Abstract
Abstract. The Indian Ocean Dipole (IOD) and the El Niño/Southern Oscillation (ENSO) are independent climate modes, which frequently co-occur, driving significant interannual changes within the Indian Ocean. We use a four-decade hindcast from a coupled biophysical ocean general circulation model, to disentangle patterns of chlorophyll anomalies driven by these two climate modes. Comparisons with remotely sensed records show that the simulation competently reproduces the chlorophyll seasonal cycle, as well as open-ocean anomalies during the 1997/1998 ENSO and IOD event. Results suggest that anomalous surface and euphotic-layer chlorophyll blooms in the eastern equatorial Indian Ocean in fall, and southern Bay of Bengal in winter, are primarily related to IOD forcing. A negative influence of IOD on chlorophyll concentrations is shown in a region around the southern tip of India in fall. IOD also depresses depth-integrated chlorophyll in the 5–10° S thermocline ridge region, yet the signal is negligible in surface chlorophyll. The only investigated region where ENSO has a greater influence on chlorophyll than does IOD, is in the Somalia upwelling region, where it causes a decrease in fall and winter chlorophyll by reducing local upwelling winds. Yet unlike most other regions examined, the combined explanatory power of IOD and ENSO in predicting depth-integrated chlorophyll anomalies is relatively low in this region, suggestive that other drivers are important there. We show that the chlorophyll impact of climate indices is frequently asymmetric, with a general tendency for larger positive than negative chlorophyll anomalies. Our results suggest that ENSO and IOD cause significant and predictable regional re-organisation of chlorophyll via their influence on near-surface oceanography. Resolving the details of these effects should improve our understanding, and eventually gain predictability, of interannual changes in Indian Ocean productivity, fisheries, ecosystems and carbon budgets.
Highlights
The El Niño/Southern Oscillation (ENSO) is well-known as a dominant mode of interannual climate variability that develops from air–sea interactions in the tropical Pacific, but affects weather patterns globally (McPhaden et al, 2006)
Applying empirical orthogonal function (EOF) analyses to 4 yr of global SeaWiFS data, which provide an estimate of surface phytoplankton biomass, Yoder and Kennelly (2003) identified two interannual modes of variability in surface chlorophyll, both of which they ascribed to ENSO control
Focus was on the response of chlorophyll, changes in thermocline depth, surface temperature and surface winds were assessed in order to gain a better understanding of physical processes driving the biological patterns
Summary
The El Niño/Southern Oscillation (ENSO) is well-known as a dominant mode of interannual climate variability that develops from air–sea interactions in the tropical Pacific, but affects weather patterns globally (McPhaden et al, 2006). Teleconnections associated with El Niño result in an overall warming of the Indian Ocean (Klein et al, 1999; Murtugudde and Busalacchi, 1999; Xie et al, 2009), due to changing cloud cover and wind patterns that relate to changes in ascending and descending branches of the Walker circulation (Du et al, 2009; Reason et al, 2000; Venzke et al, 2000) Such physical perturbations can affect the biology in local and distant oceans (Ménard et al, 2007; Spencer et al, 2000; Vinueza et al, 2006). Applying empirical orthogonal function (EOF) analyses to 4 yr of global SeaWiFS data, which provide an estimate of surface phytoplankton biomass, Yoder and Kennelly (2003) identified two interannual modes of variability in surface chlorophyll, both of which they ascribed to ENSO control
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
