Fig. 2: Heat budget analysis showing the processes contributing to the pattern asymmetry between extreme IOD events.
a Averaged MLT tendency (\({T}_{t}^{\prime }\)) in JA (°C month−1) for EXpIOD events. b Same as (a), but for EXnIOD events. c Spatially asymmetric component between (a) and (b), which is defined as the difference between the averaged EXpIOD and the reversed EXnIOD [EXpIOD − ( − EXnIOD)] (see the ‘Mixed-layer heat budget analysis’ section in 'Methods'). The meridional dipole represents the residuals of EXpIOD and EXnIOD events after cancelling each other out. The cooling tendency off Sumatra and the warming tendency off Java are highlighted by the northern black box (a parallelogram-shaped region extending 5° offshore from the coast of Sumatra) and the southern black box (105°–115°E, 12°–8°S), respectively. d Asymmetric components of heat budget terms averaged over the northern (southern) box in c are represented as the blue bars (red bars), and the green bars are calculated as their difference (blue minus red). Therefore, the green bar measures the contrast in the meridional dipole. Nonlinear advection (NA) denotes the summation of three-dimensional nonlinear oceanic advections (\(-{u}^{\prime }{T}_{x}^{\prime }\), \(-{v}^{\prime }{T}_{y}^{\prime }\) and \(-{w}^{\prime} {T}_{z}^{\prime }\)) by anomalous currents advecting SSTA, and linearised advection (LA) denotes the summation of three-dimensional linearised advection terms due to anomalous currents advecting mean SST and mean currents advecting SSTA (\(-{u}^{\prime} {\overline{T}}_{x}\), \(-\overline{u}{T}_{x}^{\prime }\), \(-{v}^{\prime }{\overline{T}}_{y}\), \(-\overline{v}{T}_{y}^{\prime }\), \(-{w}^{\prime} {\overline{T}}_{z}\) and \(-\overline{w}{T}_{z}^{\prime }\)). The LA terms generally show mirror images between phases with opposite signs, so that their magnitudes decrease remarkably after extracting the asymmetric components. On the contrary, the NA terms have the same sign for both phases, making their roles stand out. e Decomposition of NA into horizontal and vertical advection terms. f Asymmetric components of the LHF (\({Q}_{{lh}}^{\prime }\)).
