A decade-long enhance of the basin-wide sea stage and warmth content material within the subtropical southern Indian Ocean (SIO) throughout 2004–2013 ended abruptly, instantly following the onset of the sturdy 2014–2016 El Niño. Apparently, this unprecedented drop of the SIO warmth shortly recovered in the course of the weak 2017–2018 La Niña. A study recently published in Science Advances used observations and mannequin simulations to discover what precipitated the abrupt discount and restoration of the SIO warmth and sea stage throughout 2014-2018.
The interannual-to-decadal variability of warmth content material and sea stage within the SIO is strongly influenced by its reference to the Pacific and large-scale climatic forcing within the Indo-Pacific area primarily related to El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Southern Annular Mode (SAM). The distant ENSO impact on the SIO warmth content material is twofold. First, ENSO drives adjustments within the upper-ocean warmth content material within the western equatorial Pacific and, due to this fact, modulates advection into the SIO from the Pacific by way of the Indonesian Throughflow (ITF), the “ocean tunnel” impact. Second, native adjustments in wind stress curl influencing the upper-ocean warmth content material within the SIO are additionally associated to ENSO by the technique of atmospheric teleconnections by way of the Walker Circulation, the “atmospheric bridge” impact. Whereas the atmospheric bridge is a reasonably quick teleconnection, it takes years for the ocean tunnel alerts to achieve the western Indian Ocean. The IOD occasions are sometimes triggered by ENSO. Nevertheless, they don’t seem to be all the time in-phase with ENSO as a result of about one-third of IOD occasions happen independently of ENSO occasions.
In line with the new study, the 2014–2016 El Niño did contribute to the noticed cooling of the SIO by way of an uncommon mixture of each the lowered warmth advection from the Pacific (dominant within the japanese SIO) and the basin-wide cyclonic wind anomaly that led to shoaling of isotherms (dominant within the western SIO). The following restoration was primarily compelled by an anticyclonic wind anomaly related to stronger commerce winds that precipitated deepening of isotherms and thus the suppression of 2014–2016 cooling sign propagating from the japanese boundary. The primary outcomes introduced on this examine spotlight the complexity of the SIO warmth content material variability pushed by the ocean tunnel and atmospheric bridge results and their interactions.
Figure 1 from Volkov et al. (2020). Schematic displaying the dynamic processes affecting the subtropical SIO warmth content material and sea stage: Colour reveals the imply dynamic topography within the horizontal airplane and temperature climatology within the vertical planes. The atmospheric circulation within the SIO is dominated by southeasterly commerce winds. The overall ocean circulation consists of the Indonesian Throughflow (ITF) that feeds the South Equatorial Present (SEC) and the Leeuwin Present (LC), and the South Indian Counter Present (SICC). ENSO impacts the SIO warmth content material by way of the ocean route (ITF) and atmospheric route (atmospheric bridge resulting from Walker Circulation). The ITF quantity and warmth transports into the japanese SIO (ESIO) enhance throughout La Niña circumstances (stronger commerce winds within the Pacific) and reduce throughout El Niño circumstances (weaker commerce winds within the Pacific). Moreover advection, alerts generated within the Pacific attain the coast of West Australia as coastally trapped waves. Then, these alerts propagate towards the western SIO (WSIO) as eddies and Rossby waves. Native wind stress curl can modify the waves radiated from the japanese boundary and/or generate different waves.
Volkov, D., S.-Okay. Lee, A. L. Gordon, and M. Rudko, 2020: Unprecedented reduction and quick recovery of the South Indian Ocean heat content and sea level in 2014–2018. Sci. Adv. 6(36), eabc1151. https://doi.org/10.1126/sciadv.abc1151.