Abstract
The Indian Ocean Dipole (IOD) is an interannual climate mode of the tropical Indian Ocean. Although it is known that negative sea surface temperature (SST) anomalies in the eastern pole during the positive IOD are stronger than positive SST anomalies during the negative IOD, no consensus has been reached on the relative importance of various mechanisms that contribute to this asymmetry. Based on a closed mixed layer heat budget analysis using a regional ocean model, here we show for the first time that the vertical mixing plays an important role in causing such asymmetry in SST anomalies in addition to the contributions from the nonlinear advection and the thermocline feedback proposed by previous studies. A decomposition of the vertical mixing term indicates that nonlinearity in the anomalous vertical temperature gradient associated with subsurface temperature anomalies and anomalous vertical mixing coefficients is the main driver of such asymmetry. Such variations in subsurface temperature are induced by the anomalous southeasterly trade winds along the Indonesian coast that modulate the thermocline depth through coastal upwelling/downwelling. Thus, the thermocline feedback contributes to the SST asymmetry not through the vertical advection as previously suggested, but via the vertical mixing.
Highlights
The Indian Ocean Dipole (IOD)[1] is a dominant interannual climate mode of the tropical Indian Ocean that develop through the Bjerknes feedback[2]
Those over the western pole is underestimated compared to the observation especially in the positive IOD (pIOD), the zonal dipole structure and asymmetric amplitude of sea surface temperature (SST) anomalies associated with the IOD are well reproduced
We have investigated the asymmetry in SST anomalies over the eastern pole of the IOD for the first time based on a completely closed mixed layer heat budget analysis using a regional ocean model that realistically reproduces the main features the IOD
Summary
The Indian Ocean Dipole (IOD)[1] is a dominant interannual climate mode of the tropical Indian Ocean that develop through the Bjerknes feedback[2]. Understanding of the mechanisms responsible for the asymmetry is crucial In this regard, the first study on the cause of asymmetry in SST anomalies over the eastern pole of the IOD emphasized the importance of the nonlinear zonal and vertical advection based on a mixed layer heat budget analysis[11]. The first study on the cause of asymmetry in SST anomalies over the eastern pole of the IOD emphasized the importance of the nonlinear zonal and vertical advection based on a mixed layer heat budget analysis[11] They pointed out that the nonlinear zonal advection term leads to anomalous cooling during both pIOD and nIOD; negative zonal SST gradient anomalies are advected by westward current anomalies during the pIOD, while positive zonal SST gradient anomalies are advected by eastward current anomalies during the nIOD. It was pointed out that such discrepancies are due to SST biases in the ocean assimilation product and spurious trends in precipitation seen in the atmospheric reanalysis data used by previous s tudies[16]
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