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Madden-Julian Oscillation teleconnections and their influence on Northern Hemisphere winter blocking

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March 22, 2017
Stephanie Henderson
Hosted by Eric Maloney (advisor), Dave Thompson, Libby Barnes, Edwin Chong (Electrical and Computer Engineering)

Abstract

Winter blocking events are characterized by persistent and quasi-stationary patterns that re-direct precipitation and air masses, leading to long-lasting extreme winter weather. Studies have shown that the teleconnection patterns forced by the primary mode of tropical intraseasonal variability, the Madden-Julian Oscillation (MJO), influence extratropical factors associated with blocking, such as the North Atlantic Oscillation. However, the influence of the MJO on winter blocking is not well understood. Understanding this relationship may improve the mid-range forecasting of winter blocking and the associated weather extremes.

The impact of the MJO on Northern Hemisphere winter blocking is examined using a two-dimensional blocking index. Results suggest that all MJO phases demonstrate significant changes in west and central Pacific high-latitude blocking. East Pacific and Atlantic blocking are significantly suppressed following phase 3 of the MJO, characterized by anomalous convection in the tropical East Indian Ocean and suppressed convection in the west Pacific. A significant increase in east Pacific and Atlantic blocking follows the opposite-signed MJO heating during MJO phase 7. Over Europe, blocking is suppressed following MJO phase 4 and significantly increased after MJO phase 6. Results suggest that the European blocking increase may be due to two precursors: 1) a pre-existing anomalous Atlantic anticyclone, and 2) a negative Pacific North American (PNA) pattern triggered by the MJO.

The influence of the MJO on winter blocking may be different if a change occurs to the basic state and/or MJO heating, such as during El Niño – Southern Oscillation (ENSO) events. MJO teleconnections during ENSO events are examined using composite analysis and a nonlinear baroclinic model and their influence of winter high-latitude blocking is discussed. Results demonstrate that the ENSO-altered MJO teleconnection patterns significantly influence Pacific and Atlantic blocking and the impacts depend on ENSO phase. During El Niño, Pacific and Atlantic blocking is significantly increased following MJO phase 7, with maximum Atlantic blocking frequency anomalies triple the climatological winter mean blocking frequency. Results suggest that the MJO forces the initial anomalous Atlantic dipole associated with the blocking increase, and transient eddy activity aids in its persistence. During La Niña, significant changes to high-latitude blocking are mostly observed during the first half of an MJO event, with significant suppression of Pacific and Atlantic blocking following MJO phase 3.

MJO teleconnection patterns may also be altered by basic state and MJO heating biases in General Circulation Models (GCMs), important for mid-range forecasting and future climate studies of weather and climate patterns significantly altered by the MJO, such as winter blocking. Data from phase 5 of the Coupled Model Intercomparison Project (CMIP5) is used to investigate MJO teleconnection biases due to basic state and MJO biases, and a linear baroclinic model is used to interpret the results. Results indicate that poor basic state GCMs (but a good MJO) can have equally poor skill in simulating the MJO teleconnection patterns as GCMs with a poor MJO. Large biases in MJO teleconnection patterns occur in GCMs with a zonally extended Pacific subtropical jet relative to reanalysis. In good MJO GCMs, bias in the location and horizontal structure of Indo-Pacific MJO heating is found to have modest impacts on MJO teleconnection patterns. However, East Pacific heating during MJO events can influence MJO teleconnection amplitude and the pathways over North America. Results suggest that both the MJO and the basic state must be well represented in order to properly capture the MJO teleconnection patterns.