Impact of ENSO on MJO Pattern Evolution over the Maritime Continent

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

Supported by the National Natural Science Foundation of China (42088101 and 41875069), US National Science Foundation (AGS-2006553), and US NOAA Grant (NA18OAR4310298). This is SOEST contribution number 11206 and IPRC contribution number 1494

• doi: 10.1007/s13351-020-0046-2
• The modulation of Madden–Julian oscillation (MJO) pattern evolution over the Maritime Continent (MC) by El Niño–Southern Oscillation (ENSO) was investigated through a combined observational and modeling study. MJO convective branches shifted south of the equator over the MC during eastern Pacific (EP) El Niño winters, while it became relatively symmetric about the equator during La Niña winters. The impact of central Pacific (CP) El Niños to MJO pattern, on the other hand, is not statistically significant. The cause of the distinctive MJO pattern evolutions is likely attributed to the ENSO-induced changes of the background moisture and vertical shear over the MC. Idealized numerical experiments with a 2.5-layer model were carried out, and the result revealed that the background moisture change played a dominant role. An observational diagnosis of column-integrated moist static energy (MSE) budgets was further conducted. The result indicated that the MJO pattern difference was attributed to the MSE tendency asymmetry in front of MJO convection between EP El Niño and La Niña, caused by the advection of the mean MSE by anomalous meridional wind. The difference in the MJO-scale anomalous meridional wind was ultimately controlled by the change of the background meridional moisture gradient associated with EP El Niño and La Niña.
• Fig. 1.  Standard deviation (STD) of 10–90-day band-pass filtered OLR anomalies (shaded; W m−2) for (a) EP El Niño, (b) CP El Niño, and (c) La Niña. Climatological STD at each grid point has been subtracted at each panel. The green box denotes a key region (0°–20°N, 105°–145°E) where ENSO exerts a strong impact on MJO.

Fig. 2.  Lead–lag regressed patterns of 10–90-day filtered OLR anomaly (shaded; W m−2) and associated wind anomaly field at 850 hPa (vector; m s−1) from Days 0 to 6. The regression was based on the OLR anomaly at a reference point over Indian Ocean (5°S, 95°E) for EP El Niño (left) and La Niña (right) winters. The OLR anomalies that pass the significant test at 95% confidence level are stippled.

Fig. 3.  The Hovmöller diagram of the regressed OLR (shaded; W m−2) and column-integrated MSE (contour; J m−2) anomalies averaged over 20°S–20°N for (a) EP El Niño and (b) La Niña winters from Days −20 to 20. The regression was based on the OLR anomaly at 5°S, 95°E. The OLR anomalies that pass the significant test at 95% confidence level are stippled.

Fig. 4.  Background specific humidity (shaded; g kg−1) and 850-hPa wind (vector; m s−1) fields during (a) El Niño and (b) La Niña winters (DJF). Climatological mean specific humidity and 850-hPa wind fields have been subtracted at each panel. The green box shows the same region as that in Fig. 1.

Fig. 5.  Vertical profiles of (a) background zonal wind (m s−1), (b) vorticity (10−6 s−1), (c) vertical p-velocity (10−3 Pa s−1), and (d) specific humidity (g kg−1) averaged over the green box (0°–20°N, 105°–145°E) shown in Fig. 1 for EP El Niño (green) and La Niña (red) years.

Fig. 6.  Time evolutions of lower-tropospheric wind (vector; m s−1) and precipitation (shaded; mm day−1) anomaly fields of an MJO-like perturbation from Days 3 to 7, simulated by a simple atmospheric model.

Fig. 7.  Accumulated (Days 4–7) precipitation anomalies as the MJO-like disturbance passes over the MC in CTL, EXP1, and EXP2.

Fig. 8.  Horizontal patterns of the OLR anomaly (shaded; W m−2) and column-integrated MSE tendency (contour; W m−2) fields regressed onto the OLR anomaly at the reference point (5°S, 95°E) for EP El Niño (top) and La Niña (bottom) winters. The red and blue boxes are key regions for the MSE budget analysis in front of the MJO convection.

Fig. 9.  MSE tendency (W m−2) anomalies averaged over the northern (red) and southern (blue) boxes shown in Fig. 8 for EP El Niño (left) and La Niña (right).

Fig. 10.  MSE budget terms (W m−2) averaged over the northern (red) and southern (blue) boxes shown in Fig. 8 for (a) EP El Niño winters, (b) La Niña winters, and (c) their difference (La Niña minus EP El Niño). Bars from left to right represent MSE tendency (mt), zonal advection (umx), meridional advection (vmy), vertical advection (wmp), vertically integrated radiative heating (qr), surface heat flux (qt), and sum of all budget terms on rhs of the MSE budget equation (sum).

Fig. 11.  Decomposition of the anomalous meridional MSE advection term (−${\left\langle {v\partial m/\partial y} \right\rangle '}$; W m−2) averaged over the northern box (red box in Fig. 8) for (a) EP El Niño, (b) La Niña, and (c) their difference (La Niña minus EP El Niño). Bars from left to right represent total and each component of the meridional advection term: −${\left\langle {v\partial m/\partial y} \right\rangle '}$, −${\left\langle {{v_1}\partial {m_1}/\partial y} \right\rangle '}$, −${\left\langle {{v_1}\partial {m_2}/\partial y} \right\rangle '}$, −${\left\langle {{v_1}\partial {m_3}/\partial y} \right\rangle '}$, −${\left\langle {{v_2}\partial {m_1}/\partial y} \right\rangle '}$, −${\left\langle {{v_2}\partial {m_2}/\partial y} \right\rangle '}$, −${\left\langle {{v_2}\partial {m_3}/\partial y} \right\rangle '}$, −${\left\langle {{v_3}\partial {m_1}/\partial y} \right\rangle '}$, −${\left\langle {{v_3}\partial {m_2}/\partial y} \right\rangle '}$, −${\left\langle {{v_3}\partial {m_3}/\partial y} \right\rangle '}$, and the sum of the nine decomposed terms. Here subscripts 1, 2, and 3 denote the climatological annual cycle, interannual, and high frequency (< 90 days) components.

Fig. 12.  Vertical profiles of term −${\left\langle {{v_3}\partial {m_1}/\partial y} \right\rangle '}$ (i.e., advection of the mean MSE by intraseasonal meridional wind; W m−2) averaged over the northern box (red box in Fig. 8) during EP El Niño (green) and La Niña (red) winters.

Fig. 13.  Horizontal patterns of the climatological mean MSE (shaded; J m−2) and intraseasonal wind (vector; m s−1) fields averaged over 800–600 hPa. Right panels show the meridional distributions of the mean MSE (m1; blue) and anomalous meridional wind (v3; red) averaged over 130°–150°E during (a, b) EP El Niño and (c, d) La Niña winters.

Fig. 14.  Meridional distributions of the regressed vertical p-velocity anomaly (${\omega _3}$; Pa s−1) at 700 hPa averaged over 130°–150°E for EP El Niño (green) and La Niña (red) winters. The regression was based on the time series of the OLR anomaly at the reference point (5°S, 95°E).

Fig. 15.  Lead–lag regressed patterns of 10–90-day filtered OLR anomaly (shaded; W m−2) and associated wind anomaly field at 850 hPa (vector; m s−1) from Days 0 to 6. The regression was based on the OLR anomaly at a reference point over Indian Ocean (5°S, 95°E) during 2015 El Niño (left) and 2017 La Niña (right) winter (DJF), respectively. The OLR anomalies that pass the significant test at 95% confidence level are stippled.

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

Impact of ENSO on MJO Pattern Evolution over the Maritime Continent

Corresponding author: Tim LI, timli@hawaii.edu;
• 1. Key Laboratory of Meteorological Disaster, Ministry of Education (KLME)/Joint International Research Laboratory of Climate and Environmental Change (ILCEC)/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), 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, Honolulu, HI 96822, USA
Funds: Supported by the National Natural Science Foundation of China (42088101 and 41875069), US National Science Foundation (AGS-2006553), and US NOAA Grant (NA18OAR4310298). This is SOEST contribution number 11206 and IPRC contribution number 1494

Abstract: The modulation of Madden–Julian oscillation (MJO) pattern evolution over the Maritime Continent (MC) by El Niño–Southern Oscillation (ENSO) was investigated through a combined observational and modeling study. MJO convective branches shifted south of the equator over the MC during eastern Pacific (EP) El Niño winters, while it became relatively symmetric about the equator during La Niña winters. The impact of central Pacific (CP) El Niños to MJO pattern, on the other hand, is not statistically significant. The cause of the distinctive MJO pattern evolutions is likely attributed to the ENSO-induced changes of the background moisture and vertical shear over the MC. Idealized numerical experiments with a 2.5-layer model were carried out, and the result revealed that the background moisture change played a dominant role. An observational diagnosis of column-integrated moist static energy (MSE) budgets was further conducted. The result indicated that the MJO pattern difference was attributed to the MSE tendency asymmetry in front of MJO convection between EP El Niño and La Niña, caused by the advection of the mean MSE by anomalous meridional wind. The difference in the MJO-scale anomalous meridional wind was ultimately controlled by the change of the background meridional moisture gradient associated with EP El Niño and La Niña.

Reference (52)

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