A22B-01 INVITED
Zonal Jet Structure and the Leading Mode of Variability
An observational analysis of the Northern Hemisphere circulation during winter reveals that the leading mode of variability depends on longitude. In particular, the first EOF of the zonal mean circulation differs over the Atlantic and Pacific sectors. These results provide motivation for a series of model runs in which we use a simple GCM to investigate the effects of the zonal jet structure on the leading mode of variability in the Northern Hemisphere. Model results indicate that the leading mode of variability depends on the distance between the eddy-driven and subtropical jets. When the jets are well separated, the leading mode of variability describes latitudinal shifting of the eddy-driven jet. However, when the two jets are nearly collocated, pulsing of the combined jet dominates the variability. This change coincides with a weakening of the positive feedback between the eddies and zonal flow anomalies. These results provide a possible explanation for the reduced amplitude of the Northern Annular Mode in the Pacific sector relative to that in the Atlantic sector during Northern Hemisphere winter. http://www.atmos.washington.edu/~dennis/Papers.html
A22B-02
Sensitivity of tropical circulation and ITCZ to the underlying meridional gradient of SST
The meridional gradient of sea surface temperature (SST) plays an important role in setting the location of the Intertropical Convergence Zone (ITCZ) and the properties of its associated meteorological phenomena such as easterly waves, equatorially trapped waves, Hadley circulation, and tropical cyclones. The ability of the SST distribution to dictate these properties depends on the strength of its meridional gradient: a stronger gradient implies greater control of atmospheric phenomena and vice versa. Owing to the distribution of land and ocean, the present climate of Earth displays a wide range of ITCZ properties depending on longitude, as well as interannual variability such as ENSO. In certain regions, such as the Atlantic and east/central Pacific, the underlying meridional gradient of SST plays a larger role than elsewhere, and it is possible to investigate aspects of this control using an aquaplanet GCM in which longitudinal variations of externals are ignored completely. This talk will explore the role of SST meridional curvature in setting the location of the ITCZ and the meridional extent of the Hadley circulation. The number of ITCZs (one or two) is shown to depend on the SST gradient, and in an intermediate range of parameter space, both regimes are supported; a hysteresis is observed between these two states as the gradient of SST is varied slowly in time. The properties of waves and their role in spawning hurricanes in the idealized model is also discussed. It is thought that certain paleoclimatic regimes have witnessed a fundamental though transient change in ITCZ behavior resulting from changes in equator-to-pole temperature gradient. Although near-term projections of future climate change involve a smaller alteration of ITCZ properties, these changes may nevertheless be important to tropical meteorology and the impact of extreme events such as hurricanes. This will be shown explicitly using results from the idealized aquaplanet model. The behavior of the numerical model follows reasonably well the predictions of a simple theoretical model of the nonlinear angular-momentum conserving Hadley circulation.
A22B-03
Monsoon transitions in the seasonal cycle simulated with aquaplanet GCMs
Traditionally, Earth's large-scale monsoons have been interpreted as a planetary-scale sea-breeze driven by the thermal contrast between land and ocean. However, simulations with an aquaplanet GCM show that, in the course of the seasonal cycle, the tropical mean meridional circulation undergoes transitions that resemble the onset and end of Earth's large-scale monsoons even in the absence of land masses, provided the heat capacity (mixed-layer depth) of the underlying ocean is sufficiently low. A transition in early summer is associated with a rapid rearrangement in the subtropics and intensification of the major precipitation zone, an abrupt strengthening and broadening of the cross-equatorial Hadley cell, and wind reversals in both the upper and lower levels. A reverse transition occurs in late summer. It is found, consistent with similar dry simulations (Schneider and Bordoni, 2007), that the rapid rearrangements of the meridional circulations mark shifts between an equinox and summer regime, in which eddy momentum flux divergence dominates the vertically averaged zonal momentum balance near the center of the Hadley cell, and a winter, "monsoon" regime, in which the eddy momentum flux divergence is small and the mean momentum flux dominates. The summertime precipitation zones form just equatorward of the lower lever moist static energy maximum, which is colocated with the boundary between the winter and the summer cell. Factors controlling the exact locations of these precipitation zones are examined. Sensitivity to changes in the mixed layer depth and implications for Earth's monsoons will also be discussed.
A22B-04 INVITED
Scaling laws of atmospheric macroturbulence and their implications for Hadley cell dynamics and climate stability
In simulations of a wide range of circulations with an idealized general circulation model, clear scaling laws of dry atmospheric macroturbulence emerge. The scaling laws exhibit a regime transition, between a regime in which the extratropical thermal stratification and tropopause height are controlled by radiation and convection and a regime in which baroclinic entropy fluxes modify the extratropical thermal stratification and tropopause height. At the regime transition, for example, the dependence of the eddy flux of surface potential temperature and the dependence of the vertically integrated eddy momentum flux convergence on mean fields changes. This result has implications for climate stability and for the general circulation of an atmosphere, including its Hadley circulation, which is strongly influenced by eddy momentum fluxes and exhibits a similar regime transition. http://www.gps.caltech.edu/~tapio/pubs.html
A22B-05
Time scales and spatial patterns of passive ocean-atmosphere decay modes
The decay characteristics of a mixed layer ocean passively coupled to an atmospheric model are important to the response of the climate system to stochastic or external forcing. Two salient features of such decay--the spatial scale dependence of sea surface temperature anomaly (SSTA) decay timescales and the spatial inhomogeneities of SSTA decay modes--are addressed using the Quasi-equilibrium Tropical Circulation (or QTCM), an intermediate level complexity model. As expected, decay timescales increase with the spatial extent of the SSTA. Most modes decay rapidly-- with characteristic decay times of 50-100 days for a 50 m mixed layer--with the decay determined by local surface flux adjustment. Only those modes with spatial scales approaching or larger than the tropical basin scale exhibit decay timescales distinctively longer than the local decay, with the decay timescale of the most slowly decaying mode of order 250-300 days in the Tropics (500 days globally). Simple analytic prototypes of the spatial scale dependence and the effect of basic state inhomogeneities, especially the impact of nonconvecting regions, elucidate these results. Horizontal energy transport sets the transition between fast, essentially local, decay timescales and the slower decay at larger spatial scales; within the Tropics, efficient wave dynamics accounts for the small number of slowly-decaying modes. Inhomogeneities in the basic state climate, such as the presence or absence of mean tropical deep convection, strongly impact large-scale SSTA decay characteristics. For nonconvecting regions, SSTA decay is slow because evaporation is limited by relatively slow moisture divergence. The separation of convecting and nonconvecting region decay times and the closeness of the slower nonconvecting region decay timescale to the most slowly-decaying modes cause a blending between local nonconvecting modes and the large-scale modes, resulting in pronounced spatial inhomogeneity in the slow decay modes.
A22B-06
Energy of midlatitude transient eddies in idealized simulations of changed climates
As the climate changes, changes in static stability, meridional temperature gradients, and availability of moisture for latent heat release may exert competing effects on the energy of transient eddies in the midlatitude storm- tracks. We examine how the eddy energy changes as the climate changes in simulations with an idealized moist general circulation model. To mimic the effects of changes in greenhouse gas forcing, we vary the optical thickness of the longwave absorber in the model over a wide range. The eddy energy has a maximum for a climate with mean temperature similar to that of present-day Earth, with significantly smaller values both for warmer and for colder climates. The eddy energy scales linearly with the dry mean available potential energy averaged over the baroclinic zones. Changes in eddy energy can therefore be related to the changes in the atmospheric thermal structure that affect the mean available potential energy. http://www.its.caltech.edu/~pog /publications.html
A22B-07 INVITED
A parameter study of climate using an idealized GCM
The results from a series of idealized climate model runs are reviewed. For these model runs, two parameters were varied, one parameter that determines the strength of the tropical heating (H), and the other parameter that determines the width of the baroclinically unstable region (C). By varying these parameters over a range of earth-like parameter settings and then by comparing the results for different parameter values, we gain deeper insight into basic dynamical properties of the atmosphere than by analyzing individual model runs in isolation. It is found that the properties of the climatological zonal flow are closely tied to the properties of the variability of the zonal flow. For the weak C and strong H regime, it is found that the climatolological jet is comprised of a single eddy-driven jet, whereas for the strong C and weak H regime, the climatological jet is characterized by the presence of two jets, a subtropical jet and an eddy driven jet. For the former regime, it is found that the zonal index (latitudinal jet meanders) dominate, whereas for the latter regime the primary form of variability is that of poleward propagating zonal mean flow anomalies. The above results suggest that both the time-mean and the temporal variability of the zonal mean flow may be determined by two key parameters of the heating field. These results in turn allow us to examine other dynamical questions, such as what dynamical processes drive the poleward zonal mean anomaly propagation, what determines the time scale of the zonal index or annular mode, and what propertiess of the flow are essential for determining whether the flow is predictable.
A22B-08
Studies of the General Circulation with a Simplified Moist GCM
Idealized atmospheric models often ignore moisture and latent heating due to the significant complexity associated with moist convection. However, for many atmospheric phenomena, moisture is of fundamental importance. We will discuss the development of a idealized moist general circulation model, including the formulation and testing of different physicalparameterizations such as radiation and convection schemes. Then we provide a survey of some of the results we have derived using this model, involving the Hadley circulation, midlatitude static stability, jet latitude, and atmospheric energy transports. We also provide comparisons with full GCMs, to validate the results from the idealized model.
A22B-09
Response of the zonal mean atmospheric circulation to El Nino versus global warming
The change in the zonal mean atmospheric circulation under global warming is studied in comparison with the response to El Nino forcing, by examining the model simulations conducted for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. In contrast to the strengthening and contraction of the Hadley cell and the equatorward shift of the tropospheric zonal jets in response to El Nino, the Hadley cell weakens and expands poleward, and the jets move poleward in a warmed climate, despite the "El Nino- like" enhanced warming over equatorial central and eastern Pacific. Two feasible mechanisms are proposed for the zonal mean circulation response to global warming: (1) The increase in static stability of the subtropical and mid-latitude troposphere, a result of the quasi-moist adiabatic adjustment to the surface warming, can stabilize the eddy growth on the equatorward side of the storm track and plausibly push the eddy activity and the associated eddy-driven wind and subsidence poleward, leading to the poleward expansion of the Hadley cell and the shift of jet; (2) the strengthening of the mid-latitude wind at the upper-troposphere and lower-stratosphere, arguably a consequence of the rise in the height of the tropopause and the associated increase in the meridional temperature gradient, can increase the phase speed of the eddies emanating from the mid-latitudes, and thus the critical latitudes (where the eddy phase speed matches the background zonal wind, and where the eddies break and extract angular momentum from the thermally driven wind) displace poleward together with the eddy-driven circulation. Both mechanisms are somewhat, if not completely, distinct from those in response to the El Nino condition. The hydrological impacts of global warming also exhibit distinct patterns over the subtropics and mid-latitudes in comparison to the El Nino. Since both mechanisms do not essentially depend on the details of the SST warming in the equatorial oceans and the variety of model physical parameterizations, these extratropical responses to global warming may be of increased credence due to the fact that they are dynamically determined.
A22B-10 INVITED
Flexible climate modeling systems: Lessons from Snowball Earth, Titan and Mars
Climate models are only useful to the extent that real understanding can be extracted from them. Most leading- edge problems in climate change, paleoclimate and planetary climate require a high degree of flexibility in terms of incorporating model physics -- for example in allowing methane or CO2 to be a condensible substance instead of water vapor. This puts a premium on model design that allows easy modification, and on physical parameterizations that are close to fundamentals with as little empirical ad-hoc formulation as possible. I will provide examples from two approaches to this problem we have been using at the University of Chicago. The first is the FOAM general circulation model, which is a clean single-executable Fortran-77/c code supported by auxiliary applications in Python and Java. The second is a new approach based on using Python as a shell for assembling building blocks in compiled-code into full models. Applications to Snowball Earth, Titan and Mars, as well as pedagogical uses, will be discussed. One painful lesson we have learned is that Fortran-95 is a major impediment to portability and cross-language interoperability; in this light the trend toward Fortran-95 in major modelling groups is seen as a significant step backwards. In this talk, I will focus on modeling projects employing a full representation of atmospheric fluid dynamics, rather than "intermediate complexity" models in which the associated transports are parameterized.