Some Features of the Summer Climate of Interior Alaska

A comparative analysis is made of the distribution of long term monthly mean temperature and precipitation data for three representative locations in the Alaska interior basin. Examples of extreme monthly conditions over the region are selected by reference to the record at Tanana dating back to 1903. The characteristics are outlined of the mean monthly orientations of the 700-millibar troughs and ridges and also the patterns of geopotential anomaly associated with summer months which exhibit extremes of warmth, cold, moisture and dryness. A comparison is made between these patterns and those that have been postulated for the time of the maximum Wisconsin glaciation

The climate of Macquarie Island and its role in atmospheric monitoring

An analysis is made of the principal climatic elements at Macquarie Island in relation to the general circulation of the high latitudes of the southern hemisphere. The climate is characterised by a high frequency of strong west to northwesterly winds and frequent gales, low variability of temperature, a high frequency of low cloud and fog, and a high number of days with precipitation throughout the year. In general, the climate is typical of a higher mid-latitude oceanic island, and its features are compared with others in the circumpolar Southern Ocean. From a different perspective the island occupies a unique geographic site. This makes it extremely valuable as a meteorological observatory, enabling regular surface and upper air observations for day-to-day global analysis and forecasting, and providing climatic data representative of the higher southern mid-latitudes. Despite the advent of new observational techniques (such as satellite-reporting drifting ocean buoys, satellite cloud imagery and satellite-derived atmospheric temperature profiles), the observations from Macquarie Island continue to constitute essential calibration data for space-derived measurements and provide the long-term continuity only possible at a fixed baseline station. The importance of these data is stressed; not only with respect to the standard meteorological observations but also to the measurement of ozone, carbon dioxide and other atmospheric trace constituents which are becoming increasingly recognised as significant in studies of long-term climatic change

Aspects of Winter Temperatures in Interior Alaska

The interior basin between the climatic divides of the Brooks and Alaska Ranges has extremely low winter temperatures in contrast to the milder regime of both southcentral Alaska and the region on the Arctic Slope of the Brooks Range. There is a cold pole centered on the Yukon flats and extending over the Yukon and Tanana valleys. Two local factors in the occurrence of low winter temperatures are the wide expanse of relatively flat and low-lying surface suited to the development of substantial low level temperature inversions in winter when the radiation balance of the surface is strongly negative; and encirclement by a mountain mass which is effective in preventing the weaker low level synoptic scale weather systems from invading the basin. Large-scale circulation factors associated with extreme years are described.Aspects des températures hivernales en Alaska intérieur. L'auteur décrit les variations des températures mensuelles moyennes hivernales des vallées du Yukon et de la Tanana, en relation avec la topographie de la région. Il identifie les mois de froid extrême et de chaleur extrême à partir des observations recueillies en 66 ans. Pour ces mois extrêmes, les variations thermiques au jour le jour aux différentes stations révèlent l'influence des barrières topographiques sur les tendances au réchauffement. L'auteur décrit enfin les facteurs de circulation à grande échelle qui sont associés aux années extrêmes

Some Observations of Alaskan Glacier Winds in Midsummer

Discusses 21 June-12 July observations at Castner Glacier in the Alaska Range and 16 July-11 Aug 1967 at Worthington Glacier in the Chugach Range. The purpose was to study the frequency and diurnal variation of glacier winds and differences due to location and exposure. In both cases max windspeed occurs slightly before sunrise and in the mid-afternoon. In the absence of a daytime glacier wind at Castner, a regular night downslope circulation is often observed.Quelques observations sur les vents de glacier de l'Alaska au milieu de l'été. Les auteurs dévrivent les caractères des vents de glacier observés au milieu de l'été pour les glaciers Worthington et Castner. Chacun présente un double maximum quotidien de vélocité, soit juste avant l'aurore et vers le milieu de l'après-midi. Lorsqu'il n'y a pas de vent de glacier diurne à Castner, on observe souvent une circulation nocturne très régulière et descendante

The role of tropical-extratropical interaction and synoptic variability in maintaining the South Pacific Convergence Zone in CMIP5 models

The South Pacific Convergence Zone (SPCZ) is simulated as too zonal a feature in current generation climate models, including those in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). This zonal bias induces errors in tropical convective heating, with subsequent effects on global circulation. The SPCZ structure, particularly in the subtropics, is governed by the tropical-extratropical interaction between transient synoptic systems and the mean background state. However, the fidelity of synoptic-scale interactions as simulated by CMIP5 models has not yet been evaluated. In this study, analysis of synoptic variability in the simulated subtropical SPCZ reveals that the basic mechanism of tropical-extratropical interaction is generally well simulated, with storms approaching the SPCZ along comparable trajectories to observations. However, there is a broad spread in mean precipitation and its variability across the CMIP5 ensemble. Inter-model spread appears to relate to a biased background state in which the synoptic waves propagate. In particular, the region of mean negative zonal stretching deformation or "storm graveyard" in the upper troposphere?a feature previously determined to play a key role in SPCZ-storm interactions?is typically displaced in CMIP5 models to the northeast of its position in reanalysis data, albeit with individual model graveyards displaying a pronounced (25 degree) longitudinal spread. From these findings, we suggest that SPCZs simulated by CMIP5 models are not simply too zonal; rather, in models the subtropical SPCZ manifests a diagonal tilt similar to observations while SST biases force an overly zonal tropical SPCZ, resulting in a more disjointed SPCZ than observed

A dynamical framework for the origin of the diagonal South Pacific and South Atlantic convergence zones

The South Pacific Convergence Zone (SPCZ) and South Atlantic Convergence Zone (SACZ) are diagonal bands of precipitation that extend from the equator southeastward into the Southern Hemisphere over the western Pacific and Atlantic Oceans, respectively. With mean precipitation rates over 5 mm day−1, they are a major component of the tropical and global climate in austral summer. However, their basic formation mechanism is not fully understood. Here, a conceptual framework for the diagonal convergence zones is developed, based on calculations of the vorticity budget from reanalysis and Rossby wave theory. Wave trains propagate eastward along the Southern Hemisphere subtropical jet, with initially quasi-circular vorticity centres. In the zonally sheared environment on the equatorward flank of the jet, these vorticity centres become elongated and develop a northwest-southeast tilt. Ray tracing diagnostics in a non-divergent, barotropic Rossby wave framework then explain the observed equatorward propagation of these diagonal vorticity structures toward the westerly ducts over the equatorial Pacific and Atlantic. The baroclinic component of these circulations leads to destabilisation and ascent ahead of the cyclonic vorticity anomaly in the wave, triggering deep convection because of the high sea surface temperatures in this region. Latent heat release then forces additional ascent and strong upper-tropospheric divergence, with an associated anticyclonic vorticity tendency. A vorticity budget shows that this cancels out the advective cyclonic vorticity tendency in the wave train over the SPCZ, and dissipates the wave within a day. The mean SPCZ is consequently comprised of the sum of these pulses of diagonal bands of precipitation. Similar mechanisms also operate in the SACZ. However, the vorticity anomalies in the wave trains are stronger, and the precipitation and negative feedback from the divergence and anticyclonic vorticity tendency are weaker, resulting in continued propagation of the wave and a more diffuse diagonal convergence zone

The influence of diabatic heating in the South Pacific Convergence Zone on Rossby wave propagation and the mean flow

The South Pacific Convergence Zone (SPCZ) is a northwest-southeast oriented precipitation band over the South Pacific Ocean. Latent heat release from condensation leads to substantial diabatic heating, which has potentially large impacts on local and global climate. The influence of this diabatic heating within the SPCZ is investigated using the Intermediate General Circulation Model (IGCM4). Precipitation in the SPCZ has been shown to be triggered by transient Rossby waves that originate in the Australian subtropical jet and are refracted towards the equatorial eastern Pacific. A Rossby wave triggers a SPCZ 'convective event', with associated diabatic heat release and vortex stretching. Consequently, the Rossby wave is dissipated in the SPCZ region. These features are simulated well in a control integration of IGCM4. In an experiment, convective heating is prescribed to its 'climatological' value in the SPCZ region during the Rossby wave 'events' and dynamic forcing from Rossby waves is decoupled from the usual thermodynamic response. In this experiment Rossby waves over the SPCZ region are not dissipated, confirming the vortex stretching mechanism from previous studies. Furthermore, the change in Rossby wave propagation has an impact on momentum transport. Overall, the effect of the Rossby wave-induced convection in the SPCZ is to decrease the strength of the Pacific subtropical jet and the equatorial eastern Pacific upper-tropospheric westerlies, by about 2-6 m s-1. Following these changes to the basic state, two potential feedbacks in the SPCZ and larger Pacific climate system are suggested: increased SPCZ convection due to the enhancement of negative zonal stretching deformation in the SPCZ region and decreased equatorward refraction of Rossby waves into the westerly duct leading to less SPCZ 'events'. As the convective events in the SPCZ have a significant impact on Pacific mean climate, it is crucial that the SPCZ is represented correctly in climate models

Wavelet analysis of stratosphere gravity wave packets over Macquarie Island 1. Wave parameters

We describe a technique to detect gravity wave packets in high-resolution radiosonde soundings of horizontal wind and temperature. The vertical profiles of meridional and zonal wind speeds are transformed using the Morlet wavelet, and regions of high wind variance in height-wavenumber space are identified as gravity wave packets. Application of the Stokes parameter analysis to horizontal wind and temperature profiles of the reconstructed wave packets yields the wave parameters. The technique was applied to twice-daily radiosonde launches at Macquarie Island (55°S, 159°E) between 1993 and 1995. A strong seasonal cycle in the total wave variance was found, with a maximum in winter. The amount of wave energy propagating downward from the upper stratosphere also maximized in winter. Waves propagated predominately toward the southwest in winter, but in summer propagation directions were approximately isotropic. The intrinsic frequencies were close to the inertial frequency, and the waves had inferred horizontal wavelengths of several hundred kilometers.Florian Zink and Robert A. Vincen

Why the South Pacific Convergence Zone is diagonal

During austral summer, the majority of precipitation over the Pacific Ocean is concentrated in the South Pacific Convergence Zone (SPCZ). The surface boundary conditions required to support the diagonally (northwest-southeast) oriented SPCZ are determined through a series of experiments with an atmospheric general circulation model. Continental configuration and orography do not have a significant influence on SPCZ orientation and strength. The key necessary boundary condition is the zonally asymmetric component of the sea surface temperature (SST) distribution. This leads to a strong subtropical anticyclone over the southeast Pacific that, on its western flank, transports warm moist air from the equator into the SPCZ region. This moisture then intensifies (diagonal) bands of convection that are initiated by regions of ascent and reduced static stability ahead of the cyclonic vorticity in Rossby waves that are refracted toward the westerly duct over the equatorial Pacific. The climatological SPCZ is comprised of the superposition of these diagonal bands of convection. When the zonally asymmetric SST component is reduced or removed, the subtropical anticyclone and its associated moisture source is weakened. Despite the presence of Rossby waves, significant moist convection is no longer triggered; the SPCZ disappears. The diagonal SPCZ is robust to large changes (up to +/-6 degC) in absolute SST (i.e. where the SST asymmetry is preserved). Extreme cooling (change less than -6 degC) results in a weaker and more zonal SPCZ, due to decreasing atmospheric temperature, moisture content and convective available potential energy