Java-Sumatra Niño/Niña and associated regional rainfall variability

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


Introduction
In boreal fall (September-November), sea surface temperature anomalies (SSTAs) in the tropical Indian Ocean are often characterized by the Indian Ocean Dipole (IOD), a key driver of rainfall variability in Southeast Asia, southeastern Australia and eastern Africa [1][2][3][4][5][6][7][8] . The anomalous SST gradient from the southeastern equatorial Indian Ocean (90°E-110°E and 10°S-0°N) to the western equatorial Indian Ocean (50°E-70°E and 10°S-10°N), which is referred to as the Dipole Mode Index 1 , has been widely used to represent the occurrence and intensity of IOD events. However, SSTAs in the western and southeastern equatorial regions are poorly correlated 9,10 on interannual time scales, with a minimum correlation of 0.1 in boreal summer (June-August) and a maximum of only -0.29 in boreal fall (September-November, SON) for the past 70 years (See Methods). This suggests that it may not be ideal to rely only on the IOD to understand and monitor atmosphere-ocean processes over the tropical Indian Ocean and their climate impacts on the surrounding continents. It is therefore important to investigate how the western and southeastern equatorial Indian Ocean individually contribute to the IOD.
The southeastern pole of the IOD has much greater variability than the western pole 11 and is often characterized by the appearance of warm/cold SSTAs along the south and southwest coasts of Java and Sumatra, a phenomenon largely linked to wind-driven coastal upwelling 12,13 and thus referred to here as Java-Sumatra Niño/Niña (hereafter, JSN for the phenomenon in general and JS Niño/Niña for warm/cold events). Closely phase-locked to the seasonal cycle, JSN prevails largely in July-September (JAS), when the monsoonal winds are upwelling-favorable along the Java-Sumatra coast (i.e., southeasterly) and the coastal thermocline is relatively shallow 12 (Fig. 1a,d). At times JSN appears as a precursor to the IOD, which becomes mature in October 1 . In this study, JSN events are de ned based on SSTAs averaged in the Java and Sumatra coastal regions (9°S and 3°S, 95°E-105°E and 12°S and 6°S, 105°E-115°E). See Methods for more details.
Similar to the IOD, JSN events are linked to rainfall variability over the surrounding continents (Fig. 1c,f).
For the IOD, these relationships are complex and often break down, partly due to the strong remote in uence of El Niño-Southern Oscillation (ENSO) via changes in the Indo-Paci c Walker cells and extratropical stationary waves 3,4,[14][15][16] . Additionally, some IOD events are driven by ENSO [17][18][19][20][21][22] , while others are driven by atmosphere-ocean processes internal to the Indian Ocean 20,[22][23][24][25][26][27] . These may also be applicable to JSN events. For instance, the 1997 JS Niña event occurred during the developing phase of the 1997-1998 El Niño, and contributed to widespread Indonesian forest res, which resulted in massive coral reef mortality events along the Java-Sumatra coast 28 . In contrast, the 2019 JS Niña event, one of the strongest events ever recorded, developed rapidly in boreal summer under near-neutral ENSO conditions in the Paci c 29,30 , produced heavy rainfall and widespread severe ooding and landslides across South Asia 31 , and caused extreme hot and dry conditions that contributed to severe forest res over southeastern Australia 32,33 .
As brie y discussed, a signi cant gap still exists in our understanding of atmosphere-ocean variability in the tropical Indian Ocean. In particular, interannual variability of SST in the southeastern equatorial Indian Ocean associated with JSN and its impact on regional rainfall variability need to be described and explained, which is the main objective of the present study. To do so, we rst present the basic characteristics of JSN. Next, to address large inter-event variability of JSN, we present the spatiotemporal evolution patterns of the most frequently recurring JSN varieties and their links to ENSO and regional rainfall variability. We also present the potential onset mechanisms of the leading JSN varieties, followed by a discussion.

Results
Basic characteristics of JSN. As shown in Fig. 1a, JS Niña often develops as early as April-June (AMJ) along the south coast of Java, peaks in July-September (JAS) as the cold SSTAs slowly shift to the southwest coast of Sumatra, and dissipates in October-December (OND). Although weak, there is a secondary peak of JS Niña in the preceding January-March (JFM; hereafter, pre-season). This pre-season cooling pattern is closely tied to a basin-wide cooling of the tropical Indian Ocean that is largely modulated by the Indian Ocean Basin (IOB) mode, which tends to occur during the dissipation phase of La Niña in boreal winter and spring 35 , as discussed later in this study. Interestingly, relatively weak warm SSTAs develop in the central tropical South Indian Ocean in JAS, and then propagate westward to produce a positive IOD-like pattern in OND.
During the most active months (JAS), JS Niña is typically characterized by cold SSTAs, a shallower thermocline, southeasterly surface wind anomalies along the Java-Sumatra coast, a deeper thermocline farther westward in the tropical South Indian Ocean, and easterly surface wind anomalies along the eastern equatorial Indian Ocean (Fig. 1b). These anomalous atmosphere-ocean patterns indicate that local Bjerknes feedback plays an important role in the development of JS Niña 23 . More speci cally, the cold SSTAs off the Java-Sumatra coast suppress atmospheric convection aloft. The associated heat sink induces an anomalous anticyclone to its west (i.e., a descending Rossby wave response 34 ), producing southeasterly wind anomalies and anomalous upwelling off the Java-Sumatra coast, which in turn reinforce the initial cooling 23 . The heat sink also drives equatorial easterly wind anomalies (i.e., an equatorial divergent wind response 34 ), thus producing anomalous upwelling in the eastern equatorial Indian Ocean and promoting an expansion of the cold SSTAs toward the equator along the Sumatra coast (Fig. 2). JS Niño is characterized by nearly opposite patterns of anomalous SSTs, thermocline depth and surface winds compared to JS Niña (Fig. 1d,e). However, there are several noticeable differences that break the antisymmetry. JS Niño tends to develop and decay slightly earlier than JS Niña in boreal summer and fall, and the associated atmosphere-ocean anomalies are generally much weaker 36 . In contrast, the preseason peak in JFM, which is tied to a positive phase of the IOB mode and the dissipation phase of El Niño 35 , has a much larger amplitude, being on par with the peak amplitude in June-August. Additionally, unlike JS Niña, JS Niño tends to remain a monopole throughout boreal summer and fall.
JS Niña events are linked to decreased rainfall over the Maritime Continent (i.e., maritime Southeast Asia) and Malay Peninsula, and vice versa for JS Niño events (Fig. 1c,f). Some JS Niña/Niño events are also linked to increased rainfall in South Asia and mainland Southeast Asia (hereafter, South-Southeast Asia). However, these and other basic characteristics of JSN derived from the composite mean elds (Fig. 1a-f) are not applicable to all events. Indeed, the spatiotemporal evolution patterns are quite different between individual events in terms of the timing, zonal pattern, and amplitude of their onset, peak, and dissipation (Supplementary Figs. 1 and 2). Spatiotemporal diversity of JSN. As described in Methods, we carry out a spatiotemporal EOF analysis to identify the rst two preferred modes of the observed JS Niño/Niña events. Each EOF mode for warm/cold events represents two contrasting JS Niño/Niña varieties or avors (PC changing from − 1 to 1) that correspond to adding and subtracting the EOF pattern to the composite mean, leading to four sets of the most frequently recurring JS Niño/Niña varieties. See Methods and Supplementary Fig. 3 for more details.
One of the leading JS Niño/Niña varieties describes warm/cold events that form in JAS and thus is referred to as the on-season JS Niño/Niña variety (Fig. 3f,h) Occasionally, strong cold events may develop in boreal summer, persist throughout SON and spawn robust warm SSTAs in the central tropical South Indian Ocean, which in turn propagate westward and peak in OND. The spatiotemporal evolution of such strong and persistent cold events (e.g., the 2019 JS Niña) is well captured by the persistent JS Niña variety (Fig. 3b). Finally, although rare and very weak in amplitude, a pre-season warm event may switch to a cold event in the boreal summer (hereafter, the transition JS Niño variety; Fig. 3d). It is worth noting that the persistent JS Niña variety displays a positive IOD-like pattern in OND, whereas the double-peak JS Niño variety prevails largely as a monopole throughout boreal summer and fall. Both the double-peak JS Niña and on-season JS Niño varieties display an IOD-like pattern in boreal summer, while the on-season JS Niña variety has a more complex zonal tripole structure.
Interactions with ENSO. The pre-season and double-peak JS Niño/Niña varieties are strongly connected to the dissipating phase of El Niño/La Niña in boreal spring (Fig. 4a,c,e,g). Speci cally, the pre-season JS Niño/Niña variety is favored if El Niño/La Niña dissipates into an ENSO-neutral phase in boreal summer, whereas the double-peak JS Niño/Niña variety is favored if El Niño/La Niña transitions to the opposite phase and then intensi es during boreal summer and fall. However, some double-peak JS Niño/Niña events occurred under a near-neutral ENSO condition in JAS (e.g., the 2016 JS Niño; Supplementary Figs. 4 and 5). In contrast, there is no apparent ENSO signal linked to the on-season JS Niño/Niña variety in boreal summer and fall. This suggests that the on-season JS Niño/Niña variety is predominantly driven by atmosphere-ocean processes internal to the Indian Ocean. Similarly, the transition JS Niño variety seems to have little connection to ENSO.
Perhaps the most interesting case is the persistent JS Niña variety, which is characterized by the greatest intensity of cold SSTAs off the Java-Sumatra coast. During these events, strong warm SSTAs develop in the southwestern equatorial Indian Ocean around OND, so this variety can also be described as strong positive IOD. The persistent JS Niña variety is association with the developing phase of El Niño, but the regressed ENSO signal is relatively weak, especially during the onset phase of the persistent JS Niña variety around May-July (Fig. 4b). Therefore, it is unlikely that ENSO is required for the onset of the persistent JS Niña variety. Instead it is likely driven by atmosphere-ocean processes internal to the Indian Ocean.
To further study the potential role of ENSO in driving the JSN varieties, longitude-time composite maps of SSTAs along the equatorial Paci c and tropical South Indian Oceans are constructed for observed El Niño and La Niña events during 1949-2019 ( Fig. 4i,j). There is an overall weak connection between the developing phase of ENSO and SSTAs along the Java-Sumatra coast. An IOD-like pattern of SSTAs emerges around SON from the southeast equatorial Indian Ocean to the western equatorial Indian Ocean.
However, during the onset phase of JSN (i.e., prior to July), equatorial Paci c SSTAs associated with the developing phase of ENSO are relatively weak; thus, it is unlikely that ENSO drives the onset of JSN. The remote in uence of ENSO during the onset, peak and dissipation phases of JSN, and the associated regional rainfall patterns, are further investigated next.
Regional rainfall patterns linked to JSN varieties. Figure 5 shows the rainfall and 850 hPa wind anomalies associated with El Niño and the leading JS Niña varieties for the onset (AMJ), peak (JAS) and dissipation (OND) phases of JS Niña. During the peak phase of JS Niña, the rainfall anomalies associated with El Niño show a predominantly zonal see-saw pattern with decreased rainfall over the Maritime Continent and increased rainfall in the western tropical Paci c (Fig. 5b). The anomalous lowlevel westerly winds along the western tropical Paci c clearly indicate a weakened Paci c Walker cell. The rainfall and low-level wind anomalies during the peak phase of the double-peak JS Niña variety are largely consistent with those of El Niño (Fig. 5e), suggesting that the remote in uence of El Niño via changes in the Paci c Walker cell largely dictates the regional rainfall anomalies associated with the double-peak JS Niña variety.
In contrast, the rainfall anomalies during the peak phase of the persistent JS Niña variety show a meridional see-saw pattern between the Maritime Continent and South-Southeast Asia (Fig. 5h). The associated anomalous low-level winds suggest a Gill-type cross-hemispheric atmospheric response to diabatic cooling anomalies aloft in the southeastern equatorial Indian Ocean 34 . The Gill-type atmospheric circulation carries extra moisture from the southeastern equatorial Indian Ocean region to South-Southeast Asia and can be interpreted as a strengthening of the South and Southeast Asian summer monsoons. The on-season JS Niña variety during its peak phase also shows a similar meridional see-saw pattern of rainfall anomalies and the Gill-type atmospheric response, though the rainfall and low-level wind anomalies are generally weaker than those of the persistent JS Niña variety (Fig. 5k). Additionally, the teleconnection to southeastern Australia, which is known to occur in boreal summer during some IOD events 14 , is observed only during the peak phase of the persistent JS Niña variety (Fig. 5h).
During the dissipation phase (OND) of JS Niña, the rainfall anomalies associated with El Niño and the double-peak, persistent and on-season JS Niña varieties all show a zonal tripole pattern with increased rainfall from eastern Africa to the western tropical Indian Ocean, decreased rainfall over and around the Maritime Continent, and increased rainfall over the western tropical Paci c Ocean (Fig. 5c,f,i,l). The zonal tripole pattern of rainfall anomalies that appears during the dissipation phase of the persistent JS Niña variety is particularly strong, with large rainfall anomalies over eastern Africa, northern Australia and the Maritime Continent, whereas the rainfall response to the on-season JS Niña variety is generally very weak in OND. Figure 6 shows the rainfall and 850 hPa wind anomalies associated with La Niña and the leading JS Niño varieties for the onset, peak and dissipation phases of JS Niño. The rainfall and low-level wind anomalies associated with La Niña are almost mirror images of those associated with El Niño, indicating a zonal see-saw pattern of rainfall anomalies and a strengthened Paci c Walker cell during the peak phase of JS Niño (Fig. 6b). The rainfall and low-level wind anomalies associated with the peak phase of the double-peak JS Niño variety are largely consistent with those of La Niña (Fig. 6e). Interestingly, Gilltype atmospheric circulation anomalies still appear during the peak phase in response to diabatic heating anomalies over the southeastern equatorial Indian Ocean. However, these anomalies are not robust enough to produce a meridional see-saw pattern of rainfall anomalies between the Maritime Continent and South-Southeast Asia. Similarly, the anomalous low-level winds associated with the peak phase of the on-season JS Niño variety show a Gill-type atmospheric response to diabatic heating anomalies over the southeastern equatorial Indian Ocean (Fig. 6e). The associated regional rainfall anomalies are somewhat indicative of a meridional see-saw pattern of regional rainfall anomalies. However, the rainfall response to the on-season JS Niño variety is generally weak.
During the dissipation phase (OND) of JS Niño, the rainfall anomalies associated with La Niña and the double-peak and on-season JS Niño varieties are characterized by a zonal tripole pattern with decreased rainfall from eastern Africa to the western tropical Indian Ocean, increased rainfall over the Maritime Continent and over northern and eastern Australia, and decreased rainfall over the western tropical Paci c Ocean (Fig. 6c,f,i). However, the rainfall response to the on-season JS Niño variety is very weak in OND.
Onset mechanisms of JSN varieties. As discussed earlier, the pre-season and double-peak JS Niño/Niña, and persistent JS Niña varieties are connected to ENSO, while the on-season JS Niño/Niña variety is largely independent of ENSO. Here, we explore in more detail the onset mechanisms of leading JSN varieties. First, we examine the anomalous SSTs, thermocline depth and surface winds associated with El Niño/La Niña and then compare them with those of the leading JS Niña/Niño varieties during the onset, peak and dissipation phases of JSN (Figs. 7 and 8).
SSTAs in the tropical Indian Ocean associated with El Niño are generally weak (Fig. 7a-c). Although weak cold SSTAs appear in a broad region of the southeastern tropical Indian Ocean during the peak phase (JAS) of JS Niña, the surface wind anomalies are too weak to trigger or sustain JS Niña. A positive IODlike pattern of weak SSTAs develops during the dissipation phase (OND) of JS Niña, with corresponding surface wind anomalies (i.e., southeasterly) that are favorable for coastal upwelling along the Java-Sumatra coast and downwelling in the central tropical South Indian Ocean. However, since the southeasterly monsoonal wind weakens and can reverse its direction as the Australian summer monsoon develops 37 , the seasonal mean thermocline along Java and Sumatra is too deep to sustain local Bjerknes feedback in OND ( Supplementary Fig. 6).
For the double-peak JS Niña variety, cold SSTAs appear in the southeastern tropical Indian Ocean during its onset phase (AMJ), with almost no surface wind anomalies in the region (Fig. 7d). This suggests that the cold SSTAs are the residual of basin-wide cold SSTAs forced remotely during the dissipating phase of La Niña (Fig. 4e). Although still weak, the anomalous SSTs, thermocline depth and surface winds are generally larger than those of El Niño during the peak and dissipation phases of JS Niña (Fig. 7e,f), suggesting an involvement of local Bjerknes feedback in the double-peak JS Niña variety, possibly enhanced by the residual of cold SSTAs formed during the pre-season.
Unlike El Niño or the double-peak JS Niña variety, a robust pattern of upwelling favorable surface wind anomalies appears along the equatorial and southeastern equatorial Indian Ocean during the onset phase of the persistent JS Niña variety (Fig. 7g). Although weak, cold SSTAs appear along the south coast of Java, and the thermocline becomes shallower from the south coast of Java to the northeast equatorial region off Sumatra. During the peak phase, the anomalous surface winds strengthen greatly, and the cold SSTAs intensify and expand to the southwest coast of Sumatra (Fig. 7h). Additionally, the thermocline becomes shallower along the Java-Sumatra coast and in the eastern equatorial region, and the anomaly expands to the west. In contrast, warm SSTAs and a deeper thermocline develop in the central tropical South Indian Ocean. These patterns of amplifying surface wind, SST and thermocline depth anomalies from the onset to peak phase strongly suggest an active role of local Bjerknes feedback.
During the dissipation phase of the persistent JS Niña variety, the cold SSTAs off the Java-Sumatra coast persist but weaken, while the anomalous patterns of surface winds and thermocline depth further intensify and propagate westward (Fig. 7i). These surface wind and thermocline depth anomalies are accompanied by a pronounced basin-wide see-saw pattern of SSTAs between the southeast and southwestern tropical Indian Ocean, thus producing a strong positive IOD. Although the surface wind anomalies are still strong and upwelling favorable, the cold SSTs along the Java-Sumatra coast start to weaken in OND as the southeasterly monsoonal wind weakens and the seasonal mean thermocline along the Java-Sumatra coast becomes too deep to in uence the coastal SSTs. These also explain how the weakening and directional shifts in monsoonal winds in boreal winter prevent JSN from continuing beyond OND (Supplementary Fig. 6).
The spatial patterns and amplitudes of anomalous SSTs, thermocline depth and surface winds associated with the on-season JS Niña variety are very similar to those of the persistent JS Niña variety during the onset and peak phases (Fig. 7j,k). However, unlike the persistent JS Niña variety, the anomalous SSTs and surface winds diminish rapidly in OND (Fig. 7l). It is not entirely clear why some JS Niña events are persistent (i.e., the persistent JS Niña variety) and others dissipate more quickly (i.e., the on-season JS Niña variety). One possibility is that some JS Niña events are enhanced and prolonged by remote forcing from the Paci c. Another possibility is that an exceptionally strong JS Niña event may greatly weaken the Indo-Paci c Walker cells, producing warm SSTAs in the neighboring regions of the tropical Indian and Paci c Oceans. The warm SSTAs in the tropical Indian and Paci c Oceans may in turn further weaken the Indo-Paci c Walker cells to prolong the JS Niña event in OND. Consistent with this hypothesis, the persistent JS Niña variety is linked to warm SSTAs in the southwestern tropical Indian Ocean and to weak El Niño conditions (Fig. 4b), but there are almost no such warm SSTAs for the onseason JS Niña variety (Fig. 4f). This hypothesis is also in line with the idea that a strong IOD event may induce a weak (or pseudo) ENSO event 38-41 .
Anomalous SSTs, surface winds and thermocline depth associated with La Niña are more or less mirror images of those associated with El Niño (Fig. 8a-c). In other words, the anomalies are too weak to trigger or sustain JS Niño during the onset (AMJ) and peak (JAS) phases of JS Niño. During the dissipation phase (OND) of JS Niño, a negative IOD-like pattern of weak SSTAs develops with corresponding surface wind anomalies (i.e., northwesterly) that are favorable for coastal downwelling along the Java-Sumatra coast. However, as discussed earlier, due to the weakening of the southeasterly monsoonal wind and the associated deepening of the coastal thermocline after JAS (supplementary Fig. 6), the anomalous northwesterly winds along the Java-Sumatra coast in OND do not produce a pronounced JS Niño event.
Unlike La Niña, a robust pattern of downwelling favorable surface wind anomalies emerges along the equatorial and southeastern equatorial Indian Ocean during the onset phase of the double-peak JS Niño variety (Fig. 8d). As a result, the thermocline is deeper from the south coast of Java to the northeast equatorial region off Sumatra and shallower immediately southwest of the Java-Sumatra coast. Also during this early period (AMJ), warm SSTAs already prevail widely across the tropical Indian Ocean including in the southeastern equatorial region. The early season warm SSTAs are the residual of preseason warming, forced remotely during the dissipation phase of El Niño in boreal winter and spring (Fig. 4g). These surface wind and thermocline depth anomalies and the warm SSTAs along the Java-Sumatra coast intensify during the peak phase (Fig. 8e). This suggests that local Bjerknes feedback still plays an active role in the double-peak JS Niño variety, given that the remote in uence of La Niña is generally weak during the onset and peak phases of JS Niño (Fig. 8a,b). Consistently, some double-peak JS Niño events occurred under a near-neutral ENSO condition in JAS. For instance, the 2016 JS Niño event, which is the strongest warm event during 1949-2019, occurred under a very weak and short-lived La Niña event and brought extremely high rainfall over Indonesia and Australia in June-September 41 ( Supplementary Figs. 2 and 5). During the dissipation phase of the double-peak JS Niño variety, warm SSTAs off the Java-Sumatra coast quickly diminish, while the anomalous patterns of surface winds and thermocline depth further intensify and propagate westward (Fig. 8f). Additionally, warm SSTAs develop in the central tropical South Indian Ocean south of 10°S. It is likely that these atmosphere-ocean anomalies in OND are, to a certain extent, enhanced by La Niña through a strengthened Walker cell in the tropical Indian Ocean and the associated anomalous wind stress curl and Ekman pumping. Similar to the double-peak JS Niño variety, during the onset phase of the on-season JS Niño variety, downwelling favorable surface wind anomalies appear along the equatorial and southeastern equatorial Indian Ocean (Fig. 8g). Although weak, warm SSTAs also appear along the south coast of Java. During the peak phase of the on-season JS Niño variety, the anomalous surface winds intensify; thus, the thermocline is deeper along the Java-Sumatra coast and shallower in the central tropical South Indian Ocean (Fig. 8h). The anomalous SSTs and surface winds quickly diminish during the dissipation phase since the seasonal mean thermocline along the Java-Sumatra coast is too deep to sustain local Bjerknes feedback in OND (Supplementary Fig. 6).
Potential triggering mechanisms of local Bjerknes feedback. It is widely accepted that ENSO is an important forcing mechanism for the IOD [17][18][19][20][21][22] , which tends to peak in October 1 . However, as shown and discussed in this study, during the onset phase of JSN (AMJ), the remote in uence of ENSO is generally not robust enough to directly force the development of JSN. Thus, for most of the leading JSN varieties (i.e., the on-season JS Niña/Niño, double-peak JS Niño and persistent JS Niña varieties), local Bjerknes feedback 23 is a key ingredient for their emergence. As suggested in Li et al. 23 , it is likely that JSN is a weakly damped oscillation, which by de nition requires a nite amplitude perturbation to trigger and sustain the local Bjerknes feedback.
Previous studies suggested that westward propagating oceanic Rossby waves across the equatorial South Indian Ocean could directly affect atmospheric convection in the western equatorial region to produce the IOD 29,42 . The oceanic Rossby waves may also re ect from the western boundary to produce eastward propagating equatorial Kelvin waves. These oceanic waves may further disturb the Wyrtki jets, the eastward owing ocean currents along the equatorial Indian Ocean appearing in boreal spring and fall, and thus potentially trigger the IOD 41,43−52 . In line with these hypotheses, during the onset phase of the double-peak JS Niño variety, the thermocline across the Seychelles Dome in the southwestern tropical Indian Ocean is deeper (Fig. 8d). The deepened thermocline is remotely forced from the Paci c during the pre-season [53][54][55][56] (Fig. 4g). It is possible that the anomaly later propagates to the west and re ects from the western boundary to produce eastward propagating equatorial Kelvin waves, initiating local Bjerknes feedback that kick-starts the second peak. It is also possible that the residual of warm SSTAs in the southeastern tropical Indian Ocean formed during the pre-season may persist and enhance the local Bjerknes feedback (Fig. 8d). However, in some cases the pre-season anomalies completely dissipate before boreal summer (i.e., the pre-season JS Niño/Niña variety). In addition, there are little-to-no preseason anomalies for the on-season JS Niña/Niña variety or the persistent JS Niña variety. Therefore, further study is needed to understand the extent to which the pre-season anomalies of thermocline depth and SSTs can actually trigger the local Bjerknes feedback.
Other proposed onset mechanisms of the IOD involve subseasonal atmospheric processes, such as the Madden-Julian Oscillation and boreal summer intraseasonal oscillation 25,57−59 , a delayed response to the Paci c ENSO through the Indonesian Through ow 12,60 , recharge oscillation 61 and inter-hemispheric sea level pressure gradient 62 . Yet, some other studies point to atmosphere-ocean anomalies originating from the subtropical South Indian Ocean as a potential driver of the IOD 24,27 . In agreement with this hypothesis, prior to the onset phase of the persistent and on-season JS Niña varieties, an anticyclonic sea level pressure anomaly appears over the subtropical South Indian Ocean in March ( Supplementary  Fig. 7a,d). This sea level pressure anomaly slowly shifts (or expands) toward the equator, producing anomalous southeasterly winds off the Java-Sumatra coast as early as April ( Supplementary Fig. 7b,e). This may in turn serve as a trigger for local Bjerknes feedback. Similarly, prior to the onset phase of the double-peak and on-season JS Niño varieties, a cyclonic sea level pressure anomaly forms over the subtropical South Indian Ocean and then slowly migrates toward the equator (Supplementary Fig. 7g-l). Further study is needed to understand how these subtropical sea level pressure anomalies form and migrate equatorward.

Discussion
To better understand atmosphere-ocean variability in the tropical Indian Ocean, here we explore JSN and its spatiotemporal diversity, associated regional rainfall variability, and onset mechanisms. JSN is largely associated with anomalous upwelling/downwelling along the Java-Sumatra coast, and thus is closely phase-locked to the seasonal cycle, prevailing in JAS when the monsoonal winds are upwelling favorable along the Java-Sumatra coast. At times, JSN appears as a precursor to the IOD, which becomes mature in October 1 , but it often occurs without corresponding SSTAs in the western equatorial Indian Ocean. JSN is essentially an intrinsic mode of variability driven by local Bjerknes feedback and thus does not rely on remote forcing from the Paci c for its emergence. However, the timing, zonal pattern, and amplitude of onset, peak, and dissipation are quite different from event to event.
Some JSN events rst appear in the pre-season (JFM) and then, after a short pause, reorganize in JAS (the double-peak JS Niño/Niña variety). Although these JSN events do not rely on ENSO for their onset, the associated regional rainfall pattern is greatly modulated by ENSO through changes in the Paci c Walker cell (Fig. 9a). In contrast, some JS Niña events appear predominantly in JAS (the on-season and persistent JS Niña varieties) and produce a meridional see-saw pattern of rainfall anomalies that projects onto the South and Southeast Asian summer monsoons, driven by a Gill-type cross-hemispheric atmospheric circulation in response to the cold SSTAs along the Java-Sumatra coast (Fig. 9b). Some of those JS Niña events are much stronger than others and display a strong positive IOD signature (the persistent JS Niña variety). These persistent JS Niña events (e.g., the 2019 JS Niña) potentially enhanceand are enhanced by -warm SSTAs in the neighboring regions of the tropical Indian and Paci c Oceans.
They thus tend to persist longer and produce particularly strong rainfall anomalies over the surrounding continents.
As summarized in Table 1, the main results presented in this study indicate that JSN develops earlier than the IOD as either a monopole, dipole or even tripole. JSN does not rely on ENSO for its emergence, and it is also an important driver of climate variability over the tropical Indian Ocean and the surrounding continents. Given these new ndings, further analysis and modeling studies of JSN are warranted to better understand and monitor the atmosphere-ocean processes involved in the onset, peak and dissipation of JSN. These advances will likely lead to an earlier warning of extreme rainfall and ooding events in the most heavily populated areas of the world.

Methods
Data used. In this study, we analyze observational and reanalysis data sets to describe and explain JSN, and associated regional rainfall variability. Monthly SSTAs are derived from the Centennial in situ Since we are mainly interested in interannual variability, a separate 30-year averaged climatology is constructed every 5 years and used to derive SSTAs. For instance, to compute SSTAs for the 1951-1955 period, a 30-year averaged climatology for 1936-1965 is used; to compute SSTAs for 1956-1960, a 30-year averaged climatology for 1941-1970 is used; and so forth. This method de nes JSN events relative to their contemporary climatology. It is also currently being used at the National Oceanic and Atmospheric Administration's Climate Prediction Center to de ne El Niño and La Niña events. One way to de ne JSN Niño/Niña event is based on the threshold that the 3-month averaged SSTAs exceed ± 0.4°C in the Java-Sumatra coastal region (9°S and 3°S, 95°E-105°E and 12°S and 6°S, 105°E-115°E) for at least three consecutive overlapping seasons. An alternative method is to apply the same threshold separately to the 3-month averaged SSTAs in the south coast of Java ( are associated with each individual JS Niña event, and the EOFs represent a linearly independent set of longitude-time maps. The rst and second leading PCs explain 36% and 13% of the inter-event variance, respectively ( Supplementary Fig. 3b,c). The same EOF analysis is performed to identify the preferred spatiotemporal modes of the 19 observed JS Niño events for which the rst and second leading PCs explain 27% and 23% of the inter-event variance, respectively ( Supplementary Fig. 3e,f). Note that the same method was previously used to identify the leading spatiotemporal modes of the observed El Niño events in the Paci c 68,69 and Atlantic Niño events in the Atlantic 70 .
Regression analysis. In order to describe the atmosphere-ocean processes associated with different JSN varieties, the corresponding spatiotemporal patterns of ocean and atmospheric variables are obtained by linearly regressing their time series onto these PCs. For instance, we rst subsample 21 maps of precipitation anomalies averaged in any season (e.g., AMJ) for the 21 years during which JS Niña occurred. Then, they are regressed on the 21 values of PC2 for the JS Niña events. The regression coe cients are then added to the composite mean of the 21 maps of precipitation anomalies to obtain the anomalous rainfall patterns for the double peak JS Niña variety (Fig. 5d,e,f). Similarly, the regression coe cients are subtracted from the composite mean to obtain the anomalous rainfall patterns for the onseason JS Niña variety (Fig. 5j,k,l).

Code Availability
The NCL codes used to perform the spatiotemporal EOF analysis can be accessed upon request to S.-K.L.

Competing interests
The authors declare no competing interests.