Interannual Variability of Eddy Kinetic Energy in the Subtropical Southeast Indian Ocean Associated With the El Niño‐Southern Oscillation

Abstract

Interannual variability of eddy kinetic energy (EKE) in the subtropical southeast Indian Ocean (SEIO) is investigated using satellite observations in three regions in the 20°S–35°S latitude band: R1 (108°E–115°E), R2 (100°E–108°E), and R3 (60°E–100°E). The El Niño‐Southern Oscillation (ENSO) plays an important role in modulating the interannual variability of EKE in the SEIO. EKE in the three regions shows negative correlations with the Nino3.4, lagging Nino3.4 by 2, 14, and 22 months, respectively. In R1, the ENSO modulates the interannual variability of EKE through influencing baroclinic instability of meridional velocity shear between the upper poleward Leeuwin Current (LC) and the underlying equatorward Leeuwin Undercurrent (LUC). During La Niña (El Niño) year, both the poleward LC and equatorward LUC strengthened (weakened) due to the high (low) sea level anomaly (SLA) propagating from the Pacific Ocean as Rossby wave under geostrophic equilibrium, and baroclinic instability of vertical shear enhanced (slackened), further induced the strong (weak) EKE in the eastern boundary. In R2, the ENSO modulates the interannual variability of EKE through influencing baroclinic instability of zonal velocity shear between the central and southern branches of the upper eastward South Indian Countercurrent (SICC) and the lower extending westward South Equatorial Current (SEC). In R3, both the ENSO and the Southern Annual Mode modulate the interannual variability of EKE through influencing baroclinic instability of zonal velocity shear between the SICC and SEC. The interannual variability of EKE in the interior SEIO could be influenced by westward propagation of EKE originated from eastern boundary.

Interannual to Multidecadal Forcing of Mesoscale Eddy Kinetic Energy in the Subtropical Southern Indian Ocean

“…We also examined the lagged correlations of the climate mode indices with mesoscale EKE averaged in the eastern SSIO and west central SSIO (Figure ), in order to consider the aggregate impact of the climate modes in these distinct regions of mesoscale eddy variability. The time series of eastern SSIO EKE lp and EKE meso is closely aligned with the Niño3.4 index (Figure a), and the negative Niño3.4‐EKE meso correlation is very likelyless than −0.5 at lags of 3–6 months (Figure b), indicating a very robust influence from ENSO as Zheng et al () found. However, despite the localized negative correlation of SAM in part of the E SSIO region, there is no significantly robust correlation between SAM and EKE meso in the E SSIO region overall (Figures c and d), implying the dominance of the ENSO influence in this region.…”

“…The relationship of the northwestern tropical Pacific SLA and mesoscale EKE in the eastern SSIO region suggests that ENSO or decadal variability in the tropical Pacific can induce variations of eddy activity in the SSIO, a hypothesis supported by the influence of ENSO on the strength of the Leeuwin Current (Feng et al, ) where much of the eddy generation occurs. Jia, Wu, Lan, and Qiu () discussed a significant negative correlation between spatially averaged EKE meso in the southeast Indian Ocean and the SAM, while Zheng et al () explored the influence of ENSO on eddy activity in the SSIO. However, the westward extent of the influence of climate modes on EKE meso has not been clearly mapped, and the climate influence on SSIO mesoscale eddy activity has not been assessed on decadal to multidecadal time scales.…”

“…Earlier studies of the ID variability of EKE in the SSIO region (Jia, Wu, Lan, & Qiu, ; Zheng et al, ) have focused on the vertical shear of horizontal velocity ( ∂ u / ∂ z and ∂ v / ∂ z ) as an indicator of favorable conditions for baroclinic instability. Here we consider the magnitude of the lateral (horizontal) density gradient ∇ H ρ , which is related to ∂ u / ∂ z and ∂ v / ∂ z by the thermal wind relation.…”

“…Far from the Leeuwin Current in the central and western SSIO, Zheng et al () found weak correlations with vertical shear of horizontal velocity and ENSO, but otherwise, there has not been an explanation of the substantial interannual and decadal variability of EKE in the SSIO interior. The Indian Ocean Dipole (Saji et al, ) climate mode influences levels of EKE near Madagascar (Palastanga et al, ) but does not significantly correlate with EKE levels in the interior SSIO (Jia, Wu, Lan, & Qiu, ; Zheng et al, ).…”

“…An analysis by Jia, Wu, Lan, and Qiu (2011) emphasized the effect of the Southern Annular Mode (SAM) on eddy generation in the eastern SSIO; they also found a connection between the ENSO and SSIO eddy activity, which Zheng et al (2018) attributed to the vertical gradient (shear) in meridional velocity between the southward Leeuwin Current and the Leeuwin Undercurrent. Far from the Leeuwin Current in the central and western SSIO, Zheng et al (2018) found weak correlations with vertical shear of horizontal velocity and ENSO, but otherwise, there has not been an explanation of the substantial interannual and decadal variability of EKE in the SSIO interior. The Indian Ocean Dipole (Saji et al, 1999) climate mode influences levels of EKE near Madagascar (Palastanga et al, 2006) but does not significantly correlate with EKE levels in the interior SSIO (Jia, Wu, Lan, & Qiu, 2011;Zheng et al, 2018).…”