Modulation of Madden-Julian Oscillation Activity by the Tropical Pacific-Indian Ocean Associated Mode

Abstract

In this study, the impacts of the tropical Pacific-Indian Ocean associated mode (PIOAM) on Madden-Julian Oscillation (MJO) activity were investigated using reanalysis data. In the positive (negative) phase of the PIOAM, the amplitudes of MJO zonal wind and outgoing longwave radiation are significantly weakened (enhanced) over the Indian Ocean, while they are enhanced (weakened) over the central and eastern Pacific. The eastward propagation of the MJO can extend to the central Pacific in the positive phase of the PIOAM, whereas it is mainly confined to west of 160°E in the negative phase. The PIOAM impacts MJO activity by modifying the atmospheric circulation and moisture budget. Anomalous ascending (descending) motion and positive (negative) moisture anomalies occur over the western Indian Ocean and central-eastern Pacific (Maritime Continent and western Pacific) during the positive phase of the PIOAM. The anomalous circulation is almost the opposite in the negative phases of the PIOAM. This anomalous circulation and moisture can modulate the activity of the MJO. The stronger moistening over the Indian Ocean induced by zonal and vertical moisture advection leads to the stronger MJO activity over the Indian Ocean in the negative phase of the PIOAM. During the positive phase of the PIOAM, the MJO propagates farther east over the central Pacific owing to the stronger moistening there, which is mainly attributable to the meridional and vertical moisture advection, especially low-frequency background state moisture advection by the MJO’s meridional and vertical velocities.

摘要

本文利用再分析资料研究了热带太平洋—印度洋联合模(PIOAM)对MJO活动的影响。在PIOAM正(负)位相, MJO纬向风和向外长波辐射的振幅在印度洋上显著减弱(增强), 在中东太平洋上显著增强(减弱)。在PIOAM正位相, MJO的东传能到达中太平洋, 然而在PIOAM负位相, MJO的活动主要局限在160°E以西。PIOAM通过改变大气环流和水汽收支来影响MJO活动。当PIOAM处于正位相, 西印度洋和中东太平洋(西太平洋)上空出现了异常的上升(下降)运动和正(负)水汽异常。经向和垂直水汽平流输送, 特别是MJO的经向和垂直速度对低频背景场水汽的平流输送能引起中太平洋上空大气湿度正异常, 进而导致MJO的进一步东传。当PIOAM处于负位相时, 异常环流形态几乎相反。纬向和垂直水汽平流输送引起印度洋上空大气湿度正异常, 进而导致印度洋上空较强的MJO活动。