# Publications

## Revisiting the identification of wintertime atmospheric circulation regimes in the Euro‐Atlantic sector

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Atmospheric circulation is often clustered in so‐called circulation regimes, which are persistent and recurrent patterns. For the Euro‐Atlantic sector in winter, most studies identify four regimes: the Atlantic Ridge, Scandinavian Blocking and the two phases of the North Atlantic Oscillation. These results are obtained by applying k‐means clustering to the first several empirical orthogonal functions (EOFs) of geopotential height data. Studying the observed circulation in reanalysis data, it is found that when the full field data are used for the k‐means cluster analysis instead of the EOFs, the optimal number of clusters is no longer four but six. The two extra regimes that are found are the opposites of the Atlantic Ridge and Scandinavian Blocking, meaning they have a low‐pressure area roughly where the original regimes have a high‐pressure area. This introduces an appealing symmetry in the clustering result. Incorporating a weak persistence constraint in the clustering procedure is found to lead to a longer duration of regimes, extending beyond the synoptic time‐scale, without changing their occurrence rates. This is in contrast to the commonly used application of a time‐filter to the data before the clustering is executed, which, while increasing the persistence, changes the occurrence rates of the regimes. We conclude that applying a persistence constraint within the clustering procedure is a better way of stabilizing the clustering results than low‐pass filtering the data.

## Jet Latitude Regimes and the Predictability of the North Atlantic Oscillation

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In recent years, numerical weather prediction models have begun to show notable levels of skill at predicting the average winter North Atlantic Oscillation (NAO) when initialised one month ahead. At the same time, these model predictions exhibit unusually low signal-to-noise ratios, in what has been dubbed a `signal-to-noise paradox'. We analyse both the skill and signal-to-noise ratio of the Integrated Forecast System (IFS), the European Center for Medium-range Weather Forecasts (ECMWF) model, in an ensemble hindcast experiment. Specifically, we examine the contribution to both from the regime dynamics of the North Atlantic eddy-driven jet. This is done by constructing a statistical model which captures the predictability inherent to to the trimodal jet latitude system, and fitting its parameters to reanalysis and IFS data. Predictability in this regime system is driven by interannual variations in the persistence of the jet latitude regimes, which determine the preferred state of the jet. We show that the IFS has skill at predicting such variations in persistence: because the position of the jet strongly influences the NAO, this automatically generates skill at predicting the NAO. We show that all of the skill the IFS has at predicting the winter NAO over the period 1980-2010 can be attributed to its skill at predicting regime persistence in this way. Similarly, the tendency of the IFS to underestimate regime persistence can account for the low signal-to-noise ratio, giving a possible explanation for the signal-to-noise paradox. Finally, we examine how external forcing drives variability in jet persistence, as well as highlight the role played by transient baroclinic eddy feedbacks to modulate regime persistence.

## Does ENSO regularity increase in a warming climate?

Journal of Climate American Meteorological Society (0) JCLI-D-19-0545.1

<jats:p> The impact of a warming climate on El Nino-Southern Oscillation (ENSO) is investigated in large ensemble simulations of the Community Earth System Model (CESM1). These simulations are forced by historical emissions for the past and the RCP8.5-scenario emissions for future projections. The simulated variance of the Nino-3.4 ENSO index increases from 1.4<jats:sup>◦</jats:sup>C<jats:sup>2</jats:sup> in 1921-1980 to 1.9<jats:sup>◦</jats:sup>C<jats:sup>2</jats:sup> in 1981-2040 and 2.2<jats:sup>◦</jats:sup>C<jats:sup>2</jats:sup> in 2041-2100. The autocorrelation timescale of the index also increases, consistent with a narrowing of its spectral peak in the 3- to 7-yr ENSO band, raising the possibility of greater seasonal to interannual predictability in the future. Low-order linear inverse models (LIMs) fitted separately to the three 60-yr periods capture the CESM1 increase in ENSO variance and regularity. Remarkably, most of the increase can be attributed to the increase in the 23-month damping timescale of a single damped oscillatoryENSO eigenmode of these LIMs by 5 months in 1981-2040 and 6 months in 2041-2100. These apparently robust projected increases may however be compromised by CESM1 biases in ENSO amplitude and damping timescale. A LIM fitted to the 1921-1980 observations has an ENSO eigenmode with a much shorter 8-month damping timescale, similar to that of several other eigenmodes. When the mode’s damping timescale is increased by 5 and 6 months in this observational LIM, a much smaller increase of ENSO variance is obtained than in the CESM1 projections. This may be because ENSO is not as dominated by a single ENSO eigenmode in reality as it is in the CESM1. </jats:p>

## PROPERTIES OF THE ELIASSEN-PALM FLUX FOR PLANETARY SCALE MOTIONS

JOURNAL OF THE ATMOSPHERIC SCIENCES **39** (1982) 992-997

## DIAGNOSTIC STUDY OF A WAVENUMBER-2 STRATOSPHERIC SUDDEN WARMING IN A TRANSFORMED EULERIAN-MEAN FORMALISM

JOURNAL OF THE ATMOSPHERIC SCIENCES **38** (1981) 844-855

## ASPECTS OF STRATOSPHERIC SUDDEN WARMINGS STUDIED FROM A TRANSFORMED EULERIAN-MEAN VIEWPOINT

JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS **86** (1981) 9679-9687

## Gravitational energy-momentum: The Einstein pseudotensor reexamined

General Relativity and Gravitation **12** (1980) 149-154

By using a suitable two-point scalar field, a covariant formulation of the Einstein pseudotensor is given. A unique choice of scalar field is made possible by examining the role of linear and angular momentum in their correct geometric context. It is shown that, contrary to many text-book statements, linear momentum is not generated by infinitesimal coordinate transformations on space-time. Use is made of the nonintersecting lifted geodesies on the tangent bundle, TM, to space-time, to define a globally regular three-dimensional Lagrangian submanifold of TM, relative to an observer at some point z in space-time. By integrating over this submanifold rather than a necessarily singular spacelike hypersurface, gravitational linear and angular momentum, relative to z, are defined, and shown to have sensible physical properties. © 1980 Plenum Publishing Corporation.

## OBSERVATION AND SIMULATION OF A STRATOSPHERIC SUDDEN WARMING

GEOPHYSICAL JOURNAL OF THE ROYAL ASTRONOMICAL SOCIETY **61** (1980) 222-222

## DERRICKS THEOREM IN CURVED SPACE

JOURNAL OF PHYSICS A-MATHEMATICAL AND GENERAL **12** (1979) L17-L19

## Incorporation of climatology into a Backus‐Gilbert variational scheme for atmospheric temperature retrieval

Quarterly Journal of the Royal Meteorological Society **105** (1979) 803-809

A method is proposed for explicitly incorporating climatological information into a Backus‐Gilbert variational scheme for atmospheric temperature retrieval by a remote sounding radiometer. The method is applied to a study of the retrieval ability of a ground‐based microwave radiometer. Copyright © 1979 Royal Meteorological Society

## MOMENTUM-TRANSFER IN GRAVITATION THEORY

GENERAL RELATIVITY AND GRAVITATION **10** (1979) 623-629

## On the Magnetic Flux Linkage of an Electrically-Conducting Fluid: A Treatment of the Relativistic Case Using the Exterior Calculus Formalism

Geophysical & Astrophysical Fluid Dynamics **12** (1979) 177-180

Hide's (1979) theorem on the rnagnetic flux linkage of an electrically-conducting fluid is extended to the fully general relativistic case by rederiving the theorem in the elegant and succinct formalism of Cartan's exterior calculus. © Gordon and Breach Science Publishers Ltd., 1979

## CONSERVATION EQUATIONS AND GRAVITATIONAL SYMPLECTIC FORM

JOURNAL OF MATHEMATICAL PHYSICS **19** (1978) 2324-2331

## COVARIANT CONSERVATION EQUATIONS AND THEIR RELATION TO THE ENERGY-MOMENTUM CONCEPT IN GENERAL RELATIVITY

PHYSICAL REVIEW D **18** (1978) 4399-4407

## COVARIANT CONSERVATION EQUATIONS AND THEIR RELATION TO THE ENERGY-MOMENTUM CONCEPT IN GENERAL RELATIVITY

PHYSICAL REVIEW C (1978) 4399-4407

## Canonical Valuations and the Birational Section Conjecture

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We develop a notion of a `canonical $\mathcal{C}$-henselian valuation' for a class $\mathcal{C}$ of field extensions, generalizing the construction of the canonical henselian valuation of a field. We use this to show that the $p$-adic valuation on a finite extension $F$ of $\mathbb{Q}_p$ can be recovered entirely (or up to some indeterminacy of the residue field) from various small quotients of $G_F$, the absolute Galois group of $F$. In particular, it can be recovered fully from the maximal solvable quotient. We use this to prove several versions of the birational section conjecture for varieties over $p$-adic fields.

## Through a Jet Speed Darkly: The Emergence of Robust Euro-Atlantic Regimes in the Absence of Jet Speed Variability

ArXiv (0)

Euro-Atlantic regimes are typically identified using either the latitude of the eddy-driven jet, or clustering algorithms in the phase space of 500hPa geopotential height (Z500). However, while robust trimodality is visibly apparent in jet latitude indices, Z500 clusters require highly sensitive significance tests to distinguish them from autocorrelated noise. As a result, even small shifts in the time-period considered can notably alter the diagnosed regimes. Fixing the optimal regime number is also hard to justify. We argue that the jet speed, a near-Gaussian distribution projecting strongly onto the Z500 field, is the source of this lack of robustness. Once its influence is removed, the Z500 phase space becomes visibly non-Gaussian, and clustering algorithms easily recover three extremely stable regimes, corresponding to the jet latitude regimes. Further analysis supports the existence of two additional regimes, corresponding to a tilted and split jet. This framework therefore naturally unifies the two regime perspectives.

## On the shallow atmosphere approximation in finite element dynamical cores

ArXiv (0)

We provide an approach to implementing the shallow atmosphere approximation in three dimensional finite element discretisations for dynamical cores. The approach makes use of the fact that the shallow atmosphere approximation metric can be obtained by writing equations on a three-dimensional manifold embedded in $\mathbb{R}^4$ with a restriction of the Euclidean metric. We show that finite element discretisations constructed this way are equivalent to the use of a modified three dimensional mesh for the construction of metric terms. We demonstrate our approach via a convergence test for a prototypical elliptic problem.

## Compatible finite element methods for numerical weather prediction

ArXiv (0)

This article takes the form of a tutorial on the use of a particular class of mixed finite element methods, which can be thought of as the finite element extension of the C-grid staggered finite difference method. The class is often referred to as compatible finite elements, mimetic finite elements, discrete differential forms or finite element exterior calculus. We provide an elementary introduction in the case of the one-dimensional wave equation, before summarising recent results in applications to the rotating shallow water equations on the sphere, before taking an outlook towards applications in three-dimensional compressible dynamical cores.