73 Equatorial Superrotation at Earth's Surface

Monday, 26 June 2017
Salon A-E (Marriott Portland Downtown Waterfront)
Rodrigo Caballero, Stockholm Univ., Stockholm, Sweden; and H. Carlson

A growing body of recent work indicates that as tropical surface temperatures warm, the
amplitude of Madden-Julian Oscillation (MJO)-like variability increases. Since the MJO
converges zonal momentum onto the equator at upper levels, the result is a
high-temperature transition to equatorial superrotation--a state with westerly winds
along the equatorial upper troposphere. While this transition is dynamically
interesting, the effects of superrotation on Earth's climate will remain subtle and
indirect so long as the superrotation is confined to the upper troposphere. If
superrotation were to extend all the way to the surface, on the other hand, it would
lead to a radical reorganization of the tropical climate; in particular, surface
equatorial westerlies would presumably lead to a permanent El Nino state.

Here, we explore the conditions under which surface superrotation may occur, focusing on
the role of vertical momentum transport by convective motions (CMT). We assume that
lateral eddy momentum convergence peaks in the upper troposphere, as is the case for the
MJO. We present theoretical arguments to suggest that two different momentum balances
are possible: in the limit of weak CMT, eddy momentum convergence is balanced entirely
by Hadley cell transport, with zero surface wind; in the opposite limit, upper-level
eddy momentum convergence is transported to the surface and balanced by friction,
yielding surface superrotation. Experiments with an idealized axisymmetric model show
that both limits are indeed possible. Next, we explore the effects of artificially
increasing CMT in a full-complexity GCM which displays upper-level superrotation. We
find that a transition to surface superrotation fails to occur because the effect of
enhanced CMT is to decrease the MJO's amplitude and momentum convergence. We conclude
that surface superrotation is unlikely to be achieved so long as tropical variability is
dominated by waves which converge momentum predominantly in the upper troposphere.

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