Seasonal forecasting of East Asian summer monsoon based on oceanic heat sources

Abstract

We use the upper-level divergence zone at 150 hPa to define the areas of study for the East Asian summer monsoon (EASM) and to show the advances and retreats of the EASM. We find that the EASM can be subdivided into a northern and southern component with distinctly different driving mechanisms. The northern EASM (NEASM) is affected by heat sources in the tropical oceans related to El Niño events while the southern EASM (SEASM) is affected by the subtropical oceans related to a North Pacific sea surface temperature (SST) dipole mode. A stronger NEASM is related to above-normal western North Pacific (WNP) anticyclonic anomalies, while a stronger SEASM is related to below-normal WNP anticyclonic anomalies. These WNP anticyclonic anomalies are connected to SST anomalies in the tropical and subtropical Pacific during the pre-monsoon season (December∼May). We also find that NEASM precipitation can be predicted from regional oceanic heat sources, i.e. SST and ocean heat content, in the tropical Pacific and Indian Oceans during the pre-monsoon season using a linear regression model. SEASM precipitation can be predicted from pre-monsoon SST in the eastern North Pacific. The NEASM forecast model is more skillful than that for the SEASM.

Relative impact of insolation and the Indo-Pacific warm pool surface temperature on the East Asia summer monsoon during the MIS-13 interglacial

“…Yang and Lau (2004) found that the interannual variation of the summer precipitation over central eastern China and over southern coastal China is correlated with a north-south dipole mode of SST anomalies over the western North Pacific, the tropical Indian Ocean and the warm pool: when SSTs are abnormally warm over the warm pool and northern Indian Ocean and are abnormally cold over the western North Pacific, summer precipitation tends to be heavier than usual in central eastern China but to be less in southern coastal China. Lee et al (2008) found that the north EASM precipitation is negatively correlated with the tropical western Pacific SST and is positively correlated to the tropical Indian Ocean SST, and that the south EASM precipitation is positively correlated with the western North Pacific SST but has no obvious correlation with the Indian Ocean SST. However, also based on observational data, Saji and Yamagata (2003) found that the precipitation over India and southern China is enhanced during the positive IOD (Indian Ocean dipole) event, an event which is characterized by anomalous cooling in the eastern equatorial Indian Ocean and anomalous warming in the western equatorial Indian Ocean.…”

Model assessment of the observed relationship between El Niño and the northern East Asian summer monsoon using the Community Climate System Model Community Atmosphere Model‐Community Land Model version 3 (CAM‐CLM3)

“…Significant regions at the 90 and 95% levels are contoured. The relationship between NEASM precipitation and El Niño during the premonsoon season (December through May) is examined using a statistical forecast model of Lee et al [2008] (). where “SST in the TEP” is SST in the tropical eastern Pacific (5°S∼5°N, 170°W∼80°W), “SST in the TWP” is SST in the tropical western Pacific (0°∼15°N, 130°E∼160°E), “SST in the TIO” is SST in the tropical Indian Ocean (10°S∼10°N, 40°∼100°E), and “OHC in the TIO” is ocean heat content in the tropical Indian Ocean (20°S∼15°N, 50°E∼70°E) during the premonsoon season.…”

“…In order to elucidate the physical mechanisms between the NEASM and El Niño shown in the observational analysis [ Lee et al , 2008], we examine mean JJA differences in 850 hPa wind vector and vorticity between the El Niño and control simulations (Figure 5, left). For the El Niño run (i.e., warm SST anomalies in the tropical eastern Pacific), there are cold SST anomalies in the tropical western Pacific, WNP, and western South Pacific (a horseshoe pattern (see Figure 1)).…”

“…The variability of SST in the tropical Pacific and adjacent regions (i.e., El Niño−Southern Oscillation) is correlated with summer rainfall in the EASM region, covering eastern China, the Korean peninsula, Japan, and the adjacent marginal seas [ Shen and Lau , 1995; Zhang et al , 1996; Wu and Wang , 2002; Lee et al , 2005, 2008]. Previously, in an observational analysis, we found northern EASM (NEASM over 30°N∼50°N and 110°E∼145°E; see Figure 3a in section 3) precipitation is positively correlated with SST in the tropical eastern Pacific and Indian Ocean, and negatively correlated with SST in the tropical western Pacific [ Lee et al , 2008]. That paper proposed plausible physical linkages between tropical SST anomalies and the NEASM with a stronger NEASM related to enhanced western North Pacific (WNP) anticyclonic anomalies and thus more cyclonic anomalies in the NEASM region due to the Pacific–East Asian teleconnection [ Wang et al , 2000], which is connected to tropical SST anomalies.…”

The effects of ocean SST dipole on Mongolian summer rainfall

“…For example, SSTdifferenced dipoles provide stronger correlation with summer rainfall over the Loess Plateau as compared to consideration of just positive or negative SSTs (Yasuda et al, 2009). Links to the summer monsoon rainfall (JJA) of East Asia (20°-50° N, 110°-145° E) with dipoles over the Pacific Ocean at 6-month time lag (DJF) were reported by Lee et al (2008). We also selected significant dipoles to maximize the correlation and predictive ability of the relationships between SST and summer rainfall.…”