Browse

You are looking at 1 - 10 of 56 items for :

  • Years of the Maritime Continent x
  • Refine by Access: All Content x
Clear All
Hyemi Kim
and
James J. Benedict

Abstract

Studies have indicated exaggerated Maritime Continent (MC) barrier effect in simulations of the Madden-Julian oscillation (MJO), a dominant source of subseasonal predictability in the tropics. This issue has plagued the modeling and operational forecasting communities for decades, while the sensitivity of MC barrier on MJO predictability has not been addressed quantitatively. In this study, perfect-model ensemble forecasts are conducted with an aqua-planet configuration of the Community Earth System Model Version 2 (CESM2) in which both basic state and tropical modes of variability are reasonably simulated with a Warm Pool-like SST distribution. When water-covered terrain mimicking MC land masses is added to the Warm Pool-like SST framework, the eastward propagation of the MJO is disturbed by the prescribed MC aqua-mountain. The MJO predictability estimate with the perfect-model experiment is about 6 weeks but reduces to about 4 weeks when the MJO is impeded by the MC aqua-mountain. Given that the recent operational forecasts show an average of 3-4 weeks of MJO prediction skill, we can conclude that improving the MJO propagation crossing the MC could improve the MJO skill to 5-6 weeks, close to the potential predictability found in this study (6 weeks). Therefore, more effort on understanding and improving the MJO propagation is needed to enhance the MJO and MJO-related forecasts to improve the subseasonal-to-seasonal prediction.

Restricted access
Biao Geng
and
Masaki Katsumata

Abstract

This study investigated the daily cycle of the wind and divergence fields observed off the southwestern coast of Sumatra during a field campaign of the Years of the Maritime Continent pilot study. An algorithm was developed to retrieve kinematic variables from the single-Doppler data collected aboard the Research Vessel Mirai from 24 November to 13 December 2015. The observed daily cycles of the wind and divergence fields consisted of diurnal, semidiurnal, and short-term variations. Diurnal wind variation was characterized by deep and three-dimensional circulation. There was an approximate phase locking of the semidiurnal variation to the diurnal variation, both in the wind and divergence fields. The short-term wind variation occurred at a time scale of ∼1–3 h, and this pattern was associated with density currents or mesoscale gravity waves. Up to 73% of the daily vertical motion variance can be attributed to the diurnal and semidiurnal vertical motion variations with comparable strengths. Concurrently, precipitation propagated offshore in phase with density currents and mesoscale gravity waves. Our results suggest that diurnal and semidiurnal wind variations dominate the daily evolution of precipitation, whereas density currents and mesoscale gravity waves control offshore propagation. Additionally, it appears that the daily precipitation cycle is modulated by multiple-time-scale wind variabilities of less than a day, which is also responsible for the development of strong nocturnal convection off the southwestern coast of Sumatra.

Significance Statement

To improve our understanding of the daily wind and divergence cycle off the southwestern coast of Sumatra, we examined wind data collected by a shipborne Doppler radar. The observed daily cycles of the wind and divergence fields consisted of diurnal and semidiurnal variations, as well as a 1–3-h variation associated with a density current or mesoscale gravity wave. Our results suggest that diurnal and semidiurnal wind variations dominate the daily evolution of precipitation, whereas density currents and mesoscale gravity waves control offshore propagation. Thus, we highlight the role of multiple-time-scale wind variabilities of less than a day in modulating the daily precipitation cycle off the southwestern coast of Sumatra.

Open access
Chen Chen
,
Sandeep Sahany
,
Aurel F. Moise
,
Xin Rong Chua
,
Muhammad E. Hassim
,
Gerald Lim
, and
Venkatraman Prasanna

Abstract

The Maritime Continent (MC), located in the heart of the Indo-Pacific warm pool, plays an important role in the global climate. However, the future MC climate is largely unknown, in particular the ENSO-rainfall teleconnection. ENSO induces a zonal dipole pattern of rainfall variability across the Indo-Pacific Ocean, i.e., positive variability in the Tropical Pacific and negative variability towards the MC. Here new CMIP6 models robustly project that, for both land and sea rainfall, the negative ENSO teleconnection over the MC (drier/wetter during El Niño/La Niña) could intensify significantly under the SSP585 warming scenario. Strengthened teleconnection may cause enhanced droughts and flooding, leading to agricultural impacts and altering rainfall predictability over the region. Models also project that the Indo-Pacific rainfall center and the zero-crossing of dipole-like rainfall variability both shift eastward, which adjustments are more notable during boreal summer than winter. All these projections are robustly supported by the model agreement and scale up with the warming trend.

Restricted access
Yihao Zhou
,
Shuguang Wang
,
Juan Fang
, and
Da Yang

Abstract

The Maritime Continent disrupts eastward propagation of the Madden–Julian oscillation (MJO). This study surveys the impact of the disruption—often known as the barrier effect—on the MJO teleconnections. The MJO propagation may be broadly categorized based on whether the MJO precipitation crosses the Maritime Continent (MC) during extended boreal winter seasons: successfully propagating across the MC (MJO-C) or being blocked by the MC (MJO-B). Compositing atmospheric circulation upon these two categories reveals that precipitation anomalies of MJO-C are stronger and more coherent than those of MJO-B, while their phase speed and lifetime are comparable. MJO-C and MJO-B excite distinct extratropical responses due to their diabatic heating in the deep tropics. Midlatitude circulation displays stronger and long-lasting negative geopotential anomalies in the northern Pacific Ocean 5–14 days after phase 7–8 of MJO-C, but significantly weaker anomalies from MJO-B. The extratropical water vapor transport during MJO-B and MJO-C differs markedly after phase 2. The Pacific–North American (PNA) pattern and North Atlantic Oscillation (NAO) both show significant response after phase 6 of MJO-C as its precipitation anomaly over the tropical Pacific during this period is stronger, while MJO-B has little impact on both. Surface air temperatures (SAT) at high latitudes during MJO-B and MJO-C are also significantly different. SAT is weaker and delayed in MJO-B in comparison to MJO-C, likely due to different meridional eddy heat fluxes.

Restricted access
Xueli Yin
,
Dongliang Yuan
,
Xiang Li
,
Zheng Wang
,
Yao Li
,
Corry Corvianawatie
,
Adhitya Kusuma Wardana
,
Dewi Surinati
,
Adi Purwandana
,
Mochamad Furqon Azis Ismail
,
Asep Sandra Budiman
,
Ahmad Bayhaqi
,
Praditya Avianto
,
Edi Kusmanto
,
Priyadi Dwi Santoso
,
Dirhamsyah
, and
Zainal Arifin

Abstract

The mean circulation and volume budgets in the upper 1200 m of the Maluku Sea are studied using multiyear current meter measurements of four moorings in the Maluku Channel and of one synchronous mooring in the Lifamatola Passage. The measurements show that the mean current in the depth range of 60–450 m is northward toward the Pacific Ocean with a mean transport of 2.07–2.60 Sv (1 Sv ≡ 106 m3 s−1). In the depth range of 450–1200 m, a mean western boundary current (WBC) flows southward through the western Maluku Sea and connects with the southward flow in the Lifamatola Passage. The mean currents in the central-eastern Maluku Channel are found to flow northward at this depth range, suggesting an anticlockwise western intensified gyre circulation in the middle layer of the Maluku Sea. Budget analyses suggest that the mean transport of the intermediate WBC is 1.83–2.25 Sv, which is balanced by three transports: 1) 0.62–0.93 Sv southward transport into the Seram–Banda Seas through the Lifamatola Passage, 2) 0.97–1.01 Sv returning to the western Pacific Ocean through the central-eastern Maluku Channel, and 3) a residual transport surplus, suggested to upwell to the upper layer joining the northward transport into the Pacific Ocean. The dynamics of the intermediate gyre circulation are explained by the potential vorticity (PV) integral constraint of a semienclosed basin.

Significance Statement

The Indonesian Throughflow plays an important role in the global ocean circulation and climate variations. Existing studies of the Indonesian Throughflow have focused on the upper thermocline currents. Here we identify, using mooring observations, an intermediate western boundary current with the core at 800–1000-m depth in the Maluku Sea, transporting intermediate waters from the Pacific into the Seram–Banda Seas through the Lifamatola Passage. Potential vorticity balance suggests an anticlockwise gyre circulation in the intermediate Maluku Sea, which is evidenced by the mooring and model data. Transport estimates suggest northward countercurrent in the upper Maluku Sea toward the Pacific, supplied by the Lifamatola Passage transport and upwelling from the intermediate layer in the Maluku Sea. Our results suggest the importance of the intermediate Indonesian Throughflow in global ocean circulation and overturn. More extensive investigations of the Indo-Pacific intermediate ocean circulation should be conducted to improve our understanding of global ocean overturn and heat and CO2 storages.

Restricted access
Daniel Argüeso
,
A. Di Luca
,
N. C. Jourdain
,
R. Romero
, and
V. Homar

Abstract

The Maritime Continent is one of the most challenging regions for atmospheric models. Processes that modulate deep convection are poorly represented in models, which affects their ability to simulate precipitation features accurately. Thus, future projections of precipitation over the region are prone to large uncertainties. One of the key players in modeling tropical precipitation is the convective representation, and hence convection-permitting experiments have contributed to improve aspects of precipitation in models. This improvement creates opportunities to explore the physical processes that govern rainfall in the Maritime Continent, as well as their role in a warming climate. Here, we examine the response to climate change of models with explicit and parameterized convection and how that reflects in precipitation changes. We focus on the intensification of spatial contrasts as precursors of changes in mean and extreme precipitation in the tropical archipelago. Our results show that the broad picture is similar in both model setups, where islands will undergo an increase in mean and extreme precipitation in a warmer climate and the ocean will see less rain. However, the magnitude and spatial structure of such changes, as well as the projection of rainfall percentiles, are different across model experiments. We suggest that while the primary effect of climate change is thermodynamical and it is similarly reproduced by both model configurations, dynamical effects are represented quite differently in explicit and parameterized convection experiments. In this study, we link such differences to horizontal and vertical spatial contrasts and how convective representations translate them into precipitation changes.

Restricted access
Sopia Lestari
,
Alain Protat
,
Valentin Louf
,
Andrew King
,
Claire Vincent
, and
Shuichi Mori

Abstract

Jakarta, a megacity in Indonesia, experiences recurrent floods associated with heavy rainfall. Characteristics of subdaily rainfall and the local factors influencing rainfall around Jakarta have not been thoroughly investigated, primarily because of data limitations. In this study, we examine the frequency and intensity of hourly and daily rain rate, including spatial characteristics and variations across time scales. We use 6-min C-band Doppler radar and 1-min in situ data during 2009–12 to resolve spatial rain-rate characteristics at higher resolution than previous studies. A reflectivity–rain rate (Z–R) relationship is derived (Z = 102.7R 1.75) and applied to estimate hourly rain rate. Our results show that rain rate around Jakarta is spatially inhomogeneous. In the rainy season [December–February (DJF)], rain rate exhibits statistical properties markedly different from other seasons, with much higher frequency of rain, but, on average, less intense rain rate. In all seasons, there is a persistent higher hourly and daily mean rain rate found over mountainous areas, indicating the importance of local orographic effects. In contrast, for hourly rain-rate extremes, peaks are observed mostly over the coastal land and lowland areas. For the diurnal cycle of mean rain rate, a distinct afternoon peak is found developing earlier in DJF and later in the dry season. This study has implications for other analyses of mesoscale rain-rate extremes in areas of complex topography and suggests that coarse-grain products may miss major features of the rain-rate variability identified in our study.

Significance Statement

For many years, Jakarta and its surrounding regions have been repeatedly inundated by flooding triggered by short-duration heavy rainfall or rainfall accumulated over multiple days. Little is known about the distribution of local rainfall and how it differs between seasons. In this study, we used high-resolution C-band Doppler radar during 2009–12 to understand the characteristics of rainfall over this complex topography. The results demonstrate that the rainfall features vary spatially and seasonally. In the wet season, rainfall is more frequent but, on average, lighter relative to other seasons. In all seasons, the highest hourly and daily mean rain rate persistently occurs over the mountains, indicating the vital role of topography in generating rainfall in the region.

Restricted access
Yihao Zhou
,
Shuguang Wang
, and
Juan Fang

Abstract

Surface precipitation anomalies over Maritime Continent islands typically lead oceanic precipitation by a week in the form of dipolar pattern before the arrival of Madden–Julian oscillation (MJO) convective phase. The authors study this dipolar pattern over Borneo during the boreal winter MJO event in January–February 2017 using cloud-permitting modeling, observation, and reanalysis datasets. The diurnal cycles of precipitation are analyzed during the local growing and decaying stages of this MJO event. Both the observation and simulation show positive precipitation anomaly over southwestern Borneo and negative anomaly over northeastern Borneo associated with the MJO easterly in the growing stage, whereas the pattern reverses in the decaying stage. Due to relatively high terrain, the low-level flows over Borneo split near the topography on the diurnal time scale. During the late afternoon and night (1700–2000 local solar time), the splitting-flow-induced wake vortices and thermally driven sea breezes tend to converge at the leeside, both contributing to leeward convergence and precipitation, which peaks at midnight. Subsequent offshore propagation during midnight and early morning develops from the leeward inland convection, and propagates northwestwards in the growing stage over west Borneo, and eastward in the decaying stage over east Borneo. Offshore propagation lasts until the next noon when sea breezes and island convection initiate. The timing and location of the offshore propagation suggest that it is not an independent convective mode. Instead, it is tied to the dipolar distribution of island precipitation modulated by the MJO.

Restricted access
Sopia Lestari
,
Andrew King
,
Claire Vincent
,
Alain Protat
,
David Karoly
, and
Shuichi Mori

Abstract

Research on the interaction between the Madden–Julian oscillation (MJO) and rainfall around Jakarta is limited, although the influence of the MJO on increased rainfall is acknowledged as one of the primary causes of flooding in the region. This paper investigates the local rainfall response around Jakarta to the MJO. We used C-band Doppler radar in October–April during 2009–12 to study rain-rate characteristics at much higher resolution than previous analyses. Results show that the MJO strongly modulates rain rates over the region; however, its effect varies depending on topography. During active phases, MJO induces a high rain rate over the ocean and coast, meanwhile during suppressed phases, it generates a high rain rate mainly over the mountains. In phase 2 of the MJO we find the strongest increase in mean and extreme rain rate, which is earlier in the MJO cycle than most studies reported, based on lower-resolution data. This higher rain rate is likely due to increases in convective and stratiform activities. The MJO promotes more stratiform rain once it resides over Indonesia. In phase 5, over the northwestern coast and western part of the radar domain, the MJO might bring forward the peak of the hourly rain rate that occurs in the early morning. This is likely due to a strong westerly flow arising from MJO superimposed westerly monsoonal flow, blocked by the mountains, inducing a strong wind propagating offshore resulting in convection near the coast in the morning. Our study demonstrates the benefits of using high-resolution radar for capturing local responses to the larger-scale forcing of the MJO in Indonesia.

Significance Statement

Rainfall in Jakarta and its surroundings is highly variable and often heavy resulting in devastating floods. In this region, in the wet season, rainfall is influenced by large-scale climate variability including the Madden–Julian oscillation (MJO) characterized by eastward propagation of clouds near the equatorial regions on intraseasonal time scales. The MJO has been known to increase the probability of rainfall occurrence and its magnitude, but we show that the impact differs in varying topography. The frequency and intensity of rainfall increase over land areas including mountains even when MJO has not arrived in Indonesia. Meanwhile, once MJO moves through Indonesia, the frequency and magnitude of the rainfall increases over the northern coast and ocean as well as in the west of the radar domain.

Restricted access
Sang-Ki Lee
,
Hosmay Lopez
,
Gregory R. Foltz
,
Eun-Pa Lim
,
Dongmin Kim
,
Sarah M. Larson
,
Kandaga Pujiana
,
Denis L. Volkov
,
Soumi Chakravorty
, and
Fabian A. Gomez

Abstract

A phenomenon referred to here as Java–Sumatra Niño/Niña (JSN or JS Niño/Niña) is characterized by the appearance of warm/cold sea surface temperature anomalies (SSTAs) in the coastal upwelling region off Java–Sumatra in the southeastern equatorial Indian Ocean. JSN develops in July–September and sometimes as a precursor to the Indian Ocean dipole, but often without corresponding SSTAs in the western equatorial Indian Ocean. Although its spatiotemporal evolution varies considerably between individual events, JSN is essentially an intrinsic mode of variability driven by local atmosphere–ocean positive feedback, and thus does not rely on remote forcing from the Pacific for its emergence. JSN is an important driver of climate variability over the tropical Indian Ocean and the surrounding continents. Notably, JS Niña events developing in July–September project onto the South and Southeast Asian summer monsoons, increasing the probability of heavy rainfall and flooding across the most heavily populated regions of the world.

Restricted access