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Impact of 10-30-day Oscillation Intensity over the Tropical Northwest Pacific Ocean on the South China Sea Summer Monsoon
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摘要: 采用1958—2011年NCEP/NCAR再分析资料以及ERSST海温资料，分析热带西太平洋夏季对流10~30 d振荡对南海夏季风的影响。在年际变化尺度上，热带西北太平洋夏季10~30 d振荡强度指数 (TWPI) 与南海夏季风强度有很好的正相关关系。在TWPI增强年份，海温主要呈El Niño分布，南海周边区域增强的异常西风产生强的正涡度切变，导致异常气旋性环流，为季风槽的增强提供了热量和水汽，从而增强南海夏季风强度。反之，在TWPI减弱年份，海温主要呈La Niña分布，南海夏季风强度减弱。在不同的年代际背景下，垂直切变和水汽-对流的总体变化是影响TWPI总体变化的重要因子，但不能影响南海夏季风强度的总体变化。海陆热力对比的总体变化是导致南海夏季风强度总体变化的主要影响因素。Abstract: Based on the NCEP reanalysis data and ERSST sea surface temperature (SST) data, using statistical methods, 10-30-day significant sub-seasonal variability periods are extracted from the summer (Jun-Aug) convective in the tropical western Pacific, and these oscillations have different effects on the South China Sea summer monsoon intensity at different scales. At inter-annual time scale, a positive significant correlation is found between the intensity variation of 10-30-day oscillation over the tropical northwest Pacific Ocean regions (TWPI), and the correlation coefficient is 0.635. Influences of TWPI on the South China Sea summer monsoon intensity are mainly regulated by ENSO. Because of the asymmetric response of the lower troposphere Northwest Pacific atmospheric circulation to ENSO, TWPI is much more significant in El Niño developing years than in La Niña years. During strong TWPI years, the SST anomalies are El Niño pattern, which induces anomalously enhanced westerly in the South China Sea, the Philippines and the tropical northwest Pacific Ocean. The westerly anomalies generate strong positive vorticity shear, resulting in abnormal cyclonic circulation, and enhance TWPI and summer monsoon intensity through the wind-evaporation feedback mechanism. On the contrary, in La Niña years, the anticyclonic anomaly circulation result in TWPI weakening and the monsoon strength weakening. Under different inter-decadal backgrounds, TWPI does not show a significant change in the decadal trend, mainly slightly weaker (1958-1976), slightly stronger (1977-1993) and slightly weaker (1994-2011) change. The trend of the summer monsoon intensity inter-decadal changes are more obvious, namely, much stronger, slightly weaker and much weaker. The overall change of vertical shear and water vapor-convection are consistent with TWPI, but are not consistent with the summer monsoon. Vertical shear of wind field and water vapor-convection play important roles on the inter-decadal variation of TWPI, but not for the summer monsoon. The thermal contrast between sea and land is the key factor that leads to the inter-decadal change in the South China Sea summer monsoon.
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图 1 1958—2011年热带西北太平洋 (0°~20°N，135°~175°E) 850 hPa涡度逐日变化的功率谱多年平均曲线
Fig. 1 Power spectrum of daily vorticity time series averaged in the tropical western Pacific (0°-20°N, 135°-175°E) for the averaged from 1958 to 2011
图 2 1958—2011年南海夏季风强度以及TWPI标准化时间序列
(水平的实线和虚线分别为南海夏季风和TWPI 3个年代平均)
Fig. 2 Standardized time series of the South China Sea summer monsoon intensity and the TWPI
(horizonal solid line and dashed line denote two series average in 3 different interdecadal background year)
图 3 TWPI强年与弱年夏季物理量合成偏差分布 (阴影表示达到0.05显著性水平)
(a) 海温 (单位：K)，(b)850 hPa风场 (矢量) 和700 hPa水汽 (等值线，单位：g·kg-1)，(c)5°~20°N垂直环流剖面 (纬向风单位：m·s-1，垂直速度单位：10-2 Pa·s-1)，(d)135°~175°E纬向风 (单位：m·s-1)
Fig. 3 The composite differences between strong and weak years of TWPI (the shaded denotes passing the test of 0.05 level)
(a) sea surface temperature (unit:K), (b)850 hPa wind (the vector) and 700 hPa specific humidity (the contour, unit: g·kg-1), (c) vertical circulation averaged over 5°-20°N (unit of zonal wind:m·s-1, unit of vertical velocity:10-2 Pa·s-1), (d) the zonal wind averaged over 135°-175°E (unit:m·s-1)
图 4 夏季影响南海夏季风强度的关键区 (0°~20°N，135°~175°E) 10~30 d振荡对应的温度、水汽、湿静力、位温、垂直速度、散度、相对湿度以及水汽通量散度距平随高度变化
Fig. 4 Anomalies of temperature specific humidity, moist static energy, equivalent potential temperature, vertical velocity, divergence, relative humidity, and moisture flux convergence averaged over the key activity region (0°-20°N, 135°-175°E)
图 5 在3个不同年代际背景下夏季风场垂直切变距平分布 (单位：m·s-1，阴影表示达到0.05显著性水平，矩形框分别代表南海和热带西太平洋区域)
Fig. 5 Composite differences of wind vertical shear anomaly in three different interdecadal background years (unit:m·s-1, the shaded denotes passing the test of 0.05 level, rectangles denote the South China Sea and the tropical western Pacific)
图 6 夏季影响南海夏季风强度的关键区 (0°~20°N，135°~175°E) 10~30 d振荡对应的温度、水汽、湿静力、位温、垂直速度、散度、相对湿度以及水汽通量散度距平在3个不同年代际背景下随高度变化
Fig. 6 Anomalies of temperature, specific humidity, moist static energy, equivalent potential temperature, vertical velocity, divergence, relative humidity and moisture flux convergence averaged over the key activity region (0°-20°N, 135°-175°E) in three different interdecadal background years
图 7 夏季海温场 (等值线，单位：K) 和850 hPa风场 (矢量) 在3个不同年代际背景下距平分布
(阴影表示达到0.05显著性水平, 矩形框分别代表南海和热带西太平洋区域)
Fig. 7 Anomlies of sea surface temperature (the contour, unit:K) and 850 hPa wind (the vector) in three different interdecadal background years (the shaded denotes passing the test of 0.05 level, rectangles denote the South China Sea and the tropical western Pacific)
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