Biennial and Lower-Frequency Variability Observed in the Early Summer Climate in the Western North Pacific

Abstract

Early summer climate in the western North Pacific is largely represented by the baiu phenomenon. The meridional fluctuations of the baiu front on interannual time scales and the associated large-scale circulations are examined using the empirical orthogonal function (EOF) analysis and composite or correlation analyses based on the EOF time coefficients. The first EOF mode indicates a 5- or 6-yr low-frequency fluctuation (LF mode) appearing south of 35°N. The development is concurrent with horseshoe sea surface temperature anomalies (SSTAs) in the entire tropical Pacific that are associated with the El Niño–Southern Oscillation (ENSO). SSTAs in the western North Pacific control the anomalous southward expansion of the baiu front through a modification of the convection at around 20°–35°N. The LF mode is negatively correlated with the south-southeast Asian summer monsoon. The second EOF mode is characterized by a meridional seesawlike fluctuation with a node at around 28°N and a time scale of biennial oscillation (BO mode). The horseshoe SSTAs again control the anomalous meridional circulations, but with a different spatial phase through a convection off the Philippines. The spatial phase difference between the two horseshoe patterns is about 90° in both the zonal and meridional directions. The BO mode is negatively correlated with the tropical western North Pacific monsoon. SSTAs associated with the BO mode tend to be confined to the tropical western Pacific, while the signals of the LF mode extend rather broadly in the tropical Pacific–Indian Ocean sector, suggesting that the tropical BO is an aborted ENSO in the tropical central–western Pacific. The spatial phase of horseshoe SSTAs adjusts the interannual variability of the meridional fluctuation of the baiu front in the western North Pacific.

Interannual Variability of the Baiu Season near Japan Evaluated from the Equivalent Potential Temperature

“…The physical processes explaining these lag-correlations are not discussed here, but could be a question for future studies. The impacts of the ENSO, TBO, and NAO on Baiu precipitation have been examined in detail by Tomita et al (2004), Tomita et al (2010), and Yamaura and Tomita (2011).…”

“…Many previous works have investigated the interannual variability of the EA summer monsoon (EASM) with that of the M/B frontal activity (e.g., Lau 1992, Murakami and Matsumoto 1994, Chang et al 2000, Wang et al 2001, Tomita et al 2004, Yamaura and Tomita 2011. However, they mostly used monthly or seasonal mean data to demonstrate the spatiotemporal structure.…”

A quasi-biennial signal in western US hydroclimate and its global teleconnections

“…Significant correlation cells are located in regions where precipitation and/or pressure oscillations have been previously documented over the range of 2.1-2.2 years. These areas include the Arctic (Trenberth and Shin 1984;Venegas and Mysak 2000), the Aleutians (Angell et al 1969;Angell and Korshover 1974), the subtropical NW Pacific (Tomita and Yasunari 1996;Tomita et al 2004), Tasmania-New Zealand (Trenberth 1975(Trenberth , 1980, and the Pacific coast of Patagonia (Villalba et al 1998). These *26-month regional cycles appear to vary together with Western US winter precipitation as constituents of a single global-scale oscillation.…”

The North Pacic Subtropical High Characterized Separately for June, July, and August: Zonal Displacement Associated with Submonthly Variability

“…However, the relations are not clearly visible in Table 1 and might depend on how the index is defined. Tomita et al (2004) investigated the meridional fluctuations of the Baiu front on interannual timescales and the associated large-scale circulations and showed the existence of a biennial oscillation timescale. Fourier analysis of the NPSH index in Fig.…”

“…There is a possibility that the variability is an outcome of the tropical biennial oscillation (TBO) dominating in the western North Pacific (cf. Chang et al 2000, Tomita et al 2004). We used an index based on the interannual variability in the western part of the NPSH in northern summer because our focus was the difference in NPSH characteristics during its westward extension and eastward retreat in June, July, and August.…”