Abstract
The paper focuses on investigation of ‘twin’ subtropical oceanic highs of the Northern Hemisphere, i.e. the Azores High (AH) and the Hawaiian High (HH) in January and July based on gridded 2.5° × 2.5° data of Reanalysis Project of the National Center for Atmospheric Research for the period 1948–2018. The aim is to answer three questions: (1) Are there any connections between AH and HH (both within and between the systems)? (2) What is the long-term variability and trends of the basic characteristics of AH and HH? (3) Do the AH and HH move, and if so, in what directions? The most important results are as follows: (1) the long-term trend of sea level air pressure in the AH centre in January is positive, statistically significant with the increase of 0.63 hPa/10 years, (2) pressure in both centres significantly relates with the latitude of each system; variables characterising the HH in January explain 11% of variation of the variables of the AH in July, (3) the NE-SW/SW-NE index proves the shifting of the AH in January from the south-west to the north-east until the 1990s of the twentieth century and again to the south-west in the twenty-first century, (4) the HH in January and July moved generally from the north-east to the south-west until the end of the twentieth century and shifted again to north-east during the twenty-first century, (5) the AH in July was characterised by complicated displacement system with the prevalence of the shifting from the north-east to the south-west with the exception for the period 1980–1990. In winter, the AH moves towards the land area of Europe in the second half of the twentieth century, while the HH moves towards the open Pacific. The statistically significant increase of pressure in the centre of the AH in January is closely related to the shifting of the system to the north-east. The positive pressure trend in the centre of the AH in January combined with the zero trend in July is the cause of diminishing difference between summer and winter air pressure value of the high. Due to increased sea surface temperature of the Atlantic, the AH does not lose its strength in winter as it used to a few decades ago.
Highlights
Climate change research carried out in recent decades led in many cases to the conclusion that the cause of these changes are changes and variability in the macro- and mesoscale atmospheric circulation (e.g. Konrad 1998; Clark et al 1999; Marsz 1999; Slonosky et al 2001; Falarz 2007, 2013; Rasmus et al 2015)
A more precise definition can be found in modern meteorology glossaries, namely (1) atmospheric centre of action is an area of high or low pressure on the globe with a more or less permanent location, remaining for long periods (Niedźwiedź 2003) and (2) any one of the semi-permanent highs and lows that appear on mean charts of sea level pressure (Glossary of Meteorology 2018)
The Azores High (AH) in July is shifted to the west relative to the average long-term January location, while the HH is to the northwest
Summary
Climate change research carried out in recent decades led in many cases to the conclusion that the cause of these changes are changes and variability in the macro- and mesoscale atmospheric circulation (e.g. Konrad 1998; Clark et al 1999; Marsz 1999; Slonosky et al 2001; Falarz 2007, 2013; Rasmus et al 2015). Climate change research carried out in recent decades led in many cases to the conclusion that the cause of these changes are changes and variability in the macro- and mesoscale atmospheric circulation The field of research into changes in circulation includes investigation of fluctuation and interaction of atmospheric centres of action (ACAs) over the globe. A more precise definition can be found in modern meteorology glossaries, namely (1) atmospheric centre of action (centre of atmospheric activity) is an area of high or low pressure on the globe with a more or less permanent location, remaining for long periods (Niedźwiedź 2003) and (2) any one of the semi-permanent highs and lows that appear on mean charts of sea level pressure (Glossary of Meteorology 2018). The formation of atmospheric centres of action is related to the varied rate of
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