Walker circulation response to extratropical radiative forcing

Walker circulation variability and associated zonal shifts in the heating of the tropical atmosphere have far-reaching global impacts well into high latitudes. Yet the reversed high latitude-to-Walker circulation teleconnection is not fully understood. Here, we reveal the dynamical pathways of this teleconnection across different components of the climate system using a hierarchy of climate model simulations. In the fully coupled system with ocean circulation adjustments, the Walker circulation strengthens in response to extratropical radiative cooling of either hemisphere, associated with the upwelling of colder subsurface water in the eastern equatorial Pacific. By contrast, in the absence of ocean circulation adjustments, the Walker circulation response is sensitive to the forcing hemisphere, due to the blocking effect of the northward-displaced climatological intertropical convergence zone and shortwave cloud radiative effects. Our study implies that energy biases in the extratropics can cause pronounced changes of tropical climate patterns

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Department of Urban and Environmental Engineering (Environmental Science and Engineering)Significant improvement in our understanding of extratropical impacts on remote climate has been made with a flurry of evidence in paleoclimate proxies and modeling experiments. Although the theoretical framework successfully provides insight and explanation, our understanding by which interhemispheric energy imbalance constrains zonal-mean tropical precipitation is mostly developed under the steady-state condition. Here, the temporal evolution of the extratropics-forced tropical response was examined by employing an idealized general circulation model. It is found that tropical precipitation depends profoundly on the time scale of extratropical sources: unaltered by sufficiently high-frequency extratropical forcing. The transient energetic analysis is presented, showing that sensitivity to the extratropical forcing periodicity arises from the critical time required for sea surface temperature (SST) propagation toward the equator. The extratropics-to-tropics teleconnection preferentially occurs through the lower tropospheric layer. Transient eddies to diffuse moist static energy perturb the midlatitude SSTs outside the forcing region. The transient eddies weaken in the subtropics, and a further equatorward advection is accomplished by the Hadley circulation. The essential role of each dynamics is well underpinned by theoretical models. It is of the question whether the impact of extratropical thermal forcing in one hemisphere would extend far into high-latitudes of the other hemisphere. A simple sequential mechanism for tropics-to-pole teleconnection in a zonal-mean standpoint is presented via momentum balance. The extratropics-to-tropics teleconnection is expanded into a global scale, a pole-to-pole teleconnection. Our results highlight the important role of decadal-and-longer extratropical climate variability in shaping the tropical climate system and further opposite hemisphere. This potentially suggests an integrated perception for Arctic and Antarctic climate. As fundamental mechanisms that link extratropical and tropical variability are presented, the results will provide a guide to understanding teleconnection process in our planet including ocean dynamics, land surface and orography. These understanding would help to provide improve potential predictability of global climate system.ope

1.5 Ekman model response to the extratropical thermal forcing

We examine how the Ekman transport modulates the tropical response to hemispherically differential thermal forcing. The 1.5 layer ocean model is implemented to a gray atmospheric model, with no radiative feedbacks, coupled to an aquaplanet slab ocean. The control climate with no surface flux adjustment is perturbed by southern hemisphere cooling and northern hemisphere warming at the surface. We examine the sensitivity to the forcing amplitude, forcing location, and Ekman flux strength in the control climate. The tropics-to-midlatitude component of Ekman transport, which is a negative feedback that counteracts the prescribed forcing, dominates over the equatorial component, which is a positive feedback that reinforces the prescribed forcing. This leads to a reduction in the atmospheric energy transport, but by less than the Ekman heat transport, because the OLR response is also damped by the negative extratropical feedback. As a result, the damping effect of Ekman flux on the atmospheric energy transport is limited, with the atmospheric energy transport response always dominant over the Ekman heat transport. The results highlight the need to look into the role of deep circulation and subtropical gyres other than the Ekman transport in fully coupled models

Aquaplanet experiments with periodically time-varying Qflux, links to supplementary material

We conduct a suite of two experiments by employing an idealized general circulation model to examine how extratropical thermal forcing confined to one hemisphere affects the other hemisphere. The employed model is based on the Geophysical Fluid Dynamics Laboratory Atmospheric Model 2 (AM2), coupled to an aquaplanet slab ocean model with approximately 50 m depth, and governed under perpetual equinox insolation and without sea-ice. One experiment simulates the impact of thermal forcing confined in one high-latitude, north of 40°N, periodically time-varying Qflux, representing a low-frequency polar variability, called PERI. The high-latitude forced variability is propagated into the tropics and the opposite hemisphere. The distinct climate response in the opposite hemisphere is deniable evidence of pole-to-pole teleconnection via zonal-mean atmospheric dynamics. The other experiment is similarly conducted with PERI, but southern mid-to-high latitude is nudged into the climatology, denoted as FSST. This partially fixed SST experiment only removes the influence on the tropics from the southern extratropics, hence a comprehensive tool to understanding the sequence of pole-to-pole teleconnection captured in PERI. This is not only the ideal result to explore the role of high-latitude low-frequency variability, but also a useful benchmark to examine sequential pathways for high-latitude variability

How Does the High-Latitude Thermal Forcing in One Hemisphere Affect the Other Hemisphere?

Significant progress has been made in our understanding of extratropical impacts on the tropical climate via energetics framework. It is of question whether the impact of extratropical thermal forcing in one hemisphere would extend far into high-latitudes of the other hemisphere. We examine the possibility of the pole-to-pole linkage via atmospheric teleconnections by imposing a cyclic surface thermal forcing in the northern extratropics of an aquaplanet slab ocean model. We reveal a synchronous temperature response between the two poles mediated by zonal-mean atmospheric dynamics. A warming in one polar region leads to a strengthened Hadley circulation of the unforced hemisphere, fluxing more momentum toward the subtropics, thereby pulling the eddy-driven jet equatorward. A consequent anomalous descent over the polar region causes warming. The polar surface warming in the unforced hemisphere reaches 30% of that in the forced hemisphere, inferring a significance of the pole-to-pole connection

Dependence of Arctic climate on the latitudinal position of stationary waves and to high-latitudes surface warming

Previous studies suggest large uncertainties in the stationary wave response under global warming. Here, we investigate how the Arctic climate responds to changes in the latitudinal position of stationary waves, and to high-latitudes surface warming that mimics the effect of Arctic sea ice loss under global warming. To generate stationary waves in an atmospheric model coupled to slab ocean, a series of experiments is performed where the thermal forcing with a zonal wavenumber-2 (with zero zonal-mean) is prescribed at the surface at different latitude bands in the Northern Hemisphere. When the stationary waves are generated in the subtropics, the cooling response dominates over the warming response in the lower troposphere due to cloud radiative effects. Then, the low-level baroclinicity is reduced in the subtropics, which gives rise to a poleward shift of the eddy driven jet, thereby inducing substantial cooling in the northern high latitudes. As the stationary waves are progressively generated at higher latitudes, the zonal-mean climate state gradually becomes more similar to the integration with no stationary waves. These differences in the mean climate affect the Arctic climate response to high-latitudes surface warming. Additional surface heating over the Arctic is imposed to the reference climates in which the stationary waves are located at different latitude bands. When the stationary waves are positioned at lower latitudes, the eddy driven jet is located at higher latitude, closer to the prescribed Arctic heating. As baroclinicity is more effectively perturbed, the jet shifts more equatorward that accompanies a larger reduction in the poleward eddy transport of heat and momentum. A stronger eddy-induced descending motion creates greater warming over the Arctic. Our study calls for a more accurate simulation of the present-day stationary wave pattern to enhance the predictability of the Arctic warming response in a changing climate