Linking mid-latitude storms, atmospheric composition and climate variability
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Date
27/06/2016Item status
Restricted AccessEmbargo end date
31/12/2100Author
Knowland, Katherine Emma
Metadata
Abstract
In this thesis, the role of mid-latitude cyclones in air pollution transport in the
Northern Hemisphere is quantified. The storm tracking model, TRACK, is used
to study the mechanisms through which pollution, specifically ozone (O3) and
carbon monoxide (CO), are vented from the boundary layer to the free troposphere
and thus transported over large distances, as well as the introduction of O3 from
the stratosphere into the troposphere. The relationship between mid-latitude
cyclones and air pollution transport of O3 and CO is explored for the first time
using the Monitoring Atmospheric Composition and Climate (MACC) reanalysis,
a combined meteorology and composition reanalysis dataset.
A comparison between springtime surface ozone measurements at rural background
sites on the west coast of Europe and cyclone track frequency in the
surrounding regions was used to first establish the correlation between cyclone
location and surface air quality. The focus is on spring as it tends to be the season
of maximum intercontinental transport of O3. The surface observations were
compared to the MACC O3 values at the same locations and case studies of how
cyclones can influence surface O3 measurements are described. When cyclones
track north of 53°N, there is a significant probability that the surface O3 will be
high (> the 75th percentile), due to the close proximity to stratospheric intrusions
and the transport at low levels across the North Atlantic Ocean.
The most intense spring cyclones (95th percentile) were selected for two regions,
the North Atlantic and the North Pacific, for further investigation into the
mechanisms which impact O3 and CO concentrations near cyclones. These
intense cyclones ( 60 over each region) often tracked over the major emission
sources of eastern North America and East Asia. The distributions of MACC
O3 and CO within a "typical" intense cyclone are examined by compositing the
cyclones together. The cyclone-centered composites were compared to background
composites of "average conditions" created by sampling the reanalysis data of
the previous year to the cyclone locations. Mid-latitude cyclones are found to
redistribute concentrations of O3 and CO horizontally and vertically throughout
the cyclone. This is clearly shown to occur through two main mechanisms: (1)
vertical lifting of CO-rich and O3-poor air isentropically from near the surface
to the mid- to upper-troposphere in the region of the warm conveyor belt; and
(2) descent of O3-rich and CO-poor air isentropically in the vicinity of the dry
intrusion, from the stratosphere toward the mid-troposphere.
This work was expanded to identify the links between teleconnection patterns,
mainly the North Atlantic Oscillation (NAO), that affect the major storm track
pathways in the North Atlantic sector and the distribution of MACC O3 and CO
throughout the troposphere and lower stratosphere. For this analysis, TRACK
was used to calculate seasonal weighted-average O3 and CO distribution maps
based on the monthly NAO index. During positive NAO phase, the persistence
of low pressures over the North Atlantic coupled with the Azores High promotes
transport across the North Atlantic throughout the troposphere. During negative
NAO phase, blocking high pressure in the eastern North Atlantic are known to
occur, which shifts transport pathways to a more southerly zonal flow.
This work demonstrates the complex relationship between the horizontal and
vertical distribution of pollution, including surface concentrations, and synoptic-scale
systems.