Assessment of the three representative empirical models for zenith tropospheric delay(ZTD)using the CMONOC data

The precise correction of atmospheric zenith tropospheric delay(ZTD)is significant for the Global Navigation Satellite System(GNSS)performance regarding positioning accuracy and convergence time.In the past decades,many empirical ZTD models based on whether the gridded or scattered ZTD products have been proposed and widely used in the GNSS positioning applications.But there is no comprehensive evaluation of these models for the whole China region,which features complicated topography and climate.In this study,we completely assess the typical empirical models,the IGGtropSH model(gridded,non-meteorology),the SHAtropE model(scattered,non-meteorology),and the GPT3 model(gridded,meteorology)using the Crustal Movement Observation Network of China(CMONOC)network.In general,the results show that the three models share consistent performance with RMSE/bias of 37.45/1.63,37.13/2.20,and 38.27/1.34 mm for the GPT3,SHAtropE and IGGtropSH model,respectively.However,the models had a distinct performance regarding geographical distribution,elevation,seasonal variations,and daily variation.In the southeastern region of China,RMSE values are around 50 mm,which are much higher than that in the western region,approximately 20 mm.The SHAtropE model exhibits better performance for areas with large variations in elevation.The GPT3 model and the IGGtropSH model are more stable across different months,and the SHAtropE model based on the GNSS data exhibits superior performance across various UTC epochs.

A predictive model for regional zenith tropospheric delay correction

The conventional zenith tropospheric delay(ZTD)model(known as the Saastamoinen model)does not consider seasonal variations affecting the delay,giving it low accuracy and stability.This may be improved with adjustments to account for annual and semi-annual variations.This method uses ZTD data provided by the Global Geodetic Observing System to analyze seasonal variations in the bias of the Saastamoinen model in Asia,and then constructs a model with seasonal variation corrections,denoted as SSA.To overcome the dependence of the model on in-situ meteorological parameters,the SSA+GPT3 model is formed by combining the SSA and GPT3(global pressure-temperature)models.The results show that the introduction of annual and semi-annual variations can substantially improve the Saastamoinen model,yielding small and time-stable variations in bias and root mean square(RMS).In summer and autumn,the bias and RMS are noticeably smaller than those from the Saastamoinen model.In addition,the SSA model performs better in low-latitude and low-altitude areas,and bias and RMS decease with the increase of latitude or altitude.The prediction accuracy of the SSA model is also evaluated for external consistency.The results show that the accuracy of the SSA model(bias:-0.38 cm,RMS:4.43 cm)is better than that of the Saastamoinen model(bias:1.45 cm,RMS:5.16 cm).The proposed method has strong applicability and can therefore be used for predictive ZTD correction across Asia.

An ERA5 tropospheric parameters-augmented approach for improving GNSS precise point positioning

Precise Point Positioning(PPP) technology has developed into a potent instrument for geodetic positioning, ionospheric modeling, tropospheric atmospheric parameter detection, and seismic monitoring.As atmospheric reanalysis data products’ accuracy and spatiotemporal resolution have improved recently, it has become important to apply these products to obtain high-accuracy tropospheric delay parameters, like zenith tropospheric delay(ZTD) and tropospheric horizontal gradient. These tropospheric delay parameters can be applied to PPP to reduce the convergence time and to increase the accuracy in the vertical direction of the position. The European Centre for Medium-Range Weather Forecasts Reanalysis 5(ERA5) atmospheric reanalysis data is the latest product with a high spatiotemporal resolution released by the European Center for Medium-Range Weather Forecasts(ECMWF). Only a few researches have evaluated the application of ERA5 data to Global Navigation Satellite System(GNSS)PPP. Therefore, this study compared and validated the ZTD products derived from ERA5 data using ZTD values provided by 290 global International GNSS Service(IGS) stations for 2016-2017. The results indicated a stable performance for ZTD, with annual average bias and RMS values of 0.23 cm and 1.09 cm,respectively. Further, GNSS observations for one week in each of the four seasons(spring: DOY 92-98;summer: DOY 199-205;autumn: DOY 275-281;and winter: DOY 22-28) from 34 multi-GNSS experiments(MGEX) stations distributed globally in 2016 were considered to evaluate the performance of ERA5-derived tropospheric delay products in GNSS PPP. The performance of ERA5-enhanced PPP was compared with that of the two standard GNSS PPP schemes(without estimated tropospheric horizontal gradient and with estimated tropospheric horizontal gradient). The results demonstrated that ERA5-enhanced GNSS PPP showed no significant improvement in the convergence times in both the Eastern(E) and Northern(N) directions, while the average convergence time over four weeks in the vertical(U)direction improved by 53.3% and 52.7%, respectively(in the case of pngm station). The average convergence times for each week in the U direction of the northern and southern hemisphere stations indicated a decrease of 16.3%, 12.6%, 9.6%, and 9.1%, and 16.9%, 9.6%, 8.9%, and 14.5%, respectively.Regarding positioning accuracy, ERA5-enhanced PPP showed an improvement of 13.3% and 16.2% over the two standard PPP schemes in the U direction, respectively. No significant improvement in the positioning performance was observed in both the E and N directions. Thus, this study demonstrated the potential application of the ERA5 tropospheric parameters-augmented approach to Beidou navigation and positioning.

Impact of Tropospheric Delay Gradients on Total Tropospheric Delay and Precise Point Positioning

GPS signals are electromagnetic waves that are affected by the Earth’s atmosphere. The Earth’s atmosphere can be categorized, according to its effect on GPS signals, into the ionosphere (ionospheric delay) and neutral atmosphere (tropospheric delay). The first-order ionospheric delay can be eliminated by linear combination of GPS observables on different frequencies. However, tropospheric delay cannot be eliminated because it is frequency-independent. The total tropospheric delay can be divided into three components. The first is the dry component, the second part is the wet component, and the third part is the horizontal gradients which account for the azimuthal dependence of tropospheric delay. In this paper, the effect of modeling tropospheric gradients on the estimation of the total tropospheric delay and station position is investigated. Long session, one month during January 2015, of GPS data is collected from ten randomly selected globally distributed IGS stations. Two cases are studied: the first case, the coordinates of stations are kept fixed to their actual values and the tropospheric delay is estimated twice, with and without tropospheric gradients. In the second case, the station position is estimated along with the total tropospheric delay with and without tropospheric gradients. It is shown that the average bias of the estimated total tropospheric delay when neglecting tropospheric gradients ranges from ?1.72 mm to 2.14 mm while the average bias when estimating gradients are ?0.898 mm to 1.92 mm which means that the bias is reduced by about 30%. In addition, the average standard deviation of the bias is 4.26 mm compared with 4.52 mm which means that the standard deviation is improved by about 6%.

Interannual and Decadal Changes in Tropospheric Ozone in China and the Associated Chemistry–Climate Interactions: A Review

China has been experiencing widespread air pollution due to rapid industrialization and urbanization in recent decades.The two major concerns of ambient air quality in China are particulate matter(PM)and tropospheric ozone(O3).With the implementation of air pollution prevention and control actions in the last five years,the PM pollution in China has been substantially reduced.In contrast,under the conditions of the urban air pollution complex,the elevated O3 levels in city clusters of eastern China,especially in warm seasons,have drawn increasing attention.Emissions of air pollutants and their precursors not only contribute to regional air quality,but also alter climate.Climate change in turn can change chemical processes,long-range transport,and local meteorology that influence air pollution.Compared to PM,less is known about O3 pollution and its climate effects over China.Here,we present a review of the main findings from the literature over the period 2011-18 with regard to the characteristics of O3 concentrations in China and the mechanisms that drive its interannual to decadal variations,aiming to identify robust conclusions that may guide decision-making for emissions control and to highlight critical knowledge gaps.We also review regional and global modeling studies that have investigated the impacts of tropospheric O3 on climate,as well as the projections of future tropospheric O3 owing to climate and/or emission changes.

Climate Responses to Direct Radiative Forcing of Anthropogenic Aerosols,Tropospheric Ozone,and Long-Lived Greenhouse Gases in Eastern China over 1951–2000

A unified chemistry-aerosol-climate model is applied in this work to compare climate responses to changing concentrations of long-lived greenhouse gases （GHGs, CO2, CH4, N2O）, tropospheric O3, and aerosols during the years 1951-2000. Concentrations of sulfate, nitrate, primary organic carbon （POA）, secondary organic carbon （SOA）, black carbon （BC） aerosols, and tropospheric 03 for the years 1950 and 2000 are obtained a priori by coupled chemistry-aerosol-GCM simulations, and then monthly concentrations are interpolated linearly between 1951 and 2000. The annual concentrations of GHGs are taken from the IPCC Third Assessment Report. BC aerosol is internally mixed with other aerosols. Model results indicate that the sinmlated climate change over 1951-2000 is sensitive to anthropogenic changes in atmospheric components. The predicted year 2000 global mean surface air temperature can differ by 0.8℃ with different forcings. Relative to the climate simulation without changes in GHGs, O3, and aerosols, anthropogenic forcings of SO4^2-, BC, BC＋SO4^2-, BC＋SO4^2- ＋POA, BC＋SO4^2- ＋POA＋SOA＋NO3^-, O3, and GHGs are predicted to change the surface air temperature averaged over 1971-2000 in eastern China, respectively, by -0.40℃, ＋0.62℃, ＋0.18℃, ＋0.15℃, -0.78℃, ＋0.43℃, and ＋0.85℃, and to change the precipitation, respectively, by -0.21, ＋0.07, -0.03, ＋0.02, -0.24, -0.08, and ＋0.10 mm d^-1. The authors conclude that all major aerosols are as important as GHGs in influencing climate change in eastern China, and tropospheric O3 also needs to be included in studies of regional climate change in China.

Increased Tibetan Plateau Snow Depth:An Indicator of the Connection between Enhanced Winter NAO and Late-Spring Tropospheric Cooling over East Asia

The authors present evidence to suggest that variations in the snow depth over the Tibetan Plateau （TP） are connected with changes of North Atlantic Oscillation （NAO） in winter （JFM）. During the positive phase of NAO, the Asian subtropical westerly jet intensifies and the India-Myanmar trough deepens. Both of these processes enhance ascending motion over the TP. The intensified upward motion, together with strengthened southerlies upstream of the India-Myanmar trough, favors stronger snowfall over the TP, which is associated with East Asian tropospheric cooling in the subsequent late spring （April-May）. Hence, the decadal increase of winter snow depth over the TP after the late 1970s is proposed to be an indicator of the connection between the enhanced winter NAO and late spring tropospheric cooling over East Asia.

A Numerical Study of Tropospheric Ozone in the Springtime in East Asia

The Models-3 Community Multi-scale Air Quality modeling system (CMAQ) coupled with the Regional Atmospheric Modeling System (RAMS) is applied to East Asia to study the transport and photochemical transformation of tropospheric ozone in March 1998. The calculated mixing ratios of ozone and carbon monoxide are compared with ground level observations at three remote sites in Japan and it is found that the model reproduces the observed features very well. Examination of several high episodes of ozone and carbon monoxide indicates that these elevated levels are found in association with continental outflow, demonstrating the critical role of the rapid transport of carbon monoxide and other ozone precursors from the continental boundary layer. In comparison with available ozonesonde data, it is found that the model-calculated ozone concentrations are generally in good agreement with the measurements, and the stratospheric contribution to surface ozone mixing ratios is quite limited.

PM2.5 and tropospheric O_3 in China and an analysis of the impact of pollutant emission control

This study reviewed the status of PM2.5 and tropospheric O3 observations in China(15e55N, 72e136E). Initially, the distribution of tropospheric O3 over the globe and China was determined based on satellite observations made during 2010e2013. The annual mean values were 29.78 DU and 33.97 DU over the globe and China, respectively. The distribution of PM2.5 and seasonal changes in concentrations in China were then simulated using an aerosol chemistry e climate coupled model system, with an annual mean value of 0.51×10-8kg mà3. The contributions from five different aerosols to the simulated PM2.5 concentrations in different seasons were also determined. The relationships among the emissions of aerosols, greenhouse gases and their precursors and radiative forcings were determined with reference to the(IPCC AR5) Intergovernmental Panel on Climate Change the Fifth Assessment Report. From these relationships, the possible effects of controlling O3 precursors and(PM) particulate matter on the climate were considered. The influence of the control of O3 precursors was not totally clear, and reducing emissions of short-lived greenhouse gases and black carbon was considered a secondary measure for short-term(the next 50years) climate-change mitigation. Reducing emissions of CO2 is still the best strategy for meeting the target of a global average rise in surface air temperature of less than 2C. Near- and short-term emission reduction strategies are important for both effective environmental protection and climate-change mitigation.

A Modeling Study of Effective Radiative Forcing and Climate Response Due to Tropospheric Ozone

This study simulates the effective radiative forcing （ERF） of tropospheric ozone from 1850 to 2013 and its effects on global climate using an aerosol-climate coupled model, BCC_AGCM2.0. I_CUACE/Aero, in combination with OMI （Ozone Monitoring Instrument） satellite ozone data. According to the OMI observations, the global annual mean tropospheric col- umn ozone （TCO） was 33.9 DU in 2013, and the largest TCO was distributed in the belts between 30°N and 45°N and at approximately 30°S; the annual mean TCO was higher in the Northern Hemisphere than that in the Southern Hemisphere; and in boreal summer and autumn, the global mean TCO was higher than in winter and spring. The simulated ERF due to the change in tropospheric ozone concentration from 1850 to 2013 was 0.46 W m-2, thereby causing an increase in the global annual mean surface temperature by 0.36°C, and precipitation by 0.02 mm d-1 （the increase of surface temperature had a significance level above 95%）. The surface temperature was increased more obviously over the high latitudes in both hemispheres, with the maximum exceeding 1.4°C in Siberia. There were opposite changes in precipitation near the equator, with an increase of 0.5 mm d- 1 near the Hawaiian Islands and a decrease of about -0.6 mm d- 1 near the middle of the Indian Ocean.

Harmonious Inter-decadal Changes of July–August Upper Tropospheric Temperature Across the North Atlantic, Eurasian Continent,and North Pacific

The authors have developed an integral view of the inter-decadal variability of July-August （JA） tropospheric temperature across the entire subtropical Northern Hemisphere. Using reanalysis data and complementary balloon-borne measurements, the authors identify one major mode of variability for the period 1958 2001 which exhibits a significant cooling center over East Asia and warming centers over the North Atlantic and North Pacific. The cooling （warming） signals barotropically penetrate through the troposphere, with the strongest anomalies at 200-300 hPa. The amplitude of the cooling over East Asia is stronger than that of the warming over the North Atlantic （North Pacific） by a factor of 2 （3）. This dominant mode exhibits a declining tendency for the entire period examined, particularly before 1980. After the mid-1980s, the tendency has leveled off. Variations of the harmonious change of JA upper tropospheric temperature represented by the principal component of Empirical Orthogonal Function analysis exhibit significant negative （positive） correlations with SST anomalies in the eastern tropical Pacific and the western tropical Indian Ocean （mid-latitude North Pacific）. Possible mechanisms are discussed.

A zenith tropospheric delay correction model based on the regional CORS network

Tropospheric delay is a primary error source in earth observations and a variety of radio navigation technologies. In this paper, the relationship between zenith tropospheric delays and the elevation and longitude of stations is analyzed using the zenith tropospheric delay final products of International GNSS Service （IGS） stations from 2011. Two new models are proposed for estimating zenith tropospheric delays from regional CORS data without meteorological data. The proposed models are compared with the direct interpolation method and the remove-restore method using data from Guangxi CORS. The results show that the new models significantly improve the calculated precision. Finally, the root mean square （RMS） errors of the new models were used to estimate the surface precipitable water vapor （PWV） value at CORS station, which was determined to be less than 2 mm.

Relationship between Lightning Activity and Tropospheric Nitrogen Dioxide and the Estimation of Lightning-produced Nitrogen Oxides over China

To better understand the relationship between lightning activity and nitrogen oxides （NOx） in the troposphere and to estimate lightning-produced NOx （LNOx） production in China more precisely, spatial and temporal distributions of vertical column densities of tropospheric nitrogen dioxide （NO2 VCDs） and lightning activity were analyzed using satellite measure- ments. The results showed that the spatial distribution of lightning activity is greater in the east than in the west of China, as with NO2 VCDs. However, the seasonal and annual variation between lightning and NO2 density show different trends in the east and west. The central Tibetan Plateau is sparsely populated without modem industry, and NO2 VCDs across the plateau are barely affected by anthropogenic sources. The plateau is an ideal area to study LNOx. By analyzing 15 years of satellite data from that region, it was found that lightning density is in strong agreement with annual, spatial and seasonal variations of NO2 VCDs, with a correlation coefficient of 0.79 from the linear fit. Combining Beirle＇s method and the linear fit equation, LNOx production in the Chinese interior was determined to be 0.07 （0.02-0.27） TgN yr-1 for 1997-2012, within the range of 0.016-0.384 TgN yr-1 from previous estimates.

Anthropogenic Direct Radiative Forcing of Tropospheric Ozone and Aerosols in 1850 and 2000 Estimated with IPCC AR5 Emissions Inventories

This study estimates direct radiative forcing by tropospheric ozone and all aerosols between the years 1850 and 2000, using the new IPCC AR5 （the Intergovernmental Panel on Climate Change Fifth Assessment Report） emissions inventories and a fully coupled chemistry-aerosol general circulation model. As compared to the previous Global Emissions Inventory Activity （GEIA） data, that have been commonly used for forcing estimates since 1990, the IPCC AR5 emissions inventories report lower anthropogenic emissions of organic carbon and black carbon aerosols and higher sulfur and NOx emissions. The simulated global and annual mean burdens of sulfate, nitrate, black carbon （BC）, primary organic aerosol （POA）, secondary organic aerosol （SOA）, and ozone were 0.79, 0.35, 0.05, 0.49, 0.34, and 269 Tg, respectively, in the year 1850, and 1.90, 0.90, 0.11, 0.71, 0.32, and 377 Tg, respectively, in the year 2000. The estimated annual mean top of the atmosphere （TOA） direct radiative forcing of all anthropogenic aerosols based on the AR5 emissions inventories is -0.60 W m^-2 on a global mean basis from 1850 to 2000. However, this is -2.40 W m-2 when forcing values are averaged over eastern China （18-45°N and 95-125°E）. The value for tropospheric ozone is 0.17 W m^-1 on a global mean basis and 0.24 W m^-2 over eastern China. Forcing values indicate that the climatic effect of aerosols over eastern China is much more significant than the globally averaged effect.

DETERMINATION OF WEIGHTED MEAN TROPOSPHERIC TEMPERATURE USING GROUND METEOROLOGICAL MEASUREMENTS

The weighted mean tropospheric temperature is a critical parameter in the conversion of wet zenith delay to precipitable water vapor in GPS Meteorology.This parameter can not be calculated from the radiosonde data in real time through the conventional methods.In this study,we first discuss the admissible error of weighted mean temperature to enable the accuracy of the conversion better than 1 mm,then summarize the performance of some of the existing methods. An empirical formula is established that satisfies the real_time requirement in GPS meteorology using Sequential Regression Analysis method.It is shown that this real_time formula as compared with other empirical methods is more accurate for local applications.

Spatial Patterns of Tropospheric Biennial Oscillation and Its Numerical Simulation

In order to investigate the spatial patterns of the Tropospheric Biennial Oscillation （TBO） on the global scale, the Climate Prediction Center （CPC） Merged Analysis of Precipitation （CMAP） monthly averaged precipitation and the Climate Diagnostics Center （CDC） monthly outgoing long-wave radiation （OLR） and SST are used in conjunction with TBO bandpass-filtering. The results indicate active biennial variability in the tropical eastern-central Pacific regions. It is evident that observations reflect the biennial component of the ENSO rather than the TBO itself. Since some studies have pointed out that the TBO is a broad-scale phenomenon differing from the ENSO, to investigate the pure TBO the ENSO signal must be excluded. The Scale Interaction Experiment-FRCGC （SINTEX-F） coupled general circulation model （CGCM） developed at Japan Frontier Research Center for Global Change （FRCGC） can capture both the ENSO and the biennial signals. Air-sea interactions in the tropical eastern-central Pacific are decoupled to eliminate the effects of ENSO in a experiment by SINTEX-F and the results show that biennial variability still exists even without ENSO. It seems to mean that the TBO and ENSO are independent from each other. Furthermore, the model results indicate that the two key regions are southwest Sumatra and the tropical western Pacific for the TBO cycle.

Simulation of Tropospheric Ozone with MOZART-2:An Evaluation Study over East Asia

Climate changes induced by human activities have attracted a great amount of attention. With this, a coupling system of an atmospheric chemistry model and a climate model is greatly needed in China for better understanding the interaction between atmospheric chemical components and the climate. As the first step to realize this coupling goal, the three-dimensional global atmospheric chemistry transport model MOZART-2 (the global Model of Ozone and Related Chemical Tracers, version 2) coupled with CAM2 (the Community Atmosphere Model, version 2) is set up and the model results are compared against observations obtained in East Asia in order to evaluate the model performance. Comparison of simulated ozone mixing ratios with ground level observations at Minamitorishima and Ryori and with ozonesonde data at Naha and Tateno in Japan shows that the observed ozone concentrations can be reproduced reasonably well at Minamitorishima but they tend to be slightly overestimated in winter and autumn while underestimated a little in summer at Ryori. The model also captures the general features of surface CO seasonal variations quite well, while it underestimates CO levels at both Minamitorishima and Ryori. The underestimation is primarily associated with the emission inventory adopted in this study. Compared with the ozonesonde data, the simulated vertical gradient and magnitude of ozone can be reasonably well simulated with a little overestimation in winter, especially in the upper troposphere. The model also generally captures the seasonal, latitudinal and altitudinal variations in ozone concentration. Analysis indicates that the underestimation of tropopause height in February contributes to the overestimation of winter ozone in the upper and middle troposphere at Tateno.

Impacts of Global Emissions of CO,NO_x ,and CH_4 on China Tropospheric Hydroxyl Free Radicals

Using the global chemistry and transport model MOZART, the simulated distributions of tropospheric hydroxyl free radicals （OH） over China and its sensitivities to global emissions of carbon monoxide （CO）, nitrogen oxide （NOx）, and methane （CH4） were investigated in this study. Due to various distributions of OH sources and sinks, the concentrations of tropospheric OH in east China are much greater than in west China. The contribution of NO ＋ perhydroxyl radical （HOs） reaction to OH production in east China is more pronounced than that in west China, and because of the higher reaction activity of non-methane volatile organic compounds （NMVOCs）, the contributions to OH loss by NMVOCs exceed those of CO and take the dominant position in summer. The results of the sensitivity runs show a significant increase of tropospheric OH in east China from 1990 to 2000, and the trend continues. The positive effect of double emissions of NOx on OH is partly offset by the contrary effect of increased CO and CH4 emissions： the double emissions of NOx will cause an increase of OH of 18.1% 30.1%, while the increases of CO and CH4 will cause a decrease of OH of 12.2% 20.8% and 0.3% 3.0%, respectively. In turn, the lifetimes of CH4, CO, and NOx will increase by 0.3%-3.1% with regard to double emissions of CH4, 13.9% 26.3% to double emissions of CO and decrease by 15.3% 23.2% to double emissions of NOx.

Effects of South China Sea/western North Pacific summer monsoon on tropospheric biennial oscillation （TBO）

Several theories have been developed to explain tropical biennial oscillation （TBO）, as an air-sea interactive system to impact Asian and global weather and climate, and some models have been established to produce a TBO. A simple 5-box model, with almost all the key processes associated with TBO, can produce a TBO by including airsea interactions in the monsoon regions. Despite that, the South China Sea/western North Pacific summer monsoon （SCS/WNPSM）, a very important monsoon subsystem, is neglected. In this paper, based on the dynamical framework of 5-box model, the term of SCS/WNPSM has been added and a 6-box model has been developed. Comparing the difference of TBO sensibilities with several key parameters, air-sea coupling coefficient α, SST-thermocline feedback coefficient γand wind-evaporation feedback coefficient λ, between the modified model and original model, TBO is more sensible to the parameters in the new model. The results imply that the eastern Pacific and local wind-evaporation play more important roles in the TBO when including SCS /WNPSM.

Seasonal and Interannual Variations of Upper Tropospheric Water Vapor Band Brightness Temperature over the Global Monsoon Regions

The upper-troposphere water vapor (UTWV) band brightness temperature (BT) dataset derived from the High-resolution Infrared Radiation Sounder (HIRS) channel 12 of the National Oceanic and Atmospheric Administration (NOAA) polar satellites from 1979 to 1995 is used to analyze the seasonal and interannual variations for the global monsoon regions. Results show that (i) there are three major regions where the UTWV band BT varies significantly with season, i.e., South Asia, the western coastal South-North America tropical region and the low-latitude African region; (ii) UTWV band BT clearly reveals the water vapor temporal/spatial features as well as the atmospheric circulation structure over the low-latitude during the monsoon onset; and (iii) there is a remarkable relationship between the interannual variation of the UTWV band BT over the monsoon regions and the sea surface temperature anomaly in the eastern equatorial Pacific.