Six ALAMO floats, two EM-APEX floats, and a buoy were deployed in northwest

Australia on 22nd Nov 2018 (Feng et al. 2020). In the middle of December 2018, two

ALAMO floats, and two EM-APEX floats remained and recorded the ocean temperature,

salinity, and horizontal velocity during one MJO event when was from 14th to 18th Dec.

These data sets aided us in exploring the upper ocean response to the MJO and heat flux

variation to the MJO’s deep convection.

MJOs are typically associated with a westerly wind. During the MJO, the wind speed

rose to around 10 m s-1. In the upper ocean, the current velocity forced by the wind

increased up to 0.4 m s-1 above 40 m. At the same time, an upper ocean and SST cooling

event were observed since 14th Dec. The temperature in the upper 40 m dropped about

1.1 °C in one day. The cold water below the MLD should entrain into the upper ocean

and cool the SST near the sea surface. Due to the drop in SST, the difference between air

temperature and SST changed. In addition, the wind speed also favored evaporation. As

a result, air-sea heat flux was modified to rise from 100 Wm-2 to 400 Wm-2.

The MLD was deepened from 25 m to 50 m during this MJO period. By using the

float measurements, the buoyancy frequency was computed and was frequently less than

zero within the MLD. Additionally, the vertical shear was up to 3 3×104 N m-2. These

allowed the Richardson number to be less than 0.25, implying the upper ocean was

unstable. Using the Thorpe scale method, the turbulent dissipation rate was estimated by

10-8 to 10-6 W kg-1. These were larger than those within the typical thermocline. Thus,

strong turbulent mixing happened via shear instability during the MJO, resulting in MLD

deepening. This process allowed the cold water below the MLD to be entrained into the

upper ocean, resulting in SST cooling.

We used KPP in one-dimensional ROMS to simulate the MLD deepen under the buoy

wind measurement. In the model, MLD did not consist of observation MLD and did not

change significantly in the first two MJO days. Sensitive tests on the effect of vertical

resolution and mixing parameters were used to find the critical factor in simulating

turbulent mixing under MJO. By tuning the turbulence simulation parameter, increasing

the critical bulk Richardson number significantly affected the MLD simulation in that

higher critical bulk Richardson number was able to reach a deeper KPP boundary layer.

Comparing the current velocity between the observations and model, the mean

current velocity in the upper 20 m was 0.2 m s-1 and 0.1 m s-1, respectively. The difference

in current acceleration rate will result in different wind stress fluxes. The drag coefficient

is adjusted manually to perform the sensitive test, so different magnitudes of surface wind

stress will be used for forcing the ocean current. According to the model simulations, the

simulated results of MLD do not agree well with the observations unless 1.8 times of

wind drag coefficient is used. The momentum flux between the observations and model

results might be different. As a result, the linear momentum budget was used to estimate

wind stress. The results demonstrated that the difference between the observation and

model was about 0.2 N m-2 and 0.075 N m-2 in time rate change on velocity and Coriolis

force, respectively. Thus, the wind stress estimated by COARE 3.0 may be

underestimated in ROMS, resulting in the MLD being inconsistent with observation.

Even considering the buoyancy effect by tuning the heat flux with ± 50 W m-2 as the

uncertain wind speed and temperature variation, the effect is just about 2 m discrepancies.

Despite the fact that buoyancy allows MLD to get deepen. However, the wind stress effect

is more significant than buoyancy in MLD simulation under the MJO wind field.

Therefore, COARE 3.0 still underestimated wind stress on MLD simulation under MJO

when considering the buoyancy effect on turbulence simulation.

In summary, the high wind speeds during the active phase of MJOs trigger the ocean

current and thereby larger vertical shear for the growth of turbulence. The vertical velocity

shear should destabilize the upper ocean during the MJO. The turbulence mixing at the

base of MLD allows the cold water entrains to upper and cool down the SST. Due to the

SST variation, heat flux is modified and may influence the MJO convection developments.

In the model, wind stress estimation is the critical factor in MLD simulation. The wind

stress scheme from the COARE 3.0 algorithm in ROMS should be underestimated during

the MJO, which will result in the failure of the MLD simulation well. The consequence

of MLD failing to simulate may affect MJO simulation and forecast in the model. As a

result, field measurements on ocean current and wind speed are able to explore the

momentum flux between air-sea during MJOs. The correct wind stress estimation may

also improve the MJO forecast by being used in the global coupled model in the future.


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