Western North Pacific Summer Research Articles

Contrast of Rainfall–SST Relationships in the Western North Pacific between the ENSO-Developing and ENSO-Decaying Summers*

Abstract While the overall summer rainfall–sea surface temperature (SST) relationship has a negative correlation over the western North Pacific (WNP), this relationship experiences a significant interannual variation. During the ENSO-developing (decaying) summer, the rainfall–SST correlation is significantly positive (negative). The positive correlation is attributed to interplay between the anomalous Walker circulation and the cross-equatorial flows associated with the enhanced WNP summer monsoon. The former leads to negative rainfall anomalies in the western Pacific, whereas the latter leads to a cold SST anomaly resulting from enhanced surface latent heat fluxes. The negative correlation is attributed to the maintenance of an anomalous Philippine Sea anticyclone from the El Niño peak winter to the subsequent summer. The anomalous anticyclone, on one hand, suppresses the local rainfall, and on the other hand induces a warm in situ SST anomaly through both the enhanced solar radiation (resulting from a decrease in clouds) and the reduced surface latent heat flux (resulting from the decrease of the monsoon westerly). The rainfall–SST correlation is insignificant in the remaining summers. Thus, the overall weak negative rainfall–SST correlation is attributed to the significant negative correlation during the ENSO-decaying summers.

Decadal change in relationship between east Asian and WNP summer monsoons

It has been recognized that the intensity of the east Asian (EA) summer monsoon has a negative correlation with that of the western North Pacific (WNP) summer monsoon. Here we show that this relationship is much stronger in the recent decade (1994–2004) than in the epoch before 1994 (1979–1993). The first two leading modes of summer‐mean precipitation over the large region of the WNP and EA region are shown to be associated with two factors: the ENSO development and the WNP summer monsoon fluctuation. The leading mode has changed from an ENSO‐related mode in 1979–1993 to a WNP summer monsoon‐related mode in the recent decade (1994–2004). The summer‐mean mid‐tropospheric geopotential heights that are correlated with the WNP monsoon index also show a marked change in the teleconnection (wave‐train) pattern between 1994–2004 and 1979–1993. All together this evidence suggests that the relationship between the EA and the WNP summer monsoons has experienced a significant decadal change around 1993–1994.

Interannual Variability of the Western North Pacific Summer Monsoon: Differences between ENSO and Non-ENSO Years

Abstract The interannual variability of the western North Pacific (WNP) summer monsoon is examined for the non-ENSO, ENSO developing, and ENSO decaying years, respectively. The ENSO developing (decaying) year is defined as the year before (after) the mature phase of ENSO, and the non-ENSO year is defined as the year that is neither the ENSO developing year nor the ENSO decaying year. A strong (weak) WNP summer monsoon tends to occur during the El Niño (La Niña) developing year and a weak (strong) WNP summer monsoon tends to occur during the El Niño (La Niña) decaying year. In all non-ENSO, ENSO developing, and ENSO decaying years, the strong (weak) WNP summer monsoon is associated with the positive (negative) rainfall anomalies, cold (warm) sea surface temperature anomalies, warm (cold) upper-tropospheric temperature anomalies, low (high) surface pressure anomalies, and a low-level cyclonic (anticyclonic) circulation anomaly over the subtropical WNP. The 850-hPa wave train associated with the WNP and east Asian (EA) summer monsoons in the non-ENSO, ENSO developing, and ENSO decaying years extends northward and suggests a possible teleconnection between the WNP summer monsoon and the North American climate. The wave train extended into the Southern Hemisphere in the non-ENSO and ENSO developing years implies a teleconnection between the WNP summer monsoon and the Australian winter climate. The anomalous WNP monsoon in the non-ENSO and ENSO developing years exists only in summer, while the anomalous WNP monsoon in the ENSO decaying year persists from the beginning of the year to the summer season. The anomalous WNP summer monsoon exhibits a strong ocean–atmosphere interaction, especially in the ENSO decaying year. This study suggests that the anomalous WNP summer monsoon in the non-ENSO year is associated with the variation of the meridional temperature gradient in the upper troposphere, while the anomalous WNP summer monsoon in the ENSO developing and decaying years is associated with ENSO-related SST anomalies.

Rainy Season of the Asian–Pacific Summer Monsoon*

To date, the monsoon-research community has not yet reached a consensus on a unified definition of monsoon rainy season or on the linkage between the onsets over the Asian continent and the adjacent oceans. A single rainfall parameter is proposed, and a suite of universal criteria for defining the domain, onset, peak, and withdrawal of the rainy season are developed. These results reveal a cohesive spatial–temporal structure of the Asian–Pacific monsoon rainy season characteristics, which will facilitate validation of monsoon hydrological cycles simulated by climate system models and improve our understanding of monsoon dynamics. The large-scale onset of the Asian monsoon rainy season consists of two phases. The first phase begins with the rainfall surges over the South China Sea (SCS) in mid-May, which establishes a planetary-scale monsoon rainband extending from the south Asian marginal seas (the Arabian Sea, the Bay of Bengal, and the SCS) to the subtropical western North Pacific (WNP). The rainband then advances northwestward, initiating the continental Indian rainy season, the Chinese mei-yu, and the Japanese baiu in early to mid-June (the second phase). The heights of the rainy seasons occur primarily in three stepwise phases: in late June over the mei-yu/baiu regions, the northern Bay of Bengal, and the vicinity of the Philippines, in late July over India and northern China; and in mid-August over the tropical WNP. The rainy season retreats northward over east Asia, yet it moves southward over India and the WNP. Clear distinctions in the characteristics of the rainy season exist among the Indian, east Asian, and WNP summer monsoon regions. Nevertheless, the rainy seasons of the three subsystems also show close linkage. The causes of the regional distinctions and linkages are discussed. Also discussed are the atypical monsoon rainy seasons, such as the skewed and bimodal seasonal distributions found in various places of Asian monsoon domain.