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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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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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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
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
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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Timothy M. Merlis
,
Isaac M. Held
,
Georgiy L. Stenchikov
,
Fanrong Zeng
, and
Larry W. Horowitz

Abstract

Coupled climate model simulations of volcanic eruptions and abrupt changes in CO2 concentration are compared in multiple realizations of the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (GFDL CM2.1). The change in global-mean surface temperature (GMST) is analyzed to determine whether a fast component of the climate sensitivity of relevance to the transient climate response (TCR; defined with the 1% yr−1 CO2-increase scenario) can be estimated from shorter-time-scale climate changes. The fast component of the climate sensitivity estimated from the response of the climate model to volcanic forcing is similar to that of the simulations forced by abrupt CO2 changes but is 5%–15% smaller than the TCR. In addition, the partition between the top-of-atmosphere radiative restoring and ocean heat uptake is similar across radiative forcing agents. The possible asymmetry between warming and cooling climate perturbations, which may affect the utility of volcanic eruptions for estimating the TCR, is assessed by comparing simulations of abrupt CO2 doubling to abrupt CO2 halving. There is slightly less (~5%) GMST change in 0.5 × CO2 simulations than in 2 × CO2 simulations on the short (~10 yr) time scales relevant to the fast component of the volcanic signal. However, inferring the TCR from volcanic eruptions is more sensitive to uncertainties from internal climate variability and the estimation procedure.

The response of the GMST to volcanic eruptions is similar in GFDL CM2.1 and GFDL Climate Model, version 3 (CM3), even though the latter has a higher TCR associated with a multidecadal time scale in its response. This is consistent with the expectation that the fast component of the climate sensitivity inferred from volcanic eruptions is a lower bound for the TCR.

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Matthew Henry
,
Timothy M. Merlis
,
Nicholas J. Lutsko
, and
Brian E. J. Rose

Abstract

The precise mechanisms driving Arctic amplification are still under debate. Previous attribution methods compute the vertically uniform temperature change required to balance the top-of-atmosphere energy imbalance caused by each forcing and feedback, with any departures from vertically uniform warming collected into the lapse-rate feedback. We propose an alternative attribution method using a single-column model that accounts for the forcing dependence of high-latitude lapse-rate changes. We examine this method in an idealized general circulation model (GCM), finding that, even though the column-integrated carbon dioxide (CO2) forcing and water vapor feedback are stronger in the tropics, they contribute to polar-amplified surface warming as they produce bottom-heavy warming in high latitudes. A separation of atmospheric temperature changes into local and remote contributors shows that, in the absence of polar surface forcing (e.g., sea ice retreat), changes in energy transport are primarily responsible for the polar-amplified pattern of warming. The addition of surface forcing substantially increases polar surface warming and reduces the contribution of atmospheric dry static energy transport to the warming. This physically based attribution method can be applied to comprehensive GCMs to provide a clearer view of the mechanisms behind Arctic amplification.

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Anne-Sophie Fortin
,
Carolina O. Dufour
,
Timothy M. Merlis
, and
Rym Msadek

Abstract

The pattern and magnitude of the Atlantic meridional overturning circulation (AMOC) in response to an increase in atmospheric carbon dioxide (CO2) concentration greatly differ across climate models in particular due to differences in the representation of oceanic processes. Here, we investigate the response of the AMOC to an idealized climate change scenario, along with the drivers of this response, in the three configurations of a coupled climate model suite with varying resolutions in the ocean (1°, 0.25°, 0.10°). In response to the CO2 increase, the AMOC shows a reduction of similar magnitude in the low and high resolutions, while a muted response is found in the medium resolution. A decomposition of the AMOC into its geostrophic and residual components reveals that most of the AMOC reduction is due to a weakening of the geostrophic streamfunction driven by temperature anomalies, partly opposed by a strengthening of the geostrophic streamfunction driven by salinity anomalies. Changes in the AMOC due to the mesoscale eddy streamfunction contribute to 13% and 17% of the AMOC decline in the low and high resolutions, respectively, but induce very little change in the medium resolution. The similar response of the AMOC strength in the low and high resolutions hides important differences in the contribution and pattern of the geostrophic and eddy streamfunctions. The lack of sensitivity of the medium resolution to the CO2 forcing is due to a weak connection between the deep water formation regions in the northern subpolar gyre and the Deep Western Boundary Current.

Significance Statement

The Atlantic meridional overturning circulation (AMOC) is a major system of ocean currents in the Atlantic that contributes to shaping the climate at regional and global scales, notably through the transport of heat from the low to the high latitudes. A major slowdown of the AMOC over the twenty-first century is predicted by current climate models in response to increasing greenhouse gases. Yet, the magnitude and timing of this slowdown are uncertain. The purpose of this study is to investigate the expected weakening of the AMOC using state-of-the-art numerical climate models that include higher resolutions than typically used in climate change assessments. Our results provide insights into the mechanisms driving the weakening of the AMOC and into differences arising from model resolutions.

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