Changes in coastal upwelling systems, which provide about 50% of global fish catch, can lead to changes in the marine food web, affecting commercially valuable fish species so impacting the people dependent on them for food and income. The repercussions are more severe in regions where the local economy relies heavily on fisheries. 

Our study underscores the role that one of the largest ocean eddies globally, the Great Whirl, plays in the biological response of the world’s strongest seasonal upwelling system, the Somali upwelling (Figure 1). Due to a large amount of upwelled nutrients, the system is highly productive, attracting the lucrative tuna and tuna-like migratory fish species and other top marine predators. Despite the Great Whirl inducing downwelling, ostensibly reducing productivity, the absence of this eddy might intuitively be presumed to enhance productivity. However, counter-intuitively, our findings refute this assumption.

Using satellite observations, we show that the Somali upwelling productivity has been declining by about 10% for the last 25 years following decreases in upwelling strength. Changes in local winds post 2006 with a decrease in zonal wind stress during the May to September months (upwelling season) off the Somali coast, led to a decrease in the upwelling intensity. The areal extent of the Somali upwelling productivity region has also been diminishing with an abrupt decrease around 2006-07 (Figure 2a). This decline coincided with an abrupt increase in the downwelling effect (Figure 2b). This is due to the Great Whirl being weaker or absent which allowed other smaller eddies to produce stronger downwelling than that due to the Great Whirl itself.

An abrupt shift in the Indian Ocean Dipole (IOD) in 2006 (Figure 2c) is likely the cause of the regime changes in the areal extent of the upwelling productivity region and the downwelling effect of the Great Whirl and other eddies. The IOD is a measure of climatic variations in the Indian Ocean with warmer sea surface temperature in the western basin during a positive IOD phase and vice versa during a negative phase. While the 2006-07 positive IOD event was not the most intense on record, it stood out as initiating a rare sequence of three consecutive positive IODs. This was then followed by an increase in the frequency of positive IOD events.

To uncover the mechanisms behind the abrupt change, we focus on variations in remote forcing, a crucial element in the formation of the Great Whirl and strong positive IOD events. Our findings reveal a shift in the propagation of downwelling Rossby waves in the North Tropical Indian Ocean around 2006. Rossby waves are inertial oceanic waves that travel for hundreds of kilometres in a westward direction. They are known to be influenced by changes in wind fields, specifically in the anticyclonic Wind Stress Curl Anomalies, which we also observe to exhibit a change in behaviour in 2006. Additionally, we detect changes in Rossby waves excited by alterations in the anticyclonic Wind Stress Curl Anomalies in the South Tropical Indian Ocean around 2006. These changes are attributed to variations in the Indian Ocean atmospheric Walker circulation, involving anomalous easterlies over the equator and southeasterlies off Sumatra, representing the atmospheric triggers of the IOD.

Beyond understanding environmental drivers of low productivity scenarios in the Somali upwelling region, our results are also applicable to other western boundary current upwelling regions in the ocean. These too could shift to lower productivity regimes due to similar mechanisms: changes in eddies’ downwelling effects and shifts in climatic conditions. The findings outlined in this study highlight the significant role that ocean vortices can play in regulating the productivity of one of the world’s largest upwelling systems. Moreover, they demonstrate how various atmospheric factors (such as winds) or climate phenomena (such as the Indian Ocean Dipole) could influence this process, which could intensify under the accelerating impact of climate change.

Given the wide-ranging effects of climate change on global upwelling systems, it is imperative to undertake regional analysis to unravel the intricate mechanisms that drive individual systems, to ensure appropriate mitigation is taken to alleviate the impacts on vulnerable coastal communities.