Positive Indian Ocean Dipole events prevent anoxia off the west coast of India

Abstract

Abstract. The seasonal upwelling along the west coast of India (WCI) brings nutrient-rich, oxygen-poor subsurface waters to the continental shelf, favoring very low oxygen concentrations in the surface waters during late boreal summer and fall. This yearly-recurring coastal hypoxia is more severe during some years, leading to coastal anoxia that has strong impacts on the living resources. In the present study, we analyze a 1/4 • resolution coupled physical-biogeochemical regional oceanic simulation over the 1960-2012 period to investigate the physical processes influencing the oxycline interannual variability off the WCI, that being a proxy for the variability on the shelf in our model. Our analysis indicates a tight relationship between the oxycline and thermocline variations in this region on both seasonal and interannual timescales, thereby revealing a strong physical control of the oxycline variability. As in observations, our model exhibits a shallow oxycline and thermocline during fall that combines with interannual variations to create a window of opportunity for coastal anoxic events. We further demonstrate that the boreal fall oxycline fluctuations off the WCI are strongly related to the Indian Ocean Dipole (IOD), with an asymmetric influence of its positive and negative phases. Positive IODs are associated with easterly wind anomalies near the southern tip of India. These winds force downwelling coastal Kelvin waves that propagate along the WCI and deepen the thermocline and oxycline there, thus preventing the occurrence of coastal anoxia. On the other hand, negative IODs are associated with WCI thermocline and oxycline anomalies of opposite sign but of smaller amplitude, so that the negative or neutral IOD phases are necessary but not the sufficient condition for coastal anoxia. As the IODs generally start developing in summer, these findings suggest some predictability to the occurrence of coastal anoxia off the WCI a couple of months ahead.

Sea Level Interannual Variability Along the West Coast of India

“…The leading mode of interannual SLA variability in the Indian Ocean is associated with the IOD and is very well reproduced by our model. The ST contribution is indeed weaker for negative than for positive IODs, as hypothesized by Parvathi et al (2017), but only by~20%, while the total SLA is weaker by~60%. Equatorial easterly wind anomalies typical of positive IODs extend sufficiently northward to STIP region to trigger downwelling coastal Kelvin waves, which dominate WCI SLA during late summer and fall.…”

“…It is therefore important to understand the processes by which IOD influences the WCI. Along the same lines, Parvathi et al (2017) reported an asymmetric influence of positive and negative IOD events on WCI oxycline/thermocline variations, with positive IODs suppressing anoxia, while negative IODs having a less systematic impact. Here we use a linear ocean model to explore the main processes that influence interannual SLA (and hence thermocline/oxycline variations) along the WCI during an IOD event.…”

“…These two opposite signals result in overall weak BB remote forcing contribution to WCI SLA during fall and winter (Figure 3b). Parvathi et al (2017) hypothesized a key role of zonal wind variations near the southern tip of India, as that previously demonstrated for the seasonal cycle (Suresh et al, 2016) but did not demonstrate it. It is therefore important to understand the processes by which IOD influences the WCI.…”

“…The above anticyclonic circulation (Figure 2a) is also associated with upwelling-favorable alongshore winds all around the BoB rim, especially along the northwestern boundary ( Figure S3d), yielding negative ABB signals (Figure 2d). Parvathi et al (2017) recently demonstrated that IOD events could modulate the thermocline (and oxycline) depth anomalies along the WCI during fall and thus favor or inhibit anoxia along the WCI. Parvathi et al (2017) recently demonstrated that IOD events could modulate the thermocline (and oxycline) depth anomalies along the WCI during fall and thus favor or inhibit anoxia along the WCI.…”

“…A positive IOD induces negative SLA in the BoB during April-December, with a peak in fall (Aparna et al, 2012). Parvathi et al (2017) is to our knowledge the only study that investigated the WCI interannual thermocline (hence SLA) variability, owing to its strong impact on anoxic events there. Several studies used partial correlation techniques to isolate intrinsic SLA signals associated with ENSO and IOD (Currie et al, 2013;Rao & Behera, 2005;Yu et al, 2005).…”