Characteristics of sea surface temperature retrieved from MTSAT-1R and in-situ data

Multi-channel sea surface temperature (MCSST) data were retrieved from the Japanese geostationary satellite, MTSAT-1R, for East Asia in western North Pacific. The coefficients used to calculate the MCSST data were estimated by assuming a linear relationship between the brightness temperatures obtained from the satellite and the in-situ buoy SST. It is important to remove cloud contamination pixels to retrieve meaningful information from infrared data. Therefore, the cloud detection algorithm was improved by using a 10-day maximum or minimum composite map for infrared and visible channels. The RMSE of the MCSST in comparison with the two-year buoy SST was about 0.89oC. The error was the largest at mid-latitudes in summer. Additionally, the error between the two SSTs showed that diurnal variation had a positive bias during daytime and a negative bias during nighttime. Furthermore, in 2007, both SSTs showed seasonal and spatial diurnal variation. The magnitude of the daily variation in the MCSST was two times larger than that in the buoy SST, and this was attributed to diurnal heating with a weak surface wind speed.     

Multi-stage onset of the summer monsoon over the western North Pacific

 The climatological summer monsoon onset displays a distinct step wise northeastward movement over the South China Sea and the western North Pacific (WNP) (110°–160°E, 10°–20°N). Monsoon rain commences over the South China Sea-Philippines region in mid-May, extends abruptly to the southwestern Philippine Sea in early to mid-June, and finally penetrates to the northeastern part of the domain around mid-July. In association, three abrupt changes are identified in the atmospheric circulation. Specifically, the WNP subtropical high displays a sudden eastward retreat or quick northward displacement and the monsoon trough pushes abruptly eastward or northeastward at the onset of the three stages. The step wise movement of the onset results from the slow northeastward seasonal evolution of large-scale circulation and the phase-locked intraseasonal oscillation (ISO). The seasonal evolution establishes a large-scale background for the development of convection and the ISO triggers deep convection. The ISO over the WNP has a dominant period of about 20–30 days. This determines up the time interval between the consecutive stages of the monsoon onset. From the atmospheric perspective, the seasonal sea surface temperature (SST) change in the WNP plays a critical role in the northeastward advance of the onset. The seasonal northeastward march of the warmest SST tongue (SST exceeding 29.5 °C) favors the northeastward movement of the monsoon trough and the high convective instability region. The seasonal SST change, in turn, is affected by the monsoon through cloud-radiation and wind-evaporation feedbacks. Received: 19 October 1999 / Accepted: 5 June 2000     

基于AHI观测的全天气条件海表温度反演

基于搭载在日本新一代静止气象卫星Himawari-8上的先进葵花成像仪(Advanced Himawari Imager,AHI)观测资料,研究了高时空分辨率的、全天气条件的海表温度(Sea Surface Temperature,SST)反演算法。本算法包括两步:第一步,根据云检测算法划分晴空和云区,然后利用非线性SST(NLSST)方程由红外亮温估计晴空SST;第二步,在有云区,先由前5 d同一时刻的晴空SST进行初步补缺,然后再利用Barnes插值完善云区SST估计和进行异常点平滑。最终得到时间分辨率为10 min、空间分辨率为0.05°的全天气条件海温分布。利用移动浮标的观测SST验证,晴空区SST估计的均方根误差(Root Mean Square Error,RMSE)和平均误差(Mean Error,ME)分别为0.857 K和0.017 K。全天气条件SST估计的RMSE和ME分别为0.872 K和-0.005 K。     

An ocean–atmosphere interaction mechanism for the active break cycle of the Asian summer monsoon

A typical active–break cycle of the Asian summer monsoon is taken as beginning with maximum SST (pentad 0) over the north Bay of Bengal when the oceans to its west and east from longitude 40°–160°E, and between latitudes 10° and 25°N (area A) also has maximum SST. During this pentad the recently found “Cold Pool” of the Bay of Bengal (between latitudes 3°N and 10°N) has its minimum SST. An area of convection takes genesis over the Bay of Bengal immediately after pentad 0 in the zone of large SST gradient north of the Cold Pool and it pulls the monsoon Low Level Jetstream (LLJ) through peninsular India. Convection and the LLJ westerlies then spread to the western Pacific Ocean during pentads 1–4 taken as the active phase of the monsoon during which convection and LLJ have grown in a positive feed back process. The cyclonic vorticity to the north of the LLJ axis is hypothesized to act as a flywheel maintaining the convection during the long active phase against the dissipating effect of atmospheric stabilization by each short spell of deep convection. By the end of pentad 4 the SST over area A has cooled and the convection weakens there, when the LLJ turns clockwise over the Arabian Sea and flows close to the equator in the Indian ocean. A band of convection develops at pentad 5 between the equator and latitude 10°S over the Indian ocean and it is nourished by the cyclonic vorticity of the LLJ now near the equator and the moisture supply through it. This is taken as the break monsoon phase lasting for about three to four pentads beginning from pentad 5 of a composite active–break cycle of 40 day duration. With reduced wind and convection over the area A during the break phase, solar radiation and light winds make the SST there warm rapidly and a new active–break cycle begins. SST, convection, LLJ and the net heat flux at the ocean surface have important roles in this new way of looking at the active–break cycle as a coupled ocean–atmosphere phenomenon.     

Applications of AMSR-E Measurements for Tropical Cyclone Predictions Part Ⅰ： Retrieval of Sea Surface Temperature and Wind Speed

Existing satellite microwave algorithms for retrieving Sea Surface Temperature （SST） and Wind （SSW） are applicable primarily for non-raining cloudy conditions. With the launch of the Earth Observing System （EOS） Aqua satellite in 2002, the Advanced Microwave Scanning Radiometer （AMSRoE） onboard provides some unique measurements at lower frequencies which are sensitive to ocean surface parameters under adverse weather conditions. In this study, a new algorithm is developed to derive SST and SSW for hurricane predictions such as hurricane vortex analysis from the AMSRoE measurements at 6.925 and 10.65 GHz. In the algorithm, the effects of precipitation emission and scattering on the measurements are properly taken into account. The algorithm performances are evaluated with buoy measurements and aircraft dropsonde data. It is found that the root mean square （RMS） errors for SST and SSW are about 1.8 K and 1.9 m s^- 1, respectively, when the results are compared with the buoy data over open oceans under precipitating clouds （e.g., its liquid water path is larger than 0.5 mm）, while they are 1.1 K for SST and 2.0 m s^-1 for SSW, respectively, when the retrievals are validated against the dropsonde measurements over warm oceans. These results indicate that our newly developed algorithm can provide some critical surface information for tropical cycle predictions. Currently, this newly developed algorithm has been implemented into the hybrid variational scheme for the hurricane vortex analysis to provide predictions of SST and SSW fields.     

Temperature variability over the Indian Ocean and its relationship with Indian summer monsoon rainfall

Summary The present study examines the long term trend in sea surface temperatures (SSTs) of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean in the context of global warming for the period 1901–2002 and for a subset period 1971–2002. An attempt has also been made to identify the relationship between SST variations over three different ocean areas, and All-India and homogeneous region summer monsoon rainfall variability, including the role of El-Ni?o/Southern Oscillation (ENSO). Annual sea surface temperatures of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean show a significant warming trend of 0.7 °C, 0.6 °C and 0.5 °C per hundred years, respectively, and a relatively accelerated warming of 0.16 °C, 0.14 °C and 0.14 °C per decade during the 1971–2002 period. There is a positive and statistically significant relationship between SSTs over the Arabian Sea from the preceding November to the current February, and Indian monsoon rainfall during the period 1901–2002. The correlation coefficient increases from October and peaks in December, decreasing from February to September. This significant relationship is also found in the recent period 1971–2002, whereas, during 1901–70, the relationship is not significant. On the seasonal scale, Arabian Sea winter SSTs are positively and significantly correlated with Indian monsoon rainfall, while spring SSTs have no significant positive relationship. Nino3 spring SSTs have a negative significant relationship with Indian monsoon rainfall and it is postulated that there is a combined effect of Nino3 and Arabian Sea SSTs on Indian monsoon. If the Nino3 SST effect is removed, the spring SSTs over the Arabian Sea also have a significant relationship with monsoon rainfall. Similarly, the Bay of Bengal and Equatorial South Indian Ocean spring SSTs are significantly and positively correlated with Indian monsoon rainfall after removing the Nino3 effect, and correlation values are more pronounced than for the Arabian Sea. Authors’ address: Dr. D. R. Kothawale, A. A. Munot, H. P. Borgaonkar, Climatology and Hydrometeorology divisions, Indian Institute of Tropical Meteorology, Pune 411008, India.     

FY-3C/VIRR海表温度产品及质量检验

国家卫星气象中心FY-3C/VIRR（visible and infrared radiometer,可见光红外扫描辐射计）海表温度产品在云检测产品的基础上,采用多通道MCSST（multichannel SST）算法进行晴空区海温反演。该文详细介绍了海表温度产品算法、产品设计、质量控制及质量检验方法。FY-3C/VIRR海表温度产品包括5 min段原始投影海温和5 km全球等经纬度投影海温。设计逐像元的海温质量标识,将海温像元分为优、良、差3个等级,用户可根据应用目标选择海温的质量等级。与日最优插值海温OISST（optimum interpolation SST）相比,FY-3C/VIRR 2015年1月—2019年12月的5 min段海温质量检验结果表明:质量等级为优的海温,白天和夜间的偏差分别为-0.18℃和-0.06℃,均方根误差分别为0.85℃和0.8℃;白天海温均方根误差有季节性波动,夏季有的月份均方根误差大于1℃（如2015年7月、2016年7月和2019年7月）;在海温回归系数不变的条件下,夜间海温偏差的季节性波动与星上黑体温度相关显著。从一级数据质量、定位、业务运行状况等方面讨论引起海表温度产品异常的原因,为FY-3C/VIRR历史数据定位、定标和产品重处理及用户应用提供重要的参考信息。     

Applications of AMSR-E Measurements for Tropical Cyclone Predictions Part Ⅰ: Retrieval of Sea Surface Temperature and Wind Speed

Existing satellite microwave algorithms for retrieving Sea Surface Temperature(Sst)and wind(SSW)are applicable primarily for non-raining cloudy conditions.With the launch of the Earth Observing System (EOS)Aqua satellite in 2002,the Advanced Microwave Scanning Radiometer(AMSR-E)onboard provides some unique measurements at lower frequencies which are sensitive to ocean surface parameters under ad-verse weather conditions.In this study,a new algorithm is developed to derive SST and SSW for hurricane predictions such as hurricane vortex analysis from the AMSR-E measurements at 6.925 and 10.65 GHz.In the algorithm,the effects of precipitation emission and scattering on the measurements are properly taken into account.The algorithm performances are evaluated with buoy measurements and aircraft dropsonde data.It is found that the root mean square (RMS) errors for SST and SSW are about 1.8K and 1.9m s(-1),respectively,when the results are compared with the buoy data over open oceans under precipitating clouds (e.g.,its liquid water path is larger than 0.5 mm),while they are 1.1 K for SST and 2.0 ms(-1)for SSW,respectively,when the retrievals are validated against the dropsonde measurements over warm oceans.These results indicate that our newly developed algorithm catl provide some critical surface information for trop-ical cycle predictions.Currently,this newly developed algorithm has been implemented into the hybrid variational scheme for the hurricane vortex analysis to provide predictions of SST and SSW fields.     

An assessment of TropFlux and NCEP air-sea fluxes on ROMS simulations over the Bay of Bengal region

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.     

Anomalous warming over Indian Ocean during 1997 El-Niño

Summary  The year 1997 witnessed one of the most severe El-Ni?o events of the century. However, the All-India Summer Monsoon Rainfall (AISMR) was 102% of its long period average. In view of recent studies (Tourre and White, 1995, 1997) of detection of ENSO signal over Indian Ocean, the Sea-Surface Temperature (SST) variation over Indian Ocean (20° N–10° S/50° E–100° E), concurrent to El-Ni?o event of 1997 is examined. It is observed that during the developing, mature and decaying stages of El-Ni?o, the North Indian Ocean was abnormally warm. This anomalous warming may be one of the factors responsible for anomalous precipitation over India during October to December of 1997. Received August 24, 1999/Revised February 15, 2000     

Evolution of geophysical parameters over the Indian Ocean region during contrasting monsoon years of 2002 and 2003 using TRMM/TMI data

Summary The evolution of geophysical parameters over Indian Ocean during two contrasting monsoon years 2002 (drought) and 2003 (normal) were studied using TRMM/TMI satellite data. Analysis indicates that there was a lack of total water vapour (TWV) build up over Western Indian Ocean (WIO) during May 2002 (drought) when compared to 2003 (normal). Negative (positive) TWV anomalies were found over the WIO in May 2002 (2003). In 2002, negative SST anomaly of ∼1.5 °C is found over entire WIO when compared to 2003. Anomalously high sea surface wind speed (SWS) anomaly over the South West Indian Ocean (SWIO) and WIO would have resulted in cooling of the sea surface in May 2002 in comparison to 2003. In 2003 the wind speed anomaly over entire WIO and Arabian Sea (AS) was negative, whereas sea surface temperature (SST) anomaly was positive over the same region, which would have resulted in higher moisture availability over these regions. A negative (positive) TWV anomaly over Eastern Arabian Sea (EAS) and positive (negative) anomaly over WIO forms a dipole structure. In the month of June no major difference is seen in all these parameters over the Indian Ocean. In July 2002 the entire WIO and AS was drier by 10–15 mm as compared to 2003. The pentad (5 day) average TWV values shows high (>55 mm) TWV convergence over EAS and Bay of Bengal (BoB) during active periods of 2003, which gives high rainfall over these regions. However, during 2002 although TWV over BoB was >55 mm but it was ∼45–55 mm over EAS during entire July and hence less rainfall. The evaporation has been calculated from the bulk aerodynamic formula using TRMM/TMI geophysical products. It has been seen that the major portion of evaporative moisture flux is coming from southern Indian Ocean (SIO) between 15 and 25° S. Evaporation in June was more over AS and SIO in 2003 when compared to 2002 which may lead to reduce moisture supply in July 2002 and hence less rainfall compared to July 2003.     

A comparison of PMIP2 model simulations and the MARGO proxy reconstruction for tropical sea surface temperatures at last glacial maximum

Results from multiple model simulations are used to understand the tropical sea surface temperature (SST) response to the reduced greenhouse gas concentrations and large continental ice sheets of the last glacial maximum (LGM). We present LGM simulations from the Paleoclimate Modelling Intercomparison Project, Phase 2 (PMIP2) and compare these simulations to proxy data collated and harmonized within the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface Project (MARGO). Five atmosphere–ocean coupled climate models (AOGCMs) and one coupled model of intermediate complexity have PMIP2 ocean results available for LGM. The models give a range of tropical (defined for this paper as 15°S–15°N) SST cooling of 1.0–2.4°C, comparable to the MARGO estimate of annual cooling of 1.7 ± 1°C. The models simulate greater SST cooling in the tropical Atlantic than tropical Pacific, but interbasin and intrabasin variations of cooling are much smaller than those found in the MARGO reconstruction. The simulated tropical coolings are relatively insensitive to season, a feature also present in the MARGO transferred-based estimates calculated from planktonic foraminiferal assemblages for the Indian and Pacific Oceans. These assemblages indicate seasonality in cooling in the Atlantic basin, with greater cooling in northern summer than northern winter, not captured by the model simulations. Biases in the simulations of the tropical upwelling and thermocline found in the preindustrial control simulations remain for the LGM simulations and are partly responsible for the more homogeneous spatial and temporal LGM tropical cooling simulated by the models. The PMIP2 LGM simulations give estimates for the climate sensitivity parameter of 0.67°–0.83°C per Wm⁻², which translates to equilibrium climate sensitivity for doubling of atmospheric CO₂ of 2.6–3.1°C.     

Evolution of the equatorial and dipole modes of the sea-surface temperature in the Tropical Atlantic at decadal scale

Summary ?One hundred and thirty six years (1856–1991) of monthly sea-surface temperature (SST) data in the Tropical Atlantic Ocean are used to investigate the propagating signals of the SST at a decadal (DD) time scale. The first and the third evolving modes show a relationship between the equatorial and the inter-hemispheric patterns, one evolving into the other mode and vice-versa. These modes describe two different evolutions of the SST at DD time-scale. The first EEOF features a 12-year period oscillatory regime with a strong 2-year duration inter-hemispheric pattern evolving into a weak 1-year duration equatorial pattern and vice-versa. This mode exhibits also a northward displacement of the anomalies in the band 15° S–15° N. The third EEOF also shows an oscillatory regime, but with a period of 10 years and with a relatively strong 2-year duration equatorial pattern evolving into a weak 1-year duration inter-hemispheric pattern and vice-versa. For this mode, the SST anomalies show a southward displacement in the band 15° S–15° N. These results have not yet been documented in previous works and explain some of the previous findings on the DD variability in the Tropical Atlantic. Received December 31, 2001; revised April 9, 2002; accepted September 4, 2002 Published online: March 20, 2003     

Influence of vegetation changes during the Last Glacial Maximum using the BMRC atmospheric general circulation model

 The influence of different vegetation distributions on the atmospheric circulation during the Last Glacial Maximum (LGM, 21 000 years before present) is investigated. The atmospheric general circulation model of the Bureau of Meteorology Research Center was run using a modern vegetation and in a second experiment with a vegetation reconstruction for the LGM. It is found that a change from conifer to desert and tundra causes an additional LGM cooling of 1–2 °C in Western Europe, up to −4 °C in North America and −6 °C in Siberia. An expansion of dryland vegetation causes an additional annual cooling of 1–2 °C for Australia and northern Africa. On the other hand, an increase of temperature (2 °C) is found in Alaska due to changes in circulation. In the equatorial region the LGM vegetation leads to an increased modelled temperature of 0.5–1.5 °C and decreased precipitation (30%) over land due to a reduction of the tropical rainforest, mainly in Indonesia, where the reduction of precipitation over land is associated with an increase of precipitation of 30% over the western Pacific. Received: 15 December 1999 / Accepted: 10 January 2001     

Empirical prediction of Indian summer monsoon rainfall with different lead periods based on global SST anomalies

Summary The main objective of this study was to develop empirical models with different seasonal lead time periods for the long range prediction of seasonal (June to September) Indian summer monsoon rainfall (ISMR). For this purpose, 13 predictors having significant and stable relationships with ISMR were derived by the correlation analysis of global grid point seasonal Sea-Surface Temperature (SST) anomalies and the tendency in the SST anomalies. The time lags of the seasonal SST anomalies were varied from 1 season to 4 years behind the reference monsoon season. The basic SST data set used was the monthly NOAA Extended Reconstructed Global SST (ERSST) data at 2° × 2° spatial grid for the period 1951–2003. The time lags of the 13 predictors derived from various areas of all three tropical ocean basins (Indian, Pacific and Atlantic Oceans) varied from 1 season to 3 years. Based on these inter-correlated predictors, 3 predictor sub sets A, B and C were formed with prediction lead time periods of 0, 1 and 2 seasons, respectively, from the beginning of the monsoon season. The selected principal components (PCs) of these predictor sets were used as the input parameters for the models A, B and C, respectively. The model development period was 1955–1984. The correct model size was derived using all-possible regressions procedure and Mallow’s “Cp” statistics. Various model statistics computed for the independent period (1985–2003) showed that model B had the best prediction skill among the three models. The root mean square error (RMSE) of model B during the independent test period (6.03% of Long Period Average (LPA)) was much less than that during the development period (7.49% of LPA). The performance of model B was reasonably good during both ENSO and non-ENSO years particularly when the magnitudes of actual ISMR were large. In general, the predicted ISMR during years following the El Ni?o (La Ni?a) years were above (below) LPA as were the actual ISMR. By including an NAO related predictor (WEPR) derived from the surface pressure anomalies over West Europe as an additional input parameter into model B, the skill of the predictions were found to be substantially improved (RMSE of 4.86% of LPA).     

Coupled ocean-atmosphere surface variability and its climate impacts in the tropical Atlantic region

 This study examines time evolution and statistical relationships involving the two leading ocean-atmosphere coupled modes of variability in the tropical Atlantic and some climate anomalies over the tropical 120 °W–60 °W region using selected historical files (75-y near global SSTs and precipitation over land), more recent observed data (30-y SST and pseudo wind stress in the tropical Atlantic) and reanalyses from the US National Centers for Environmental Prediction (NCEP/NCAR) reanalysis System on the period 1968–1997: surface air temperature, sea level pressure, moist static energy content at 850 hPa, precipitable water and precipitation. The first coupled mode detected through singular value decomposition of the SST and pseudo wind-stress data over the tropical Atlantic (30 °N–20 °S) expresses a modulation in the thermal transequatorial gradient of SST anomalies conducted by one month leading wind-stress anomalies mainly in the tropical north Atlantic during northern winter and fall. It features a slight dipole structure in the meridional plane. Its time variability is dominated by a quasi-decadal signal well observed in the last 20–30 ys and, when projected over longer-term SST data, in the 1920s and 1930s but with shorter periods. The second coupled mode is more confined to the south-equatorial tropical Atlantic in the northern summer and explains considerably less wind-stress/SST cross-covariance. Its time series features an interannual variability dominated by shorter frequencies with increased variance in the 1960s and 1970s before 1977. Correlations between these modes and the ENSO-like Nino3 index lead to decreasing amplitude of thermal anomalies in the tropical Atlantic during warm episodes in the Pacific. This could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales. Overall the relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes at the ocean/atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the north–south movement of the seasonal cycle. This appears to be consistent with the intrinsic non-dipole character of the tropical Atlantic surface variability at the interannual time step and over the recent period, but produces abnormal amplitude and/or delayed excursions of the intertropical convergence zone (ITCZ). Connections with continental rainfall are approached through three (NCEP/NCAR and observed) rainfall indexes over the Nordeste region in Brazil, and the Guinea and Sahel zones in West Africa. These indices appear to be significantly linked to the SST component of the coupled modes only when the two Atlantic modes+the ENSO-like Nino3 index are taken into account in the regressions. This suggests that thermal forcing of continental rainfall is particularly sensitive to the linear combinations of some basic SST patterns, in particular to those that create meridional thermal gradients. The first mode in the Atlantic is associated with transequatorial pressure, moist static energy and precipitable water anomaly patterns which can explain abnormal location of the ITCZ particularly in northern winter, and hence rainfall variations in Nordeste. The second mode is more associated with in-phase variations of the same variables near the southern edge of the ITCZ, particularly in the Gulf of Guinea during the northern spring and winter. It is primarily linked to the amplitude and annual phase of the ITCZ excursions and thus to rainfall variations in Guinea. Connections with Sahel rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region but the second Atlantic mode and the ENSO-like Pacific variability are clearly involved in the Sahel climate interannual fluctuations: anomalous dry (wet) situations tend to occur when warmer (cooler) waters are present in the eastern Pacific and the gulf of Guinea in northern summer which contribute to create a northward (southward) transequatorial anomaly gradient in sea level pressure over West Africa. Received: 14 April 1998 / Accepted: 24 December 1998     

Climate changes and its impact on tundra ecosystem in Qinghai-Tibet Plateau,China

Alpine ecosystems in permafrost region are extremely sensitive to climate change. The headwater regions of Yangtze River and Yellow River of the Qinghai-Tibet plateau permafrost area were selected. Spatial-temporal shifts in the extent and distribution of tundra ecosystems were investigated for the period 1967–2000 by landscape ecological method and aerial photographs for 1967, and satellite remote sensing data (the Landsat’s TM) for 1986 and 2000. The relationships were analyzed between climate change and the distribution area variation of tundra ecosystems and between the permafrost change and tundra ecosystems. The responding model of tundra ecosystem to the combined effects of climate and permafrost changes was established by using statistic regression method, and the contribution of climate changes and permafrost variation to the degradation of tundra ecosystems was estimated. The regional climate exhibited a tendency towards significant warming and desiccation with the air temperature increased by 0.4–0.67°C/10a and relative stable precipitation over the last 45 years. Owing to the climate continuous warming, the intensity of surface heat source (HI) increased at the average of 0.45 W/m² per year, the difference of surface soil temperature and air temperature (DT) increased at the range of 4.1°C–4.5°C, and the 20-cm depth soil temperature within the active layer increased at the range of 1.1°C–1.4°C. The alpine meadow and alpine swamp meadow were more sensitive to permafrost changes than alpine steppe. The area of alpine swamp meadow decreased by 13.6–28.9%, while the alpine meadow area decreased by 13.5–21.3% from 1967 to 2000. The contributions of climate change to the degradation of the alpine meadow and alpine swamp was 58–68% and 59–65% between 1967 and 2000. The synergic effects of climate change and permafrost variation were the major drivers for the observed degradation in tundra ecosystems of the Qinghai-Tibet plateau.     

Ice-sheet growth and high-latitudes sea-surface temperature

The response of the LLN 2-D climate model to the insolation and CO₂ forcings during the Eemian interglacial is compared to reconstructions obtained from deep-sea cores drilled in the Norwegian Sea and in the North Atlantic. Both reconstructions and modeling results show a decrease of sea-surface temperature (SST) in the higher latitudes (70–75 °N zonal belt for the model and the Norwegian Sea for the proxy records), associated with a more moderate cooling at lower latitudes (50–55 °N and North Atlantic), at the middle of isotopic substage 5e, several millenia before the beginning of continental ice-sheet growth. Such a comparison between the simulated SST and ice volume of the Northern Hemisphere has been extended to the whole last glacial-interglacial cycle. The influence of the insolation forcing on SST and the shortcomings of the model due to its zonal character are discussed. Received: 6 July 1995/Accepted: 19 December 1995     

冬季黑潮延伸体区域海表温度锋对北太平洋风暴轴的影响

利用NOAA最优插值逐日海表温度资料和NCEP/NCAR的逐日大气再分析资料,分析了冬季黑潮延伸体区域海表温度锋的变化及其对北太平洋风暴轴的影响。结果表明,冬季黑潮延伸体区域海表温度锋强度和纬度位置既存在年际变化,也存在年代际变化,且强度和位置的变化是相互独立的。冬季黑潮延伸体区域海表温度锋强度的年际变化对北太平洋风暴轴没有显著的影响,而其年代际变化则对北太平洋风暴轴具有非常显著的影响,当冬季海表温度锋偏强时,大气斜压性在鄂霍次克海及阿拉斯加附近区域上空增强,而在海表温度锋下游至东太平洋区域上空显著减弱,平均有效位能向涡动有效位能的斜压能量转换在45°N以北的太平洋区域上空有所增多,而在30°-45°N的太平洋区域上空有所减少,涡动有效位能向涡动动能的斜压能量转换在35°N以北的西太平洋区域以及45°N以北的东太平洋区域都显著增加,而仅在其南部边缘存在东西带状的减弱区域,导致40°N以北海区北太平洋风暴轴增强,40°N以南海区北太平洋风暴轴减弱,冬季海表温度锋偏弱时则有与之相反的结果。冬季黑潮延伸体区域海表温度锋纬度位置的变化对北太平洋风暴轴也存在较显著的影响,当海表温度锋位置偏北时,在其下游45°N以南的太平洋区域上空大气斜压性减弱,45°N以南的中东太平洋区域上空区域平均有效位能向涡动有效位能、以及涡动有效位能向涡动动能的斜压能量转换都减少;而在45°N以北的太平洋区域上空大气斜压性增强,在阿拉斯加湾附近上空尤其显著,在黑潮延伸体区域附近以及45°N以北的中东太平洋上空平均有效位能向涡动有效位能、以及涡动有效位能向涡动动能的斜压能量转换都显著增加,导致北太平洋风暴轴在其气候平均态轴线两侧呈现北正南负的偶极子形态;海表温度锋位置偏南时则有与之相反的结果。冬季黑潮延伸体区域海表温度锋强度和位置的变化均对北太平洋风暴轴具有显著的影响,其具体的物理机制还需要进一步的研究。      

Intense rainfall events over the west coast of India

Summary The west coast of the Indian peninsula receives very heavy rainfall during the summer Monsoon (June–September) season with average rainfall over some parts exceeding 250 cm. Heavy rainfall events with rainfall more than 15 cm day⁻¹ at one or more stations along the west coast of India occur frequently and cause considerable damage. A special observational programme, Arabian Sea Monsoon Experiment, was carried out during the monsoon season of 2002 to study these events. The spatial and temporal distributions of intense rainfall events, presented here, were used for the planning of this observational campaign. The present study using daily rainfall data for summer monsoon season of 37 years (1951–1987) shows that the probability of getting intense rainfall is the maximum between 14° N–16° N and near 19° N. The probability of occurrence of these intense rainfall events is high from mid June to mid August, with a dip in early July. It has been believed for a long time that offshore troughs and vortices are responsible for these intense rainfall events. However, analysis of the characteristics of cloud systems associated with the intense rainfall events during 1985–1988 using very high resolution brightness temperature data from INSAT-IB satellite shows that the cloud systems during these events are characterized by large spatial scales and high cloud tops. Further study using daily satellite derived outgoing longwave radiation (OLR) data over a longer period (1975–1998) shows that, most of these events (about 62%) are associated with systems organized on synoptic and larger scales. We find that most of the offshore convective systems responsible for intense rainfall along the west coast of India are linked to the atmospheric conditions over equatorial Indian Ocean.