Modulation of Boreal Summer Intraseasonal Oscillations over the Western North Pacific by ENSO

Abstract

The authors investigate the effects of El Niño and La Niña on the intraseasonal oscillation (ISO) in the boreal summer (May–October) over the western North Pacific (WNP). It is found that during El Niño summers, the ISO is dominated by a higher-frequency oscillation with a period of around 20–40 days, whereas during La Niña summers the ISO is dominated by a lower-frequency period of around 40–70 days. The former is characterized by northwestward-propagating convection anomalies in the WNP, and the latter is characterized by northward- and eastward-propagating convective signals over the tropical Indian Ocean/Maritime Continent. The possible mechanisms through which El Niño–Southern Oscillation (ENSO)-induced background mean state changes influence the ISO behavior are examined through idealized numerical experiments. It is found that enhanced (weakened) mean moisture and easterly (westerly) vertical wind shear in the WNP during El Niño (La Niña) are the main causes of the strengthened (weakened) 20–40-day northwestward-propagating ISO mode, whereas the 40–70-day ISO initiated from the Indian Ocean can only affect the WNP during La Niña years because the dry (moist) background moisture near the Maritime Continent during El Niño (La Niña) suppresses (enhances) the ISO over the Maritime Continent, and the ISO propagates less over the Maritime Continent during El Niño years than in La Niña years.

Impacts of ENSO on the Bay of Bengal Summer Monsoon Onset via Modulating the Intraseasonal Oscillation

“…The initial perturbation is centered at 70°E over the equator, with a zonal length of 4,000 km (refer to the Figure 7 in Liu et al, 2016). Under pentad mean states in the climatology, the same initial perturbations are specified to have a theoretical Kelvin wave structure (i.e., a baroclinic perturbation with negative pressure anomalies in the lower troposphere and positive anomalies in the upper troposphere).…”

Empirical Subseasonal Prediction of Summer Rainfall Anomalies over the Middle and Lower Reaches of the Yangtze River Basin Based on Atmospheric Intraseasonal Oscillation

“…Six pentad previous predictors and six pentad succeeding predictands are concatenated in the ESVD to conduct the 5-30-day (5, 10, 15, 20, 25 and 30 days, respectively) lead-time forecasts simultaneously. For example, at the forecast time of early pentad 31, the predictors at pentads 25-30 are concatenated to forecast the predictand at pentads [31][32][33][34][35]. In the present study, the forecast time starts at early pentad 31 and ends at early pentad 55.…”

“…As indicated in Figure 5a The intensity (variance) of ISO has been suggested to be related to tropical SSTs [32]; therefore, the year-to-year Niño3.4 index and Oceanic Niño Index (ONI) in boreal winter (December, January and February) are plotted in Figure 7b to check their relationship with ISO intensity. The results show that the high TCC skill of summer MLRYB rainfall anomalies in 2003 and 2010 follows the two strong El Niño events.…”

“…The rainfall condensation heating (cooling) over the Indian Ocean (Pacific Ocean) could excite a Matsuno-Gill-type Kelvin (Rossby) wave response [33,34], forming an anticyclonic circulation anomaly at lower levels over the East Asia-western North Pacific sector. The anomalous The intensity (variance) of ISO has been suggested to be related to tropical SSTs [32]; therefore, the year-to-year Niño3.4 index and Oceanic Niño Index (ONI) in boreal winter (December, January and February) are plotted in Figure 7b to check their relationship with ISO intensity. The results show that the high TCC skill of summer MLRYB rainfall anomalies in 2003 and 2010 follows the two strong El Niño events.…”

The Influence of Tropical Forecast Errors on Higher Latitude Predictions

“…The midlatitude response to tropical heating is very rapid in both theory and models, with a substantial response even after 2 days, with its character depending on the season and frequency of the tropical forcing (Branstator, ; Matthews et al, ; Newman & Sardeshmukh, ). Over time the response initially spreads poleward and eastward, with lower frequency forcing such as that associated with the Madden and Julian Oscillation (MJO) and El Ni o Southern Oscillation (ENSO) being particularly effective at producing teleconnections (Berbery & Nogues‐Paegle, ; Hsu, ; Liu et al, ; Roundy et al, ). Of particular relevance is that even short‐lived tropical convective pulses might also have a systematic imprint on subsequent midlatitude weather days and weeks later (Branstator, , their Figures 3 and 5).…”