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- Author or Editor: Sarah M. Kang x
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Abstract
This study investigates the formation mechanism of the ocean surface warming pattern in response to a doubling CO2 with a focus on the role of ocean heat uptake (or ocean surface heat flux change, ΔQ net). We demonstrate that the transient patterns of surface warming and rainfall change simulated by the dynamic ocean–atmosphere coupled model (DOM) can be reproduced by the equilibrium solutions of the slab ocean–atmosphere coupled model (SOM) simulations when forced with the DOM ΔQ net distribution. The SOM is then used as a diagnostic inverse modeling tool to decompose the CO2-induced thermodynamic warming effect and the ΔQ net (ocean heat uptake)–induced cooling effect. As ΔQ net is largely positive (i.e., downward into the ocean) in the subpolar oceans and weakly negative at the equator, its cooling effect is strongly polar amplified and opposes the CO2 warming, reducing the net warming response especially over Antarctica. For the same reason, the ΔQ net-induced cooling effect contributes significantly to the equatorially enhanced warming in all three ocean basins, while the CO2 warming effect plays a role in the equatorial warming of the eastern Pacific. The spatially varying component of ΔQ net, although globally averaged to zero, can effectively rectify and lead to decreased global mean surface temperature of a comparable magnitude as the global mean ΔQ net effect under transient climate change. Our study highlights the importance of air–sea interaction in the surface warming pattern formation and the key role of ocean heat uptake pattern.
Abstract
This study investigates the formation mechanism of the ocean surface warming pattern in response to a doubling CO2 with a focus on the role of ocean heat uptake (or ocean surface heat flux change, ΔQ net). We demonstrate that the transient patterns of surface warming and rainfall change simulated by the dynamic ocean–atmosphere coupled model (DOM) can be reproduced by the equilibrium solutions of the slab ocean–atmosphere coupled model (SOM) simulations when forced with the DOM ΔQ net distribution. The SOM is then used as a diagnostic inverse modeling tool to decompose the CO2-induced thermodynamic warming effect and the ΔQ net (ocean heat uptake)–induced cooling effect. As ΔQ net is largely positive (i.e., downward into the ocean) in the subpolar oceans and weakly negative at the equator, its cooling effect is strongly polar amplified and opposes the CO2 warming, reducing the net warming response especially over Antarctica. For the same reason, the ΔQ net-induced cooling effect contributes significantly to the equatorially enhanced warming in all three ocean basins, while the CO2 warming effect plays a role in the equatorial warming of the eastern Pacific. The spatially varying component of ΔQ net, although globally averaged to zero, can effectively rectify and lead to decreased global mean surface temperature of a comparable magnitude as the global mean ΔQ net effect under transient climate change. Our study highlights the importance of air–sea interaction in the surface warming pattern formation and the key role of ocean heat uptake pattern.
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.
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.
Abstract
A subtropical continent is added to two aquaplanet atmospheric general circulation models (AGCMs) to better understand the influence of land on tropical circulation and precipitation. The first model, the gray-radiation moist (GRaM) AGCM, has simplified physics, while the second model, the GFDL Atmospheric Model version 2.1 (AM2.1), is a fully comprehensive AGCM. Both models have a continent that is 60° wide in longitude from 10° to 30°N, in an otherwise slab-ocean-covered world. The precipitation response varies with cloudy- and clear-sky feedbacks and depends on continental albedo. In GRaM simulations with a continent, precipitation in the Northern Hemisphere decreases mostly as a result of decreased evaporation. In AM2.1 simulations, precipitation also shifts southward via Hadley circulation changes due to increasing albedo, but the radiative impact of clouds and moisture creates a more complex response. Results are similar when a seasonal cycle of insolation is included in AM2.1 simulations. The impact of a large, bright subtropical continent is to shift precipitation to the opposite hemisphere. In these simulations, the hemisphere of greater tropical precipitation is better predicted by the hemisphere with greater atmospheric energy input, as has been shown in previous literature, rather than the hemisphere that has higher surface temperature.
Abstract
A subtropical continent is added to two aquaplanet atmospheric general circulation models (AGCMs) to better understand the influence of land on tropical circulation and precipitation. The first model, the gray-radiation moist (GRaM) AGCM, has simplified physics, while the second model, the GFDL Atmospheric Model version 2.1 (AM2.1), is a fully comprehensive AGCM. Both models have a continent that is 60° wide in longitude from 10° to 30°N, in an otherwise slab-ocean-covered world. The precipitation response varies with cloudy- and clear-sky feedbacks and depends on continental albedo. In GRaM simulations with a continent, precipitation in the Northern Hemisphere decreases mostly as a result of decreased evaporation. In AM2.1 simulations, precipitation also shifts southward via Hadley circulation changes due to increasing albedo, but the radiative impact of clouds and moisture creates a more complex response. Results are similar when a seasonal cycle of insolation is included in AM2.1 simulations. The impact of a large, bright subtropical continent is to shift precipitation to the opposite hemisphere. In these simulations, the hemisphere of greater tropical precipitation is better predicted by the hemisphere with greater atmospheric energy input, as has been shown in previous literature, rather than the hemisphere that has higher surface temperature.
Abstract
This study explores the dependence of the climate response on the altitude of black carbon in the northern subtropics by employing an atmospheric general circulation model coupled to an aquaplanet mixed layer ocean, with a focus on the pattern changes in the temperature, hydrological cycle, and large-scale circulation. Black carbon added below or within the subtropical low-level clouds tends to suppress convection, which reduces the low cloud amount, resulting in a positive cloud radiative forcing. The warmer northern subtropics then induce a northward shift of the intertropical convergence zone (ITCZ) and a poleward expansion of the descending branch of the northern Hadley cell. As the black carbon–induced local warming is amplified by clouds and is advected by the anomalous Hadley circulation, the entire globe gets warmer. In contrast, black carbon added near the surface increases the buoyancy of air parcels to enhance convection, leading to an increase in the subtropical low cloud amount and a negative cloud radiative forcing. The temperature increase remains local to where black carbon is added and elsewhere decreases, so that the ITCZ is shifted southward and the descending branch of the northern Hadley cell contracts equatorward. Consistent with previous studies, the authors demonstrate that the climate response to black carbon is highly sensitive to the vertical distribution of black carbon relative to clouds; hence, models have to accurately compute the vertical transport of black carbon to enhance their skill in simulating the climatic effects of black carbon.
Abstract
This study explores the dependence of the climate response on the altitude of black carbon in the northern subtropics by employing an atmospheric general circulation model coupled to an aquaplanet mixed layer ocean, with a focus on the pattern changes in the temperature, hydrological cycle, and large-scale circulation. Black carbon added below or within the subtropical low-level clouds tends to suppress convection, which reduces the low cloud amount, resulting in a positive cloud radiative forcing. The warmer northern subtropics then induce a northward shift of the intertropical convergence zone (ITCZ) and a poleward expansion of the descending branch of the northern Hadley cell. As the black carbon–induced local warming is amplified by clouds and is advected by the anomalous Hadley circulation, the entire globe gets warmer. In contrast, black carbon added near the surface increases the buoyancy of air parcels to enhance convection, leading to an increase in the subtropical low cloud amount and a negative cloud radiative forcing. The temperature increase remains local to where black carbon is added and elsewhere decreases, so that the ITCZ is shifted southward and the descending branch of the northern Hadley cell contracts equatorward. Consistent with previous studies, the authors demonstrate that the climate response to black carbon is highly sensitive to the vertical distribution of black carbon relative to clouds; hence, models have to accurately compute the vertical transport of black carbon to enhance their skill in simulating the climatic effects of black carbon.
Abstract
This 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 midlatitude 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.
Abstract
This 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 midlatitude 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.
Abstract
The polar region has been one of the fastest warming places on Earth in response to greenhouse gas (GHG) forcing. Two distinct processes contribute to the observed warming signal: (i) local warming in direct response to the GHG forcing and (ii) the effect of enhanced poleward heat transport from low latitudes. A series of aquaplanet experiments, which excludes the surface albedo feedback, is conducted to quantify the relative contributions of these two physical processes to the polar warming magnitude and degree of amplification relative to the global mean. The globe is divided into zonal bands with equal area in eight experiments. For each of these, an external heating is prescribed beneath the slab ocean layer in the respective forcing bands. The summation of the individual temperature responses to each local heating in these experiments is very similar to the response to a globally uniform heating. This allows the authors to decompose the polar warming and amplification signal into the effects of local and remote heating. Local polar heating that induces surface-trapped warming due to the large tropospheric static stability in this region accounts for about half of the polar surface warming. Cloud radiative effects act to enhance this local contribution. In contrast, remote nonpolar heating induces a robust polar warming pattern that features a midtropospheric peak, regardless of the meridional location of the forcing. Among all remote forcing experiments, the deep tropical forcing case contributes most to the polar-amplified surface warming pattern relative to the global mean, while the high-latitude forcing cases contribute most to enhancing the polar surface warming magnitude.
Abstract
The polar region has been one of the fastest warming places on Earth in response to greenhouse gas (GHG) forcing. Two distinct processes contribute to the observed warming signal: (i) local warming in direct response to the GHG forcing and (ii) the effect of enhanced poleward heat transport from low latitudes. A series of aquaplanet experiments, which excludes the surface albedo feedback, is conducted to quantify the relative contributions of these two physical processes to the polar warming magnitude and degree of amplification relative to the global mean. The globe is divided into zonal bands with equal area in eight experiments. For each of these, an external heating is prescribed beneath the slab ocean layer in the respective forcing bands. The summation of the individual temperature responses to each local heating in these experiments is very similar to the response to a globally uniform heating. This allows the authors to decompose the polar warming and amplification signal into the effects of local and remote heating. Local polar heating that induces surface-trapped warming due to the large tropospheric static stability in this region accounts for about half of the polar surface warming. Cloud radiative effects act to enhance this local contribution. In contrast, remote nonpolar heating induces a robust polar warming pattern that features a midtropospheric peak, regardless of the meridional location of the forcing. Among all remote forcing experiments, the deep tropical forcing case contributes most to the polar-amplified surface warming pattern relative to the global mean, while the high-latitude forcing cases contribute most to enhancing the polar surface warming magnitude.
Abstract
This paper examines the seasonal dependence of the effect of Arctic greening on tropical precipitation. In CAM3/CLM3 coupled to a mixed layer ocean, shrub and grasslands poleward of 60°N are replaced with boreal forests. With darker Arctic vegetation, the absorption of solar energy increases, but primarily in boreal spring and summer since little insolation reaches the Arctic in boreal winter. The net energy input into the northern extratropics is partly balanced by southward atmospheric energy transport across the equator by an anomalous Hadley circulation, resulting in a northward shift of the tropical precipitation. In contrast, in boreal fall, the slight increase in insolation over the Arctic is more than offset by increased outgoing longwave radiation and reduced surface turbulent fluxes in midlatitudes, from the warmer atmosphere. As a result, the Northern Hemisphere atmosphere loses energy, which is compensated by a northward cross-equatorial atmospheric energy transport, leading to a southward shift of the tropical precipitation in boreal fall. Thus, although Arctic vegetation is changed throughout the year, its effect on tropical precipitation exhibits substantial seasonal variations.
Abstract
This paper examines the seasonal dependence of the effect of Arctic greening on tropical precipitation. In CAM3/CLM3 coupled to a mixed layer ocean, shrub and grasslands poleward of 60°N are replaced with boreal forests. With darker Arctic vegetation, the absorption of solar energy increases, but primarily in boreal spring and summer since little insolation reaches the Arctic in boreal winter. The net energy input into the northern extratropics is partly balanced by southward atmospheric energy transport across the equator by an anomalous Hadley circulation, resulting in a northward shift of the tropical precipitation. In contrast, in boreal fall, the slight increase in insolation over the Arctic is more than offset by increased outgoing longwave radiation and reduced surface turbulent fluxes in midlatitudes, from the warmer atmosphere. As a result, the Northern Hemisphere atmosphere loses energy, which is compensated by a northward cross-equatorial atmospheric energy transport, leading to a southward shift of the tropical precipitation in boreal fall. Thus, although Arctic vegetation is changed throughout the year, its effect on tropical precipitation exhibits substantial seasonal variations.
Abstract
Tropical climate response to greenhouse warming is to first order symmetric about the equator but climate models disagree on the degree of latitudinal asymmetry of the tropical change. Intermodel spread in equatorial asymmetry of tropical climate response is investigated by using 37 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). In the simple simulation with CO2 increase at 1% per year but without aerosol forcing, this study finds that intermodel spread in tropical asymmetry is tied to that in the extratropical surface heat flux change related to the Atlantic meridional overturning circulation (AMOC) and Southern Ocean sea ice concentration (SIC). AMOC or Southern Ocean SIC change alters net energy flux at the top of the atmosphere and sea surface in one hemisphere and may induce interhemispheric atmospheric energy transport. The negative feedback of the shallow meridional overturning circulation in the tropics and the positive low cloud feedback in the subtropics are also identified. Our results suggest that reducing the intermodel spread in extratropical change can improve the reliability of tropical climate projections.
Abstract
Tropical climate response to greenhouse warming is to first order symmetric about the equator but climate models disagree on the degree of latitudinal asymmetry of the tropical change. Intermodel spread in equatorial asymmetry of tropical climate response is investigated by using 37 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). In the simple simulation with CO2 increase at 1% per year but without aerosol forcing, this study finds that intermodel spread in tropical asymmetry is tied to that in the extratropical surface heat flux change related to the Atlantic meridional overturning circulation (AMOC) and Southern Ocean sea ice concentration (SIC). AMOC or Southern Ocean SIC change alters net energy flux at the top of the atmosphere and sea surface in one hemisphere and may induce interhemispheric atmospheric energy transport. The negative feedback of the shallow meridional overturning circulation in the tropics and the positive low cloud feedback in the subtropics are also identified. Our results suggest that reducing the intermodel spread in extratropical change can improve the reliability of tropical climate projections.
Abstract
This study investigates the transient responses of atmospheric energy and momentum fluxes to a time-invariant extratropical thermal heating in an atmospheric model coupled to an aquaplanet mixed layer ocean with the goal of understanding the mechanisms and time scales governing the extratropical-to-tropical connection. Two distinct stages are observed in the teleconnection: 1) A decrease in the meridional temperature gradient in midlatitudes leads to a rapid weakening of the eddy momentum flux and a slight reduction of the Hadley cell strength in the forced hemisphere. 2) The subtropical trades in the forced hemisphere decrease and reduce evaporation. The resulting change to sea surface temperature leads to the development of a cross-equatorial Hadley cell, and the intertropical convergence zone shifts to the warmer hemisphere. The Hadley cell weakening in the first stage is related to decreased eddy momentum flux divergence, and the response time scale is independent of the mixed layer depth. In contrast, the time taken for the development of the cross-equatorial cell in the latter stage increases as the mixed layer depth increases. Once developed, the deep tropical cross-equatorial cell response is an order of magnitude stronger than the initial subtropical response and dominates the anomalous circulation. The analysis combines the momentum and energetic perspectives on this extratropical-to-tropical teleconnection and moreover shows that the subtropical circulation changes associated with the momentum budget occur with a time scale that is distinct from the deep tropical response determined by the thermal inertia of the tropical ocean.
Abstract
This study investigates the transient responses of atmospheric energy and momentum fluxes to a time-invariant extratropical thermal heating in an atmospheric model coupled to an aquaplanet mixed layer ocean with the goal of understanding the mechanisms and time scales governing the extratropical-to-tropical connection. Two distinct stages are observed in the teleconnection: 1) A decrease in the meridional temperature gradient in midlatitudes leads to a rapid weakening of the eddy momentum flux and a slight reduction of the Hadley cell strength in the forced hemisphere. 2) The subtropical trades in the forced hemisphere decrease and reduce evaporation. The resulting change to sea surface temperature leads to the development of a cross-equatorial Hadley cell, and the intertropical convergence zone shifts to the warmer hemisphere. The Hadley cell weakening in the first stage is related to decreased eddy momentum flux divergence, and the response time scale is independent of the mixed layer depth. In contrast, the time taken for the development of the cross-equatorial cell in the latter stage increases as the mixed layer depth increases. Once developed, the deep tropical cross-equatorial cell response is an order of magnitude stronger than the initial subtropical response and dominates the anomalous circulation. The analysis combines the momentum and energetic perspectives on this extratropical-to-tropical teleconnection and moreover shows that the subtropical circulation changes associated with the momentum budget occur with a time scale that is distinct from the deep tropical response determined by the thermal inertia of the tropical ocean.
Abstract
This study investigates the transient evolution of tropical Pacific sea surface temperature (SST) responses to a constant northern high-latitude solar heating in fully coupled CESM 1.2. The study identifies two stages through multiple ensemble runs. 1) In the first 3 years, a hemispherically asymmetric pattern emerges, caused by air–sea interactions associated with the anomalous cross-equatorial Hadley cell. The northern tropics experience warming that is blocked north of the equator by the intertropical convergence zone. The southeast Pacific cooling reaches the equatorial region and is amplified by the equatorial Ekman divergence. 2) Within a decade, the equatorial cooling is replaced by warming in the eastern equatorial basin. The anomalous warming that appears faster than the time scales of the oceanic ventilation is attributed to anomalous meridional heat convergence and weakening of the northern subtropical cell. Our findings highlight the influence of ocean dynamics on the temporal and spatial evolution of tropical SST response to hemispherically asymmetric heating. The initial cooling caused by Ekman divergence delays the arrival of slow warming, while initial wind and temperature anomalies set the stage for the weakening of the subtropical cell. The results have important implications for understanding the evolution of tropical SST patterns in observational records and future climate change simulations, as they show strong interhemispheric temperature asymmetry in the extratropics.
Abstract
This study investigates the transient evolution of tropical Pacific sea surface temperature (SST) responses to a constant northern high-latitude solar heating in fully coupled CESM 1.2. The study identifies two stages through multiple ensemble runs. 1) In the first 3 years, a hemispherically asymmetric pattern emerges, caused by air–sea interactions associated with the anomalous cross-equatorial Hadley cell. The northern tropics experience warming that is blocked north of the equator by the intertropical convergence zone. The southeast Pacific cooling reaches the equatorial region and is amplified by the equatorial Ekman divergence. 2) Within a decade, the equatorial cooling is replaced by warming in the eastern equatorial basin. The anomalous warming that appears faster than the time scales of the oceanic ventilation is attributed to anomalous meridional heat convergence and weakening of the northern subtropical cell. Our findings highlight the influence of ocean dynamics on the temporal and spatial evolution of tropical SST response to hemispherically asymmetric heating. The initial cooling caused by Ekman divergence delays the arrival of slow warming, while initial wind and temperature anomalies set the stage for the weakening of the subtropical cell. The results have important implications for understanding the evolution of tropical SST patterns in observational records and future climate change simulations, as they show strong interhemispheric temperature asymmetry in the extratropics.
