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Nicholas M. J. Hall
and
Jacques Derome

Abstract

A dry primitive equation model is used to investigate the remote response to a fixed tropical heat source. The basic forcing for the model takes the form of time-independent terms added to the prognostic equations in two configurations. One produces a perturbation model, in which anomalies grow on a fixed basic state. The other gives a simple GCM, which can be integrated for a long time and delivers a realistic climate simulation with realistic storm tracks. A series of experiments is performed, including 15-day perturbation runs, ensemble experiments, and long equilibrium runs, to isolate different dynamical influences on the fully developed Pacific–North American (PNA) type response to an equatorial heating anomaly centered on the date line.

The direct linear response is found to be very sensitive to changes in the basic state of the same order as the atmosphere’s natural variability, and to the natural progression of the basic state over the time period required to set up the response. However, interactions with synoptic-scale noise in the ambient flow are found to have very little systematic effect on the linear response. Nonlinear interactions with a fixed basic state lead to changes in the position, but not the amplitude, of the response. Feedback with finite-amplitude transient eddies leads to downstream amplification of the PNA pattern, both within the setup time for the response and in a fully adjusted equilibrium situation.

Nonlinearity of the midlatitude dynamics gives rise to considerable asymmetry between the response to tropical heating and the response to an equal and opposite cooling.

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Stephanie Leroux
and
Nicholas M. J. Hall

Abstract

This idealized modeling study investigates how convectively triggered African easterly waves (AEWs) are influenced by the intraseasonal variability of the African easterly jet (AEJ). A set of 10-day averaged zonally varying basic states is constructed with the NCEP-2 reanalysis (1979–2006). A primitive equation model is used to simulate linear AEWs on each of these basic states using the same idealized convective heating localized over the Darfur mountains as an initial trigger. It is shown that the transient response depends strongly on the basic state. With the same trigger, many configurations of the AEJ fail to produce a wave disturbance, while others produce strong easterly wave structures. Necessary conditions for the development of strong waves can be characterized by a strong jet, a strong vertical shear, or a strong and extended potential vorticity reversal. In strong-wave cases the jet is extended to the south and west, and the jet core is aligned with the maximum of surface westerlies, maximizing the vertical shear. The pattern that is optimal for generating easterly waves also closely resembles the dominant mode of variation of the AEJ revealed by an empirical orthogonal function (EOF) analysis of the set of basic states.

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Nicholas M. J. Hall

Abstract

A dry spectral primitive equation model is used to simulate the global atmospheric circulation during northern winter. The resolution is T21 in the horizontal, with five equally spaced sigma layers. The only additional terms in the equations are those describing linear damping, linear scale-selective diffusion, and time-independent forcing. The damping and diffusion act on temperature and momentum. The forcing acts on all prognostic variables. It is calculated objectively from the tendencies produced when the model is initialized with a long time series of observational analyses, and separated into components to ease comparison with time-dependent perturbation models.

The simulated climate reproduces observed features of the circulation, both time-mean fields and transient-eddy covariances, with remarkable success. The accurate simulation of tropical divergent flow is a particularly useful result. The main deficiencies are an underestimation of transient-eddy kinetic energy and a lack of transient activity in the Southern Hemisphere.

In an attempt to reduce the forcing of divergent flow, a modified vertical scheme and modified forcing functions based on a calculation of balanced flow are introduced. The former still has significant divergence forcing and makes little difference to the final result. The latter tends to give solutions that are unrealistic in the Tropics. The model’s sensitivity to variations in forcing functions and damping parameters is further explored. The Southern Hemisphere transient behavior can be improved by boosting the local forcing of baroclinicity by up to a factor of 2, and a simulation of the Southern Hemisphere winter is relatively successful.

The applications and limitations of such a simple fast-running climate model with a relatively realistic simulated climate are discussed.

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Nicholas M. J. Hall

Abstract

The northwest corners of the major ocean basins are characterized by seaward jets, flanked by tight, nonlinear gyres exhibiting closed potential vorticity contours. At deep levels, isolated from surface forcing, areas of homogeneous potential vorticity are apparent. A model is presented describing these ”recirculation“ regions extending the quasi-geostrophic layer models of Marshall and Nurser to the continuously stratified two-dimensional case.

The study is diagnostic, concentrating on numerical inversions of idealized quasi-geostrophic potential vorticity distributions in a vertical, meridional section through a free inertial gyre. An iterative approach is used to find the “bowl” of the circulation. the free boundary between the deep recirculating homogenized water and the stagnant water below.

It is shown that the homogenized recirculation has a finite depth penetration, possibly not extending to the ocean floor. In cases where the flow reaches the bottom, the recirculation can be divided into two regions: a “core” region, where bottom currents exist and a baroclinic “fringe” to the south. The surface intensified part of the eastward jet is recirculated in the broad, westward flowing fringe, while the component of the transport returned within the core itself is largely depth independent. The enhanced man transport of the Gulf Stream can be accounted for by the model. Its magnitude is sensitive to the upper-level potential vorticity imposed. For realistic parameters, the core carries the greater proportion of the transport.

The structure of the recirculation is dependent on the value assumed for the deep homogeneous potential vorticity. If a positive deep potential vorticity anomaly is imposed, the upper-level gyre interface moves northward while a cyclonic gyre becomes dominant in the abyssal flow. If the anomaly exceeds a certain limit, solutions can no longer be found.

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Nicholas M. J. Hall
and
Paul J. Valdes

Abstract

Two 10-yr integrations of the UGAMP GCM are presented. Each has a full seasonal cycle, T42 resolution, interactive land and sea ice, and prescribed sea surface temperatures. They differ in that one integration represents present day climate (PD) and the other has a perturbed orbit and reduced atmospheric concentrations of CO2 appropriate to the climate of 6000 years ago (6 kyr, hereafter 6k). The 6k integration produces enhanced continental warmth during summer and cold during winter. Changes in atmospheric temperature gradients brought about by the surface response lead to altered jet stream structures and transient eddy activity, which in turn affect precipitation patterns. Tropical “monsoon”-type circulation patterns are also affected, also leading to altered precipitation. Many of the changes in hydrology mimic the geological record remarkably well: the Sahel is much wetter, as are the midwestern United States and the Mediterranean regions; California and northern Europe are drier. Processes leading to the model’s surface responses in both temperature and hydrology are described in detail. Finally, the sensitivity of the results to an alternative, objective definition of the 6k calendar is investigated. This sensitivity is found to be smaller than the overall signal to the extent that the principal conclusions are not altered.

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Nicholas M. J. Hall
and
Prashant D. Sardeshmukh

Abstract

The dynamical stability of the Northern Hemisphere wintertime mean atmosphere is investigated in a linearized primitive equation model. In the absence of any damping on the perturbation, exponentially growing modes are found for the zonal-mean and zonally varying basic states. Their growth rates are 0.41 and 0.38 days−1, respectively. Both have the form of midlatitude baroclinic wave trains.

Three distinct idealized profiles of linear damping are then imposed on the perturbation vorticity and temperature. The damping is strongest below 800 mb and weak or nonexistent in the rest of the troposphere. It is specified to be proportional at all levels to a single parameter, R s , the strength of damping at the surface.

For the zonal-mean basic state, as R s is increased linearly, the growing modes decrease their growth rates almost linearly, and change their structure only slowly. For an average damping timescale in the boundary layer of about one day (R s = 2 days−1), the growing baroclinic modes are effectively neutralized. The wavy basic state is also rendered neutral when R s reaches this value. It is argued that this magnitude of damping is within the range of observable parameters in the atmosphere. However, the precise position of the neutral point is sensitive to the relative magnitudes of temperature and vorticity damping. The latter is more efficient in stabilizing the system.

For the wavy basic state, a second mode replaces the undamped mode as the fastest growing just before the neutral point is reached. This mode also resembles a midlatitude baroclinic wave train, but has a longer zonal wavelength. Zonal-mean transient fluxes of eddy temperature and momentum, and eddy kinetic energy calculated for this mode, show an improvement over the undamped and zonal-mean modes when compared with observations. It is argued that this improvement may be meaningful, particularly in an atmosphere that is close to neutral.

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Victor M. Torres
,
Chris D. Thorncroft
, and
Nicholas M. J. Hall

Abstract

This paper explores a new mechanism for in situ genesis of easterly waves (EWs) over the tropical eastern Pacific Ocean (EPAC). Using an idealized primitive equation model, it is shown that EWs can be triggered by finite-amplitude transient heating close to the midlevel jet at about 15°N over the EPAC and intra-Americas sea region. The atmospheric response to heating initiates EWs downstream, showing an EW structure within 4 days, with a wavelength and propagation speed of about 2000 km and 4.6 m s−1, respectively, resembling EWs described in the literature. The most sensitive location for EW initiation from finite-amplitude transient heating is located over the northern part of South America and extends to the EPAC. The closer the heating is to the jet, the bigger the response is. A stratiform heating profile is the most efficient at triggering EPAC EWs. Comparisons of simulated EWs over the EPAC and West Africa reveal similar structures but with a shorter wavelength and much weaker amplitudes over the EPAC. EPAC EWs are dominated by horizontal tilts against the shear on the equatorial side of the jet, consistent with barotropic growth, with weaker low-level amplitudes relative to those seen over West Africa. These differences arise from differences in the mean state EPAC having a shorter and weaker midlevel jet with less baroclinicity.

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Nicholas M. J. Hall
,
Paul J. Valdes
, and
Buwen Dong

Abstract

A 5-yr simulation of the last glacial maximum using the UGAMP GCM is presented. It has a full seasonal cycle, T42 resolution, and interactive land surface and sea ice. Boundary conditions of SST, sea ice extent and land ice elevation are taken from the CLIMAP dataset and orbital parameters and carbon dioxide concentration are adjusted. It is compared with a 10-yr simulation of present-day climate using the same model.

The results are analyzed in terms of processes leading to the maintenance of the atmospheric circulation and temperature structure, midlatitude transient behavior, precipitation, and eventually accumulation of ice over the glaciers. The model responds in a similar manner to previous studies in global mean statistics but differs in its treatment of regional climates. Changes in sea ice and orography are equally important in determining the positions of the upper-level jets. The Atlantic jet and storm track in particular are much stronger than in the present-day simulation, and the associated distribution of precipitation and snowfall changes accordingly. Both major ice sheets are maintained by snowfall at the center and ablation at the edges at a reasonable rate through the annual cycle.

The results with a full seasonal cycle are compared to perpetual integrations by the authors and found to be very similar in most measures.

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Chris D. Thorncroft
,
Nicholas M. J. Hall
, and
George N. Kiladis

Abstract

This paper promotes the view that African easterly waves (AEWs) are triggered by localized forcing, most likely associated with latent heating upstream of the region of observed AEW growth. A primitive equation model is used to show that AEWs can be triggered by finite-amplitude transient and localized latent heating on a zonally varying basic state that is linearly stable. Heating close to the entrance region of the African easterly jet (AEJ) is shown to initiate AEWs downstream. The heating leads to an initial trough that reaches the West African coast about 5–7 days later, depending on the nature of the heating profile. After this, a structure that projects strongly onto the leading linear normal mode of the basic state becomes established, characterized by a number of westward-propagating disturbances that strongly resemble AEWs. The sensitivity of the forced AEWs to the nature and location of the heating profile is examined. AEWs are most efficiently triggered by heating profiles that establish lower tropospheric circulations close to the entrance region of the AEJ. In the present study, this was best achieved by lower tropospheric heating from shallow convection or upper-level heating and lower-level cooling from a stratiform precipitation profile. Both profiles have significant heating gradients in the vertical in the mid-to-lower troposphere. This triggering paradigm for the genesis of AEWs has consequences for the variability and predictability of AEWs at weather and climate time scales. In addition to the nature of the AEJ, often emphasized, it is crucial to consider the nature and variability of upstream heating triggers.

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George N. Kiladis
,
Chris D. Thorncroft
, and
Nicholas M. J. Hall

Abstract

The mean structure of African easterly waves (AEWs) over West Africa and the adjacent Atlantic is isolated by projecting dynamical fields from reanalysis and radiosonde data onto space–time-filtered satellite-derived outgoing longwave radiation. These results are compared with previous studies and an idealized modeling study in a companion paper, which provides evidence that the waves bear a close structural resemblance to the fastest-growing linear normal mode of the summertime basic-state flow over Africa. There is a significant evolution in the three-dimensional structure of AEWs as they propagate along 10°N across West Africa. At this latitude, convection occurs in northerly flow to the east of the Greenwich meridian, then shifts into the wave trough, and finally into southerly flow as the waves propagate offshore into the Atlantic ITCZ. In contrast, to the north of the African easterly jet along 15°N convection remains in southerly flow throughout the waves’ trajectory. Along 10°N over West Africa, the location of convection is consistent with the adiabatic dynamical forcing implied by the advection of perturbation vorticity by the mean thermal wind in the zonal direction, as in the companion paper. Offshore, and along 15°N, the relationship between the convection and dynamics is more complex, and not as easily explained in terms of dynamical forcing alone.

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