Unravelling the Indian Ocean dipole from the oceanic perspective

Author: Ankit Kavi

Kavi, Ankit, 2020 Unravelling the Indian Ocean dipole from the oceanic perspective, Flinders University, College of Science and Engineering

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This dissertation investigates the physical oceanographic processes related to Indian Ocean dipole mode events, specifically focusing on wind-driven ocean surface circulation and sea level anomalies. The atmospheric and oceanic physical properties are investigated to reveal the dominant processes. Highly unstable nature of tropical climatic conditions attracts the attention of researcher from many years and number of studies have been performed worldwide. Surface wind variability in a tropical climate is one of the most complex phenomena which drive the surface circulation as well. The role of synoptic-scale tropical atmospheric processes triggered from tropical convection is examined which are erratic.

Although, the variability of winds as ocean surface forcing was analysed previously in several studies though the source of its variability and related processes is still unexplored particularly within the eastern tropical Indian Ocean region. Multitemporal and multiresolution satellite data of atmospheric and oceanic properties at the interface is analysed for as long as 30 years of period to precisely disclose the processes involved in the regional wind variability and subsequently the variability of the ocean surface circulation and the ocean surface temperature variability. The in-situ subsurface database arrived later from 2005 for the tropical Indian Ocean regime; hence, subsurface physical properties are incorporated from that year.

The primary results indicate the high interannual and intraseasonal variability of winds along the southern Sumatra coastline along with zonal wind variability over the eastern equatorial Indian Ocean during boreal summer-fall of each year in the study period (1980-2016), where intraseasonal variations are captured using wavelet spectrum for higher precision. The intraseasonal zonal wind variability at eastern equatorial Indian Ocean is known in previous studies though no previous study claimed a precise process which triggers these wind events. This study reveals that high frequency intraseasonal convective oscillations over the southeastern tropical Indian Ocean (SETIO) triggers low latitude synoptic-scale cyclonic circulation in the lower atmosphere which consecutively triggers zonal wind events at the eastern equatorial Indian Ocean. This study is also discovered the intraseasonal wind reversal along the southern Sumatra coastline during boreal summer-fall under the influence of synoptic-scale low-pressure cyclones which I called “SETIO cyclone”.

Simplified coastal upwelling theory along the coast of a finite horizontal length is developed under the direction my supervisor, to explain the coastal upwelling process precisely under the highly varying wind field conditions along the southern Sumatra coastline. This theory requires subsurface stratification and mixing scheme of the subsurface water in the region precisely, though as mentioned above due to the lack of subsurface data in the last century in the study region; hence, decadal seasonal average values from ARGO data of 2005 to 2016 are used to define mixing depths of the study region. The theory of coastal upwelling defined here discloses why the southern coastal Java upwelling has a more frequent signature in surface oceanic cooling and on the other hand the adjacent southern Sumatra coastline has a rare appearance in surface cooling.

Moreover, I proposed a convective dipole over the same region of IOD boxes, eastern IOD box and western IOD box, and suggest that reversal in the convective dipole is a precursor and not the aftermath of the Indian Ocean Dipole events.

Keywords: Indian Ocean Dipole, cyclonic circulation, quasi-biweekly oscillation, atmospheric tropical convection, java coastal upwelling, sumatra coastal upwelling, sst anomaly IOD, intraseasonal atmospheric oscillations, MJO, QBWO, wind reversal

Subject: Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: AProf Jochen Kaempf