Experimental Assessments on Impacts of Salinity Anomalies on the Positive Indian Ocean Dipole

Abstract

Impacts of salinity anomalies associated with the positive Indian Ocean Dipole (pIOD) are assessed through novel sensitivity experiments using a regional ocean model (Regional Ocean Model System) and detailed diagnostics of heat and momentum budget. During the pIOD, density stratification in the eastern equatorial Indian Ocean is enhanced due to anomalous surface freshening and subsurface saltening. This causes momentum inputs from the wind forcing to be more strongly trapped in the surface layer, and zonal and vertical current anomalies to be more confined to the upper layer. As a result, upward transports of cold water from below the thermocline to the surface layer are significantly suppressed, and the cooling in the eastern equatorial Indian Ocean is suppressed by as much as 1.0°C. The above arguments are further corroborated by a set of sensitivity experiments using a linear continuously stratified ocean model, which can isolate the effect of stratification change caused by salinity anomalies associated with the pIOD in the Regional Ocean Model System simulation. Our results suggest that salinity does play an active role in the evolution of the pIOD, rather than being passively affected by large-scale anomalous atmospheric and oceanic conditions. Plain Language Summary Salinity modulates the upper ocean circulation and temperature by changing the density of seawater and dynamical/themodynamical processes that control them. Recent studies have demonstrated that the tropical Indian Ocean experiences significant salinity variation associated with the Indian Ocean Dipole (IOD), which is a dominant climate mode in the tropical Indian Ocean and exerts widespread impacts on the global climate. However, how these salinity anomalies affect the upper ocean properties and feedback onto the IOD are not quantitatively understood. Here, we investigate the roles played by salinity variation associated with the positive IOD (pIOD) by conducting a series of sensitivity experiments using a regional ocean model. The results from our experiments show that these pIOD-related salinity anomalies act to shift the current anomalies upward and weaken the surface cooling by about 1.0°C in the eastern equatorial Indian Ocean. This is because enhanced salinity stratification traps momentum imparted by wind to the upper layer and alters the structure of the equatorial circulation. The validity of this mechanism is supported by detailed heat and momentum budget analyses, as well as sensitivity experiments using a simplified ocean model. These results provide clear evidences that salinity plays an active role in the evolution of the pIOD.

Asymmetric Response of Sea Surface Salinity to Extreme Positive and Negative Indian Ocean Dipole in the Southern Tropical Indian Ocean

“…This paper shows that the stronger asymmetries of SSS response to the IOD occur in the central southern TIO, mainly caused by the vertical processes and local precipitation. Meanwhile, the SSS changes the MLD and affects the temperature and ocean circulation by altering density fields and further impacting the IOD (Kido et al., 2019a, 2019b; Li et al., 2017; Masson, 2004). Whether the asymmetries in SSS impact the asymmetries of IOD needs to be further studied in the future.…”

Correlation between subsurface salinity anomalies in the Bay of Bengal and the Indian Ocean Dipole and governing mechanisms

“…The first is the global quality-controlled monthly ocean temperature and salinity objective analyses of version 4.2.1 of the Met Office Hadley Centre "EN" series (Good et al, 2013), hereafter EN4. The second is an ocean synthesis, which is the German contribution of the Estimating the Circulation and Climate of the Ocean project GECCO2 (Köhl, 2015). The third is the free run of the mixed resolution of MPI-ESM (Jungclaus et al, 2013) under historical conditions, hereafter MPI-ESM-MR.…”

Air‐Sea Interaction in the Western Tropical Pacific and its Impact on Asymmetry of the Ningaloo Niño/Niña

“…To quantify the main contributors to SST variability in the western-central tropical Pacific and its asymmetry, we have conducted a simulation using the Regional Ocean Modeling System (ROMS; Shchepetkin & McWilliams, 2005). This model has been extensively used to investigate the upper ocean variability in the tropics (Bingham et al, 2020;Kido et al, 2019aKido et al, , 2019bLee et al, 2019). The model domain covers the global tropical ocean between 12°S and 12°N.…”

“…To evaluate the impact of wind stress variability on SSH variability and its amplitude asymmetry, we have used an LCSM (McCreary, 1981). It has been widely used to investigate dynamical mechanisms responsible for variability of SSH and the upper ocean circulation in the Indo-Pacific region (Chen et al, 2015;Han et al, 2014;Kido et al, 2019b;Trenary & Han, 2012;Yuan et al, 2011;Zhang & Han, 2020).…”