Fig. 6: Mass stream function anomalies of overturning circulation for Boreal Summer Intraseasonal Oscillation propagation modes.

For the days 0–5 after initiation in the equatorial Indian Ocean (EIO), the overturning circulation structure is analyzed. The first column (a, e, i) shows the composited zonal circulation, meridionally averaged from 0° S to 10° N, the second column (b, f, j) the zonally dependent circulation \({\bar{{{\Psi }}}}_{u}\) averaged between 400 hPa and 600 hPa. The color shading denotes the mass stream function in zonal direction. Red (blue) indicates irrotational lower-level easterlies (westerlies) and upper-level westerlies (easterlies). The third column (c, g, k) displays the zonally averaged (between 70^∘E-80^∘E) meridional circulation in the Central Indian Ocean. The fourth column (d, h, I) depicts the meridionally dependent circulation \({\bar{{{\Psi }}}}_{v}\) averaged between 400 hPa and 600 hPa. The color shading denotes the mass stream function in the meridional direction. Here, red (blue) indicates irrotational lower-level northerlies (southerlies) and upper-level southerlies (northerlies). The wind fields in the zonal (meridional) circulation plots are estimated using the meridionally (zonally) averaged u (v) components, measured in m/s, and the vertical velocity w in the horizontal direction, measured in hPa/s. For visual clarity, only every 3rd wind arrow is plotted. Stipples denote anomalies that are significant at a 95% confidence level using Student’s t-test.