Theories on Formation of an Anomalous Anticyclone in Western North Pacific during El Niño: A Review

• Corresponding author: Tim LI, timli@hawaii.edu
• Funds:

Supported by the National Key Research and Development Program (2017YFA0603802 and 2015CB453200), National Natural Science Foundation of China (41630423, 41475084, 41575043, and 41375095), United States National Science Foundation (AGS-1565653), Jiangsu Province Natural Science Foundation Key Project (BK20150062), Jiangsu Shuang-Chuang Team Fund (R2014SCT001), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This is SOEST contribution number 10280, IPRC contribution number 1297, and ESMC contribution 196

• doi: 10.1007/s13351-017-7147-6
• The western North Pacific anomalous anticyclone (WNPAC) is an important atmospheric circulation system that conveys El Niño impact on East Asian climate. In this review paper, various theories on the formation and maintenance of the WNPAC, including warm pool atmosphere–ocean interaction, Indian Ocean capacitor, a combination mode that emphasizes nonlinear interaction between ENSO and annual cycle, moist enthalpy advection/Rossby wave modulation, and central Pacific SST forcing, are discussed. It is concluded that local atmosphere–ocean interaction and moist enthalpy advection/Rossby wave modulation mechanisms are essential for the initial development and maintenance of the WNPAC during El Niño mature winter and subsequent spring. The Indian Ocean capacitor mechanism does not contribute to the earlier development but helps maintain the WNPAC in El Niño decaying summer. The cold SST anomaly in the western North Pacific, although damped in the summer, also plays a role. An inter-basin atmosphere–ocean interaction across the Indo-Pacific warm pool emerges as a new mechanism in summer. In addition, the central Pacific cold SST anomaly may induce the WNPAC during rapid El Niño decaying/La Niña developing or La Niña persisting summer. The near-annual periods predicted by the combination mode theory are hardly detected from observations and thus do not contribute to the formation of the WNPAC. The tropical Atlantic may have a capacitor effect similar to the tropical Indian Ocean.
• Fig. 1.  Time evolution of Niño 3.4 SSTA (°C) for individual El Niño events during 1956–2000. From Wang et al. (2000).

Fig. 2.  Monthly mean anomalous 850-hPa wind fields regressed against an area-averaged rainfall index in boreal summer (May–August) over the Yangtze River Valley (YRV) basin. Station rainfall data in China and NCEP reanalysis I datasets for the period of 1951–96 were used for the regression analysis. From Chang et al. (2000a).

Fig. 3.  Seasonal evolving patterns of 850-hPa wind (vector, m s–1) and SST (shading, °C) anomalies associated with El Niño turnabout from the developing summer JJA(0), to the decaying summer JJA(1) (see labels on upper left of each panel) calculated based on the SS-SVD analysis. From Wang et al. (2003).

Fig. 4.  A schematic diagram illustrating the effect of a positive air–sea feedback between the WNPAC and cold SSTA in WNP. The double arrow denotes the background mean trade wind, and heavy lines with black arrows represent the anomalous wind. The long (short) dashed lines represent a positive (negative) SSTA. From Wang et al. (2000).

Fig. 5.  Seasonal evolution patterns of (left) anomalous 850-hPa wind (vector, m s–1) and 500-hPa vertical p-velocity (10–2 hPa s–1), (middle) SST (°C), and (right) 200-hPa velocity potential (10–6 m2 s–1) fields derived from 12-El Niño composite from El Niño mature winter to the subsequent summer during 1950–2006. From Li and Hsu (2017).

Fig. 6.  The first EOF mode derived from summer (JJA) mean 850-hPa geopotential height (H850) in the Asian–Australian monsoon domain (20°S–40°N, 30°E–180°) for 1979–2009. (a) Spatial pattern (contour) and the correlated precipitation (shading) in the Indo-Pacific domain with correlations significant at 90% confidence (r > 0.3). (b) The correlated SSTA (shading) and 850-hPa wind anomalies with reference to the principal component of EOF-1. (c) Simulated JJA mean H850 (contour, m), precipitation (shading, mm day–1), H850 (contours, m), and 1000-hPa wind anomaly (m s–1) in response to an initial SST perturbation over the WNP in a coupled model experiment. Adopted from Wang et al. (2013).

Fig. 7.  Schematic diagrams showing the interaction between the WNPAC and Indo-Pacific SST dipole. The figure is drawn based on the composite anomalies associated with 8 strong El Niño events during 1957–2016. “A” in black indicates the anticyclonic circulation. Adopted from Wang et al. (2017).

Fig. 8.  (a, b) Composite SSTA (left, °C) and precipitation anomaly (right, mm day–1) fields during El Niño mature phase (DJF). The composite is based on 1980–2013. (c, d) Simulated 850-hPa wind anomaly fields (vectors, m s–1) from ECHAM4 in response to a specified SSTA and a diabatic heating anomaly forcing in tropical Indian Ocean as shown in (a, b). The blue (red) vector denotes anomalous easterly (westerly) wind response. From Chen M.-C. et al. (2016).

Fig. 9.  Schematic diagram illustrating the impact of Indian Ocean basin-wide heating on WNP anomalous anticyclone during El Niño decaying summer. From Wu et al. (2009).

Fig. 10.  (a) Geographic domains for the tropical Indian Ocean and western Pacific, where the SST anomalies are specified as the model lower boundary condition for the western Pacific (WP) and Indian Ocean (IND) runs. (b) Temporal evolutions of area-averaged vorticity anomalies (10–6 m2 s–1) over the region of 10°–35°N, 110°–160°E for the WP (dashed line) and IND (solid line) SSTA forcing runs.

Fig. 11.  Power spectra for sea level pressure (SLP) anomalies averaged in (a) northern Indian Ocean (5°–20°N, 60°–100°E), (b) western North Pacific (5°–20°N, 120°–160°E), (c) central North Pacific (5°–20°N, 180°–140°W), (d) southern Indian Ocean (20°–5°S, 60°–100°E), (e) Australia (20°–5°S, 120°–160°E), and (f) central South Pacific (20°–5°S, 180°–140°W). Black lines indicate the power density and colored lines indicate the respective confidence interval (CI) based on a red noise null hypothesis. From Li et al. (2016).

Fig. 12.  (a) Seasonal evolution of SLP (shading, hPa) and 850-hPa wind (vectors, m s–1) anomaly fields from boreal summer to winter in 1997. The anomaly fields were derived by subtracting a climatological annual cycle and then applying a 12-month low-pass filter. (b) Same as (a) but for 13–19-month band-pass filtered anomalies. (c) Same as (a) but for 19–84-month band-pass filtered anomalies. The Butterworth filter was applied in the above calculations. From Li et al. (2016).

Fig. 13.  (a, c, e) Seasonal mean precipitation (shading, mm day–1) and 925-hPa stream function anomalies (contours, 0.3×106 m2 s–1) regressed against the D(0)JF(1)-mean Niño 3.4 index for JJA(0), SON(0), and D(0)JF(1), respectively. (b, d, f) Same as the left panels but for SST anomalies (K). For the precipitation and SST anomalies, only values reaching the 5% significance level are shown. The red box in (e) (1°–14°N, 125°–160°E) denotes the key negative precipitation anomaly center for driving the WNPAC during the El Niño mature winter. From Wu et al. (2017a).

Fig. 14.  Schematic of the moist enthalpy advection mechanism responsible for the formation and maintenance of the WNPAC during the El Niño mature winter and the following spring. Warm SSTAs in the central–eastern equatorial Pacific (red line) enhance local convection (green shading), and thus stimulate cyclonic anomalies to the northwest (black solid line). The northerly anomalies at western flank of the cyclonic anomalies advect dry (low moist enthalpy) air into the tropical WNP and thus suppress convection there (orange shading). The suppressed convection further stimulates the WNPAC (black dashed line) to the northwest. From Wu et al. (2017a).

Fig. 15.  Climatological monthly mean 925-hPa specific humidity (shading, g kg–1) and anomalous wind in (a) August(0), (b) September(0), (c) October(0), (d) November(0), and (e) December(0), obtained by regression against the D(0)JF(1)-mean Niño 3.4 index (vectors, m s–1). The area-averaged meridional gradient of 925-hPa climatological specific humidity (g kg–1 m–1) over 1°–14°N, 125°–160°E is given on the top right corner of each panel. From Wu et al. (2017b).

Fig. 16.  Climatological 850-hPa relative vorticity (10–6 s–1) in (a–e) August–December derived from the ECMWF interim reanalysis (ERA-Interim) dataset. The box area-averaged meridional gradient of relative vorticity (m–1 s–1) over 5°–20°N, 120°–160°E (black box) is given on the top right corner of each panel. From Wu et al. (2017b).

Fig. 17.  850-hPa stream function anomalies (shading, 106 m2 s–1) simulated by an anomaly AGCM with specified background mean state (a–e) from August to December. Contours are the horizontal distributions of an anomalous heating field used to drive the dry anomaly AGCM (contour: 1 K day–1). From Wu et al. (2017b).

Fig. 18.  Schematic of the combined moist enthalpy advection/Rossby wave modulation mechanism. During the El Niño developing phase [Sep(0)], westward stretch of the cyclonic anomalies is enhanced by the positive meridional gradient of the mean relative vorticity (${\partial _y}\bar \zeta > 0$). With the formation of negative ${\partial _y}\bar \zeta$ over the tropical WNP in Nov(0), the cyclonic anomalies withdraw eastward and leave space for the onset of the WNPAC. Meanwhile, ${\partial _y}\bar q$ changes its sign from positive to negative over the region. These two factors, combining with underlying cold SSTAs, cause the WNPAC to form in Nov(0) and maintain throughout the winter and spring. From Wu et al. (2017b).

Fig. 19.  The second EOF mode derived from summer (JJA) mean 850-hPa geopotential height (H850) in the Asian–Australian monsoon domain (20°S–40°N, 30°E–180°) for 1979–2009. (a) Spatial pattern (contour) and the correlated precipitation (shading) in the Indo-Pacific domain with correlations significant at 90% confidence (r > 0.3). (b) The correlated SSTA (shading) and 850-hPa wind anomalies with reference to the principal component of the EOF mode. (c) Simulated JJA H850 (contour, m) and precipitation (shading, mm day–1) anomalies using an AGCM model forced by a prescribed cold SSTA over the equatorial central Pacific (blue contour, °C). Adopted from Wang et al. (2013).

Fig. 20.  Schematic diagrams showing the WNPAC forced by a central Pacific cooling. Green (yellow) represents positive (negative) rainfall anomalies; black arrows represent low-level wind direction; and “A” in black indicates the anticyclonic circulation. Adopted from Wang et al. (2017).

Fig. 21.  Anomalous SST (shaded, °C) and 850-hPa wind (m s–1) fields regressed against the Niño 3.4 index for (a) JJS0, (b) SON0, (c) DJF+1, (d) MAM+1, and (e) JJA+1 of 1980–2015. Year 0 denotes El Niño developing year, and year +1 denotes El Niño decaying year. Dotted areas indicate that SSTA signals exceed the 95% confidence level.

Fig. 22.  Lead–lag correlations between SSTA (°C) averaged over eastern equatorial Pacific (180°–120°W, 10°S–10°N) and over the tropical Atlantic (80°–20°W, 5°–25°N) for the period of 1980–2015. Dashed line denotes the 95% confidence level.

通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

Theories on Formation of an Anomalous Anticyclone in Western North Pacific during El Niño: A Review

Corresponding author: Tim LI, timli@hawaii.edu;
• 1. Key Laboratory of Meteorological Disaster, Ministry of Education/Joint International Research Laboratory of Climate and Environmental Change/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China
• 2. International Pacific Research Center and Department of Atmospheric Sciences, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA
• 3. Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
• 4. University of Chinese Academy of Sciences, Beijing 100049, China
• 5. Naval Postgraduate School, Monterey, CA 93943, USA
• 6. Institute of Atmospheric Sciences, Fudan University, Shanghai 200433, China
Funds: Supported by the National Key Research and Development Program (2017YFA0603802 and 2015CB453200), National Natural Science Foundation of China (41630423, 41475084, 41575043, and 41375095), United States National Science Foundation (AGS-1565653), Jiangsu Province Natural Science Foundation Key Project (BK20150062), Jiangsu Shuang-Chuang Team Fund (R2014SCT001), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This is SOEST contribution number 10280, IPRC contribution number 1297, and ESMC contribution 196

Abstract: The western North Pacific anomalous anticyclone (WNPAC) is an important atmospheric circulation system that conveys El Niño impact on East Asian climate. In this review paper, various theories on the formation and maintenance of the WNPAC, including warm pool atmosphere–ocean interaction, Indian Ocean capacitor, a combination mode that emphasizes nonlinear interaction between ENSO and annual cycle, moist enthalpy advection/Rossby wave modulation, and central Pacific SST forcing, are discussed. It is concluded that local atmosphere–ocean interaction and moist enthalpy advection/Rossby wave modulation mechanisms are essential for the initial development and maintenance of the WNPAC during El Niño mature winter and subsequent spring. The Indian Ocean capacitor mechanism does not contribute to the earlier development but helps maintain the WNPAC in El Niño decaying summer. The cold SST anomaly in the western North Pacific, although damped in the summer, also plays a role. An inter-basin atmosphere–ocean interaction across the Indo-Pacific warm pool emerges as a new mechanism in summer. In addition, the central Pacific cold SST anomaly may induce the WNPAC during rapid El Niño decaying/La Niña developing or La Niña persisting summer. The near-annual periods predicted by the combination mode theory are hardly detected from observations and thus do not contribute to the formation of the WNPAC. The tropical Atlantic may have a capacitor effect similar to the tropical Indian Ocean.

Reference (77)

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