The mysteries of tropical upwelling regions have long fascinated oceanographers and climate scientists alike. These dynamic zones, coupled by their production of rich biodiversity and robust fisheries, play a pivotal role in global climate regulation and marine biogeochemical cycles. Yet, despite decades of research, the intricate processes that govern upwelling systems remain only partially understood, making it difficult to predict how these vital ecosystems will respond to escalating anthropogenic pressures such as climate change, intensive fishing, and pollution.
Eastern boundary upwelling systems are traditionally explained through the mechanism of along-shore winds. In many coastal regions, prevailing winds blow parallel to the coastline and toward the equator, effectively pushing the warm surface waters offshore. This displacement facilitates the upward movement of cold, nutrient-dense waters from the deep ocean, fueling prolific phytoplankton blooms that underpin the entire marine food web. In this classic picture, the pulse of coastal productivity corresponds closely with wind patterns and intensity.
However, recent observations highlight that this framework falls short of explaining the entire picture in the Benguela upwelling system, located off the coast of Angola and Namibia. Strikingly, seasonal upwelling events are recorded even when winds are conspicuously weak or absent. It is this anomaly that sparked the latest scientific expedition aboard the research vessel METEOR, which departed from Las Palmas with the mission to investigate the unique oceanographic forces at play and their broader climatological implications.
The crux of this enigma lies in the dynamics of coastal Kelvin waves, which are an essential yet often underappreciated component of ocean circulation along eastern boundaries. These waves originate from wind fluctuations near the equator and propagate poleward, traveling thousands of kilometers along the continental margins. Unlike the surface phenomena evident in wind-driven upwelling, Kelvin waves modulate deeper ocean currents and can cause vertical displacements in water masses without significantly altering the sea surface height. This subtle modulation affects the vertical transport of cold, nutrient-rich waters to the surface, contributing to upwelling even in near windless conditions.
Complementing these wave-induced effects is the role of vertical mixing, primarily driven by tidal forces. Turbulence within the upper ocean layers enhances the exchange between colder deep waters and the surface, providing an alternate pathway for sustaining upwelling processes independently from surface wind stress. The interplay of these forces—wave propagation and tidal mixing—is a major focus for Dr. Marcus Dengler and his team aboard METEOR during the M217 “BOCABENO” expedition, which aims to dissect these complex physical oceanographic interactions.
Further compounding the scientific intrigue is the phenomenon known locally as Benguela Niños—periodic marine heatwaves characterized by abrupt sea surface temperature anomalies up to 3°C above average. These thermal events have profound regional impacts, triggering flooding across Angola and Namibia, increasing precipitation in the typically arid Namib Desert, and disrupting fragile marine ecosystems built on the upwelling productivity. Understanding the triggers and progression of Benguela Niños is crucial for predicting their recurrence and mitigating their detrimental ecological and socio-economic consequences.
One of the most significant recent Benguela Niño events occurred in 2021, exhibiting unusual timing by rising late in the upwelling season and notably suppressing phytoplankton proliferation. This disruption cascaded through the food web, resulting in a marked reduction in fish stocks dependent on primary productivity. The genesis of these heatwaves remains contested, with theories proposing influences ranging from the triggering coastal Kelvin wave impulses arriving from equatorial zones, alterations in regional wind regimes, to variations in freshwater discharge, especially from the Congo River basin. The incomplete understanding of these mechanisms underscores the urgency and relevance of ongoing investigations.
Methodologically, the BOCABENO expedition employs a multi-pronged approach to collect high-resolution, multidisciplinary data. Instruments moored to the ocean floor provide continuous time series of physical parameters such as currents, temperature, salinity, pressure, and oxygen content extending down to depths of around 1,200 meters. These data are invaluable for capturing temporal dynamics spanning months and years, facilitating the disentanglement of regular seasonal patterns versus anomalous events.
In concert with fixed moorings, vertical profiling using CTD rosette casts complements the dataset with high-resolution snapshots of the water column at strategically chosen stations. This technique enables meticulous measurement of temperature and salinity gradients alongside oxygen levels, while allowing direct collection of water samples for detailed biochemical analyses—nutrients, dissolved gases, and biological components—which collectively illuminate the ecosystem’s health and functionality.
Adding to the expedition’s technical arsenal are specialized turbulence sensors deployed to quantify small-scale mixing processes in situ. These devices capture the intensity and distribution of turbulent energy dissipation, which governs the upward flux of cold deep water that is vital for sustaining the upwelling phenomenon, especially under low-wind conditions. By integrating these datasets, researchers aim to unravel the complex synergy between physical ocean dynamics and biological productivity.
The expedition’s trajectory traces a vital corridor in the tropical Atlantic, departing from the port of Las Palmas and concluding in Walvis Bay, Namibia. Over nearly a month of data collection, the researchers focus on the continental slope areas off Angola and Namibia, regions particularly sensitive to the confluence of atmospheric, oceanic, and terrestrial forces shaping the Benguela system. This comprehensive survey offers new opportunities to validate numerical models and enhance predictive capabilities for climate-driven changes in upwelling intensity and marine heatwave occurrences.
As climate change accelerates, with anticipated alterations in wind patterns, ocean stratification, and freshwater inputs, understanding the nuanced mechanisms governing the Benguela system is more critical than ever. Insights gained from this expedition could not only shed light on regional climate impacts but also inform broader discussions about resilience and adaptation of marine ecosystems highly dependent on upwelling processes worldwide.
Ultimately, the METEOR’s journey represents a significant stride toward deciphering one of the ocean’s most compelling puzzles—how a system seemingly defying classical principles maintains its productivity and how its future trajectory might unfold in the face of a rapidly changing global climate.
Subject of Research: Tropical Atlantic upwelling systems and Benguela Niños marine heatwaves
Article Title: Unraveling the Enigma of Wind-Independent Upwelling and Benguela Niños Off Southwest Africa
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Image Credits: Photo by Philipp Henning, GEOMAR
Keywords
Ocean circulation, Ocean currents, Gyres, Ocean temperature, Ocean waves, Tides, Oceans, Ocean physics, Climate variability, El Nino, Seasonal changes
