Modelling the dynamics of the upper ocean in the Eastern Tropical Pacific and its interactions with the tropical atmosphere

Abstract

The physical behaviour of the central and eastern tropical Pacific regions is characterized by a complex variety of multi-scale interacting processes. The Madden-Julian Oscillation (MJO) is the primary mode of intraseasonal variability in the tropical atmosphere. Although convection, together with related cloudiness and precipitation, tends to dissipate east of the 180º meridian, the MJO wind signal continues to progress eastward across the eastern Pacific and South America and into the tropical Atlantic. This research explores numerically the response of the upper ocean in the central and eastern tropical Pacific regions to MJO forcing. We use a global, intermediate resolution configuration of the Nucleus for European Modelling of the Ocean (NEMO) framework version 3.6, referred to as ORCA1-LIM3, forced with daily atmospheric forcing from the Coordinated Oceanic Reference Experiments (CORE) dataset version 2, for the period 1990 to 2000. The results show a strong influence of the MJO on temperature, salinity, zonal currents, and vertical currents in the first 300 m of depth, particularly at the equator. The intraseasonal wind is the key forcing factor that modulates the impact of the MJO. This is except for the temperatures at the mixed layer depth, the halocline, and the pycnocline, in the regions of the Central American Pacific coast and the southwestern Mexican coast where the influence of the MJO can be observed even in the absence of intraseasonal wind forcing. The impact of the MJO on the ocean's internal intraseasonal variability is analysed in the case of tropical instability waves. We show that the barotropic and baroclinic conversion terms that control the eddy kinetic energy levels in the region vary in magnitude with the phases of the MJO. These results have ramifications for understanding ocean intraseasonal variability, which is a critical step towards improving our ability to make more reliable mid-range ocean weather and ocean climate forecasts for the region.