RESUMO
The Indian Ocean Dipole (IOD) is a prominent mode of ocean-atmosphere interannual variability with great climate and socioeconomic impacts. Early positive IOD (pIOD), a newly discovered type of pIOD, induces pronounced rainfall anomalies in boreal summer more than canonical pIOD. It also contributes to more frequent consecutive pIODs, causing devastating droughts and floods. How early pIOD responds to global warming remains unknown. Here, we show that early pIOD has increased substantially in the past decades, reaching the same frequency as canonical pIOD. The increase is caused by intensified Bjerknes feedback and an early summer monsoon onset, which is the major trigger for early pIOD. Model simulations suggest that the increased frequency of early pIOD is likely to continue under greenhouse warming by the same mechanisms as in the observations, increasing boreal summer climate variability and leading to more climate extremes in affected regions.
RESUMO
While the effects of the Southern Annular Mode (SAM), a dominant climate variability mode in the Southern Ocean, on ocean acidification have been examined using models, no consensus has been reached. Using observational data from south of Tasmania, we show that during a period with positive SAM trends, surface water pH and aragonite saturation state at 60°-55° S (Antarctic Zone) decrease in austral summer at rates faster than those predicted from atmospheric CO2 increase alone, whereas an opposite pattern is observed at 50°-45° S (Subantarctic Zone). Together with other processes, the enhanced acidification at 60°-55° S may be attributed to increased westerly winds that bring in more "acidified" waters from the higher latitudes via enhanced meridional Ekman transport and from the subsurface via increased vertical mixing. Our observations support climatic modulation of ocean acidification superimposed on the effect of increasing atmospheric CO2.