Bastin, Swantje (2018) Ingredients for blocking in a dry dynamical atmospheric model. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 86, XIV pp.

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Abstract

Using the Portable University Model of the Atmosphere (PUMA), a dry, spectral, primitive equation atmospheric model, the influence of idealised orography and tropospheric heating anomalies on Northern Hemisphere winter blocking is investigated. By means of sensitivity experiments, the impact of several atmospheric features and phenomena on blocking is separately examined, which is difficult to achieve with, for example, reanalysis data. The analysis is carried out in two parts: first, the contributions of mid-latitude orography and continent-ocean heat contrasts to blocking are studied, followed by an investigation of blocking changes due to tropical heating anomalies, including idealised tropical heating based on the El Niño/Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO) and the tropical precipitation bias of the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Orography is found to be necessary for mid-latitude blocking in the model to occur, because it forces stationary planetary waves that disrupt the westerly mean flow and form jet exit regions where blocking can be maintained against zonal advection. A mid-latitude continent-ocean heating dipole causes a strengthening of the synopticscale activity in the jet stream, which does not lead to blocking on its own, but can, in combination with orography, strongly enhance mid-latitude blocking downstream of its location. Generally, a negative tropical heating anomaly is found to impact mid-latitude blocking more than a positive heating anomaly of the same amplitude. The negative heating leads to reduced mid-latitude blocking at the longitude of the heating and increased mid-latitude blocking west of the longitude of the heating. For El Niño, increased high-latitude blocking over Siberia and Greenland and slightly reduced mid-latitude blocking over the Pacific and Europe is found; whereas La Niña is associated with reduced high-latitude blocking over Siberia and increased (decreased) mid-latitude blocking over the Atlantic and Europe (the Pacific). MJO phase 2 leads to a weak increase (decrease) of high-latitude blocking over Greenland (Siberia), and MJO phase 6 is associated with strongly increased high-latitude blocking over Siberia and Greenland as well as strongly increased European-Atlantic mid-latitude blocking. Tropical heating based on the CMIP5 multi-model mean precipitation bias leads to reduced (increased) high-latitude blocking over Greenland (Siberia), and to strongly reduced mid-latitude blocking over Europe, Atlantic and Pacific, offering a possible explanation for the tendency to find reduced blocking in these models compared to observations, especially over the Atlantic and Europe.

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