1 Introduction
The Madden-Julian Oscillation ( MJO ) [ Madden and Julian, 1972 ] is the prevailing manner of tropical intraseasonal variability that is distinguished from other convectively coupled equatorial waves [ Kiladis et al., 2009 ] by its huge zonal scale ( i, wave numbers 1–3 ), 30–60 day period, and eastbound propagation over the Indo-Pacific warm pool at about 5 m s−1. Despite its meaning in the global weather-climate organization [ Zhang, 2013 ], understanding of MJO ‘s dynamics has remained incomplete [ Wang, 2011 ] and many contemporaneous global climate models ( GCMs ) suffer from poor representation of it [ Kim et al., 2009 ; Hung et al., 2013 ; Jiang et al., 2015 ; Ahn et al., 2017 ] .
The GCMs have detail difficulty in their representation of the MJO propagation in and around the Maritime Continent ( MC ), which is the area located between the indian and Pacific Oceans, including the archipelago of Indonesia, New Guinea, Philippines, the Malay Peninsula, and the surrounding seas. When initiated over the indian Ocean, the MJO propagates toward the MC. The eastbound propagation of the MJO is sometimes disrupted over the MC [ for example, D. Kim et al., 2014 ], and models tend to overestimate this probability of MJO break over the MC [ for example, Wang et al., 2014 ; H. M. Kim et al., 2014 ]. Our inability to understand and correct the poor model representation of MJO generation over the MC is partially due to our incomplete agreement of the propagation mechanism, and particularly, the function of the MC has remained ailing understand .
many previous studies have focused on the weakening of the MJO over the MC. During its passing from the amerind Ocean to the west Pacific, the MJO weakens over the MC area and restrengthens when it reaches the west Pacific. previous studies have attempted to relate the weakening of the MJO over the central MC area to the complex topography and land-sea contrast, and the prevail diurnal bicycle over the MC [ Inness and Slingo, 2006 ; Hsu and Lee, 2005 ; Wu and Hsu, 2009 ; Sobel et al., 2010 ; Oh et al., 2012 ; Hagos et al., 2016 ]. Inness and Slingo [ 2006 ] argued by using a put of idealized GCM experiments that the north-south orient exorbitant orology over the Sumatra island blocks MJO ‘s eastbound generation and prevents the development of subsequent convective anomalies in the central MC area. Sobel et al. [ 2010 ] suggested that the near-zero heat capacity of kingdom surface would prevent convection over farming to have an intraseasonal time scale, hence weaken the MJO signal over the MC. however, the MJO-related convection weakens not entirely over land but besides over the ocean in the central MC sphere [ for example, Climate Variability and Predictability Madden-Julian Oscillation Working Group, 2009, Figure 12 ] ( besides see design 1 five hundred below ). Based on modeling results, Oh et al. [ 2012 ] proposed that the strong diurnal cycle of convection over the MC competes against the MJO for the finite come of energy available. According to their argumentation, development of the MJO is inhibited over the MC because diurnal bicycle consumes most of the available energy.
Figure 1Open in figure viewer PowerPoint−2) regressed against a reference time series that is constructed by area-averaging intraseasonal OLR anomalies over the central Indian Ocean (70–100°E, 15°S–15°N). (a–f) Lag days −5, 0, 5, 10, 15, and 20, respectively. The two rectangles in Figure Maps of intseaseasonal OLR anomalies ( W megabyte ) regressed against a citation clock serial that is constructed by area-averaging intraseasonal OLR anomalies over the central amerind Ocean ( 70–100°E, 15°S–15°N ). ( a–f ) Lag days −5, 0, 5, 10, 15, and 20, respectively. The two rectangles in Figure 1 five hundred indicate central MC ( 100–140°E, 5°S–5°N ) and southerly MC ( 100–140°E, 15°S–5°S ) areas .
The MJO exhibits a considerable seasonal worker variability [ Zhang and Dong, 2004 ]. During boreal summer, the primary convective sign of the MJO resides in the Northern Hemisphere [ Zhang and Dong, 2004 ], and the east propagation of the MJO often accompanies north propagating components over the amerind Ocean and the west Pacific [ Wang and Rui, 1990 ; Hsu et al., 2004 ; Jiang et al., 2004 ]. During austral summer, on the other hired hand, the MJO-associated convection anomaly is centered in the Southern Hemisphere. The latitudinal placement of the MJO ‘s passage has a practical meaning to the countries in the passage because the convection associated with the MJO provides the water system resource to those countries. The seasonality of the MJO besides offers a challenge to the existing MJO theories and modeling—a complete hypothesis for the MJO would need to be able to explain it, and the general circulation models would be required to represent it correctly .
The MC is the area in which the latitudinal migration of the passage through which the MJO moves is most pronounce. During austral summer ( December-January-February ), when the erratic scale envelope of enhance convective natural process associated with the MJO stays over the MC area, the convection anomalies over south of Sumatra and Java island and over the Timor Sea become stronger than those in the Indonesia ( Figure 1 five hundred ). As a consequence, it appears that the MJO “ detours ” the Maritime Continent south. This curious behavior of MJO propagation during austral summer is the focus of this newspaper. This survey aims to enhance understand of MJO propagation over the MC by employing the recent development in the MJO hypothesis. We find that the basic state moisture distribution plays a critical function in the MJO detour .
This paper is organized as follows. part 2 gives data and methodology used in this study. The results of the column-integrated damp electrostatic energy budget are given in section 3, followed by summary and conclusions in part 4 .
2 Data and Method
The MJO generation and the associated column-integrated MSE ( damp inactive energy ) budget are analyzed by using daily-averaged fields in a 32 year ( 1979–2010 ) time period. We focus on MJO generation during the DJF season. Outgoing longwave radiation ( OLR ) is obtained from the advanced identical high resolution radiometer [ Liebmann and Smith, 1996 ] as a proxy of tropical bass convection. three-dimensional atmospheric states including zonal and meridional winds ( 
), pressure speed ( ω ), geopotential acme ( Z ), temperature ( T ), specific humidity ( q ), airfoil coerce ( ps ), and the radiative and churning fluxes at the surface and top of atmosphere are obtained from the European Centre for Medium-Range Weather Forecasts Interim reanalysis product [ Dee et al., 2011 ]. All data are interpolated into 2.5° longitude × 2.5° latitude horizontal grids. A 201-point Lanczos trickle [ Duchon, 1979 ] is used to isolate intraseasonal ( 20–100 days ) clock scale unevenness from OLR and the MSE budget terms .
m = CpT + gZ + Lvq is MSE; Cp is the specific heat of dry air at constant pressure; g is the gravitational constant; Lv is the latent energy of vaporization; LH and SH are surface latent and sensible heat flux, respectively; and LW and SW are longwave and shortwave radiative heating rates, respectively. The prime (′) indicates an intraseasonal anomaly, and the angle brackets denote the mass-weighted vertical integral (
, where pb = 1000 hPa, pt = 100 hPa). The MSE budget analysis has been used to examine the propagation and maintenance of the MJO in observations and model simulations [e.g., Maloney, 2009Andersen and Kuang, 2012Arnold et al., 2013D. Kim et al., 2014 To examine the moistening process during the MJO propagation across the MC, we use the intraseasonal, vertically integrate MSE budget equality : whereis MSE ; is the specific heat of dry air at changeless press ; is the gravitational constant ; is the latent energy of vaporization ; LH and SH are surface latent and sensible inflame flux, respectively ; and LW and SW are longwave and shortwave radiative heat rates, respectively. The premier ( ′ ) indicates an intraseasonal anomaly, and the angle brackets denote the mass-weighted erect integral (, where= 1000 hPa, = 100 hPa ). The MSE budget analysis has been used to examine the propagation and sustenance of the MJO in observations and model simulations [ for example ,. ,., ] .
3 Results
In club to examine MJO propagation from the indian Ocean to the MC, a reference time series is constructed by area-averaging intraseasonal OLR anomalies over the central indian Ocean ( 70–100°E, 15°S–15°N ). Intraseasonal anomalies of OLR and MSE budget terms are lag-regressed against the character time serial. Figure 1 shows that the MJO exhibits pronounced changes in its latitudinal social organization as it propagates from the indian Ocean to the MC. When the enhance convection anomalies of the MJO are over the cardinal indian Ocean ( ~85°E ), the negative OLR anomalies peak near the equator ( Figure 1 a ). The center of the negative OLR anomalies shifts southbound as the MJO moves eastbound ( Figures 1 boron and 1 c ), and it locates at around 10°S in between Indonesia and Australia when the MJO is over the MC ( Figures 1 d– 1 farad ). A marked contrast between cardinal and southerly MC areas is clearly seen in Figure 1 d—negative OLR anomalies are much stronger in the southern MC ( sMC ) area than in the central MC ( cMC ) sphere .
figure 2 a compares eastbound propagation of the MJO in two 10° wide-eyed latitude bands : one encompassing near-equatorial area ( 5°S–5°N, upper ) and another enclosing southerly tropical area ( 15°S–5°S, lower ). bill that the two latitude bands cover the cMC and sMC areas, respectively. At interim days between −10 and −5, when the enhance convective anomaly is centered over the western indian Ocean, an anomalously oppress condition prevails over the MC area in both bands. As the enhance convective anomaly propagates toward the MC ( lag days between −5 and +15 ), MC areas experience a conversion to an enhance circumstance in both latitude bands. The order of magnitude of the fault shows a punctuate contrast between the two latitude bands. In the southerly band, the transition is potent enough to yield at stave day +10 an anomalously enhanced discipline ( i, negative OLR anomalies ) that is comparable in magnitude to the amerind Ocean anomalies at slowdown day 0. In line, the development of anomalously enhanced stipulate is much decrepit in the near-equatorial band, and as a result, the MJO experiences a significant weakening. This suggests that the MJO detours the MC southerly because the transition from a suppressed to an enhanced phase over the MC is weaker in the cMC than in the sMC area .
Figure 2Open in figure viewer PowerPoint ( a ) Longitude-lag diagram of OLR ( W m−2, contour ) and column-integrated MSE ( ×106 J m−2, shaded ) anomalies for near-equatorial ( 5°S–5°N, top ) and southern Tropical band ( 15°S–5°S, bottom ). ( bel ) Intraseasonal column-integrated MSE ( ×2 × 106 J m−2, total darkness ), and column-integrated specific humidity ( ×0.2 gigabyte kg−1, red ), and OLR ( W m−2, blue ) anomalies averaged over cardinal ( top ) and southern ( bottom ) MC areas .
What are the processes that are responsible for the suppressed-to-active transition over the MC ? What makes the regional contrast in the suppressed-to-active transition ? To address these questions, we examine the column-integrated MSE budget of the MJO. The analysis of the column-integrated MSE budget is inspired by a recently developed theoretical framework for the MJO—the moisture modality theory [ Raymond, 2001 ; Sobel and Maloney, 2012, 2013 ; Adames and Kim, 2016 ]. The hypothesis focuses on the expression of tight copulate of tropical convection with environmental moisture [ Bretherton et al., 2004 ] and the unaccented temperature gradient in the tropics [ Sobel et al., 2001 ], and it explains the generation and care of the MJO convective anomalies by those of moisture anomalies. Because more than 90 % of the intraseasonal variability of column-integrated MSE is explained by that of column-integrated moisture over tropical oceans ( not shown ), examination of the column-integrated MSE budget could provide insights into the development of moisture anomalies, and consequently anomalous convection. diagnosis of the MSE budget of the MJO has helped better understand the authoritative features of the MJO propagation in observations [ for example, Kiranmayi and Maloney, 2011 ; D. Kim et al., 2014 ] and in model simulations [ for example, Maloney, 2009 ; Jiang, 2017 ] .
Intraseasonal anomalies of column-integrated MSE expose propagation characteristics that are like to those of OLR anomalies with an opposite polarity ( Figure 2 a, contour ), affirming the tight pair between column-integrated MSE and convection in the intraseasonal time scales. There is a phase stave of 0–5 days between column-integrated MSE and OLR anomalies, presumably due to OLR anomalies primarily reflecting high-cloud anomalies and hence being amplified during the late ( more stratiform cloud ) part of an enhance MJO phase. In both cMC and sMC areas, MSE anomalies undergo a negative-to-positive phase conversion between imprison days −15 and +5 ( Figure 2 bacillus ). And the order of magnitude of the chemise is much larger in the southerly function of the MC, resulting in a greater MSE anomalies in the sMC area at stave sidereal day +5. calculate 2 bel besides shows that changes in tropospheric moisture anomalies explain most of the changes in MSE anomalies. In other words, moisture recharging from the interim days −10 to +10 is more significant in the sMC area than in the cMC area, suggesting that the stronger moisture recharge in sMC leads the more vigorous anomalous convection there. Understanding the difference in the moisture inclination between the two regions is consequently the key to understanding the MJO detour .
figure 3 presents proportional contributions of each MSE budget term to the entire moisten ( i.e., convinced MSE tendency ) before and during the attack of the MJO in cMC and sMC areas. The leftmost bars in Figure 3 read that the drizzle is greater in sMC ( blue ) than in cMC ( bolshevik ). The zonal and meridional MSE advection terms exhibit a luminary difference between the two areas, with a greater drizzle in the sMC area than in the cMC area, suggesting that these terms are the primary cause of the MJO detour. On the reverse, surface latent hotness flux decreases with a greater amplitude in the sMC area than in the cMC area before and during the onset of the MJO, counteracting horizontal advective tendencies. This is because the mean zonal surface winds are westerly over the sMC. The easterly anomalies over the sMC area during the drizzle period consequently would effectively reduce the hoist speed and LH .
Figure 3Open in figure viewer PowerPoint Column-integrated MSE budget over the MC during the suppressed-to-enhanced transition ( lag days between −15 and 5 ). The blue and bolshevik bar represents the sMC and cMC area, respectively. The leftmost bars indicate total tendency, and the perch represents individual MSE budget terms .
other terms seem to play a secondary or a negligible function on the MJO detour. When averaged over the moisten period, the column-integrated longwave radiative heating system exhibits negative anomalies over the MC, indicating an enhanced longwave cool. The enhanced longwave cool induces anomalous sinking motion under the WTG, thereby drying the troposphere. This dry is weaker in the sMC sphere than in the cMC, contributing positively to the difference of the moisture or MSE leaning between the two regions. erect advection weakly counteracts the deviation, exhibiting a greater drizzle or weaker drying in the sMC area. When these two terms are combined by considering that anomalous erect gesture and anomalous cloud radiative pull are physically coupled, the regional difference becomes negligible. The residual of the MSE budget is nonnegligible in order of magnitude. however, it exhibits a relatively little difference between the two areas, suggesting that the function of the residual term on the MJO detour is not significant. The nonnegligible budget residual in reanalysis products has been documented in previous studies [ Kiranmayi and Maloney, 2011 ; Mapes and Bacmeister, 2012 ; H. M. Kim et al., 2014 ] .
Our analysis of the column-integrated MSE budget of the MJO underlines the zonal and meridional advection terms as the key to understanding the MJO detour. A closer probe of the horizontal MSE advection terms reveals that the advection of the basic state moisture gradient by anomalous MJO scale wind has the dominant allele contribution to both zonal and meridional advection terms ( not shown ). eminence that the advection of the setting moisture by anomalous winds was emphasized as the key drizzle serve during MJO propagation across the MC in holocene studies [ for example, D. Kim et al., 2014 ; Feng et al., 2015 ]. In rate to understand the big difference between the two areas in contribution from zonal and meridional advection terms, we show in Figure 4 the DJF climatological mean 700 hPa specific humidity. We chose the 700 hPa horizontal surface where the moisture variation is in maximal associated with MJO. note that the results presented under is not medium to the degree choose. During austral summer, the hateful low-tropospheric specific humidity maximize near the MC area. When the zonal moisture gradient in the cMC and sMC areas are compared, a pronounce positive gradient exists in the sMC sphere while no large-scale gradient exists in the cMC area. In regarding meridional moisture gradient, large-scale positivist and damaging gradient exists in the sMC and cMC area, respectively .
Figure 4Open in figure viewer PowerPoint−1, contour) and intraseasonal zonal wind anomaly (m s−1, shaded) averaged over the recharging period (lag days between −15 and 5) at 700 hPa. (b) Same as Figure −1). (c) Intraseasonal OLR (W m−2, contour) and zonal advection by intraseasonal zonal wind anomalies acting upon climatological zonal moisture gradient (×10−10 g kg−1 s−1, shaded) during the recharging period. (d) Same as Figure −10 g kg−1 s−1). Two rectangles in each panel indicate central MC and southern MC areas. In Figures −2, the dashed lines indicate negative values, and the zero line is omitted. ( a ) December-January-February climatological particular humidity ( deoxyguanosine monophosphate kilogram, contour ) and intraseasonal zonal hoist anomaly ( megabyte s, shaded ) averaged over the recharge period ( lag days between −15 and 5 ) at 700 hPa. ( bacillus ) Same as Figure 4 a exclude that intraseasonal meridional wind anomaly is shaded ( ×0.2 megabyte randomness ). ( c ) Intraseasonal OLR ( W molarity, shape ) and zonal advection by intraseasonal zonal fart anomalies acting upon climatological zonal moisture gradient ( ×10g kilogram, shaded ) during the recharge period. ( vitamin d ) Same as Figure 4 coulomb except that color shading indicates meridional advection by intraseasonal meridional fart anomalies acting upon climatological meridional moisture gradient ( ×10g kilogram ). Two rectangles in each empanel indicate central MC and southerly MC areas. In Figures 4 c and 4 d, contour interval is 1.5 W megabyte, the daunt lines indicate negative values, and the zero line is omitted .
The deviation in zonal advection condition ( Figure 3 ) is chiefly due to that a large-scale zonal gradient of specific humidity exists over the sMC area and not over the cMC area. During the recharge period ( between imprison days −15 and 5 ), when maximum and minimal convective anomalies locate over the eastern indian Ocean ( IO ) and easterly MC, respectively, easterly wind anomalies prevail over the entire MC ( Figure 4 a ). Both the Kelvin wave reply to the anomalous inflame over the easterly IO and the global wave response to the anomalous cool over the eastern MC provide easterly anomalies over the MC [ Matsuno, 1966 ; Gill, 1980 ]. Although the magnitude of the anomalous easterly is greater in the cMC area, the easterly anomaly is ineffective to induce advective moisten in the cMC sphere due to the lack of large-scale zonal moisture gradient. In contrast, in the sMC area, relatively weak easterly anomaly could cause a much greater moisten by acting upon the large zonal moisture gradient. The boastfully zonal moisture gradient in the sMC area is associated with the australian monsoon, which is active during the target season. The australian monsoon brings rain and damp air over the northern australian region, and this sets up the large-scale zonal moisture gradient over the sMC area .
Regarding meridional structure, the climatological mean moisture field peaks at closely 5°S and the two MC areas are characterized by a large negative ( cMC ) and positive ( sMC ) meridional moisture gradient. During the moisten period, poleward flows in both hemispheres appear in between the enhance and suppressed convective anomalies ( Figure 4 bel, contour ). The poleward hang can be understood as the forced, westward propagating planetary brandish response to the suppressed convective anomalies [ Matsuno, 1966 ; Gill, 1980 ]. The north menstruation over the sMC area brings moist air out from the north and therefore moisten the sMC area. The meridional fart anomaly over the cMC sphere, unlike that over the sMC area, exhibits a crinkled feature, presumably due to the bearing of the MC topography such as Sumatra and Borneo islands. The zonal asymmetry in the meridional fart anomalies weakens the area averaged meridional advection over the cMC area. In line, over the sMC area, the zonally uniform anomalous northerly acts upon the positive climatological moisture gradient to produce relatively big meridional advection .
4 Summary and Conclusions
The southerly detour of the MJO around the MC during austral summer was studied. When the MJO is in the MC, the MJO convective anomalies peak in the southerly MC area ( 15°S–5°S, 100°–140°E ) with much weaker anomalies in the central MC area ( 5°S–5°N, 100°–140°E ). The column-integrated MSE budget of the MJO was examined to identify processes responsible for the MJO detour. Moisture recharging ahead and during the onset of the MJO is greater in the southerly MC area than in the central MC area, causing the line in convective anomalies between the southerly and the central MC areas .
The column-integrated MSE budget psychoanalysis reveals horizontal moisture advection as a critical summons that is responsible for the deviation in the moisture recharging between the southern and central MC areas. Zonal and meridional advection of lower tropospheric moisture moistens the standard atmosphere at a fast rate in the southerly MC area than in the central MC area before and during the attack of the MJO. Further analysis indicates that the zonal and meridional advection by MJO wind anomalies acting upon the climatological moisture gradient plays a significant character. The large-scale zonal gradient of the background moisture airfield is much larger in the southerly MC area than in the central MC sphere, leading to a greater zonal advection in the southerly MC area. While a large-scale meridional gradient of the background moisture field is deliver in both the southerly and cardinal MC areas, meridional moisture advection is weaker in the central MC area because the MC island topography distort the meridional MJO weave anomalies in the central MC area .
Before and during the onset of the MJO in the MC, meridional wind instrument anomalies exhibit a zonally inhomogeneous convention in the cardinal MC area, weakening area-averaged meridional moisture advection. This zonal heterogeneity in the meridional wind instrument anomalies might be a result of ( i ) the planetary-scale flow associated with the MJO being distorted by the MC topography and ( two ) the intraseasonal modulation of the synoptic-scale unevenness. Chang et al. [ 2005 ] showed that frequency of the dominant synoptic unevenness in the cardinal MC area—the cold surge [ Chang et al., 1983 ] and the Borneo whirl [ Cheang, 1977 ] —is modulated by the MJO. This suggests that the circulation patterns of the synoptic unevenness appear in the MJO anomalies through a rectification. The nature of the heterogeneity in the MJO meridional wind anomalies in the central MC area warrants further examination .
Our results highlight the character of the basic state of matter moisture distribution on the propagation of the MJO over the MC. This suggests that understanding the background—seasonal mean—moisture distribution is a necessary step toward a arrant understand of the MJO generation over the MC. One factor that is tightly related to the basic express moisture is the basic state precipitation. diurnal cycle of haste prevails over the MC, and it interacts with the seasonal ( i.e., monsoonal ) flow to determine the seasonal worker mean precipitation. This emphasizes the character of the MC ‘s building complex topography, land-sea contrast, and diurnal hertz on shaping the beggarly state. It besides suggests that if a GCM fails to correctly represent the diurnal hertz of precipitation over the MC, the diagonal might cause errors in the basic state, which would degrade the MJO generation. Although not discussed in depth in the current study, the role of the MJO-associated poleward moisture fluxes in shaping the entail submit moisture model besides warrants far probe .
The authors are presently investigating GCM fidelity of representing the southbound MJO detouring around the MC using the climate model simulations collected in a late model intercomparison study [ Jiang et al., 2015 ]. preliminary results showed that the observe relationship between the basic department of state moisture gradient and the MJO detour holds in model simulations. The role of the basic state on the MJO generation characteristics could besides be investigated in the context of different climate regimes by using simulations of future or former climates .
Acknowledgments
This work was supported by the Korea Meteorological Administration Research and Development Program under grant KMIPA 2016-6010. D. Kim was besides supported by the startup grant from the University of Washington .
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