Azores High and Hawaiian High: correlations, trends and shifts (1948–2018)

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 longterm 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 southwest to the northeast from the 1990s of the twentieth century and again to the southwest in the twentyfirst century, (4) the HH in January and July moved generally from the northeast to the southwest until the end of the twentieth century and shifted again to northeast during the twenty-first century, (5) the AH in July was characterised by complicated displacement system with the prevalence of the shifting from the northeast to the southwest 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 northeast. 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.

Cloud cover changes driven by atmospheric circulation in Europe during the last decades

“…The high‐pressure centres in the East are weaker, compared to the previous GoCT, enabling more southerly tracks of Icelandic lows over Scandinavia and northeastern Europe. The patterns in sea level pressure for both groups of circulations together with their trend are in line with the shift towards south‐west of the Azores High after 1990 observed by Falarz (2019).…”

“…The highpressure centres in the East are weaker, compared to the previous GoCT, enabling more southerly tracks of Icelandic lows over Scandinavia and northeastern Europe. The patterns in sea level pressure for both groups of circulations together with their trend are in line with the shift towards south-west of the Azores High after 1990 observed by Falarz (2019). The difference between the composite maps of SSRD and TTR corresponding to those two major circulation groups (Figure 12c,d) outlines the overall effect of changes in atmospheric circulation on radiation fluxes.…”

The 2018 summer heatwaves over northwestern Europe and its extended-range prediction

“…Substantial variability in the central of the Azores High during summer was observed through a comparison of different study periods 42 , 43 . We adapted the area for the variations in the center of the summertime Azores High determined by Falarz 42 . Linear regressions of 2-m temperature anomalies and 500-hPa geopotential height anomalies in July onto the monthly AZH index revealed similar spatial patterns to those of CPC-obtained monthly SCA index (Supplementary Fig.…”

“…To measure the variability of the Azores High, we calculated the regional mean of the normalized geopotential height anomalies at 500 hPa over the area 24°–54°W and 30°–45°N. Substantial variability in the central of the Azores High during summer was observed through a comparison of different study periods 42 , 43 . We adapted the area for the variations in the center of the summertime Azores High determined by Falarz 42 .…”

Using Portuguese stalagmites to constrain Holocene arid and humid intervals and drivers of regional hydroclimate

“…The intensity, size, and location of AH can vary substantially . The intensity of the AH (particularly in January) has strengthened in recent decades, and due to its impact on weather and climate in the region, it is important to constrain variations in the pressure field of the AH as well as latitudinal and longitudinal changes (Falarz et al, 2019;. Variability in the AH has far-reaching effects, impacting northwest Europe's precipitation with north/south shifts Rashid et al (2012) and the influence of the NAO on precipitation of the Middle East is primarily through changes in the AH .…”