Thomas Spengler

The Modulation of the Subtropical and Extratropical Atmosphere in the Pacific Basin in Response to the Madden–Julian Oscillation

Abstract The 40-yr ECMWF Re-Analysis (ERA-40) data are combined with a number of novel climatologies to conduct a comprehensive examination of the response of the subtropical and extratropical atmosphere over the Pacific basin to an evolving Madden–Julian oscillation (MJO) event. The adopted approach constitutes a symbiosis of a climatological analysis during the Northern Hemisphere winter from 1979 to 2002 and a case study analysis of a distinct MJO event that occurred in January–February 1993. The former is designed to obtain the general characteristics observed during a composite MJO life cycle, while the latter is used to provide insight into the instantaneous mechanisms responsible for the observed composite evolution. A primary component of the study involves the diagnosis of anomalous wave breaking activity in response to MJO forcing in the form of tropical convection and/or upper-level divergence. Wave breaking events are separated by their characteristic life cycles: LC1 (anticyclonic) and LC2 (c...

The Norwegian IPY–THORPEX: Polar Lows and Arctic Fronts during the 2008 Andøya Campaign

From a weather forecasting perspective, the Arctic poses particular challenges for mainly two reasons: 1) The observational data are sparse and 2) the weather phenomena responsible for severe weather, such as polar lows, Arctic fronts, and orographic influences on airflow, are poorly resolved and described by the operational numerical weather prediction (NWP) models. The Norwegian International Polar Year (IPY)– The Observing System Research and Predictability Experiment (THORPEX) project (2007–10) sought to significantly improve weather forecasts of these phenomena through a combined modeling and observational effort. The crux of the observational effort was a 3-week international field campaign out of northern Norway in early 2008, combining airborne and surface-based observations. The main platform of the field campaign was the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) research aircraft Falcon, equipped with lidar systems for profiling of aerosols, humidity, and wind, in addition to in situ measu...

Rossby waves, extreme fronts, and wildfires in southeastern Australia

The most catastrophic fires in recent history in southern Australia have been associated with extreme cold fronts. Here an extreme cold front is defined as one for which the maximum temperature at 2 m is at least 17°C lower on the day following the front. An anticyclone, which precedes the cold front, directs very dry northerlies or northwesterlies from the interior of the continent across the region. The passage of the cold front is followed by strong southerlies or southwesterlies. European Centre for Medium-Range Weather Forecasts ERA-Interim Reanalyses show that this regional synoptic pattern common to all strong cold fronts, and hence severe fire conditions, is a consequence of propagating Rossby waves, which grow to large amplitude and eventually irreversibly overturn. The process of overturning produces the low-level anticyclone and dry conditions over southern Australia, while simultaneously producing an upper level trough and often precipitation in northeastern Australia.

Climatology of Polar Lows over the Sea of Japan Using the JRA-55 Reanalysis

AbstractPolar lows are intense meso-α-scale cyclones that develop over the oceans poleward of the main baroclinic zone. A number of previous studies have reported polar low formation over the Sea of Japan within the East Asian winter monsoon. To understand the climatology of polar lows over the Sea of Japan, a tracking algorithm for polar lows is applied to the recent JRA-55 reanalysis. The polar low tracking is applied to 36 cold seasons (October–March) from October 1979 to March 2015. The polar lows over the Sea of Japan reach their maximum intensity on the southeastern side of the midline between the Japanese islands and the Asian continent. Consistent with previous case studies, composite analysis demonstrates that the polar low development is associated with the enhanced northerly flow on the western side of a synoptic-scale extratropical cyclone, with the cold trough in the midtroposphere and with increased heat fluxes from the sea surface. Furthermore, the present climatological study has revealed ...

A Lagrangian Climatology of Wintertime Cold Air Outbreaks in the Irminger and Nordic Seas and Their Role in Shaping Air–Sea Heat Fluxes

AbstractUnderstanding the climatological characteristics of marine cold air outbreaks (CAOs) is of critical importance to constrain the processes determining the heat flux forcing of the high-latitude oceans. In this study, a comprehensive multidecadal climatology of wintertime CAO air masses is presented for the Irminger Sea and Nordic seas. To investigate the origin, transport pathways, and thermodynamic evolution of CAO air masses, a novel methodology based on kinematic trajectories is introduced.The major conclusions are as follows: (i) The most intense CAOs occur as a result of Arctic outflows following Greenland’s eastern coast from the Fram Strait southward and west of Novaya Zemlya. Weak CAOs also originate in flow across the SST gradient associated with the Arctic Front separating the Greenland and Iceland Seas from the Norwegian Sea. A substantial fraction of Irminger CAO air masses originate in the Canadian Arctic and overflow southern Greenland. (ii) CAOs account for 60%–80% of the wintertime ...

Polar Lower-Latitude Linkages and Their Role in Weather and Climate Prediction

International Workshop on Polar-lower Latitude Linkages in Weather and Climate Prediction What: Eighty experts from twenty different countries met to assess recent progress in, and new directions for, our understanding of the mechanisms governing polar-lower latitude linkages and their role in weather and climate prediction including services. When: 10–12 December 2014 Where: Barcelona, Spain

Forward and Reverse Shear Environments during Polar Low Genesis over the Northeast Atlantic

AbstractThe synoptic and subsynoptic environments associated with polar low genesis are examined. Ambient pre–polar low environments are classified as forward or reverse shear conditions based on the angle between the thermal and mean wind. Forward shear environments are associated with a synoptic-scale ridge over Scandinavia, featuring a zonally oriented baroclinic zone extending throughout the troposphere with a wind speed maximum at the tropopause. Similar to typical midlatitude cyclogenesis, concurrent wavelike development occurs both in the lower and upper troposphere along the baroclinic zone and the mean propagation direction is eastward, parallel to isolines of sea surface temperature. Reverse shear environments exhibit a distinctly different structure and are characterized by a trough over Scandinavia, associated with a synoptic-scale, occluded cyclone. The genesis area exhibits strong cold air advection on its right-hand side and polar low development occurs on the warm side of an intense low-le...

Complementing Scientific Monsoon Definitions with Social Perception in Bangladesh

AbstractThe monsoon onset is a critical event in the Bangladesh calendar, especially for the domestic agricultural sector. Providing information about the monsoon onset for the past, present, and future has potential benefit for a country so vulnerable to changes in climate. But, when does the monsoon start? To produce any scientific information about monsoon onsets, lengths, and withdrawals, we first need to apply a monsoon definition to our data. Choosing a scientific definition is not such a simple exercise in Bangladesh. Different definitions lead to different monsoon onsets and thereby also monsoon lengths. If a climate application aims to provide information about the monsoon onset, then we need to understand how the people who might use this information perceive the monsoon onset. We then need to understand how their perceptions compare with previous scientific work. In this study we carried out a structured questionnaire in six rural regions around Bangladesh and asked the local agriculturists how...

Diabatic heating and jet stream shifts: A case study of the 2010 negative North Atlantic Oscillation winter

The role of extratropical diabatic heating in the variability of storm tracks and jet streams remains an important open question. This paper analyzes the role of diabatic heating in observationally constrained analysis data for the 2010 winter, which was notable for an extreme southward shift of the North Atlantic eddy-driven jet. An isentropic slope framework is employed by which the contribution of diabatic terms to the maintenance of seasonal mean baroclinicity can be quantified. This reveals a striking contrast between the eastern North Atlantic, where the latent heating shifted south along with the storm track in 2010, and the western North Atlantic, where the latent heating remained fixed over the Gulf Stream. This motivates the hypothesis that the latent heating may contribute to the anchoring of the storm track entrance over the Gulf Stream but provide a very different feedback on the jet variability downstream.

A New Look at Deformation as a Diagnostic for Large-Scale Flow

AbstractDeformation plays a key role in atmospheric dynamics because it provides a dynamical measure of the interaction between different scales, such as in frontogenesis. A climatology of deformation constructed from Interim ECMWF Re-Analysis (ERA-Interim) data (1979–2013) reveals four main processes associated with deformation: 1) frontogenesis at lower levels, 2) movement and evolution of jet streams in the upper troposphere, 3) orographic blocking, and 4) Rossby wave breaking. The merits of deformation as an additional perspective are discussed for these processes on the basis of case studies and composite analyses in conjunction with analytic solutions.This study shows that deformation can be used to unambiguously detect orographic blocking through the local strength of the flow diversion around orography. Moreover, the deformation signature for orographic blocking observed in case studies and composites closely resembles the analytic solution for two-dimensional flow around an obstacle.The climatolo...