Extended Data Fig. 8: Seasonal longitudinal shift in the Walker circulation due to the distance effect.
From: Two annual cycles of the Pacific cold tongue under orbital precession
The difference between the distance-only run and zero annual forcing run (former minus latter) for various climate fields averaged over March–June (following aphelion) in the left column, and September–December (following perihelion) in the right column. (a-b) Precipitation (shaded) and wind stress (vectors). (c-d) Zonal overturning circulation at the equator displayed as vectors, with the x-component being the divergent component of the zonal wind (in m/s) averaged 10°S-10°N and y-component being the pressure vertical velocity (in Pa/s) multiplied by 250, also averaged 10°S-10°N. The green bar in (c-d) indicates the approximate longitudes of the Maritime Continent. (e-f) 200mb velocity potential. (g-h) Surface pressure. The precipitation in panels (a-b) show a shift in the location of equatorial rainfall between the Maritime Continent and western equatorial Pacific between March–June and September–December, associated with changing equatorial trades over the western equatorial Pacific. The zonal overturning circulation in panels (c-d) show anomalous subsidence over the Maritime continent and anomalous uplift over the western equatorial Pacific in March–June, indicating an eastward shift in the main uplift region of the Walker circulation; the opposite occurs for September–December. The velocity potential change in panels (e-f) shows a predominantly zonal wavenumber 1 pattern with the nodal point over the Maritime continent, reversing in sign between March–June and September–December. The surface pressure change in panels (g-h) show a see-saw in atmospheric mass between Africa/Indian ocean and the Pacific, again with the nodal point at the Maritime continent. Thus, all fields shown are consistent with a seasonal longitudinal shift of the Walker circulation towards the east in March–June and towards the west in September–December. The M_Map package41 is used to generate the maps, using coastline data from the Global Self-consistent, Hierarchical, High-resolution Geography Database42.
