Abstract
During the CINDY/DYNAMO field campaign, exceptionally large upper ocean responses to strong westerly wind events associated with the Madden–Julian oscillation (MJO) were observed in the central equatorial Indian Ocean. Strong eastward equatorial currents in the upper ocean lasted more than one month from late November 2011 to early January 2012. The remote ocean response to these unique MJO events are investigated using a high resolution (1/25°) global ocean general circulation model along with the satellite altimeter data. The local ocean response to the MJO events are realistically simulated by the global model based on the comparison with the data collected during the field campaign. The satellite altimeter data show that anomalous sea surface height (SSH) associated with the strong eastward jets propagated eastward as an equatorial Kelvin wave. The positive SSH anomalies then partly propagate westward as a reflected Rossby wave. The SSH anomalies associated with the reflected Rossby wave in the southern hemisphere propagate all the way to the western boundary. These remote ocean responses are well simulated by the global model. The analysis of the model simulation indicates the significant influence of reflected Rossby waves on sub-seasonal variability of Somali current system near the equator. The analysis further suggests that the reflected Rossby wave causes a substantial change in the structure of the Seychelles–Chagos thermocline ridge, which contributes to significant SST anomalies.
Highlights
The Cooperative Indian Ocean experiment on intraseasonal variability (CINDY)/Dynamics of the MJO (DYNAMO) international field campaign was conducted in boreal fall 2011 through winter 2011/2012 with the goal of expediting our understanding of the physical and dynamical processes key to Madden–Julian Oscillation (MJO) initiation in the Indian Ocean [1]
While large variations of thermocline depth are not observed during this period at these locations, occasional small changes are captured by the model
The maximum of upward Ekman pumping velocity is found around 6◦ S, 79◦ E–85◦ E during this period, which is mostly due to the strong westerly winds associated with the MJO. These analyses demonstrate that a meridional migration of Seychelles–Chagos thermocline ridge (SCTR) in the DYNAMO area is caused by a combination of annual Rossby wave and MJO-induced wind stress. These SCTR changes are not generated by the reflected Rossby wave such as those discussed in the previous section, the results suggest that intraseasonal variation of wind stress produced by the MJO can largely alter the SCTR structure
Summary
The Cooperative Indian Ocean experiment on intraseasonal variability (CINDY)/Dynamics of the MJO (DYNAMO) international field campaign was conducted in boreal fall 2011 through winter 2011/2012 with the goal of expediting our understanding of the physical and dynamical processes key to Madden–Julian Oscillation (MJO) initiation in the Indian Ocean [1]. Three episodes of large-scale convection associated with the MJO propagated eastward over the Indian Ocean. E.g., [1,2,3] These westerly wind events provide a significant source of fluxes of momentum and heat into the ocean, e.g., [4], causing large upper ocean responses including strong anomalous currents, changes of thermocline depth, and temperature. The upper ocean variability produced by these MJO events were well monitored by the arrays of surface mooring and ship observations in the central Indian Ocean during the intensive observing period of the field campaign (1 October 2011–15 January 2012; [1,3,5]). This study investigates remote ocean responses to the MJO events observed during the field campaign, in which the local upper ocean response was exceptionally strong
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