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Timothy M. Merlis
,
Nicole Feldl
, and
Rodrigo Caballero

Abstract

The midlatitude poleward atmospheric energy transport increases in radiatively forced simulations of warmed climates across a range of models from comprehensive coupled general circulation models (GCMs) to idealized aquaplanet moist GCMs to diffusive moist energy balance models. These increases have been rationalized from two perspectives. The energetic (or radiative) perspective takes the atmospheric energy budget and decomposes energy flux changes (radiative forcing, feedbacks, or surface fluxes) to determine the energy transport changes required by the budget. The diffusive perspective takes the net effect of atmospheric macroturbulence to be a diffusive energy transport down-gradient, so transport changes can arise from changes in mean energy gradients or turbulent diffusivity. Here, we compare these perspectives in idealized moist, gray-radiation GCM simulations over a wide range of climates. The energetic perspective has a dominant role for radiative forcing in this GCM, with cancellation between the temperature feedback components that account for the GCM’s nonmonotonic energy transport changes in response to warming. Comprehensive CMIP5 simulations have similarities in the Northern Hemisphere to the idealized GCM, although a comprehensive GCM over several CO2 doublings has a distinctly different feedback evolution structure. The diffusive perspective requires a non-constant diffusivity to account for the idealized GCM-simulated changes, with important roles for the eddy velocity, dry static stability, and horizontal energy gradients. Beyond diagnostic analysis, GCM-independent a priori theories for components of the temperature feedback are presented that account for changes without knowledge of a perturbed climate state, suggesting that the energetic perspective is the more parsimonious one.

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Nicole Feldl
,
Simona Bordoni
, and
Timothy M. Merlis

Abstract

The response of atmospheric heat transport to anthropogenic warming is determined by the anomalous meridional energy gradient. Feedback analysis offers a characterization of that gradient and hence reveals how uncertainty in physical processes may translate into uncertainty in the circulation response. However, individual feedbacks do not act in isolation. Anomalies associated with one feedback may be compensated by another, as is the case for the positive water vapor and negative lapse rate feedbacks in the tropics. Here a set of idealized experiments are performed in an aquaplanet model to evaluate the coupling between the surface albedo feedback and other feedbacks, including the impact on atmospheric heat transport. In the tropics, the dynamical response manifests as changes in the intensity and structure of the overturning Hadley circulation. Only half of the range of Hadley cell weakening exhibited in these experiments is found to be attributable to imposed, systematic variations in the surface albedo feedback. Changes in extratropical clouds that accompany the albedo changes explain the remaining spread. The feedback-driven circulation changes are compensated by eddy energy flux changes, which reduce the overall spread among experiments. These findings have implications for the efficiency with which the climate system, including tropical circulation and the hydrological cycle, adjusts to high-latitude feedbacks over climate states that range from perennial or seasonal ice to ice-free conditions in the Arctic.

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Timothy M. Merlis
,
Tapio Schneider
,
Simona Bordoni
, and
Ian Eisenman

Abstract

The response of the monsoonal and annual-mean Hadley circulation to orbital precession is examined in an idealized atmospheric general circulation model with an aquaplanet slab-ocean lower boundary. Contrary to expectations, the simulated monsoonal Hadley circulation is weaker when perihelion occurs at the summer solstice than when aphelion occurs at the summer solstice. The angular momentum balance and energy balance are examined to understand the mechanisms that produce this result. That the summer with stronger insolation has a weaker circulation is the result of an increase in the atmosphere’s energetic stratification, the gross moist stability, which increases more than the amount required to balance the change in atmospheric energy flux divergence necessitated by the change in top-of-atmosphere net radiation. The solstice-season changes result in annual-mean Hadley circulation changes (e.g., changes in circulation strength).

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Timothy M. Merlis
,
Tapio Schneider
,
Simona Bordoni
, and
Ian Eisenman

Abstract

The response of the monsoonal and annual-mean Hadley circulation to orbital precession is examined in an idealized atmospheric general circulation model with a simplified representation of land surface processes in subtropical latitudes. When perihelion occurs in the summer of a hemisphere with a subtropical continent, changes in the top-of-atmosphere energy balance, together with a poleward shift of the monsoonal circulation boundary, lead to a strengthening of the monsoonal circulation. Spatial variations in surface heat capacity determine whether radiative perturbations are balanced by energy storage or by atmospheric energy fluxes. Although orbital precession does not affect annual-mean insolation, the annual-mean Hadley circulation does respond to orbital precession because its sensitivity to radiative changes varies over the course of the year: the monsoonal circulation in summer is near the angular momentum-conserving limit and responds directly to radiative changes; whereas in winter, the circulation is affected by the momentum fluxes of extratropical eddies and is less sensitive to radiative changes.

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Timothy M. Merlis
,
Tapio Schneider
,
Simona Bordoni
, and
Ian Eisenman

Abstract

Orbital precession changes the seasonal distribution of insolation at a given latitude but not the annual mean. Hence, the correlation of paleoclimate proxies of annual-mean precipitation with orbital precession implies a nonlinear rectification in the precipitation response to seasonal solar forcing. It has previously been suggested that the relevant nonlinearity is that of the Clausius–Clapeyron relationship. Here it is argued that a different nonlinearity related to moisture advection by the atmospheric circulation is more important. When perihelion changes from one hemisphere’s summer solstice to the other’s in an idealized aquaplanet atmospheric general circulation model, annual-mean precipitation increases in the hemisphere with the brighter, warmer summer and decreases in the other hemisphere, in qualitative agreement with paleoclimate proxies that indicate such hemispherically antisymmetric climate variations. The rectification mechanism that gives rise to the precipitation changes is identified by decomposing the perturbation water vapor budget into “thermodynamic” and “dynamic” components. Thermodynamic changes (caused by changes in humidity with unchanged winds) dominate the hemispherically antisymmetric annual-mean precipitation response to precession in the absence of land–sea contrasts. The nonlinearity that enables the thermodynamic changes to affect annual-mean precipitation is a nonlinearity of moisture advection that arises because precession-induced seasonal humidity changes correlate with the seasonal cycle in low-level convergence. This interpretation is confirmed using simulations in which the Clausius–Clapeyron relationship is explicitly linearized. The thermodynamic mechanism also operates in simulations with an idealized representation of land, although in these simulations the dynamic component of the precipitation changes is also important, adding to the thermodynamic precipitation changes in some latitudes and offsetting it in others.

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Malte F. Jansen
,
Louis-Philippe Nadeau
, and
Timothy M. Merlis

Abstract

Much of the existing theory for the ocean’s overturning circulation considers steady-state equilibrium solutions. However, Earth’s climate is not in a steady state, and a better understanding of the ocean’s nonequilibrium response to changes in the surface climate is urgently needed. Here, the time-dependent response of the deep-ocean overturning circulation to atmospheric warming is examined using a hierarchy of idealized ocean models. The transient response to surface warming is characterized by a shoaling and weakening of the Atlantic meridional overturning circulation (AMOC)—consistent with results from coupled climate simulations. The initial shoaling and weakening of the AMOC occurs on decadal time scales and is attributed to a rapid warming of northern-sourced deep water. The equilibrium response to warming, in contrast, is associated with a deepening and strengthening of the AMOC. The eventual deepening of the AMOC is argued to be associated with abyssal density changes and driven by modified surface fluxes in the Southern Ocean, following a reduction of the Antarctic sea ice cover. Full equilibration of the AMOC requires a diffusive adjustment of the abyss and takes many millennia. The equilibration time scale is much longer than most coupled climate model simulations, highlighting the importance of considering integration time and initial conditions when interpreting the deep-ocean circulation in climate models. The results also show that past climates are unlikely to be an adequate analog for changes in the overturning circulation during the coming decades or centuries.

Open access
Pei-Ning Feng
,
Hai Lin
,
Jacques Derome
, and
Timothy M. Merlis

Abstract

The prediction skill of the North Atlantic Oscillation (NAO) in boreal winter is assessed in the operational models of the WCRP/WWRP Subseasonal-to-Seasonal (S2S) prediction project. Model performance in representing the contribution of different processes to the NAO forecast skill is evaluated. The S2S models with relatively higher stratospheric vertical resolutions (high-top models) are in general more skillful in predicting the NAO than those models with relatively lower stratospheric resolutions (low-top models). Comparison of skill is made between different groups of forecasts based on initial condition characteristics: phase and amplitude of the NAO, easterly and westerly phases of the quasi-biennial oscillation (QBO), warm and cold phases of ENSO, and phase and amplitude of the Madden–Julian oscillation (MJO). The forecasts with a strong NAO in the initial condition are more skillful than with a weak NAO. Those with negative NAO tend to have more skillful predictions than positive NAO. Comparisons of NAO skill between forecasts during easterly and westerly QBO and between warm and cold ENSO show no consistent difference for the S2S models. Forecasts with strong initial MJO tend to be more skillful in the NAO prediction than weak MJO. Among the eight phases of MJO in the initial condition, phases 3–4 and phase 7 have better NAO forecast skills compared with the other phases. The results of this study have implications for improving our understanding of sources of predictability of the NAO. The situation dependence of the NAO prediction skill is likely useful in identifying “windows of opportunity” for subseasonal to seasonal predictions.

Open access
Yan-Ting Chen
,
Yi Huang
, and
Timothy M. Merlis

Abstract

The radiative forcing of carbon dioxide (CO2) at the top of the atmosphere (TOA) has a rich spatial structure and has implications for large-scale climate changes, such as poleward energy transport and tropical circulation change. Beyond the TOA, additional CO2 increases downwelling longwave at the surface, and this change in flux is the surface CO2 forcing. Here we thoroughly evaluate the spatiotemporal variation of the instantaneous, longwave CO2 radiative forcing at both the TOA and surface. The instantaneous forcing is calculated with a radiative transfer model using ERA5 reanalysis fields. Multivariate regression models show that the broadband forcing at the TOA and surface are well predicted by local temperatures, humidity, and cloud radiative effects. The difference between the TOA and surface forcing, the atmospheric forcing, can be either positive or negative and is mostly controlled by the column water vapor, with little explicit dependence on the surface temperature. The role of local variables on the TOA forcing is also assessed by partitioning the change in radiative flux to the component emitted by the surface versus that emitted by the atmosphere. In cold, dry regions, the surface and atmospheric contribution partially cancel out, leading to locally weak or even negative TOA forcing. In contrast, in the warm, moist regions, the surface and atmospheric components strengthen each other, resulting in overall larger TOA forcing. The relative contributions of surface and atmosphere to the TOA forcing depend on the optical thickness in the current climate, which in turn is controlled by the column water vapor.

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Eric Bembenek
,
Timothy M. Merlis
, and
David N. Straub

Abstract

A large fraction of tropical cyclones (TCs) are generated near the intertropical convergence zone (ITCZ), and barotropic instability of the related wind shear has been shown to be an important generation mechanism. The latitudinal position of the ITCZ shifts seasonally and may shift poleward in response to global warming. Aquaplanet GCM simulations have shown TC-generation frequency to vary with position of the ITCZ. These results, and that moisture plays an essential role in the dynamics, motivate the present study on the growth rates of barotropic instability in ITCZ-like zonal wind profiles. Base-state zonal wind profiles are generated by applying a prescribed forcing (representing zonally averaged latent heat release in the ITCZ) to a shallow-water model. Shifting the latitudinal position of the forcing alters these profiles, with a poleward shift leading to enhanced barotropic instability. Next, an examination of how latent release impacts the barotropic breakdown of these profiles is considered. To do this, moisture is explicitly represented using a tracer variable. Upon supersaturation, precipitation occurs and the related latent heat release is parameterized as a mass transfer out of the dynamically active layer. Whether moisture serves to enhance or reduce barotropic growth rates is found to depend on how saturation humidity is represented. In particular, taking it to be constant or a function of the layer thickness (related to temperature) leads to a reduction, whereas taking it to be a specified function of latitude leads to an enhancement. Simple arguments are given to support the idea that moisture effects should lead to a reduction in the moist shallow-water model and that a poleward shift of the ITCZ should lead to an enhancement of barotropic instability.

Full access
Eric Bembenek
,
David N. Straub
, and
Timothy M. Merlis

Abstract

The effects of moisture on the energetics of a statistically stationary, baroclinically unstable jet representing the midlatitude atmosphere are examined using a two-layer, β-plane shallow-water model. Flow is driven by a relaxation of the interface between the two layers to a baroclinically unstable profile. Moisture is input to the lower layer by evaporation. When supersaturation occurs, precipitation is triggered and the related latent heat release drives a mass transfer between the two layers. A comparison between dry and moist reference atmospheres shows that precipitation reduces eddy kinetic energy. This is related to the meridional distribution of precipitation, which occurs on the poleward side of the jet (where the interface field is raised). This latitudinal structure of precipitation is related to a correlation between poleward flow and ascent, which is analyzed using a shallow-water analog to the ω equation. The precipitation effect on the energy budget is predominately due to zonal- and time-averaged terms. Because of this, dry simulations in which the thermal forcing is modified to mimic the effect of zonally averaged precipitation are carried out and compared with their precipitating counterparts. These simulations show a similar reduction of baroclinic eddy kinetic energy; however, the barotropic eddy kinetic energy response shows a larger difference.

Free access