Ocean Response to CINDY/DYNAMO MJOs in Air-Sea-Coupled COAMPS

Abstract

The response of the ocean to three Madden-Julian Oscillation (MJO) events during the fall of 2011 is simulated by the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) in a fully coupled mode with high resolution in the atmosphere and ocean. The model simulates the cooler sea surface temperature (SST) and disappearance of the diurnal cycle in SST during the active phase of the MJO and it compares well with the observed SST from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoys. The most striking direct response to the westerly zonal wind stress associated with the onset of the MJO is a rapidly accelerating Yoshida jet in the ocean mixed layer with equatorial zonal currents exceeding 1 m s -1 . These jets are found in the model as well as the RAMA buoy observations. In the model, the Yoshida jet is superimposed on the seasonal Wyrtki jet that has a subsurface local maximum between 50 and 150 m. A shear layer separates the subsurface seasonal jet and the surface jet forced by the MJO. The sea surface elevation response and upper ocean heat content show wind generated westward propagating Rossby waves symmetric around the equator and an associated eastward propagating equatorial Kelvin wave response. After the third MJO event, the Yoshida jet spans most of the equatorial Indian Ocean. Upon reaching the Indonesian coast, the associated equatorial Kelvin wave reflects and generates additional westward propagating equatorial Rossby waves. The volume transport associated with these waves causes the westward advection of low-salinity north and south of the equator, impacting the tropical ocean circulation.

Remote Ocean Response to the Madden–Julian Oscillation during the DYNAMO Field Campaign: Impact on Somali Current System and the Seychelles–Chagos Thermocline Ridge

“…This distribution of zonal current anomaly is consistent with the spatial pattern of westerly winds, in which the stronger westerlies are found in the eastern Indian Ocean in late December ( Figure 5, right panel), [2]. Note that the eastward propagation of zonal current can be detected, but it is not as clear as that of SSH because equatorial zonal currents often include other large signals such as Rossby waves and Yoshida jet [33,36].…”

“…Nevertheless, the model is able to simulate observed thermocline variability reasonably well. Westerly winds associated with the MJO events during the field campaign generated strong equatorial jets [2,5,10,33]. In order to validate the simulation of equatorial jets, the model surface zonal currents are compared with the RAMA and DYNAMO mooring data in the central equatorial Indian Ocean (Figure 3).…”

Modeling Salinity Exchanges Between the Equatorial Indian Ocean and the Bay of Bengal

“…SSS variability in the northern BoB is influenced by significant freshwater contributions from to the equatorial region. When eastwardflowing equatorial jets associated with intraseasonal oscillations are present, for instance the westerly wind bursts during the active phase of the MJO (e.g., Jensen et al, 2015), these intrusions into the BoB can take place (Wijesekera et al, 2015).…”

“…Jensen et al, 2015). Very high vertical resolution is needed to resolve the thin surface freshwater lenses that result from river plumes and intense rainfall, which drive formation of a barrier layer between the mixed layer and the thermocline.…”

Coupled ocean–atmosphere modeling and predictions

“…High vertical resolution is needed when coupled to an atmospheric model to capture freshwater lenses from precipitation and the diurnal cycle during calm conditions. To model the diurnal cycle of SST during the inactive phase of the MJO, a 0.5 m resolution in the upper 10 m was used Jensen et al 2015), and fluxes between the models were exchanged every 6 min using an ESMF coupler. An additional challenge is the formation of high density (cold and saline) water under growing sea ice on the shelf areas in the Arctic.…”