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Interannual variations of both the Baiu precipitation and tropical cyclone (TC) activity in the western North Pacific (WNP) are controlled by large-scale atmospheric circulations associated with the El Niño/Southern oscillation (ENSO) and the tropospheric biennial oscillation (TBO) of the Asian monsoon. This work examines covariability between the Baiu precipitation and the TC activity in the WNP through the ENSO and the TBO. In years when sea surface temperature anomalies (SSTAs) are negative in the eastern tropical Pacific with respect to the ENSO, the number of TCs increases near the Philippines in the Baiu season, June and July. On the other hand, in years of negative SSTAs in the eastern tropical Pacific related to the TBO, the strength of TCs is enhanced to the southeast of Japan. Each of these two TC activities enhances a cyclonic circulation there, which shifts the axis of monsoon southwesterlies and contributes to form the peculiar Baiu precipitation anomalies. These modifications are, however, dependent on the phase of the ENSO and the TBO. In years of positive SSTAs in the eastern tropical Pacific, the anomalous TC activity is small and sometimes has opposing effects on the atmospheric circulations of the ENSO and TBO. Thus, the covariability is strong between the Baiu precipitation and TC activity in the WNP in the specific phase of the ENSO and the TBO, although both covary in large-scale atmospheric circulations. region. To manage water resources and mitigate the impacts of natural disasters associated with the Baiu precipitation, countries in EA need to improve their prediction scheme for the Baiu frontal activity more comprehensively. Many studies have examined the physical mechanisms of the interannual variability in the Baiu frontal activity (e.g., Tanaka 1997; Kawamura and Murakami 1998; Chang et al. 2000a, 2000b; Tomita et al. 2004; Yamaura and Tomita 2011). Several studies have investigated correlation between the Baiu precipitation and El Niño/Southern oscillation (ENSO; Rasmusson and Carpenter 1982), which is established while the ENSO, with its periodicity of 3 to 4 years, affects the global climate. Tanaka Corresponding author and present affiliation: Tsuyoshi Yamaura, RIKEN Advanced Institute for Computational Science, 7-1-26, Minatojima-minami-machi, Chuo-ku, Kobe, Hyogo, 650-0047, Japan. E-mail: info@spherewind.com ©2012, Meteorological Society of Japan Journal of the Meteorological Society of Japan Vol. 90, No. 4 450 (1997) showed that in onset years of El Niño (La Niña), the Baiu precipitation tended to increase (decrease). Tomita et al. (2004) and Yamaura and Tomita (2011) examined more detailed spatial patterns of the Baiu precipitation anomalies formed by the Pacific– East Asian (PEA) teleconnection of the ENSO (Wang et al. 2000). The tropospheric biennial oscillation (TBO) of the Asian monsoon (Meehl 1987, 1993, 1997) also modifies the interannual variations in Baiu precipitation. Although the sea surface temperature anomalies (SSTAs) associated with the TBO are similar to those of the ENSO (e.g., Chang and Li 2000; Meehl and Arblaster 2002; Kawamura et al. 2003; Meehl et al. 2003; Li et al. 2006), they have a different spatial phase which is shifted by approximately π/2 in the WNP, thus enabling the specific PEA-like teleconnection (Tomita et al. 2004). Anomalous meridional circulations associated with the SSTAs alter precipitation, particularly in the western part of the Baiu front between 110°−140°E (Yamaura and Tomita 2011). Shen and Lau (1995) reported that interannual variation in precipitation over China, relating to the western Baiu precipitation, displayed a strong biennial tendency caused by an anomalous anticyclone near the Philippines and the modification of water vapor flow. Many researchers have investigated the relationship between the ENSO and TC activity in the WNP (e.g., Chan 2000, 2007; Wang and Chan 2002; Camargo and Sobel 2005). For instance, Chan (2000) and Camargo and Sobel (2005) revealed that in the boreal fall when an El Niño event developed, the number of TCs tended to decrease in the South China Sea, but increase in the eastern part of the western tropical Pacific (0°−20°N, 140°E−180°). Wang and Chan (2002) also noted that TCs in the WNP tend to occur at lower latitudes, move farther northward, and have longer lifetime during an El Niño event than at the other times. In the years following an El Niño event, TC activity is suppressed from January to July. With a similar distribution of SSTAs and anomalous atmospheric circulations, the TBO probably alters TC activity in the WNP, although this still remains to be confirmed. The TCs in the EA−WNP sector have potential for changing the strength and spatial distribution of the Baiu precipitation as they approach the Baiu front. However, the TCs may not influence the total Baiu precipitation because the number of TC geneses is not large in the WNP during June−July (6 TCs on average) and therefore the effects are restricted in time and space. This work examines this covariation in more detail, that is, if event-like TCs in the WNP significantly modify the amount and distribution of the Baiu precipitation in a year by diagnosing covariability between the TC activity and the Baiu precipitation from the large-scale point of view. The interannual tendency is particularly investigated in large-scale atmospheric variations associated with the ENSO and the TBO. The aim of this work is to understand the interannual variability in Baiu precipitation more comprehensively. The specific targets are summarized in Fig. 1, which illustrates the flows diagnosed in this work. We primarily re-examined whether large-scale atmospheric circulations associated with the ENSO and the TBO control the interannual variations of the Baiu precipitation. We then reveal how the ENSO and the TBO modify TC activity in the WNP during the Baiu season. Finally, we evaluated covariability between the Baiu precipitation and the TC activity in the WNP through the large-scale atmospheric circulations modified by the ENSO and the TBO. The rest of this paper is structured as follows. Section 2 presents data and methodologies used in this work. Section 3 demonstrates how the ENSO and the TBO modify the Baiu precipitation on interannual time scales. Section 4 discusses the interannual variability of TC activity in the WNP with respect to the ENSO and the TBO. Section 5 examines how large-scale atFig. 1. Schematic flow diagram showing the relationship among the Baiu precipitation, TC activity, and large-scale atmospheric circulations associated with the ENSO and the TBO. Covariability between the Baiu precipitation and TC activity illustrated by a dotted line is mainly examined in this work. Numbers in a circle indicate the flow of discussions in this work. T. YAMAURA and T. TOMITA August 2012 451 mospheric circulations modified by the ENSO and the TBO connect the Baiu precipitation anomalies and the anomalous TC activity in the WNP. Finally, a summary of this work and further discussions are given in Section 6. 2. Data and methodology This work uses data from the following five datasets: (1) the Global Precipitation Climatology Project (GPCP) monthly data for precipitation rate, which are set on global grids with a 2.5° interval (Adler et al. 2003); (2) the Japanese 25-year Reanalysis (JRA-25)/ Japan Meteorological Agency Climate Data Assimilation System (JCDAS) 6-hourly and monthly data for atmospheric parameters covering the globe with a spatial resolution of 1.25° (Onogi et al. 2007); (3) the Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) data edited by the United Kingdom Meteorological Office for monthly SST, which cover almost the entire globe with 1° grids (Rayner et al. 2003); (4) daily data of global outgoing long-wave radiation (OLR) compiled by the National Oceanic and Atmospheric Administration (NOAA) with a 2.5° grid interval (Liebmann and Smith 1996); and (5) 6-hourly data of TCs in the WNP (100°E−180°, 0°−50°N) collected by the Joint Typhoon Warning Center (JTWC). This work uses the location and maximum wind speed of all TCs, which includes tropical depressions whose maximum wind speed is less than 17.2 m s-1. To diagnose atmospheric circulations associated with interannual variations in the Baiu precipitation, this work employs data collected during the Baiu season (June and July) for a time period from 1979 to 2008 (30 years). In general, the existence of TCs has a large impact on daily mean atmospheric circulations. However, these impacts may be small when considering monthly mean atmospheric circulation because TCs occur infrequently and therefore their impacts are diluted over the longer time frame. To evaluate the effect of TCs in a region, we estimated the number of TCs (NTC) and the accumulated cyclone energy (ACE; Bell et al. 2000; Camargo and Sobel 2005). NTC is simply formulated as follows: NTC N

Covariability between the Baiu Precipitation and Tropical Cyclone Activity through Large-Scale Atmospheric Circulations