Vertical structure of the 6 CENS events observed during the YMC-CSO2021
Figure 1 shows the horizontal distributions of precipitation, sea level pressure (SLP), and 10-m winds and their anomalies from the climatological mean averaged for the YMC-CSO2021 period from January 8 to March 8, 2021. During the YMC-CSO2021, northerly winds, as the basic state, were steadily dominant over the southern South China Sea, Karimata Strait, and Java Sea between the SLP ridge meridionally extending from the Indochina Peninsula and the low SLP area to northern Australia, and large precipitation areas were distributed in the vicinity of Java Island (Fig. 1a). The northerly winds around the CGK and PGK stations were significantly stronger than the climatological mean (Fig. 1b). In fact, an extreme precipitation event (> 200 mm/day) inducing widespread flooding occurred in association with the CENS event around the northern coast of Central Java Island on 5-6 February 7. In addition, positive precipitation and negative SLP anomalies were distributed over the eastern Indian Ocean and western Pacific Ocean and were associated with active convection of the MJO.
During the YMC-CSO2021 period, the following 6 events were extracted using the CENS index 4, as shown in Table 1. CENS1 (Jan. 18﹣20), CENS2 (Jan. 29﹣30), CENS3 (Feb. 2﹣5), CENS4 (Feb. 5﹣9), CENS5 (Feb. 18﹣20), and CENS6 (Feb. 25﹣26) each had different conditions for the CS index 3 and the phase and amplitude of the MJO index 13. Among the observed 6 CENS events, CENS1, CENS2, CENS4, and CENS5 were associated with CS conditions, whereas CENS3 and CENS6 were not. In addition, CENS1, CENS2, CENS3, CENS4, CENS5, and CENS6 occurred during MJO phases 4, 6, 6, 7, 7, and 6, respectively. Specifically, CENS3, CENS4, CENS5, and CENS6 occurred during the convective active phase of the MJO (the amplitude of the MJO index >= 1).
The vertical structure of winds in the 6 CENS events occurring under such different environmental conditions showed distinct characteristics in the radiosonde observations at PGK and CGK (Fig. 2ab). At PGK, which was located in the northern part of the CENS, when examining the northerly wind layer thickness using a 5 m/s threshold, CENS1 and CENS5 were confined to the lower atmosphere from the surface up to 800 hPa, whereas the northerly winds associated with CENS2, CENS3, CENS4, and CENS6 extended above 800 hPa (Fig. 2a). At CGK, which was located in the southern part of the CENS, the northerly wind layer thickness for CENS1, CENS2, CENS3, CENS4, and CENS5 was limited to below 800 hPa from the surface, but for CENS6, it reached 400 hPa (Fig. 2b). Furthermore, when examining the layer of strong westerlies exceeding 8 m/s at CGK, the characteristics of the zonal winds varied significantly by event, especially for CENS2, CENS3, and CENS4, which extended to 300 hPa. This fact indicates that CENS2, CENS3, and CENS4 occurred under environmental conditions in which westerly wind bursts developed due to enhanced convective activity associated with the MJO over the western Pacific Ocean (corresponding to Phases 6-7 with the MJO index). The vertical structure of the potential temperature (PT) in the radiosonde observations at PGK and CGK revealed the thermal characteristics of the 6 CENS events (Fig. 3ab). CENS1, CENS2, CENS3, and CENS5 were associated with cold anomalies of -0.5 K to -1 K, whereas CENS4 and CENS6 did not exhibit such cold anomalies.
By comparing the vertical profiles at the PGK and CGK locations from JRA-55 with the radiosonde data, it was possible to quantitatively and accurately represent both the differences and similarities in the vertical structures of the observed CENS northerly winds, westerly winds, and PT anomalies (Figs. 2cd and 3cd). This fact is very important as a premise for the statistical analysis with the JRA-55 shown in the next section.
Figure 4 shows the time–latitude cross-section of the SLP anomalies referenced to the SLP at the equator and northerly winds averaged over 105-120°E during the YMC-CSO2021. Focusing on the SLP anomalies to the north of 10°N, during the periods of CENS1, CENS2, CENS4, and CENS5, which were associated with the CS, there were significant positive SLP anomalies of more than 2 hPa. However, in CENS4, the positive SLP anomaly decayed northward, and in CENS6, it became a negative SLP anomaly. This fact indicates that the CENS did not simply occur as the southern edge of the southward-expanded CS. Focusing on the SLP anomalies to the south of 10°S, significant negative SLP anomalies corresponded to all 6 CENS events. In CENS4 and CENS6, which were not associated with the CS, the negative SLP anomalies significantly deepened to -2 to -3 hPa at 10°S, and the northerly wind expanded southward beyond 15°S. This common feature of significant negative SLP anomalies corresponding to all 6 CENS events implies that a southward pressure gradient near the low-SLP area between 15°S and 10°S was necessary to enhance the significant northerly winds of the CENS.
Statistical features of past CENS events
In this section, to investigate the representativeness of the 6 CENS events observed during the YMC-CSO2021, the statistical characteristics of CENS occurrences and the vertical structure from composite analysis are described. The pressure anomaly fields around the MC significantly fluctuate due to the El Niño-Southern Oscillation (ENSO) 14, and it is known that the ascending region of the Walker circulation is enhanced during La Niña years 15. Table 2 summarizes the CENS occurrences by ENSO phase and month for the 160 CENS events, which were extracted from 64 seasons—December, January, February, and March 1959-2022. Over 80% of the 160 CENS events occurred in January and February, showing a clear relationship with the ENSO phase. That is, the number of CENS occurrences was the lowest during El Niño years and the greatest during La Niña years across all months. However, there was no significant difference in the number of CS and non-CS CENS occurrences in any ENSO phase or monthly classification.
As shown in Fig. 5a, 2021 was a La Niña year, with 7 CENS occurrences over 4 months, making it the 3rd highest in the past 64 years. After extracting the 3 years with the greatest number of CENS occurrences, we found 9 in 1974, 8 in 1967, and 7 each in 1970, 2008, and 2021—all of which were La Niña years. The number of CENS occurrences in relation to the positive/negative DMI was 80/80, and there was no significant difference in the number of occurrences when the DMI thresholds were set at ±0.1, ±0.2, and ±0.3. Since the Indian Ocean Dipole (IOD)16 is a phenomenon with a larger amplitude from August to November, the impact of the IOD on the occurrence of CENSs could be small.
As shown in Fig. 5b, the duration of most CENSs ranged from 1 to 4 days, and there was no significant difference in the duration of CENSs with or without a CS. As indicated in Table 2, the highest number of CENS occurrences occurred in January, but all long-lived CENS events (lasting from 7 to 10 days) occurred in February. Compared to these statistical characteristics, the durations of the 6 CENS events observed during the YMC-CSO2021 were 3, 2, 3, 4, 2, and 2 days, which can be considered typical durations.
Fig. 5c shows the occurrences of 102 CENS events classified by the phase and amplitude of the MJO over a 48-year period from December 1974 to March 2022. First, it is evident that the CENS occurrences were significantly greater during periods when the MJO was active (amplitude of 1 or higher) than during periods when the MJO was inactive (amplitude less than 1). Examining the relationship between the CENS occurrences and the MJO phase, 35% (36 events) of the events occurred in Phases 4 and 5 when the MJO was positioned over the MC, and 52% (53 events) of the events occurred in Phases 6 and 7 when the MJO was positioned over the Pacific Ocean. As shown in Table 1, the 6 CENS events observed during the YMC-CSO2021 occurred 4 times when the MJO was active and 2 times when it was inactive, with 3 CENS events in MJO Phase 6, 2 CENS events in MJO Phase 7, and 1 CENS event in MJO Phase 4, which is consistent with the statistical characteristics indicated by the past 102 events.
Fig. 6 shows the statistical composited 3-dimensional structure of the past CENS events. Examining the distribution of the SLP anomalies referenced to the SLP at the equator shown in Fig. 6a, the surface northerly winds flowing across the MC were noticeable between the positive SLP anomalies of 9﹣15 hPa over southeastern Eurasia and the negative SLP anomalies of -3 hPa over northwestern Australia. In particular, the negative SLP anomalies below -3 hPa were distributed in the eastern Indian Ocean to the south of the Java and Sumatra Islands, and this feature is consistent with the fact that all 6 CENS events shown in Fig. 4 corresponded with the decreasing SLP in the Southern Hemisphere. A large precipitation area of 10 mm/day (indicated by the red contour), which corresponds to significant positive precipitation anomalies, was distributed ahead of the northerly winds of the CENS (southeastern Sumatra, Java, and southwestern Borneo Islands).
Examining the PT anomalies at the surface shown in Fig. 6b, the PT over the southern South China Sea, Karimata Strait, and Java Sea were relatively 0.5﹣1 K cooler than the PT over the Indian Ocean and Pacific Ocean. In other words, there was a locally significant zonal PT gradient crossing Sumatra and the Malay Peninsula at approximately 100°E.
Examining the latitudinal vertical cross-section shown in Fig. 6c, the northerly wind associated with CENS was deepest over the equator (reaching 700 hPa at the 5 m/s threshold) and extended to approximately 15°S. However, the cold air dome (indicated by the 298 K contour) originating from the Eurasian continent became indistinct near the equator. The lower-level convergence at the southern edge of the CENS was located between 15°S and 10°S, which coincided with the region of prominent ascending motion.
Examining the longitudinal vertical cross-section shown in Fig. 6d, in the western part of the northerly wind associated with the CENS (100°E﹣110°E), there were PT anomalies of -0.1﹣-0.5 K (indicated by the blue contours) against the westerly inflow from the Indian Ocean, accompanied by clear lower-level convergence (indicated by the red contour) and stronger ascending motion. These negative PT anomalies are consistent with the radiosonde observations shown in Fig. 3. Such a localized zonal PT gradient associated with the CENS suggests that the "weak temperature gradient approximation" often used in tropical wave theory 17–19 cannot be partially applicable over the MC.