4.2 Monsoon onset and soil moisture
The standardized SAH, the WB, and the XV indices are shown in Fig. 3. The positive WB and XV anomalies indicate late monsoon onset, and their negative anomalies indicate early monsoon onset, respectively. The XV index shows a negative correlation of -0.56 with the SAH index, the correlation between the WB index and the SAH is -0.61; both correlation coefficients have passed the significance test with a 95% confidence level. Thus, it is suggested that the SAH and the SA monsoon may have different mechanisms, and the SAH negatively affects the SA monsoon onset. These findings complement the previous studies and suggest that the SAH intensity may enhance the divergence aloft accompanied by lower-level cyclonic and convective activities, leading to monsoon onset vortex (MOV) in the SA monsoon domain (Liu et al. 2013; Wei et al. 2014). The TP thermal heating and energy fluxes are known to link with the SAH; however, the soil moisture relation with SAH remains unknown, although it is known to play an essential role in land surface energy fluxes partitioning.
Hence, a pixel-wise correlation analysis is conducted to understand the TP soil moisture relations with the WB, the XV, and the SAH index, respectively, and the results are shown in Fig. 4. The average soil moisture from April to June (AMJ) indicates springtime soil moisture over the TP (Ullah et al. 2020). It appears that both XV (Fig. 4a) and WB (Fig. 4b) indices are negatively correlated with springtime soil moisture across the TP, with correlation coefficients ranging from − 0.50 to -0.70. However, the SAH index is positively correlated with the springtime soil moisture across the plateau, and the correlation coefficients are ≥ 0.90 (α = 5%) in most areas. The TP soil moisture is hypothesized to modulate the SAH intensity through a certain mechanism, affecting the SA monsoon’s onset. Several studies have indicated that soil moisture affects the boundary layer structure and the upper troposphere by altering the vertical thermal processes (Sanchez-Mejia and Papuga 2014; Koster et al. 2016; Berg et al. 2017; Schwingshackl et al. 2018). In the following sections, we have shown the spring soil moisture influence on the TP thermal profile and linked them with SAH movement and SA monsoon onset.
4.3 Spring soil moisture influence on TP thermal forcing
We conducted the composite analysis to understand the possible thermal processes that link the TP soil moisture to the SAH intensity and the SA monsoon onset. As shown in Fig. 3, the positive and negative values of the z-score indicate the deviation of the monsoon onset date from its climatological mean onset date. In this regard, the years with negative z-score values were referred to as early-onset composite (1990, 1999, 2002, and 2004) and late-onset composite (1992, 1995, 1997, and 2003) for positive z-score values, respectively. The respective average onset date/pentad for the early and late-onset years are further shown in Table 1. The onset timing estimated from the two indices has an obvious difference, implying that the onset inferred from zonal wind maybe earlier than tropospheric temperature. The onset composites have an evident difference of 20–25 days between the early and late-onset composite. The monsoon onset’s regionally indicating the reversal mechanism of the wind and tropospheric temperature, preceded by convective activities and abrupt precipitation in the Indian ocean, which moves towards continental regions in the following days (Liu et al. 2013). Based on these two composites, the TP’s spring season thermodynamics is expressed as a precursor of the processes leading to the SAH modulation and the monsoon onset.
Table 1
Average monsoon onset days and the average monsoon onset pentad based on the composites.
Composite name
|
Years
|
XV index (onset day)
|
WB index (onset day)
|
Average onset pentad
|
|
1990
|
134
|
128
|
26
|
|
1999
|
140
|
130
|
27
|
Early onset
|
2002
|
135
|
127
|
26
|
|
2004
|
136
|
126
|
26
|
|
1992
|
162
|
157
|
32
|
Late onset
|
1995
|
152
|
159
|
31
|
|
1997
|
156
|
163
|
32
|
|
2003
|
152
|
163
|
32
|
The soil moisture, sensible heat, and latent heat anomalies in the spring season are analyzed in the early- and late-onset composites, respectively, shown in Fig. 5. In the early-onset composites, the TP soil moisture shows mostly positive anomalies (Fig. 5a) over the plateau; during the late-onset composites, the TP soil moisture shows negative anomalies across the plateau, except for the southeast corner (Fig. 5b). The associated thermal processes of the contrary composites are further studied. In the early-onset composite, the sensible heat fluxes (Fig. 5c) show negative anomalies of < -9 W m2 in the eastern and southern TP, where the latent heat (Fig. 5e) shows strong positive anomalies with a magnitude of about 12 W m− 2. In the late-onset composite, the sensible heat (Fig. 5d) exhibited positive anomalies of 6 ~ 8 W m− 2 in the eastern and the southern TP and negative anomalies from − 2 to -4 W m− 2 in the northern TP. On the contrary, the latent heat (Fig. 5f) shows anomalies opposite to those of the TP’s sensible heat.
Soil moisture plays an essential role in partitioning the available net radiation into sensible and latent heat, and the composite results agree with previous studies (Berg et al. 2015; Ford and Schoof 2016; Koster et al. 2016). Soil moisture deficit is suggested to increase the sensible heat and decrease the latent heat, although other factors such as solar forcing may affect these relations. However, the soil moisture influence on near-surface energy fluxes is rarely linked with the atmospheric profiles, local and remote-scale circulations using observational data. Hence, we further derived the vertical sensible and latent heat profiles in the early- and late-onset composites. Previous studies have indicated that the interannual variations of SAH are significantly affected by the condensation heating from the eastern TP and Yangtze river valley (Wei et al. 2015; Zhang et al. 2016; Ge et al. 2019). Figure 6 shows the vertical profile of latent and sensible heat fluxes averaged for 27N to 37N and 85E to 103E. The vertical profile of latent and sensible heat fluxes is for the same region as used in the above studies, but consider the soil moisture effect rather than the precipitation-induced heating. In the early-onset composites (Fig. 6a), both sensible and latent heat anomalies are positive (0.8 K day− 1) near the surface; however, at mid-troposphere from 500 to 300 hPa, the sensible heat anomalies reduce to zero, and the latent heat anomalies are the strongest, reaching 2.5 K day− 1. In the late-onset composites (Fig. 6b), there are negative latent heat anomalies in the lower and upper troposphere from 400 to 200 hPa with a magnitude of <-0.7 K day− 1. The sensible heat flux during the late-onset composite is positive at near-surface (550 hPa) pressure levels (0.6 K day− 1). The sensible heat flux near the surface pressure level (600hPa) exhibited negative anomalies attributed to the surface and boundary layer induced gradients and uncertainties of the reanalysis products over TP (Cui and Wang 2009). During the frozen and transition period, the reanalysis products have shown deviations and uncertain states of the energy and water fluxes over the TP (Ullah et al. 2018) mainly due to deficiencies in model structures (Bi et al. 2016).
From Fig. 6, significant changes in latent and sensible heat fluxes in the early- and late-onset years are obvious from surface pressure levels till 200hPa. Considering the soil moisture role in modifying the energy fluxes in the overlying atmosphere through the convective processes (Taylor et al. 2011), a drier soil moisture profile may influence diabatic heating in the overlying atmosphere through enhancing the sensible heat flux and vice versa for the latent heat. On the other hand, the wetter soil enhances latent heat, releasing the absorbed excess energy upon condensation through an adiabatic ascent aloft (Koster et al. 2016). Over the TP, both sensible and latent heat fluxes are critically linked with the soil moisture’s freeze-thaw stage during the transitional spring season (Cui and Wang 2009). They hence can actively influence the vertical profile of the diabatic heating over the plateau and produce divergent motion with lower/upper level cyclonic/anticyclonic pattern (Ullah et al. 2020).
The influence of soil moisture anomalies and, consequently, heat fluxes are also associated with the TP diabatic heating’s vertical column. Figure 7 shows the vertical profiles of diabatic heating averaged from 27N to 38N in spring of the early-onset (Fig. 7a) and late-onset (Fig. 7b) composites. The diabatic heating has shown positive anomalies in the early-onset composites, which are more than > 1.2 K day− 1 at central and eastern TP. On the contrary, in the late-onset composites, the diabatic heating shows negative anomalies of -0.4 K day− 1 in the upper troposphere, whereas a consistent positive anomaly is evident in the eastern TP. The soil moisture anomalies affect the vertical profile of diabatic heating by partitioning the energy fluxes, which affects the overlying atmosphere where the excess energy absorbed is released (Koster et al. 2016). Such thermal influences are attributed to the land surface energy fluxes, which are sensitive to soil moisture anomalies (Cui and Wang 2009), and the enhanced latent heat anomalies result in a decreased boundary layer diabatic heating due to vertical adiabatic ascent. The diabatic heating anomalies in the eastern plateau favor unstable atmosphere and vertical ascent followed by descent in the western plateau. The mechanical forcing can be seen over the eastern plateau in the late-onset years that potentially triggers a microscale wind ascent, and thus an increase in diabatic heating, as suggested by Wu et al. (2012) suggested.
Figure 8 shows the SAH anomalies at 200 hPa in the spring of the early-onset and the late-onset composites. In the early-onset composites (Fig. 8a), a high-pressure system prevails (> 18 gpm) over southwestern TP, which stretches towards Iranian Plateau (IP) and the Middle East. Another high-pressure system appears over the Eastern flank of TP, with the center over the Mongolian and Siberian regions. In the late-onset composites (Fig. 8b), the geopotential height in the SAH region shows obvious negative anomalies of <-16 gpm. Over the Indian Ocean and Northern Eurasian Continent, the strong high-pressure system is persistent with an increased geopotential height of > 10gpm. The high-pressure system apparent during the early monsoon onset composite is symmetrically replaced by a low-pressure system centered over the southern and northern TP with an intensity of <-18gpm each, respectively. The SAH (Fig. 8) has a consistently high and low-pressure mode in the spring of both early- and late-onset composites, varying in a tripole pattern. In the early-onset composite, the SAH exhibits two distinct high-pressure centers located at the TP’s southern and northern sides, respectively. There is a low-pressure system over the Eurasian region, and hence a tripole pattern is formed. In the early-onset composite, the spring soil moisture-induced thermal forcing may intensify the SAH intensity. The wet soil moisture influences the vertical thermal column aloft through latent heat of condensation released, as Koster et al. (2016) described. In the late-onset composite’s spring season, the vertical heating and energy fluxes over TP are weaker, which could be associated with surface energy and negative soil moisture anomalies. In response to the reduced thermal heating, the high-pressure centers at southern and northern TP are replaced by the low-pressure systems, whereas a high-pressure system replaces the Eurasian low-pressure system. The tripole pattern mechanism in monsoon onset is further explored with composites of vertical velocity and meridional wind components over TP.
Figure 9 shows the vertical velocity and meridional wind in the spring of the early- (Fig .9a) and late-onset (Fig .9b) composite. In the early-onset composite, an obvious ascending motion exists over the eastern plateau, which produces lower tropospheric cyclonic motion and diverges towards the western plateau. In the late-onset composite, an ascending motion is obvious at the western plateau, followed by a sinking motion at the eastern plateau. Figure 9 infers that in the spring season of the early monsoon onset composites, the intensified SAH favors wind ascent towards TP as evident in the eastern TP during early monsoon onset composites and vice versa for the delayed onset composites. The ascent/descent is vastly linked with the convergence of winds from the Indian ocean, including a land-atmosphere ocean thermal contrast, leading to monsoon onset and precipitation (Wu et al. 2012; Pathak et al. 2017a). In the following section, a pentad-scale movement of the SAH and its associated ascent over the TP is linked with the transition of the equatorial zonal easterlies into prevailing westerlies resulting in monsoon onset due to strong convective activities and precipitation in the SA monsoon domain.
4.4 SAH zonal movement and monsoon onset
Figure 10 shows the pentad evolution of the SAH for early and late monsoon onset composites during the SA monsoon onset phase. For both early and late-onset composites, two pentads, including the pre-onset (onset pentad: -1) and onset-pentad (onset pentad: 0), were selected as an indicator of the SAH location and short-term evolution. During the early onset composite, in the pre-onset pentad (Fig. 10a), the SAH center is located over TP stretched zonally with a weak low located westward of the plateau. Such a high(low) combination indicates a strong ascending motion over TP accompanied by an intensified sinking west of the plateau. From the onset pentad (Fig. 10b), the SAH centers of the SAH move northeastward with secondary high generated over Iranian Plateau. The northeastward movement indicates the downstream convective activities initiation in the Bay of Bengal (BOB) due to monsoon onset and establishment of the westerlies and easterlies into the region (Liu et al. 2013). During the late-onset composite, in the post-onset (Fig. 10c) the SAH low is centered over the Iranian plateau extending into the SA in the east, indicating a stable high pressure in the lower troposphere. The TP high-pressure system is farther northwest of the plateau, indicating a weaker ascent. During the onset pentad (Fig. 10d), the two low-pressure systems located eastward of the plateau and over the Iranian plateau replace the SAH centers, as evident during the early onset pentad. The northwest high center of the SAH moves into TP, but weaker intensity indicates an ascent over continental regions located westward of the Bay of Bengal. In conclusion, two aspects of the SAH are evident during the early and late-onset composites. The first aspect is its potential relationship with TP soil moisture induced thermal forcing that maintains its intensity and controls the monsoon onset. The second aspect of the SAH is its expansion into the high latitudes, which intensify the meridional circulations. Hence, it can be the potential reason for early monsoon onset triggered by TP soil moisture-induced thermal forcing. The findings complement the previous studies reporting similar characteristics of the SAH (Wu 2002; Wei et al. 2014); however, a model study will further be conducted to validate the findings.
Figure 11 shows the TP meridional wind component, and vertical velocity averaged for the latitudinal range of 27N to 38N across the longitude. During the pre-onset pentad (Fig. 11a), the western and central plateau experience an intensified ascent accompanied by descent in the western plateau, which during the onset-pentad (Fig. 11b) moves eastward into the eastern plateau and Bay of Bengal region. In the onset-pentad, the ascending motion over the eastern plateau results in the regional westerlies and easterlies movement towards the SA domain that initiates in the Bay of Bengal and pushes the SAH to the north. A similar pattern of the ascending motion and upper-tropospheric divergent motion was also reported by Ullah et al. (2020) and Liu et al. (2013), attributed to the plateau soil moisture thermal profile and condensation heating release. During late-onset composite, the pre-onset pentad (Fig. 11c) meridional wind and vertical velocity anomalies are weaker with obvious descending motion evident over the plateau, inferring a weaker thermal profile of the plateau. The onset pentad (Fig. 11d) has experienced a relatively weaker ascent over the western plateau and Yangtze river basin but rather a stronger descent over the eastern plateau. Such weaker ascent and partly descent over the eastern plateau can potentially be attributed to soil moisture negative anomalies that can lead to weaker thermal profile and westward shift in the ascent and associated precipitation. To explore the regional westerlies and easterlies convergence into the SA monsoon domain forced by TP soil moisture-laden thermal forcing, the lower tropospheric wind anomalies and vertical velocity are shown at 850 hPa. The wind anomalies shown are a representation of the monsoon onset due to wind shear in the pre-defined monsoon index (WB-index) representing the westerlies evolution replacing the zonal easterlies.
Figure 12 shows the wind components (vectors) and vertical velocity (shaded) for the pre-onset, onset, and post-onset pentads for the early and late-onset composites at 850 hPa. The wind anomalies are used to show the transition of the westerlies and establishment of the subtropical jet stream associated with enhanced moisture contents, enhanced convective activities, and ascent showed by vertical velocity anomalies. During early-onset composites, in the pre-onset pentad (Fig. 12a), the prominent features include zonal easterlies from the pacific warm pool and north-easterlies from South China Sea (SCS) enters into Indian ocean and Bay of Bengal (BOB). These two easterlies merge into the continental westerlies and form prevailing continental easterlies gusting towards eastern Africa. In the onset pentad (Fig. 12c), the equatorial zonal easterlies after entering into Indian ocean changes into zonal westerlies, and the SCS currents after entering into BOB, join the zonal westerlies and advances into the continental landmasses. In the onset pentad, a clear transition of the easterlies into westerlies is evident with stronger intensities and ascent (shaded) in the Indian ocean and peninsular India, which is relatively weaker in the continental regions. During the post-onset pentad (Fig. 12e), the south-westerlies from the Indian ocean and zonal easterlies intensify the ascent, evident from vertical velocity in the continental regions. During the pre-onset pentad of late-onset (Fig. 12b), the zonal equatorial easterlies are much weaker, and south-westerlies also replace the SCS north-easterlies with stable atmosphere and descending motion evident in BOB and Arabian sea. In the onset-pentad (Fig. 12d), the zonal easterlies change into westerlies and advance towards SA, but the BOB easterlies are suppressed, and hence a south-westerlies jet into SCS prevails. In the post-onset pentad (Fig. 12f), the westerlies are fully established but rather weaker and limited to the western parts of SA with stable conditions over BOB. In conclusion, the TP soil moisture can influence the ascent and SAH intensity and modulation in zonal and meridional domains that exhibit teleconnections with downstream zonal and meridional wind and modulate the monsoon onset. The findings agree with previous studies suggesting that TP thermal profile has a strong influence on the Asian monsoon by changing the circulation patterns and thus precipitation magnitude (Wu et al. 2012; Rajagopalan and Molnar 2013; Ullah et al. 2020). From the composite analysis, it can be deduced that spring soil moisture anomalies over TP influence the near-surface energy balance, which can further affect TP energy fluxes, the vertical profile of diabatic heating aloft (Wu et al. 2012; Wang et al. 2014). Such thermal conditions can affect the SAH intensity, which results in the high/low-pressure centers across southern/northern TP. The SAH variability can trigger vertical ascending/descending motion, leading to cyclonic/anticyclonic activities, and thus modulate the SA monsoon onset.