Ocean motion can occur at scales as large as global circulation patters that last for thousands of years down to small ephemeral waves breaking at the sea surface to even the motion of a tiny bubble in the water. “Middle” sized ocean motions are known as mesoscale features, which are typically mesoscale eddies.
Mesoscale eddies are loops of swirling water that typically have broken off of a larger current, such as the Gulf Stream. They can spin clockwise or counterclockwise and are found in both the northern and southern hemispheres. Eddies are usually around 100 km (62 miles) wide and can last weeks to months, traveling long distances in the ocean before dissipating. Having broken off a current means that the characteristics of the water (e.g., temperature and salinity) within an eddy can be very different than the water surrounding it. In some cases, a mesoscale eddy can be an "oasis," bringing a swirl of highly productive water into a barren ocean desert.
For decades, scientists have used a suite of in-water instruments and satellites to track eddies by homing in on their unique signatures of sea surface height, temperature, and salinity. NASA's latest physical oceanography satellite, Surface Water and Ocean Topography (SWOT), launched in December 2021 is poised to provide novel insight into not just mesoscale features, but even submesoscales! With 10 times better resolution than previous sea surface higher missions, SWOT's global ocean coverage provides unprecedented detail to better understand how mesoscale eddies and submesoscale features form and dissipate over time.
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Mesoscale mixing has important impacts ocean circulation, water mass formation, and transport oxygen, carbon, and nutrients through the ocean. Little is known, however, about how this mixing varies across climate timescales. Not only is this important in terms of how we understand these ocean processes, but it is also a key missing piece in climate models as we seek to understand dynamics of the ocean in the past and future climate projections. This study presents a novel look at mesoscale mixing at the ocean surface based on long-term global observations.
Busecke, J.J.M. and Abernathey, R.P. (2019). Read the full article.
There are many reasons why salinity may change in the ocean - proximity to river inputs, seasonal and climatological cycles, or the water cycle (i.e., the balance between evaporation and precipitation). Salinity is also impacted by mixing processes from eddies. In this study, researchers examined how ocean salinity is changed as a result of eddy transport. Interestingly, eddy transport of salinity tends to counteract some of the water cycle impacts. For example, areas where the water cycle processes lead to higher salinity – for example, the subtropics where evaporation exceeds precipitation – are counterbalanced by eddies moving low-salinity water into the region. Conversely, in the tropics where precipitation exceeds evaporation, eddy transport of dissolved salt helps to mitigate decreases in salinity.
Melnichenko, O., Hacker, P., and Müller, V. (2021). Read the full article
The Bay of Bengal has a strong seasonal cycle due to the annual monsoon. This study examined the impacts of the monsoon cycle on eddies within the Bay of Bengal, particularly in relation to the larger Indian Ocean Dipole cycle. Overall, they found a significant impact of the monsoon on eddies and interestingly, these impacts yield dramatically different eddy fields between the eastern and western portions of the Bay of Bengal.
Greaser, S.R., Subrahmanyam, B., Trott, C.B., and Roman-Stork, H.L (2020) Read the full article
