Dynamical and Thermodynamical Influences of the Maritime Continent on ENSO Evolution

Abstract

El Niño-Southern Oscillation (ENSO) exerts tremendous influences on the global climate. Through dynamic lifting and thermal forcing, the Maritime Continent (MC) plays an important role in affecting global atmospheric circulation. In spite of the extensive studies on ENSO mechanisms, the influence of MC on the characteristics of ENSO life cycle remains unclear. Our coupled model experiments reveal that the absence of the MC land contributes to a strong ENSO asymmetry and a weakened nonlinear atmospheric response to the combined seasonal and interannual SST variations (i.e. the combination mode) that prolongs the warm events, resulting in a reduction of ENSO frequency. On the other hand, our experiments suggest that the global climate model applied (NCAR CESM) overestimates the MC topographic uplifting effect on ENSO simulation. Overall, this study provides a new physical insight into the nature of the MC influence on ENSO evolution.

Figure 1

First combined EOF modes of 925-hPa wind, representing the ENSO mode. (left) Spatial pattern and (right) corresponding principal component (PC) for (a,b) NCEP CFSR, (c,d) CTL experiment, (e,f) NOTOPO experiment, and (g,h) NOLAND experiment, respectively. Shadings in the left panels represent the pattern of zonal wind. In the right panels, solid black lines denotes the corresponding PC1s, and dashed red lines denotes Niño-3.4 indices.

Figure 2

Second combined EOF modes of 925-hPa wind, representing the combined mode. Same as Fig. 1, but for the second leading combination EOF modes.

Figure 3

Composite of El Niño and La Niña events. Composite patterns of 3-month running mean Niño-3.4 SST anomalies in (a) El Niño and (b) La Niña events in observation (black line), CTL experiment (red line), NOTOPO experiment (orange line), and NOLAND experiment (blue line). El Niño (La Niña) events are defined as 3 month running mean of Niño-3.4 SST anomaly exceeding 0.5 (−0.5) for 5 consecutive overlapping seasons. The x-coordinate indicates corresponding months from ENSO Year 0 to Year 1. The curves are normalized by dividing each with its peak value.

Figure 4

Ocean-atmosphere anomalies related to ENSO life cycle. Correlations of SST and 10 m wind with the first PCs for (left) lag 0-month and (right) lag 5-month from (a,b) NCEP CFSR, (c,d) CTL experiment, (e,f) NOTOPO experiment, and (g,h) NOLAND experiment, respectively. Significant values exceeding the 90% confidence level are shown.

Figure 5

ENSO decaying process. Correlations of SLP and 10 m wind with the second PCs for (left) lead 4-month, (middle) lead 2-month, and (right) lag 0-month from (a–c) NCEP CFSR, (d–f) CTL experiment, (g–i) NOTOPO experiment, and (j–l) NOLAND experiment, respectively. Significant values exceeding the 90% confidence level are shown.