Arctic warming under increased CO2 peaks in winter, but is influenced by summer forcing via seasonal ocean heat storage. Yet changes in atmospheric heat transport into the Arctic have mainly been investigated in the annual mean or winter, with limited focus on other seasons. We investigate the full seasonal cycle of poleward heat transport modelled with increased CO2 or with individually applied Arctic sea-ice loss and global sea-surface warming. We find that a winter reduction in dry heat transport is driven by Arctic sea-ice loss and warming, while a summer increase in moist heat transport is driven by sub-Arctic warming and moistening. Intermodel spread in Arctic warming controls spread in seasonal poleward heat transport. These seasonal changes and their intermodel spread are well-captured by down-gradient diffusive heat transport. While changes in moist and dry heat transport compensate in the annual-mean, their opposite seasonality may support non-compensating effects on Arctic warming.

A document by Pradeebane VAITTINADA''AYAR. Click on the document to view its contents.

Canonical understanding based on general circulation models (GCMs) is that the atmospheric circulation response to midlatitude sea-surface temperature (SST) anomalies is weak compared to the larger influence of tropical SST anomalies. However, the horizontal resolution of modern GCMs, ranging from roughly 300 km to 25 km, is too coarse to fully resolve mesoscale atmospheric processes such as weather fronts. Here, we investigate the large-scale atmospheric circulation response to idealized Gulf Stream SST anomalies in Community Atmosphere Model (CAM6) simulations with 14-km regional grid refinement over the North Atlantic, and compare it to the response in simulations with 28-km regional refinement and uniform 111-km resolution. The highest resolution simulations show a large positive response of the wintertime North Atlantic Oscillation (NAO) to positive SST anomalies in the Gulf Stream, a 0.8-standard-deviation anomaly in the seasonal-mean NAO for 2°C SST anomalies. The lower-resolution simulations show a weaker response with a different spatial structure. The enhanced large-scale circulation response results from an increase in resolved vertical motions with resolution and an associated increase in the influence of SST anomalies on transient-eddy heat and momentum fluxes in the free troposphere. In response to positive SST anomalies, these processes lead to a stronger North Atlantic jet that varies less in latitude, as is characteristic of positive NAO anomalies. Our results suggest that the atmosphere responds differently to midlatitude SST anomalies in higher-resolution models and that regional refinement in key regions offers a potential pathway to improve multi-year regional climate predictions based on midlatitude SSTs.

The lapse-rate feedback is the dominant driver of stronger warming in the Arctic than the Antarctic in simulations with increased CO2. While Antarctic surface elevation has been implicated in promoting a weaker Antarctic lapse-rate feedback, the mechanisms in which elevation impacts the lapse-rate feedback are still unclear. Here we suggest that weaker Antarctic warming under CO2 forcing stems from shallower, less intense climatological inversions due to limited atmospheric heat transport above the ice sheet elevation and elevation-induced katabatic winds. In slab ocean model experiments with flattened Antarctic topography, stronger climatological inversions support a stronger lapse-rate feedback and annual-mean Antarctic warming comparable to the Arctic under CO2 doubling. Unlike the Arctic, seasonality in warming over flat Antarctica is mainly driven by a negative shortwave cloud feedback which exclusively dampens summer warming, with a smaller contribution from the winter-enhanced lapse-rate feedback.

Earth’s tropical and subtropical rainbands, such as Intertropical Convergence Zones (ITCZs) and monsoons, are complex systems, governed by both large-scale constraints on the atmospheric general circulation and regional interactions with continents and orography, and coupled to the ocean. Monsoons have historically been considered as regional large-scale sea breeze circulations, driven by land-sea contrast. More recently, a perspective has emerged of a Global Monsoon, a global-scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation. This results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone. Regional subsystems are embedded in this global monsoon, localized by surface boundary conditions. Parallel with this, much theoretical progress has been made on the fundamental dynamics of the seasonal Hadley cells and convergence zones via the use of hierarchical modeling approaches, including aquaplanets. Here we review the theoretical progress made, and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings. After summarizing the dynamical and energetic balances that distinguish an ITCZ from a monsoon, we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets. Limitations of current theories are discussed, including the need for a better understanding of the influence of zonal asymmetries and transients on the large-scale tropical circulation.

This study assesses the effective climate sensitivity (EffCS) and transient climate response (TCR) derived from global energy budget constraints within historical simulations of 8 CMIP6 global climate models (GCMs). These calculations are enabled by use of the Radiative Forcing Model Intercomparison Project (RFMIP) simulations, which permit accurate quantification of the historical effective radiative forcing. We find that long-term historical energy budget constraints generally underestimate EffCS from CO2 quadrupling and TCR from CO2 ramping, both by 12%, owing to changes in radiative feedbacks and changes in ocean heat uptake efficiency. Atmospheric GCMs forced by observed warming patterns produce lower values of EffCS that are more in line with those inferred from observed historical energy budget constraints. Understanding the discrepancies between modeled and observed historical surface warming patterns remains critical for constraining EffCS and TCR from the historical record.

We evaluate the longitudinal variation in meridional shifts of the tropical rainband in response to natural and anthropogenic forcings using a large suite of coupled climate model simulations. We find that the energetic framework of the zonal mean Hadley cell is generally not useful for characterizing shifts of the rainband at regional scales, regardless of the characteristics of the forcing. Forcings with large hemispheric asymmetry such as extratropical volcanic forcing and meltwater forcing give rise to robust zonal mean shifts of the rainband, however the direction and magnitude of the shift varies strongly as a function of longitude. Even the Pacific rainband doesn't shift uniformly under any forcing considered. Forcings with weak hemispheric asymmetry such as CO and mid-Holocene forcing give rise to zonal mean shifts that are small or absent, but the rainband does shift regionally in coherent ways across models that may have important dynamical consequences.

Join us in an exploration of the climate of Northland, a world where the entire northern hemisphere is covered by a continent, and the entire southern hemisphere is covered by an ocean! On the continent, we will visit the seasonally moist tropics, the subtropical desert, and the Great Northern Swamp. We explore the interplay between water, energy, land, ocean, and atmosphere in this idealized climate model study. We find that the presence of a continent greatly increases the poleward extend of the ITCZ over both the land and ocean hemispheres compared to an aquaplanet, as a result of hemispheric energy imbalances introduced by (a) the small heat capacity of land and (b) large reductions in atmospheric water vapor (and thus reduced longwave trapping) over the continent. A combination of moisture transport from the tropics and local water recycling results in a polar swamp over the continent. We explore how the climate state responds to changes in the albedo and evaporative resistance of the continent. While making the land surface darker leads to warming, we find that decreasing evaporation from the land surface leads to global-scale cooling. This is in contrast to past studies, where reduced terrestrial evaporation leads to warming as a result of suppressed evaporative cooling of the land surface. In the case of Northland, the lack of an ocean to provide water to the northern hemisphere means that decreasing land evaporation leads to large reductions in water vapor over the northern hemisphere, in turn reducing strength of the greenhouse effect, resulting in cooling of near-surface air temperatures. This cooling signal is strongest over the continent, but cools air temperatures over the ocean hemisphere as well. We hypothesize that a threshold exists in the temperature response to reduced terrestrial evaporation: for small decreases in evaporation, reduced latent cooling dominates and near-surface temperatures warm, while for large decreases in evaporation, reduced longwave trapping from reduced atmospheric water vapor dominate, cooling near-surface temperatures. Through this idealized study of a hypothetical, Earth-like planet, we gain valuable insight into the connections between water, energy, land surface properties, and continental distribution in controlling global-scale climate.

Stratospheric volcanic aerosol can have major impacts on global climate. Despite a consensus among studies on an El Niño–like response in the first or second post-eruption year, the mechanisms that trigger a change in the state of El Niño-Southern Oscillation (ENSO) following volcanic eruptions are still debated. Here, we shed light on the processes that govern the ENSO response to tropical volcanic eruptions through a series of sensitivity experiments with an Earth System Model where a uniform stratospheric volcanic aerosol loading is imposed over different parts of the tropics. Three tropical mechanisms are tested: the “ocean dynamical thermostat” (ODT); the cooling of the Maritime Continent; and the cooling of tropical northern Africa (NAFR). We find that the NAFR mechanism plays the largest role, while the ODT mechanism is absent in our simulations as La Niña-like rather than El-Niño-like conditions develop following a uniform radiative forcing over the equatorial Pacific.

Observed surface temperature trends over recent decades are characterized by (i) intensified warming in the Indo-Pacific Warm Pool and slight cooling in the eastern equatorial Pacific, consistent with strengthening of the Walker circulation, and (ii) cooling in the Southern Ocean. In contrast, state-of-the-art coupled climate models generally project Walker circulation weakening, enhanced warming in the eastern equatorial Pacific, and warming in the Southern Ocean. Here we investigate the ability of 16 climate model large ensembles to reproduce observed sea-surface temperature and sea-level pressure trends over 1979-2020 through a combination of externally forced climate change and internal variability. We find large-scale differences between observed and modeled trends that are very unlikely (<5% probability) to occur due to internal variability as represented in models. Disparate trends are found even in regions with weak multi-decadal variability, suggesting that model biases in the transient response to anthropogenic forcing constitute part of the discrepancy.