Search Results

You are looking at 1 - 10 of 46 items for

  • Author or Editor: Darryn W. Waugh x
  • Refine by Access: All Content x
Clear All Modify Search
Darryn W. Waugh

Abstract

High-resolution simulations of a polar vortex disturbed by quasi-topographic forcing are performed using the method of “contour surgery”, a numerical method for inviscid flows wherein arbitrarily steep vorticity gradients can be formed and wherein scales of motion can vary over an extensive range. Simulations are performed of inviscid, incompressible, barotropic motion on the hemisphere, the sphere, and the plane. Comparisons with a hemispherical pseudospectral simulation show that accurate contour surgery simulations can be performed using a moderate number of contours to represent each hemisphere. Intermodel comparisons show that, although the overall evolution of the flow is qualitatively similar, there are noticeable differences. There is a significant difference between spherical and hemispherical calculations but, surprisingly, a remarkable agreement between spherical and planar calculations when a spatially varying planetary vorticity is used in the planar calculations.

Full access
Thando Ndarana and Darryn W. Waugh

Abstract

A 30-yr climatology of Rossby wave breaking (RWB) on the Southern Hemisphere (SH) tropopause is formed using 30 yr of reanalyses. Composite analysis of potential vorticity and meridional fluxes of wave activity show that RWB in the SH can be divided into two broad categories: anticyclonic and cyclonic events. While there is only weak asymmetry in the meridional direction and most events cannot be classified as equatorward or poleward in terms of the potential vorticity structure, the position and structure of the fluxes associated with equatorward breaking differs from those of poleward breaking. Anticyclonic breaking is more common than cyclonic breaking, except on the lower isentrope examined (320 K). There are marked differences in the seasonal variations of RWB on the two surfaces, with a winter minimum for RWB around 350 K but a summer minimum for RWB around 330 K. These seasonal variations are due to changes in the location of the tropospheric jets and dynamical tropopause. During winter the subtropical jet and tropopause at 350 K are collocated in the Australian–South Pacific Ocean region, resulting in a seasonal minimum in the 350-K RWB. During summer the polar front jet and 330-K tropopause are collocated over the Southern Atlantic and Indian Oceans, inhibiting RWB in this region.

Full access
Dieter Peters and Darryn W. Waugh

Abstract

The characteristics of Rossby wave propagation and breaking in the Southern Hemisphere upper troposphere during winter are examined. Although the Southern Hemisphere subtropical jet is more zonally symmetric than that of the Northern Hemisphere, there are still significant zonal variations in the upper-tropospheric flow. In particular, the flow within a given sector (≈120° longitude) can generally be characterized into one of four different configurations: (i) a single jet, (ii) a “broken” subtropical jet, (iii) a polar jet at the upstream end of the subtropical jet, or (iv) a polar jet at the downstream end of the subtropical jet. Using “potential vorticity thinking” and barotropic wind shear arguments, it is argued that the characteristics of the Rossby wave propagation and breaking will differ between each flow configuration. Consistent with these arguments, examination of potential vorticity maps and contour advection calculations show differing wave-breaking characteristics. In particular, there is &ldquo=uatorward” wave breaking with cyclonic behavior when a single strong jet exists, “poleward” breaking with anticyclonic behavior when a broken subtropical jet or a polar jet is downstream of a subtropical jet, and more “symmetric” wave breaking when a polar jet is upstream of a subtropical jet. Some of the flow configurations have preferred geographical locations, and this results in different geographical sectors having differing preferred configurations and variability, and, hence, characteristics of the Rossby wave propagation. For example, a broken subtropical jet or polar jet with poleward wave breaking is most common within the Australian and Pacific Ocean sectors.

Full access
Dieter Peters and Darryn W. Waugh

Abstract

The characteristics of the poleward advection of upper-tropospheric air are investigated using meteorological analyses and idealized numerical models. Isentropic deformations of the tropopause during Northern Hemisphere winter are examined using maps of Ertel's potential vorticity together with contour advection calculations. Large poleward excursions of upper-tropospheric air are observed during Rossby wave breaking events. These “poleward” breaking events occur in regions of diffluence (over the eastern Atlantic Ocean-Europe region, and over the eastern Pacific Ocean-North America region), and the evolution of the tropospheric air depends on the local, meridional shear: in anticyclonic (or weak cyclonic) shear the tropospheric air tilts downstream, broadens, and wraps up anticyclonically, whereas in cyclonic shear the tropospheric air tilts upstream, thins, and is advected cyclonically. The role of ambient barotropic flow is further examined by considering the flow in two numerical models: a planar, equivalent-barotropic, contour dynamics model and a simplified general circulation model. In both models, the variation of the poleward wave breaking with the zonal and meridional shear is consistent with that in the analyses.

Full access
Darryn W. Waugh and Timothy M. Hall

Abstract

The propagation of a range of tracer signals in a simple model of the deep western boundary current is examined. Analytical expressions are derived in certain limits for the transit-time distributions and the propagation times (tracer ages) of tracers with exponentially growing or periodic concentration histories at the boundary current’s origin. If mixing between the boundary current and the surrounding ocean is either very slow or very rapid, then all tracer signals propagate at the same rate. In contrast, for intermediate mixing rates tracer ages generally depend on the history of the tracer variations at the origin and range from the advective time along the current to the much larger mean age. Close agreement of the model with chlorofluorocarbon (CFC) and tritium observations in the North Atlantic deep western boundary current (DWBC) is obtained when the model is in the intermediate mixing regime, with current speed around 5 cm s−1 and mixing time scale around 1 yr. In this regime anomalies in temperature and salinity of decadal or shorter period will propagate downstream at roughly the current speed, which is much faster than the spreading rate inferred from CFC or tritium–helium ages (approximately 5 cm s−1 as compared with 2 cm s−1). This rapid propagation of anomalies is consistent with observations in the subpolar DWBC, but is at odds with inferences from measurements in the tropical DWBC. This suggests that observed tropical temperature and salinity anomalies are not simply propagated signals from the north. The sensitivity of the tracer spreading rates to tracer and mixing time scales in the model suggests that tight constraints on the flow and transport in real DWBCs may be obtained from simultaneous measurements of several different tracers—in particular, hydrographic anomalies and steadily increasing transient tracers.

Full access
Darryn W. Waugh and William J. Randel

Abstract

The climatological structure, and interannual variability, of the Arctic and Antarctic stratospheric polar vortices are examined by analysis of elliptical diagnostics applied to over 19 yr of potential vorticity data. The elliptical diagnostics define the area, center, elongation, and orientation of each vortex and are used to quantify their structure and evolution. The diagnostics offer a novel view of the well-known differences in the climatological structure of the polar vortices. Although both vortices form in autumn to early winter, the Arctic vortex has a shorter life span and breaks down over a month before the Antarctic vortex. There are substantial differences in the distortion of the vortices from zonal symmetry; the Arctic vortex is displaced farther off the pole and is more elongated than the Antarctic vortex. While there is a midwinter minimum in the distortion of the Antarctic vortex, the distortion of the Arctic vortex increases during its life cycle. There are also large differences in the interannual variability of the vortices: the variability of the Antarctic vortex is small except during the spring vortex breakdown, whereas the Arctic vortex is highly variable throughout its life cycle, particularly in late winter. The diagnostics also reveal features not apparent in previous studies. There are periods when there are large zonal shifts (westward then eastward) in the climatological locations of the vortices: early winter for the Arctic vortex, and late winter to spring for the Antarctic vortex. Also, there are two preferred longitudes of the center of the lower-stratospheric Arctic vortex in early winter, and the vortex may move rapidly from one to the other. In the middle and upper stratosphere large displacements off the pole and large elongation of the vortex are both associated with a small vortex area, but there is very little correlation between displacement off the pole and elongation of the vortex.

Full access
Chaim I. Garfinkel and Darryn W. Waugh

A typo was included in Eq. (2) of Garfinkel et al. (2013), Garfinkel and Waugh (2014), and Garfinkel and Harnik (2017). The equation should read as follows:

All modeling experiments in Garfinkel et al. (2013), Garfinkel and Waugh (2014), and Garfinkel and Harnik (2017) were forced with as formulated here, and hence this error did not affect the results or conclusions of any of the three original papers. Note that the last term on the right-hand side of the equation leads to a temperature perturbation in

Full access
Darryn W. Waugh and R. Alan Plumb

Abstract

We present a trajectory technique, contour advection with surgery (CAS), for tracing the evolution of material contours in a specified (including observed) evolving flow. CAS uses the algorithms developed by Dritschel for contour dynamics/surgery to trace the evolution of specified contours. The contours are represented by a series of particles, which are advected by a specified, gridded, wind distribution. The resolution of the contours is preserved by continually adjusting the number of particles, and finescale features are produced that are not present in the input data (and cannot easily be generated using standard trajectory techniques). The reliability, and dependence on the spatial and temporal resolution of the wind field, of the CAS procedure is examined by comparisons with high-resolution numerical data (from contour dynamics calculations and from a general circulation model), and with routine stratospheric analyses. These comparisons show that the large-scale motions dominate the deformation field and that CAS can accurately reproduce small scales from low-resolution wind fields. The CAS technique therefore enables examination of atmospheric tracer transport at previously unattainable resolution.

Full access
Ilai Guendelman, Darryn W. Waugh, and Yohai Kaspi

Abstract

Zonal jets are common in planetary atmospheres. Their character, structure, and seasonal variability depend on the planetary parameters. During solstice on Earth and Mars, there is a strong westerly jet in the winter hemisphere and weak, low-level westerlies in the ascending regions of the Hadley cell in the summer hemisphere. This summer jet has been less explored in a broad planetary context, both due to the dominance of the winter jet and since the balances controlling it are more complex, and understanding them requires exploring a broader parameter regime. To better understand the jet characteristics on terrestrial planets and the transition between winter- and summer-dominated jet regimes, we explore the jet’s dependence on rotation rate and obliquity. Across a significant portion of the parameter space, the dominant jet is in the winter hemisphere, and the summer jet is weaker and restricted to the boundary layer. However, we show that for slow rotation rates and high obliquities, the strongest jet is in the summer rather than the winter hemisphere. Analysis of the summer jet’s momentum balance reveals that the balance is not simply cyclostrophic and that both boundary layer drag and vertical advection are essential. At high obliquities and slow rotation rates, the cross-equatorial winter cell is wide and strong. The returning poleward flow in the summer hemisphere is balanced by low-level westerlies through an Ekman balance and momentum is advected upward close to the ascending branch, resulting in a midtroposphere summer jet.

Restricted access
Andrea Molod, Haydee Salmun, and Darryn W. Waugh

Abstract

Heterogeneities in the land surface on scales smaller than the typical general circulation model (GCM) grid size can have a profound influence on the grid-scale mean climate. There exists observational and modeling evidence that the direct effects of surface heterogeneities may be felt by the atmosphere well into the planetary boundary layer. The impact of including an “extended mosaic” (EM) scheme, which accounts for the vertical influence of land surface heterogeneities in a GCM, is evaluated here by comparing side-by-side GCM simulations with EM and with the more standard mosaic formulation (M).

Differences between the EM and M simulations are observed in the boundary layer structure, in fields that link the boundary layer and the general circulation, and in fields that represent the general circulation itself. Large EM − M differences are found over the eastern United States, eastern Asia, and southern Africa in the summertime, and are associated with a boundary layer eddy diffusion feedback mechanism. The feedback mechanism operates as a positive or negative feedback depending on the local Bowen ratio. Significant EM − M differences are also found in the region of the Australian monsoon and in the strength of the stationary Pacific–North America pattern in the northern Pacific.

Full access