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Ken Takahashi

Abstract

The spread among the predictions by climate models for the strengthening of the global hydrological cycle [i.e., the global mean surface latent heat flux (LH), or, equivalently, precipitation] at a given level of CO2-induced global warming is of the same magnitude as the intermodel mean. By comparing several climate models from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) database under idealized CO2 forcings, it is shown that differences in the increase in global atmospheric shortwave heating (SWabs) induced by clear-sky absorption, presumably by water vapor, partly explains this spread. The increases in SWabs and LH present similar spreads across models but are anticorrelated, so the sum SWabs + LH increases more robustly than either alone. This is consistent with a recently proposed theory (Takahashi) that predicts that this sum (or, equivalently, the net longwave divergence minus the surface sensible heat flux) is constrained by energy conservation and robust longwave physics.

The intermodel scatter in SWabs changes is explained neither by differences in the radiative transfer models nor in intermodel differences in global water vapor content change, but perhaps by more subtle aspects of the changes in the water vapor distribution. Nevertheless, the fact that the radiative transfer models generally underestimate the increase in SWabs relative to the corresponding line-by-line calculation for a given change in water vapor content suggests that the climate models might be overestimating the rate of increase in the global hydrological cycle with global warming.

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Ken Takahashi

Abstract

The radiative constraints on the partitioning of the surface energy budget and, hence, on the strength of the hydrological cycle are analyzed in an idealized one-dimensional radiative–convective equilibrium model formulated in terms of the energy budgets at the top of the atmosphere, the subcloud layer, and the free atmosphere, which enables it to predict both surface relative humidity and the air–sea temperature difference. Using semigray radiative transfer, a semianalytical solution was obtained that explicitly shows how the surface latent heat flux (LHF) is related to the radiative properties of the atmosphere. This solution was also used in conjunction with a full radiative transfer code and was found to provide reasonably realistic quantitative estimates.

In the model the LHF is fundamentally constrained by the net longwave flux divergence above the level of condensation by lifting (LCL) and by the atmospheric absorption of shortwave radiation, with only a weak indirect control by near-surface moisture. The latter implies that the Clausius–Clapeyron relation does not directly constrain the strength of the hydrological cycle. Under radiative perturbations, the changes in LHF are determined by the changes in the net longwave fluxes at the LCL, associated mainly with the changes in the longwave transmissivity, and by the changes in shortwave absorption by the atmosphere (e.g., by increased water vapor).

Using a full radiative transfer model with interactive water vapor feedback with the semianalytical solution indicates a rate of change in LHF with greenhouse forcing of around 2 W m−2 K−1 of surface warming, which corresponds to the Planck feedback (∼3.2 W m−2 K−1) multiplied by a coefficient of order one that, to first approximation, depends only on the relative magnitudes of the net longwave radiation fluxes at the LCL and the top of the atmosphere (i.e., on the shape of the vertical profile of the net longwave flux).

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Ken Takahashi

Abstract

The relative importance of the processes responsible for the annual cycle in the upper-ocean heat content in the Peru Current, in the southeastern tropical Pacific, was diagnosed from an oceanic analysis dataset. It was found that the annual cycle of heat content is forced mainly by insolation. However, the ocean dynamical processes play an important role in producing different regional budget characteristics.

In a band 500 km from the coast of Peru, the annual heat content changes in this region are relatively large and can be approximated as sea surface temperature (SST) changes in a fixed-depth mixed layer. The annual cycle of the albedo associated with low-level clouds enhances the annual cycle in insolation, which explains the relatively strong annual cycle of heat content. These clouds, to a large extent, act as a feedback to SST, but a small additional forcing, which is proposed to be cold air advection in this paper, is needed to explain the fact that the maximum cloudiness leads the lowest SST by around a month. Ocean dynamics is important closer to the coast, where upwelling acts partly as damping of the heat content changes and forces it to peak earlier than farther offshore.

In a band farther to the southwest, locally wind-forced thermocline motions, which become shallower (deeper) in the warm (cool) season, partially cancel the effect of net surface heat fluxes, whose annual cycle is comparable to that in the region previously mentioned, producing a relatively small annual cycle of heat content. The local forcing appears to be associated with the annual meridional displacements of the South Pacific anticyclone. The annual cycle in SST is also relatively small, which is probably due to the changes in the temperature of the water entrained into the mixed layer associated with the thermocline motions, but also to a mixed layer deeper than that closer to the coast.

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Ken Takahashi and David S. Battisti

Abstract

The nature of the South Pacific convergence zone (SPCZ) is addressed by focusing on the dry (and cool) zone bounded by it and the coast of South America through numerical experiments. As shown in a companion paper, this dry zone is due, to a large extent, to orographically forced subsidence. Here it is shown that the northwestward expansion of this dry zone can be explained by advection of low moist static energy by the trade winds. These results provide an explanation of the geometry of the western edge of the dry zone and, therefore, of the eastern edge of the adjacent SPCZ. Sea surface temperature underneath the SPCZ is enhanced by relatively high near-surface humidity through evaporative processes, which feeds back into its organization. However, in this model, this feedback is not critical for the existence of the SPCZ. The subsidence associated with the ITCZ in the North Hemisphere negatively affects the precipitation rate in the SPCZ. It was also found that the sensitivity of the forced response is largest for peak orographic heights below 3000 m, which indicates that the exact representation of the Andes in numerical models might not be as critical as that of lower orography such as that in southern Africa.

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Nicholas J. Lutsko and Ken Takahashi

Abstract

The relationship between climate models’ internal variability and their response to external forcings is investigated. Frequency-dependent regressions are performed between the outgoing top-of-atmosphere (TOA) energy fluxes and the global-mean surface temperature in the preindustrial control simulations of the CMIP5 archive. Two distinct regimes are found. At subdecadal frequencies the surface temperature and the outgoing shortwave flux are in quadrature, while the outgoing longwave flux is linearly related to temperature and acts as a negative feedback on temperature perturbations. On longer time scales the outgoing shortwave and longwave fluxes are both linearly related to temperature, with the longwave continuing to act as a negative feedback and the shortwave acting as a positive feedback on temperature variability. In addition to the different phase relationships, the two regimes can also be seen in estimates of the coherence and of the frequency-dependent regression coefficients. The frequency-dependent regression coefficients for the total cloudy-sky flux on time scales of 2.5 to 3 years are found to be strongly (r 2 > 0.6) related to the models’ equilibrium climate sensitivities (ECSs), suggesting a potential “emergent constraint” for Earth’s ECS. However, O(100) years of data are required for this relationship to become robust. A simple model for Earth’s surface temperature variability and its relationship to the TOA fluxes is used to provide a physical interpretation of these results.

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Ken Takahashi and David S. Battisti

Abstract

The question of why the intertropical convergence zone (ITCZ) is generally north of the equator in the tropical Pacific is addressed. Experiments with an atmospheric general circulation model coupled to idealized representations of the ocean show that the presence of the Andes is enough to lower sea surface temperature (SST) off the west coast of South America through evaporation, thus promoting a north–south asymmetry, with the ITCZ north of the equator, which is amplified by interactions between the ocean and the atmosphere. The evaporative cooling results mainly from the subsidence of low specific humidity air, which is due in turn to the mechanical effect of the Andes on the zonal mean flow. The positive feedback from low-level clouds on SST is an important factor for the efficiency of the mechanism described.

West of 120°W, the presence of the Rockies and Himalayas produces a comparable forcing to that of the Andes, but this is not enough to reverse or neutralize the north–south asymmetry set by the Andes. It is shown that the longitudinal offset between the forcings in both hemispheres allows the Andes to preferentially set the north–south asymmetry, which propagates westward into the rest of the Pacific.

Asymmetry in the observed ocean heat transports (more heat transport convergence in the Northern Hemisphere) associated with the Kuroshio was found to reinforce the effect of the Andes, although it is not a strong forcing by itself. Sensitivity experiments indicate that the north–south asymmetry of the ITCZ caused (evaporatively) by the Andes is robust to the presence of a strong equatorial cold tongue and to seasonality in insolation.

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Michael Winton, Ken Takahashi, and Isaac M. Held

Abstract

This article proposes a modification to the standard forcing–feedback diagnostic energy balance model to account for 1) differences between effective and equilibrium climate sensitivities and 2) the variation of effective sensitivity over time in climate change experiments with coupled atmosphere–ocean climate models. In the spirit of Hansen et al. an efficacy factor is applied to the ocean heat uptake. Comparing the time evolution of the surface warming in high and low efficacy models demonstrates the role of this efficacy in the transient response to CO2 forcing. Abrupt CO2 increase experiments show that the large efficacy of the Geophysical Fluid Dynamics Laboratory’s Climate Model version 2.1 (CM2.1) sets up in the first two decades following the increase in forcing. The use of an efficacy is necessary to fit this model’s global mean temperature evolution in periods with both increasing and stable forcing. The intermodel correlation of transient climate response with ocean heat uptake efficacy is greater than its correlation with equilibrium climate sensitivity in an ensemble of climate models used for the third and fourth Intergovernmental Panel on Climate Change (IPCC) assessments. When computed at the time of doubling in the standard experiment with 1% yr−1 increase in CO2, the efficacy is variable amongst the models but is generally greater than 1, averages between 1.3 and 1.4, and is as large as 1.75 in several models.

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Juan Sulca, Mathias Vuille, Yamina Silva, and Ken Takahashi

ABSTRACT

Extreme precipitation events in the Peruvian Andes have significant socioeconomic impacts, yet their atmospheric dynamics are poorly understood. Here austral summer (December–March) wet and dry spells and their continental- and large-scale teleconnections are analyzed using reanalysis, gridded, and in situ precipitation data. Dry and wet spells in the Peruvian Andes show a pervasive dipole pattern with precipitation anomalies of the opposite sign over northeastern Brazil. Composite anomalies of various atmospheric fields during extreme precipitation events indicate that this dipole is related to large-scale adjustments in the upper-tropospheric Bolivian high–Nordeste low system, which in turn are modulated by northward-propagating extratropical Rossby wave trains. At upper- and midtropospheric levels, westerly wind anomalies over the Peruvian Andes suppress moisture flux from the Amazon during dry events, while wet events are characterized by opposite conditions. Yet, while easterly wind anomalies appear to be a prerequisite for heavy precipitation events in the region, they are not a sufficient forcing, as dry days can still occur during such periods. Dry spells in the Peruvian Andes appear to be linked to weakened convective activity over the western tropical Pacific, consistent with the previously documented El Niño influence over the region. Extreme dry and wet spells in northeastern Brazil only show a weak link to precipitation anomalies of the opposite sign over Peru but are strongly coupled with changes in the position and strength of the Nordeste low and the South Atlantic convergence zone.

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Yechul Shin, Sarah M. Kang, Ken Takahashi, Malte F. Stuecker, Yen-Ting Hwang, and Doyeon Kim

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 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.

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Isaac M. Held, Michael Winton, Ken Takahashi, Thomas Delworth, Fanrong Zeng, and Geoffrey K. Vallis

Abstract

The fast and slow components of global warming in a comprehensive climate model are isolated by examining the response to an instantaneous return to preindustrial forcing. The response is characterized by an initial fast exponential decay with an e-folding time smaller than 5 yr, leaving behind a remnant that evolves more slowly. The slow component is estimated to be small at present, as measured by the global mean near-surface air temperature, and, in the model examined, grows to 0.4°C by 2100 in the A1B scenario from the Special Report on Emissions Scenarios (SRES), and then to 1.4°C by 2300 if one holds radiative forcing fixed after 2100. The dominance of the fast component at present is supported by examining the response to an instantaneous doubling of CO2 and by the excellent fit to the model’s ensemble mean twentieth-century evolution with a simple one-box model with no long times scales.

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