Intraseasonal sea surface salinity variability in the equatorial Indo‐Pacific Ocean induced by Madden‐Julian oscillations

Abstract

Intraseasonal sea surface salinity (SSS) variability in the equatorial Indo-Pacific Ocean is investigated using the Aquarius/SAC-D satellite measurements and Hybrid Coordinate Ocean Model (HYCOM). Large-scale SSS variations at 20-90 day time scales induced by Madden-Julian oscillations (MJOs) are prominent in the central-to-eastern Indian Ocean (IO) and western Pacific Ocean (PO) with a standard deviation of $0.15 psu. The relationship between SSS anomaly and freshwater flux is nearly in phase in the central-toeastern IO and out of phase in the western PO during a MJO cycle. A series of HYCOM experiments are performed to explore the causes for SSS variability. In most areas of the equatorial Indo-Pacific Ocean, wind stress-forced ocean dynamical processes act as the main driver of intraseasonal SSS, while precipitation plays a secondary role. In some areas of the western PO and western IO, however, precipitation effect is the leading contributor. In comparison, evaporation effect induced by radiation and wind speed changes is generally much smaller. Besides the external forcing by MJOs, ocean internal variability can also cause considerable intraseasonal SSS changes, explaining 10-20% of the total variance in some regions. Composite analysis for MJO events reveals that the effects of wind stress, precipitation, and evaporation vary at different phases of a MJO cycle. The MJO-induced SSS signature is the result of complicated superimposition and interaction of these effects. The effect of wind stress also varies significantly from event to event. It affects SSS variability primarily through horizontal ocean current advection and to a lesser extent through vertical entrainment.

Origins of Eddy Kinetic Energy in the Bay of Bengal

“…However, the EKE patterns in the NoISO (Figure b) and NoSTRESS (Figure c) experiments are strikingly similar. This demonstrates that the effects of surface heat and freshwater fluxes on EKE are small, although their impacts on ocean temperature and salinity are large (Li et al, ). In Regions 1, EKE is 6.6% enhanced in the NoSTRESS experiment compared with the NoISO experiment, and in Region 2, EKE is 8.6% stronger in NoISO than in NoSTRESS (Figure ).…”

Bay of Bengal salinity stratification and Indian summer monsoon intraseasonal oscillation: 1. Intraseasonal variability and causes

“…The incoherent intraseasonal variability of precipitation and SSS were also observed in the eastern Indian Ocean and western Pacific Ocean [ Guan et al ., ]. Data analysis and model experiments demonstrate that these large‐scale intraseasonal SSS anomalies are produced by ocean current advection driven by intraseasonal winds and therefore do not follow the freshwater flux changes [ Matthews et al ., ; Li et al ., ; Li and Han , ].…”

Causes for intraseasonal sea surface salinity variability in the western tropical Pacific Ocean and its seasonality

“…Li et al . [] provided a detailed comparison between HYCOM MR solution and observations (both satellite and in‐situ) and demonstrated that HYCOM is able to realistically simulate the mean state and intraseasonal variability of the tropical Indo‐Pacific Ocean (their Figures ). Here we provide further validations but only for the western tropical PO.…”

“…In this study HYCOM is configured to the tropical and subtropical PO basin (110°E–70°W, 48°S–48°N) with 1/3°×1/3° horizontal resolution. Three 5° sponge layers are applied to the western, southern, and northern open‐ocean boundaries, where model temperature and salinity are relaxed to WOA09 climatology [see Antonov et al ., ; Locarnini et al ., ; Li et al ., , Figure b]. The model has 26 vertical layers, and the top layer thickness is ∼2.6 m. Realistic bathymetry from the National Geophysical Data Center (NGDC) 2′ digital data are used after a 1.5°×1.5° smoothing.…”

“…[] and Li et al . [] mainly studied the box area of 155°E–165°E, 5°S–5°N. A comprehensive investigation of SSS ISV for the entire western tropical PO has not yet been done.…”