Edvin Aldrian

THE MARITIME CONTINENT MONSOON

The Maritime Continent is situated between the Asian and Australian summer monsoons, with monsoon rainfall generally peaking during boreal winter. The seasonal asymmetries are geographically complex and reflect multiscale interactions. Monsoon rainfall exhibits pronounced variability on all time scales from diurnal to interannual and longer, and is well correlated with ENSO during the dry and transition (i.e. June-November) seasons. Monsoon onset is substantially delayed during EI Nino years, while the monsoon retreat is less impacted. Regional model simulations reveal increased monsoon rainfall intensities over orography during EI Nino events, tied to strengthened diurnal landsea and mountain breeze circulations, associated with weaker large-scale winds conditions during EI Nino events. On intraseasonal and synoptic scales the region is heavily influenced by the MJO and cold surges, which can interact with each other as well as with in situ synoptic systems such as the Borneo vortex, often leading to torrential rainfall, flash floods, and severe storms, including one rare case, a typhoon. 85

Twentieth century sea surface temperature and salinity variations at Timor inferred from paired coral δ18O and Sr/Ca measurements

The Indonesian Throughflow (ITF), which represents the global ocean circulation connecting the Pacific Warm Pool to the Indian Ocean, strongly influences the Indo-Pacific climate. ITF monitoring since the late 1990s using mooring buoys have provided insights on seasonal and interannual time scales. However, the absence of longer records limits our perspective on its evolution over the past century. Here, we present sea surface temperature (SST) and salinity (SSS) proxy records from Timor Island located at the ITF exit passage via paired coral δ18O and Sr/Ca measurements spanning the period 1914–2004. These high-resolution proxy based climate data of the last century highlights improvements and cautions when interpreting paleoclimate records of the Indonesian region. If the seasonality of SST and SSS is not perfectly in phase, the application of coral Sr/Ca thermometry improves SST reconstructions compared to estimates based on coral δ18O only. Our records also underline the importance of ocean advection besides rainfall on local SSS in the region. Although the El Nino/Southern Oscillation (ENSO) causes larger anomalies relative to the Indian Ocean Dipole (IOD), Timor coral-based SST and SSS records robustly correlate with IOD on interannual time scales, whereas ENSO only modifies Timor SST. Similarly, Timor SST and SSS are strongly linked to Indian Ocean decadal-scale variations that appear to lead Timor oceanographic conditions by about 1.6–2 years. Our study sheds new light on the complex signatures of Indo-Pacific climate modes on SST and SSS dynamics of the ITF.

Coral Sr/Ca-based sea surface temperature and air temperature variability from the inshore and offshore corals in the Seribu Islands, Indonesia

The ability of massive Porites corals to faithfully record temperature is assessed. Porites corals from Kepulauan Seribu were sampled from one inshore and one offshore site and analyzed for their Sr/Ca variation. The results show that Sr/Ca of the offshore coral tracked SST, while Sr/Ca variation of the inshore coral tracked ambient air temperature. In particular, the inshore SST variation is related to air temperature anomalies of the urban center of Jakarta. The latter we relate to air-sea interactions modifying inshore SST associated with the land-sea breeze mechanism and/or monsoonal circulation. The correlation pattern of monthly coral Sr/Ca with the Niño3.4 index and SEIO-SST reveals that corals in the Seribu islands region respond differently to remote forcing. An opposite response is observed for inshore and offshore corals in response to El Niño onset, yet similar to El Niño mature phase (December to February). SEIO SSTs co-vary strongly with SST and air temperature variability across the Seribu island reef complex. The results of this study clearly indicate that locations of coral proxy record in Indonesia need to be chosen carefully in order to identify the seasonal climate response to local and remote climate and anthropogenic forcing.

ENSO modulation of seasonal rainfall and extremes in Indonesia

This paper provides a detailed description of how ENSO events affect seasonal and extreme precipitation over Indonesia. Daily precipitation data from 97 stations across Indonesia covering the period from 1981 to 2012 were used to investigate the effects of El Niño and La Niña on extreme precipitation characteristics including intensity, frequency and duration, as defined based on a subset of the Expert Team on Climate Change Detection and Indices (ETCCDI). Although anomalous signals in these three indices were consistent with those of total rainfall, anomalies in the duration of extremes [i.e., consecutive dry days (CDD) and consecutive wet days (CWD)] were much more robust. El Niño impacts were particularly prominent during June–July–August (JJA) and September–October–November (SON), when anomalously dry conditions were experienced throughout the country. However, from SON, a wet anomaly appeared over northern Sumatra, later expanding eastward during December–January–February (DJF) and March–April–May (MAM), creating contrasting conditions of wet in the west and dry in the east. We attribute this apparent eastward expansion of a wet anomaly during El Niño progression to the equatorial convergence of two anti-cyclonic circulations, one residing north of the equator and the other south of the equator. These anti-cyclonic circulations strengthen and weaken according to seasonal changes and their coupling with regional seas, hence shaping moisture transport and convergence. During La Niña events, the eastward expansion of an opposite (i.e., dry) anomaly was also present but less prominent than that of El Niño. We attribute this to differences in regional ocean—atmosphere coupling, which result in the contrasting seasonal evolution of the two corresponding anomalous cyclonic circulations and in turn suggests the strong nonlinearity of El Niño and La Niña responses over the Maritime Continent. Based on the seasonal behaviour of anomalous CDD and CWD, we propose five sub-divisions of the Indonesian region for both El Niño and La Niña.

Spatio-temporal characteristics of temperature and precipitation extremes in Indonesian Borneo

This study aims to investigate the characteristics of temperature and precipitation extremes in Indonesian Borneo both in space and time. Using daily data of 15 weather stations, a subset of 12 climate extreme indices has been calculated to clarify whether the frequency, intensity and duration of temperature and precipitation extremes have changed over the last three decades. Results show that the island has clearly warmed up for the last three decades. The annual average of maximum temperature (TXmean) and minimum temperature (TNmean) increased significantly by 0.22 (0.36) °C per 10 years during the studied period. The annual number of warm days (TX90p) and warm nights (TN90p) significantly increased while the number of cool days (TX10p) and cool nights (TN10p) decreased significantly. In contrast, the trends of precipitation extremes were not clearly observed. The changes in the extreme rainfall events are generally less consistent between the different stations. However, the tendency of the island to b...