A biological Indian Ocean Dipole event in 2019

Abstract

The 2019 positive Indian Ocean Dipole (IOD) event in the boreal autumn was the most serious IOD event of the century with reports of significant sea surface temperature (SST) changes in the east and west equatorial Indian Ocean. Observations of the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (SNPP) between 2012 and 2020 are used to study the significant biological dipole response that occurred in the equatorial Indian Ocean following the 2019 positive IOD event. For the first time, we propose, identify, characterize, and quantify the biological IOD. The 2019 positive IOD event led to anomalous biological activity in both the east IOD zone and west IOD zone. The average chlorophyll-a (Chl-a) concentration reached over ~ 0.5 mg m−3 in 2019 in comparison to the climatology Chl-a of ~ 0.3 mg m−3 in the east IOD zone. In the west IOD zone, the biological activity was significantly depressed. The depressed Chl-a lasted until May 2020. The anomalous ocean biological activity in the east IOD zone was attributed to the advection of the higher-nutrient surface water due to enhanced upwelling. On the other hand, the dampened ocean biological activity in the west IOD zone was attributed to the stronger convergence of the surface waters than that in a normal year.

Subsurface Water Property Structures Along 80°E Under the Positive Indian Ocean Dipole Mode in December 2019

“…Particularly, around the upper part of the salinity decreases (approximately 100 dbar), the opposite changes of DO (increase), nitrate, and DIC (decreases) were observed on the isobar surfaces, where the vertical gradient of the biogeochemical variables just below the mixed layers and heaving associated with the pIOD event were too large to clearly show the effect of the horizontal advection, and the downward heaving suppressed the increases in nutrients. This might explain why chlorophyll-a increases were limited to the narrow region in the eastern part near the equatorial region in the biological IOD (Shi and Wang, 2021). In addition, the heaving effect seemed to cause apparent changes in anthropogenic carbon estimations, particularly in the region of 10°S-15°S, which was far away from the enhanced upwelling associated with the pIOD, and thus heaving may not have been caused by the direct influence of the pIOD.…”

“…For example, mooring and satellite observations at approximately 0°and, 80.5°E have shown highfrequency chlorophyll-a increases at sea surfaces associated with strong winds, intensified ocean current shear, and southward flow anomalies (Strutton et al, 2015). Satellite observations have captured increases in the chlorophyll-a concentration in the southeastern equatorial region (90°E-110°E, 10°S-0°) and decreases in the western equatorial region (50°E-70°E), which are caused by upwelling in the eastern region and depression of subsurface nutrient-rich water in the western region (Shi and Wang, 2021). The upwelling and depression have been associated with pIOD mode event.…”

ENSO Explains the Link Between Indian Ocean Dipole and Meridional Ocean Heat Transport

“…For example, the extreme positive 2019 IOD event which occurred without El Niño may have been associated with different climate impacts than ENSO forced IOD events, such as those in 1982 and 1997 (Doi et al., 2020; Ratna et al., 2021; Takaya et al., 2020; L. Zhang et al., 2021). Additionally, the biological impacts of the 2019 IOD event were caused by increased upwelling off of Sumatra (Shi & Wang, 2021). IOD events in NoENSO lack the upwelling‐favorable wind pattern seen in CTRL (Figure 2), suggesting that more study is needed to understand whether internal and ENSO forced IOD have different biological impacts.…”

“…Zhang et al, 2021). Additionally, the biological impacts of the 2019 IOD event were caused by increased upwelling off of Sumatra (Shi & Wang, 2021). IOD events in NoENSO lack the upwelling-favorable wind pattern seen in CTRL (Figure 2), suggesting that more study is needed to understand whether internal and ENSO forced IOD have different biological impacts.…”

Honing in on bioluminescent milky seas from space

“…During its positive phase, the IOD corresponds to warm/wet conditions with warm pooling waters on the western side of the Indian Ocean, and cool/dry conditions with strong easterly winds on the eastern side. These winds generate upwelling of cool, nutrient-rich coastal waters which drift offshore with the currents, leading to algal blooms over a broad region 22 , and potentially, conditions favourable for milky sea genesis.…”