Influence of ENSO on tropical cyclone intensity in the Fiji region

One of the most dominant climatic processes affecting severe weather events in the southwestern tropical Pacific, particularly tropical cyclones (TCs), is the El Niño Southern Oscillation (ENSO) phenomenon. A study is carried out in the Fiji region (defined as the area between 5-25°S and 170°E-170°W) to determine the relationship between accumulated cyclone energy (ACE) and ENSO. Results suggest that the seasonal variation of ACE is strongly affected by ENSO. High correlations of ACE can be found with several ENSO indices even as far back as the May-July preseason, thus having possible implications for the development of seasonal prediction schemes.

To further examine the effect of large-scale environmental forcing on ACE, various environmental conditions that are known to affect TC characteristics (i.e., 850-hPa relative vorticity, environmental vertical wind shear, 200-hPa divergence, sea surface temperature, mid-tropospheric relative humidity and equivalent potential temperature between surface and 500-hPa) are subjected to empirical orthogonal function (EOF) analysis. Only the principal components that are significantly related to ENSO are examined. Results clearly show a zonal oscillation of the spatial patterns for each condition about a zero isoline that straddles approximately 15°S latitude. This indicates that in El Niño years, conditions are more favourable to sustain high ACE values equatorward of 15°S. The reverse is true for La Niña years in the region poleward of 15°S.

After removing the effect of the contributions of TC frequency and TC days from ACE so that ACE is only dependent on TC intensity, it is found that cyclones tend to have higher intensity poleward of 15°S in La Niña years than in El Niño years. This is because as TCs track poleward of 15°S, they encounter large-scale conditions that are found to be less favourable to sustain TC intensity in El Niño years than in La Niña years. As a result, the rate of decay of TCs poleward of 15°S is larger in El Niño years than in La Niña years. Consequently, TCs are likely track further south in La Niña years than in El Niño years.