Geoscience Reference
In-Depth Information
Generally, the onset of the SCSSM is accompanied by a well-defined
vertical southward-tilting moisture structure in the southern and central
SCS in early and middle May (Fig. 2). In 2003, the enhanced convective
activities occur in middle May in the central SCS around 12
◦
N-15
◦
N
(Fig. 2(b)), signaling the onset of the SCSSM. Then, the rainfall maximum
moves northward in late May (Fig. 2(c)). In 2004, strong rainfall starts in
the southern SCS in early May (Fig. 2(d)), and then the rainfall maximum
moves northward in middle and later May (Figs. 2(f) and (g)). In 2005, the
convective activities in May are much weaker compared to those in 2003
and 2004, but the northward propagation of the convective activities from
the southern to northern SCS is still seen clearly.
The most important feature depicted in Fig. 2 is that the rainfall
maximum is preceded by water vapor maximum in the atmospheric
boundary layer (ABL, around 900-1000 hPa) and is followed by a dry phase.
The former is overlapping with easterlies, and latter with westerlies. In
2003, the vapor maximum in the low-to-middle troposphere (850-500 hPa)
is clearly concurrent with the rainfall maximum (Figs. 2(a) and (b)). But
near the surface the water vapor maximum occurs in the front of the rainfall
maximum and is confined in the easterly prevailing region. The feature can
also be clearly identified in 2004 and 2005 (Figs. 2(d), (e), (g), and (h)).
This indicates that the enhanced moisture accumulation in the ABL in front
of the monsoon convection plays an active role in leading the northward
movement of the convection.
In order to identify what causes an enhanced moisture accumulation
in ABL in front of the monsoon convection, we plot the south-north
distributions of the AIRS humidity at 1000 hPa, TMI SST, ECMWF
analyzed latent heat flux, QuikSCAT wind divergence, and TMI rain rate
along 105
◦
E-120
◦
E during early, middle, and late May. Figure 3 presents
the result in 2003 and the following features are noteworthy:
(1) The 1000 hPa humidity experiences a sharply decrease from early to
middle May (Fig. 3(a), thin solid and dashed lines), when strong
monsoon rainfall occurs in the southern SCS (south of 10
◦
N). But it
recovers in late May (Fig. 3(a) thick solid line), as the monsoon rainfall
moves into the northern SCS with maximum in the region of 10
◦
N-
15
◦
N. In the northern SCS (north to 10
◦
N), the decrease of humidity
in ABL is coincident with a maximum rainfall moving from the southern
SCS (Figs. 3(a) and (e), thick solid line).
(2) A decrease of SST occurs from early to late May in the southern
SCS is concurrent with a rapid increase of SST in the northern SCS
