The Interannual Variations of the Summer Monsoon Onset overthe South China Sea (original) (raw)
Related papers
The Role of Bay of Bengal Convection in the Onset of the 1998 South China Sea Summer Monsoon
Monthly Weather Review, 2002
Assimilated analysis fields from the South China Sea Monsoon Experiment and the outgoing longwave radiation data from the National Center for Atmospheric Research (NCAR) have been employed to describe the largescale and synoptic features of the subtropical circulation during the Bay of Bengal (BOB; 6Њ-20ЊN, 80Њ-100ЊE) and South China Sea (SCS; 7Њ-20ЊN, 110Њ-120ЊE) monsoon onsets in 1998. The results show that the Asian monsoon onset during May 1998 exhibited a typical eastward development from the BOB region to the SCS domain. The weakening and retreat of the subtropical anticyclone from the SCS were preceded by the intrusion of westerlies and the development of convective activities over the northern part of the SCS (NSCS; 15Њ-20ЊN, 110Њ-120ЊE). As the vertical shear of zonal wind changes in sign, the ridge surface of the subtropical anticyclone tilted northward and the summer pattern was established over the SCS. Based on these observational results, version 4 of the NCAR climate model (CCM3) is used to investigate the physical link between the convection associated with the BOB monsoon vortex and the SCS summer monsoon onset, as well as the mechanism of the evolution of the low-level subtropical anticyclone over the SCS. Introduction of heating over the BOB results in vigorous convection over the BOB, and the BOB monsoon onset, as well as the development of westerlies and vertical ascent over the NSCS region due to an asymmetric Rossby wave response. Together with the low-level moisture advection, convection is induced over the NSCS. It is the condensation heating over the NSCS that causes the overturning of the meridional gradient of temperature over the SCS. Consequently the subtropical anticyclone in the lower troposphere over the SCS weakened gradually. Eventually as convection develops over the entire SCS domain, the subtropical anticyclone moves out of the region.
Influences of subseasonal to interannual oscillations on the SCS summer monsoon onset in 2018
Terrestrial, Atmospheric and Oceanic Sciences, 2020
The seasonal transition from December 2017 to May 2018 occurred during the final decaying stage of the La Niña phase following the 2015/16 El Niño. In this report we documented the anomalous cyclonic flow that persisted over the South China Sea (SCS) in winter and over the western North Pacific in spring was maintained by the anomalous heating in the equatorial Pacific and the extratropical influences that consist of weakened Aleutian low and subtropical wind-SST coupled air-sea fluxes over the NW Pacific. Persistent anomalous north-easterlies along with negative OLR and warm SST anomalies over the SCS in spring was found to be sustained by the cold and dry air advected from East Asia into the lower latitudes through the anomalous easterlies associated with the cyclonic flow to the south and anticyclonic flow to the north at 20-30°N. Besides the above interannual scale influences, heating over tropical Indian Ocean (IO) associated with two intra-seasonal oscillation (ISO) episodes in April and May caused strong easterly flow over eastern IO and Maritime Continent. The interannual and intra-seasonal influences together maintained an anticyclonic flow from SCS to Bay of Bengal in April and May, and delayed the SCS monsoon onset. A dry Equatorial Rossby wave that arrived at the SCS in late May further enhanced the delay and resulted in a late SCS monsoon onset in early June.
In this work, the authors investigate changes in the interannual relationship between the East Asian summer monsoon (EASM) and the tropical Indian Ocean (IO) in the late 1970s. By contrasting the correlations of the EASM index (EASMI) with the summer IO sea surface temperature anomaly (SSTA) between 1953-1975 and 1978-2000, a pronounced different correlation pattern is found in the tropical IO. The SSTA pattern similar to the positive Indian Ocean Dipole (IOD) shows a strongly positive correlation with the EASMI in 1953-1975. But in 1978-2000, significant negative correlation appears in the northern IO and the IOD-like correlation pattern disappears. It is indicated that the summer strong IOD events in 1953-1975 can cause a weaker-than-normal western North Pacific (WNP) subtropical high, which tends to favor a strong EASM. In 1978-2000, the connection between the summer IOD and the WNP circulation is disrupted by the climate shift. Instead, the northern IO shows a close connection with the WNP circulation in 1978-2000. The warming over the northern IO is associated with the significant enhanced 500 hPa geo-potential height and an anomalous anticyclone over the WNP. The change in the IO-EASM relationship is attributed to the interdecadal change of the background state of the ocean-atmosphere system and the interaction between the ENSO and IO. In recent decades, the tropical IO and tropical Pacific have a warmer mean SST, which has likely strengthened (weakened) the influence of the northern IO (IOD) on the EASM. In addition, due to the increase in the ENSO variability along with the higher mean equatorial eastern Pacific SST in 1978-2000, the influence of ENSO on the East Asian summer circulation experiences a significant strengthening after the late 1970s. Because the warming over the northern IO is associated with the significant warming in the equatorial eastern Pacific, the strengthened ENSO-EASM relationship has likely also contributed to the strengthened relationship between the northern IO and the EASM in 1978-2000.
Sea surface temperature associations with the late Indian summer monsoon
Climate Dynamics, 2003
Recent gridded and historical data are used in order to assess the relationships between interannual variability of the Indian summer monsoon (ISM) and sea surface temperature (SST) anomaly patterns over the Indian and Pacific oceans. Interannual variability of ISM rainfall and dynamical indices for the traditional summer monsoon season (June–September) are strongly influenced by rainfall and circulation anomalies observed during August and September, or the late Indian summer monsoon (LISM). Anomalous monsoons are linked to well-defined LISM rainfall and large-scale circulation anomalies. The east-west Walker and local Hadley circulations fluctuate during the LISM of anomalous ISM years. LISM circulation is weakened and shifted eastward during weak ISM years. Therefore, we focus on the predictability of the LISM. Strong (weak) (L)ISMs are preceded by significant positive (negative) SST anomalies in the southeastern subtropical Indian Ocean, off Australia, during boreal winter. These SST anomalies are mainly linked to south Indian Ocean dipole events, studied by Besera and Yamagata (2001) and to the El Niño-Southern Oscillation (ENSO) phenomenon. These SST anomalies are highly persistent and affect the northwestward translation of the Mascarene High from austral to boreal summer. The southeastward (northwestward) shift of this subtropical high associated with cold (warm) SST anomalies off Australia causes a weakening (strengthening) of the whole monsoon circulation through a modulation of the local Hadley cell during the LISM. Furthermore, it is suggested that the Mascarene High interacts with the underlying SST anomalies through a positive dynamical feedback mechanism, maintaining its anomalous position during the LISM. Our results also explain why a strong ISM is preceded by a transition in boreal spring from an El Niño to a La Niña state in the Pacific and vice versa. An El Niño event and the associated warm SST anomalies over the southeastern Indian Ocean during boreal winter may play a key role in the development of a strong ISM by strengthening the local Hadley circulation during the LISM. On the other hand, a developing La Niña event in boreal spring and summer may also enhance the east–west Walker circulation and the monsoon as demonstrated in many previous studies.
Geophysical Research Letters
The Bay of Bengal (BoB) summer monsoon is often triggered by northward propagating intraseasonal oscillations (ISOs) in the eastern Indian Ocean. The timing of summer monsoon onset, with a mean date of 1 May and standard deviation of 13 days, is substantially advanced (delayed) following a cold (warm) ENSO event. In this study, the significant ENSO-onset anomaly relationship is understood by investigating the ENSO modulations on the pivotal monsoon trigger. In the decaying spring of a cold (warm) ENSO event, pronounced easterly (westerly) vertical wind shear anomalies are induced in the northern Indian Ocean due to the enhanced (suppressed) convection anomalies in the northwestern Pacific Ocean. The above interannual vertical shear difference dominantly (more than 60%) contributes to a distinct ISO intensity contrast in the northern BoB. Consequently, the strengthened (weakened) ISOs associated with cold (warm) ENSO events lead to advanced (delayed) onsets of BoB summer monsoon. Plain Language Summary Prediction of the summer monsoon onset is crucial for agricultural and economic practices in the Asian monsoon regions. The Bay of Bengal experiences the earliest Asian summer monsoon, which is often triggered by tropical intraseasonal oscillations. At the interannual timescale, the timing of the Bay of Bengal monsoon onset is substantially advanced (delayed) following a cold (warm) ENSO event. However, the intrinsic relationship is not currently well understood. Targeting the close ENSO-onset anomaly relationship, we provide a perspective from the interannual variability of the critical monsoon trigger. In the year following a cold (warm) ENSO event, a striking enhanced (suppressed) convection anomaly persists in the northwestern Pacific Ocean. The wind responses to the anomalous convection create a favorable (unfavorable) environmental condition for the development of an intraseasonal oscillation in the Bay of Bengal, which then contributes to an advanced (delayed) monsoon onset.
This study examines the decadal change in the relationship between two major Indian Ocean (IO) sea surface temperature patterns, namely the Indian Ocean dipole (IOD) and northern IO and the East Asia summer monsoon (EASM) in the early 2000s. In 1991-1999, the former epoch, the interannual variability of EASM was associated with the IOD-like pattern in the original paper and its relationship weakened in 2000-2016. There are two possible causes for this decadal change; stronger land-sea thermal contrast as a local forcing in latter epoch, which may result in the weakening of the relationship between the IO and the EASM. In addition, the influence of El Niño-southern Oscillation (ENSO) on the western North Pacific subtropical high (WNPSH) could be changed depending on the frequency of ENSO. In the 2000s, the intensity of the low frequency (LF)-type ENSO (42-86 months period) events was weaker compared to the former epoch but that of quasi-biennial (QB)-type ENSO (16-36 months period) remained persistent. This could explain that the QB-type ENSO is remote forcing that modulates the change in the relationship between the tropical IO patterns and EASM in the 2000s.
Onset of the Summer Monsoon over the Indochina Peninsula: Climatology and Interannual Variations*
Journal of Climate, 2002
The temporal and spatial structures of the atmospheric circulation associated with the climatology and interannual variations of the summer monsoon onset over the Indochina Peninsula were studied using the observed daily rainfall at 30 stations and the NCEP-NCAR reanalysis from 1951 to 1996. The climatological monsoon onset over Indochina is on 9 May, with a standard deviation of 12 days. The monsoon onset is characterized by the pronounced northeastward progression of the low-level southwesterlies over the Indian Ocean and the intensification and northward extension of the tropical convection from Sumatra. It coincides with the weakening of the midlatitude westerly over south Asia, and the westward propagation of the intraseasonal oscillation (ISO) originated in the South China Sea (SCS) and the western Pacific with a dominant timescale of 12-25 days. A close relationship between the interannual variations of the monsoon onset and El Niño/La Niña was identified. Years with warm (cold) sea surface temperature (SST) anomalies in the western Pacific and cold (warm) SST anomalies in the central-eastern Pacific in the preceding spring have an early (late) onset. For an early onset year, strong convective activities appear over the southern Indochina Peninsula and the southern SCS in the preceding winter and spring. Associated with the changes of the Walker circulation and the local Hadley circulation related to La Niña, strong convective activities were maintained by the convergence between the anomalous southwesterlies in the Indian Ocean and northeasterlies over the northern SCS. The anomalous southwesterlies in the Indian Ocean were induced by both the anomalous Walker circulation associated with La Niña and anomalous land-sea thermal contrast. The anomalous northeasterlies over the northern SCS were originated in northern winter due to the combined effects of the cold east China land and the warm Philippine Sea, and further maintained by a positive thermodynamic air-sea feedback mechanism related to La Niña. An opposite scenario is found for a late onset year with warm SST anomalies in the central-eastern Pacific (El Niño).
International Journal of Climatology, 2018
This manuscript analyses the climatological characteristics (especially the synoptic phenomena and changes) of the South China Sea (SCS) summer monsoon withdrawal (SCSSMW) based on monsoon retreat dates from the National Climate Center of China Meteorological Administration. The SCSSMW is mainly defined as the westerly to easterly shift in the zonal winds due to the westwards intrusion of the western North Pacific (WNP) subtropical high in the northern SCS region. At the low level (850 hPa), the weakening and retreat of the intertropical convergence zone (ITCZ) and rain belt in the SCS-WNP are pertinent, and the southwesterly winds over the north Indian Ocean and the SCS also weaken and retreat. The anomalous anticyclone centred over the northern SCS resembles a Rossby wave response to the reduced precipitation over the SCS and the Philippine Sea. Changes in the upper-level (200 hPa) circulations include the deceleration of the tropical easterly stream (from the Indo-China Peninsula to the Arabian Sea) and northerly cross-equatorial flow (around the equatorial eastern Indian Ocean and Maritime Continent). At the mid-level (500 hPa), the ascending centre moves equatorwards, shifting from the northern to the southern SCS. There appears an anomalous vertical circulation cell (descending/ascending in the northern/southern SCS), which links the anomalous horizontal circulation through the Sverdrup balance. In addition, the upper-level divergent centre is shifted southeastwards, as is the generating centre of tropical cyclones. Moreover, an anomalous convergent centre over Taiwan Island is prominent in the upper level. The time series of several atmospheric variables (e.g., zonal wind, precipitation, convection) also exhibited abrupt changes during the SCSSMW. The reasons for not simultaneous retreat of monsoonal westerlies and rainy season in the SCS are also explored.
2006
The observed sequence of events leading to the onset of the summer monsoon in the South China Sea (SCS) is described, with a particular focus on conditions during the South China Sea monsoon experiment (SCSMEX) in May-June 1998. During SCSMEX, SCS monsoon onset occurred within the context of a multitude of scale interactions within the ocean-atmosphere system on intraseasonal time scales. Results from the 1998 SCSMEX case study illustrate that SCS monsoon onset is preceded by the development of an eastwardpropagating Madden-Julian Oscillation (MJO) in the Indian Ocean, as suggested by previous authors, and the subsequent emanation of a convectively coupled Kelvin wave into the Pacific. Remarkably similar results are obtained in an independent composite of 25 years of data. Since both the MJO and Kelvin waves generate westerly surface winds in their wake, it is suggested that these waves may accelerate or trigger the monsoon onset process in the southern SCS. A detailed analysis of the Kelvin wave that propagated through the SCS during SCSMEX shows that it was responsible for a large portion of the surface wind shift leading to monsoon onset in 1998. Finally, easterly wind anomalies in the eastern Pacific associated with the Indian Ocean MJO event during the SCSMEX period are shown to result in the sudden demise of the 1997-1998 El Niño event.
Pure and Applied Geophysics, 2018
An examination of the contrasting behaviour of the Indian summer monsoon during peak monsoon months of July and August 2002 in relation to the South China Sea (SCS) SST variations on intra-seasonal time scale have been carried out in this study. Monsoon rainfall over India was a deficit during July whereas enhanced rainfall activity was evident during August. Interestingly, SST over the SCS also exhibited similar contrasting behaviour with less variation during July and strong biweekly intraseasonal (BWI) oscillation with 10-20 day periodicity during August. During August, the BWI oscillation in the SCS SST could induce anomalous cyclonic circulation and enhancement in rainfall over the SCS, suggestive of a strong air-sea interaction process. Moreover, a strong relationship between the Indian monsoon and SCS SST is evident during August, at 3-day lead time of SST. The SCS SST variations could enhance westward propagation of moisture flux from the SCS to the Indian subcontinent and thereby influences the Indian monsoon. The SST variations and air-sea interaction processes over the SCS, also westward propagation of BWI moisture flux were weak during July. Our analysis suggests that SST variations over the SCS could modulate the monsoon circulation as well as the moisture flux from the SCS to the Indian subcontinent and thereby influence the Indian monsoon, particularly on biweekly time scale at least 3 days in advance. The study indicates a potential role of the SCS SST in foreshadowing the biweekly intra-seasonal oscillation during the Indian summer monsoon period.