Abstract
The New Zealand-scale atmospheric circulation patterns reflect the variability of large-scale atmospheric circulation modes. These large-scale circulation modes are subject to change in frequency and intensity under the current climate warming. Exactly how these large-scale atmospheric circulation modes drive the New Zealand-scale circulation patterns is not well understood, particularly in the interest of the looming effects of climate change. Larger variability in the circulation patterns of New Zealand can result in consequential impacts on the precipitation and temperature patterns that, in turn, impact the socio-economic pillars.
To examine the occurrence of past trends in the circulation patterns over New Zealand, the Trenberth indices are applied through linear regression to discern trends over four time periods: 1949 – 2018, 1979 – 2018, 1979 – 2021, and 1962 – 2023. A distinct seasonality exists in the zonal and meridional circulation, which aligns with the seasonal variations in the large-scale circulation patterns. In particular, the westerly circulation over New Zealand has decreased through summer with a subsequent increase of the westerly circulation to the south of New Zealand through winter, while the southerly circulation to the south and west of New Zealand has increased through spring, along with an autumn decrease in the southerly airflow to the east of New Zealand. Pearson’s correlation analysis was applied to the Trenberth and large-scale circulation indices, divulging positive correlations between the Trenberth indices and Interdecadal Pacific Oscillation (IPO), and negative correlations between the Trenberth indices and Southern Annular Mode (SAM), Zonal Wavenumber Three (ZW3), and Southern Oscillation (SO).
The relationship between the New Zealand-scale and large-scale circulation patterns is investigated through two key approaches: the New Zealand-scale to large- scale and the large-scale to New Zealand-scale approach. Both approaches embody an index and spatial composite analysis technique across the three-month seasonal groups. The Trenberth index values corresponding to high and low large-scale circulation phases revealed that the summer easterly airflow over New Zealand is driven by the positive SAM polarity and La Niña phase. Concurrently, the spring southerly airflow to the east and west of New Zealand is driven by the El Niño and positive IPO phase. The spatial composite analysis of southerly airflow revealed a low- pressure system over New Zealand, consistent with the El Niño and positive IPO phase. The large-scale to New Zealand-scale approach supports these findings while adding the winter positive SAM phase drives the westerly airflow to the south of New Zealand, which the spatial composite analysis revealed is related to a southward shift in the high and low-pressure gradients to the north and south of New Zealand. Collectively, the results highlight several interesting trends and interactions between circulation patterns that are important for understanding the relationship between the New Zealand-scale and large-scale circulation patterns.