Search Results

You are looking at 11 - 18 of 18 items for

  • Author or Editor: Maarten H. P. Ambaum x
  • Refine by Access: All Content x
Clear All Modify Search
Lenka Novak
,
Maarten H. P. Ambaum
, and
Rémi Tailleux

Abstract

The North Atlantic eddy-driven jet exhibits latitudinal variability with evidence of three preferred latitudinal locations: south, middle, and north. Here the authors examine the drivers of this variability and the variability of the associated storm track. The authors investigate the changes in the storm-track characteristics for the three jet locations and propose a mechanism by which enhanced storm-track activity, as measured by upstream heat flux, is responsible for cyclical downstream latitudinal shifts in the jet. This mechanism is based on a nonlinear oscillator relationship between the enhanced meridional temperature gradient (and thus baroclinicity) and the meridional high-frequency (periods of shorter than 10 days) eddy heat flux. Such oscillations in baroclinicity and heat flux induce variability in eddy anisotropy, which is associated with the changes in the dominant type of wave breaking and a different latitudinal deflection of the jet. The authors’ results suggest that high heat flux is conducive to a northward deflection of the jet, whereas low heat flux is conducive to a more zonal jet. This jet-deflecting effect was found to operate most prominently downstream of the storm-track maximum, while the storm track and the jet remain anchored at a fixed latitudinal location at the beginning of the storm track. These cyclical changes in storm-track characteristics can be viewed as different stages of the storm track’s spatiotemporal life cycle.

Full access
Ben Harvey
,
John Methven
, and
Maarten H. P. Ambaum

Abstract

The amplitude of ridges in large-amplitude Rossby waves has been shown to decrease systematically with lead time during the first 1–5 days of operational global numerical weather forecasts. These models also exhibit a rapid reduction in the isentropic gradient of potential vorticity (PV) at the tropopause during the first 1–2 days of forecasts. This paper identifies a mechanism linking the reduction in large-scale meander amplitude on jet streams to declining PV gradients. The mechanism proposed is that a smoother isentropic transition of PV across the tropopause leads to excessive PV filamentation on the jet flanks and a more lossy waveguide. The approach taken is to analyze Rossby wave dynamics in a single-layer quasigeostrophic model. Numerical simulations show that the amplitude of a Rossby wave propagating along a narrow but smooth PV front does indeed decay transiently with time. This process is explained in terms of the filamentation of PV from the jet core and associated absorption of wave activity by the critical layers on the jet flanks, and a simple method for quantitatively predicting the magnitude of the amplitude reduction without simulation is presented. Explicitly diffusive simulations are then used to show that the combined impact of diffusion and the adiabatic rearrangement of PV can result in a decay rate of Rossby waves that is 2–4 times as fast as could be expected from diffusion acting alone. This predicted decay rate is sufficient to explain the decay observed in operational weather forecasting models.

Full access
Maarten H. P. Ambaum
,
Brian J. Hoskins
, and
David B. Stephenson

Abstract

The definition and interpretation of the Arctic oscillation (AO) are examined and compared with those of the North Atlantic oscillation (NAO). It is shown that the NAO reflects the correlations between the surface pressure variability at its centers of action, whereas this is not the case for the AO. The NAO pattern can be identified in a physically consistent way in principal component analysis applied to various fields in the Euro-Atlantic region. A similar identification is found in the Pacific region for the Pacific–North American (PNA) pattern, but no such identification is found here for the AO. The AO does reflect the tendency for the zonal winds at 35° and 55°N to anticorrelate in both the Atlantic and Pacific regions associated with the NAO and PNA. Because climatological features in the two ocean basins are at different latitudes, the zonally symmetric nature of the AO does not mean that it represents a simple modulation of the circumpolar flow. An increase in the AO or NAO implies strong, separated tropospheric jets in the Atlantic but a weakened Pacific jet. The PNA has strong related variability in the Pacific jet exit, but elsewhere the zonal wind is similar to that related to the NAO. The NAO-related zonal winds link strongly through to the stratosphere in the Atlantic sector. The PNA-related winds do so in the Pacific, but to a lesser extent. The results suggest that the NAO paradigm may be more physically relevant and robust for Northern Hemisphere variability than is the AO paradigm. However, this does not disqualify many of the physical mechanisms associated with annular modes for explaining the existence of the NAO.

Full access
Maarten H. P. Ambaum
,
Brian J. Hoskins
, and
David B. Stephenson
Full access
Lenka Novak
,
Maarten H. P. Ambaum
, and
Ben J. Harvey

Abstract

The steady-state response of a midlatitude storm track to large-scale extratropical thermal forcing and eddy friction is investigated in a dry general circulation model with a zonally symmetric forcing. A two-way equilibration is found between the relative responses of the mean baroclinicity and baroclinic eddy intensity, whereby mean baroclinicity responds more strongly to eddy friction whereas eddy intensity responds more strongly to the thermal forcing of baroclinicity. These seemingly counterintuitive responses are reconciled using the steady state of a predator–prey relationship between baroclinicity and eddy intensity. This relationship provides additional support for the well-studied mechanism of baroclinic adjustment in Earth’s atmosphere, as well as providing a new mechanism whereby eddy dissipation controls the large-scale thermal structure of a baroclinically unstable atmosphere. It is argued that these two mechanisms of baroclinic adjustment and dissipative control should be used in tandem when considering storm-track equilibration.

Full access
Paolo Ruggieri
,
Maarten H. P. Ambaum
, and
Jonas Nycander

Abstract

Large-scale overturning mass transport in the stratosphere is commonly explained through the action of potential vorticity (PV) rearrangement in the flank of the stratospheric jet. Large-scale Rossby waves, with their wave activity source primarily in the troposphere, stir and mix PV and an overturning circulation arises to compensate for the zonal torque imposed by the breaking waves. In this view, any radiative heating is relaxational and the circulation is mechanically driven. Here we present a fully thermodynamic analysis of these phenomena, based on ERA-Interim data. Streamfunctions in a thermodynamic, log(pressure)–temperature space are computed. The sign of a circulation cell in these coordinates directly shows whether it is mechanically driven, converting kinetic energy to potential and thermal energy, or thermally driven, with the opposite conversion. The circulation in the lower stratosphere is found to be thermodynamically indirect (i.e., mechanically driven). In the middle and upper stratosphere thermodynamically indirect and direct circulations coexist, with a prominent semiannual cycle. A part of the overturning in this region is thermally driven, while a more variable indirect circulation is mechanically driven by waves. The wave driving does not modulate the strength of the thermally direct part of the circulation. This suggests that the basic overturning circulation in the stratosphere is largely thermally driven, while tropospheric waves add a distinct indirect component to the overturning. This indirect overturning is associated with poleward transport of anomalously warm air parcels.

Free access
Jan A. Kamieniecki
,
Maarten H. P. Ambaum
,
Robert S. Plant
, and
Steven J. Woolnough

Abstract

A thermodynamic analysis is presented of an overturning circulation simulated by two cloud-resolving models, coupled by a weak temperature gradient parameterization. Taken together, they represent two separated regions over different sea surface temperatures, and the coupling represents an idealized large-scale circulation such as the Walker circulation. It is demonstrated that a thermodynamic budget linking net heat input to the generation of mechanical energy can be partitioned into contributions from the large-scale interaction between the two regions, as represented by the weak temperature gradient approximation, and from convective motions in the active warm region and the suppressed cool region. Model results imply that such thermodynamic diagnostics for the aggregate system are barely affected by the strength of the coupling, even its introduction, or by the SST contrast between the regions. This indicates that the weak temperature gradient parameterization does not introduce anomalous thermodynamic behavior. We find that the vertical kinetic energy associated with the large-scale circulation is more than three orders of magnitude smaller than the typical vertical kinetic energy in each region. However, even with very weak coupling circulations, the contrast between the thermodynamic budget terms for the suppressed and active regions is strong and is relatively insensitive to the degree of the coupling. Additionally, scaling arguments are developed for the relative values of the terms in the mechanical energy budget.

Full access
Adam H. Monahan
,
John C. Fyfe
,
Maarten H. P. Ambaum
,
David B. Stephenson
, and
Gerald R. North

Abstract

Empirical orthogonal function (EOF) analysis is a powerful tool for data compression and dimensionality reduction used broadly in meteorology and oceanography. Often in the literature, EOF modes are interpreted individually, independent of other modes. In fact, it can be shown that no such attribution can generally be made. This review demonstrates that in general individual EOF modes (i) will not correspond to individual dynamical modes, (ii) will not correspond to individual kinematic degrees of freedom, (iii) will not be statistically independent of other EOF modes, and (iv) will be strongly influenced by the nonlocal requirement that modes maximize variance over the entire domain. The goal of this review is not to argue against the use of EOF analysis in meteorology and oceanography; rather, it is to demonstrate the care that must be taken in the interpretation of individual modes in order to distinguish the medium from the message.

Full access