Celeste

Satellite retrievals of tropospheric temperature reveal strong mid-latitude warming

during 1979-2005 relative to the global mean temperature trend. Over the same period,

the stratosphere shows strong mid-latitude cooling relative to its global mean trend.

These zonally symmetric bands of enhanced warming and cooling in the troposphere and

stratosphere respectively may indicate a broadening of the Hadley circulation and

a poleward shift of the jet stream. The pattern is strongest in the summer months

in both hemispheres and may have wide-spread implications for both drought persistence

and rainfall patterns.

The partition of heat transport in a simple coupled

climate model

Brian Rose_

Program in Atmospheres, Oceans and Climate

MIT

January 30, 2006

Dynamical heat transports in the atmosphere and ocean play a central role in

the climate system by greatly reducing the pole-to-equator temperature difference. However, the mechanisms that set the partition of heat transport between the two fluids are not well understood. We seek a better understanding of this partition in order to make sense of the widely varying climate states of the Earth’s past. Since the oceans are set in motion by the wind stress, it seems reasonable that the surface wind should play an important role in any theory of the partition. I will describe a very simple, zonally averaged atmospheric model with eddy processes parameterized according to a mixing length theory for potential vorticity. This model is simple to solve and obeys global momentum and energy constraints; and yet it is capable of reproducing many of the gross features of the general circulation, including the surface wind pattern. This simplest of GCMs is coupled to an equally simplified ocean model via the surface wind stress and the heating. We will look at equilibrium solutions of this coupled model and try to glean some insight into the partition problem. In particular, we find that the atmospheric heat flux dominates over the oceanic flux in mid-latitudes due to its much more intense residual overturning circulation.

_brose@mit.edu

Heat Transport in Simple Models of Meridional Overturning Circulation

Eleanor Williams1 and Ken Takahashi2

Abstract

Sandstrom's theorem states that a closed steady circulation can only be maintained in a fluid body if the heating is applied at a lower level than the cooling in the absence of mixing. We investigate meridional overturning circulation in two simple models, Welander's one-dimensional loop model and a two-dimensional Boussinesq flow in a box. Both models have heating and cooling applied at the same level. We find that heat transport and circulation intensity depend on the type of boundary conditions, fixed flux or relaxation, even with negligible mixing. In the two-dimensional model, the intensity and geometry of heat transport--deep or confined to the surface layers--are affected by the intensity and vertical profile of mixing. Using these two simple models, with complex behavior, we draw conclusions about the use of fully three-dimensional models and the real ocean meridional overturning circulation.

Title: A rigorous reduction method applied to a model of atmospheric low-frequency variability (LFV)

Joel Culina

Climate systems are multi-scale processes. Scale separation is the basis for reduction of high-dimensional climate models to low-dimensional models. High-dimensional models generally give the most accurate prediction of climate, but computation is extensive and the underlying dynamics is obscured. For example, the path of a slower-evolving 'climate' variable is muddled by the faster-evolving 'weather' variable, which is noise-like on the scale of the climate variable.

Low-dimensional models, then, have a place along-side high-dimensional models. Since the connection between the two model types is important, it is best to simplify high-dimensional models rather than work from scratch. It is also desirable that the reduction method be systematic and rigorously-based. I applied such a procedure, a stochastic reduction procedure, to a model of atmospheric low-frequency variability (LFV). This procedure is derived from theorems that prove for certain multi-scale systems, fast-evolving processes are reduced to a stochastic term. The power of this method lies in its rigour, in contrast to the more common ad hoc methods of stochastic representation.

LFV is characterised by a maximum in power among low-frequency waves, at around wavenumbers 2,3, and 4, in the wintertime extratropical troposphere. There is substantial evidence that this behaviour is driven by an essentially low-dimensional system, key elements of which are persistent, recurrent and distinct wave patterns of planetary scale ('regimes'). The faster-evolving, synoptic-scale patterns are known to be necessary to the existence of LFV, but the dynamical relationship between scales is unknown. Applying the rigorous reduction method to a baroclinic, quasigeostrophic model of LFV, I will deduce these dynamics based on which of the reduced equations best reflects the full-dimensional model.

Camille Li

The existence of a different circulation regime in the presence of large ice sheets has implications for the stability of glacial climates. In a simulation of the Last Glacial Maximum from the NCAR Community Climate Systems Model CCSM3, we observe a strong, zonally oriented Atlantic jet, and associated with it, a decrease in wintertime storminess. Given the relatively short (on the order of weeks) adjustment time scale of the mid-latitudes, one could imagine the possibility of abrupt climate change due to a shift between a fast, stable Atlantic jet regime and one more like today’s. These jet shifts, along with changes in storminess and heat transport, the amplifying effect of sea ice and positive feedbacks with the tropics, could cause dramatic changes in the North Atlantic region as suggested by the paleoclimate record.

Aaron

Coupled general circulation models with Last Glacial Maximum ice sheet topography, orbital forcing, carbon dioxide suggest that the Last Glacial Maximum atmosphere supported fewer storms than the present day atmosphere. This result is counter intuitive given that there is more energy in the Last Glacial Maximum climate system to feed the storms: the enhanced equator to pole temperature gradient would be expected to support stronger baroclinic eddies and the stronger and more localized jets should support stronger barotropic eddies. The mechanisms leading to weaker storms during the Last Glacial Maximum are explored using normal mode analysis on the sphere and a simple rectangular three dimensional model with the general circulation output jet structure.

Celeste

aaron

2006-03-27T16:04:00