₂ ) only over land in a global coupled climate model. We show that this causes a pronounced transient cooling of the eastern and equatorial Pacific. A transient 1%-per-year CO ₂ increase over land produces decadal cooling, suggesting relevance to the observed eastern Pacific cooling. Targeted simulations with locally increased CO ₂ concentrations over different land regions reveal three drivers of this cooling: a northward intertropical convergence zone shift, a westward convection shift over the western Pacific, and strengthened subtropical highs from Rossby waves. These insights recast the commonly used ocean dynamical thermostat mechanism as a feedback rather than a driver of transient cooling and contribute to understanding and predicting Pacific temperature patterns.

The land–ocean warming contrast—where global land mean warms more than the global ocean mean —is one of the most prominent features of global warming. In idealized CO₂-increase experiments, although there is inter-model spread, it is known that land warming is typically about 1.6 times larger than ocean warming. However, it remains unclear what determines the magnitude of this land–ocean warming contrast. For the 1980–2014 trend, observed land warming exceeds ocean warming by more than a factor of 2.3, and such a large observed land–ocean warming contrast cannot be reproduced by CMIP6 historical simulations. It is also well known that during this period there is a mismatch in the SST trend pattern between observations and historical simulations. In this study, we investigated how differences in sea surface temperature (SST) patterns affect the magnitude of land–ocean warming contrast. Using the climate model MPI-ESM, we conducted AGCM experiments forced by (i) a globally uniform SST warming (+2 K, +4 K, and +6 K), (ii) the same global-mean SST warming superimposed with the observed 1980–2014 SST trend pattern, and (iii) the same global-mean SST warming with the sign of the 1980–2014 SST trend pattern reversed. The results show that, despite having the same global-mean SST warming, land warming differs significantly among the SST patterns, and that the observed SST trend pattern tends to enhance the global-mean land warming. Under the observed SST pattern experiments, warming tends to be amplified across the entire Eurasian continent, which is a major contributor to the enhanced global-mean land warming. The strong warming over the mid-to-high-latitude Eurasian continent is explained primarily by pronounced Atlantic warming and warming in the northwestern Pacific, whereas the cooling tendency in the eastern equatorial Pacific affects the land-warming pattern over the North America through teleconnections. This study demonstrates that SST patterns exert a substantial influence on the factors controlling the magnitude of the land–ocean warming contrast, and suggests that the coupling between ocean and land temperature changes varies markedly depending on the future SST pattern change.

Stratospheric sulfate aerosols, produced by explosive volcanic eruptions or through artificial stratospheric aerosol injection for solar radiation management, can perturb Earth’s radiative budget for several years. However, the understanding of the state dependence of aerosol forcing and its effect on radiative feedback remains incomplete.We use a one-dimensional radiative–convective equilibrium model of the tropical atmosphere to quantify the clear-sky forcing and feedback contributions from aerosol absorbing and re-emitting longwave radiation, stratospheric heating, and enhanced stratospheric water vapor.We show that aerosol forcing exhibits a stronger surface temperature dependence than CO2 forcing. Between 280 and 300 K, aerosol forcing becomes less negative with increasing surface temperature because its longwave component becomes increasingly positive. Additionally, the radiative feedback is less negative in the presence of the aerosol. Both the feedback’s dependence on aerosol concentration and the forcing’s dependence on temperature arise from aerosol absorption in optically thin spectral regions, which masks temperature-dependent surface emission.This highlights the critical role of the spectral nature of aerosol longwave absorption in determining the surface temperature dependence of stratospheric sulfate forcing and in weakening radiative feedbacks compared to an atmosphere without stratospheric aerosol.

The Walker circulation's rising branch is located over the warm water in the Western Pacific Warm Pool, and the air sinks over the cold Eastern Pacific. It is usually taken for granted that the Walker circulation exists because the dynamic ocean induces this SST gradient: efficient dynamical cooling by upwelling water keeps the SST cold in the East, while the warm water piles up in the West.Here, we revisit this paradigm and offer a new perspective on the origin of the Walker circulation. We show that a Walker circulation arises even in climate model simulations with a zonally symmetric slab ocean where there is no oceanically forced zonal temperature gradient. Instead, a zonally asymmetric land distribution is sufficient to elicit a realistic Walker circulation. We find that the presence of South America alone can cause an atmospheric heating profile which forces a pan-tropical wave response leading to a Walker circulation. Rather than the oceanically induced SST gradient, we emphasize the importance of cold/dry advection from the subtropical anticyclones for explaining the climatological existence of the Walker circulation. Furthermore, we demonstrate the importance of water vapor and cloud feedbacks in amplifying the perturbation that creates the Walker circulation.Our results show that the canonical coupled air-sea framework of the Walker circulation is incomplete, and that land-driven atmospheric teleconnections play a fundamental role in setting up the climatological Walker circulation.

Extratropical forcing can generate strong non-local responses via teleconnections. For example, the Hadley circulation responds to high-latitude forcing by shifting its ascending branch, the intertropical convergence zone (ITCZ), towards the warmer hemisphere. However, the ITCZ location has been shown to modulate the inter-hemispheric communication of an extratropical surface anomaly through the ITCZ blocking mechanism.In this study, we investigate how the climatological ITCZ position affects the climate response to extratropical forcing. We conduct aquaplanet slab ocean simulations in MPI-ESM by imposing a southern hemispheric extratropical cooling of 50 Wm-2 to five control states, each differing in the ITCZ location. Results show that the Hadley cell response and consequent ITCZ northward shift are the largest when the climatological ITCZ is in the same hemisphere as the forcing. Both responses progressively weaken as the climatological ITCZ is displaced northward.The amplitude and progressive weakening of the atmospheric response are shaped by the cloud radiative effect (CRE). If the ITCZ lies in the forced hemisphere, extratropical low-cloud formation enhances the imposed cooling locally, thus increasing the atmospheric compensation for the energetic imbalance. However, when the ITCZ is in the opposite hemisphere, a weak but positive low-cloud anomaly extending equatorward from the forced extratropics results in a dampened atmospheric compensation.Locking the clouds mutes the atmospheric response, further highlighting the role of cloud feedbacks for ITCZ shifts.Furthermore, we show that the negative sea surface temperature (SST) anomaly originating in the forced extratropics does not extend substancially beyond the ITCZ, reinforcing the idea that the ITCZ location limits the propagation of surface signals. We propose that changes in latent heat fluxes tied to the surface-wind response to the forcing are at the core of the ITCZ blocking mechanism, as an anomalous increase (decrease) in wind speed southward (northward) of the new ITCZ location leads to an enhancement (reduction) of the negative SST anomaly.Our findings reveal that the ITCZ location and blocking effect strongly modulate extratropical-tropical interactions, implying that model biases in the ITCZ location might produce inaccurate responses to high-latitude forcing.

Stratospheric sulfate aerosols from tropical volcanic eruptions alter surface air temperatures. Fundamentally, this is due to two different properties of the aerosols: Their ability to reflect incoming solar shortwave radiation, and their ability to absorb terrestrial longwave radiation. However, the relatedness and dynamical consequences of the two properties are unclear. Here we show that in a state‐of‐the art climate model the two properties have independent and fundamentally different impacts on zonal asymmetries of Northern Hemisphere extra‐tropical winter temperatures: The shortwave properties induce a tropospheric Rossby wave train apparently connected to changes in tropical convection. The longwave properties imprint a wavenumber 1‐like signal on surface temperatures related to altered stratospheric winds. The independence of these responses provides a useful starting point for analyzing the extra‐tropical temperature response to volcanic eruptions in both observations and models.