ALBERT

Description: During stratospheric sudden warming (SSW) periods large changes in the low latitude vertical drift have been observed at Jicarmarca as well as in other longitudinal sectors. In general a strengthening of the daytime maximum vertical drift with a shift from pre-noon to the afternoon is observed. During the January 2013 stratospheric warming significant longitudinal differences in the equatorial vertical drift were observed. At Jicarmarca the previously reported SSW behavior prevails, however no shift of the daytime maximum drift was exhibited in the African sector. Using the National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) the possible causes for the longitudinal difference are examined. The timing of the strong SSW effect in the vertical drift (15–20 January) coincides with moderate geomagnetic activity. The simulation indicates that approximately half of the daytime vertical drift increase in the American sector may be related to the moderate geophysical conditions (Kp=4) with the effect being negligible in the African sector. The simulation suggests that the wind dynamo accounts for approximately 50% of the daytime vertical drift in the American sector and almost 100% in the Africansector. The simulation agrees with previous findings that the migrating solar tides and the semidiurnal westward propagating tide with zonal wave number 1 (SW1) mainly contribute to the daytime wind dynamo and vertical drift. Numerical experiments suggest that the neutral wind and the geomagnetic main field contribute to the presence (absence) of a local time shift in the daytime maximum drift in the American (African) sector.