New research using detailed 3D ocean models from the Community Earth System Model (CESM) has found that the slowdown in the ocean's overturning circulation and the global mean sea level rise is responsible for the recently observed increase in the amplitude of the semi-diurnal internal tide in the South Pacific Ocean. The findings were recently published in the journal Geophysical Research Letters.

The ocean tides are mainly driven by the gravitational forces from the Sun and Moon. In the open ocean, the largest signal is seen for the semi-diurnal tide with a period of half a day, which is usually around 0.2 meters high in the deep ocean. However, in some coastal areas with particular geographical and bathymetric conditions, such as the English Channel between the UK and France and the Bay of Fundy in Canada, the tidal range can be greatly magnified by the resonance of the semi-diurnal internal tides.

The research team including Dr. Chao and Dr. Wen poured through a massive amount of ocean data produced by the CESM. CESM is a state-of-the-art climate model developed at the National Center for Atmospheric Research (NCAR), which simulates the Earth's climate system and tracks changes over time. By meticulously analyzing 3D ocean variables produced by CESM under two different scenarios, one representing the present-day climate and one simulating the future climate conditions assuming a large amount of greenhouse gas emissions, they managed to identify the fingerprints of internal tide changes hidden in the huge datasets.

The results suggested that the semi-diurnal internal tides in the South Pacific Ocean have strengthened over the last few decades and will continue to grow stronger under future climate warming scenarios. The scientists pointed out that the slowdown of the ocean overturning circulation and global mean sea level rise are the two key factors responsible for the internal tide changes, highlighting the important impacts of climate change on ocean dynamics.

The strengthening of the semi-diurnal internal waves can have important implications for coastal regions, particularly those already vulnerable to high tidal ranges. Furthermore, internal tides are known to influence ocean mixing processes and nutrient transportation in the ocean. Therefore, the changes of the internal tides suggest that the ocean's ability to redistribute heat and nutrients might also be impacted due to climate change, potentially leading to shifts in marine ecosystems.