An assessment of hourly rainfall on Sub-Antarctic Marion Island and the implications for climate change

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Date

2015

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University of Fort Hare

Abstract

Hourly rainfall data from the South African Weather Service for the meteorological station on the eastern side of Marion Island were assessed (1995-2009) to investigate trends in rainfall intensity. For all categories the annual frequency of hourly rainfall shows no statistically significant increase or decrease over the recording period. This is in contrast to long-term change over the last forty years where the climate of Marion Island has changed significantly. The lack of change over the last 15 years may indicate stability with regards to annual rainfall. However, even though the frequency of hours with no rainfall for the recording period shows no linear temporal trend, significant increase in hours with no rainfall occurred during the summer months. From synoptic charts and data it is suggested that high intense hourly rainfall is associated with a strong cut-off low pressure system over Marion Island extending meridionally in a north-westerly direction. Mean air pressure is low and the meridional component in the dominant airflow explains the high humidity and suggests the potential to bring in a northerly maritime tropical air from lower latitudes west or north-east of the island. Climate change implications for hourly rainfall on Marion Island show various opposing trends. A decrease in low pressure systems and increase in anticyclonic conditions will increase the number of hours without rainfall (as seen from the data) especially in summer. A further implication is that the number of intensive hourly rainfall events could also decrease. However the possibility exist that Marion Island is becoming wetter and the data indicate that hourly rainfall over the last decade show an increase during the summer months. Further research is needed to ascertain the current rainfall trends on Marion Island.

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Keywords

Climatic changes, Rainfall frequencies

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