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1.
Summary The atmospheric and oceanic conditions associated with the southwest monsoon during the contrasting monsoon years of 2002 and 2003 over the Arabian Sea have been analyzed in the present study. Early onset of southwesterlies and reduced net heat gain due to low solar radiation were responsible for low sea-surface temperatures (SSTs) over the Arabian Sea during 2002 pre-monsoon (particularly in May). Conversely, light winds and an increased net heat gain set up the pre-monsoon warming in 2003. The development and intensification of deep convection over a large area of the Arabian Sea prior to the onset of the monsoon was observed during 2003, but was absent in 2002. Weak cross equatorial flow and a weak low level jet over the Arabian Sea reduced moisture transport towards the Indian subcontinent in July 2002. This scenario helped to contribute to a prolonged break in monsoon conditions during July. However, no such break in conditions occurred during July 2003. In 2002, the summer monsoon cooling of the Arabian Sea occurred well before July, whereas in 2003 cooling occurred during July. Estimates of wind driven Ekman (horizontal) and vertical transports showed maximum values in the month of June (July) in 2002 (2003). These estimates clearly show the importance of horizontal and vertical advection in the summer cooling of the Arabian Sea. During the southwest monsoon period, the Arabian Sea was warmer in 2003 than in 2002. Late onset of the southwesterlies in June, late cooling of the Arabian Sea in July, and downwelling Rossby wave propagation were responsible for the warm SSTs in 2003. Weak wind stress curl in July dampened the westward propagating sea surface height anomaly signals (Rossby waves) before they reached the western Arabian Sea in 2002, whereas, in 2003 strong wind stress curl enhanced Rossby wave propagation. During the summer monsoon period, subsurface temperatures in the south central Arabian Sea were warmer in 2003 than in 2002, particularly in July and August. Strong Ekman convergence, solar penetration, and downwelling (downward velocities) are responsible for the enhanced subsurface warming in 2003.  相似文献   

2.
This study presents an assessment of the TropFlux and the National Centers for Environmental Prediction (NCEP) reanalysis air-sea fluxes in simulating the surface and subsurface oceanic parameters over the Bay of Bengal (BoB) region during 2002–2014 using the Regional Ocean Modelling System (ROMS). The assessment has been made by comparing the simulated fields with in-situ and satellite observations. The simulated surface and subsurface temperatures in the TropFlux forced experiment (TropFlux-E) show better agreement with the Research Moored Array for African-Asian-Australian Monsoon Analysis (RAMA) and Argo observations than the NCEP forced experiment (NCEP-E). The BoB domain averaged sea surface temperature (SST) simulated in the NCEP-E is consistently cooler than the satellite SST, with a root mean square error (RMSE) of 0.79 °C. Moreover, NCEP-E shows a limitation in simulating the observed seasonal cycle of the SST due to substantial underestimation of the pre-monsoon SST peak. These limitations are mostly due to the lower values of the NCEP net heat flux. The seasonal and interannual variations of SST in the TropFlux-E are better comparable to the observations with correlations and skills more than 0.80 and 0.90 respectively. However, SST is overestimated during summer monsoon periods mainly due to higher net heat flux. The superiority of TropFlux forcing over the NCEP reanalysis can also be seen when simulating the interannual variabilities of the magnitude and vertical extent of Wyrtki jets at two equatorial RAMA buoy locations. The jet is weaker in the NCEP-E relative to the TropFlux-E and observations. The simulated sea surface height anomalies (SSHA) from both the experiments are able to capture the regions of positive and negative SSHA with respect to satellite-derived altimeter data with better performance in the TropFlux-E. The speed of the westward propagating Rossby wave along 18°N in the TropFlux-E is found to be about 4.7 cm/s, which is close to the theoretical phase speed of Rossby waves.  相似文献   

3.
The epochal changes in the seasonal evolution of El Niño induced tropical Indian Ocean (TIO) warming in the context of mid-1970s regime shift is investigated in this study. El Niño induced warming is delayed by one season in the northern TIO during epoch-2 (post mid-1970) and southern TIO during epoch-1 (pre mid-1970). Significant spatiotemporal changes in TIO (especially in the north) warming are apparent during the developing phase of El Niño. The ocean dynamics is the major driver in the basin wide warming during epoch-2 whereas heat fluxes are the dominant processes during epoch-1. Strong coupling between thermocline and sea surface temperature (SST) in epoch-2 indicates that El Niño induced oceanic changes are very significant in the seasonal evolution of basin-wide warming. The thermocline-SST coupling is strengthened by the upward propagating subsurface warming in epoch-2. The westward propagating barrier layer over southern TIO supports persistence of warm SST (over southwest TIO in epoch-2), which in turn induce spring asymmetric mode in winds and precipitation. The asymmetric wind pattern and persistent subsidence over maritime continent are primarily responsible for stronger spring warming in epoch-2. The strong east equatorial Indian Ocean cooling in epoch-2 is mainly driven by coastal upwelling over Java–Sumatra coast, whereas in epoch-1 the weak cooling is controlled by the latent heat flux. The spatiotemporal changes in TIO SST warming and their evolution have strong impact on atmospheric circulation and rainfall distribution over the Indian Oceanic rim through local air–sea interaction.  相似文献   

4.
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5.
A numerical simulation is performed to understand the features and development processes of the arc-shaped precipitation system that dominates in Bangladesh during the pre-monsoon (March–May) period. An arc-shaped precipitation system of 26 April 2002 is simulated using the Cloud Resolving Storm Simulator (CReSS) with a horizontal grid increment of 2 km. The Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model is used for downscaling. Hourly outputs of the finest domain (grid increment of 5 km) of MM5 and National Oceanic and Atmospheric Administration Reynolds weekly mean sea surface temperature data are used as the initial and boundary conditions for CReSS. Younger and more intense cells are formed in the southwestern end of the system. These cells move northeastward and merge with the system producing intense rainfalls. Simulation results indicate that low-level southwesterly or southerly wind brings warm moist air from the Bay of Bengal and helps develop new cells. The propagation speed of the system is 8 m/s, and the northeastern end moves faster than the southwestern end, creating clockwise rotation of the system. The propagation speed and the rotation of the simulated system coincide well with radar observations. The clockwise rotation of the system can be explained by the stronger (weaker) outflow and weaker (stronger) inflow in the northeastern (southwestern) end. The propagation of the system is attributable to the weak (≤7 m/s, storm relative) rear-to-front flow in the moist environment. Thus, the arc-shaped precipitation system common to the pre-monsoon period in Bangladesh develops through a balance of strong southwesterly or southerly moist inflow in the low altitudes below 2 km and relatively weak outflow in the rear of the system.  相似文献   

6.
We propose a dynamical interpretation of the inverse relationship between the tropical eastern Pacific annual-cycle (AC) amplitude and the El Niño-Southern Oscillation (ENSO) amplitude, based on a pre-industrial simulation of Geophysical Fluid Dynamics Laboratory Couple climate model 2.0 with a fixed concentration of greenhouse gases spanning approximately 500 years. The slowly varying background conditions over more than a decade alternately provided favorable conditions for two opposite regimes, namely the ‘strong AC—weak ENSO regime’ and the ‘weak AC—strong ENSO regime’. For the weak AC—strong ENSO regime, the tropical eastern Pacific shows meridional-asymmetric surface warming with an emphasis on the southern part, leading to weakening of both the zonal trade wind and the cross equatorial southerly wind, as well as deepening of both the thermocline and mixed layer. The deeper mixed layer, weaker southerly wind, and reduced zonal gradient of the mean sea surface temperature due to tropical eastern Pacific warming all acts to reduce the AC. Conversely, the ENSO was intensified by the deeper mixed layer and deeper thermocline depth (thermocline feedback), but suppressed by the deeper thermocline depth (Ekman feedback) and the reduced zonal temperature gradient. We also computed the coupling strengths of the ENSO and AC, defined as the linear regression coefficients of the zonal and meridional wind stresses against the eastern Pacific SST, respectively. The coupling strengths of both the AC and ENSO are larger when they are intensified, and vice versa. All processes for the weak AC—strong ENSO regime operate in the opposite manner for the strong AC—weak ENSO regime.  相似文献   

7.
This study investigates the seasonal scale variability of the East Asian winter monsoon (EAWM), which is distinguished from the seasonal cycle with temporal variation throughout winter. Winters lasting 120 days (Nov. 17–Mar. 16) for a period of 64 years from the NCEP daily reanalysis data set are used to study the seasonal scale variability of the EAWM. Cyclostationary empirical orthogonal function (CSEOF) analysis is adopted to decompose the variability of the EAWM. The second CSEOF mode of 850-hPa temperature exhibits a seasonal scale variation, the physical mechanism of which is explained in terms of physically consistent variations of temperature, geopotential height, sea level pressure, wind, and surface heat fluxes. The seasonal-scale EAWM exhibits a weak subseasonal and a strong interannual variability and has gradually weakened during the 64 years. In a weak EAWM phase, the land-sea contrast of sea level pressure declines in East Asia. Consistent with this change, low-level winds decrease and warm thermal advection increases over the eastern part of mid-latitude East Asia. Latent and sensible heat fluxes are reduced significantly over the marginal seas in East Asia. However, during a strong EAWM phase, the physical conditions in East Asia reverse. A large fraction of the variability of the EAWM is explained by the seasonal cycle and the seasonal scale variation. A two-dimensional EAWM index was developed to explain these two distinct components of the EAWM variability. The new index appears to be suitable for measuring both the subseasonal and the interannual variability of the EAWM.  相似文献   

8.
The role of spring Wyrtki jets in modulating the equatorial Indian Ocean and the regional climate is an unexplored problem. The source of interannual variability in the spring Wyrtki jets is explored in this study. The relationship between intraseasonal and interannual variability from 1958 to 2008 and its relation with Indian Summer Monsoon is further addressed. Analysis reveals that the interannual variability in spring Wyrtki jets is controlled significantly by their intraseasonal variations. These are mostly defined by a single intraseasonal event of duration 20 days or more which either strengthens or weakens the seasonal mean jet depending on its phase. The strong spring jets are driven by such intraseasonal westerly wind bursts lasting for 20-days or more, whereas the weak jets are driven by weaker intraseasonal westerlies. During the years of strong jets, the conventional westward phase propagation of Wyrtki jets is absent and instead there is an eastward phase propagation indicating the possible role of Madden Julian Oscillation (MJO) in strengthening the spring Wyrtki jets. These strong intraseasonal westerly wind bursts with eastward phase propagation during strong years are observed mainly in late spring and have implications on June precipitation over the Indian and adjoining land mass. Anomalously strong eastward jets accumulate warm water in the eastern equatorial Indian Ocean (EIO), leading to anomalous positive upper ocean heat content and supporting more local convection in the east. This induces subsidence over the Indian landmass and alters monsoon rainfall by modulating monsoon Hadley circulation. In case of weak current years such warm anomalies are absent over the eastern EIO. Variations in the jet strength are found to have strong impact on sea level anomalies, heat content, salinity and sea surface temperature over the equatorial and north Indian Ocean making it a potentially important player in the north Indian Ocean climate variability.  相似文献   

9.
吴磊  陈海山  周洋 《气象科学》2019,39(4):427-436
本文分析了夏季东亚中纬度近地面温度和春、夏北极海冰时空变化特征,探讨了格陵兰海、巴伦支海海冰异常变化与夏季东亚中纬度陆面热力异常在年际上的可能联系。结果表明:(1)1950—2014年,东亚中纬度夏季近地面温度明显增暖,并伴有明显的年际变化,年际变率最大值的区域主要位于40°N以北至贝加尔湖地区;春、夏格陵兰海和巴伦支海的海冰也呈现明显的减少趋势,同时表现出较强的年际变化特征。(2)春、夏格陵兰海、巴伦支海海冰异常对东亚中纬度夏季陆面热力异常具有一定的指示作用:春、夏格陵兰海、巴伦支海海冰异常偏多,通常对应夏季东亚中纬度近地面的东亚中纬度夏季增暖现象;反之亦然。(3)春、季格陵兰海、巴伦支海北极海冰指数(Arctic Sea Ice Index,ASII)高值年(海冰异常偏多年份),贝加尔湖及西南的蒙古高原地区通常为大范围的异常高压控制,有利于近地面温度升高;同时由于乌拉尔山阻塞高压减弱,极地南下的冷空气减弱,有利于东亚中纬度区域的温度升高。而ASII低值年的情形则相反,贝加尔湖以南地区受异常低压控制,乌拉尔山阻塞高压增强,冷空气易向南侵袭,不利于东亚中纬度近地面升温。  相似文献   

10.
利用欧洲中期天气预报中心第五代再分析数据产品,归类分析了藏东南雅鲁藏布大峡谷地区水汽输送类别.选取大峡谷地区排龙站、墨脱站两个站点2019年涡动相关系统观测数据,分析不同水汽条件下雅鲁藏布大峡谷地区不同位置近地面水热交换通量的日变化特征.结果表明:高原季风期对应大峡谷地区水汽强输送期和温湿期,高原非季风期则相反.墨脱站...  相似文献   

11.
The surface flux exchange associated with the exchange coefficients and upper ocean conditions is essential to the development of tropical cyclones (TCs). Using the Weather Research and Forecasting (WRF) model, the present study has investigated the impact of exchange coefficients and ocean coupling during Super Typhoon Saomai (2006). Firstly, two experiments with different formula of roughness are conducted. The experiment with the Donelan formula for drag coefficient (Cd) and ramped formula for enthalpy coefficient (Ck) can simulate stronger intensity compared to other experiments due to the increased surface wind and enthalpy fluxes. That is because the new formulas allows for a smaller Cd and larger Ck in the high wind regime than the former formulas did. Moreover, two coupled simulations between WRF and a one-dimensional ocean model are conducted to examine the feedback of sea surface cooling to the TC. In the experiments with a horizontal uniform mixed layer depth of 70 m, the sea surface cooling is too weak to change the evolution of TC. While in the experiment with an input mixed layer calculated using the Hybrid Coordinate Ocean Model (HYCOM) data, the significant sea surface cooling induces obvious impact on TC intensity and structure. Under the negative feedback of sea surface cooling, the sensible and latent heat fluxes decreases, especially in the right part of Saomai (2006). The negative feedback with coupled ocean model plays a vital role in simulating the intensity and structure of TC.  相似文献   

12.
The evolution of sea surface temperature (SST) and thermocline (represented by 20 °C isotherm depth, D20) in the east equatorial Indian Ocean (EIO) associated with the Indian Ocean Dipole (IOD) years is studied for the period of 50 years from 1958 to 2007. A new IOD index based on combined anomalies of surface winds, D20 and SST over the equatorial Indian Ocean is defined to identify strong and weak IOD events. It is found that the evolution of strong IOD events is driven by ocean dynamics in the form of thermocline–SST coupling and is strongly interactive with the atmosphere, whereas the weak IOD events are mere response to surface winds without such dynamical coupling. The easterly wind anomalies extend up to the western equatorial Indian Ocean (WIO) during strong IOD years and support enhanced EIO air–sea interactions. On the other hand, the evolution of zonal wind anomalies is weak during the weak IOD years. Thermocline–SST coupling is robust in both EIO and WIO during strong IOD years, which is primarily responsible for the enhanced SST gradient, strong enough to establish anomalous Walker circulation within the Indian Ocean. The strong convection over the WIO associated with the Indian Ocean Walker cell triggers a secondary cell with subsidence over the African landmass. This double cell structure over the equatorial Indian Ocean is not reported before. Such double cell structure is not evident in weak IOD years and instead the convection over WIO extends up to African landmass. These are well supported by the spatial pattern of anomalous precipitable water during strong and weak IOD years. Strengthening of monsoon flow and local Hadley cell associated with strong IOD events enhances precipitation over the Indian subcontinent, whereas weak IOD years have less impact on the Indian summer monsoon circulation and rainfall. Analysis reveals that the EIO thermocline index and combined index could be potential predictors for the central Indian rainfall during summer.  相似文献   

13.
We synthesize variability and trends in multiple analyses of Antarctic near-surface temperature representing several independent source datasets and spatially complete reconstructions, and place these into the broader context of the behavior of other components of the climate system during the past 30–50?years. Along with an annual-mean trend during the past 50?years of about 0.1°C/decade averaged over Antarctica, there is a distinct seasonality to the trends, with insignificant change (and even some cooling) in austral summer and autumn in East Antarctica, contrasting with warming in austral winter and spring. Apart from the Peninsula, the seasonal warming is largest and most significant in West Antarctica in the austral spring since the late 1970s. Concurrent trends in sea ice are independent evidence of the observed warming over West Antarctic, with the decrease in sea ice area in the Amundsen and Bellingshausen Seas congruent with at least 50% of the inland warming of West Antarctica. Trends in near surface winds and geopotential heights over the high-latitude South Pacific are consistent with a role for atmospheric forcing of the sea ice and air temperature anomalies. Most of the circulation trend projects onto the two Pacific South American (PSA) modes of atmospheric circulation variability, while the Southern Annular Mode lacks a positive trend in spring that would otherwise cause a cooling tendency. The largest circulation trend is associated with the PSA-1 mode, a wave-train extending from the tropics to the high Southern latitudes. The PSA-1 mode is significantly correlated with SSTs in the southwestern tropical and subtropical Pacific. The increased SSTs in this region, together with the observed increase in rainfall, suggest that anomalous deep convection has strengthened or increased the occurrence of the Rossby wave-train associated with PSA-1. This hypothesis is supported by results from two ensembles of SST-forced atmospheric general circulation model simulations. Finally, the implications of the seasonality, timing, and spatial patterns of Antarctic temperature trends with respect to interpreting the relative roles of stratospheric ozone depletion, SSTs and increased atmospheric concentrations of greenhouse gasses are discussed.  相似文献   

14.
The modern Asian monsoon system exhibits strong interannual variation, which has profound environmental and economical impacts. It has been well-documented that the mean Asian monsoon state underwent significant changes in the Late Miocene (11–5 Ma ago). But how the interannual variability of the monsoon climate evolved during this period is still largely unknown. In this study, a long-term simulation of the Late Miocene with a fully coupled atmosphere–ocean general circulation model (ECHAM5/MPI-OM) at T31L19 resolution is used to explore the interannual variation of the Indian summer monsoon (ISM) in the Late Miocene. The regional climate model COSMO–CLM with a higher spatial resolution (~1° × 1°) is further employed to better characterize the spatial patterns of these variations. Our results show that although the mean ISM circulation is weaker in the Late Miocene runs, its interannual variation is as strong as or even stronger than at present and the dominant periods (~2.6–2.7 years) are shorter than at present (~3.4–8.4 years). It is noticed that while the extratropical influence on the ISM variability is weaker-than-present, a persistent El Niño-Southern Oscillation with stronger-than-present interannual variability is observed in our Late Miocene run. This may have maintained a strong interannual variation of the ISM with a shorter period in the Late Miocene. Our findings do not only improve our understanding of the Asian monsoon evolution in the Late Miocene, but also shed light on the future changes in the interannual variability of the ISM.  相似文献   

15.
Turbulent and mean meteorological data collected at five levels on a 20-m tower over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are analyzed to examine different regimes of the stable boundary layer (SBL). Eleven months of measurements during SHEBA cover a wide range of stability conditions, from the weakly unstable regime to very stable stratification. Scaling arguments and our analysis show that the SBL can be classified into four major regimes: (i) surface-layer scaling regime (weakly stable case), (ii) transition regime, (iii) turbulent Ekman layer, and (iv) intermittently turbulent Ekman layer (supercritical stable regime). These four regimes may be considered as the basic states of the traditional SBL. Sometimes these regimes, especially the last two, can be markedly perturbed by gravity waves, detached elevated turbulence (‘upside down SBL’), and inertial oscillations. Traditional Monin–Obukhov similarity theory works well in the weakly stable regime. In the transition regime, Businger–Dyer formulations work if scaling variables are re-defined in terms of local fluxes, although stability function estimates expressed in these terms include more scatter compared to the surface-layer scaling. As stability increases, the near-surface turbulence is affected by the turning effects of the Coriolis force (the turbulent Ekman layer). In this regime, the surface layer, where the turbulence is continuous, may be very shallow (< 5 m). Turbulent transfer near the critical Richardson number is characterized by small but still significant heat flux and negligible stress. The supercritical stable regime, where the Richardson number exceeds a critical value, is associated with collapsed turbulence and the strong influence of the earth’s rotation even near the surface. In the limit of very strong stability, the stress is no longer a primary scaling parameter.  相似文献   

16.
The sea surface temperature （SST） anomaly of the eastern Indian Ocean （EIO） exhibits cold anomalies in the boreal summer or fall during E1 Nino development years and warm anomalies in winter or spring following the E1 Nino events. There also tend to be warm anomalies in the boreal summer or fall during La Nina development years and cold anomalies in winter or spring following the La Nina events. The seasonal phase-locking of SST change in the EIO associated with E1 Nino/Southern Oscillation is linked to the variability of convection over the maritime continent, which induces an atmospheric Rossby wave over the EIO. Local air-sea interaction exerts different effects on SST anomalies, depending on the relationship between the Rossby wave and the mean flow related to the seasonal migration of the buffer zone, which shifts across the equator between summer and winter. The summer cold events start with cooling in the Timor Sea, together with increasing easterly flow along the equator. Negative SST anomalies develop near Sumatra, through the interaction between the atmospheric Rossby wave and the underneath sea surface. These SST anomalies are also contributed to by the increased upwelling of the mixed layer and the equatorward temperature advection in the boreal fall. As the buffer zone shifts across the equator towards boreal winter, the anomalous easterly flow tends to weaken the mean flow near the equator, and the EIO SST increases due to the reduction of latent heat flux from the sea surface. As a result, wintertime SST anomalies appear with a uniform and nearly basin-wide pattern beneath the easterly anomalies. These SST anomalies are also caused by the increase in solar radiation associated with the anticyclonic atmospheric Rossby wave over the EIO. Similarly, the physical processes of the summer warm events, which are followed by wintertime cold SST anomalies, can be explained by the changes in atmospheric and oceanic fields with opposite signs to those anomalies described above.  相似文献   

17.
ABSTRACT

South Indian Ocean Rossby waves (SIO-RW) are identified in the Global Ocean Data Assimilation System (GODAS) 1.5–7?yr filtered sea surface height (SSH) time series. There is a persistent three-year oscillation in the 5°–15°S latitude band from 55° to 85°E. Field correlations show little coupling at 90°E, but as the SIO-RW undulates westward at approximately 0.19?m?s?1 across the mid-basin, a northwest–southeast axis of warm sea surface temperatures (SSTs) and deep convection forms. Many teleconnections in earlier work are confirmed: interannual pulses of zonal wind in the eastern basin trigger the SIO-RW via anticyclonic wind stress curl. New insights derive from an understanding of links with the upper troposphere. As the SIO-RWs move westward with the onset of an El Niño in the Pacific, increased convection over the north Indian Ocean corresponds to reduced evaporation and SST warming. Mid-tropospheric heating T′?>?2°C over the northwest Indian Ocean accelerates the southern sub-tropical jet to greater than 10?m?s?1 over the southeast Indian Ocean, reinforcing the anticyclonic vorticity. The downstream acceleration of the jet generates upper-level divergence and moist convection over the western basin, anchoring an atmospheric Rossby wave in a northwest–southeast alignment underpinned by differential propagation of the SIO-RW. As the ocean Rossby wave reaches Africa, the coupling fades and transitions. What distinguishes Indian Ocean from Pacific Ocean Rossby waves are their southern latitude and higher frequency. The tropical mid-tropospheric heating that accelerates the southern sub-tropical jet shifts westward in tandem with the SIO-RW.  相似文献   

18.
In this study, statistical techniques are employed to decompose climate signals around southern Africa into the dominant temporal frequencies, with the aim of modelling and predicting area-averaged rainfall. In the rainfall time series over the period 1900–1999, the annual cycle accounts for 83% of variance. Residual spectral energy cascades from biennial (42%) to interannual (20%) to decadal bands (3%). Regional climate signals are revealed through a multi-taper singular value decomposition analysis of sea surface temperature and sea level pressure fields over the Atlantic and Indian Oceans, in conjunction with southern Africa rainfall. Rossby wave action in the South Indian Ocean dominates the biennial scale variability. El Niño-Southern Oscillation (ENSO) and related Indian Ocean dipole patterns are important for interannual variability. Significant sea temperature and pressure fluctuations occurring 6–12 months prior to rainfall contribute biennial and interannual indices to a multi-variate model that demonstrates useful predictive skill.  相似文献   

19.
Summary The Tropospheric Biennial Oscillation (TBO), a major interannual variation phenomenon in the Indo-Pacific region, is the result of strong ocean-atmosphere coupling over the Asian-Australian monsoon area. Along with other meteorological and oceanographic parameters, the tropical circulation also exhibits interannual oscillations. Even though the TBO is the result of strong air–sea interaction, the circulation cells during TBO years are, as yet, not well understood. In the present study, an attempt has been made to understand the interannual variability of the mean meridional circulation and local monsoon circulation over south Asia in connection with the TBO. The stream function computed from the zonal mean meridional wind component of NCEP/NCAR reanalysis data for the years 1950–2003 is used to represent the mean meridional circulation. Mean meridional mass transport in the topics reverses from a weak monsoon to a strong monsoon in the presence of ENSO, but in normal TBO years mean transport remains weak across the Northern Hemisphere. The meridional temperature gradient, which drives the mean meridional circulation, also shows no reversal during the normal TBO cycle. The local Hadley circulation over the monsoon area follows the TBO cycle with anomalous ascent (descent) in strong (weak) monsoon years. During normal TBO years, the Equatorial region and Indian monsoon areas exhibit opposite local Hadley circulation anomalies. Authors’ addresses: Prasanth A. Pillai, Research Scholar, Department of Atmospheric Sciences, Cochin University of Science and Technology, Lakeside Campus, Cochin 682016, India; K. Mohan Kumar, Professor & Dean, Department of Atmospheric Sciences, Faculty of Marine Sciences, Cochin University of Science and Technology, Lakeside Campus, Cochin 682016, India.  相似文献   

20.
Summary Climatological characteristics associated with summer monsoon onset over the eastern Bay of Bengal (BOB) are examined in terms of the westerly-easterly boundary surface (WEB). The vertical tilt of the WEB depends on the horizontal meridional temperature gradient (MTG) near the WEB, under the constraint of the thermal wind balance. The switch in the WEB tilt firstly occurs between 90 and 100°E during the first pentad of May. At this time the 850 hPa ridgeline splits over the BOB and heavy rainfall commences over the eastern BOB, indicating the onset of the BOB summer monsoon (BOBSM). The area-averaged MTG (200–500 hPa) is proposed as an index to define the BOBSM onset. A comparison of the onset determined by the MTG, 850 hPa zonal wind, and outgoing longwave radiation (OLR) shows that the MTG index is the most effective in characterizing the interannual variability of the BOBSM onset. Strong precursor signals are found prior to an anomalous BOBSM onset. Composite results show that early (late) BOBSM onset follows excessive (deficient) rainfall over the western Pacific and anomalous lower tropospheric cyclonic circulation which extends zonally from the northern Indian Ocean into the western Pacific, and strong (weak) equatorial westerly anomalies in the preceding winter and spring. Prior to an early (late) BOBSM onset, significant positive (negative) thickness anomalies exist around the Tibetan Plateau, accompanied by anomalous upper tropospheric anticyclonic (cyclonic) circulation. The interannual variations of the BOBSM onset are significantly correlated with anomalous sea surface temperature related to ENSO. These occurs through changes in the Walker circulation and local Hadley circulation, leading to middle and upper tropospheric temperature anomalies over the Asian sector. The strong precursor signals around the Tibetan Plateau may be partly caused by local snow cover anomalies, and an early (late) BOBSM onset is preceded by less (more) snow accumulation over the Tibetan Plateau during the preceding winter.  相似文献   

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