Signatures of Indian Ocean Dipole and El Niño-Southern Oscillation events in sea level variations in the Bay of Bengal

• Published in: Journal of Geophysical Research: Oceans Vol. 117; no. C10
• Main Authors: Aparna, S. G., McCreary, J. P., Shankar, D., Vinayachandran, P. N.
• Format: Journal Article
• Language: English
• Published: Washington, DC Blackwell Publishing Ltd 01-10-2012; American Geophysical Union
• Subjects: Earth sciences; Earth, ocean, space; El Niño; Exact sciences and technology; Indian Ocean Dipole; Marine; sea-level variability; Indian Ocean; Asia; Bay of Bengal; India; ISW, Bangladesh, Bengal Bay; ISW, Indian Ocean; ISW, North Indian Ocean; Indian ocean dipole; Tide gauge; Stratified model; Wind effect; Sea surface temperature; anomalies; lead; Rossby wave; Relaxation; coastal zone; El Nino; solution; winds; Southern oscillation; bays; covariance analysis; Forcing; Composite analysis; La Nina; sea level; altimetry
• ISSN: 0148-0227; 2169-9275; 2156-2202; 2169-9291
• DOI: 10.1029/2012JC008055

We investigate the impact of the Indian Ocean Dipole (IOD) and El Niño and the Southern Oscillation (ENSO) on sea level variations in the North Indian Ocean during 1957–2008. Using tide‐gauge and altimeter data, we show that IOD and ENSO leave characteristic signatures in the sea level anomalies (SLAs) in the Bay of Bengal. During a positive IOD event, negative SLAs are observed during April–December, with the SLAs decreasing continuously to a peak during September–November. During El Niño, negative SLAs are observed twice (April–December and November–July), with a relaxation between the two peaks. SLA signatures during negative IOD and La Niña events are much weaker. We use a linear, continuously stratified model of the Indian Ocean to simulate their sea level patterns of IOD and ENSO events. We then separate solutions into parts that correspond to specific processes: coastal alongshore winds, remote forcing from the equator via reflected Rossby waves, and direct forcing by interior winds within the bay. During pure IOD events, the SLAs are forced both from the equator and by direct wind forcing. During ENSO events, they are primarily equatorially forced, with only a minor contribution from direct wind forcing. Using a lead/lag covariance analysis between the Niño‐3.4 SST index and Indian Ocean wind stress, we derive a composite wind field for a typical El Niño event: the resulting solution has two negative SLA peaks. The IOD and ENSO signatures are not evident off the west coast of India.
Key Points
We identify the signatures of IOD and ENSO in sea level variability
Distinguish the signatures of IOD with signatures of ENSO
Identify the forcing mechanisms of the sea‐level variability