Abstract

The persistence of multiple jets is investigated with a quasigeostrophic, two-layer, β-plane channel model. Linearly unstable normal modes are found to be capable of qualitatively describing the eddy fluxes of the nonlinear model. For a persistent double jet (PDJ) state, the most unstable normal mode has its largest amplitude located between the two jets, with a downshear tilt that acts to keep the jets separated. The opposite tilt occurs for a double jet state that is intermittent. An analysis of these normal modes, which utilized the concept of counterpropagating Rossby waves (CRWs), suggests that the downshear tilt in the interjet region hinges on the presence of critical latitudes only in the lower layer. This conclusion in turn suggests that the initial generation of the persistent jets requires L/C _gy < r ⁻¹, where L is the distance between the wave source (jet) and sink (interjet), C _gy is the meridional group velocity, and r is the linear damping rate. Similar CRW analysis for a conventional normal mode, which has its largest amplitude at the jet centers, suggests that the downshear tilt adjacent to the jet maxima is associated with the presence of critical latitudes only in the upper layer. The PDJ is found to be accompanied by potential vorticity (PV) staircases in the upper layer, characterized by a strong PV gradient at the jet centers and a broad region of homogenized PV between the jets. This PV mixing is realized through baroclinic waves that propagate slowly westward in the interjet region. Nonlinear evolution of the most unstable normal mode of the PDJ shows that northward heat flux by these waves is crucial for broadening the interjet PV mixing zone necessary for producing the PV staircase.

Abstract

During boreal winter, the climatological stationary wave plays a key role in the poleward transport of heat in mid- and high latitudes. Latent heating is an important driver of boreal-winter stationary waves. In this study, the temporal relationship between tropical and extratropical heating and transient–stationary wave interference is investigated by performing observational data analyses and idealized model experiments. In line with stationary wave theory, the observed heating anomaly fields during constructive interference events have a spatial structure that reinforces the zonal asymmetry of the climatological heating field. The observational analysis shows that about 10 days prior to constructive interference events, tropical heating anomalies are established, and within 1 week North Pacific and then North Atlantic heating anomalies follow. This result suggests that constructive interference involves a heating–circulation relay: tropical latent heating drives circulation anomalies that transport moisture in such a manner as to increase latent heating in the North Pacific; circulation anomalies driven by this North Pacific heating similarly lead to enhanced latent heating in the North Atlantic. This heating–circulation relay picture is supported by initial-value model calculations in which the observed heating anomalies are used to drive model circulations. Our results also show that the constructive interference driven by both tropical and extratropical diabatic heating generates a relatively large-amplitude wave in high latitudes and leads to particularly prolonged Arctic warming episodes, whereas when both the tropical and extratropical diabatic heating are weak, constructive interference is confined to midlatitudes and does not lead to Arctic warming.

Abstract

The dynamical mechanisms that lead to wintertime Arctic warming during the planetary-scale wave (PSW) and synoptic-scale wave (SSW) life cycles are identified by performing a composite analysis of ERA-Interim data. The PSW life cycle is preceded by localized tropical convection over the western Pacific. Upon reaching the midlatitudes, the PSWs amplify as they undergo baroclinic conversion and constructively interfere with the climatological stationary waves. The PSWs flux large quantities of sensible and latent heat into the Arctic, which produces a regionally enhanced greenhouse effect that increases downward IR and warms the Arctic 2-m temperature. The SSW life cycle is also capable of increasing downward IR and warming the Arctic 2-m temperature, but the greatest warming is accomplished in the subset of SSW events with the most amplified PSWs. Consequently, during both the PSW and SSW life cycles, wintertime Arctic warming arises from the amplification of the PSWs.

Abstract

Future projections of the poleward eddy heat flux by the atmosphere are often regarded as being uncertain because of the competing effect between surface and upper-tropospheric meridional temperature gradients. Previous idealized modeling studies showed that eddy heat flux response is more sensitive to the variability of lower-tropospheric temperature gradient. However, observational evidence is lacking. In this study, observational data analyses are performed to examine the relationships between eddy heat fluxes and temperature gradients during boreal winter by constructing daily indices. On the intraseasonal time scale, the surface temperature gradient is found to be more effective at regulating the synoptic-scale eddy heat flux (SF) than is the upper-tropospheric temperature gradient. Enhancements in surface temperature gradient, however, are subject to an inactive planetary-scale eddy heat flux (PF). The PF in turn is dependent on the zonal gradient in tropical convective heating. Consistent with these interactions, over the past 40 winters, the zonal gradient in tropical heating and PF have been trending upward, while the surface temperature gradient and SF have been trending downward. These results indicate that for a better understanding of eddy heat fluxes, attention should be given to zonal convective heating gradients in the tropics as much as to meridional temperature gradients.

Abstract

This paper describes the evolution of global angular momentum (GAM) on intraseasonal timescales in data from two general circulation model (GCM) runs: an aquaplanet GCM and a fully “realistic” GCM that includes continents, topography, and observed climatological sea surface temperatures.

For both GCMS, the angular momentum budget is quite well balanced. Composites of various quantities are calculated at different lags relative to the maximum GAM and GAM tendency. In both GCMS, this composite analysis shows that the GAM tendency is largest as a precipitation anomaly propagates eastward along the equator. Associated with this precipitation anomaly is a tropical circulation that shows some of the characteristics of the Gill model, particularly in the aquaplanet GCM, and a Rossby wave train that propagates from the Tropics into midlatitudes. It is the anomalous midiatitude surface wind field associated with this Rossby wave train that is primarily responsible for the anomalous friction torques in both models. In the realistic GCM, this Rossby wave train has the appropriate structure to induce a large mountain torque, particularly at the Rocky Mountains. Also, it is found that the friction and mountain torques contribute about equally to the intraseasonal evolution of GAM, with the anomalous friction torque leading the anomalous mountain torque by three days.

After the precipitation anomaly weakens, the Rossby wave train completely propagates out of the Tropics and leaves behind a pattern resembling that of a Kelvin wave. Consistent with this wave propagation, in both GCMS, eddies transport the angular momentum gained at the surface in midlatitudes toward the equator. Lastly, the effects of zonal inhomogencities on the wave dynamics associated with GAM evolution are discussed.

Abstract

An initial-value problem is employed with a GCM to investigate the role of the convectively driven Rossby and Kelvin waves for tropopause transition layer (TTL) upwelling in the tropics. The convective heating is mimicked with a prescribed heating field, and the Lagrangian upwelling is identified by examining the evolution of passive tracer fields whose initial distribution is identical to the initial heating field. This study shows that an overturning circulation, induced by the tropical Rossby waves, is capable of generating the TTL upwelling. Even when the heating is placed in the eastern Pacific, the TTL upwelling occurs only over the western tropical Pacific, indicating that the background flow plays a crucial role. The results from a Rossby wave source analysis suggest that a key feature of the background flow is the strong absolute vorticity gradient associated with the Asian subtropical jet. In addition, static stability is relatively weak over the western Pacific, suggesting that this may also contribute to the TTL upwelling in that region.

The background flow also modulates the internal Kelvin waves in such a manner that the coldest region in the TTL (resembling the observed “cold trap”) occurs over the western tropical Pacific. As a consequence, the upwelling air, induced by the meridional momentum flux of the Rossby wave, passes through the cold trap generated by the Kelvin wave. Since in reality the background flow is shaped by the convective heating, the climatological western tropical Pacific heating is ultimately responsible for both the TTL upwelling and the cold trap; however, both processes are realized indirectly through its impact on the background flow and the generation of the tropical waves.

Abstract

The structure of westerly jets in a statistically steady state is investigated with both dry and moist models on the sphere. The dry model is forced with an idealized radiative equilibrium temperature profile that consists of a global-scale base profile plus both localized tropical heating and high-latitude cooling. The tropical heating controls the intensity of the subtropical jet, while the high-latitude cooling modulates the meridional width of the extratropical baroclinic zone.

The jet structure is analyzed with a large number of dry model runs in which the tropical heating and high-latitude cooling rates are systematically varied. This parameter study shows that, in a regime with weak tropical heating and strong high-latitude cooling, the polar-front jet separates itself from the subtropical jet, forming a double-jet state. In contrast, if the tropical heating rate is greater than a certain value, a strong single jet emerges, indicating that the presence of one or two jets in a statistically steady state is dependent upon the relative values of both the tropical heating and the baroclinic zone width.

A set of moist model runs is examined in which the moisture content is systematically varied. For a relatively small moisture content, the circulation prefers a double-jet state. However, for a moisture content that is greater than a certain threshold value, the two jets collapse into a single jet. With the aid of the aforementioned dry model results, an explanation for this nonlinear response exhibited by the moist model is provided. Based on the results of the dry and moist model calculations, this paper discusses various physical interpretations of the circulation responses to global warming presented in the literature.

Abstract

A heat-engine analysis of a climate system requires the determination of the solar absorption temperature and the terrestrial emission temperature. These temperatures are entropically defined as the ratio of the energy exchanged to the entropy produced. The emission temperature, shown here to be greater than or equal to the effective emission temperature, is relatively well known. In contrast, the absorption temperature requires radiative transfer calculations for its determination and is poorly known.

The maximum material (i.e., nonradiative) entropy production of a planet’s steady-state climate system is a function of the absorption and emission temperatures. Because a climate system does no work, the material entropy production measures the system’s activity. The sensitivity of this production to changes in the emission and absorption temperatures is quantified. If Earth’s albedo does not change, material entropy production would increase by about 5% per 1-K increase in absorption temperature. If the absorption temperature does not change, entropy production would decrease by about 4% for a 1% decrease in albedo. It is shown that, as a planet’s emission temperature becomes more uniform, its entropy production tends to increase. Conversely, as a planet’s absorption temperature or albedo becomes more uniform, its entropy production tends to decrease. These findings underscore the need to monitor the absorption temperature and albedo both in nature and in climate models.

The heat-engine analyses for four planets show that the planetary entropy productions are similar for Earth, Mars, and Titan. The production for Venus is close to the maximum production possible for fixed absorption temperature.

Abstract

Spatial structure of annular modes shows a remarkable resemblance to that of the recent trend in the observed circulation (Thompson et al.). This study performs a series of multilevel primitive equation model simulations to examine the extent to which the annular mode is capable of predicting changes in the zonal-mean flow response to external heat perturbations. Each of these simulations represents a statistically steady state and differs from each other in the values of the imposed tropical heating (H) and high-latitude cooling (C).

Defining the annular mode as the first empirical orthogonal function (EOF1) of zonal-mean tropospheric zonal wind, it is found that the “climate predictability” is generally high in the small C–large H region of the parameter space, but is markedly low in the large C–small H region. In the former region, EOF1 represents meridional meandering of the midlatitude jet, while in the latter region, EOF1 and EOF2 combine to represent coherent poleward propagation of zonal-mean flow anomalies. It is also found that the climate predictability tends to be higher with respect to changes in C than to changes in H. The implications of these findings for the Southern Hemisphere climate predictability are also presented.

Abstract

A two-layer quasi-geostrophic model forced by surface friction and radiative relaxation to a jetlike wind profile can exist in either a wave-free state or in a finite-amplitude wave state, over a substantial region of the model's parameter space. The friction on the lower layer must be much stronger than the thermal relaxation, and the upper layer must be nearly inviscid, for this behavior to be observed.

Consistent with this behavior, weakly unstable waves are found that do not stabilize the flow; instead, their growth rate increases with wave amplitude. We attempt to provide a physical explanation for this behavior in terms of 1) the competition between the stabilizing effect of the lower-layer potential vorticity fluxes and the destabilizing effect of nonlinear critical layer formation associated with the upper-layer fluxes, and 2) the tendency of surface drag to restore the vertical shear at the center of the jet by damping the surface westerlies generated by the baroclinic instability.