Theories on the Tropospheric Biennial Oscillation :
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
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.