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Chia-chi Wang
and
Gudrun Magnusdottir

Abstract

The intertropical convergence zone (ITCZ) is observed to undulate and at times break down into a series of tropical disturbances in several days. Some of these disturbances may develop into tropical cyclones and move to higher latitudes, while others dissipate, and the ITCZ may reform in the original region. It has been proposed that the ITCZ may break down because of its heating-induced potential vorticity (PV) anomalies. Here this process is examined in three-dimensional simulations using a primitive equation model. A simulation of the ITCZ in a background state of rest is compared to simulations in different background flows. The effect of different vertical structures of the prescribed heating is also examined.

Deep heating induces a positive PV anomaly in the lower troposphere, leading to a reversal of the PV gradient on the poleward side of the heating, while the induced PV anomaly at upper levels is negative, leading to a reversal of the PV gradient on the equatorward side of the heating. The response at upper levels leads to a weaker PV gradient change, but the response is greater in areal extent than the lower-tropospheric response. For shallow heating, the lower-tropospheric PV response is greater than that for deep heating, and there is no upper-tropospheric PV response. The ITCZ lasts longer before breaking in this case than in the deep heating case.

Effects of the background flow are mainly felt in the deep heating cases. When the background flow enforces the PV-induced wind field, ITCZ breakdown occurs more rapidly, whereas when the background flow is opposite to the PV-induced flow, ITCZ breakdown takes longer and the ITCZ may dissipate before breakdown.

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John T. Abatzoglou
and
Gudrun Magnusdottir

Abstract

Planetary wave breaking (PWB) over the subtropical North Atlantic is observed over 45 winters (December 1958–March 2003) using NCEP–NCAR reanalysis data. PWB is manifested in the rapid, large-scale and irreversible overturning of potential vorticity (PV) contours on isentropic surfaces in the subtropical upper troposphere. As breaking occurs over the subtropical North Atlantic, an upper-tropospheric PV tripole anomaly forms with nodes over the subtropical, midlatitude, and subpolar North Atlantic. The northern two nodes of this tripole are quite similar to the spatial structure of the North Atlantic Oscillation (NAO), with positive polarity.

Nonlinear reflection is identified in approximately a quarter of all PWB events. Following breaking, two distinct circulation regimes arise, one in response to reflective events and the other in response to nonreflective events. For reflective events, anomalies over the North Atlantic rapidly propagate away from the breaking region along a poleward arching wave train over the Eurasian continent. The quasi-stationary wave activity flux indicates that wave activity is exported out of the Atlantic basin. At the same time, the regional poleward eddy momentum flux goes through a sign reversal, as does the polarity of the NAO. For nonreflective events, the dipole anomaly over the North Atlantic amplifies. Diagnostics for nonreflective events suggest that wave activity over the Azores gets absorbed, allowing continued enhancement of both the regional poleward eddy momentum flux and the positive NAO.

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John T. Abatzoglou
and
Gudrun Magnusdottir

Abstract

Forty-six years of daily averaged NCEP–NCAR reanalysis data are used to identify the occurrence of planetary wave breaking (PWB) in the subtropical upper troposphere. As large-amplitude waves propagate into the subtropics where the zonal flow is weak, they may break. PWB is diagnosed by observing the large-scale meridional overturning of potential vorticity (PV) contours on isentropic surfaces near the subtropical tropopause. PWB occurs most often during summer, and almost exclusively over the subtropical ocean basins in the Northern Hemisphere. The seasonal evolution of the zonal flow (and the associated latitudinal PV gradient) regulates the location and frequency of PWB. Significant interannual variability in PWB is associated with well-known modes of climate variability.

One of the most interesting dynamical consequences of PWB is the possibility of nonlinear reflection poleward out of the wave-breaking region. Modeling studies have found nonlinear reflection following PWB. Observations show that about 36% of all PWB events are followed by nonlinear reflection back into midlatitudes. In these cases, a poleward-arching wave train can be seen propagating away from the wave-breaking region following breaking. It is suggested that a sufficiently strong latitudinal PV gradient must be present downstream of the wave-breaking region for reflection to take place. The proportion of PWB events that is reflective stays rather constant through the year, with slightly higher numbers in spring and fall compared to those in winter and summer.

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N. Joss Matthewman
and
Gudrun Magnusdottir

Abstract

The relationship between North Pacific sea ice and the Western Pacific (WP) pattern is examined using wintertime observational data between 1978 and 2008. Weekly averaged data are chosen to capture the characteristically short time scale of the WP. A clear relationship is found between the WP and sea ice concentrations in the Bering Sea, where the positive polarity of the WP is accompanied by increasing sea ice concentrations and the negative WP by decreasing sea ice concentrations. Sea ice concentrations in the Sea of Okhotsk, however, are shown to be largely insensitive to the strength of the WP. Feedback of Bering Sea sea ice concentrations onto the WP is tested by fitting weekly averaged observations to a vector autoregressive (VAR) model. Results from the VAR model indicate that feedback of Bering Sea sea ice onto the WP plays a significant role in the dynamics of the WP and that this feedback is positive; that is, WP-induced changes in Bering Sea sea ice concentrations help sustain existing WP conditions, thereby lengthening the time scale of variability of the WP.

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Yi-Hui Wang
and
Gudrun Magnusdottir

Abstract

Several studies have found an eastward shift in the northern node of the North Atlantic Oscillation (NAO) during the winters of 1978–97 compared to 1958–77. This study focuses on the connection between this shift of the northern node of the NAO and Rossby wave breaking (RWB) for the period 1958–97. It is found that the region of frequent cyclonic RWB underwent a northeastward shift at high latitudes in the latter 20-yr period. On a year-to-year basis, the cyclonic RWB region moves along a southwest–northeast (SW–NE)-directed axis. Both latitude and longitude of the winter maximum frequency of cyclonic RWB occurrence are positively correlated with the NAO index.

To investigate the role of location of cyclonic RWB in influencing the NAO pattern, the geographical location of frequent cyclonic RWB is divided into two subdomains located along the SW–NE axis, to the south (SW domain) and east (NE domain) of Greenland. Two composites are assembled as one cyclonic RWB occurrence is detected in one of the two subdomains in 6-hourly instantaneous data. The forcing of the mean flow due to cyclonic RWB within individual subdomains is found to be locally restricted to where the breaking occurs, which is usually near the jet exit region and far removed from the jet core. The difference in the jet between the NE and SW composites resembles the difference in the mean jet between the 1978–97 and 1958–77 periods, which suggests that the change in cyclonic RWB occurrence in the two subdomains is associated with the wobbling of the jet on the decadal time scale.

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Ashley E. Payne
and
Gudrun Magnusdottir

Abstract

A large-scale analysis of landfalling atmospheric rivers (ARs) along the west coast of North America and their association with the upper-tropospheric flow is performed for the extended winter (November–March) for the years 1979–2011 using Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis data. The climatology, relationship to the El Niño–Southern Oscillation and the Madden–Julian oscillation, and upper-level characteristics of approximately 750 landfalling ARs are presented based on the 85th percentile of peak daily moisture flux. AR occurrence along the West Coast is dominated by early season events. In composites of upper-level fields during AR occurrences, certain characteristics stand out irrespective of the tropical climate indices. This suggests that extratropical dynamical processes play a key role in AR dynamics.

The influence of the large-scale circulation on AR intensity prior to landfall is examined by objectively selecting an extreme subset of 112 landfalling AR dates representing the 95th percentile of strongest cases. Each landfalling AR date that is identified is traced backward in time using a novel semiautomated tracking algorithm based on spatially and temporally connected organized features in integrated moisture transport. Composites of dynamical fields following the eastward progression of ARs show a close relationship of the location of the jet, Rossby wave propagation, and anticyclonic Rossby wave breaking in the upper troposphere of the eastern Pacific and moisture transport in the lower troposphere. Comparison between the strongest and the weakest ARs within the most extreme subset shows differences in both the intensity of moisture transport and the scale and development of anticyclonic Rossby wave breaking in the eastern Pacific.

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Yi-Hui Wang
and
Gudrun Magnusdottir

Abstract

An objective analysis of tropospheric anticyclonic- and cyclonic-breaking Rossby waves is performed for the Southern Hemisphere in austral summer (December–February) of 1979–2009. The climatology of both anticyclonic and cyclonic Rossby wave breaking (RWB) frequency is presented. The frequency of anticyclonic RWB is highest in an extended region of the Eastern Hemisphere on the anticyclonic side of the jet, while that of cyclonic RWB is highest on the cyclonic side of the jet. A composite analysis of anticyclonic and cyclonic RWB shows how they contribute to a positive and negative southern annual mode (SAM) index, respectively. The time series of austral summer anticyclonic RWB occurrence has a trend that closely matches the trend in the SAM index.

Regions of RWB that are significantly correlated with the SAM index are objectively determined. Even though several such regions are identified, only two regions (anticyclonic and cyclonic) covering 17% of the area of the hemisphere are required in a linear regression model of the SAM index. The anticyclonic RWB region is zonally extended at 45°S and explains 78% of the variability of the summer-mean SAM index. The cyclonic region is located at high latitudes somewhat decoupled from the jet, in the longitudinal sector of the Indian Ocean. On synoptic time scales, transitions of the SAM index respond to RWB without time lag.

ENSO cycles present an interesting zonal asymmetry to the distribution of Southern Hemispheric RWB in the central Pacific. Anticyclonic RWB is increased in the tropical/subtropical central Pacific during La Niña compared to El Niño. This increase is related to the strong local decrease in zonal wind. At the same time, anticyclonic RWB outside the central Pacific is increased in frequency poleward and decreased in frequency equatorward of 42°S, corresponding to a positive SAM index.

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Gudrun Magnusdottir
and
Wayne H. Schubert

Abstract

We develop here the isentropic–geostrophic coordinate version of semigeostrophic theory on a midlatitude β-plane. This approach results in a simple mathematical form in which the horizontal ageostrophic velocities are implicit and the entire dynamics reduces to a predictive equation for the potential pseudodensity and an invertibility relation. Linearized versions of the theory lead to a generalized Charney–Stern theorem for combined barotropic–baroclinic instability and to Rossby wave solutions with a meridional structure different from that in quasi-geostrophic theory.

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Gudrun Magnusdottir
and
Wayne H. Schubert

Abstract

This paper presents the combined isentropic and spherical geostrophic coordinate version of semigeostrophic theory. This is accomplished by first proposing a spherical coordinate generalization of the geostrophic momentum approximation and discussing its associated conservation principles for absolute angular momentum, total energy, potential vorticity and potential pseudodensity. We then show how the use of the spherical geostrophic coordinates allows the equations of the geostrophic momentum approximation to be written in a canonical form that makes ageostrophic advection implicit. This leads to a simple equation for the prediction of the potential pseudodensity. The potential pseudodensity can then be inverted to obtain the associated wind and mass fields. In this way the more general semigeostrophic theory retains the same simple mathematical structure as quasi-geostrophic theory—a single predictive equation which does not explicitly contain ageostrophic advection and an invertibility principle. The combined use of isentropic and spherical geostrophic coordinates is crucial to retaining this simplicity.

In order to demonstrate how the theory applies to problems of barotropic–baroclinic instability and Rossby–Haurwitz wave dispersion, we derive the semigeostrophic generalization of the Charney–Stern theorem and compare the semigeostrophic Rossby–Haurwitz wave frequencies with those of Laplace's tidal equations. The agreement between these frequencies is generally better than 0.5%. Thus, the theory appears to encompass a wide range of meteorological phenomena including both planetary-scale and synoptic-scale waves, along with their finer scale aspects such as fronts and jets.

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Gudrun Magnusdottir
and
Peter H. Haynes

Abstract

Wave activity diagnostics are calculated for four different baroclinic wave life cycles, including the LC1 and LC2 cases studied by Thorncroft, Hoskins, and McIntyre. The wave activity is a measure of the disturbance relative to some zonally symmetric, time-independent basic state, which need not be the initial zonally averaged state and which satisfies a finite-amplitude conservation relation. The wave activity density and fluxes may be calculated in terms of Eulerian variables provided that the potential vorticity is a monotonic function of latitude on isentropic surfaces in the basic state. The LC1 and LC2 experiments used initial states in which the potential vorticity (PV) did not satisfy this monotonicity condition. Therefore two approaches are taken. The first is to define a basic state that is not the initial state and use this to calculate the wave activity diagnostics. The second is to carry out new LC1- and LC2-type experiments on initial states in which the monotonicity condition is satisfied. New basic states are generated by PV rearrangement and inversion.

The results allow quantification of the difference between LC1- and LC2-type life cycles. They also show that LC1- and LC2-type behavior occurs for different initial states other than those used by Thorncroft, Hoskins, and McIntyre and that the classification is therefore robust in terms of the potential vorticity field and wave activity diagnostics. If one were to consider only eddy kinetic energy, the distinction is no longer clear. In fact, in the evolution of eddy kinetic energy the modified LC1-type life cycle resembles LC2 and the modified LC2 more than it resembles LC1.

The results also shed new light on the role of wave propagation in baroclinic life cycles. In particular, it is found that during the later stages of the life cycle the pattern of equatorward wave activity flux that has often been interpreted as associated with equatorward wave propagation in the subtropical upper troposphere is in fact associated primarily with advective transport of wave activity.

New finite-amplitude expressions are presented for the wave activity associated with potential temperature gradients on the lower boundary. Problems with using PV rearrangement techniques are discussed.

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