The Madden‐Julian Oscillation's Impact on the Makassar Strait Surface Layer Transport

Abstract

The Indonesian Throughflow (ITF) surface layer dynamics within the Makassar Strait, responsible for ~77% of the ITF, are investigated using in situ and satellite‐derived observations from January 2004 to August 2011 and August 2013 to December 2016. Surface layer southward transport attains its minima during boreal winter in response to atmospheric and oceanic processes attributed to Australian‐Indonesian monsoon as well as the Madden‐Julian Oscillation (MJO). While the monsoon's impact on seasonal variability of the ITF transport has been well documented, the MJO's role to modulate the variability of the ITF transport is less studied. Eleven MJO events traversed from Indian Ocean to Western Pacific during boreal winter months over the course of the Makassar Strait time series. Intensified along‐strait wind stress, reduced outgoing longwave radiation, increased sea surface height in the southern Makassar Strait, and a reduction in the surface layer ITF transport by up to 4 Sv marked the impact of the active phase of the MJO. Analysis of the momentum budget in the surface layer indicates that the excess of northward momentum due to the along‐channel pressure gradient over northward momentum attributed to the vertical divergence of Reynold stress governs the northward acceleration of the surface layer during the MJO active phase.

Detecting Change in the Indonesian Seas

“…Increased turbulent stress at the base of the upper layer also acts to decelerate the anomalous ITF transport toward the Pacific Ocean during MJO events. This contributes to the seasonal reduction of the southward throughflow (Napitu, 2017;Napitu et al, 2019). Indian Ocean Kelvin waves are identifiable from the observations as northward flows beneath 200 m (Figure 2), acting to regulate the intraseasonal and semi-annual (May and November) variability in Makassar Strait (Pujiana et al, 2009(Pujiana et al, , 2013Susanto et al, 2012).…”

“…The Madden-Julian Oscillation (MJO) and shear instabilitygenerated eddies together dictate intraseasonal variations in the upper 200 m of the Makassar Strait throughflow (Pujiana et al, 2012;Napitu, 2017;Napitu et al, 2019). The MJO is strongest during boreal winter monsoon across the Indonesian seas (Napitu et al, 2015).…”

Makassar Strait Throughflow Seasonal and Interannual Variability: An Overview

“…During the active phase of the MJO over the central Indonesian maritime continent, MJO westerly winds‐driven Ekman transport increases sea surface height in the southern Makassar Strait, leading to a reduction in the southward along‐strait pressure gradient (Napitu et al, this issue; in review). The suppressed southward along‐strait pressure gradient and the northward component of the MJO winds, together, account for an increase in northward momentum transfer, contributing to the weakening of the southward flow in the upper 100 m during boreal winter (Napitu et al, ; in review).…”

“…The seasonal signal is modulated by interannual ENSO forcing, with weaker southward flow and a deeper subsurface V‐max during El Niño; stronger southward flow with a shallower V‐max during La Niña (Figure ). Interannual differences in the winter surface layer transport are associated with MJO passing over the Maritime Continent, with strong winter reversals occurring in 2014/2015 and 2016/2017 (Figure ; Napitu et al, ). The strong weakening of the Makassar throughflow in observed in June 2016 (Figures a and b) can be attributed to a strong negative IOD (Pujiana et al, ).…”

Mean Seasonal Sea Surface Height Variations in and around the Makassar Strait

“…Many researchers have investigated the ITF variations and related them to the sea level differences, or the pressure gradient force, at various time scales. Daily gridded satellite altimetry sea surface height (SSH) products are used in [2] to obtain the SSH anomaly (SSHA) difference between the Celebes Sea and the Java Sea to discuss the Madden-Julian Oscillation (MJO). Term balances in the equation of motion in the along-strait direction reveal that both northward along-strait wind stress and northward pressure gradient over the Makassar Strait during the active phase of the MJO events will accelerate the northward ITF in the upper layers.…”