Abstract
Increasing greenhouse gas concentration and ozone depletion are generally considered two important factors that affect the variability of the Antarctic Oscillation (AAO). Here, we find that the first leading mode of sea surface temperature (SST) variability (rotated empirical orthogonal functions) shows a long-term upward trend from 1901 to 2004 and is closely related to the AAO index that is obtained using the observationally constrained reanalysis data. Further, regressions of the sea level pressure and the 500-hPa geopotential height anomalies, against the principle component associated with the long-term SST anomalies, display a seesaw behavior between the middle and high latitudes of the Southern Hemisphere in austral winter, which is similar to the high polarity of the AAO. The circulation responses to the long-term oceanic warming in three numerical models are consistent with the observed results. This finding suggests that the long-term oceanic warming is partly responsible for the upward trend of the AAO in austral winter. The thermal wind response to the oceanic warming in South Indian and South Atlantic Ocean may be a possible mechanism for this process.
Highlights
The Antarctic Oscillation (AAO) is one of the most important extratropical atmospheric circulation modes in the Southern Hemisphere, and it has strong effects on the Southern ocean temperature, marine ecosystems and even climate variability in the Northern Hemisphere[1,2,3,4,5]
To explore the potential impact of the long-term oceanic warming on the AAO in austral winter, we examine the relationship between the long-term sea surface temperature (SST) anomalies and the spatio-temporal variability of the AAO
The principal component (PC) associated with the longterm SST anomalies is closely related to the AAO index, with a correlation coefficient of 0.71 (Fig. 1b)
Summary
The Antarctic Oscillation (AAO) is one of the most important extratropical atmospheric circulation modes in the Southern Hemisphere, and it has strong effects on the Southern ocean temperature, marine ecosystems and even climate variability in the Northern Hemisphere[1,2,3,4,5]. Several studies suggested that anthropogenic greenhouse gas emissions and ozone play a critical role in the AAO trend[8,9,10] This is due to the changes in direct radiative forcing by greenhouse-gas accumulation and stratospheric ozone depletion[8,9,10]. Seviour et al.[14] found that the Antarctic ozone depletion lead to a change in the AAO index, corresponding to a significant warming in the tropical oceans and a cooling in the middle latitudes of the Southern Hemisphere oceans. Global oceans exhibited a long-term warming, especially in the South Indian Ocean and South Atlantic (Fig. 1a) This would generate a change in the surface temperature gradient in the southern hemisphere, which might contribute to a prominent change in AAO. The objective of this study is to investigate the decisive influence of the long-term oceanic warming on the AAO in austral winter, based on reanalysis dataset and numerical experiments
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