Interdecadal Variability in Large and Small Warm Pools in Western Pacific and Their Association with Rainfall Anomalies

ZHOU Lian-Tong

1 Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190, China

2 Key Laboratory of Global Change and Marine-Atmospheric Chemistry, Xiamen 361005, China

1 Introduction

The highest sea surface temperature (SST) in the global ocean surface is located over tropical western Paci fi c. Thus, it is known as “the warm pool” (Cornejo-Garrido and Stone, 1977; Nitta, 1987; Huang and Li,1988). Moreover, the air-sea interaction is very strong and the ascending branch of the Walker circulation lies over this region due to its warm state of this region. This leads to significant convergence of the air and moisture and results in strong convective activity and heavy rainfall in this region (Cornejo-Garrido and Stone, 1977; Hartmann et al., 1984). The summer rainfall anomalies in Southeast China have been analyzed extensively (Zhu et al., 2011;Feng et al., 2011; Li et al., 2011, 2012b; Li and Zhou,2012). Moreover, previous studies have shown that the air-sea flux impacts the summer rainfall in southern China(Wu et al., 1999; Ren et al., 2000; Li et al., 2012a). Huang and Sun (1994) suggested that the western Pacific warm pool impacted the climate variability through convective activity. Dong et al. (1994) studied the western Pacific warm pool’s impact on winter circulation variability. Li et al. (2004) examined the interdecadal and interannual characteristics of tropical western Pacific warm pool’s heat status. Wu and Zhang (2007) discussed the interdecadal shift the western North Pacific summer SST anomalies in the late 1980s by using the empirical orthogonal function(EOF) analysis. Besides, the fact that the tropical western Pacific’s thermal state also generally satisfies the thermal condition of tropical cyclone (TC) genesis. Every year,more than 10 TCs or typhoons make landfall in China,Korea, and Japan, which cause huge economic losses and death of several hundred people in these countries (Huang and Chen, 2007; Chen et al., 2012). Thus, the tropical western Pacific has an important thermal effect on local and remote climate variability. Other studies in the past have focused on SST in warm pool and their influence on climate variability (e.g., Wu and Zhang, 2007). However,these studies did not consider the large and small warm pools in western Pacific and their associated climate variability. Wang et al. (2006, 2007, 2008) pointed out that the large and small warm pools in northern Atlantic have a significant influence on climate variability. Therefore, the current study focused on interdecadal variability in large and small warm pools in western Pacific and their associated rainfall anomalies.

2 Datasets

In this study, monthly rainfall data was collected at 160 stations in China from 1958 to 2008. The data was provided by the Chinese Meteorological Data Center. The monthly mean wind, omega and special humidity were derived from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEPNCAR) reanalysis for the period of 1958–2008 (Kalnay et al., 1996; Kistler et al., 2001). The surface latent heat flux is derived from the Multidecade Global Flux Datasets from the Objectively Analyzed Air-Sea Fluxes (OAFlux)Project for the period of 1958–2008 (Yu et al., 2008).The global precipitation data were derived from the Global Precipitation Climatology Project (GPCP) for the period of 1979–2008. The monthly mean SST data were obtained from the Met Office Hadley Center for the period of 1958–2008 (Rayner et al., 2003).

3 Results

3.1 Interdecadal variability in the warm pool in western Pacific

In this study, the warm pool in the western Pacific was defined as the region covered by water warmer than 29.0°C, shown in Fig. 1a. This figure represents the climatological monthly means in SST around the world. The regions of water temperature above 29.0°C occurred over western Pacific and northern Atlantic. The warm temperature regions in the western Pacific were much bigger than that in northern Atlantic. Therefore, the climatological regional monthly means in SST were averaged for 0–20°N, 120–180°E (Fig. 1b). Figure 1b shows that there was a significant seasonal variation in SST in the warm pool in western Pacific. The SST anomalies were sharp upward during March–June and downward during October–February, with maximum values during June–October.

Figure 1 (a) Global JJASO (June–October) climatological monthly means in sea surface temperature (SST). The shading denotes regions with SST above 29°C. (b) The climatological monthly means in SST for the regional averaged 0–20°N, 120–180°E. (c) The large and small warm pools were calculated as the anomalies of the region of SST warmer than 29.0°C divided by the climatological warm pool region in JJASO.

Previous studies showed that the large and small warm pools in the Atlantic Ocean could impact local and remote climate variability (Wang et al., 2006, 2007, 2008).Therefore, the large and small warm pools in the western Pacific were examined in this study. The large and small warm pools were calculated as anomalies of the region of SST that was warmer than 29.0°C divided by the climatological warm pool region from June to October (JJASO)(Fig. 1c). Figure 1c shows that the large and small warm pool shows an interdecadal shift around the late 1980s.The large warm pool years over western Pacific were found after 1986, whereas the small warm pool years were often seen throughout the periods before 1986.Therefore, the period of 1958–1985 was defined as the small warm pool years, and the period of 1986–2008 was defined as the large warm pool years.

3.2 Interdecadal variability in rainfall

Previous studies showed that the thermal state of warm pool in western Pacific influences on rainfall variability(Nitta, 1987; Huang and Li, 1988; Huang and Sun, 1992).The interdecadal variability in rainfall in JJASO in China was analyzed using observations at 160 stations for the period of 1958–2008. Results from the previous section supported the appearance of the interdecadal shift around 1986. Therefore, the differences in JJASO rainfall anomalies from 1986 to 2008 and from 1958 to 1985 in China(Fig. 2a) showed that the increase of JJASO rainfall was the largest in Southeast China. The present results are consistent with Wu and Zhang (2007), who focused on summer (JJA) rainfall anomalies. To examine large-scale rainfall changes, the GPCP rainfall data was used in this study. Figures 2b–d show the JJASO rainfall average anomalies for 1979–1985, 1986–2008, and the differences between 1986–2008 and 1979–1985 in Eurasia. During the period of 1979–1985 (Fig. 2b), the percent decrease in JJASO rainfall occurred over Southeast China and warm pool in western Pacific. There was an opposite spatial distribution between 1986–2008 and 1979–1985 (Fig. 2c).The differences in JJASO precipitation anomalies between 1986–2008 and 1979–1985 showed a significant interdecadal variability in rainfall (Fig. 2d). The increase in JJASO rainfall occurred over Southeast China and warm pool in western Pacific. The distribution of rainfall in China was similar to that in Fig. 2a. The above results suggested that the changes over Southeast China could possibly part of the large-scale climate changes in relation to the SST anomalies in the warm pool.

3.3 Interdecadal variability of water vapor flux

Figure 2 (a) The differences in JJASO rainfall anomalies between 1986–2008 and 1958–1985 in China (units: mm). The JJASO rainfall anomalies averaged for (b)1979–1985, (c) 1986–2008, and (d) the difference between 1986–2008 and 1979–1985 in Eurasia (units: mm d–1).The shading denotes regions with positive values.

It was known that the rainfall anomalies were associated with water vapor flux anomalies. The water vapor flux alone hardly forms rainfall, so, the water vapor flux convergence is also crucial. Therefore, the water vapor flux convergence and vertical motion were examined to understand the interdecadal variability in JJASO rainfall on Southeast China and warm pool in western Pacific.Figure 3 shows JJASO 850 hPa vertical velocity (omega)and water vapor flux convergence anomalies averages for 1958–1985, 1986–2008, and the difference between 1986–2008 and 1958–1985. During the period of 1958–1985(Figs. 3a and 3d), there was significant sink motion and tropospheric moisture divergence over Southeast China and warm pool in western Pacific. However, during the period of 1986–2008 (Figs. 3b and 3e), a significant enhancement of ascending motion and tropospheric moisture convergence was seen over Southeast China and warm pool in western Pacific. The difference between 1987–2008 and 1958–1985 in JJASO omega and water vapor flux show enhancement of ascent motion and tropospheric moisture convergence over Southeast China and warm pool in western Pacific (Figs. 3c and 3f), which could support the increase in JJASO rainfall in these regions.

3.4 Interdecadal variability of surface latent heat flux

Surface latent heat flux is defined as the rate (per unit area), which is related to the moisture fl ux, and its energy associated with the phase change of water that is transferred from the ocean to the atmosphere. In the Tropics,the surface latent heat fl ux is typically an order of magnitude greater than the sensible heat fl ux. Zhou (2013)pointed out that the thermal state of warm pool in western Pacific influence on surface sensible heat flux. Thus, the warm pool in western Pacific maybe influence on surface latent heat flux in this region. Besides, Cayan (1992a, b)pointed out the interannual variation in sea surface latent heat flux was association with sea surface temperature anomalies. Thus, it can be concluded that the sea surface latent heat flux is associated with SST anomalies. The large warm pool means larger region of SST above 29.0°C that may contribute to increase in surface latent heat flux. Moreover, the change in water vapor anomalies was associated with changes in surface latent heat flux. Therefore, in this study, the JJASO sea surface latent heat flux anomalies were averaged for 1958–1985, 1986–2008, and the difference between 1986–2008 and 1958–1985 are shown in Fig. 4. During the period of 1958–1985 (Fig. 4a), a prominent feature is that the negative anomalies in surface latent heat flux dominated over warm pool in most parts of western Pacific. However,during the period of 1986–2008, there were significant positive anomalies over western Pacific (Fig. 4b). The difference between 1987–2008 and 1958–1985 in JJASO sea surface latent heat flux show there was an obvious interdecadal increase in sea surface latent heat flux over western Pacific (Fig. 4c). The results indicated that the large warm pool contributed to the increase of surface latent heat flux over the warm pool in the western Pacific.Thus, there was more water vapor over western Pacific,which contributed to the increase in the amount of rainfall in these regions.

4 Summary

Figure 3 JJASO 850 hPa vertical velocity (omega) averaged for (a) 1958–1985, (b) 1986–2008, and (c) the difference between 1986–2008 and 1958–1985 (units: m s–1). (d), (e), and (f) were the same as (a), (b), and (c) except for water vapor flux convergence anomalies (integration 1000–100 hPa) (units: g m–2 s–1). The shading denotes regions with negative values.

Figure 4 The JJASO surface latent heat flux anomalies averaged for (a) 1958–1985, (b) 1986–2008, and (c) the difference between 1986–2008 and 1958–1985.The shading denotes regions with positive values (units: W m–2).

This study analyzed and identified the interdecadal variability of JJASO large and small warm pools in the western Pacific and their associated with rainfall anomalies using station and reanalysis data for the period of 1958–2008. The large and small warm pools were calculated as the anomalies of region of SST warmer than 29.0°C divided by the climatological warm pool region during JJASO. The results of the analysis showed that the large and small warm pools showed an interdecadal shift around the late 1980s. The large warm pool years over western Pacific were found after 1986, whereas the small warm pool years were often seen throughout the periods before 1986. Therefore, the period of 1958–1985 was defined as the small warm pool years, and the period of 1986–2008 was defined as the large warm pool years.

The analysis results also showed that there were obvious interdecadal variability in JJASO rainfall in Southeast China and western Pacific; namely, during 1958–1985(small warm pool years), the rainfall decreased. However,during 1986–2008 (large warm pool years), the rainfall increased. Further analysis showed that the rainfall anomalies were associated with changes in water vapor flux and omega anomalies. During 1958–1985 (small warm pool years), there was significant tropospheric moisture divergence and sink motion over South-east China and warm pool in western Pacific. However, during 1986–2008 (large warm pool years), a significant enhancement of tropospheric moisture convergence and ascending motion was seen over Southeast China and warm pool in western Pacific. These results also indicated that the large warm pool contributed to the increase in surface latent heat flux over warm pool in western Pacific.Thus, there was more water vapor over western Pacific,which contributed to increased rainfall over these regions.

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