A qualitative study on sea surface temperature over the tropical Indian Ocean and performance of Indian summer monsoon

A careful analysis of the sea surface temperature (SST) over the tropical Indian Ocean using the available SST data sets (namely, Hadley Center Ice SST, tropical rainfall measuring mission microwave imager SST, and optimum interpolation SST) at different time scales has been presented in the present study. By simple visual inspection of the SST plots, it has been shown that the qualitative prediction of Indian summer monsoon condition (weak/normal) and northern limit of monsoon (NLM) can be possible a month in advance using SST. The present qualitative study may be useful for common man to know the behavior of summer monsoon well a month in advance. Therefore, the qualitative study may enable the common man to show the application of satellite data to bring out the information regarding the onset of summer monsoon and related performance of Indian summer monsoon well in advance.     

Evaluation of the surface heat budget over the tropical Indian Ocean in two versions of FGOALS

热带印度洋海表热收支平衡极大地影响着周边的气候变化,对此的精确模拟是提高海气耦合模式模拟区域气候能力的重要一方面。本文使用不同数据来源的格点资料集OAFlux和NOCS-V2的海表通量产品,评估了FGOALS-g2和FGOALS-s2两个模式对热带印度洋海表热收支平衡的模拟结果。发现模拟净热通量存在海盆尺度的低估,来源于潜热通量的高估。相对而言FGOALS-s2模拟的辐射通量误差更大。模式无法模拟出净热通量的减少趋势,主因在于对潜热通量趋势模拟能力不足。     

Aerosol over the Southern Ocean

In a first attempt to assess a proposed climatic change feedback process involving cloud condensation nuclei (CCN) and cloud albedo, CCN concentrations N as a function of supersaturation S were measured on a voyage from latitude 43 to 65°S in October–November 1988. The usual relationship N=CS^k, with k=0.5 and C a constant was a fair apprraximation for S in the range 0.3–0.7% implying that CCN concentrations should largely determine cloud drop concentrations and hence albedo for clouds with S in that range. South of latitude 50°S and at smaller S,k was 1 or larger on average, which would lead to reduced dependence of albedo on CCN for the relevant clouds. N varied very widely for separations of the order of 100 km or 6 hours in time, particularly when the sea was partly ice-covered, suggesting strong local influences. During a large increase in N 60°S, unaccompanied by an increase in condensation nuclei (CN), cloud drops grew more rapidly than usual. In a subsidiary experiment particles were collected and examined by transmission electron microscopy. For particles less than 0.2 μm diameter, 80–90% appeared to consist of ammonium sulfate, the remainder being sea salt or an unknown substance which was more liquid and heat-resistant. Dialysis showed that the sulfate particles contained a few percent of insoluble material. Particles which formed cloud drops in vapours other water, were also studied. Comparison of these and water CCN and the rates of droplet suggested that the water insoluble portion of the particles was ethanol-soluble and surface-active. CN concentrations decreased by a factor of about 2 between 43 and 65°S, a change closely paralleled by ethanol CCN concentrations.     

Changes in the frequency of tropical cyclones over the North Indian Ocean

Summary  Changes in the frequency of tropical cyclones developing over the Arabian Sea and the Bay of Bengal have been studied utilizing 122 year (1877–1998) data of tropical cyclone frequency. There have been significant increasing trends in the cyclone frequency over the Bay of Bengal during November and May which are main cyclone months. During transitional monsoon months; June and September however, the frequency has decreased. The results have been presented for five months, i.e., May-November which are relevant as far as tropical cyclone frequency over the Arabian Sea and the Bay of Bengal are concerned. The tropical cyclone frequency in the Arabian Sea has not shown any significant trend, probably due to small normal frequency. The frequency time series has been subjected to the spectral analysis to obtain the significant periods. The cyclone frequency over the Bay of Bengal during May has shown a 29 year cycle. A significant 44 year cycle has been found during November. Over the Arabian Sea significant cycles of 13 and 10 years have been observed during May-June and November, respectively. The tropical cyclone frequency in the North Indian Ocean has a prominent El Ni?o-Southern Oscillation (ENSO) scale cycle (2–5 years) during all above five months. The annual cyclone frequency exhibits 29 year and ENSO scale (2–4 years) oscillations. There is a reduction in tropical cyclone activity over the Bay of Bengal in severe cyclone months May and November during warm phases of ENSO. Examination of the frequencies of severe cyclones with maximum sustained winds ≥ 48 knots has revealed that these cyclones have become more frequent in the North Indian Ocean during intense cyclone period of the year. The rate of intensification of tropical disturbances to severe cyclone stage has registered an upward trend. Received June 7, 1999/Revised March 20, 2000     

Annual mean statistics of the surface fluxes of the tropical Indian Ocean

The computed long-term annual mean and intramonthly variances of air and sea surface temperature, wind stress, effective radiation at the surface, heat gain over the ocean and the total heat loss for the tropical Indian Ocean between 30 °N and 30 °S are presented. These estimates, which are based on about one million weather reports for the period 1948–1972, indicate a mean annual meridional heat transport in agreement with previous estimates in direction though different in magnitude. The annual mean E-P chart shows that the Bay of Bengal region is highly conducive to large-scale convergence.     

A study of climate and weather variability over the tropical southwest Indian Ocean

Summary The climatology and variability of summer convection and circulation over the tropical southwest Indian Ocean is investigated using satellite imagery, routine synoptic observations, outgoing longwave radiation (OLR) data, sea surface temperatures (SST) and areal averaged rainfall departures. OLR has a –0.90 correlation with rainfall departures and the OLR minimum (ITCZ) in January and February lies across the 10°S latitude, extending further south near Madagascar. The intensity of ITCZ convection is greatest in the longitudes 20–35°E over northern Zambia and is considerably reduced over the SW Indian Ocean. Spatial correlations are analyzed for standardized departures of OLR, rainfall and SST. The correlations change sign in a coherent fashion, creating a climatic dipole between southern Africa and the SW Indian Ocean. Interannual trends are examined through analysis of January–February zonal and meridional wind indices constructed from significantly correlated variables at Zimbabwe, Madagascar and Mauritius. Circulation variability is dominated by quasi-decadal cycles and a trend of inereasing westerly winds. Zonal wind shear alternates from easterly (barotropic) to westerly and together with SST appears to regulate the frequency and intensity of tropical cyclogenesis. Areally averaged rainfall departures exhibit 6.25 year cycles in NE Madagascar and 12.5 and 18.75 year cycles in SW Madagascar and Zimbabwe, respectively. Summer rainfall and meridional winds in NE Madagascar and Zimbabwe are out of phase and negatively correlated in most summers. The presence of synoptic weather systems is assessed using daily Hovmoller-type satellite imagery composites. Convective structure is dominated by transient waves in the 10°–20°S latitude band, with periods of 15–20 days common. The waves are more prominent in summers with increased easterly shear and contribute to fluctuations in rainfall over SE Africa.With 8 Figures     

Structures and mechanisms of the first-branch northward-propagating intraseasonal oscillation over the tropical Indian Ocean

The first-branch northward-propagating intraseasonal oscillation (FNISO) over the tropical Indian Ocean (IO) often triggers the onset of the Asian summer monsoon. In this study we investigate the structures and mechanisms associated with FNISO through the diagnosis of ERA-Interim reanalysis data for the period of 1990–2009. A composite analysis is conducted to reveal the structure and evolution characteristics of the FNISO and associated background circulation changes. It is found that the FNISO convection originates from the southwestern IO and propagates eastward. After reaching the eastern IO, the major convective branch moves northward toward the northern Bay of Bengal (BoB). Two possible mechanisms may contribute to the northward propagation of the FNISO. One is the meridional asymmetry of the background convective instability. A greater background convective instability over the northern BoB may destabilize Rossby waves and cause convection to shift northward. The other is the meridional phase leading of perturbation humidity in the planetary boundary layer (PBL). Maximum PBL moisture appears to the north of the convection center, which promotes a convectively unstable stratification ahead of the convection and leads to the northward propagation of the FNISO. A PBL moisture budget analysis reveals that anomalous zonal advection is a dominant process in contributing to the moisture asymmetry.     

Roles of Indian and Pacific Ocean air–sea coupling in tropical atmospheric variability

Sea surface temperature (SST) variations include negative feedbacks from the atmosphere, whereas SST anomalies are specified in stand-alone atmospheric general circulation simulations. Is the SST forced response the same as the coupled response? In this study, the importance of air–sea coupling in the Indian and Pacific Oceans for tropical atmospheric variability is investigated through numerical experiments with a coupled atmosphere-ocean general circulation model. The local and remote impacts of the Indian and Pacific Ocean coupling are obtained by comparing a coupled simulation with an experiment in which the SST forcing from the coupled simulation is specified in either the Indian or the Pacific Ocean. It is found that the Indian Ocean coupling is critical for atmospheric variability over the Pacific Ocean. Without the Indian Ocean coupling, the rainfall and SST variations are completely different throughout most of the Pacific Ocean basin. Without the Pacific Ocean coupling, part of the rainfall and SST variations in the Indian Ocean are reproduced in the forced run. In regions of large mean rainfall where the atmospheric negative feedback is strong, such as the North Indian Ocean and the western North Pacific in boreal summer, the atmospheric variability is significantly enhanced when air–sea coupling is replaced by specified SST forcing. This enhancement is due to the lack of the negative feedback in the forced SST simulation. In these regions, erroneous atmospheric anomalies could be induced by specified SST anomalies derived from the coupled model. The ENSO variability is reduced by about 20% when the Indian Ocean air–sea coupling is replaced by specified SST forcing. This change is attributed to the interfering roles of the Indian Ocean SST and Indian monsoon in western and central equatorial Pacific surface wind variations.     

Intraseasonal Oscillation in the Tropical Indian Ocean

1. Introduction The intraseasonal oscillation (ISO or Madden- Julian Oscillation, MJO) in the tropical atmosphere has been studied extensively, including its existence, structure, evolution and propagation (Madden and Ju- lian, 1971; Murakami, et al., 198…     

热带印度洋夏季水汽输送特征及对南亚季风区降水的影响

利用NCEP/NCAR再分析环流资料、CMAP降水量和NOAA海温资料研究了热带印度洋夏季水汽输送的时空变化特征,并考察其对南亚季风区夏季降水的影响.热带印度洋夏季异常水汽输送第一模态表现为异常水汽从南海向西到达孟加拉湾后分成两支,其中一支继续往西到达印度次大陆和阿拉伯海,对应印度半岛南端和中南半岛的西风水汽输送减弱,导致这些区域降水减少;第二模态表现为异常水汽从赤道东印度洋沿赤道西印度洋、阿拉伯海、印度半岛、中南半岛的反气旋输送,印度和孟加拉湾南部为反气旋异常水汽输送,水汽辐散、降水减少,而印度东北部为气旋性水汽输送,水汽辐合、降水增多.就水汽输送与局地海温的关系而言,水汽输送第一模态与热带印度洋海温整体增暖关系密切,而第二模态与同期印度洋偶极子关系密切.     

Improved tropical cyclone forecasts over north Indian Ocean with direct assimilation of AMSU-A radiances

The Weather Research and Forecasting (WRF-ARW) model and its three-dimensional variational data assimilation (3D-Var) system are used to investigate the impact of the Advanced Microwave Sounding Unit-A (AMSU-A) radiances on the prediction of Indian Ocean tropical cyclones. Three tropical cyclones are selected for this study: cyclone Mala (April 2006; Bay of Bengal), cyclone Gonu (June 2007; Arabian Sea), and cyclone Sidr (November 2007; Bay of Bengal). For each case, observing system experiments are designed, by producing two sets of analyses from which forecasts are initialized. Both sets of analyses contain all conventional and satellite observations operationally used, including, but not limited to, Quick Scatterometer (QuikSCAT) surface winds, Special Sensor Microwave/Imager (SSM/I) surface winds, Meteosat-derived atmospheric motion vectors (AMVs), and differ only in the exclusion (CNT) or inclusion (EXP) of AMSU-A radiances. Results show that the assimilation of AMSU-A radiances changes the large-scale thermodynamic structure of the atmosphere, and also produce a stronger warm core. These changes cause large forecast track improvements. In particular, without AMSU-A assimilation, most forecasts do not produce landfall. On the contrary, the forecasts initialized from improved EXP analyses in which AMSU-A data are included produce realistic landfall. In addition, intensity forecast is also improved. Even if the analyzed cyclone intensity is not affected by the assimilation of AMSU-A radiances, the predicted intensity improves substantially because of the development of warm cores which, through creation of stronger gradients, helps the model in producing intense low centre pressure.     

Relationship of the interannual variability of the Indonesian Throughflow with the IOD over the tropical Indian Ocean

Based on the Simple Ocean Data Assimilation (SODA) reanalysis product, the interannual variability of the upper-ocean ITF volume transport from 1958 to 2001 is investigated. The wavelet analysis shows a second prominent interannual oscillation with a period of about 2–4 years. To reveal any relationship between this band-scale oscillation of the upper-ocean ITF and Indian Ocean Dipole (IOD), correlation and wavelet analyses are used. The correlation coefficient between the upper-ocean ITF and IOD reaches –0.35 with the upper-ocean ITF lagging the IOD index by 8 months. The dipole structure of IOD event is reproduced by the correlation with the upper-ocean ITF lagging the SST anomaly over the tropical Indian Ocean by 8 months from 1958 to 2001. The upper-ocean ITF and IOD show high coherency from about 1975 to 2001. The fact that the wavelet power spectrum of the upper-ocean ITF shows similar structure to that of IOD index supports this high coherency. These analyses suggest that the 2–4-year band-scale oscillation of the upper-ocean ITF is uniquely related to IOD over the tropical Indian Ocean.     

The association of surface wind stresses over Indian Ocean with monsoon rainfall

Summary Variability of Indian summer monsoon rainfall is examined with respect to variability of surface wind stresses over Indian Ocean. The Indian Ocean region extending from 40°–120° E, and 30° S–25° N, has been divided into 8 homogeneous subregions, viz (1) Arabian Sea (AS), (2) Bay of Bengal (BB), (3) West-equatorial Indian Ocean (WEIO), (4) Central-equatorial Indian Ocean (CEIO), (5) East-equatorial Indian Ocean (EEIO), (6) South-west Indian Ocean (SWIO), (7) South-central Indian Ocean (SCIO), and (8) South-east Indian Ocean (SEIO). The period of study extends for 13 years from 1982–1994. Monthly NCEP surface wind stress data of five months – May through September, have been used in the study. The spatial variability of seasonal and monthly surface wind stresses shows very low values over CEIO and EEIO and very high values over AS, SWIO, and SEIO regions. On the seasonal scale, all India summer monsoon rainfall (AISMR) shows concurrent positive relationships with the surface wind stresses over AS, BB, WEIO, SWIO and SCIO and negative relationships with the surface wind stresses over EEIO and SEIO. The relationships of AISMR with the surface wind stresses over AS and WEIO are significant at 5% level. The concurrent relationships between monthly surface wind stresses over these 8 oceanic sub-regions and monthly subdivisional rainfalls over 29 sub-divisions have been studied. The rainfalls over the subdivisions in the central India and on the west coast of India are found to be significantly related with surface wind stresses over AS, SWIO, SCIO. Monthly subdivisional rainfalls of four subdivisions in the peninsular India show negative relationship with BB surface wind stresses. May surface wind stresses over AS, BB, WEIO, CEIO and SWIO have been found to be positively related with ensuing AISMR. The relationship with AS wind stresses is significant at 5% level and hence may be considered as a potential predictor of AISMR. Received May 21, 2001 Revised October 8, 2001     

冬季北大西洋涛动对热带印度洋海表热通量的影响

利用1979—2017年TropFlux海气热通量资料、ERA5再分析资料及HadISST资料,分析了冬季北大西洋涛动(North Atlantic Oscillation, NAO)与同期热带印度洋海气热通量的关系。结果表明,NAO指数与热带印度洋海气净热通量整体上呈负相关,意味着NAO为正位相时,海洋向大气输送热量,其显著区域主要位于热带西印度洋(50°～70°E,10°S～10°N)。净热通量的变化主要依赖于潜热通量和短波辐射的变化;潜热通量和短波辐射在NAO正(负)位相事件期间的贡献率分别为72.96%和61.48%(71.72%和57.06%)。NAO可通过Rossby波列影响印度洋地区局地大气环流,进而影响海气热通量;当NAO为正位相时,波列沿中低纬路径传播至印度洋地区,在阿拉伯海北部对流层高层触发异常反气旋环流。该异常反气旋性环流加强了阿拉伯高压,使得北印度洋偏北风及越赤道气流加强。伴随风速的加强,海面蒸发增强,同时加强的越赤道气流导致热带辐合带强度偏强,深对流加强引起对流层水汽和云量增多,进而引起海表下行短波辐射减少。     

Transient convective waves in the tropical SW Indian Ocean

Summary Wave-organized convective features in the southwest Indian Ocean are described using Hovmoller composites of satellite imagery, OLR anomalies and ECMWF precipitable water departures during the southern summer. Westward movement of large convective elements is noted in the 10–20°S latitude band in about half of the years between 1970 and 1984. A study of 47 convective systems from satellite imagery establishes the climatological features, including zonal propagation speeds for maritime systems in the range –2 to –4 m s^–1, wavelengths of 25–35° longitude (3,000 km), lifespans of 10–20 days and convective areas of 7–10° longitude (800 km). Transient convective waves over the tropical SW Indian Ocean are slower and more diverse than their northern hemisphere counterparts. Interannual tendencies in the frequency and mode are studied. Wet summers over SE Africa correspond with an increased frequency of westward moving convective systems, whereas in dry summers convective systems tend to be quasi-stationary. INSAT data composites provide additional insight into the convective structure and show that tropical waves penetrated into southern Africa in February 1988. A more quantitative assessment of transient convective waves is provided by Hovmoller composites of OLR anomalies and precipitable water departures. Both display westward moving systems in 1976 and 1984 and highlight the wide variety and mixed mode character of convective waves. A case study is analyzed which illustrates the deepening of a moist, unstable layer coincident with the westward passage of a convective wave.With 12 Figures     

Asymmetry in the tropical Indian Ocean subsurface temperature variability

The empirical orthogonal function (EOF) analysis of subsurface temperature shows a dominant north-south mode of interannual variability in the Tropical Indian Ocean (TIO) at around 100 m depth (thermocline). This subsurface mode (SSM) of variability evolves in September-November (SON) as a response to Indian Ocean Dipole and intensifies during December-February (DJF) reinforced by El Niño and Southern Oscillation (ENSO) forcing. The asymmetry in the evolution of positive and negative phases of SSM and its impacts on the modulation of surface features are studied. The asymmetry in the representation of anomalous surface winds along the equator and off-equatorial wind stress curl anomalies are primarily responsible for maintaining the asymmetry in the subsurface temperature through positive and negative phases of the SSM. During the positive phase of SSM, downwelling Rossby waves generated by anticyclonic wind stress curl propagate towards the southwestern TIO (SWTIO), the thermocline ridge region of mean upwelling. The warmer subsurface water associated with the downwelling Rossby waves upwells in the region of mean upwelling and warms the surface resulting in strong subsurface-surface coupling. Such interaction processes are however weak during the negative phase of SSM. The asymmetry in the subsurface-surface interaction during the two phases of SSM and its impact on the modulation of surface features of TIO are also reported. In addition to the ENSO forcing, self-maintenance of SSM during DJF season is evident in the positive SSM (PSSM) years through modulation of subsurface surface coupling and air-sea coupling. This positive feedback during PSSM years is maintained by the deepening thermocline, warm SSTs and convection. The asymmetry in the thermocline evolution is more evident in the SWTIO and southern TIO.