Shift of peak in summer monsoon rainfall over Korea and its association with El Niño-Southern Oscillation

The annual cycle of rainfall over the Korean Peninsula is marked by two peaks: one during July and the other during August. Since the mid-1970s, the maximum rainfall over the Korean Peninsula has shifted from July to August. This shift in rainfall peak was caused by a significant increase of August rainfall after the mid-1970s. The basic reason for this shift has been traced to a change in teleconnection between El Niño-Southern Oscillation (ENSO) and August rainfall. The relationship between August rainfall over Korea and ENSO changed from 1954-1975 (PI) to 1976-2002 (PII). The variability of August rainfall was significantly associated with sea surface temperature (SST) variation over the eastern equatorial Pacific during PI, but this relationship is absent during the PII period. In El Niño years during PI, low-level westerly and southerly wind anomalies are dominant around the East China Sea, which relates to strong August rainfall. In La Niña years during PI, easterly and northerly wind anomalies are dominant. During the PII period, however, westerly and southerly wind anomalies around the East China Sea were responsible for the high August rainfall over the East Asian region, even though La Niña SST conditions were in effect over the eastern Pacific

Source of low frequency modulation of ENSO amplitude in a CGCM

We study the relationship between changes in equatorial stratification and low frequency El Niño/Southern Oscillation (ENSO) amplitude modulation in a coupled general circulation model (CGCM) that uses an anomaly coupling strategy to prevent climate drifts in the mean state. The stratification is intensified at upper levels in the western and central equatorial Pacific during periods of high ENSO amplitude. Furthermore, changes in equatorial stratification are connected with subsurface temperature anomalies originating from the central south tropical Pacific. The correlation analysis of ocean temperature anomalies against an index for the ENSO modulation supports the hypothesis of the existence of an oceanic "tunnel" that connects the south tropical Pacific to the equatorial wave guide. Further analysis of the wind stress projection coefficient onto the oceanic baroclinic modes suggests that the low frequency modulation of ENSO amplitude is associated with a significant contribution of higher-order modes in the western and central equatorial Pacific. In the light of these results, we suggest that, in the CGCM, change in the baroclinic mode energy distribution associated with low frequency ENSO amplitude modulation have its source in the central south tropical Pacific

(a) 300 hPa zonal wind (ms⁻¹), (c) 500 hPa geopotential height (m), and (e) SLP (hPa) during the boreal winter in AFT1985

<p><strong>Figure 2.</strong> (a) 300 hPa zonal wind (ms⁻¹), (c) 500 hPa geopotential height (m), and (e) SLP (hPa) during the boreal winter in AFT1985. Differences of (b) 300 hPa zonal wind, (d) 500 hPa geopotential height, and (f) SLP between AFT1985 and BEF1985. Contour intervals in (b) and (d) are 0.5 ms⁻¹ and 5 m, respectively. Contour interval in (f) is 0.5 hPa. Shadings in (b), (d), and (f) represent the values significant at the 95% confidence level.</p> <p><strong>Abstract</strong></p> <p>An analysis of the interannual variability of surface air temperature during the boreal winter in the East Asian (EA) region from 1960 to 2009 reveals that the East Asian winter monsoon (EAWM) significantly weakens after the mid-1980s. The robust warming over the EA region in the lower and middle troposphere as well as at the surface is caused mainly by changes in circulations over the North Pacific and Eurasian continent. The 300 hPa East Asian jet and 500 hPa trough over the EA region, which are closely linked to cold surges, significantly weaken after the mid-1980s. The weakened northerly wind in the Siberian high region and north of the EA region interfere with cold advection toward the EA region. The anomalous southeasterlies over the East China Sea due to an enhanced North Pacific oscillation (NPO)-like sea level pressure (SLP) pattern lead to anomalous warm advection over the EA region. It is also found that the advection of mean temperature by anomalous wind and the advection of anomalous temperature by mean wind mainly contribute to the anomalous warm advection in the EA region after the mid-1980s. Consequently, these anomalous circulations provide a more favorable environment for weakening of the EAWM.</p

(a) Difference in temperature advection (K day⁻¹) at 1000 hPa between AFT1985 and BEF1985

<p><strong>Figure 4.</strong> (a) Difference in temperature advection (K day⁻¹) at 1000 hPa between AFT1985 and BEF1985. The contributions of (b) the advection of mean temperature by anomalous wind (K day⁻¹), (c) the advection of anomalous temperature by mean wind (K day⁻¹), and (d) the nonlinear advection (K day⁻¹) to the temperature advection difference in (a).</p> <p><strong>Abstract</strong></p> <p>An analysis of the interannual variability of surface air temperature during the boreal winter in the East Asian (EA) region from 1960 to 2009 reveals that the East Asian winter monsoon (EAWM) significantly weakens after the mid-1980s. The robust warming over the EA region in the lower and middle troposphere as well as at the surface is caused mainly by changes in circulations over the North Pacific and Eurasian continent. The 300 hPa East Asian jet and 500 hPa trough over the EA region, which are closely linked to cold surges, significantly weaken after the mid-1980s. The weakened northerly wind in the Siberian high region and north of the EA region interfere with cold advection toward the EA region. The anomalous southeasterlies over the East China Sea due to an enhanced North Pacific oscillation (NPO)-like sea level pressure (SLP) pattern lead to anomalous warm advection over the EA region. It is also found that the advection of mean temperature by anomalous wind and the advection of anomalous temperature by mean wind mainly contribute to the anomalous warm advection in the EA region after the mid-1980s. Consequently, these anomalous circulations provide a more favorable environment for weakening of the EAWM.</p

Differences in 1000 hPa wind (ms⁻¹) (vector) and geopotential height (m) (contour) during the boreal winter between AFT1985 and BEF1985

<p><strong>Figure 3.</strong> Differences in 1000 hPa wind (ms⁻¹) (vector) and geopotential height (m) (contour) during the boreal winter between AFT1985 and BEF1985. Arrows significant at the 90% the confidence level are shown. Contour interval is 5 m. Shadings represent the values of geopotential height significant at the 95% confidence level.</p> <p><strong>Abstract</strong></p> <p>An analysis of the interannual variability of surface air temperature during the boreal winter in the East Asian (EA) region from 1960 to 2009 reveals that the East Asian winter monsoon (EAWM) significantly weakens after the mid-1980s. The robust warming over the EA region in the lower and middle troposphere as well as at the surface is caused mainly by changes in circulations over the North Pacific and Eurasian continent. The 300 hPa East Asian jet and 500 hPa trough over the EA region, which are closely linked to cold surges, significantly weaken after the mid-1980s. The weakened northerly wind in the Siberian high region and north of the EA region interfere with cold advection toward the EA region. The anomalous southeasterlies over the East China Sea due to an enhanced North Pacific oscillation (NPO)-like sea level pressure (SLP) pattern lead to anomalous warm advection over the EA region. It is also found that the advection of mean temperature by anomalous wind and the advection of anomalous temperature by mean wind mainly contribute to the anomalous warm advection in the EA region after the mid-1980s. Consequently, these anomalous circulations provide a more favorable environment for weakening of the EAWM.</p

(a) The normalized time series of anomalous winter mean 2 m air temperature over East Asia (25–45°N, 105–145°E)

<p><strong>Figure 1.</strong> (a) The normalized time series of anomalous winter mean 2 m air temperature over East Asia (25–45°N, 105–145°E). Differences in air temperature (K) between AFT1985 and BEF1985 (b) at surface, (c) in latitude–height cross section averaged along 105–145°E, and (d) in longitude–height cross section averaged along 25–45°N. Contour intervals in (b) and ((c), (d)) are 0.5 K and 0.2 K, respectively. Shadings in (b), (c) and (d) represent the values significant at the 95% confidence level.</p> <p><strong>Abstract</strong></p> <p>An analysis of the interannual variability of surface air temperature during the boreal winter in the East Asian (EA) region from 1960 to 2009 reveals that the East Asian winter monsoon (EAWM) significantly weakens after the mid-1980s. The robust warming over the EA region in the lower and middle troposphere as well as at the surface is caused mainly by changes in circulations over the North Pacific and Eurasian continent. The 300 hPa East Asian jet and 500 hPa trough over the EA region, which are closely linked to cold surges, significantly weaken after the mid-1980s. The weakened northerly wind in the Siberian high region and north of the EA region interfere with cold advection toward the EA region. The anomalous southeasterlies over the East China Sea due to an enhanced North Pacific oscillation (NPO)-like sea level pressure (SLP) pattern lead to anomalous warm advection over the EA region. It is also found that the advection of mean temperature by anomalous wind and the advection of anomalous temperature by mean wind mainly contribute to the anomalous warm advection in the EA region after the mid-1980s. Consequently, these anomalous circulations provide a more favorable environment for weakening of the EAWM.</p