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Gudrun Magnusdottir

Abstract

The response of the atmospheric winter circulation in both hemispheres to changes in the meridional gradient of sea surface temperature (SST) is examined in an atmospheric general circulation model. Climatological SSTs are employed for the control run. The other runs differ in that a zonally symmetric component is added to or subtracted from the climatological SST field. The meridional structure of the variation in SST gradient is based on the observed change in zonally averaged SST over the last century. The SST trend has maxima of about 1 K at high latitudes of both hemispheres. Elsewhere, the increase in SST over the last century is fairly uniform at about 0.5 K.

In both hemispheres the response to decreased SST gradients is decreased baroclinity in the lower troposphere and increased baroclinity in the upper troposphere, with the reverse response when the SST gradient is increased. Because the cases with decreased SST gradients correspond to warmer SSTs everywhere, they are accompanied by an increase in moisture and a general expansion of the troposphere. The warming cases in the Northern Hemisphere (NH) winter are marked by greatly increased tropical convection, a stronger subtropical jet that is shifted upward and equatorward, and a robust stationary-wave response. Many aspects of the response are remarkably consistent among the different warming experiments, both in pattern and amplitude. The storm-track response is weaker but still consistent among the different warming experiments. Despite general decrease in storm-track activity, there is a tendency for the upper-level NH storm tracks to strengthen at their downstream end and to weaken at their upstream and northward end. When the zonally symmetric SST anomaly field is subtracted from the climatological SST (resulting in lower SST with increased latitudinal gradient), the response is different in many fields and is considerably weaker.

In the winter Southern Hemisphere the change in baroclinity of the low-level flow plays a greater role in the response than in the winter NH. The response in the storm track is zonal with a decrease in midlatitude storm-track activity in the warming cases and an increase in the case that has an increased SST gradient (and cooler SST). There is close correspondence between the pattern of response in all the experiments, irrespective of the sign of the SST anomaly field.

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Wenchang Yang and Gudrun Magnusdottir

Abstract

The intertropical convergence zone (ITCZ) in the east Pacific is located north of the equator during most of the year. In daily data it is most variable in March–April when it may be located north of the equator (nITCZ), on both sides of the equator (dITCZ), or south of the equator (sITCZ), or it may be absent (when convection does not take on a zonally elongated form). Additionally, in strong El Niño years it is located on the equator during the boreal winter half-year. Here the focus is on conditions when the ITCZ has a presence south of the equator (dITCZ, sITCZ) and composites of various fields are compared to “normal conditions” [i.e., when the ITCZ is north of the equator (nITCZ)]. Composites of sea surface temperature (SST), precipitation, outgoing longwave radiation, and the upper-level circulation show very similar patterns for dITCZ and sITCZ days, where the latter cases have almost double the amplitude of the former. The sITCZ state is viewed as an extreme case of the dITCZ state. Both are found to be related to the central Pacific (CP) La Niña with anomalous positive SST and atmospheric heating over the western tropical Pacific and anomalous negative SST and cooling over the central tropical Pacific. Ocean–atmosphere interaction plays an important role in developing the dITCZ and sITCZ anomalies. These daily composite patterns can be reproduced by the regression of monthly fields on the cold CP El Niño–Southern Oscillation mode, suggesting that the interannual rather than day-to-day variability dominates in contributing to the patterns of the composites.

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Courtenay Strong and Gudrun Magnusdottir

Abstract

The role of Rossby wave breaking (RWB) is explored in the transient response of an atmospheric general circulation model to boundary forcing by sea ice anomalies related to the North Atlantic Oscillation (NAO). When the NCAR Community Climate Model, version 3, was forced by an exaggerated sea ice extent anomaly corresponding to one arising from a positive NAO, a localized baroclinic response developed and evolved into a larger-scale equivalent barotropic pattern resembling the negative polarity of the NAO. The initial baroclinic response shifted the phase speeds of the dominant eddies away from a critical value equal to the background zonal flow speed, resulting in significant changes in the spatial distribution of RWB. The forcing of the background zonal flow by the changes in RWB accounts for 88% of the temporal pattern of the response and 80% of the spatial pattern of the zonally averaged response. Although results here focus on one experiment, this “RWB critical line mechanism” appears to be relevant to understanding the equilibrium response in a broad class of boundary forcing experiments given increasingly clear connections among the northern annular mode, jet latitude shifts, and RWB.

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Courtenay Strong and Gudrun Magnusdottir

Abstract

The leading pattern of extratropical Pacific sea surface temperature variability [the Pacific decadal oscillation (PDO)] is shown to depend on observed variability in the spatiotemporal distribution of tropospheric Rossby wave breaking (RWB), where RWB is the irreversible overturning of potential vorticity on isentropic surfaces. Composite analyses based on hundreds of RWB cases show that anticyclonic (cyclonic) RWB is associated with a warm, moist (cool, dry) column that extends down to a surface anticyclonic (cyclonic) circulation, and that the moisture and temperature advection associated with the surface circulation patterns force turbulent heat flux anomalies that project onto the spatial pattern of the PDO. The RWB patterns that are relevant to the PDO are closely tied to El Niño–Southern Oscillation, the Pacific–North American pattern, and the northern annular mode. These results explain the free troposphere-to-surface segment of the atmospheric bridge concept wherein El Niño anomalies emerge in summer and modify circulation patterns that act over several months to force sea surface temperature anomalies in the extratropical Pacific during late winter or early spring. Leading patterns of RWB account for a significant fraction of PDO interannual variability for any month of the year. A multilinear model is developed in which the January mean PDO index for 1958–2006 is regressed upon the leading principal components of cyclonic and anticyclonic RWB from the immediately preceding winter and summer months (four indexes in all), accounting for more than two-thirds of the variance.

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Courtenay Strong and Gudrun Magnusdottir

Abstract

Objective analysis of several hundred thousand anticyclonic and cyclonic breaking Rossby waves is performed for the Northern Hemisphere (NH) winters of 1958–2006. A winter climatology of both anticyclonic and cyclonic Rossby wave breaking (RWB) frequency and size (zonal extent) is presented for the 350-K isentropic surface over the NH, and the spatial distribution of RWB is shown to agree with theoretical ideas of RWB in shear flow.

Composites of the two types of RWB reveal their characteristic sea level pressure anomalies, upper- and lower-tropospheric velocity fields, and forcing of the upper-tropospheric zonal flow. It is shown how these signatures project onto the centers of action and force the velocity patterns associated with the North Atlantic Oscillation (NAO) and Northern Hemisphere annular mode (NAM). Previous studies have presented evidence that anticyclonic (cyclonic) breaking leads to the positive (negative) polarity of the NAO, and this relationship is confirmed for RWB over the midlatitudes centered near 50°N. However, an opposite and statistically significant relationship, in which cyclonic RWB forces the positive NAO and anticyclonic RWB forces the negative NAO, is shown over regions 20° to the north and south, centered at 70° and 30°N, respectively.

On a winter mean basis, the frequency of RWB over objectively defined regions covering 12% of the area of the NH accounts for 95% of the NAO index and 92% of the NAM index. A 6-hourly analysis of all the winters indicates that RWB over the objectively defined regions affects the NAO/NAM without a time lag. Details of the objective wave-breaking analysis method are provided in the .

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Yannick Peings and Gudrun Magnusdottir

Abstract

The wintertime Northern Hemisphere (NH) atmospheric circulation response to current (2007–12) and projected (2080–99) Arctic sea ice decline is examined with the latest version of the Community Atmospheric Model (CAM5). The numerical experiments suggest that the current sea ice conditions force a remote atmospheric response in late winter that favors cold land surface temperatures over midlatitudes, as has been observed in recent years. Anomalous Rossby waves forced by the sea ice anomalies penetrate into the stratosphere in February and weaken the stratospheric polar vortex, resulting in negative anomalies of the northern annular mode (NAM) that propagate downward during the following weeks, especially over the North Pacific. The seasonality of the response is attributed to timing of the phasing between the forced and climatological waves. When sea ice concentration taken from projections of conditions at the end of the twenty-first century is prescribed to the model, negative anomalies of the NAM are visible in the troposphere, both in early and late winter. This response is mainly driven by the large warming of the lower troposphere over the Arctic, as little impact is found in the stratosphere in this experiment. As a result of the thermal expansion of the polar troposphere, the westerly flow is decelerated and a weak but statistically significant increase of the midlatitude meanders is identified. However, the thermodynamical response extends beyond the Arctic and offsets the dynamical effect, such that the stronger sea ice forcing has limited impact on the intensity of cold extremes over midlatitudes.

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Wayne H. Schubert and Gudrun Magnusdottir

Abstract

A potential pseudodensity principle is derived for the quasi-static primitive equations on the sphere. An important step in the derivation of this principle is the introduction of “vorticity coordinates”—that is, new coordinates whose Jacobian with respect to the original spherical coordinates is the dimensionless absolute isentropic vorticity. The vorticity coordinates are closely related to Clebsch variables and are the primitive equation generalizations of the geostrophic coordinates used in semigeostrophic theory. The vorticity coordinates can be used to transform the primitive equations into a canonical form. This form is mathematically similar to the geostrophic relation. There is flexibility in the choice of the potential function appearing in the canonical momentum equations. This flexibility can be used to force the vorticity coordinates to move with some desired velocity, which results in an associated simplification of the material derivative operator. The end result is analogous to the way ageostrophic motions become implicit when geostrophic coordinates are used in semigeostrophic theory.

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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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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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Christopher C. Walker and Gudrun Magnusdottir

Abstract

The nonlinear behavior of planetary waves excited by midlatitude topography is considered in an atmospheric GCM. The GCM is run at standard resolution (T42) and includes all of the complexity normally associated with a GCM. Only two simplifications are made to the model. First, it is run in perpetual January mode, so that the solar radiation takes the diurnally varying value associated with 15 January. Second, the lower boundary is simplified so that it is entirely ocean with zonally symmetric SSTs. Planetary waves are excited by Gaussian-shaped topography centered at 45°N, 90°W. As in earlier studies, the excited wave train propagates toward low latitudes where, for sufficiently large forcing amplitude (i.e., height of topography), the wave will break. Several different experiments are run with different mountain heights. Each experiment is run for a total of 4015 days.

The response of the model depends on the height of the mountain. For the small-amplitude mountain (500 m), the wave is dissipated at low latitudes near its critical latitude. For large-amplitude mountains (2000, 3000, and 4000 m), wave breaking and nonlinear reflection out of the wave breaking region is observed. The spatial character of the reflected wave train is similar to that detected in earlier studies with more idealized models.

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