The Red Sea outflow regulated by the Indian monsoon

To investigate why the Red Sea water overflows less in summer and more in winter, we have developed a locally high-resolution global OGCM with transposed poles in the Arabian peninsula and India. Based on a series of sensitivity experiments with different sets of idealized atmospheric forcing, the present study shows that the summer cessation of the strait outflow is remotely induced by the monsoonal wind over the Indian Ocean, in particular that over the western Arabian Sea. During the southwest monsoon (May–September), thermocline in the Gulf of Aden shoals as a result of coastal Ekman upwelling induced by the predominantly northeastward wind in the Gulf of Aden and the Arabian Sea. Because this shoaling is maximum during the southwest summer monsoon, the Red Sea water is blocked at the Bab el Mandeb Strait by upwelling of the intermediate water of the Gulf of Aden in late summer. The simulation also shows the three-dimensional evolution of the Red Sea water tongue at the mid-depths in the Gulf of Aden. While the tongue meanders, the discharged Red Sea outflow water (RSOW) (incoming Indian Ocean intermediate water (IOIW)) is always characterized by anticyclonic (cyclonic) vorticity, as suggested from the potential vorticity difference.     

Simulation of the time-space evolution of an extratropical cyclonic precipitation field over USA

The time-space evolution of an extratropical cyclonic precipitation field over U S A is simulated in a stochastic setting as outlined in Kavvas et al. (1988). The birth of a cyclonic storm is characterized by the simultaneous birth of a cyclone center and births of subsynoptic precipitation areas (SPA) at preferred locations around the cyclone center. The precipitation cores and cells which are used as the fundamental building blocks of the SPAs are approximated by circular precipitation areas (CPA) of different sizes. The time space evolution of the precipitation field after the birth is governed by (1) the movement of the synoptic cyclone described by the cyclone center trajectory, (2) independent nonidentically distributed random velocities of the individual CPAs relative to the cyclone center, (3) the births of new CPAs in time and space relative to the cyclone center, (4) the independent evolution in time of the individual spatially uniform intensities of the existing CPAs, (5) the expansion and shrinkage of the existing CPAs in the course of movement and (6) the dissipation (death) of a random number of existing CPAs within the cyclonic system. The computer simulation, the results of which are presented in this paper, successfully reproduced the general mesoscale and synoptic scale features of the radar detected cyclonic rain fields as observed by Austin and Houze (1972), Houze et al. (1976), Hobbs (1978), Hobbs and Locatelli (1978), Houze (1981), Houze and Hobbs (1982) and others.     

Results of the studies of monsoon circulation with radon as a tracer

Radon-222 activity levels have been measured at deck level in regions of the Arabian Sea, Indian Ocean, and Bay of Bengal during the summer monsoon periods of 1973, 1977, and 1979, as part of the Monex programme. The aim of the measurements was to find the source regions of the monsoon air and the variations in its composition under different synoptic conditions. The radon data confirm that the monsoon air is predominantly of southern-hemisphere origin, with a small continental component. The continental component, as indicated by radon values, increases at higher latitudes and seems to vary with different circulation patterns in the synoptic scale. The use of radon as a tracer in monsoon studies is thus demonstrated.     

Geostrophic balance over the Arabian Sea and Bay of Bengal regions

Geostrophic balance over the Arabian Sea and Bay of Bengal regions has been studied using the wind data obtained from the AVRO (HS-748) aircraft during the FGGE-MONEX-79.In the Arabian Sea and the Bay of Bengal regions, the observed wind south of 20°N was found to be sub-geostrophic. In the Arabian Sea region the departure at 1500 m was 75–95% and at 3050 m it was 60–65%. In the Bay of Bengal region the departure was 85–95%. In a few cases north of 20°N the observed winds at 3050 m were found to be super-geostrophic in the regions of enhanced monsoon activity, cyclonic circulation reaching up to 6000 m and in the region of monsoon trough.     

The effects of asymmetric potential vorticity forcing on the instability of South Asia High and Indian summer monsoon onset

Based on the theory of potential vorticity(PV),the unstable development of the South Asia High(SAH)due to diabatic heating and its impacts on the Indian Summer Monsoon(ISM)onset are studied via a case diagnosis of 1998.The Indian Summer Monsoon onset in 1998 is related to the rapidly strengthening and northward moving of a tropical cyclone originally located in the south of Arabian Sea.It is demonstrated that the rapid enhancement of the cyclone is a consequence of a baroclinic development characterized by the phase-lock of high PV systems in the upper and lower troposphere.Both the intensification of the SAH and the development of the zonal asymmetric PV forcing are forced by the rapidly increasing latent heat released from the heavy rainfall in East Asia and South East Asia after the onsets of the Bay of Bengal(BOB)monsoon and the South China Sea(SCS)monsoon.High PV moves southwards along the intensified northerlies on the eastern side of the SAH and travels westwards on its south side,which can reach its northwest.Such a series of high PV eddies are transported to the west of the SAH continuously,which is the main source of PV anomalies in the upper troposphere over the Arabian Sea from late spring to early summer.A cyclonic curvature on the southwest of the SAH associated with increasing divergence,which forms a strong upper tropospheric pumping,is generated by the anomalous positive PV over the Arabian Sea on 355 K.The cyclone in the lower troposphere moves northwards from low latitudes of the Arabian Sea,and the upper-layer high PV extends downwards and southwards.Baroclinic development thus occurs and the tropical low-pressure system develops into an explosive vortex of the ISM,which leads to the onset of the ISM.In addition,evolution of subtropical anticyclone over the Arabian Peninsula is another important factor contributing to the onset of the ISM.Before the onset,the surface sensible heating on the Arabian Peninsula is very strong.Consequently the subtropical anticyclone which dominated the Arabian Sea in spring retreats westwards to the Arabian Peninsula and intensifies rapidly.The zonal asymmetric PV forcing develops gradually with high PV eddies moving southwards along northerlies on the eastern side of the anticyclone,and a high PV trough is formed in the middle troposphere over the Arabian Sea,which is favorable to the explosive barotropic development of the tropical cyclone into the vortex.Results from this study demonstrate that the ISM onset,which is different from the BOB and the SCS monsoon onset,is a special dynamical as well as thermodynamic process occurring under the condition of fully coupling of the upper,middle,and lower tropospheric circulations.     

Investigation on the Influence of the Column Ozone Anomaly on the Energetics of Tropical Cyclones Over NIO and Related Air–Sea Interaction

An investigation on the temporal and spatial variation of ozone using the total column ozone (TCO) values during the cyclonic activities over North Indian Ocean (NIO) is carried out during the period from 1997 to 2012. The stepwise variation of TCO during the passage of the tropical cyclones over the Bay of Bengal and the Arabian Sea of the NIO is examined. The anomalies in TCO are estimated at each step of the life span of the cyclones starting from the genesis to landfall stages. The result reveals that the TCO values are quite high prior to the formation of the depression over NIO; however, at the stage of cyclogenesis it decreases which, with the increase in the intensity of the cyclones, further decreases and becomes minimum near the coast during the landfall. The maximum negative anomaly in TCO is observed for maximum intensity of the tropical cyclones as well as during the landfall. The result further shows that when the cyclones die out after the landfall the TCO regains the normal value. It is further observed that the reduction in TCO enhances the accumulated cyclone energy over NIO. The result finally shows that, the higher the energy of the cyclones, the lower becomes the stratospheric warming, that is, the higher the stratospheric cooling.     

The significance of rainstorm variations to shallow translational hillslope failure

Landsliding in eastern Scotland results from high-magnitude rainstorms generated under either cyclonic or anticyclonic conditions, particularly during the summer. Data from Aviemore indicated that cyclonic storms produce higher rainfall totals than anticyclonic storms, as well as being of longer duration and lower intensity. The distribution of rain during individual storms also varies with the synoptic conditions under which the storms are produced. These different rainfall characteristics produce different geomorphic responses, which can be investigated in detail using physical based modelling. In this paper, a physically based coupled hydrology–stability model is used to assess the significance of these rainfall characteristics to soil moisture response and slope instability for mature podsols. The results provide evidence that rainstorms of different synoptic origin produce varying hydrological response, involving both the extent and the timing of moisture content change. This affects the depth and timing of slope failure, with anticyclonic storms promoting a large, rapid response in the factor of safety at shallow depths within the soil. Cyclonic storms produce a more gradual response, with the region of probable failure being deeper. Futhermore, each of these storm types is associated with different rainfall distributions, and this is also shown to have a significant effect on the timing and depth of slope instability.     

Variability of Convective Activity over the North Indian Ocean and its Associations with Monsoon Rainfall over India

The present study is an attempt to examine the variability of convective activity over the north Indian Ocean (Bay of Bengal and Arabian Sea) on interannual and longer time scale and its association with the rainfall activity over the four different homogeneous regions of India (viz., northeast India, northwest India, central India and south peninsular India) during the monsoon season from June to September (JJAS) for the 26 year period (1979 to 2004). The monthly mean Outgoing Long-wave Radiation (OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this study and the 26-year period has been divided into two periods of 13 years each with period-i from 1979 to 1991 and period -ii from 1992 to 2004. It is ascertained that the convective activity increases over the Arabian Sea and the Bay of Bengal in the recent period (period -ii; 1992 to 2004) compared to that of the former period (period -i; 1979 to 1991) during JJAS and is associated with a significantly increasing trend (at 95% level) of convective activity over the north Bay of Bengal (NBAY). On a monthly scale, July and August also show increase in convective activity over the Arabian Sea and the Bay of Bengal during the recent period and this is associated with slight changes in the monsoon activity cycle over India. The increase in convective activity particularly over the Arabian Sea during the recent period of June is basically associated with about three days early onset of the monsoon over Delhi and relatively faster progress of the monsoon northward from the southern tip of India. Over the homogeneous regions of India the correlation coefficient (CC) of OLR anomalies over the south Arabian Sea (SARA) is highly significant with the rainfall over central India, south peninsular India and northwest India, and for the north Arabian Sea (NARA), it is significant with northwest India rainfall and south peninsular rainfall. Similarly, the OLR anomalies over the south Bay of Bengal (SBAY) have significant CC with northwest India and south peninsular rainfall, whereas the most active convective region of the NBAY is not significantly correlated with rainfall over India. It is also found that the region over northeastern parts of India and its surroundings has a negative correlation with the OLR anomalies over the NARA and is associated with an anomalous sinking (rising) motion over the northeastern parts of India during the years of increase (decrease) of convective activity over the NARA.     

THE CHARACTERISTICS OF RAINFALL AND DISASTERS OF DEBRIS FLOW IN BEIJING AREA

LINTRODUCTIONDisastersofdebrisflotvoccurfrequentlyinChina.Mostofthemareinrainstormtype.TherainStormdebrisflowiscausedbyStormrainfallthatinducesastrongStreamflowsonloosematerialsinwatershed.Theeffectsofprecipitationonthedebrisflowareasfollows(ChengduResearchInstituteC;DisasterandEnvironment,1989)f(l)Precipitationacceleratesthematerialsofloosedebristogather,(2)PrecipitationsuPPlywatercomponentofdebrisflow,(3)Precipitationprovidesdynamicconditionsfordebrisflow,(4)Precipitationisatrigg…     

Intraseasonal Variability of the Summer Monsoon over the North Indian Ocean as Revealed by the BOBMEX and ARMEX Field Programs

During the summer monsoon season over India a range of intraseasonal modulations of the monsoon rains occur due to genesis of weather disturbances over the Bay of Bengal (BOB) and the east Arabian Sea. The amplitudes of the fluctuations in the surface state of the ocean (sea-surface temperature and salinity) and atmosphere are quite large due to these monsoonal modulations on the intraseasonal scale as shown by the data collected during the field programs under Bay of Bengal Monsoon Experiment (BOBMEX) and Arabian Sea Monsoon Experiments (ARMEX). The focus of BOBMEX was to understand the role of ocean-atmospheric processes in organizing convection over the BOB on intra-seasonal scale. ARMEX-I was aimed at understanding the coupled processes in the development of deep convection off the West Coast of India. ARMEX-II was focused on the formation of the mini-warm pool across the southeast Arabian Sea in April-May and its role in the abrupt onset of the monsoon along the Southwest Coast of India and its further progress along the West Coast of India. The paper attempts to integrate the results of the observational studies and brings out an important finding that atmospheric instability is prominently responsible for convective organization whereas the upper ocean parameters regulate the episodes of the intraseasonal oscillations.     

Continental influence on the spatial distribution of particulate loading over the Indian Ocean during winter season

Advection of continental aerosols over to the Arabian Sea and Indian Ocean regions during winter (when the prevailing synoptic wind is north-easterly) has been studied by examining the spatial distribution of the near-surface aerosol mass concentration (M_C). In the northern part of Arabian Sea and north of the inter-tropical convergence zone (ITCZ) M_C is significantly larger than that in the southern side of ITCZ. A prominent high in M_C is observed near the mid-Arabian Sea region ∼12° to 15°N. Associated with local convergences introduced by mesoscale weather systems, pockets of high M_C are observed at different locations over the oceanic regions. Significantly low values of M_C are observed in the ITCZ. Surface streamlines and wind patterns indicate advection of continental aerosols from the adjoining landmass over to the oceanic environment. Aerosol mass concentration in the far oceanic region is also influenced by in situ production of sea-spray aerosols, which depends on wind speed. The nature of advection of continental aerosols has been studied by separating the wind-induced component of M_C and chemical nature of the aerosols. The spatial variation of the wind-independent component matches well with that of the non-sea-salt aerosols estimated from chemical composition.     

The kinetic energy budget of monsoon circulation over the Indian region during ISMEX-1973

A kinetic energy budget over the Indian region is computed for the period 4–9 July 1973, when a twin monsoon depression-one in the Bay of Bengal and another in the Arabian sea were the dominant synoptic features. The generation term caused by the cross-contour flow is a dominant source to the kinetic energy. The dissipation term is computed as a residual and is a major sink for the kinetic energy. The horizontal flux divergence is also a sink term but is much smaller in magnitude than other major source and sink terms. From the results it may be inferred that the generation term is the most important for the maintenance of monsoon disturbances.     

Seasonal changes in aerosol characteristics over Arabian Sea and their consequence on aerosol short-wave radiative forcing: Results from ARMEX field campaign

Extensive and collocated measurements of several aerosol parameters were made over the eastern Arabian Sea, during the inter-monsoon and summer monsoon seasons of 2003 as a part of the Arabian Sea Monsoon Experiment (ARMEX). Associated with the seasonal changes in the synoptic wind fields from northeasterly/easterly to westerly/northwesterly, the aerosol characteristics and columnar optical depth show large variations. Consequently, the atmospheric forcing is found to increase from March to April and then to decrease consistently towards June. However, the magnitude of the forcing efficiency of aerosols continuously decreases from winter to summer. Such temporal changes in radiative forcing need to be accounted for in reducing the uncertainty in aerosol climate impact.     

A Procedure for Calculating Extreme Characteristics of the Northern Caspian Sea Level

A procedure is proposed for calculating extreme characteristics of the level of a sea with allowance for positive and negative setups. Analysis is made of past storm events in the Northern Caspian Sea that have caused strong setups. Sixty-three storm weather patterns are chosen from a period of 45 years. Time ring synoptic maps are used to digitize the atmospheric pressure fields and calculate the field of its gradient and the wind near water surface. Based on these data, the sea level values and currents are calculated through two- and three-dimensional hydrodynamic models. A probabilistic model along with computer-aided data treatment procedures are used to calculate the fields of extreme characteristics of the sea level at the Lagan gage with the occurrence of once per N years at the average Caspian Sea level of 27 m below SL.     

Aspects of Low Frequency Oscillation During the Northern Winter as Inferred From Nonlinear Energy Interactions

— The work deals with the computation and analysis of spectral energetics in the frequency domain at 850?hPa and 200?hPa over the tropics (20°S–20°N) and extratropics (20°N–60°N). The data for the winter months, i.e., November, December and January of 1995, 1996 and 1997 are selected for this purpose. The results suggest that much of the low frequency variability of the Northern Hemisphere wintertime general circulation is associated with disturbances which derive their energy from the time-mean flow through barotropic instability. Low frequency fluctuations tend to be larger in horizontal scale and their kinetic energy is largely confined to the upper troposphere. At 850?hPa, strong energy interaction south of 5°N is noticed due to a southward shift of major inflow channel, originating from the Bay of Bengal and entering the ITCZ from the western Arabian Sea. The energy balances in the tropics and the extratropics during winter have different characteristics from those during summer. In contrast to the summer circulation, instead of a downscale decascade as in the case of the extratropics, kinetic energy is transferred in an opposite sense, namely from transients of shorter to those of longer time scales in the tropics during winter. The strong nonlinear energy interactions associated with low frequency waves over the Indian Ocean (5°N–5°S) during winter is the manifestation of the deep convection due to warm water coupled with the crossequatorial low level flow along the ITCZ over this region. Forcing from this region readily excites a large response in terms of nonlinear energy interaction over the extratropical northeast Pacific.     

大气季节内振荡的活动与江淮流域夏季旱涝

观测资料的分析极为清楚地表明,江淮流域的夏季降水有着极为明显的低频变化,周期为30-60d和近20d的振荡是其最基本的特征,尤其是在多雨的年份.对应江淮夏季多雨(涝)年和少雨(旱)年,大气环流的分析表明其大气季节内振荡(IS0)的形势有着显著的差异.例如在多雨(少雨)年,在长江以南的850hPa上为一个低频(IS0)反气旋(气旋)性环流控制,而中国北部和日本一带为气旋(反气旋)性环流,从而在江淮流域形成较强的低频辐合(辐散)气流;在200hPa的青藏高原上却为一个低频气旋(反气旋)性环流所控制.分析还表明,对应多雨年,在江淮流域有明显的由中高讳度向南传播和由低玮度向北传播的大气低频振荡的汇合情况;而对应于少雨年,由中高纬度向南传播的低频系统较不明显,在江淮流域低频系统的汇合也较为不清楚.     

Numerical simulations of mesoscale variability in the Straits of Florida

The circulation in the Straits of Florida is dominated by the throughflow of the Florida Current, as modified by tidal flows, responses to atmospheric cold front and extratropical cyclone (easterly wave and tropical cyclone) passages in winter (summer), and intrinsic mesoscale variability due to instabilities of the Florida Current front and jet system. Monthly meanders of the Florida Current, persistent oceanic fronts associated with the Florida Current’s baroclinic jet, and frontal eddies shed weekly by the Florida Current are the primary mesoscale features. A limited area model (Princeton Ocean Model: POM) is implemented to cover the Straits of Florida with a curvilinear grid that resolves the mesoscale structure, especially where the baroclinic flow is locked to steep topography in a 90 degree bend of the Straits. Florida Current cyclonic frontal eddies are spawned spontaneously, grow as they translate downstream, interact with shelf waters, and exhibit the same space-time attributes that characterize their observed counterparts, as evidenced by satellite imagery, shipboard synoptic mapping, coastal HF radar, and moored time series. Here, a deeper understanding is attempted for the frontal eddy kinematics and dynamics by examining, for example, their sensitivity to model parameter values, synoptic versus monthly atmospheric forcing, and other determinants of the flow. The mean flow shears are concentrated along the shelfbreak, where these frontal eddies are trapped, favoring the formation of the eddies by mean flow instabilities. In particular, it is found that the Florida Current frontal eddies exist independent of the wind-forcing considered (i.e., no winds, monthly winds, and synoptic (but not mesoscale) winds); however, they are modulated by the synoptic wind-forcing. Nevertheless, intriguingly, the frontal eddies have the same weekly time scale as the weather cycle.     

An enhanced depth-averaged tidal model for morphological studies in the presence of rotary currents

A simple and efficient method to improve morphological predictions using depth-averaged tidal models is presented. The method includes the contribution of secondary flows in sediment transport using the computed flow field from a depth-averaged model. The method has been validated for a case study using the 3D POLCOMS model and ADCP data. The enhanced depth-averaged tidal model along with the SWAN wave model are applied to morphological prediction around the Lleyn Peninsula and Bardsey Island as a case study in the Irish Sea. Due to the presence of a headland in this area two asymmetrical tidal eddies are developed in which the cyclonic eddy is stronger as a result of Coriolis effects. The results show that the enhanced model can effectively predict formation of sand banks at the centre of cyclonic eddies, while the depth-averaged model, due to its inability to accommodate secondary flow, is inadequate in this respect.