1. Introduction
The Maritime Continent (MC) is situated in the tropical “warm pool” region, bridging the tropical Indian Ocean and the Pacific Ocean. The MC contains numerous islands and shallow/deep seas, and is the largest rainy area on Earth. Its complex land–sea distribution leads to significant diurnal variation in deep convection (e.g., Yang and Slingo 2001; Nesbitt and Zipser 2003; Mori et al. 2004; Qian 2008). Latent heating released from the deep convection plays an important role in driving the global atmospheric circulation and modulating climate variability (e.g., Ramage 1968; Sardeshmukh and Hoskins 1988; Jin and Hoskins 1995; Neale and Slingo 2003; Yamanaka et al. 2018). Understanding the multiscale convective processes over the MC region is critical to improve numerical simulation and prediction of weather and climate in the tropics. Because of its importance, the MC region is the main focus of the Years of the Maritime Continent (YMC) project (Yoneyama and Zhang 2020), an international field campaign targeting multiscale tropical convective processes in this region.
Precipitation over the MC islands can be significantly modulated by the Madden–Julian oscillation (MJO), the dominant intraseasonal variability in the tropics (Madden and Julian 1971, 1972). Ichikawa and Yasunari (2006) found that rainfall activity over the MC islands propagates to the leeside of the island between midnight and morning, and reasoned that propagation causes a leeward enhancement of rainfall. Rauniyar and Walsh (2011) showed that both the amplitude of the diurnal cycle and daily mean precipitation over MC islands are the strongest when the oceanic convection associated with the MJO is inhibited. Oh et al. (2012) found that the precipitation on the MC islands is the weakest in the local active phase of MJO, while it increases after the convective envelope of MJO leaves MC. Peatman et al. (2014) introduced the notion of the “precipitation vanguard” that precipitation on the MC islands appears about one week prior to the arrival of the large-scale MJO convection, while Coppin and Bellon (2019) suggested that the vanguard occurs even earlier than the results from Peatman et al. (2014). Further modeling studies examined details of precipitation in these islands, and suggested that individual characteristics of island topography/location/geometry can play important roles (Vincent and Lane 2017; Ruppert and Chen 2020; Ruppert et al. 2020; Tan et al. 2020; Wei et al. 2020; Wei and Pu 2022).
The diurnal cycle, as one of the most prominent atmospheric modes, plays important roles in shaping the regional climate over the MC. Nevertheless, how it interacts with the MJO is a subject of debate. Some authors suggested that these islands compete for moisture and reduce whatever is available for MJO oceanic convection (Oh et al. 2012; Zhang and Ling 2017; Ling et al. 2019), while others argued that they can inhibit the MJO propagation over the MC region through upscale effect (Majda and Yang 2016; Ajayamohan et al. 2021). Cloud-permitting modeling study by Zhou et al. (2021) suggested that presence of the MC islands changes the spatial distribution of mean moisture, and reduces the surface fluxes and horizontal advection of moisture, thereby inhibiting the smooth propagation of the MJO over MC region. Regardless of the distinct roles of the islands on the MJO propagation, it is of interest to understand what causes the precipitation vanguard (Peatman et al. 2014), not only for the sake of scientific understanding of the MJO–MC interaction, but also for the benefit to the local community.
Closer inspection of the precipitation vanguard reveals that it appears in the organized form of a dipolar pattern over the large islands. Qian (2020) showed positive precipitation anomalies on the west and negative anomalies on the east of the MC islands during the MJO easterly phase. These anomalies are asymmetric. The positive anomalies are stronger than the negative ones, leading to overall precipitation enhancement. The opposite occurs during the MJO westerly phase. The dipolar pattern differs from what would happen if the terrain forced ascent at the windward side is the main cause of precipitation. Bai and Schumacher (2022) also found that the positive anomalies on the east side of large MC islands lag the anomalies on the west side at different MJO phases (their Fig. 2), consistent with Qian’s finding. Qian (2020) suggested it occurs due to the same “wake effect” as discussed in Qian et al. (2013), which examined El Niño–Southern Oscillation (ENSO) modulation on the precipitation anomalies over the MC islands. The “wake effect” hypothesis while appealing is vague. The phrase is generally used to describe what happens to airflow after it passes an obstacle, in the general form of either waves or vortices, or a combination of both. It is studied almost exclusively without moist processes. In the context of island convection, moist convective processes and diurnal cycles add up considerable degree of complexity. These factors need to be taken into account to understand precipitation dipolar pattern.
We postulate that the key to understand rain over islands is the local mesoscale circulations—either thermally or mechanically forced—that trigger convection and/or interact with it. Wang and Sobel (2017) performed simulations of precipitation over idealized small tropical islands. They found that prevailing wind controls the flow regime over the island, and sea breeze dominates at weak prevailing wind conditions and produces more rain, while gravity waves become more important under stronger wind conditions. Zhu et al. (2017) showed that sea breeze plays crucial roles in the rainy season over the Hainan island. Vincent and Lane (2017) showed that the sensitivity of diurnal cycle to the MJO modulation may vary among the MC islands in numerical simulations, possibly due to different topography and location of islands. Wei et al. (2020) investigated the different features of diurnal cycle and offshore propagation of precipitation over several typical MC islands modulated by MJO, and compared the roles of different processes including topography, land–sea breeze, and gravity waves. Results from these studies indicate that one may gain significant insight from high-resolution simulations that resolve both convection and local mesoscale circulation.
The present study will focus on Borneo—the largest island in Asia at the center of MC, home to one of the oldest rain forest in the world. Precipitation on Borneo features a dipolar spatial pattern over different MJO phases (Qian 2020). The climatological precipitation near Borneo is shown in Fig. 1a. The maximum is found immediately offshore to the northwestern coast of Borneo, while the relative weak precipitation is evenly distributed over Borneo. The composites of precipitation anomaly during MJO phases 2–4 and 5–7 are shown in Figs. 1c and 1e, respectively, both characterized by evident dipolar pattern with opposite signs in different phases. Positive precipitation anomaly during phases 2–4 is located over western Borneo and negative anomaly over northeastern Borneo, while the distribution is opposite during phases 5–7, consistent with the results in Qian (2020). The climatology of 850-hPa wind is shown in Fig. 1b, and the composites of wind anomaly during MJO phases 2–4 and 5–7 are shown in Figs. 1d and 1f, respectively. The MJO-induced easterly and westerly anomaly dominates over Borneo during phases 2–4 and 5–7, respectively, which (we argue) is responsible for the dipolar distribution of precipitation. More details of the composites and datasets can be found in section 2a.
Composites of (a) observed precipitation (mm day−1) and (b) 850-hPa wind (vectors; m s−1) and corresponding wind speed (shading; m s−1, positive and negative values represent westerly and easterly wind, respectively) near Borneo in December–February of 2000–18. (c),(d) As in (a) and (b), but for the anomalies based on days when the amplitudes of the OMI index are greater than 1 during OMI phases 2–4. (e),(f) As in (c) and (d), but for results during OMI phases 5–7.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Composites of (a) observed precipitation (mm day−1) and (b) 850-hPa wind (vectors; m s−1) and corresponding wind speed (shading; m s−1, positive and negative values represent westerly and easterly wind, respectively) near Borneo in December–February of 2000–18. (c),(d) As in (a) and (b), but for the anomalies based on days when the amplitudes of the OMI index are greater than 1 during OMI phases 2–4. (e),(f) As in (c) and (d), but for results during OMI phases 5–7.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Composites of (a) observed precipitation (mm day−1) and (b) 850-hPa wind (vectors; m s−1) and corresponding wind speed (shading; m s−1, positive and negative values represent westerly and easterly wind, respectively) near Borneo in December–February of 2000–18. (c),(d) As in (a) and (b), but for the anomalies based on days when the amplitudes of the OMI index are greater than 1 during OMI phases 2–4. (e),(f) As in (c) and (d), but for results during OMI phases 5–7.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Because the state-of-the-art global reanalysis data cannot adequately resolve the local diurnal cycles over the MC islands (e.g., Jiang et al. 2019), and the mesoscale circulation from the reanalysis is likely unreliable at present, we will conduct cloud-permitting numerical simulation with an atmosphere–ocean coupled model. Due to computational cost, our simulation will focus on one MJO event—the January–February 2017 MJO event. This MJO event initiated in the Indian Ocean and successfully propagated across the MC to reach the western Pacific Ocean, maintaining vigorous large-scale convection throughout its life cycle, as discussed in Zhou et al. (2021). This event belongs to the “MC-crossing” category (e.g., Feng et al. 2015; Barrett et al. 2021; Kim et al. 2014; Zhou et al. 2021; Zhang and Ling 2017). We select this event partly because the MJO wind signal and its modulation of local diurnal cycles are well defined. Here, the simulation reasonably reproduces the dipolar precipitation feature over Borneo in this MJO event. The diurnal cycle of local flow and rain will be our focus below. It will be shown that the flows splitting/detouring around the terrain favor the convergence on the leeside and trigger convection there. Subsequent nocturnal offshore propagation of precipitation ensues from the leeward convection peak. The propagation features suggest that leeward convection plays a crucial role in not only the island convection but also offshore rain propagation.
The rest of this paper is organized as follows. Section 2 documents the numerical model setup and datasets used in this study. Section 3 presents the results, including the large-scale features over MC and the diurnal variations near Borneo during different MJO stages. The potential mechanism responsible for the dipolar precipitation and related offshore propagation is also examined in this part through the diagnosis of the Froude number and flows over vertical cross section, respectively. The summary is given in section 4. The paper is concluded with a schematic representation of the main processes related to the dipolar precipitation over Borneo.
2. Model and datasets
a. Datasets
The ERA-Interim dataset (Dee et al. 2011) and Navy Coupled Ocean Data Assimilation (NCODA) analysis (Cummings 2005; Cummings and Smedstad 2013) are used to drive the coupled model simulation (see section 2b). The 3-hourly ERA5 dataset (Hersbach et al. 2020) with horizontal grid spacing of 0.25° × 0.25° is used to provide the environmental fields (e.g., wind and water vapor) and validate our simulation. The 1° × 1° daily outgoing longwave radiation (OLR) version 1.2 (Lee 2011) from NOAA Climate Data Record (CDR) is used to represent the large-scale convection of the MJO. The 0.1° × 0.1° observational precipitation with 0.5‐hourly interval from Integrated Multi‐satellitE Retrievals for Global Precipitation Mission (IMERG) version 6 (Huffman et al. 2015) is used to show the observed dipolar pattern over Borneo and validate the simulation. The OMI index (Kiladis et al. 2014) is utilized to determine the life cycle and phases of the MJO.
Composite of anomalies from these datasets in Fig. 1 proceeds as follows. The first three harmonics of the annual cycle of precipitation are removed from climatology (computed as average from 2000 to 2018), and a 30–90-day Butterworth bandpass filter is applied to 850 hPa wind during the same period. The composites from resultant anomalies are calculated based on the strong MJO days when amplitudes of the OMI index are greater than 1 during different phases from December to February.
b. Coupled model and configurations
The coupled numerical simulation in this study is based on the coupler developed by Chen and Curcic (2016). The Weather Research and Forecasting (WRF) Model version 4.1.3 (Skamarock et al. 2019) and Hybrid Coordinate Ocean Model (HYCOM) version 2.2.34 (Wallcraft et al. 2009) are used as the atmosphere and ocean model component in this coupled system, respectively. The feedback of sea surface temperature (SST) from ocean model passing to the atmosphere model is allowed in this coupled configuration, through the Earth System Modeling Framework (ESMF) coupler.
Two computational domains (D01 and D02) are adopted for the atmosphere using the two-way nesting technique (Fig. 2), with horizontal grid spacing of 9 and 3 km, respectively. D01 (20°S–20°N, 60°E–180°) covers Indian Ocean and western Pacific Ocean where the MJO is mostly active, and D02 (9°S–11°N, 104°–124°E) is located over the region near Borneo. The model has 45 vertical levels with the top at 20 hPa. The WRF double-moment 6-class (WDM6) scheme is used for microphysics, and the Yonsei University (YSU) PBL scheme is used for boundary layer. No cumulus parameterization is used for both domains. The detailed choices of the dynamical and physical parameterization schemes are referred to Wang et al. (2015) and Zhou et al. (2021).
The WRF 9-km domain D01 (in the upper-right corner) and 3-km domain D02 with terrain height (shading; m). Red and purple parallel lines represent the transects used to diagnose the vertical cross sections and offshore propagation of precipitation across Borneo. Black boxes indicate western and eastern Borneo (WB and EB), respectively.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
The WRF 9-km domain D01 (in the upper-right corner) and 3-km domain D02 with terrain height (shading; m). Red and purple parallel lines represent the transects used to diagnose the vertical cross sections and offshore propagation of precipitation across Borneo. Black boxes indicate western and eastern Borneo (WB and EB), respectively.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
The WRF 9-km domain D01 (in the upper-right corner) and 3-km domain D02 with terrain height (shading; m). Red and purple parallel lines represent the transects used to diagnose the vertical cross sections and offshore propagation of precipitation across Borneo. Black boxes indicate western and eastern Borneo (WB and EB), respectively.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
The computational domain of HYCOM is slightly larger than the outermost domain of WRF. Its horizontal grid spacing is 0.08°. There are 41 vertical levels with terrain-following coordinate in coastal regions, z level in shallow waters, and isopycnal level in deep waters. The K-profile parameterization (KPP; Large et al. 1994) is used for vertical mixing.
The coupled simulation is initialized at 0000 UTC 15 January 2017 and integrated for one month to 0000 UTC 15 February 2017. The initial and boundary conditions are obtained from 6-hourly 0.75° × 0.75° ERA-Interim (Dee et al. 2011) and 0.08° daily analysis fields (GLBa0.08-91.2) from NCODA system (Cummings 2005; Cummings and Smedstad 2013) for WRF and HYCOM model, respectively.
3. Results
a. Large-scale features over MC
Similar to Hung and Sui (2018), we break down this MJO event into three stages based on the following two variables averaged over the MC region (10°S–8°N, 95°–155°E): the bandpass filtered (30–90-day) 1000–700-hPa vertically integrated water vapor and OLR anomalies. Figure 3a shows the time series of these anomalies, as well as the intraseasonal zonal wind anomaly at 850 hPa averaged over the same area. The suppressed stage (4–20 January 2017) is characterized by the positive OLR and weak moisture anomalies. In the growing stage (21–30 January 2017), moisture increases and OLR decreases, indicating that intraseasonal convection develops during this period. The opposite occurs in the decaying stage (31 January–12 February 2017), with reduced moisture and increased OLR. The growing and decaying stages correspond to the MJO phases 2–4 and 5–7, as defined by the OMI index (Kiladis et al. 2014), respectively, which roughly correspond to the easterly and westerly phases of the MJO, respectively, as the 850 hPa wind is about in quadrature with convection. Figures 3b and 3c show the observed precipitation anomaly (its annual cycle has been removed similar to Figs. 1c,e) over Borneo in the growing and decaying stage, respectively. The composite rain anomalies show contrasting pattern between the western and northeastern Borneo in both stages but with opposite sign. There are large positive anomalies (peak value ∼10 mm day−1) to the west of Borneo in the growing stage, while negative anomalies (peak value ∼−10 mm day−1) are found there in the decaying stage. The dipolar pattern is very similar to the climatological pattern of precipitation anomalies composited upon the MJO phases in Figs. 1c and 1e. The method of grouping the MJO local stages is not unique. It will also be interesting to explore to what extent the MJO tracking method in Wang et al. (2019) and Kerns and Chen (2020) can help understand island precipitation, but this topic is beyond the scope of the present study. We will next examine this event, focusing on the characteristics of precipitation, winds, and their diurnal cycles during these two stages.
(a) Time evolution of the 30–90‐day filtered 1000–700-hPa vertically integrated water vapor (blue), OLR (green), and 850 hPa zonal wind (red) anomalies from observation/reanalysis averaged over MC region (10°S–8°N, 95°–155°E) during January–February 2017. The red, blue, and plum colors denote the suppressed, growing (phases 2–4), and decaying (phases 5–7) stages of the MJO, respectively. (b),(c) Composites of observed precipitation anomaly (mm day−1) over Borneo during the growing and decaying stage of 2017 MJO event, respectively.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
(a) Time evolution of the 30–90‐day filtered 1000–700-hPa vertically integrated water vapor (blue), OLR (green), and 850 hPa zonal wind (red) anomalies from observation/reanalysis averaged over MC region (10°S–8°N, 95°–155°E) during January–February 2017. The red, blue, and plum colors denote the suppressed, growing (phases 2–4), and decaying (phases 5–7) stages of the MJO, respectively. (b),(c) Composites of observed precipitation anomaly (mm day−1) over Borneo during the growing and decaying stage of 2017 MJO event, respectively.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
(a) Time evolution of the 30–90‐day filtered 1000–700-hPa vertically integrated water vapor (blue), OLR (green), and 850 hPa zonal wind (red) anomalies from observation/reanalysis averaged over MC region (10°S–8°N, 95°–155°E) during January–February 2017. The red, blue, and plum colors denote the suppressed, growing (phases 2–4), and decaying (phases 5–7) stages of the MJO, respectively. (b),(c) Composites of observed precipitation anomaly (mm day−1) over Borneo during the growing and decaying stage of 2017 MJO event, respectively.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Figures 4a–d show the daily averaged precipitation and 850-hPa wind vectors over the MC region during the growing and decaying stages of this MJO event. The large-scale spatial patterns are similar between observation/reanalysis and the model simulation. In the growing stage, vigorous convection develops near the central MC, preceding the large-scale convective envelope of the MJO, and the precipitation is mainly found at western Borneo. The 850-hPa easterly (westerly) wind prevails over the northern (southern) part of equatorial region, and also dominates the northern (southern) Borneo, while the meridional wind is relatively weak near the equator and Borneo. This wind pattern is present largely due to the seasonal monsoonal wind; as a result, it does not change substantially during the two stages (Figs. 4a–d). In the decaying stage, precipitation is mainly found on the northeastern side of Borneo, but its amplitude weakens by ∼50%. Anomalies of precipitation and 850-hPa wind are derived by subtracting their averages of the two stages (Figs. 4e–h). It becomes evident that intraseasonal wind anomaly is the dominant easterly during the growing stage over the MC area (Figs. 4e,g), and turns into westerly during the decaying stage (Figs. 4f,h).
Composites of daily mean precipitation (shading; mm day−1) and 850-hPa wind (vectors; m s−1) over MC during the (a) growing and (b) decaying stage from observation/reanalysis. (c),(d) As in (a) and (b), but for the WRF–HYCOM coupled simulation. (e)–(h) As in (a)–(d), but for the anomalies of daily mean precipitation (shading; mm day−1) and 850-hPa wind (vectors; m s−1).
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Composites of daily mean precipitation (shading; mm day−1) and 850-hPa wind (vectors; m s−1) over MC during the (a) growing and (b) decaying stage from observation/reanalysis. (c),(d) As in (a) and (b), but for the WRF–HYCOM coupled simulation. (e)–(h) As in (a)–(d), but for the anomalies of daily mean precipitation (shading; mm day−1) and 850-hPa wind (vectors; m s−1).
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Composites of daily mean precipitation (shading; mm day−1) and 850-hPa wind (vectors; m s−1) over MC during the (a) growing and (b) decaying stage from observation/reanalysis. (c),(d) As in (a) and (b), but for the WRF–HYCOM coupled simulation. (e)–(h) As in (a)–(d), but for the anomalies of daily mean precipitation (shading; mm day−1) and 850-hPa wind (vectors; m s−1).
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
The key result from the composite analysis is that the precipitation is concentrated on the leeside of Borneo during both stages. There are some local differences between observation and simulation. For example, the IMERG precipitation is stronger over the eastern ocean of Philippines and near the Java island in the decaying stage compared with simulation (Figs. 4b,d), and the westerly wind at the southern part of Borneo is also stronger in reanalysis. However, the simulated precipitation maximum at the leeside is broadly similar to observation despite the model bias. Both the IMERG data and the simulation show the dipolar pattern of precipitation anomalies at the two stages. That is, positive anomaly over the western Borneo and negative anomaly over the northeastern Borneo during the growing stage accompanied by the easterly wind anomalies. Both the dipolar pattern and zonal wind switch sign at the decaying stage. In general, the model simulation reasonably reproduces the precipitation distribution and MJO circulation over MC and Borneo in both stages.
Figure 5 shows the spatial structure of column-integrated moisture over the MC region during the growing and decaying stages of this MJO event. In the growing stage, more moisture is found in the southern equatorial MC region in both reanalysis and simulation, although column moisture in the simulation is less, indicating either model bias or inaccuracy in the reanalysis. In the decaying stage, the moisture maximum reaches the New Guinea and western Pacific, and the dry advection induced by the MJO westerly (e.g., Hung and Sui 2018; Tan et al. 2020; Zhou et al. 2021) near Borneo may provide an unfavorable condition for vigorous convection. Based upon the results of Figs. 4 and 5, we suggest that the large-scale environmental moisture associated with the MJO might partly influence the amplitude of precipitation over Borneo but unlikely contribute to the formation of the dipolar pattern.
Composites of column-integrated water vapor (shading; kg m−2) over the MC region during (a) growing and (b) decaying stage from ERA5. (c),(d) As in (a) and (b), but for the WRF–HYCOM coupled simulation.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Composites of column-integrated water vapor (shading; kg m−2) over the MC region during (a) growing and (b) decaying stage from ERA5. (c),(d) As in (a) and (b), but for the WRF–HYCOM coupled simulation.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Composites of column-integrated water vapor (shading; kg m−2) over the MC region during (a) growing and (b) decaying stage from ERA5. (c),(d) As in (a) and (b), but for the WRF–HYCOM coupled simulation.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
b. Diurnal cycles over Borneo
1) Dipolar structure in precipitation and wind
Figure 6 shows the composited diurnal cycles of anomalous precipitation and near-surface (10-m) zonal wind averaged over the western and eastern Borneo (the land points in the black boxes of Fig. 2). In the growing stage, positive precipitation maximum is found at the western Borneo (WB), the leeside with respect to the intraseasonal easterly anomaly. Precipitation peaks (∼0.2 mm h−1) at the local night between 2000 and 2300 local solar time (LST; Fig. 6c), and reaches the minimum at local noon time. The local peak time of simulated precipitation occurs slightly earlier than IMERG. Both model bias and potential flaws in satellite-derived product (e.g., Kim et al. 2017) may contribute to this discrepancy, suggesting the need for high-quality independent local observation. Diurnal amplitude is about 0.25 mm h−1 in both the observation and simulation. In contrast, the amplitude of diurnal cycle and mean precipitation at the eastern Borneo (EB) is much weaker—it is 0.15 mm h−1 in the simulation and even less in the observation (Fig. 6e). In the decaying stage, the positive precipitation peak is larger at EB, also the leeside with respect to the intraseasonal westerly anomaly, but its amplitude of diurnal cycle and daily mean is weaker than that in the growing stage (Figs. 6c,f). Surface wind at WB (Fig. 6c) is easterly anomaly in the local morning and westerly in the local afternoon, while the wind direction reverses at EB (Fig. 6e). This is the signature of land–sea breeze at either side of Borneo. In general, the enhanced positive precipitation anomaly generally follows the establishment of the local sea breeze during the afternoon, while suppressed convection or negative anomaly follows land breeze during the morning, as expected.
Composite of diurnal cycles of precipitation anomaly (mm h−1; solid lines with left vertical axis) and 10-m zonal wind anomaly (m s−1; dashed lines with right vertical axis; negative values indicate easterly) averaged over the land grid points in Borneo in the (a) growing and (b) decaying stages. (c),(d) and (e),(f) As in (a) and (b), but for averages over the land grid points of western Borneo (WB) and eastern Borneo (EB), respectively, as indicated in Fig. 2. Black and red lines indicate the results from observation/reanalysis and the WRF-HYCOM coupled simulation, respectively.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Composite of diurnal cycles of precipitation anomaly (mm h−1; solid lines with left vertical axis) and 10-m zonal wind anomaly (m s−1; dashed lines with right vertical axis; negative values indicate easterly) averaged over the land grid points in Borneo in the (a) growing and (b) decaying stages. (c),(d) and (e),(f) As in (a) and (b), but for averages over the land grid points of western Borneo (WB) and eastern Borneo (EB), respectively, as indicated in Fig. 2. Black and red lines indicate the results from observation/reanalysis and the WRF-HYCOM coupled simulation, respectively.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Composite of diurnal cycles of precipitation anomaly (mm h−1; solid lines with left vertical axis) and 10-m zonal wind anomaly (m s−1; dashed lines with right vertical axis; negative values indicate easterly) averaged over the land grid points in Borneo in the (a) growing and (b) decaying stages. (c),(d) and (e),(f) As in (a) and (b), but for averages over the land grid points of western Borneo (WB) and eastern Borneo (EB), respectively, as indicated in Fig. 2. Black and red lines indicate the results from observation/reanalysis and the WRF-HYCOM coupled simulation, respectively.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Figure 7 shows the diurnal composite for the spatial pattern of anomalous precipitation over Borneo during the growing stage from IMERG. Figure 8 shows the diurnal composite for both the anomalous precipitation and near-surface wind from simulation. The simulated daily mean precipitation agrees with the observation overall (Figs. 7a and 8a). The surface wind from ERA5 is not shown here because its local feature is generally not well validated due to its limited resolution, and also because the observational data are only available for data assimilation every 6 h, not enough to resolve the diurnal cycles (see online supplemental material Figs. S1, S2 and text S1). As a result, the ERA5 surface winds should be treated with caution. We suggest this is one of the reasons for the lack of correspondence between convergence and convection, e.g., at 0200 and 0500 LST (Fig. S1).
Diurnal composites of spatial patterns of precipitation anomaly (shading; mm h−1) during the growing stage near Borneo from IMERG observation. (a) The averages over all hours. The color bar at the top is for (a) and for (b)–(i) is at the bottom.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Diurnal composites of spatial patterns of precipitation anomaly (shading; mm h−1) during the growing stage near Borneo from IMERG observation. (a) The averages over all hours. The color bar at the top is for (a) and for (b)–(i) is at the bottom.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Diurnal composites of spatial patterns of precipitation anomaly (shading; mm h−1) during the growing stage near Borneo from IMERG observation. (a) The averages over all hours. The color bar at the top is for (a) and for (b)–(i) is at the bottom.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Diurnal composites of spatial patterns of precipitation anomaly (shading; mm h−1) and 10-m perturbation wind (vectors; m s−1) during the growing stage near Borneo from the WRF-HYCOM coupled simulation. (a) The averages over all hours. The color bar and reference vector for the diurnal mean for (a) are shown at the top and for (b)–(i) are at the bottom.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Diurnal composites of spatial patterns of precipitation anomaly (shading; mm h−1) and 10-m perturbation wind (vectors; m s−1) during the growing stage near Borneo from the WRF-HYCOM coupled simulation. (a) The averages over all hours. The color bar and reference vector for the diurnal mean for (a) are shown at the top and for (b)–(i) are at the bottom.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Diurnal composites of spatial patterns of precipitation anomaly (shading; mm h−1) and 10-m perturbation wind (vectors; m s−1) during the growing stage near Borneo from the WRF-HYCOM coupled simulation. (a) The averages over all hours. The color bar and reference vector for the diurnal mean for (a) are shown at the top and for (b)–(i) are at the bottom.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Low-level wind anomaly over Borneo is dominated by easterly during this growing stage. Because of topography, low-level intraseasonal easterly flow is unable to cross the topography; instead, the flow at eastern side splits into two branches near 115°E and appears to converge at western Borneo (Fig. 8a). The meridional component is relatively strong over southwest of Borneo. Mean positive precipitation anomaly in IMERG and simulation both maximize at the western Borneo, but the IMERG precipitation is larger near the northwestern coastal region while the simulated precipitation appears to spread over the western flank of topography (Figs. 7a and 8a).
The land breeze over Borneo is established around 0200 LST due to the cumulated radiative cooling at night, and dominates in the morning until around 1100 LST. Enhanced convection develops mainly over the sea surrounding Borneo during this period. As the land surface temperature increases significantly at early afternoon, the sea breeze appears to initiate around 1400 LST off the coast (Fig. 8f). The sea breeze gradually grows and marches toward the coast and inland in the late afternoon, and reaches the maximum around 2000 LST (e.g., at the southwestern coast of Borneo). Easterly flow splits over the topography and detours around there near 115°E during late afternoon. Well-defined wake vortices can be found at the southern (near 1°S, 111°E) and northern ends (near 5°N, 115°E) of Borneo after 1700 LST (Figs. 8g,h). The low-level convergence (see supplemental material Fig. S3) and convection develop at the leeside as the sea breeze dominates near the southwestern coast and west slope of topography. The positive precipitation anomaly continues to propagate toward inland and reaches the maximum during 2000–2300 LST. This late-night peak of precipitation over Borneo is consistent with some previous studies (e.g., Oh et al. 2012; Kanamori et al. 2013; Vincent and Lane 2017; Qian 2020). During the subsequent midnight and morning, propagation of the northern precipitation anomaly (∼2°N, 114°E) reverses its direction, and propagates toward the northwestern coast and then further develops near the coastal sea (Figs. 7b–e and 8b–e). This corresponds to the offshore propagation as commonly observed in tropical coastal area and documented in previous studies (e.g., Qian et al. 2013; Peatman et al. 2014; Wei et al. 2020), while the southern precipitation anomaly gradually decays and disappears. There are some differences in the northwestward offshore propagation between observation and simulation. The observed positive precipitation anomaly during 0200–0500 LST is larger than simulation, which might explain their difference in the anomalous daily mean pattern (Figs. 7a and 8a).
The diurnal cycles during the decaying stage are similarly shown in Figs. 9 and 10. The surface anomalous wind is reversed to westerly. Similar to the growing stage, the transition from land breeze to sea breeze occurs during 1100–1400 LST due to the strong solar heating over land in the daytime. The westerly wind at western Borneo grows to ∼2 m s−1 in the afternoon as the onshore wind is intensified under the MJO anomalous westerly, while the flow at eastern Borneo is weak easterly. During 1700–2000 LST, the westerly splits and detours around the topography. Wake vortices mainly develop at the northern end of the Borneo, contributing to the leeward convergence (see supplemental material Fig. S4) and positive precipitation anomaly (Figs. 10g,h) along with the thermally driven sea breeze. The leeward convection reaches the maximum during the late night (2300–0200 LST). Afterwards convection propagates toward the eastern coastline gradually, and continues its propagation over the sea. The offshore-propagating precipitation anomaly is also noticeable during midnight and early morning in both observation and simulation, but more evident at eastern coast instead of the northwestern coast, indicating that offshore propagation originates from the inland convection.
As in Fig. 7, but for the decaying stage.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
As in Fig. 7, but for the decaying stage.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
As in Fig. 7, but for the decaying stage.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
As in Fig. 8, but for decaying stage.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
As in Fig. 8, but for decaying stage.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
As in Fig. 8, but for decaying stage.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
One may notice that there is light and scattered convection initiated over western Borneo in the simulation during 1700–2000 LST (Figs. 10g,h), but it is less evident in observation (Figs. 9g,h). This discrepancy might be attributed to both the model bias and observation flaw. Since our focus in decaying stage is the northeastern precipitation on Borneo, we argue that the potential model bias at western Borneo may not change our conclusion of the positive precipitation anomaly over northeastern Borneo and the overall dipolar structure in the decaying stage.
Surface precipitation over the island may be considered as a result of interaction between the land–sea-breeze induced low-level convergence and deep convection. To see this, it is instructive to further examine the vertical velocity. Figure 11 shows the diurnal cycles of vertical velocity averaged over WB and EB in the two stages. In the growing stage, the diurnal cycle of the upward motion over WB is strong (Fig. 11a). The low-level shallow ascent associated with the sea breeze occurs after 1400 LST, and continues to deepen and grow into deep ascent afterward. The deep ascent reaches the maximum in the mid- and upper troposphere (∼9 km) between 2000 and 2300 LST, with a peak value about 0.06 m s−1. In contrast, the diurnal cycle over EB is weaker (Fig. 11c). The upper tropospheric maximum ascent appears earlier than the low levels, and peaks around 1700 LST (∼0.04 m s−1), much earlier than in WB. In the decaying stage, the diurnal cycle of low-level ascent (below 2 km) is stronger (between 1400 and 2300 LST) at EB than WB (Figs. 11b,d). While the late afternoon (1700–2000 LST) upper-level ascent in EB appears to be weaker in the decaying stage (i.e., comparing the local maximum at 10–12 km in Figs. 11b,d), the deep ascent in the late evening is notably stronger in the bulk of the troposphere (3–9 km), indicating more precipitation at EB in the decaying stage.
Diurnal composites of vertical motion (10−2 m s−1) averaged over land grids of western Borneo (WB) in the (a) growing and (b) decaying stages in the WRF-HYCOM coupled simulation as a function of local time and height. (c),(d) As in (a) and (b), but for averages over land grids of eastern Borneo (EB).
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Diurnal composites of vertical motion (10−2 m s−1) averaged over land grids of western Borneo (WB) in the (a) growing and (b) decaying stages in the WRF-HYCOM coupled simulation as a function of local time and height. (c),(d) As in (a) and (b), but for averages over land grids of eastern Borneo (EB).
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Diurnal composites of vertical motion (10−2 m s−1) averaged over land grids of western Borneo (WB) in the (a) growing and (b) decaying stages in the WRF-HYCOM coupled simulation as a function of local time and height. (c),(d) As in (a) and (b), but for averages over land grids of eastern Borneo (EB).
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
We next discuss the effect of topography. Figure 2 shows that the ridge in the central Borneo reaches ∼1500 m. A variety of flow regimes may exist depending on the impinging wind speed, and mountain height and width. These factors may be summarized in a nondimensional number known as the Froude number (Fr), which is defined as Fr = W/(NH), where W is the characteristics horizontal wind speed, N is the Brunt–Väisälä frequency, and H is the terrain height. Fr is commonly used to characterize the mountain flow regime (e.g., Smolarkiewicz and Rotunno 1989; Reisner and Smolarkiewicz 1994). For Fr > 1, the airflow tends to ascend over the windward slope and pass the mountain, generally resulting in upward-propagating gravity waves and downslope wind storms. For Fr < 1, the airflow is typically blocked by the mountain and leads to windward stagnant flow if the mountain is quasi-two dimensional, or splitting into two branches around the three-dimensional mountain, which could further lead to the leeward vortices and even reversed upslope flow.
We examine the daily mean 850-hPa zonal wind, wind speed and corresponding Fr, all averaged over the land grids of Borneo in both stages (Table 1), given the estimated values of N (0.011 s−1) and H (1500 m) over Borneo. The value of Fr is small (∼0.2) at both the growing and decaying stages. The estimates of Fr are consistent with some recent studies (e.g., Qian 2020; Tan et al. 2021), which also indicated that the Fr over the MC islands during the MJO is small (<0.2). Riley Dellaripa et al. (2020) discussed precipitation and mesoscale flow over the Luzon island, and suggested that the flow blocking (Fr < 1) due to the topography prevents convection from propagating to the island valley region. Nevertheless, while the dipolar pattern reverses in different MJO phases, Fr estimated from the total wind may not be adequate to explain the difference in the two stages. This should be pursed in the future study.
Summary of the total 850-hPa zonal wind (U; m s−1, negative values indicate easterly), wind speed (W; m s−1), and Froude number (Fr) averaged over the land grids of Borneo during the growing and decaying stages in the WRF–HYCOM coupled simulation.
The above result suggests that the airflow can hardly pass through the Borneo topography under such a low-Fr condition. As a result, the processes like the hydraulic jump that may contribute to the formation or amplification of the leeward vortices (e.g., Schär and Smith 1993; Epifanio and Durran 2002a,b) are unlikely to occur since they typically require a higher Fr (e.g., Fr > 0.5). In contrast, the splitting and detouring flows around the topography are favored over the Borneo. The thermally forced land–sea breeze in modulating the local precipitation can become important under sufficiently small Fr (e.g., Smolarkiewicz et al. 1988; Kirshbaum et al. 2018), especially at the leeside (i.e., WB in the growing stage or EB in decaying stage) with respect to the prevailing wind. Consequently, the leeward convergence around 2000–2300 LST over Borneo may be caused by the combined effects of the detouring flow and thermally induced sea breeze (see supplemental material Figs. S3, S4 and text S2), which lead to the intraseasonal dipolar precipitation over Borneo shown in Figs. 7–10.
2) Offshore propagation of precipitation and vertical structure
It is shown from Figs. 7–10 that there are evident offshore-propagating precipitation anomalies over Borneo during midnight and early morning, and their initialization and propagation vary during different MJO stages. We will show in this section that these offshore propagation features are closely tied to the dipolar structure of island precipitation. We first examine characteristics of the northwestward offshore propagation during the growing stage, which has been noted in several studies (e.g., Qian et al. 2013; Peatman et al. 2014; Wei et al. 2020). Here, we further examine the propagation feature (original precipitation is used here because its propagation signal is already strong and no need to use intraseasonal anomalies). Figure 12 shows diurnal cycles of precipitation averaged along the corresponding southeast–northwest transects crossing Borneo (seven purple lines in Fig. 2) during both stages. The simulation broadly captures the observed propagation feature, indicating that the local land–sea breeze is reasonably simulated. The offshore propagation near the northwestern coast of Borneo initiates at 0200 LST, approximately when the island precipitation reaches the maximum in WB, and propagates offshore until 1400 LST, 400 km away from coast, while the amplitude becomes weaker. The amplitude is about 3 times larger in the growing stage than the decaying stage due to the stronger precipitation in this period. Despite of the difference, an important result is that the propagating mode in both stages initiates over the island after rain reaches the maximum there, indicating the crucial role of the island convection for the offshore propagating features. We then further estimate their propagation speed by tracking the maximized mean precipitation. The estimated propagation speeds in observation and simulation are quite close, which are 10.5 and 9.8 m s−1 in the growing stage, respectively, while 7.8 and 8.2 m s−1 in the decaying stage, respectively. These values are similar to the estimate over Borneo (8.1 m s−1) in Wei et al. (2020).
Diurnal composites of precipitation (mm h−1) averaged along transects over Borneo (seven purple lines in Fig. 2) during (a) growing and (b) decaying stages from IMERG observation. (c),(d) As in (a) and (b), but for the WRF-HYCOM coupled simulation. Two diurnal cycles are shown in vertical axis for clarity. Vertical thick lines indicate the approximate south and north edges of the Borneo. Dashed lines indicate the offshore propagation of precipitation.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Diurnal composites of precipitation (mm h−1) averaged along transects over Borneo (seven purple lines in Fig. 2) during (a) growing and (b) decaying stages from IMERG observation. (c),(d) As in (a) and (b), but for the WRF-HYCOM coupled simulation. Two diurnal cycles are shown in vertical axis for clarity. Vertical thick lines indicate the approximate south and north edges of the Borneo. Dashed lines indicate the offshore propagation of precipitation.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Diurnal composites of precipitation (mm h−1) averaged along transects over Borneo (seven purple lines in Fig. 2) during (a) growing and (b) decaying stages from IMERG observation. (c),(d) As in (a) and (b), but for the WRF-HYCOM coupled simulation. Two diurnal cycles are shown in vertical axis for clarity. Vertical thick lines indicate the approximate south and north edges of the Borneo. Dashed lines indicate the offshore propagation of precipitation.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Previous studies suggested that a number of physical processes may contribute to the offshore propagation of precipitation over islands. The coastal convergence caused by the land breeze at midnight may lead to the northwestward offshore propagation of Borneo precipitation (Houze et al. 1981; Qian 2008). The cold pool due to evaporation from land precipitation can excite gravity currents that promote the offshore propagation and dissipation of precipitation (e.g., Bryan and Rotunno 2008; Zhu et al. 2017). Some authors also hypothesized that convection propagates offshore as horizontally propagating gravity waves (e.g., Nicholls et al. 1991; Mapes 1993; Li and Carbone 2015), and the second baroclinic mode gravity waves may resonate with the stratiform-type precipitation mode (e.g., Mapes et al. 2003; Love et al. 2011; Vincent and Lane 2016; Short et al. 2019; Ruppert et al. 2020; Wei et al. 2020; Bai et al. 2021). These studies suggested that offshore propagation might be viewed as an independent convective mode. Here, we suggest that offshore propagation near the coastal area of Borneo may not be interpreted as an independent mode; instead, it is organically tied to island convection—no offshore propagation without the diurnal island convection.
We next examine diurnal variations of the vertical structure. Figure 13 shows the vertical cross sections of temperature and wind vector anomalies averaged along these transects. In the growing stage, the latent heating from deep convection and radiative feedback leads to warming in the column at 1700–2000 LST (Fig. 13a), which maximizes in the upper troposphere (∼11 km). The stratiform heating occurs during 2300–0200 LST as the precipitation reaches its peak, and features second baroclinic mode in vertical structure. Subsequently, the cold anomaly at about 5 km strengthens and extends offshore, shown by the magenta arrow in Fig. 13. The center of ascent and precipitation propagates gradually toward the ocean during 0200–1100 LST (Figs. 13c–f), with a phase lag with respect to the temperature anomaly. Its propagation speed (∼10 m s−1) is slower than the typical phase speed of the second-mode dry gravity waves (about 25 m s−1). In the decaying stage, the warm anomaly maximum also occurs around 2000 LST but peaks at about 5 km (Fig. 13g), much lower than that in the growing stage. The shallower structure is associated with a weaker second-mode gravity wave structure (the cold anomaly is at about 2 km) during 2300–0200 LST (Figs. 13h,i). The propagation speed is ∼8 m s−1 in the decaying stage, slightly slower than that in the growing stage.
Composites of vertical cross sections averaged along transects over Borneo (seven purple lines in Fig. 2) for temperature (shading; K) and meridional and vertical wind (υ, w) anomalies at 2000–1100 LST during (left) growing and (right) decaying stages in the WRF–HYCOM coupled simulation. Vertical velocity is scaled by 100 for visualization. Black contours indicate vertical velocity at 0.03, 0.06, 0.09, and 0.12 m s−1 during the growing stage, and 0.01, 0.02, 0.03, and 0.04 m s−1 during the decaying stage. Vertical thick lines indicate the approximate south and north edges of the Borneo. Black shading at the bottom of each panel indicates the averaged terrain height. The magenta arrows roughly denote the offshore propagation of cold anomaly.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Composites of vertical cross sections averaged along transects over Borneo (seven purple lines in Fig. 2) for temperature (shading; K) and meridional and vertical wind (υ, w) anomalies at 2000–1100 LST during (left) growing and (right) decaying stages in the WRF–HYCOM coupled simulation. Vertical velocity is scaled by 100 for visualization. Black contours indicate vertical velocity at 0.03, 0.06, 0.09, and 0.12 m s−1 during the growing stage, and 0.01, 0.02, 0.03, and 0.04 m s−1 during the decaying stage. Vertical thick lines indicate the approximate south and north edges of the Borneo. Black shading at the bottom of each panel indicates the averaged terrain height. The magenta arrows roughly denote the offshore propagation of cold anomaly.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Composites of vertical cross sections averaged along transects over Borneo (seven purple lines in Fig. 2) for temperature (shading; K) and meridional and vertical wind (υ, w) anomalies at 2000–1100 LST during (left) growing and (right) decaying stages in the WRF–HYCOM coupled simulation. Vertical velocity is scaled by 100 for visualization. Black contours indicate vertical velocity at 0.03, 0.06, 0.09, and 0.12 m s−1 during the growing stage, and 0.01, 0.02, 0.03, and 0.04 m s−1 during the decaying stage. Vertical thick lines indicate the approximate south and north edges of the Borneo. Black shading at the bottom of each panel indicates the averaged terrain height. The magenta arrows roughly denote the offshore propagation of cold anomaly.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Aside from the offshore propagation at the northwestern coast of Borneo, precipitation at northeastern side of Borneo also shows nocturnal offshore propagation in the decaying stage in both the observation and simulation (Figs. 9 and 10), as the precipitation is mainly located there during this period. Figure 14 similarly shows the diurnal cycles of precipitation along the southwest–northeast transects crossing Borneo (seven red lines in Fig. 2). In the growing stage, there are minor discrepancies near the southwestern coast of Borneo (∼109°E) and southeastern coast of Sumatra (∼105°E) between observation and simulation, likely due to the bias around these complex coastal areas. The precipitation between Sumatra and Borneo shows intricate and uneven propagation features. However, there is no evident offshore propagation over neither southwestern nor northeastern coast of Borneo (Figs. 14a,c). In contrast, during the decaying stage, there is an evident eastward offshore propagation near the northeastern coast of Borneo in both observation and simulation (Figs. 14b,d). This propagation occurs mainly during 0200–0800 LST and convection can reach 200–300 km away from the coast. The estimated propagation speed is about 10 and 9.3 m s−1 in observations and the simulation, respectively.
As in Fig. 12, but for composites averaged along seven red lines in Fig. 2. Vertical thick lines accordingly indicate the approximate west and east edges of the Borneo.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
As in Fig. 12, but for composites averaged along seven red lines in Fig. 2. Vertical thick lines accordingly indicate the approximate west and east edges of the Borneo.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
As in Fig. 12, but for composites averaged along seven red lines in Fig. 2. Vertical thick lines accordingly indicate the approximate west and east edges of the Borneo.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
The corresponding vertical structures are shown in Fig. 15. In the growing stage, warm anomaly is found at the upper levels, while cold anomaly at the lower levels during 2000–0200 LST (Figs. 15a–c), and the strong upward motion is located over the western part of Borneo. There is also another ascent with a similar structure near the southeastern coast of Sumatra (∼105°E). During 0500–1100 LST (Figs. 15d–f), the ascending flow on Borneo and Sumatra decays and disappears, while ascent over the ocean is enhanced, but its offshore propagation feature seems to be ambiguous. In the decaying stage, convection is mainly located over northeastern Borneo, and the warm anomaly around 2000 LST is confined to mid- and low-level troposphere (Fig. 15g). The presence of mountain near the eastern coast can induce the thermally forced mountain solenoid flow at night, which might amplify the land breeze and promotes the offshore propagation. The main ascent and corresponding precipitation at the northeastern side propagate toward the ocean during 0200–0800 LST (Figs. 15i–k).
As in Fig. 13, but for composites averaged along seven red lines in Fig. 2. Wind vectors indicate the anomalies of zonal and vertical flows (u, w), and vertical thick lines accordingly indicate the approximate west and east edges of the Borneo.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
As in Fig. 13, but for composites averaged along seven red lines in Fig. 2. Wind vectors indicate the anomalies of zonal and vertical flows (u, w), and vertical thick lines accordingly indicate the approximate west and east edges of the Borneo.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
As in Fig. 13, but for composites averaged along seven red lines in Fig. 2. Wind vectors indicate the anomalies of zonal and vertical flows (u, w), and vertical thick lines accordingly indicate the approximate west and east edges of the Borneo.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
In summary, the nocturnal offshore propagation over Borneo is tied to the island precipitation, and both are associated with the dipolar distribution of the precipitation during different MJO stages. In the growing stage, the northwestward offshore propagation dominates as the island precipitation is mainly located over western Borneo. In the decaying stage, the eastward offshore propagation at the northeastern side is more evident as the dipolar precipitation distribution is reversed, while offshore propagation near the northwestern coast is much weaker. These results are consistent with the observed leeward propagation of convection over the MC islands modulated by the low-level winds as suggested by Ichikawa and Yasunari (2006). Gravity waves, cold pool, land breeze, and even interactions between islands may all play some roles in the propagation at different stages. We suggest that the improved understanding of the offshore propagation of precipitation and the coupling of islands and oceanic convection can provide useful insight for large-scale precipitation over MC and its interactions with MJO.
4. Summary
Precipitation over the Maritime Continent region is significantly modulated by the MJO. It is enhanced about 1 week prior to the arrival of the large-scale MJO convection. The enhancement over the islands is not uniform, but appears in the form of a dipolar pattern with positive anomaly to the west of the islands and negative anomaly to the east (Qian 2020). Here, we focus on this phenomenon over Borneo, one of the largest islands over MC. Positive and negative precipitation anomaly during the MJO phases 2–4 is typically located in the western and northeastern side of Borneo, respectively, while the spatial pattern is opposite during phases 5–7 (Figs. 1c,e). We selected a boreal winter MJO event during January and February 2017 and conducted a cloud-permitting simulation using the WRF-HYCOM, an air–sea coupled model (e.g., Chen and Curcic 2016; Wang et al. 2021). The local development of this MJO event is divided into three stages based on the 30–90-day filtered low-level water vapor and OLR anomalies: the suppressed, growing, and decaying stages, and the last two correspond to the MJO phases 2–4 and 5–7, respectively. Precipitation during these stages shows the dipolar pattern over Borneo, similar to the climatological composites. The simulation reasonably reproduces both the large-scale characteristics of the MJO and local features over Borneo (e.g., diurnal cycles) during these two stages. The main findings are summarized as follows:
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Positive precipitation anomaly is mainly distributed at the leeside of the Borneo, that is, the western (northeastern) side of Borneo in the growing (decaying) stage when the large-scale low-level easterly (westerly) wind prevails. The asymmetric distribution of precipitation anomalies during this event agrees with the climatology at different MJO phases.
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The amplitude of precipitation anomalies over Borneo in the decaying stage is about 50% less than that in the growing stage, which might be partly due to the large-scale distribution of moisture associated with MJO. Moisture is concentrated in the center of MC and slightly shifted to the Southern Hemisphere in the growing stage, while it maximizes over the eastern MC and western Pacific in the decaying stage.
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During the afternoon and night (about 1700–2300 LST), the sea breeze builds up and strengthens over Borneo gradually. The local Froude number (Fr) is small, and the terrain-induced splitting/detouring flows are favored at the windward side, while the flow directly passing over the terrain is negligible. Wake vortices are found at both southern and northern ends of Borneo. The thermally forced sea breezes at the leeside and wave vortices can be both critical to the leeward convergence and precipitation around 2000 LST.
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During midnight and the morning (about 0200–0800 LST), precipitation following the midnight maximum propagates offshore over the Borneo coast and surrounding ocean, accompanied by the land breeze. Accordingly, the northwestward offshore propagation dominates in the growing stage over northwestern coast of Borneo, while the eastward offshore propagation achieves the maximum near the northeastern coast during the decaying stage. The offshore propagation speed is approximately 9 m s−1.
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The timing and location of offshore propagation are tied to the leeward convection, indicating that it is not a mode independent of island convection. Still, aside from the gravity waves associated with the island convection (e.g., Love et al. 2011), the cold pool and land breeze may also contribute to the propagation (e.g., Bryan and Rotunno 2008; Qian 2008), especially when the diabatic heating is confined to lower levels (e.g., in decaying stage).
Figure 16 presents the schematic diagram of the dynamical processes associated with the diurnal cycles and dipolar precipitation pattern over Borneo in the growing and decaying stages. The topography-induced splitting/detouring flows and thermal-induced sea breezes tend to converge at the leeside over Borneo during the local early night (about 1700–2300 LST), contributing to the low-level convergence and convective initiation. This leads to the dipolar precipitation over Borneo at the intraseasonal time scale during various phases of the MJO. Under a low-Fr condition, while the leeward sea breeze is considered to be critical, we also suggest the important contribution from the splitting and detouring flows to the leeward convergence. However, as the strength of anomalous wind associated with the MJO may vary in different cases, some dynamical processes such as tilting of the baroclinically produced horizontal vorticity (Smolarkiewicz and Rotunno 1989) and hydraulic jump (Schär and Smith 1993; Epifanio and Durran 2002a,b) might also become important for the leeward convergence in some particular MJO events. Regardless of possible complex flow regimes in the absence of moisture processes, we consider that the low Fr is critical for the primary convective processes around Borneo. That is, the late night island convection and the offshore-propagating precipitation during midnight and morning (about 0200–0800 LST) occur in succession, which might be responsible for the climatological precipitation maximized near the northwestern coast of Borneo (Fig. 1a).
Schematic diagrams of main processes associated with the diurnal cycles over Borneo during (left) growing and (right) decaying stages, respectively. Rows show the flow during (top) morning and noon (about 0800–1400 LST), (middle) afternoon and early night (about 1700–2300 LST), and (bottom) midnight and morning (about 0200–0800 LST). Terrain greater than 500 m is shown with brown shading. Thick blue (red) arrow indicates the intraseasonal easterly (westerly) wind. Light blue and red arrows represent detouring flows around topography, and black arrows represent the land–sea breeze. The clouds with ascending flows (green arrows) denote the convection and precipitation. Yellow arrows indicate the offshore propagation of precipitation.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Schematic diagrams of main processes associated with the diurnal cycles over Borneo during (left) growing and (right) decaying stages, respectively. Rows show the flow during (top) morning and noon (about 0800–1400 LST), (middle) afternoon and early night (about 1700–2300 LST), and (bottom) midnight and morning (about 0200–0800 LST). Terrain greater than 500 m is shown with brown shading. Thick blue (red) arrow indicates the intraseasonal easterly (westerly) wind. Light blue and red arrows represent detouring flows around topography, and black arrows represent the land–sea breeze. The clouds with ascending flows (green arrows) denote the convection and precipitation. Yellow arrows indicate the offshore propagation of precipitation.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Schematic diagrams of main processes associated with the diurnal cycles over Borneo during (left) growing and (right) decaying stages, respectively. Rows show the flow during (top) morning and noon (about 0800–1400 LST), (middle) afternoon and early night (about 1700–2300 LST), and (bottom) midnight and morning (about 0200–0800 LST). Terrain greater than 500 m is shown with brown shading. Thick blue (red) arrow indicates the intraseasonal easterly (westerly) wind. Light blue and red arrows represent detouring flows around topography, and black arrows represent the land–sea breeze. The clouds with ascending flows (green arrows) denote the convection and precipitation. Yellow arrows indicate the offshore propagation of precipitation.
Citation: Journal of the Atmospheric Sciences 79, 8; 10.1175/JAS-D-21-0258.1
Finally, while we have suggested possible mechanisms for the MJO-mediated dipolar precipitation over Borneo, sensitivities of the dipolar pattern to the island (e.g., terrain height) or other factors (e.g., prevailing winds and solar radiative heating) are unclear. These issues warrant further investigation for better understanding of the local distribution of island precipitation. The issues will be addressed in a future study through a series of sensitivity experiments based on the cloud-permitting simulation presented in the current study.
Acknowledgments.
The authors acknowledge the funding support of National Natural Science Foundation of China 41875066 and 41875067. The authors are grateful to three anonymous reviewers for their insightful and thoughtful comments, which led to a much improved manuscript. We are grateful to the High Performance Computing Center (HPCC) of Nanjing University for doing the numerical calculations in this paper on its blade cluster system.
Data availability statement.
The IMERG precipitation data version 6 (https://disc.gsfc.nasa.gov/datasets/GPM_3IMERGHH_06/summary) are available from the National Aeronautics and Space Administration (NASA). The OLR data version 1.2 (https://www.ncei.noaa.gov/access/metadata/landing-page/bin/iso?id=gov.noaa.ncdc:C00875) are available from National Oceanic and Atmospheric Administration (NOAA) Climate Data Center (CDR). The ERA-Interim (https://apps.ecmwf.int/datasets) and ERA5 data (https://cds.climate.copernicus.eu/) are provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). The NCODA analysis data (https://www.hycom.org/data/glba0pt08/expt-91pt2) and OMI index (https://psl.noaa.gov/mjo/mjoindex) are available from the developers. The code and modeling data that support the findings of this study are available from the authors upon reasonable request.
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