Tropical cyclones and depressions over the South Pacific Ocean since the late 19th century: Assessing synergistic relationship between the El Nino Southern Oscillation and Interdecadal Pacific Oscillation

Tropical cyclones (TCs) and tropical depressions (TDs), hereafter collectively referred to as tropical storms, often exhibit large year-to-year variability in the South Pacific Ocean basin. Many past studies have examined this variability in relation to the El Nino Southern Oscillation (ENSO) phenomenon, particularly using observational data from the post-satellite era (i.e., after the 1970s when TC observations became more consistent). However, less emphasis is placed on how tropical storms are modulated at interdecadal and decadal time scales such as due to Interdecadal Pacific Oscillation (IPO). This is because post-satellite data are available for relatively short time period (i.e., post-1970s), limiting our understanding of the IPO-TC relationship in the South Pacific. Here, using NOAA-CIRES 20th Century Reanalysis (20CR) dataset, we reconstruct historical records (1871-2014) of TC and depression proxies for the South Pacific Ocean basin, and then utilize these reconstructed proxies to first understand the connections between TC-ENSO and TC-IPO over the 20th century, and then investigate the combined effects of ENSO-IPO effects on TCs and depressions. Results show that La Nina (El Nino) is more dominant on TC activity than El Nino (La Nina) over the western subregion 140-170 degrees E (eastern sub-region, 170-220 degrees E) as expected. We also show that TC numbers are strongly modulated by the IPO phenomenon with, on average, more TCs occurring during the positive phase than during the negative phase of the IPO in both western and eastern sub-regions. We show for the first time (using a long-term reconstructed TC dataset) that the combined phases of El Nino and + IPO account for increased TC activity, as opposed to the combined phase of La Nina and -IPO, in the eastern sub-region. Similarly, the combined phase of La Nina and + IPO, as opposed to the combined phase of El Nino and -IPO, account for increased TC activity in the western sub-region. However, unlike TCs, the patterns of ENSO variability seem to be reversed for TDs. Changes in large-scale environmental conditions, such as environmental vertical wind shear, low-level cyclonic relative vorticity, mid-tropospheric relative humidity and sea surface temperature are linked to the various modes of variability patterns and their synergistic relationships.