Marine climate change over the eastern Agulhas Bank of South Africa

The rate of change in the marine environment over the eastern Agulhas Bank along the south coast of South Africa (32–37∘ S, 20–30∘ E) is studied using reanalysis observations for 1900–2015 and coupled ensemble model projections for 1980–2100. Outcomes are influenced by resolution and time span: ∼ 1∘ datasets covering the whole period capture large-scale changes, while ∼ 0.5∘ datasets in the satellite era better distinguish the cross-shelf gradients. Although sea surface temperatures offshore are warming rapidly (0.05 ∘C yr−1 since 1980), a trend toward easterly winds and a locally stronger Agulhas Current have intensified nearshore upwelling (−0.03 ∘C yr−1). The subtropical ridge is gradually moving poleward, leading to a drier climate.

Midlatitude Fronts and Variability in the Southern Hemisphere Tropical Width

This study examines the relationship between midlatitude synoptic activity and variations in the width of the tropics in the Southern Hemisphere for the period 1979–2016. The edge of the tropical belt is defined here in terms of the latitude of the subtropical ridge (STR) of sea level pressure, and eddy activity in the midlatitudes is characterized by the behavior of atmospheric fronts. It is shown that the location and intensity of the STR are significantly correlated with the number of cold fronts between 20° and 40°S and that these relationships exhibit seasonal and zonal asymmetry. The link between the STR and the number of fronts is analyzed in five sectors of the Southern Hemisphere to reveal regional differences in their behavior and relationship with the southern annular mode. Some earlier studies on the widening of the tropics suggest that such changes may be caused by a shift in the location of midlatitude eddies. Our analysis explores the connection between these on a synoptic time scale. It shows that the variability of the width of the tropics is indeed strongly influenced by changes in the midlatitude synoptic activity, and that changes in synoptic activity lead those in the edge of the tropical belt by approximately one day.

The relationship between the subtropical ridge and Australian temperatures

The intensity and latitude of the subtropical ridge over eastern Australia is strongly associated with southeast Australian rainfall, particularly during the cool months of the year. We show that the subtropical ridge also exerts a strong influence on temperatures across much of Australia, with warmer daytime temperatures and more warm extremes across southern Australia when the subtropical ridge is stronger than average, which is largely independent of the relationship between the subtropical ridge and rainfall. A strong subtropical ridge is also linked to warmer than average minimum temperatures over southern Australia throughout much of the year, except from May to August when a strong ridge is associated with cooler mean minimum temperatures and an increased frequency of cool nights. This relationship, and the observed strengthening of the subtropical ridge during autumn and winter in recent decades, can partially explain the weaker warming trends in minimum temperatures in southeast Australia compared to elsewhere in the country over the period 1960-2016.

Southeast Australia Autumn Rainfall Reduction: A Climate-Change-Induced Poleward Shift of Ocean–Atmosphere Circulation

Since the 1950s annual rainfall over southeastern Australia (SEA) has decreased considerably with a maximum decline in the austral autumn season (March–May), particularly from 1980 onward. The understanding of SEA autumn rainfall variability, the causes, and associated mechanisms for the autumn reduction remain elusive. As such, a new plausible mechanism for SEA autumn rainfall variability is described, and the dynamics for the reduction are hypothesized. First, there is no recent coherence between SEA autumn rainfall and the southern annular mode, discounting it as a possible driver of the autumn rainfall reduction. Second, weak trends in the subtropical ridge intensity cannot explain the recent autumn rainfall reduction across SEA, even though a significant relationship exists between the ridge and rainfall in April and May. With a collapse in the relationship between the autumn subtropical ridge intensity and position in recent decades, a strengthening in the influence of the postmonsoonal winds from north of Australia has emerged, as evident by a strong post-1980 coherence with SEA mean sea level pressure and rainfall. From mid to late autumn, there has been a replacement of a relative wet climate in SEA with a drier climate from northern latitudes, representing a climate shift that has contributed to the rainfall reduction. The maximum baroclinicity, as indicated by Eady growth rates, has shifted poleward. An associated poleward shift of the dominant process controlling SEA autumn rainfall has further enhanced the reduction, particularly across southern SEA. This observed change over the past few decades is consistent with a poleward shift of the ocean and atmosphere circulation.