PROPAGATION OF COLD SURGE DURING CROSS-EQUATORIAL NORTHERLY SURGE EVENTS BASED ON VERTICAL STRUCTURE OF THE ATMOSPHERE (CASE STUDY: JANUARY 2021)

• Main Author: Tika Simanjuntak, Juni
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/85182
• Institution: Institut Teknologi Bandung

Cross-Equatorial Northerly Surge (CENS) is an atmospheric phenomenon characterized by the strengthening of northerly winds from the South China Sea across the equator. One of the triggers of CENS is the extension of the Cold Surge (CS) phenomenon from the surface atmosphere over the South China Sea. CS propagation can affect the vertical structure of the atmosphere in the area it passes through. However, the mechanism of CS propagation into CENS is not yet known. Therefore, this study will identify the propagation of CS into CENS (CENS-CS) based on the vertical structure of the atmosphere and the causes of variations in CS propagation in the January 2021 CENS event. Analysis of changes in the vertical structure of the atmosphere using ERA5 reanalysis data and radiosonde observations in Hong Kong, Ranai, Pangkalpinang, and Jakarta. The vertical atmospheric structure parameters observed are meridional wind, temperature, and specific humidity in the 1000 hPa to 500 hPa layer. CS propagation on the three parameters is displayed in the cross section diagram based on ERA5 data and the Skew T Log P diagram based on radiosonde observation data. This data is analyzed every 12 hours in the period D-3 to D+1 of the peak CENS event. The results indicate that the propagation of CS into CENS can be observed based on changes in the vertical structure of the atmosphere from the northern South China Sea to western Java. The propagation of the CS is marked by the southward propagation of the negative anomaly in all three parameters, starting from Hong Kong on D-2 and reaching Jakarta on D+1 of the peak of the CENS event. The characteristics of this vertical structure of the atmosphere are also observed at four radiosonde observation locations, starting on D-2 in Hong Kong, D-1 in Ranai, D-0 in Pangkalpinang, and reaching Jakarta on D+1 of peak of the CENS event. The strengthening of the meridional wind, the decrease in temperature and the decrease in relative humidity significantly occurred in the upper layer of the atmosphere in the 850 hPa to 700 hPa layer in Hong Kong. However, in Ranai, Pangkalpinang, and Jakarta, this significant change occurred in the lower layer of the atmosphere in the 925 hPa to 850 hPa layer. The propagation of CS into CENS observed using ERA5 data and radiosonde observation data, generally shows consistent results. However, there is a difference in the specific humidity parameter in the lower layer of the atmosphere in the 1000 hPa to 925 hPa layer. The CENS event in January 2021 indicates variations in the duration and intensity of CS propagation. In the CENS-CS 1 event, the southward propagation of CS is clearly influenced by the high-pressure in the northern latitudes. The presence of a strong meridional gradient of pressure anomalies caused a strengthening of the north wind and a greater decrease in temperature in the CENS-CS 1 event. However, in the CENS-CS 2 event, the southward propagation of CS is less clear compared to the CENS-CS 1 event. This is due to a weaker high-pressure in the northern latitudes during the CENS-CS 2 event. The strengthening of the north wind in the CENS-CS 2 event is influenced by the presence of low pressure in the southeastern Indian Ocean which strengthened until the peak of the CENS event and the MJO phase 6 phenomenon that occurred at the same time. This resulted in a less intense meridional wind strengthening and a smaller temperature decrease, but these effects persisted for a longer duration during the CENS-CS 2 event.