Theories on the
Tropospheric Biennial Oscillation :
A Review
Tim LI. Bin w
ANG
Department 0£
Meteorology and International Paci£ic Research Center, University 0£ Hawaii.
Honolulu, Hawaii, USA
and C. -P. CHANG
Department 0£
Meteorology, Naval Postgraduate School. Monterey, Cali£ornia, USA
Abstract- The tropospheric Biennial Oscillation (TBO) is the most pronounced
component of climate vari- ability of the Asian-Australian Monsoon System. Two
essential observational features that need to be ex- plained by TBO theories
are the phase persistence from South Asian in boreal summer to northern
Australia in boreal wint~r and the phase reversal of the TBO from boreal winter
to next summer. In this review pa- per, we examine various TBO theories based
on the aforementioned two features. For convenience of dis- cussions, we
categorize these TBO theories into three groups, namely, local air-sea
interactions, remote ocean-atmosphere interactions in the tropics, and the
tropic&midlatitude teleconnection.
The local air-sea feedback mechanism proposed by
Nicholls and Meehl explains why SST anomalies change signs following a monsoon
or maximum convective season. However, it fails to explain the phase
persistence and seasonal progression of the TBO. Clarke et al. ' s work points
to the crLCial importance of a time lag ( 1 -3 months) between the local SST
and wind in sustaining a biennial oscillation. However, the physical argument
that winds respond to deep convective heating is inconsistent with the 1 -3
month time
lag.
To
explain the phase persistence of the TBO from northern summer to winter, Meehl
proposed an anomalous convection southeastward progression hypothesis. Three
premises are involved in this hypothe- sis. First, the convection associated
with the annual cycle must be locally strong during one season per year.
Second, SST anomalies in the equatorial western Pacific and Indian Ocean must
have the same sign. Third, the ocean must have long-Iasting memory through
three inactive seasons. Chang and Li, on the other hand, proposed that the
remote impact of the South Asian monsoon on the western Pacific/maritime conti-
nent SST through larg&scale east-west circulation is a key in linking the
Asian and Australian monsoons. While Meehl emphasized the effect of the eastern
Pacific SST on the phase reversal of the South Asian mon- soon, Chang and Li
excluded this effect by arguing that a small cold SST anomaly (order of -1 K)
associ- ated with TBO in the eastern Pacific is unlikely to pull air mass out
of the Asian monsoon region. Instead, they emphasized the importance of the
Walker circulation that allows the western Pacific/maritime continent region to
playa role in sustaining the Indian Ocean SST anomaly. The latter is
responsible for the phase re-
versal of the Asian monsoon.
The midlatitud&tropics
teleconnection owing to the remote impact of tropical forcing on midlatitude
cir- culation and land surface temperature may possibly cause the TBO phase
transition from northern winter to the following summer. SLCh a remote impact,
however, is complicated by midlatitude chaotic processes and is often uncertain
and model-dependent. Its validity deserves further investigations.