Summer Monsoon Trough Research Articles

ENSO Modulation of the Interannual and Intraseasonal Variability of the East Asian Monsoon—A Model Study

Abstract The impacts of ENSO on the evolution of the East Asian monsoon have been studied using output from a general circulation model experiment. Observed monthly variations of the sea surface temperature (SST) field have been prescribed in the tropical eastern and central Pacific, whereas the atmosphere has been coupled to an oceanic mixed layer model beyond this forcing region. During the boreal summer of typical El Niño events, a low-level cyclonic anomaly is simulated over the North Pacific in response to enhanced condensational heating over the equatorial central Pacific. Advective processes associated with the cyclone anomaly lead to temperature tendencies that set the stage for the abrupt establishment of a strong Philippine Sea anticyclone (PSAC) anomaly in the autumn. The synoptic development during the onset of the PSAC anomaly is similar to that accompanying cold-air surges over East Asia. The air–sea interactions accompanying the intraseasonal variations (ISV) in the model atmosphere exhibit a strong seasonal dependence. During the summer, the climatological monsoon trough over the subtropical western Pacific facilitates positive feedbacks between the atmospheric and oceanic fluctuations. Conversely, the prevalent northeasterly monsoon over this region in the winter leads to negative feedbacks. The onset of the PSAC anomaly is seen to be coincident with a prominent episode of the leading ISV mode. The ENSO events could influence the amplitude of the ISV by modulating the large-scale flow environment in which the ISV are embedded. Amplification of the summer monsoon trough over the western Pacific during El Niño enhances air–sea feedbacks on intraseasonal time scales, thereby raising the amplitudes of the ISV. A weakening of the northeasterly monsoon in El Niño winters suppresses the frequency and strength of the cold-air surges associated with the leading ISV mode in that season. Many aspects of the model simulation of the relationships between ENSO and the East Asian monsoon are in agreement with observational findings.

Interrelationship between cloud cover and sensible heat flux over land during MONTBLEX-1990

Micro-meteorological tower observations of MONTBLEX (Monsoon Trough Boundary Layer Experiment)-1990, combined with routine surface observations at Jodhpur in the dry convective sector of Indian summer monsoon trough are used to examine the interrelationship between total cloud cover (TCC) and surface sensible heat flux (SHF) during the summer monsoon of 1990. A significant inverse relationship between TCC and SHF is found during various Intensive Observation Periods of the experiment. This relationship holds for the various methods of estimation of SHF.

Simulation of Boundary Layer Structure over the Indian Summer Monsoon Trough during the Passage of a Depression

The planetary boundary layer (PBL) structure over the Indian summer monsoon trough region has been simulated using a regional numerical model during the passage of a monsoon depression along the monsoon trough. Monin–Obukhov similarity theory for the surface layer coupled with turbulent kinetic energy closure scheme (1½-order closure) for the mixed layer is used for parameterizing the PBL in the model. The results show that the model is able to simulate the evolution and the structure of the monsoon trough boundary layer and its associated features. The main observed characteristic features of the monsoon trough, such as the southward tilt of the trough with height and warmer air mass toward the northern side of the trough line, are simulated well by the model. Numerical results also indicate that the dynamic and thermodynamic characteristics of the monsoon trough boundary layer are modified by the passage of monsoon depressions.

Study of rain episode in the desert region of the Indian summer monsoon trough

A rain episode over the desert region of the summer monsoon trough during the period 1–4 July 1990 is discussed in the study. The fine structure observations of this unique event reveal that the atmosphere was hot like that of over a desert, but very moist like the monsoon air mass with specific humidity, constant or even increasing with height in the planetary boundary layer. A cold core by adiabatic cooling from vertical advection, abrupt increase of specific humidity and latent heat flux as the disturbance moved in, are some of the features noticed. The equivalent potential temperature surface was found to be raised at all levels above the surface with virtually no overturning, like that of tropical cyclones. The role of the turbulent eddies in the surface layer in destroying the moisture gradients within and making it homogeneous during pre-rain period and pumping out moisture during rain-episode are highlighted. A study into analysis of such anomalies over the past several years, and follow up through numerical modelling is suggested.

Estimation of drag coefficient over the western desert sector of the Indian summer monsoon trough

In the estimation of momentum fluxes over land surfaces by the bulk aerodynamic method, no unique value of the drag coefficient (C D) is found in the literature. The drag coefficient is generally estimated from special observations at different parts of the world. In this study an attempt is made to estimate drag coefficient over the western desert sector of India using data sets of Monsoon Trough Boundary Layer Experiment (MONTBLEX) during the summer monsoon season of 1990. For this purpose, the fast and slow response data sets obtained simultaneously from a 30 m high micro-meteorological tower at Jodhpur are used. All the observations used in this study are confined to a wind speed regime of 2.5–9.0 ms−1. A comparison of momentum fluxes computed by eddy correlation (direct estimation) with profile and bulk aerodynamic (C D = 3.9 × 10−3, Garratt, 1977) methods revealed that though the nature of variation of the fluxes by all these methods is almost similar, both the indirect methods give an under-estimated value of the fluxes. The drag coefficient is estimated as a function of wind speed and surface stability by a multiple regression approach. An average value of the estimated drag coefficient is found to be of the order of 5.43 × 10−3. The estimated value ofC D is validated with a set of independent observations and found to be quite satisfactory. The recomputed momentum fluxes by bulk aerodynamic method using the estimated drag coefficient are in close agreement with the directly estimated fluxes.

El Nino/southern oscillation modification to the structure of the monsoon and tropical cyclone activity in the Australasian region

AbstractStreamfunction and velocity potential fields, calculated from monthly averaged station data sets, are composited for ENSO and anti‐ENSO extremes in the Australasian region. Four January and four February data sets are used for each of the composites. Gridded sea‐surface temperature (SST) and highly reflective cloud (HRC) data are composited in a similar manner. These data are used to investigate the effect of the phase of ENSO on the Australasian summer monsoon and tropical cyclone activity in the region.Changes in the structure of the monsoon are found to be consistent with composited tropical cyclogenesis and cyclone track data. These effects extend to the north and west of Australia, in addition to the signal previously identified in the Queensland region. During ENSO phases, the summer monsoon trough is weak and displaced equatorward, vertical wind shear is reduced and warmer SSTs are found to the north‐west of Australia and in the central equatorial Pacific. Tropical cyclone activity is reduced to the north‐east of Australia, but increases to the north and north‐west of the continent. Owing to the associated changes in the steering flow, coastal crossings are enhanced in Western Australia and the Northern Territory, but suppressed in Queensland. In the anti‐ENSO composite, the reverse generally is observed. Features specific to the anti‐ENSO are that far fewer storms occur to the north of Australia and storms off the west coast tend to track off‐shore, persisting further south. This southward persistence also is evident on the east coast, but here the likelihood of coastal crossing is enhanced compared with the ENSO composite.

The Structure and Vorticity Budget of an Early Summer Monsoon Trough (Mei-Yu) over Southeastern China and Japan

One of the most persistent rain-making events over East Asia is the development of an early summer monsoon trough (Mei-Yu) which extends from southeastern China to southern Japan. This work studies the structure and vorticity budget of a Mei-Yu system for the period 10–15 June 1975. Subjectively analyzed grid-point data are time composited with respect to the trough axis along three cross sections over southeastern China (western section), southern East China Sea (central section) and southern Japan (eastern section), respectively, during the mature and decaying stages of the trough. The results indicate that the structure of the eastern and central sections resembles a typical midlatitude baroclinic front with strong vertical tilt toward an upper level cold core and a strong horizontal temperature gradient. On the other band, the western section resembles a semitropical disturbance with an equivalent barotropic, warm core structure, a weak horizontal temperature gradient, and a rather strong horizontal wind shear in the lower troposphere. Cumulus convection activity south of the 850 mb trough is significant in all three sections and contributes substantially to the thermally direct secondary circulation, but the large-scale organizing mechanism differs from one section to another. In the eastern and central sections it is mainly due to differential vorticity advection while in the western section it is due to Ekman pumping (CISK). The generation of cyclonic vorticity is counteracted by cumulus damping in the eastern section and by boundary layer friction in the mountainous western section.