Abstract

The authors investigate the processes acting during a cyclonic “blocking” anomaly that dominated the upper troposphere in the New Zealand-Tasman Sea region during the very stormy month of August 1990. The authors use European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and a simple, unambiguous flux form of the isentropic vorticity equation that makes possible a robust physical interpretation of the budget and that does not suffer from the problems of strong cancellation between pairs of terms that mar the traditional, isobaric approach. The simplicity of the equation also makes tractable an analysis of the errors in each of the terms in the monthly mean budget.

On the western and northern flanks of the anomaly at 310 K (≈350 hPa) the tendency of the time-mean flow to advect the anomaly downstream is countered (within the limits imposed by the relatively short averaging period) by mean stretching and the repeated influx of subpolar air by the storms during the month. However, on the eastern side of the anomaly the error analysis gives confidence that the dominant mean stretching term is only partially balanced by the mean and eddy advection terms. Two regions were found, overlying the ends of surface storm tracks, where large-scale and statistically significant residuals are required to balance the isentropic vorticity budget. The authors note that the whole eastern side of the anomaly is characterised by a large-scale drag on the prevailing flow, with isentropic gradients in the drag being responsible for the vorticity residual. Interestingly, embedded within each area of significant residual a smaller region exhibiting a significant acceleration of the zonal flow in the upper troposphere is found.

The authors argue that these features are caused by distinct physical processes and are not the result of systematic errors in vorticity tendencies, diabatic flux divergences, or errors in the analyzed divergent wind field. Instead, the authors conclude that during August 1990 there are several dynamically significant processes operating that are not resolved by 2.5°×2.5° ECMWF analyses: namely, the vertical transport of horizontal momentum associated with convective activity, the quasi-borizontal transport of horizontal momentum by breaking Rossby waves, and the ageostrophic motions associated with frontogenesis.

Abstract

Recently, the effects of nonlinearity on waves forced by sinusoidal orography in severely truncated barotropic and baroclinic models have been explored. Multiple equilibria were found for fixed forcing and these have been associated with zonal and blocked states of the global circulation, although the contrast between states was less marked in the baroclinic model.

The presence of multiple equilibria is dependent on instability of the basic forced solution. This instability in barotropic and baroclinic models is the subject of this study. In the barotropic case, the instability seems to be new but the baroclinic counterpart is shown to be a variation of the dynamics exhibited by Simmons in his study of planetary-scale waves in the polar winter stratosphere. These two instabilities are shown to play important, but different, roles in determining the behavior of simple models in the presence of forcing and dissipation.

An extension is made to a five-layer, σcoordinate, primitive equation model on the sphere, using more degrees of freedom. Taking as the basic state the Northern Hemisphere winter zonal mean flow, orographically unstable modes are found. In all but one case, the associated growth rates are much smaller than those corresponding baroclinic instability, even for rather large mountains. This is in contrast with the results from simple, highly truncated β-plane models and suggests that in more realistic situations, orographically induced instabilities may not be so important. However, the work has deepened our understanding of some of the possible interactions of Rossby waves with mountains.

Abstract

Atmospheric blocking events over the South Pacific are investigated using a 39-yr record of 500-hPa height fields from the NCEP–NCAR reanalysis dataset. The analysis extends earlier work using a 16-yr record and confirms that the occurrence of blocking over the southeast Pacific is strongly modulated by the ENSO cycle during austral spring and summer. Comparison of results at 500 hPa with the 300-hPa meridional wind component showed that blocking events are associated with large-scale wave trains lying across the South Pacific from the region of Australia to southern South America. Similar wave trains are evident in both hemispheres in singular value decomposition analyses between 300-hPa meridional wind components and tropical Pacific outgoing longwave radiation (OLR) anomalies.

The hypothesis that the divergence associated with tropical OLR anomalies forces an extratropical wave response that results in enhanced blocking over the southeast Pacific was tested using a linearized, barotropic vorticity equation (BVE) model. Observed 300-hPa mean flow fields and divergence forcing that matched the anomalous OLR were used to drive the BVE model. The resulting pattern of meridional wind and streamfunction anomalies agrees closely with observations. When the tropical OLR anomaly is given an eastward phase speed of 5° per day, the extratropical response agrees even better with observations. This suggests that linear Rossby wave propagation provides an important link between anomalous convection in the Tropics and the occurrence of blocking over the southeast Pacific Ocean.

Abstract

Characteristic patterns of cyclogenesis in the southwest Pacific region are identified from a sample of 40 developing cyclones during 1990–94. Cases were chosen objectively to ensure a realistic sampling of typical rather than “ideal” events. A subjective classification based on synoptic-scale upper-tropospheric flow signatures prior to cyclone intensification suggested four classes into which all but three cases fell. Three categories, each containing about a quarter of the population, involved direct coupling with the upper jet. They represent cyclone formation beneath (i) the poleward exit region of a 300-hPa jet upstream from a diffluent trough (class U), (ii) the confluent equatorward entrance region of the upper wind maximum (E), and (iii) the upper jet exit region where the jet is downstream from the upper trough (class D). These are analogous to previously identified categories for the western North Atlantic region. A fourth class involved cyclones forming beneath a preexisting intense upper-level trough (class T) located poleward of the upper-level jet.

Class U cyclones, forming within diffluent airflow, exhibited strong cold fronts, weak warm fronts, and a meridional configuration while class E cyclones forming in confluent flow attained a more zonally elongated structure marked by stronger warm fronts and weak cold fronts. Class U cyclones featured frontal evolution similar to the Norwegian cyclone model while class E and D cyclones exhibited characteristics of the Shapiro–Keyser model. These results provide further observational support for the emerging paradigm of contrasting frontal and cyclone structure resulting from confluent versus diffluent large-scale flow.

Abstract

The principal modes of Southern Hemisphere low-frequency variability have recently been calculated using a 39-yr record of 300-hPa streamfunction fields from the NCEP–NCAR reanalysis dataset. The authors attempt to interpret these modes as the rotational response to some divergent forcing. For a range of mean states the linearized barotropic vorticity equation (BVE) is used to solve for the divergent wind that would generate (or at least be consistent with) the observed vorticity modes. Several of these low-frequency modes can be generated by forcing the BVE with fairly simple divergent wind fields that could easily be interpreted as resulting from anomalous tropical convection. In particular this is found to be true for streamfunction anomalies with El Niño–Southern Oscillation (ENSO), high-latitude mode, South Pacific wave, and Madden–Julian oscillation structure. The authors speculate that it may be possible to relate these calculated divergent wind fields to recently observed OLR fields and hence explain some of the variance of the next month's 300-hPa streamfunction by solving the inverse problem.

These results are further evidence that linear Rossby wave propagation provides an important link between anomalous convection in the Tropics and the occurrence of circulation anomalies in higher latitudes.

Abstract

Patterns of outgoing longwave radiation (OLR) have been analyzed over the tropical Indian and Pacific Oceans in order to identify the varying influence of their associated convective anomalies on the circulation at higher latitudes. Particular attention has been given to the changes related to El Niño–Southern Oscillation (ENSO) events.

The two leading EOFs (emperical orthogonal functions) of monthly OLR anomaly patterns for the region between 20°N–20°S and 70°E–120°W, express complementary variations between centers located 1) near 170°W just south of the equator and over the Philippines, and 2) slightly south of the equator near 145°W and slightly north of the equator near 165°E. Cluster analysis over a smaller area between 10°S–10°N and 140°E–140°W has highlighted ENSO-related changes with two of the six clusters associated with “moderate” (EN) and “strong” (EN+) El Niño events, and a third including most La Niña (LN) events. The OLR anomaly patterns associated with the 1986/87 and 1991/92 warm events fell within the moderate category, whereas those for the mature and decaying phases of the 1982/83 and 1997/98 events were associated with the strong pattern.

For the EN cluster, composites of the global 1000-hPa height and 300-hPa streamfunction showed wave trains propagating poleward and eastward in each hemisphere from the main area of enhanced convection. These originated approximately 20° farther east for the EN+ composites. Apart from a deeper Aleutian low in the EN+ composites, the differences over North America were comparatively small. Significant changes were observed over New Zealand, where EN events were associated with weak southwesterly anomalies, but strong west-southwesterly anomalies were observed for the months included in the EN+ class. The overall circulation anomaly patterns associated with La Niña clusters were generally weak, and similar to those of EN but with the opposite sign. The composite patterns show little change between summer and winter months in the Southern Hemisphere, but the influence of the anomalous tropical convection on the Northern Hemisphere during the boreal summer is weak.

The responses to the anomalous convection indicated by the OLR anomalies have also been modeled by applying a linearized version of the barotropic vorticity equation at the 300-hPa level. The results obtained support a number of the key differences observed in the streamfunction composites and highlight the lack of summertime Rossby wave sources in the Northern Hemisphere.

For regions with climate variability sensitive to the location of tropical convection, these results suggest that a single index of tropical circulation, such as the Southern Oscillation index, is not sufficient on its own to specify ENSO-forced climate anomalies.

Abstract

The Deep Propagating Gravity Wave Experiment (DEEPWAVE) was designed to quantify gravity wave (GW) dynamics and effects from orographic and other sources to regions of dissipation at high altitudes. The core DEEPWAVE field phase took place from May through July 2014 using a comprehensive suite of airborne and ground-based instruments providing measurements from Earth’s surface to ∼100 km. Austral winter was chosen to observe deep GW propagation to high altitudes. DEEPWAVE was based on South Island, New Zealand, to provide access to the New Zealand and Tasmanian “hotspots” of GW activity and additional GW sources over the Southern Ocean and Tasman Sea. To observe GWs up to ∼100 km, DEEPWAVE utilized three new instruments built specifically for the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) Gulfstream V (GV): a Rayleigh lidar, a sodium resonance lidar, and an advanced mesosphere temperature mapper. These measurements were supplemented by in situ probes, dropsondes, and a microwave temperature profiler on the GV and by in situ probes and a Doppler lidar aboard the German DLR Falcon. Extensive ground-based instrumentation and radiosondes were deployed on South Island, Tasmania, and Southern Ocean islands. Deep orographic GWs were a primary target but multiple flights also observed deep GWs arising from deep convection, jet streams, and frontal systems. Highlights include the following: 1) strong orographic GW forcing accompanying strong cross-mountain flows, 2) strong high-altitude responses even when orographic forcing was weak, 3) large-scale GWs at high altitudes arising from jet stream sources, and 4) significant flight-level energy fluxes and often very large momentum fluxes at high altitudes.