A31A-0008
Time scale and feedback of zonal mean flow variability
The physical processes which determine the time scale of zonal mean flow variability are examined with an idealized numerical model that has zonally symmetric lower boundary (Son and Lee, 2005). In the part of the parameter space where the time-mean zonal flow is characterized by a single (double) jet, the dominant form of zonal mean flow variability is the zonal index (poleward propagation), and the time-mean potential vorticity gradient is found to be strong and sharp (weak and broad). The e-folding time scale of the zonal index is found to be close to 55 days, much longer than the observed 10-day time scale. The e-folding time scale of the poleward propagation is about 40 days. The long e-folding time scales for the zonal index are found to be consistent with an unrealistically strong and persistent eddy-zonal mean flow feedback. A calculation of the refractive index indicates that the background flow supports eddies that are trapped within midlatitudes, undergoing relatively little meridional propagation. Additional model runs are performed with an idealized mountain to investigate whether zonal asymmetry can disrupt the eddy feedback. For single jet states, the time scale is reduced to about 30 days if the mountain height is 4 km or less. The reduction in the time scale occurs because the stationary eddies excited by the mountain alter the background flow in a manner which leads to the replacement of zonal index events by shorter time-scale poleward propagation. With a 5-km mountain, the time scale reverts and increases to 105 days. This threshold behavior is again attributed to a sharpening of the background zonal jet which arises from an extremely strong stationary wave momentum flux convergence. In contrast, for double jet states, the time scale changes only slightly and the poleward propagation is maintained in all mountain runs.
A31A-0009
Potential Vorticity Perspective on the Stratospheric Origin of Cold Surge in East Asia
Traditionally, the coupling between the troposphere and the stratosphere has been considered as one-way influence of the troposphere on the stratosphere, with the troposphere being regarded as a source of Rossby waves and gravity waves, which propagate upward into the stratosphere. However, observational and modelling evidence is now strong that the stratosphere may play an active role in the tropospheric circulation leading to changes in the troposphere. Jeong et al. (2006) found a precursory signal in the stratosphere prior to the cold surge occurrence in East Asia. Over northern Eurasia, about one week before the cold surge occurrence, strong stratospheric negative potential vorticity and rising of geopotential height are observed. From this, we hypothesized the upper-level signal is really linked to the amplification of Siberian High through the deep penetrating circulation anomalies of stratospheric PV. To prove this, we separated entire PV anomaly into two pieces which are above and below the tropopause and examined the balanced circulation of each piece. Piecewise PV inversion technique introduced by Davis (1994) has been used for the calculation. From the inversion results, we could successfully verify that the Siberian High amplification largely depends on the circulation anomaly originating from stratospheric PV. East Asian cold surge is accompanied by the upper tropospheric short-wave trough over Lake Baikal that grows as propagating toward East Asia. However, the processes causing the upstream development of the upper-level trough are not clear. Given the reliable stratospheric influence, therefore, it is worth inquiring the subsequent cold surge event after the amplification of Siberian High. For this purpose, I constructed a simple model which has an embedded life cycle of artificial Siberian High. The model is designed to be relaxed toward radiative equilibrium temperature of northern Hemisphere Winter (perpetual winter run). Several diabatic heating anomalies are also prescribed over Tropical ocean to obtain realistic model climatology. The repeated life cycle of Siberian High was simply controlled by periodic diabatic cooling over Siberia. From the result, the organized baroclinic eddies started to appear downstream of Siberia when the Siberia is sufficiently cold due to the local baroclinic instability. The increase of Eady growth rate and downstream sequence of wave activity vector supports this fact. The links revealed in this study may also offer scope for improving the skill of extended-range tropospheric forecasts.
A31A-0010
An Idealized General Circulation Model for the Study of Stratospheric-Tropospheric Coupling
An idealized General Circulation Model of the atmosphere is developed to investigate the dynamical coupling between the stratosphere and troposphere. The model is similar to that of Polvani and Kushner (2002, GRL), but with the addition of topographically induced stationary planetary waves, which prove to be essential for coupling between the spheres on intraseasonal timescales. The frequency of sudden warming events and the timescale at which events descend through the stratosphere -- and sometimes into the troposphere -- is explored. We find that the mean structure and variability of the polar vortex is very sensitive to the amplitude of the topography, and Northern Hemisphere-like variability, with the correct frequency of warming events, occurs only for a relatively narrow range of topographic heights. The addition of a seasonal cycle also allows for the analysis of final warming events. The timing of the final warming is of particular interest, as it has been difficult to capture in more sophisticated models.
A31A-0011
Low Cloud Climate Feedback Mechanisms in Idealized Simulations with the Community Atmospheric Model (CAM3)
This study investigates the physical mechanism of low cloud feedback in the Community Atmospheric Model (CAM3) through idealized experiments over the subtropical eastern oceans. Negative cloud feedback is simulated from stratus and stratocumulus that is consistent with previous diagnostics of cloud feedbacks in CAM3 and its predecessor versions. The negative feedback is attributed to the increased lower-level convective instability due to the Clausius-Clapeyron dependence of surface moisture on temperature, and the relatively small sensitivity of cloud water to temperature than rain production. The former is responsible for the larger amount of in-cloud water in stratiform clouds, while the latter is responsible for a longer life time of clouds, both of which contributed to the negative cloud feedback. The feedback is materialized through the interaction of a suite of parameterized processes rather than from any single process. These include convection, boundary layer turbulence, radiation, cloud fraction, and the cloud macrophysical and microphysical schemes. The thermodynamic effect is found to dominate the negative cloud feedback in the model. The dynamic effect of weaker subsidence in a warmer climate also contributes to the negative cloud feedback, but with about one quarter of the magnitude of the thermodynamic effect.
A31A-0012
The Response of the ITCZ to Extratropical Thermal Forcing: Idealized Slab-Ocean Experiments with Idealized and Comprehensive GCMs
We study the mechanism by which displacements of the ITCZ are forced from the extratropics in both idealized and comprehensive atmospheric GCMs, in an aqua-planet configuration coupled to a slab ocean. Many recent paleoclimatic studies, both observational and modeling, have pointed to the influence of high latitudes on the tropical precipitation distribution. In this study, we use both idealized and comprehensive atmospheric GCMs to develop a better understanding of what controls this tropical response. The idealized moist GCM is that described by Frierson et al (2006), which has no cloud or water vapor feedbacks and includes a simplified Betts-Miller convection scheme. One of the convection scheme parameters can be varied to alter the ITCZ response. The results from the idealized model are compared with those from AM2, an atmospheric general circulation model developed at the Geophysical Fluid Dynamics Laboratory (GFDL). Heating is imposed poleward of 40°S with equal and opposite cooling added poleward of 40°N, equivalent to an imposed cross-equatorial heat flux in the ocean. As an intermediate step in understanding the tropical response, we focus on the degree of compensation between the imposed oceanic flux and the resulting response in the atmospheric energy transport. The idealized model produces a low level of compensation of about 25%. An energy balance model is constructed to support the claim that this low level of compensation is expected if the primary communication between the extratropics and the Hadley cell is through eddy fluxes of moist static energy. This claim can also be confirmed from the experiments of varying meridonal gradient of solar radiation that show increasing baroclinic eddy activities increases the degree of compensation. A simple theory is developed that predicts the precipitation response, given this degree of compensation and a measure of the gross moist stability of the model tropics. The gross moist stability can be modified by altering the convection scheme, providing a test of this theory. In AM2, all cases show a much greater shift of the ITCZ than in the idealized model, related to the fact that in AM2 the compensation of the implied oceanic transport by the atmospheric energy transport is much larger (~65% rather than ~25%). We argue that this enhancement of the response is due to changes in cloud and water vapor feedbacks. This can be confirmed from the experiments with a different parameter in the convection scheme that limits the entrainment into convective plumes, which affects the extratropical-tropical interactions in the model primarily by modifying the response of clouds to the extratropical forcing. The dependence of the ITCZ response on cloud feedbacks suggests an important way in which uncertainties in cloud modeling can create uncertainties in regional responses to climatic perturbations.
A31A-0013
Hurricane Formation in Diabatic Ekman Turbulence
This paper examines a simple representation of turbulent flow in the tropical troposphere, which occasionally produces a hurricane. In this paradigm, the flow is essentially two-dimensional (2D) turbulence under the cooperative influence of Ekman pumping and deep cumulus convection. After an incubation period, diabatic Ekman pumping can supercede ideal 2D mechanisms of self-organization, such as vortex merger. A strong cyclone-anticyclone asymmetry can develop, with very intense convective cyclones dominating the system. Diabatic Ekman Turbulence (DET), as described above, is readily studied with a 3-layer model of the troposphere. The model used here includes a frictional boundary layer (BL), a lower troposphere (LT), and an upper troposphere (UT). The parameterizations of surface fluxes, deep convection and radiative cooling are similar to those used by Ooyama in his seminal study of axisymmetric tropical cyclone intensification [Ooyama, K., J. Atmos. Sci., 26, 3 (1969)]. The results given below are from numerical simulations in a 2000km-by-2000km periodic box. In all simulations, the initial turbulence is concentrated in the BL and LT, whereas the UT starts at rest. In general, we find that DET freely evolves into a Frictional Radiative Convective Equilibrium (FRCE), in which there is an approximate balance between convective energy input and energy output by surface drag and radiation. The predictability of the FRCE that emerges from random noise varies with sea-surface temperature (SST), the Coriolis parameter f, and the ratio CE/CD, in which CE and CD are surface-exchange coefficients for moist entropy and momentum, respectively. At low values of these control parameters, DET tends to dissipate. As the control parameters increase to typical tropical values, the FRCE bifurcates into a metastable synoptic-scale gyre or a hurricane. At higher values of the control parameters, the FRCE is always a hurricane. The hurricane that emerges from DET is realistic in several ways. During rapid intensification, the hurricane typically develops polygonal eyewalls and mesovortices. In the FRCE, the hurricane exhibits moderate intensity oscillations that resemble eyewall breakdown and regeneration cycles. The time-averaged intensity of the hurricane increases with the SST and the ratio of surface-exchange coefficients CE/CD [ibid; Emanuel, K.A., J. Atmos. Sci., 43, 585 (1986)].
A31A-0014
The poleward extent of the midlatitude storm tracks and Hadley circulation in an idealized GCM
Simulations from an idealized, dry GCM are analyzed to investigate how the location of the midlatitude storm tracks varies over a large range of insolation gradients, planetary radii, and rotation rates. Over a wide range of meridional insolation gradients and extratropical stability values, both the location of the midlatitude storm tracks and the edge of the Hadley circulation move poleward in tandem, maintaining a constant spatial separation for much of the parameter range. We find that the supercriticality (Sc), a measure of ratio of the meridional temperature gradient to the bulk stability, assumes a constant value of Sc ~ 0.7 at the edge of the Hadley circulation for most parameter choices, which suggests that the Hadley cell extent may be controlled by the subtropical stability. We repeat this analysis for simulations from an idealized moist model and using reanalysis data. The idealized model provides an excellent framework to test these relationships over a wide range of climate regimes
A31A-0015
Precipitation and large scale circulation change with global warming in the coupled GCMs : effect of the SST forcing
The features of the Sea Surface Temperature (SST) changes which may explain the response of the IPSL-CM4 atmosphere-ocean coupled models (AOGCM) in terms global circulation and precipitation patterns are diagnosed when the carbon dioxide concentration is doubled. Simulation using the atmospheric component of the IPSL-CM4 AOGCM, called LMDZ4, forced by prescribed SST are run. The AOGCM shows a global weakening of the Hadley/Walker circulation, accompanied with a comparatively smaller increase of the precipitation for southern hemisphere winter. For northern hemisphere winter an increase of the Hadley circulation is diagnosed. To explain the nature of these changes, we run specific atmospheric simulations in response respectively to the global mean SST changes, the zonal mean SST anomalies and the longitudinal anomalies of the SST fields. When forced by the uniform changes of the SST, the AGCM LMDZ4 fails to represent the horizontal pattern of the Hadley circulation changes. It is able to reproduce roughly the modifications in the intensity of the hydrological cycle, but fail to capture the main regional changes. Prescribing the zonal anomalies of SST changes is enough to represent fairly well the horizontal changes in the Hadley circulation simulated by the coupled model, even if the main thermodynamical state of the tropical atmosphere does not change. The longitudinal SST anomalies are shown to have a much lesser impact on the precipitation and on the large scale tropical circulation. The results show that the geographical (meridional and longitudinal) pattern of the SST warming are crucial to assess the Hadley circulation and the precipitation changes in the AGCM.
A31A-0016
Two types of baroclinic instability in Southern Hemisphere Summer
The energetics of mid-latitude baroclinic eddy life cycles is investigated with NCEP/NCAR Reanalysis data. A composite analysis is performed for the Southern Hemisphere summer for the years 1980 through 2004. Individual life cycles are identified by local maxima in synoptic-scale eddy energy. Two types of baroclinic life cycles are examined, each defined by the strength of the barotropic energy conversion two days prior to the maximum baroclinic growth. For one life cycle, the barotropic conversion is anomalously weak before the maximum baroclinic growth, and for the other life cycle, the barotropic conversion is anomalously strong. These two life cycles are referred to as the weak barotropic (WB) and strong barotropic (SB) life cycles, respectively. The composite calculation for the WB (SB) life cycle finds that a strengthening (weakening) of the poleward wave activity flux relative to that of the climatology takes place before the initial growth of the synoptic scale eddies. The source region for these fluctuations in the wave activity flux is found to occur within the Southern Hemisphere tropics. For the WB (SB) life cycle, this change in the wave activity flux, and the corresponding eddy momentum flux, is shown to drive a weakening and broadening (strengthening and narrowing) of the midlatitude zonal mean jet. Consistent with the barotropic governor mechanism, where a reduction in the horizontal shear coincides with more rapidly growing baroclinic eddies, the baroclinic growth and the maximum energy attained for the WB life cycle exceeds that for the SB life cycle. Furthermore, it is found that the WB (SB) life arises when the baroclinicity is weaker (stronger) than that of the climatology. This suggests that the influence of the barotropic governor and baroclinicity oppose each other at the beginning of the life cycle, with the latter being dominant. Both the WB and SB life cycles coincide with an active Madden-Julian Oscillation (MJO). For the WB (SB) life cycle, the MJO convection corresponds to a strengthening (weakening) of the convection in the western tropical Pacific. These results suggest that the above changes in the wave activity flux are driven by the MJO. This, in turn, further suggests that the two types of baroclinic life cycles are ultimately triggered by MJO convection.
A31A-0017
Formation of extratropical jets in baroclinic atmospheres
It is commonly held that multiple extratropical jets in baroclinic atmospheres form as a result of an inverse cascade of eddy kinetic energy to large scales, with the jet separation scale given by the Rhines scale. Here we show that multiple jets in baroclinic atmospheres can form when the nonlinear eddy-eddy interactions that give rise to an inverse energy cascade are weak or entirely absent. In simulations with an idealized GCM, multiple extratropical jets form when the Rossby radius is smaller than the planetary radius, for example, when the planetary radius or rotation rate are sufficiently large or the static stability of the atmosphere is sufficiently small. The meridional jet scale is similar to the Rossby radius and, if the flow is sufficiently baroclinic, is also similar to the Rhines scale (otherwise the Rhines scale is smaller). When nonlinear eddy-eddy interactions are eliminated in the GCM, multiple jets continue to form, with meridional scales that are similar to, albeit somewhat smaller than, those in the fully nonlinear GCM simulations. This demonstrates that eddy-mean flow interactions alone can account for the formation of multiple jets in baroclinic atmospheres and that eddy-eddy interaction are not essential; however, eddy-eddy interactions lead to isotropization of eddies and modify meridional jet scales by O(1) factors.
A31A-0018
Sensitivity Of The Low Frequency Variability Of The Atmosphere To The Surface Friction In An Idealized Model
The low frequency variability of the atmosphere can play an important role in the climate response to external forcing. The persistence of the jet vacillation can often vary with the latitudinal shift of the climatological mean jet. We study the sensitivity of the low frequency variability of surface westerlies associated with the poleward shift of surface westerlies when the surface friction is reduced in the Held and Suarez model (1994, BAMS). In spite of the longer frictional damping scale, the westerlies are less persistent, as the barotropic shear of zonal flow gets stronger and shears out the jet vacillation. We further examine this mean flow modification from the viewpoint of the zonal mean momentum balance in both the equilibrated response and instantaneous response to a change in surface friction. Additionally, a simple shallow water model of upper troposphere is constructed, suggesting that the effect of the mean flow on eddies, possibly through changes in eddy phase speeds, may be important in generating this low frequency vacillation.
A31A-0019
The Effect of Moisture and Ocean Heat Transport on the Hadley Circulation: Simulations with an Idealized GCM
In simulations of global change, the tropical Hadley circulation expands as the temperature increases. We investigate dynamical mechanisms responsible for the response of the Hadley circulation to climate changes. In simulations with an idealized dry GCM, in a climate resembling that of present-day Earth, the strength and extent of an equinox Hadley cell are controlled by eddy fluxes and baroclinic instability in the subtropics, leading to an expansion of a Hadley cell with increasing tropical static stability (Walker & Schneider 2006). We investigate the extent to which these results carry over to moist atmospheres with simulations with an idealized moist GCM coupled to an oceanic mixed layer. Varying the optical thickness of the longwave absorber, we study configurations with no ocean heat transport and with an idealized ocean heat transport coupled to the atmospheric wind stress. The Hadley circulation generally expands as the optical thickness increases. The response of the strength of the Hadley circulation is more complex and non-monotonic but can be rationalized by considering the zonal momentum balance of the mean meridional circulation and its interaction with eddies.
A31A-0020
Coupled versus Uncoupled Atmospheric Model Integrations
The utility of atmospheric general circulation model (GCM) integrations with prescribed sea surface temperatures (SSTs) is increasingly being questioned in the contexts of climate diagnosis, climate model error diagnosis, and short-term climate predictions. The basic issue is to what extent the errors in surface heat fluxes caused by decoupling air-sea interactions in this manner affect climate variability and the mean climate. This issue is addressed here by generating and comparing multi-century coupled GCM simulations with corresponding atmospheric GCM simulations with prescribed SSTs obtained from the coupled simulations. When the SST time series is prescribed at the full (half-hourly) temporal resolution of the coupled model output, the uncoupled simulations have a negligibly small mean climate bias, small variance errors on subseasonal scales, and slightly larger variance errors on interannual and decadal scales. Even on decadal scales, however, the errors are notably smaller than anticipated from altered local thermal damping considerations alone. When the SSTs are prescribed at monthly temporal resolution, the mean bias remains small but the variance errors become larger, especially on subseasonal scales. When only the long-term mean SST seasonal cycle is prescribed, the variance errors become large, not surprisingly, also on the interannual and decadal scales. There is now also an appreciable mean bias, arising mainly from the atmospheric GCM's response to El Nino SST anomalies being larger than that to La Nina SST anomalies. Overall, these results show that the errors introduced by prescribing SSTs in atmospheric GCMs, though not negligible, are generally much smaller than the atmospheric response to the SSTs themselves. To that extent, they justify performing and using such uncoupled integrations for diagnostic and prediction purposes.