Dependence of Climate Response on Meridional Structure of External Thermal Forcing

Abstract This study shows that the magnitude of global surface warming greatly depends on the meridional distribution of surface thermal forcing. An atmospheric model coupled to an aquaplanet slab mixed layer ocean is perturbed by prescribing heating to the ocean mixed layer. The heating is distributed uniformly globally or confined to narrow tropical or polar bands, and the amplitude is adjusted to ensure that the global mean remains the same for all cases. Since the tropical temperature is close to a moist adiabat, the prescribed heating leads to a maximized warming near the tropopause, whereas the polar warming is trapped near the surface because of strong atmospheric stability. Hence, the surface warming is more effectively damped by radiation in the tropics than in the polar region. As a result, the global surface temperature increase is weak (strong) when the given amount of heating is confined to the tropical (polar) band. The degree of this contrast is shown to depend on water vapor– and cloud–radiative feedbacks that alter the effective strength of prescribed thermal forcing.

Download Full-text

The Response of the ITCZ to Extratropical Thermal Forcing: Idealized Slab-Ocean Experiments with a GCM

Journal of Climate ◽

10.1175/2007jcli2146.1 ◽

2008 ◽

Vol 21 (14) ◽

pp. 3521-3532 ◽

Cited By ~ 388

Author(s):

Sarah M. Kang ◽

Isaac M. Held ◽

Dargan M. W. Frierson ◽

Ming Zhao

Keyword(s):

Mixed Layer ◽

Energy Transport ◽

Meridional Overturning Circulation ◽

Convection Scheme ◽

Thermal Forcing ◽

Tropical Precipitation ◽

Model Configuration ◽

Mixed Layer Ocean ◽

Meridional Overturning ◽

The Tropics

Abstract Using a comprehensive atmospheric GCM coupled to a slab mixed layer ocean, experiments are performed to study the mechanism by which displacements of the intertropical convergence zone (ITCZ) are forced from the extratropics. The northern extratropics are cooled and the southern extratropics are warmed by an imposed cross-equatorial flux beneath the mixed layer, forcing a southward shift in the ITCZ. The ITCZ displacement can be understood in terms of the degree of compensation between the imposed oceanic flux and the resulting response in the atmospheric energy transport in the tropics. The magnitude of the ITCZ displacement is very sensitive to a parameter in the convection scheme that limits the entrainment into convective plumes. The change in the convection scheme affects the extratropical–tropical interactions in the model primarily by modifying the cloud response. The results raise the possibility that the response of tropical precipitation to extratropical thermal forcing, important for a variety of problems in climate dynamics (such as the response of the tropics to the Northern Hemisphere ice sheets during glacial maxima or to variations in the Atlantic meridional overturning circulation), may be strongly dependent on cloud feedback. The model configuration described here is suggested as a useful benchmark helping to quantify extratropical–tropical interactions in atmospheric models.

Download Full-text

Sensitivity of Intertropical Convergence Zone Movement to the Latitudinal Position of Thermal Forcing

Journal of Climate ◽

10.1175/jcli-d-13-00691.1 ◽

2014 ◽

Vol 27 (8) ◽

pp. 3035-3042 ◽

Cited By ~ 53

Author(s):

Jeongbin Seo ◽

Sarah M. Kang ◽

Dargan M. W. Frierson

Keyword(s):

General Circulation ◽

Circulation Model ◽

Atmospheric Model ◽

Intertropical Convergence Zone ◽

Convergence Zone ◽

Mixed Layer Ocean ◽

Latitudinal Position ◽

Radiative Feedbacks ◽

Extratropical Forcing ◽

The Tropics

Abstract A variety of recent studies have shown that extratropical heating anomalies can be remarkably effective at causing meridional shifts in the intertropical convergence zone (ITCZ). But what latitudinal location of forcing is most effective at shifting the ITCZ? In a series of aquaplanet simulations with the GFDL Atmospheric Model, version 2 (AM2), coupled to a slab mixed layer ocean, it is shown that high-latitude forcing actually causes a larger shift in the ITCZ than when equivalent surface forcing is applied in the tropics. Equivalent simulations are run with an idealized general circulation model (GCM) without cloud and water vapor feedbacks, also coupled to an aquaplanet slab ocean, where the ITCZ response instead becomes weaker the farther the forcing is from the equator, indicating that radiative feedbacks must be important in AM2. In the absence of radiative feedbacks, the tendency for anomalies to decrease in importance the farther away they are from the equator is due to the quasi-diffusive nature of energy transports. Cloud shortwave responses in AM2 act to strengthen the ITCZ response to extratropical forcing, amplifying the response as it propagates toward the equator. These results emphasize the great importance of the extratropics in determining the position of the ITCZ.

Download Full-text

A Tropical and Subtropical Land–Sea–Atmosphere Drought Oscillation Mechanism

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2186.1 ◽

2007 ◽

Vol 64 (12) ◽

pp. 4458-4468 ◽

Cited By ~ 7

Author(s):

Dorian S. Abbot ◽

Kerry A. Emanuel

Keyword(s):

Soil Moisture ◽

Mixed Layer ◽

Atmospheric Model ◽

Sustained Oscillation ◽

Model Parameters ◽

Qualitative Behavior ◽

Oscillation Mechanism ◽

Mixed Layer Ocean ◽

Land Hydrology

Abstract A two-column atmospheric model on a land–sea interface is studied. The model has sophisticated convection, cloud, and radiation schemes, a mixed layer ocean, and a bucket model to simulate land hydrology. A self-sustained oscillation in soil moisture with a period on the order of months is found. This oscillation is strongest when the model is run with parameters chosen to correspond to the arid subtropics. The effect of changing model parameters on the oscillation is explored. The existence and qualitative behavior of the oscillation are relatively robust to changes in model parameters.

Download Full-text

Trans-basin Atlantic-Pacific connections further weakened by common model Pacific mean SST biases

Nature Communications ◽

10.1038/s41467-020-19338-z ◽

2020 ◽

Vol 11 (1) ◽

Author(s):

Chen Li ◽

Dietmar Dommenget ◽

Shayne McGregor

Keyword(s):

Surface Temperature ◽

Climate Models ◽

Atmospheric Stability ◽

Sea Surface ◽

Surface Warming ◽

The Pacific ◽

Mean Sea Surface ◽

Cooling Trend ◽

Global Surface ◽

Mean State

Abstract A robust eastern Pacific surface temperature cooling trend was evident between ~1990–2013 that was considered as a pronounced contributor to the global surface warming slowdown. The majority of current climate models failed to reproduce this Pacific cooling trend, which is at least partly due to the underrepresentation of trans-basin teleconnections. Here, we investigate whether common Pacific mean sea surface temperature biases may further diminish the Atlantic-Pacific trans-basin induced Pacific cooling. Our results suggest that background Pacific SST biases act to weaken the trans-basin teleconnection by strengthening the Atlantic atmospheric stability and reducing Atlantic convection. These Pacific SST biases also act to substantially undermine the positive zonal wind-SST feedback. Furthermore, when combined, the Pacific and Atlantic SST biases led to Pacific cooling response that is almost non-existent (underestimated by 89%). Future efforts aim at reducing the model mean state biases may significantly help to improve the simulation skills of trans-basin teleconnections.

Download Full-text

Intermodel Spread in the Pattern Effect and Its Contribution to Climate Sensitivity in CMIP5 and CMIP6 Models

Journal of Climate ◽

10.1175/jcli-d-19-1011.1 ◽

2020 ◽

Vol 33 (18) ◽

pp. 7755-7775 ◽

Cited By ~ 2

Author(s):

Yue Dong ◽

Kyle C. Armour ◽

Mark D. Zelinka ◽

Cristian Proistosescu ◽

David S. Battisti ◽

...

Keyword(s):

Time Scales ◽

Climate Sensitivity ◽

Atmospheric Model ◽

Warm Pool ◽

Cmip5 Models ◽

Fast Time ◽

Pattern Effect ◽

Surface Warming ◽

Early Portion ◽

Radiative Feedbacks

AbstractRadiative feedbacks depend on the spatial patterns of sea surface temperature (SST) and thus can change over time as SST patterns evolve—the so-called pattern effect. This study investigates intermodel differences in the magnitude of the pattern effect and how these differences contribute to the spread in effective equilibrium climate sensitivity (ECS) within CMIP5 and CMIP6 models. Effective ECS in CMIP5 estimated from 150-yr-long abrupt4×CO2 simulations is on average 10% higher than that estimated from the early portion (first 50 years) of those simulations, which serves as an analog for historical warming; this difference is reduced to 7% on average in CMIP6. The (negative) net radiative feedback weakens over the course of the abrupt4×CO2 simulations in the vast majority of CMIP5 and CMIP6 models, but this weakening is less dramatic on average in CMIP6. For both ensembles, the total variance in the effective ECS is found to be dominated by the spread in radiative response on fast time scales, rather than the spread in feedback changes. Using Green’s functions derived from two AGCMs shows that the spread in feedbacks on fast time scales may be primarily due to differences in atmospheric model physics, whereas the spread in feedback evolution is primarily governed by differences in SST patterns. Intermodel spread in feedback evolution is well explained by differences in the relative warming in the west Pacific warm-pool regions for the CMIP5 models, but this relation fails to explain differences across the CMIP6 models, suggesting that a stronger sensitivity of extratropical clouds to surface warming may also contribute to feedback changes in CMIP6.

Download Full-text

Sensitivity of Polar Amplification to Varying Insolation Conditions

Journal of Climate ◽

10.1175/jcli-d-17-0627.1 ◽

2018 ◽

Vol 31 (12) ◽

pp. 4933-4947 ◽

Cited By ~ 6

Author(s):

Doyeon Kim ◽

Sarah M. Kang ◽

Yechul Shin ◽

Nicole Feldl

Keyword(s):

Energy Transport ◽

Longwave Radiation ◽

Atmospheric Model ◽

Static Stability ◽

Lapse Rate ◽

Polar Surface ◽

Surface Warming ◽

Polar Amplification ◽

Rate Feedback ◽

Radiative Feedbacks

The mechanism of polar amplification in the absence of surface albedo feedback is investigated using an atmospheric model coupled to an aquaplanet slab ocean forced by a CO2 doubling. In particular, we examine the sensitivity of polar surface warming response under different insolation conditions from equinox (EQN) to annual mean (ANN) to seasonally varying (SEA). Varying insolation greatly affects the climatological static stability. The equinox condition, with the largest polar static stability, exhibits a bottom-heavy vertical profile of polar warming response that leads to the strongest polar amplification. In contrast, the polar warming response in ANN and SEA exhibits a maximum in the midtroposphere, which leads to only weak polar amplification. The midtropospheric warming maximum, which results from an increased poleward atmospheric energy transport in response to the tropics-to-pole energy imbalance, contributes to polar surface warming via downward clear-sky longwave radiation. However, it is cancelled by negative cloud radiative feedbacks locally. Furthermore, the polar lapse rate feedback, calculated from radiative kernels, is negative due to the midtropospheric warming maximum, and hence is not able to promote the polar surface warming. On the other hand, the polar lapse rate feedback in EQN is positive due to the bottom-heavy warming response, contributing to the strong polar surface warming. This contrast suggests that locally induced positive radiative feedbacks are necessary for strong polar amplification. Our results demonstrate how interactions among climate feedbacks determine the strength of polar amplification.

Download Full-text

Evolution of the Tropical Response to Periodic Extratropical Thermal Forcing

Journal of Climate ◽

10.1175/jcli-d-20-0493.1 ◽

2021 ◽

pp. 1-53

Author(s):

Yechul Shin ◽

Sarah M. Kang ◽

Ken Takahashi ◽

Malte F. Stuecker ◽

Yen-Ting Hwang ◽

...

Keyword(s):

Mixed Layer ◽

Mixed Layer Depth ◽

Critical Time ◽

Hadley Cell ◽

Balance Model ◽

Layer Depth ◽

Thermal Forcing ◽

Extratropical Forcing ◽

The Tropics

AbstractThis study examines the temporal evolution of the extratropically forced tropical response in an idealized aquaplanet model under equinox condition. We apply a surface thermal forcing in the northern extratropics that oscillates periodically in time. It is shown that tropical precipitation is unaltered by sufficiently high-frequency extratropical forcing. This sensitivity to the extratropical forcing periodicity arises from the critical time required for sea surface temperature (SST) adjustment. Low-frequency extratropical forcing grants sufficient time for atmospheric transient eddies to diffuse moist static energy to perturb the mid-latitude SSTs outside the forcing region, as demonstrated by a one-dimensional energy balance model with a fixed diffusivity. As the transient eddies weaken in the subtropics, a further equatorward advection is accomplished by the Hadley circulation. The essential role of Hadley cell advection in connecting the subtropical signal to the equatorial region is supported by an idealized thermodynamical-advective model. Associated with the SST changes in the tropics is a meridional shift of the Intertropical Convergence Zone. Since the time needed for SST adjustment increases with increasing mixed layer depth, the critical forcing period at which the extratropical forcing can affect the tropics scales linearly with the mixed layer depth. Our results highlight the important role of decadal-and-longer extratropical climate variability in shaping the tropical climate system. We also raise the possibility that the transient behavior of a tropical response forced by extratropical variability may be strongly dependent on cloud radiative effects.

Download Full-text