Cycle 24 Onset: Why more delayed than predicted?
Mausumi Dikpati [dikpati@ucar.edu], Giuliana de Toma, Roger K. Ulrich, and Peter A.
Gilman, High Altitude Observatory (HAO), National Center for Atmospheric Research
(NCAR), Boulder, Colorado
In 2004, Dikpati and colleagues predicted about 12-18 months delay in the onset of cycle
24, by incorporating into a flux-transport dynamo model the then available information of
slow-down of meridional flow during 1996-2003. We now have more detailed information
about the latitudinal pattern and time variation of the surface meridional flow during cycle 22
and 23 from Mt. Wilson. These observations show that, during most of cycle 22 the
poleward surface meridional flow had a maximum at low latitudes (~30 degrees), with either
a weak flow or a reverse cell at high latitudes, whereas in cycle 23 the poleward flow had its
maximum at higher latitudes (>50 degrees). From these two distinct patterns, we can infer
that in cycle 22 the circulation had a shorter pathway to the bottom of the convection zone
compared to cycle 23. Given that in spherical coordinates a poleward surface flow with a
maximum speed at high-latitudes will carry a smaller mass towards the pole compared to a
flow that peaks at low latitudes, the equatorward return-flow will be reduced accordingly.
We demonstrate that the longer conveyor belt together with reduction in equatorward returnflow is consistent with a 24-month delay in cycle 24 onset. When the meridional flow is
maximum at high latitudes, an increase in flow speed causes a faster transport of and polar
convergence effects on fields coming from the follower spots producing a polar field
increase. On the other hand, if the meridional flow is maximum at low latitudes – as in the
surface flux-transport model of NRL scientists – an increase in poleward flow speed gives
faster transport of both leader and follower spots and enhanced self-annihilation, reducing the
polar fields. Thus, the NRL surface flux-transport model and our flux-transport dynamo
model are NOT IN CONFLICT.
The National Center for Atmospheric Research is sponsored by the National Science Foundation. This
work is partially supported by NASA grant NNX08AQ34G.