Abstract
Abstract The large scale tropical circulation, commonly named the Hadley
circulation, is a key element in the global heat and moisture transport.
Traditionally it is defined as the meridional circulation of the zonally
averaged flow in the tropics, but in recent years studies have shown the
importance of looking at the decomposition of the three-dimensional
atmospheric flow into local meridional and zonal circulations. These
studies gave useful analysis on the regionality and variability of the
meridional circulation in different time scales, but were mostly limited
to examining the regional strengthening/weakening of the circulation.
Here we study the interannual variability of the
longitudinally-dependent meridional circulation (LMC), with a focus on
its zonal shift. We use hierarchical clustering to objectively determine
the 5 main modes of the LMC interannual variability, and apply a
Lagrangian air parcel tracking method to reveal the detailed patterns of
the circulation. We find that the most prominent interannual variability
of the LMC is an east-west shift, which plays a dominant role in the
overall interannual variability of the tropical circulation. In
addition, the LMC variability is found to be strongly related to other
atmospheric variables such as the sea surface temperature, precipitation
and air temperature. Using multiple linear regression we analyze these
dependencies and discuss their implications for the tropical climate
system. We also relate the LMC interannual variability to the
Madden-Julian Oscillation (MJO) and find that the 2 La-Nina related
modes are significantly correlated with 2 different MJO phases.