New Research: Impacts of the Kuroshio large meander on ocean biogeochemistry explained

Impact of the Kuroshio large meander on the nutrients and primary productivity in the region

Employing sophisticated computer models to simulate numerous meander events, the recent study by Hakase Hayashida and team sheds light on the impact of the Kuroshio large meander on the nutrients and primary productivity in the region (Figure). These findings underscore the critical need to comprehend the multifaceted effects of the Kuroshio large meander, not only on marine life but also on the intricate carbon cycle. As we strive to unravel the intricacies of our planet's oceans, this study serves as a pivotal step forward, offering valuable insights into the mechanisms driving oceanic variability and its broader ecological implications. 

The Kuroshio, known as the "black current" in Japanese, is one of the strongest ocean currents in the world ocean and has significant impacts on the global climate system and local marine industries. What sets it apart from other western boundary currents is its ability to alternate between two stable paths every few years to decades. One path follows closely along Japan's coast (referred to as a non-large meander), while the other, the focus here, veers offshore south of the Tokai region (referred to as a large meander). Since August 2017, a large meander has persisted for over six years, marking it as the longest event ever recorded since consistent observations began in the 1960s by the Japan Meteorological Agency. This extended large meander has captured considerable media attention in Japan due to its association with severe extratropical cyclones featuring heavy rainfall, discomfort during Tokyo's summers, and alterations in the seasonal abundance and distribution of commercial fish. Despite its noticeable effects, its impact on biogeochemistry remains poorly understood, prompting this study to address this knowledge gap. This study analyzed the effects of the Kuroshio large meander on marine environments by examining data from a long-term simulation using a sophisticated global sea ice-ocean model. Despite some model biases, the findings in the study align well with observations. Hakase Hayashida and team discovered that during large meander events, there's increased availability of winter nutrients offshore south of Japan, leading to a larger spring bloom of phytoplankton compared to non-large meander phases. However, nearshore regions experience lower nutrient levels during large meanders due to weakened Kuroshio effects. These large meander-induced variations have significant implications for primary production, carbon uptake, and marine ecosystems, shedding new light on the North Pacific's western boundary current.

Figure: Composite-mean distributions of nitrate, one of the main nutrients, in the ocean surface south of Japan during the large meander (a) and the non-large meander (b), and their difference (c). Adapted from Hayashida et al. (2023).

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