# Publications

## Impact of ocean dynamics on the simulation of the Neoproterozoic "snowball Earth"

GEOPHYSICAL RESEARCH LETTERS **28** (2001) 1575-1578

## Physical Climate Processes and Feedbacks

in *Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change*, Cambridge University Press (2001) 7

Climate Change 2001: The Scientific Basis is the most comprehensive and up-to-date scientific assessment of past, present and future climate change.

## Transition to geostrophic turbulence in the laboratory, and as a paradigm in atmospheres and oceans

SURVEYS IN GEOPHYSICS **22** (2001) 265-317

## Bifurcations and instabilities in rotating two-layer fluids: I. f-plane

Nonlinear Processes in Geophysics **8** (2001) 21-36

In this paper, we show that the behaviour of weakly nonlinear waves in a 2-layer model of baroclinic instability on an f-plane with varying viscosity is determined by a single, degenerate codimension three bifurcation. In the process, we show how previous studies, using the method of multiple scales to derive evolution equations for the slowly varying amplitude of the growing wave, arise as special limits of the general evolution description. A companion study will extend the results to a β-plane.

## Achievements and directions in nonlinear geophysics - Editorial

NONLINEAR PROCESSES IN GEOPHYSICS **8** (2001) 191-192

## An evaluation of Eulerian and Semi-Lagrangian advection schemes in simulations of rotating, stratified flows in the laboratory. Part I: Axisymmetric flow

Monthly Weather Review **128** (2000) 2835-2852

A series of numerical simulations of steady, thermally stratified flow of a Boussinesq, incompressible fluid in a rotating, cylindrical fluid annulus were carried out over ranges of spatial resolution, grid stretch, and rotation rate. A range of different numerical advection schemes were used for the representation of heat transport, including a conventional conservative second-order Eulerian scheme and three different variants of a semi-Lagrangian scheme used either for temperature advection alone, or for both thermal and momentum advection. The resulting simulations were compared both with each other, and with high precision measurements of velocity, temperature, and total heat transport in the laboratory. The performance of the semi-Lagrangian scheme was found to be quite strongly sensitive to the spatial interpolation algorithm. A basic tensor cubic scheme generally produced good simulations of steady 2D and 3D flows, although the somewhat more accurate tensor quintic scheme (which is, however, also significantly more expensive) appeared to offer some detectable improvements in accuracy and performance in some cases. A split cubic scheme (which is computationally cheaper but formally less accurate) gave generally poor results in practice and is not recommended. In all cases considered, both the fully Eulerian and most forms of the semi-Lagrangian schemes gave good quantitative agreement with the laboratory measurements when extrapolated to very high resolution. Some significant systematic errors in the simulated heat transport and zonal momentum were found with all schemes, however, when run at moderate (though by no means very low) resolution. The semi-Lagrangian schemes had a tendency to overestimate heat transport relative to the laboratory measurements compared with the Eulerian schemes, but the latter tended to overestimate zonal momentum relative to the laboratory flows compared with the fully semi-Lagrangian simulations.

## A mechanistic model of the quasi-quadrennial oscillation in Jupiter's stratosphere

PLANET SPACE SCI **48** (2000) 637-669

An analytical model, previously developed for investigating the propagation of equatorially-trapped waves on an equatorial beta-plane in a uniform zonal flow in the presence of Rayleigh friction and Newtonian cooling in the Earth's stratosphere, is applied to Jupiter's upper troposphere and lower stratosphere. By analogy with the Earth, a 'spectral window' is identified for each of the main classes of equatorial wave mode, suggesting a mode-selection criterion for the dominant modes observed in association with strong wave-zonal flow interactions in the stratosphere. The modes favoured by this approach are compared with recent observations of wave activity and the quasi-quadrennial oscillation (QQO) in Jupiter's tropical atmosphere. Two modes with zonal wavenumber k similar to 8-11 are identified which may correspond to: (i) an equatorial Rossby mode moving eastward at around 100 m s(-1): and (ii) a mixed Rossby-gravity mode which is similar to stationary in System III, apparently excited by a wave source moving with the zonal wind in the deep troposphere. A Kelvin mode is also predicted to be present, but observational evidence for this mode is lacking to date. A numerical model, capable of solving for wave structures and wave-zonal flow interactions in arbitrary zonal flows using a Hermite spectral method, is adapted to conditions in Jupiter's stratosphere. The latter numerical model is shown to successfully simulate a plausible QQO with a period around four Earth years, given a single pair of forced Kelvin and MRG modes with tropospheric amplitudes consistent with observations. This model demonstrates that the QQO may indeed result, at least in principle, from interactions of a small number of equatorially-trapped wave modes with the zonal flow in the stratosphere. The selection of wave modes taking part in this process is not unique, however, and the precise identification of the relevant modes from observations remains elusive. (C) 2000 Elsevier Science Ltd. All rights reserved.

## 'Equability' in an unequal world: The early Eocene revisited

GFF **122** (2000) 101-102

## Lattice models of advection-diffusion.

Chaos (Woodbury, N.Y.) **10** (2000) 61-74

We present a synthesis of theoretical results concerning the probability distribution of the concentration of a passive tracer subject to both diffusion and to advection by a spatially smooth time-dependent flow. The freely decaying case is contrasted with the equilibrium case. A computationally efficient model of advection-diffusion on a lattice is introduced, and used to test and probe the limits of the theoretical ideas. It is shown that the probability distribution for the freely decaying case has fat tails, which have slower than exponential decay. The additively forced case has a Gaussian core and exponential tails, in full conformance with prior theoretical expectations. An analysis of the magnitude and implications of temporal fluctuations of the conditional diffusion and dissipation is presented, showing the importance of these fluctuations in governing the shape of the tails. Some results concerning the probability distribution of dissipation, and concerning the spatial scaling properties of concentration fluctuation, are also presented. Though the lattice model is applied only to smooth flow in the present work, it is readily applicable to problems involving rough flow, and to chemically reacting tracers. (c) 2000 American Institute of Physics.

## Spatially correlated and inhomogeneous random advection

PHYSICS OF FLUIDS **12** (2000) 822-834

## Atmospheric pCO(2) sensitivity to the biological pump in the ocean

GLOBAL BIOGEOCHEMICAL CYCLES **14** (2000) 1219-1230

## Generation of inertia-gravity waves in a baroclinically unstable fluid

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY **126** (2000) 3233-3254

## Climate change and the tropical Pacific: the sleeping dragon wakes.

Proceedings of the National Academy of Sciences of the United States of America **97** (2000) 1355-1358

## An evaluation of Eulerian and semi-Lagrangian advection schemes in simulations of rotating, stratified flows in the laboratory. Part I: Axisymmetric flow

MONTHLY WEATHER REVIEW **128** (2000) 2835-2852

## POD analysis of baroclinic wave flows in the thermally-driven, rotating annulus experiment

Physics and Chemistry of the Earth, Part B: Hydrology, Oceans and Atmosphere **24** (1999) 449-453

The Proper Orthogonal Decomposition (POD) is a procedure to compute an orthogonal basis from a time series of spatial fields. This basis is optimal among all linear decompositions, in the sense that for a given number of modes, the projection of the original signal onto the subspace will contain the most variance on average. This algorithm is applied to streamfunction fields derived from measurements of the flow in the thermally forced rotating annulus experiment. Results of this analysis are presented, and a method to derive low-dimensional models of the flow by projecting the equations of motion onto these empirical eigenfunctions is discussed.

## Mixing of an advected-diffused tracer in the lower stratosphere: probability distribution functions of tracer gradients and differences

12TH CONFERENCE ON ATMOSPHERIC AND OCEANIC FLUID DYNAMICS (1999) 303-306

## Huascaran delta O-18 as an indicator of tropical climate during the Last Glacial Maximum

GEOPHYSICAL RESEARCH LETTERS **26** (1999) 1345-1348

## Improved general circulation models of the Martian atmosphere from the surface to above 80 km

JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS **104** (1999) 24155-24175

## Subtropical water vapor as a mediator of rapid global climate change

in *Geophysical Monograph Series*, **112** (1999) 339-361

© 1999 by the American Geophysical Union. This article surveys the essential features of atmospheric water vapor dynamics needed to address current issues regarding the possible role of water vapor changes in mediating climate fluctuations on the millennial to Milankovic time scales. The focus is on the subtropics, which afford the most interesting possibilities for significant feedbacks. The observed distribution of water vapor, the amount by which water vapor must change in order to cause a significant temperature change, and the physical factors that determine the water vapor content of the subtropical atmosphere are discussed. It is shown that halving the subtropical relative humidity would lead to a 2.5K cooling of the tropics, while doubling it would lead to a 3K warming. The humidity content of the subtropics could be reduced by enhancing subsidence, reducing transient eddy activity, or contracting the convective region. Further work is needed to determine which, if any, of these changes occur in concert with the observed millennial and longer scale climate fluctions.

## Flow-field and point velocity measurements in a barotropically unstable shear layer

PHYSICS AND CHEMISTRY OF THE EARTH PART B-HYDROLOGY OCEANS AND ATMOSPHERE **24** (1999) 461-466