An oceanic teleconnection between the equatorial and southern Indian Ocean

Abstract

Sequences of Kelvin and Rossby waves are found to rapidly carry sea surface height anomalies across the Indian Ocean, and have an impact on Indian to Atlantic interocean exchange. Satellite altimeter data reveal an oceanic teleconnection between equatorial winds and variability of the interocean exchange. Four times per year, we observe an equatorial Kelvin wave to hit Indonesia, forced by monsoon variability. The signal then propagates southward along the Indonesian coast and triggers Rossby waves that propagate westward across the subtropical Indian Ocean. On reaching the Madagascar and Mozambique Channel regions, large rings form at the same four per year frequency. These drift towards the Agulhas retroflection where they control the shedding of Agulhas rings. Disturbances of this pin‐ball‐like propagating signal can be traced from Indian Ocean Dipole/El Niño events in 1994 and 1997/1998, to decreases of Indian‐Atlantic ocean exchange by Agulhas rings over two years later.

A link between low‐frequency mesoscale eddy variability around Madagascar and the large‐scale Indian Ocean variability

“…Farther south, this frequency is reduced to 4 -5 times/year [Schouten et al, 2003]. Schouten et al [2002b] proposed variations in the process of eddy formation via a highfrequency teleconnection with the IO equatorial winds. This study shows that the dominant effect of the equatorialtropical signals from the IOD on the mesoscale variability in the MZC is by variations in the flows around Madagascar due to the arrival of large-scale SSH anomalies.…”

“…The two energetic areas around Madagascar are in turn influenced by the SSH variability along two zonal bands extending east of Madagascar across the IO, around 12°S and 24°S ( Figure 2a). These bands show significant Rossby wave activity at frequencies of around 4 times/year [Schouten et al, 2002a[Schouten et al, , 2002b. While the variability along the 24°S band from off Australia up to 90°E also represents signals of westward propagating eddies from the Leewin Current [Fang and Morrow, 2003], its origin in the center of the subtropical gyre is related to baroclinic instability [Palastanga et al, 2006].…”

“…The North region (NM) lies in an area where eddies are formed due to instabilities of the NMCjet [Quadfasel and Swallow, 1986] and where SSH anomalies arrive from the IO at interannual and shorter timescales [Perigaud and Delecluse, 1993;Schouten et al, 2002b]. In the MZC between 18°-24°S (MC) the high-frequency SSH variability is related to eddies traveling regularly southward [Schouten et al, 2003].…”

“…Analysis of in situ and satellite observations revealed interannual variability in this frequency during 1996-2000; for instance, hardly any eddies were detected in the first half of 1996 and 1998. Schouten et al [2002b] proposed that the rate of eddy formation in the MZC is modulated by varying equatorial winds through an oceanic teleconnection across the IO: equatorial Kelvin waves arrive at the Sumatra coast at the same 4 times/year frequency, they continue southward as coastal Kelvin waves, and subsequently trigger Rossby waves at $12°S. Anomalous equatorial easterlies during the IOD and ENSO interrupted the chain of events in 1994 and 1997, leading to less eddies in the MZC after a lag related to the time it takes the Rossby waves to cross the IO along 12°S to Madagascar.…”

Three decades of research on the greater Agulhas Current

“…The importance of this process of ring shedding at the Agulhas retroflection has thus been demonstrated during the past decade as have the mechanisms that control it. The latter may have to do with the triggering of Natal Pulses and these in turn may be influenced by eddy formation in the source regions of the Agulhas Current (Schouten et al, 2002b). About this more in Sect.…”