Impacts of Precipitation–Evaporation–Salinity coupling on upper ocean stratification and momentum over the tropical pacific prior to onset of the 2018 El Niño

Precipitation affects upper ocean salinity and temperature over a wide range of temporal and spatial scales. Although the effects of freshwater on the diurnal mixing and formation of barrier layer have been shown in previous studies, the impacts of freshwater from the large-scale precipitation associated with the Madden–Julian Oscillation (MJO) on the ocean from the western to central-eastern Pacific are not well understood. This study investigates the coupling of precipitation, evaporation, and salinity (P–E–S) and its impact on the upper ocean stratification and momentum on time scales from days to weeks (intraseasonal) and beyond. The Unified Wave INterface – Coupled Model (UWIN-CM) is used to simulate multiscale air–sea interactions during January–July 2018, including multiple MJO events leading up to the onset of the 2018–2019 El Niño. The atmospheric component of the UWIN-CM is constrained by reanalysis and reinitialized daily, while the ocean component evolves continuously and is fully coupled with the atmosphere. The initial and lateral boundary conditions for the atmosphere and ocean are from the ECMWF ERA5 and HYCOM global analysis, respectively. The UWIN-CM simulation captures the surface winds, rainfall, and the evolution of the upper ocean compared with in situ and satellite observations. To determine the effect of P–E–S coupling, the UWIN-CM simulation with full P–E–S coupling is compared with a model experiment in which the P–E input to the ocean is set to zero (NoPE). The difference between the two increases with time and becomes particularly large during the 3rd MJO event in April 2018. Two key results emerge. First, heavy precipitation and strong winds from the MJO events result in a locally fresher, warmer western Pacific warm pool with a shallower mixed layer and thicker barrier layer. Second, the fresh, warm water mass extends into the central-eastern Pacific from mid-April to late May with up to 0.5 m s−1 eastward currents. A well-defined salinity/density front, propagating eastward at about 0.3 m s−1, is located at the leading edge of the warm pool. The P–E–S coupling plays an important role in the eastward extension of the warm pool prior to the onset of El Niño in July 2018.