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Amy C. Clement

Abstract

The influence of ocean heat transport on the seasonal cycle of the Hadley circulation is investigated using idealized experiments with a climate model. It is found that ocean heat transport plays a fundamental role in setting the structure and intensity of the seasonal Hadley cells. The ocean’s influence can be understood primarily via annual mean considerations. By cooling the equatorial regions and warming the subtropics in a year-round sense, the ocean heat transport allows for regions of SST maxima to occur off the equator in the summer hemisphere. This leads to large meridional excursions of convection over the ocean and a seasonal Hadley circulation that is strongly asymmetric about the equator. The broadening of the latitudinal extent of the SST maximum and the convecting regions by the ocean heat transport also weakens the annual mean Hadley circulation in a manner that is consistent with simpler models. The results are discussed in the context of prior studies of the controls on the strength and structure of the Hadley circulation. It is suggested that a complete understanding of the seasonal Hadley circulation must include both oceanic and atmospheric processes and their interactions.

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Amy C. Clement
and
Brian Soden

Abstract

A key disagreement exists between global climate model (GCM) simulations and satellite observations of the decadal variability in the tropical-mean radiation budget. Measurements from the Earth Radiation Budget Experiment (ERBE) over the period 1984–2001 indicate a trend of increasing longwave emission and decreasing shortwave reflection that no GCM can currently reproduce. Motivated by these results, a series of model sensitivity experiments is performed to investigate hypotheses that have been advanced to explain this discrepancy. Specifically, the extent to which a strengthening of the Hadley circulation or a change in convective precipitation efficiency can alter the tropical-mean radiation budget is assessed. Results from both model sensitivity experiments and an empirical analysis of ERBE observations suggest that the tropical-mean radiation budget is remarkably insensitive to changes in the tropical circulation. The empirical estimate suggests that it would require at least a doubling in strength of the Hadley circulation in order to generate the observed decadal radiative flux changes. In contrast, rather small changes in a model’s convective precipitation efficiency can generate changes comparable to those observed, provided that the precipitation efficiency lies near the upper end of its possible range. If, however, the precipitation efficiency of tropical convective systems is more moderate, the model experiments suggest that the climate would be rather insensitive to changes in its value. Further observations are necessary to constrain the potential effects of microphysics on the top-of-atmosphere radiation budget.

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Amy C. Clement
,
Richard Seager
, and
Raghu Murtugudde

Abstract

Tropical warm pools appear as the primary mode in the distribution of tropical sea surface temperature (SST). Most previous studies have focused on the role of atmospheric processes in homogenizing temperatures in the warm pool and establishing the observed statistical SST distribution. In this paper, a hierarchy of models is used to illustrate both oceanic and atmospheric mechanisms that contribute to the establishment of tropical warm pools. It is found that individual atmospheric processes have competing effects on the SST distribution: atmospheric heat transport tends to homogenize SST, while the spatial structure of atmospheric humidity and surface wind speeds tends to remove homogeneity. The latter effects dominate, and under atmosphere-only processes there is no warm pool. Ocean dynamics counter this effect by homogenizing SST, and it is argued that ocean dynamics is fundamental to the existence of the warm pool. Under easterly wind stress, the thermocline is deep in the west and shallow in the east. Because of this, poleward Ekman transport of water at the surface, compensated by equatorward geostrophic flow below and linked by equatorial upwelling, creates a cold tongue in the east but homogenizes SST in the west, creating a warm pool. High clouds may also homogenize the SST by reducing the surface solar radiation over the warmest water, but the strength of this feedback is quite uncertain. Implications for the role of these processes in climate change are discussed.

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Eleanor A. Middlemas
and
Amy C. Clement

Abstract

The causes of decadal time-scale variations in global mean temperature are currently under debate. Proposed mechanisms include both processes internal to the climate system as well as external forcing. Here, the robustness of spatial and time scale characteristics of unforced (internal) decadal variability among phase 5 of the Coupled Model Intercomparison Project (CMIP5) preindustrial control runs is examined. It is found that almost all CMIP5 models produce an interdecadal Pacific oscillation–like pattern associated with decadal variability, but the frequency of decadal-scale change is model dependent. To assess the roles of atmosphere and ocean dynamics in producing decadal variability, two preindustrial control Community Climate System model (version 4) configurations are compared: one with an atmosphere coupled to a slab ocean and the other fully coupled to a dynamical ocean. Interactive ocean dynamics are not necessary to produce an IPO-like pattern but affect the magnitude and frequency of the decadal changes primarily by impacting the strength of El Niño–Southern Oscillation. However, low-frequency El Niño–Southern Oscillation variability and skewness explains up to only 54% of the spread in frequency of decadal swings in global mean temperature among CMIP5 models; there may be other internal mechanisms that can produce such diversity. The spatial pattern of decadal changes in surface temperature are robust and can be explained by atmospheric processes interacting with the upper ocean, while the frequency of these changes is not well constrained by models.

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Sarah M. Larson
,
Martha W. Buckley
, and
Amy C. Clement

Abstract

Variations in the Atlantic meridional overturning circulation (AMOC) driven by buoyancy forcing are typically characterized as having a low-frequency time scale, interhemispheric structure, cross-equatorial heat transport, and linkages to the strength of Northern Hemisphere gyre circulations and the Gulf Stream. This study first tests whether these attributes ascribed to the AMOC are reproduced in a coupled model that is mechanically decoupled and, hence, is only buoyancy coupled. Overall, the mechanically decoupled model reproduces these attributes, with the exception that in the subpolar gyre, buoyancy drives AMOC variations on interannual to multidecadal time scales, yet only the multidecadal variations penetrate into the subtropics. A stronger AMOC is associated with a strengthening of the Northern Hemisphere gyre circulations, Gulf Stream, and northward oceanic heat transport throughout the basin. We then determine whether the characteristics in the mechanically decoupled model can be recovered by low-pass filtering the AMOC in a fully coupled version of the same model, a common approach used to isolate the buoyancy-driven AMOC. A major conclusion is that low-pass filtering the AMOC in the fully coupled model reproduces the buoyancy-driven AMOC pattern and most of the associated attributes, but not the statistics of the temporal variability. The strength of the AMOC–Gulf Stream connection is also not reproduced. The analyses reveal caveats that must be considered when choosing indexes and filtering techniques to estimate the buoyancy-driven AMOC. Results also provide insight on the latitudinal dependence of time scales and drivers of ocean circulation variability in coupled models, with potential implications for measurement and detection of the buoyancy-driven AMOC in the real world.

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Eleanor A. Middlemas
,
Amy C. Clement
,
Brian Medeiros
, and
Ben Kirtman

Abstract

Cloud radiative feedbacks are disabled via “cloud-locking” in the Community Earth System Model, version 1.2 (CESM1.2), to result in a shift in El Niño–Southern Oscillation (ENSO) periodicity from 2–7 years to decadal time scales. We hypothesize that cloud radiative feedbacks may impact the periodicity in three ways: by 1) modulating heat flux locally into the equatorial Pacific subsurface through negative shortwave cloud feedback on sea surface temperature anomalies (SSTA), 2) damping the persistence of subtropical southeast Pacific SSTA such that the South Pacific meridional mode impacts the duration of ENSO events, or 3) controlling the meridional width of off-equatorial westerly winds, which impacts the periodicity of ENSO by initiating longer Rossby waves. The result of cloud-locking in CESM1.2 contrasts that of another study, which found that cloud-locking in a different global climate model led to decreased ENSO magnitude across all time scales due to a lack of positive longwave feedback on the anomalous Walker circulation. CESM1.2 contains this positive longwave feedback on the anomalous Walker circulation, but either its influence on the surface is decoupled from ocean dynamics or the feedback is only active on interannual time scales. The roles of cloud radiative feedbacks in ENSO in other global climate models are additionally considered. In particular, it is shown that one cannot predict the role of cloud radiative feedbacks in ENSO through a multimodel diagnostic analysis. Instead, they must be directly altered.

Open access
Richard Seager
,
Amy C. Clement
, and
Mark A. Cane

Abstract

This paper is a modeling study of possible roles for tropospheric water vapor, surface wind speed, and boundary layer processes in glacial cooling in the Tropics. The authors divide the Tropics into a region of persistent deep convection and a subtropical region with no deep convection. The regions are coupled via a radiatively driven Hadley cell and a wind-driven meridional overturning cell in the ocean. Radiation and the convective boundary layer (CBL) are treated in some detail.

The amount of tropical cooling depends on the height of the tropospheric drying and on the extent to which cloud water in the CBL is converted into rainwater. In the most realistic case where the CBL clouds precipitate, variations in CBL depth are small, and the tropical SST becomes most sensitive to drying immediately above the CBL. Reducing the relative humidity of the entire troposphere above the subcloud layer by about 10%–20% cools the tropical SST by just over 2 K. It is shown that this climate sensitivity arises from a complex balance of processes that control the depth of the CBL, its greenhouse trapping, and the albedo of boundary layer clouds. An increase in surface wind speed, such as occurs in simulations of the last glacial maximum with coupled general circulation models, substantially reduces the SST although the change in surface air temperature is less. The Milankovitch cycles are expected to cause changes in atmosphere and ocean circulation. It appears that a circulation change that causes the lower midtroposphere to dry would be an effective way to induce strong cooling of tropical climate.

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Amy C. Clement
,
Mark A. Cane
, and
Richard Seager

Abstract

Paleoclimatic data are increasingly showing that abrupt change is present in wide regions of the globe. Here a mechanism for abrupt climate change with global implications is presented. Results from a tropical coupled ocean–atmosphere model show that, under certain orbital configurations of the past, variability associated with El Niño–Southern Oscillation (ENSO) physics can abruptly lock to the seasonal cycle for several centuries, producing a mean sea surface temperature (SST) change in the tropical Pacific that resembles a La Niña. It is suggested that this change in SST would have a global impact and that abrupt events such as the Younger Dryas may be the outcome of orbitally driven changes in the tropical Pacific.

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Michael P. Erb
,
Anthony J. Broccoli
, and
Amy C. Clement

Abstract

Radiative feedbacks influence Earth's climate response to orbital forcing, amplifying some aspects of the response while damping others. To better understand this relationship, the GFDL Climate Model, version 2.1 (CM2.1), is used to perform idealized simulations in which only orbital parameters are altered while ice sheets, atmospheric composition, and other climate forcings are prescribed at preindustrial levels. These idealized simulations isolate the climate response and radiative feedbacks to changes in obliquity and longitude of the perihelion alone. Analysis shows that, despite being forced only by a redistribution of insolation with no global annual-mean component, feedbacks induce significant global-mean climate change, resulting in mean temperature changes of −0.5 K in a lowered obliquity experiment and +0.6 K in a NH winter solstice perihelion minus NH summer solstice perihelion experiment. In the obliquity experiment, some global-mean temperature response may be attributable to vertical variations in the transport of moist static energy anomalies, which can affect radiative feedbacks in remote regions by altering atmospheric stability. In the precession experiment, cloud feedbacks alter the Arctic radiation balance with possible implications for glaciation. At times when the orbital configuration favors glaciation, reductions in cloud water content and low-cloud fraction partially counteract changes in summer insolation, posing an additional challenge to understanding glacial inception. Additionally, several systems, such as the Hadley circulation and monsoons, influence climate feedbacks in ways that would not be anticipated from analysis of feedbacks in the more familiar case of anthropogenic forcing, emphasizing the complexity of feedback responses.

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Jeremy M. Klavans
,
Amy C. Clement
, and
Mark A. Cane

Abstract

North Atlantic sea surface temperatures (SST) exhibit a lagged response to the North Atlantic Oscillation (NAO) in both models and observations, which has previously been attributed to changes in ocean heat transport. Here we examine the lagged relationship between the NAO and Atlantic multidecadal variability (AMV) in the context of the two other major components of the AMV: atmospheric noise and external forcing. In preindustrial control runs, we generally find that after accounting for spurious signals introduced by filtering, the SST response to the NAO is only statistically significant in the subpolar gyre. Further, the lagged SST response to the NAO is small in magnitude and offers a limited contribution to the AMV pattern, statistics, or predictability. When climate models include variable external forcing, the relationship between the NAO and AMV is obscured and becomes inconsistent. In these historically forced runs, knowledge of the prior NAO offers reduced predictability. The differences between the preindustrial and the historically forced ensembles suggest that we do not yet have enough observational data to surmise the true NAO–AMV relationship and add evidence that external forcing plays a substantial role in producing the AMV.

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