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Stratospheric Temperature Climate Data Record from Merged SSU and AMSU-A ObservationsAbstract
Observations from the Stratospheric Sounding Unit (SSU) on board historical NOAA polar-orbiting satellites have played a vital role in investigations of long-term trends and variability in the middle- and upper-stratospheric temperatures during 1979–2006. The successor to SSU is the Advanced Microwave Sounding Unit-A (AMSU-A) starting from 1998 until the present. Unfortunately, the two observations came from different sets of atmospheric layers, and the SSU weighting functions varied with time and location, posing a challenge to merge them with sufficient accuracy for development of an extended SSU climate data record. This study proposes a variational approach for the merging problem, matching in both temperatures and weighting functions. The approach yields zero means with a small standard deviation and a negligible drift over time in the temperature differences between SSU and its extension to AMSU-A. These features made the approach appealing for reliable detection of long-term climate trends. The approach also matches weighting functions with high accuracy for SSU channels 1 and 2 and reasonable accuracy for channel 3. The total decreases in global mean temperatures found from the merged dataset were from 1.8 K in the middle stratosphere to 2.4 K in the upper stratosphere during 1979–2015. These temperature drops were associated with two segments of piecewise linear cooling trends, with those during the first period (1979–97) being much larger than those of the second period (1998–2015). These differences in temperature trends corresponded well to changes of the atmospheric ozone amount from depletion to recovery during the respective time periods, showing the influence of human decisions on climate change.
Abstract
Observations from the Stratospheric Sounding Unit (SSU) on board historical NOAA polar-orbiting satellites have played a vital role in investigations of long-term trends and variability in the middle- and upper-stratospheric temperatures during 1979–2006. The successor to SSU is the Advanced Microwave Sounding Unit-A (AMSU-A) starting from 1998 until the present. Unfortunately, the two observations came from different sets of atmospheric layers, and the SSU weighting functions varied with time and location, posing a challenge to merge them with sufficient accuracy for development of an extended SSU climate data record. This study proposes a variational approach for the merging problem, matching in both temperatures and weighting functions. The approach yields zero means with a small standard deviation and a negligible drift over time in the temperature differences between SSU and its extension to AMSU-A. These features made the approach appealing for reliable detection of long-term climate trends. The approach also matches weighting functions with high accuracy for SSU channels 1 and 2 and reasonable accuracy for channel 3. The total decreases in global mean temperatures found from the merged dataset were from 1.8 K in the middle stratosphere to 2.4 K in the upper stratosphere during 1979–2015. These temperature drops were associated with two segments of piecewise linear cooling trends, with those during the first period (1979–97) being much larger than those of the second period (1998–2015). These differences in temperature trends corresponded well to changes of the atmospheric ozone amount from depletion to recovery during the respective time periods, showing the influence of human decisions on climate change.
Abstract
Green’s eddy diffusive transfer representation is used to parameterize the meridional eddy heat flux. The structural function obtained by Branscome for the diagonal component K yy in the tensor of the transfer coefficients is adopted. A least squares method that uses the observed data of eddy heat flux is proposed to evaluate the magnitude of K yy and the structure of the nondiagonal component K yz in the transfer coefficient tensor. The optimum motion characteristic at the steering level is used as a constraint for the relationship between K yy and K yz . The obtained magnitude of K yy is two to three times larger than that of the Branscome’s, which is obtained in a linear analysis with the assumption of K yz = 0.
Green’s vertically integrated expression for the meridional eddy momentum flux is used to test the coefficients obtained in the eddy heat flux. In this parameterization, the eddy momentum flux is related to the eddy fluxes of two conserved quantities: potential vorticity and potential temperature. The transfer coefficient is taken to be the sum of that obtained in the parameterization of eddy heat flux, plus a correction term suggested by Stone and Yao, which ensures the global net eddy momentum transport to be zero. What makes the present method attractive is that, even though only the data of eddy heat flux are used to evaluate the magnitude of the transfer coefficients, the obtained magnitude of the eddy momentum flux is in good agreement with observations. For the annual mean calculation, the obtained peak values of eddy momentum flux are 94% of the observation for the Northern Hemisphere and 101% for the Southern Hemisphere. This result significantly improves the result of Stone and Yao, who obtained 34% for the Northern Hemisphere and 16% for the Southern Hemisphere in a similar calculation, but in which K yz = 0 was assumed.
Abstract
Green’s eddy diffusive transfer representation is used to parameterize the meridional eddy heat flux. The structural function obtained by Branscome for the diagonal component K yy in the tensor of the transfer coefficients is adopted. A least squares method that uses the observed data of eddy heat flux is proposed to evaluate the magnitude of K yy and the structure of the nondiagonal component K yz in the transfer coefficient tensor. The optimum motion characteristic at the steering level is used as a constraint for the relationship between K yy and K yz . The obtained magnitude of K yy is two to three times larger than that of the Branscome’s, which is obtained in a linear analysis with the assumption of K yz = 0.
Green’s vertically integrated expression for the meridional eddy momentum flux is used to test the coefficients obtained in the eddy heat flux. In this parameterization, the eddy momentum flux is related to the eddy fluxes of two conserved quantities: potential vorticity and potential temperature. The transfer coefficient is taken to be the sum of that obtained in the parameterization of eddy heat flux, plus a correction term suggested by Stone and Yao, which ensures the global net eddy momentum transport to be zero. What makes the present method attractive is that, even though only the data of eddy heat flux are used to evaluate the magnitude of the transfer coefficients, the obtained magnitude of the eddy momentum flux is in good agreement with observations. For the annual mean calculation, the obtained peak values of eddy momentum flux are 94% of the observation for the Northern Hemisphere and 101% for the Southern Hemisphere. This result significantly improves the result of Stone and Yao, who obtained 34% for the Northern Hemisphere and 16% for the Southern Hemisphere in a similar calculation, but in which K yz = 0 was assumed.
Abstract
Warm target effect and diurnal drift errors are the main sources of uncertainties in the trend determination from the NOAA Microwave Sounding Unit (MSU) observations. Currently, there are two methods to correct the warm target effect: 1) finding a best root-level (level-1c) calibration nonlinearity using simultaneous nadir overpass (SNO) matchups to minimize this effect for each scene radiance, and 2) finding a best-fit empirical relationship between the correction term of the end-level gridded brightness temperature and warm target temperature and then removing the best fit from the unadjusted time series. The former corrects the warm target effect before the diurnal drift adjustment and provides more accurate, warm target effect–minimized, level-1c scene radiances for reanalysis applications. The latter corrects the warm target effect at the end-level merging step, which depend on the diurnal drift correction that occurred at a previous step. Although minimized, the first method still leaves small residual warm target–related errors due to imperfect calibrations. This study demonstrates that when the diurnal drift effect is negligible, a combination of the two methods completely removes warm target effect and produces an invariant trend that is independent of the level-1c calibration in the SNO framework. The conclusion is directly applicable to the MSU channel-2 oceanic midtropospheric temperature (T 2) and global channel-3 upper-tropospheric temperature (T 3) and channel-4 lower-stratospheric temperature (T 4), which satisfy the condition of negligible diurnal drift effect. On the basis of these results, version 1.2 of the National Environmental Satellite, Data, and Information Service (NESDIS)–Center for Satellite Applications and Research (STAR) multisatellite MSU time series was constructed, including all T 2, T 3, and T 4 products. In addition, a diurnal drift correction based on the Remote Sensing Systems diurnal anomalies was applied to the T 2 product, which produces consistent climate trends between land and ocean. The global long-term climate trends for T 2 and T 4 derived from the STAR V1.2 dataset are, respectively, 0.18 ± 0.05 and −0.39 ± 0.36 K decade−1 during 1979–2006; the T 3 trend is 0.11 ± 0.08 K decade−1 for 1981–2006.
Abstract
Warm target effect and diurnal drift errors are the main sources of uncertainties in the trend determination from the NOAA Microwave Sounding Unit (MSU) observations. Currently, there are two methods to correct the warm target effect: 1) finding a best root-level (level-1c) calibration nonlinearity using simultaneous nadir overpass (SNO) matchups to minimize this effect for each scene radiance, and 2) finding a best-fit empirical relationship between the correction term of the end-level gridded brightness temperature and warm target temperature and then removing the best fit from the unadjusted time series. The former corrects the warm target effect before the diurnal drift adjustment and provides more accurate, warm target effect–minimized, level-1c scene radiances for reanalysis applications. The latter corrects the warm target effect at the end-level merging step, which depend on the diurnal drift correction that occurred at a previous step. Although minimized, the first method still leaves small residual warm target–related errors due to imperfect calibrations. This study demonstrates that when the diurnal drift effect is negligible, a combination of the two methods completely removes warm target effect and produces an invariant trend that is independent of the level-1c calibration in the SNO framework. The conclusion is directly applicable to the MSU channel-2 oceanic midtropospheric temperature (T 2) and global channel-3 upper-tropospheric temperature (T 3) and channel-4 lower-stratospheric temperature (T 4), which satisfy the condition of negligible diurnal drift effect. On the basis of these results, version 1.2 of the National Environmental Satellite, Data, and Information Service (NESDIS)–Center for Satellite Applications and Research (STAR) multisatellite MSU time series was constructed, including all T 2, T 3, and T 4 products. In addition, a diurnal drift correction based on the Remote Sensing Systems diurnal anomalies was applied to the T 2 product, which produces consistent climate trends between land and ocean. The global long-term climate trends for T 2 and T 4 derived from the STAR V1.2 dataset are, respectively, 0.18 ± 0.05 and −0.39 ± 0.36 K decade−1 during 1979–2006; the T 3 trend is 0.11 ± 0.08 K decade−1 for 1981–2006.
Abstract
The Advanced Microwave Sounding Unit-A (AMSU-A, 1998–present) not only continues but surpasses the Microwave Sounding Unit’s (MSU, 1978–2006) capability in atmospheric temperature observation. It provides valuable satellite measurements for higher vertical resolution and long-term climate change research and trend monitoring. This study presented methodologies for generating 11 channels of AMSU-A-only atmospheric temperature data records from the lower troposphere to the top of the stratosphere. The recalibrated AMSU-A level 1c radiances recently developed by the Center for Satellite Applications and Research group were used. The recalibrated radiances were adjusted to a consistent sensor incidence angle (nadir), channel frequencies (prelaunch-specified central frequencies), and observation time (local solar noon time). Radiative transfer simulations were used to correct the sensor incidence angle effect and the National Oceanic and Atmospheric Administration-15 (NOAA-15) channel 6 frequency shift. Multiyear averaged diurnal/semidiurnal anomaly climatologies from climate reanalysis as well as climate model simulations were used to adjust satellite observations to local solar noon time. Adjusted AMSU-A measurements from six satellites were carefully quality controlled and merged to generate 13+ years (1998–2011) of a monthly 2.5° × 2.5° gridded atmospheric temperature data record. Major trend features in the AMSU-A-only atmospheric temperature time series, including global mean temperature trends and spatial trend patterns, were summarized.
Abstract
The Advanced Microwave Sounding Unit-A (AMSU-A, 1998–present) not only continues but surpasses the Microwave Sounding Unit’s (MSU, 1978–2006) capability in atmospheric temperature observation. It provides valuable satellite measurements for higher vertical resolution and long-term climate change research and trend monitoring. This study presented methodologies for generating 11 channels of AMSU-A-only atmospheric temperature data records from the lower troposphere to the top of the stratosphere. The recalibrated AMSU-A level 1c radiances recently developed by the Center for Satellite Applications and Research group were used. The recalibrated radiances were adjusted to a consistent sensor incidence angle (nadir), channel frequencies (prelaunch-specified central frequencies), and observation time (local solar noon time). Radiative transfer simulations were used to correct the sensor incidence angle effect and the National Oceanic and Atmospheric Administration-15 (NOAA-15) channel 6 frequency shift. Multiyear averaged diurnal/semidiurnal anomaly climatologies from climate reanalysis as well as climate model simulations were used to adjust satellite observations to local solar noon time. Adjusted AMSU-A measurements from six satellites were carefully quality controlled and merged to generate 13+ years (1998–2011) of a monthly 2.5° × 2.5° gridded atmospheric temperature data record. Major trend features in the AMSU-A-only atmospheric temperature time series, including global mean temperature trends and spatial trend patterns, were summarized.
Abstract
A technique that uses satellite-based surface wind and temperature soundings for deriving three-dimensional atmospheric wind fields is developed for climate studies over the middle- and high-latitude oceans. In this technique, the thermal wind derived from the satellite soundings is added to the surface wind to obtain a first-guess, nonmass-conserved atmospheric wind profile. Then a Lagrange multiplier in a variational formalism is used to force the first-guess wind to conserve mass. Two mass conservation schemes are proposed. One is to use the meridional mass transport conservation equation as a constraint to derive the meridional wind first, and then the vertically integrated mass conservation equation is used to infer the zonal wind. The zonal and meridional winds are obtained separately in this approach. The second scheme is to use the vertically integrated mass conservation equation as a constraint to retrieve the zonal and meridional winds simultaneously from the first-guess field.
Temperature soundings from the Television and Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder Path A dataset and a Special Sensor Microwave Imager (SSM/I) satellite-based surface wind field are used to derive the wind fields. The two mass conservation schemes yield two different wind fields. They are compared with the European Centre for Medium-Range Weather Forecasts (ECMWF) and National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses and radiosonde observations over the Southern Ocean. The general circulation structure of both wind fields is similar to the reanalysis winds. However, the annual-mean bias of the first method is small in both the zonal and meridional winds compared to radiosonde observations, while the zonal wind bias of the second method is as large as −4 m s−1. The main reason for the difference is that the second method requires that the Lagrange multiplier be zero on the latitudinal boundaries. This forces the retrieved zonal wind to approach the first-guess zonal wind. In contrast, the first method does not require latitudinal boundary conditions, allowing a larger correction to the first-guess zonal wind.
Abstract
A technique that uses satellite-based surface wind and temperature soundings for deriving three-dimensional atmospheric wind fields is developed for climate studies over the middle- and high-latitude oceans. In this technique, the thermal wind derived from the satellite soundings is added to the surface wind to obtain a first-guess, nonmass-conserved atmospheric wind profile. Then a Lagrange multiplier in a variational formalism is used to force the first-guess wind to conserve mass. Two mass conservation schemes are proposed. One is to use the meridional mass transport conservation equation as a constraint to derive the meridional wind first, and then the vertically integrated mass conservation equation is used to infer the zonal wind. The zonal and meridional winds are obtained separately in this approach. The second scheme is to use the vertically integrated mass conservation equation as a constraint to retrieve the zonal and meridional winds simultaneously from the first-guess field.
Temperature soundings from the Television and Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder Path A dataset and a Special Sensor Microwave Imager (SSM/I) satellite-based surface wind field are used to derive the wind fields. The two mass conservation schemes yield two different wind fields. They are compared with the European Centre for Medium-Range Weather Forecasts (ECMWF) and National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses and radiosonde observations over the Southern Ocean. The general circulation structure of both wind fields is similar to the reanalysis winds. However, the annual-mean bias of the first method is small in both the zonal and meridional winds compared to radiosonde observations, while the zonal wind bias of the second method is as large as −4 m s−1. The main reason for the difference is that the second method requires that the Lagrange multiplier be zero on the latitudinal boundaries. This forces the retrieved zonal wind to approach the first-guess zonal wind. In contrast, the first method does not require latitudinal boundary conditions, allowing a larger correction to the first-guess zonal wind.
Abstract
In recognizing the importance of Stratospheric Sounding Unit (SSU) onboard historical NOAA polar-orbiting satellites in assessment of long-term stratospheric temperature changes and limitations in previous available SSU datasets, this study constructs a fully documented, publicly accessible, and well-merged SSU time series for climate change investigations. Focusing on methodologies, this study describes the details of data processing and bias corrections in the SSU observations for generating consistent stratospheric temperature data records, including 1) removal of the instrument gas leak effect in its CO2 cell; 2) correction of the atmospheric CO2 increase effect; 3) adjustment for different observation viewing angles; 4) removal of diurnal sampling biases due to satellite orbital drift; and 5) statistical merging of SSU observations from different satellites. After reprocessing, the stratospheric temperature records are composed of nadirlike, gridded brightness temperatures that correspond to identical weighting functions and a fixed local observation time. The 27-yr reprocessed SSU data record comprises global monthly and pentad layer temperatures, with grid resolution of 2.5° latitude by 2.5° longitude, of the midstratosphere (TMS), upper stratosphere (TUS), and top stratosphere (TTS), which correspond to the three SSU channel observations. For 1979–2006, the global mean trends for TMS, TUS, and TTS, are respectively −1.236 ± 0.131, −0.926 ± 0.139, and −1.006 ± 0.194 K decade−1. Spatial trend pattern analyses indicated that this cooling occurred globally with larger cooling over the tropical stratosphere.
Abstract
In recognizing the importance of Stratospheric Sounding Unit (SSU) onboard historical NOAA polar-orbiting satellites in assessment of long-term stratospheric temperature changes and limitations in previous available SSU datasets, this study constructs a fully documented, publicly accessible, and well-merged SSU time series for climate change investigations. Focusing on methodologies, this study describes the details of data processing and bias corrections in the SSU observations for generating consistent stratospheric temperature data records, including 1) removal of the instrument gas leak effect in its CO2 cell; 2) correction of the atmospheric CO2 increase effect; 3) adjustment for different observation viewing angles; 4) removal of diurnal sampling biases due to satellite orbital drift; and 5) statistical merging of SSU observations from different satellites. After reprocessing, the stratospheric temperature records are composed of nadirlike, gridded brightness temperatures that correspond to identical weighting functions and a fixed local observation time. The 27-yr reprocessed SSU data record comprises global monthly and pentad layer temperatures, with grid resolution of 2.5° latitude by 2.5° longitude, of the midstratosphere (TMS), upper stratosphere (TUS), and top stratosphere (TTS), which correspond to the three SSU channel observations. For 1979–2006, the global mean trends for TMS, TUS, and TTS, are respectively −1.236 ± 0.131, −0.926 ± 0.139, and −1.006 ± 0.194 K decade−1. Spatial trend pattern analyses indicated that this cooling occurred globally with larger cooling over the tropical stratosphere.
Abstract
Poleward meridional moisture transport across the Southern Ocean during 1988 is investigated by applying conservation of mass to the wind derivation approach of Slonaker and Van Woert. The moisture field is from the Television and Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder A dataset. The wind field is first derived from a combination of the TOVS temperature profiles and a satellite-based surface wind field using the thermal wind relationship. Then a Lagrange multiplier is introduced in a variational procedure to constrain the wind to conserve mass.
The introduction of the conservation of mass reduces the estimates of the moisture flux and net precipitation dramatically in comparison with the nonmass-conserved method in Slonaker and Van Woert. For instance, the estimates of the zonally averaged, vertically integrated moisture flux across 50°S are reduced by 56% and the net precipitation between the 50°S and 60°S latitude belt are reduced by 63%. The reason for the difference is that the nonmass-conserved approach leads to unrealistically strong annual-mean winds in the lower troposphere, which results in an exaggerated mean moisture transport. In contrast, the mass-conserved annual-mean wind compares favorably with the radiosonde observations at Macquarie Island and European Centre for Medium-Range Weather Forecasts and National Centers for Environmental Prediction–National Center for Atmospheric Research reanalyses, and it yields a mean moisture flux consistent with historical estimates.
In contrast, the satellite-derived eddy moisture flux is underestimated by about 45% when compared with the radiosonde and analysis studies. This underestimation is probably due to the lower spatial and temporal resolutions of the satellite observations and lack of certain types of ageostrophic winds in the wind derivation.
Abstract
Poleward meridional moisture transport across the Southern Ocean during 1988 is investigated by applying conservation of mass to the wind derivation approach of Slonaker and Van Woert. The moisture field is from the Television and Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder A dataset. The wind field is first derived from a combination of the TOVS temperature profiles and a satellite-based surface wind field using the thermal wind relationship. Then a Lagrange multiplier is introduced in a variational procedure to constrain the wind to conserve mass.
The introduction of the conservation of mass reduces the estimates of the moisture flux and net precipitation dramatically in comparison with the nonmass-conserved method in Slonaker and Van Woert. For instance, the estimates of the zonally averaged, vertically integrated moisture flux across 50°S are reduced by 56% and the net precipitation between the 50°S and 60°S latitude belt are reduced by 63%. The reason for the difference is that the nonmass-conserved approach leads to unrealistically strong annual-mean winds in the lower troposphere, which results in an exaggerated mean moisture transport. In contrast, the mass-conserved annual-mean wind compares favorably with the radiosonde observations at Macquarie Island and European Centre for Medium-Range Weather Forecasts and National Centers for Environmental Prediction–National Center for Atmospheric Research reanalyses, and it yields a mean moisture flux consistent with historical estimates.
In contrast, the satellite-derived eddy moisture flux is underestimated by about 45% when compared with the radiosonde and analysis studies. This underestimation is probably due to the lower spatial and temporal resolutions of the satellite observations and lack of certain types of ageostrophic winds in the wind derivation.
Abstract
Accurate three-dimensional wind fields are essential for diagnosing a variety of important climate processes in the Arctic, such as the advection and deposition of heat and moisture, changes in circulation features, and transport of trace constituents. In light of recent studies revealing significant biases in upper-level winds over the Arctic Ocean from reanalyses, new daily wind fields are generated from 22.5 yr of satellite-retrieved thermal-wind profiles, corrected with a recently developed mass-conservation scheme. Compared to wind measurements from rawinsondes during the Surface Heat Budget of the Arctic (SHEBA) experiment, biases in satellite-retrieved winds are near zero in the meridional direction, versus biases of over 50% for reanalyses. Errors in the zonal component are smaller than those observed in reanalysis winds in the upper troposphere, while in the lower troposphere the effects of Greenland introduce uncertainty in the mass-conservation calculation. Further reduction in error may be achieved by incorporating winds retrieved from feature-tracking techniques using satellite imagers. Overall, satellite-retrieved winds are superior to reanalysis products over the data-sparse Arctic Ocean and provide increased accuracy for analyses requiring wind information.
Trends and anomalies for the 22.5-yr record are calculated for both meridional and zonal winds at eight levels between the surface and 300 hPa. Annual mean trends are similar at varying levels, reflecting the relatively barotropic nature of the Arctic troposphere. Zonal winds are more westerly over Eurasia and the western Arctic Ocean, while westerlies have weakened over northern Canada. Combined with the corresponding pattern in meridional winds, these results suggest that the polar vortex has, on average, shifted toward northern Canada. Seasonal trends show that some changes persist throughout the year while others vary in magnitude and sign. Most striking are spring patterns, which differ markedly from the other seasons. Changes in meridional winds are consistent with observed trends in melt-onset date and sea ice concentration in the marginal seas. Anomalies in zonal wind profiles exhibit decadal-scale cyclicity in the eastern Arctic Ocean, while overall shifts in anomaly signs are evident and vary by region. The winter North Atlantic Oscillation (NAO) index correlates moderately with meridional wind anomalies in the Atlantic sector of the Arctic Ocean: positively (0.48) in the Barents Sea and negatively (−0.59) in the Lincoln Sea. These observed trends and anomalies are expected to translate to changes in advected heat and moisture into the Arctic basin, which are likely linked to trends in sea ice extent, melt onset, cloud properties, and surface temperature.
Abstract
Accurate three-dimensional wind fields are essential for diagnosing a variety of important climate processes in the Arctic, such as the advection and deposition of heat and moisture, changes in circulation features, and transport of trace constituents. In light of recent studies revealing significant biases in upper-level winds over the Arctic Ocean from reanalyses, new daily wind fields are generated from 22.5 yr of satellite-retrieved thermal-wind profiles, corrected with a recently developed mass-conservation scheme. Compared to wind measurements from rawinsondes during the Surface Heat Budget of the Arctic (SHEBA) experiment, biases in satellite-retrieved winds are near zero in the meridional direction, versus biases of over 50% for reanalyses. Errors in the zonal component are smaller than those observed in reanalysis winds in the upper troposphere, while in the lower troposphere the effects of Greenland introduce uncertainty in the mass-conservation calculation. Further reduction in error may be achieved by incorporating winds retrieved from feature-tracking techniques using satellite imagers. Overall, satellite-retrieved winds are superior to reanalysis products over the data-sparse Arctic Ocean and provide increased accuracy for analyses requiring wind information.
Trends and anomalies for the 22.5-yr record are calculated for both meridional and zonal winds at eight levels between the surface and 300 hPa. Annual mean trends are similar at varying levels, reflecting the relatively barotropic nature of the Arctic troposphere. Zonal winds are more westerly over Eurasia and the western Arctic Ocean, while westerlies have weakened over northern Canada. Combined with the corresponding pattern in meridional winds, these results suggest that the polar vortex has, on average, shifted toward northern Canada. Seasonal trends show that some changes persist throughout the year while others vary in magnitude and sign. Most striking are spring patterns, which differ markedly from the other seasons. Changes in meridional winds are consistent with observed trends in melt-onset date and sea ice concentration in the marginal seas. Anomalies in zonal wind profiles exhibit decadal-scale cyclicity in the eastern Arctic Ocean, while overall shifts in anomaly signs are evident and vary by region. The winter North Atlantic Oscillation (NAO) index correlates moderately with meridional wind anomalies in the Atlantic sector of the Arctic Ocean: positively (0.48) in the Barents Sea and negatively (−0.59) in the Lincoln Sea. These observed trends and anomalies are expected to translate to changes in advected heat and moisture into the Arctic basin, which are likely linked to trends in sea ice extent, melt onset, cloud properties, and surface temperature.
Abstract
The Microwave Sounding Unit (MSU) onboard the National Oceanic and Atmospheric Administration polar-orbiting satellites measures the atmospheric temperature from the surface to the lower stratosphere under all weather conditions, excluding precipitation. Although designed primarily for monitoring weather processes, the MSU observations have been extensively used for detecting climate trends, and calibration errors are a major source of uncertainty. To reduce this uncertainty, an intercalibration method based on the simultaneous nadir overpass (SNO) matchups for the MSU instruments on satellites NOAA-10, -11, -12, and -14 was developed. Due to orbital geometry, the SNO matchups are confined to the polar regions, where the brightness temperature range is slightly smaller than the global range. Nevertheless, the resulting calibration coefficients are applied globally to the entire life cycle of an MSU satellite.
Such intercalibration reduces intersatellite biases by an order of magnitude compared to prelaunch calibration and, thus, results in well-merged time series for the MSU channels 2, 3, and 4, which respectively represent the deep layer temperature of the midtroposphere (T2), tropopause (T3), and lower stratosphere (T4). Focusing on the global atmosphere over ocean surfaces, trends for the SNO-calibrated T2, T3, and T4 are, respectively, 0.21 ± 0.07, 0.08 ± 0.08, and −0.38 ± 0.27 K decade−1 from 1987 to 2006. These trends are independent of the number of limb-corrected footprints used in the dataset, and trend differences are marginal for varying bias correction techniques for merging the overlapping satellites on top of the SNO calibration.
The spatial pattern of the trends reveals the tropical midtroposphere to have warmed at a rate of 0.28 ± 0.19 K decade−1, while the Arctic atmosphere warmed 2 to 3 times faster than the global average. The troposphere and lower stratosphere, however, cooled across the southern Indian and Atlantic Oceans adjacent to the Antarctic continent. To remove the stratospheric cooling effect in T2, channel trends from T2 and T3 (T23) and T2 and T4 (T24) were combined. The trend patterns for T23 and T24 are in close agreement, suggesting internal consistencies for the trend patterns of the three channels.
Abstract
The Microwave Sounding Unit (MSU) onboard the National Oceanic and Atmospheric Administration polar-orbiting satellites measures the atmospheric temperature from the surface to the lower stratosphere under all weather conditions, excluding precipitation. Although designed primarily for monitoring weather processes, the MSU observations have been extensively used for detecting climate trends, and calibration errors are a major source of uncertainty. To reduce this uncertainty, an intercalibration method based on the simultaneous nadir overpass (SNO) matchups for the MSU instruments on satellites NOAA-10, -11, -12, and -14 was developed. Due to orbital geometry, the SNO matchups are confined to the polar regions, where the brightness temperature range is slightly smaller than the global range. Nevertheless, the resulting calibration coefficients are applied globally to the entire life cycle of an MSU satellite.
Such intercalibration reduces intersatellite biases by an order of magnitude compared to prelaunch calibration and, thus, results in well-merged time series for the MSU channels 2, 3, and 4, which respectively represent the deep layer temperature of the midtroposphere (T2), tropopause (T3), and lower stratosphere (T4). Focusing on the global atmosphere over ocean surfaces, trends for the SNO-calibrated T2, T3, and T4 are, respectively, 0.21 ± 0.07, 0.08 ± 0.08, and −0.38 ± 0.27 K decade−1 from 1987 to 2006. These trends are independent of the number of limb-corrected footprints used in the dataset, and trend differences are marginal for varying bias correction techniques for merging the overlapping satellites on top of the SNO calibration.
The spatial pattern of the trends reveals the tropical midtroposphere to have warmed at a rate of 0.28 ± 0.19 K decade−1, while the Arctic atmosphere warmed 2 to 3 times faster than the global average. The troposphere and lower stratosphere, however, cooled across the southern Indian and Atlantic Oceans adjacent to the Antarctic continent. To remove the stratospheric cooling effect in T2, channel trends from T2 and T3 (T23) and T2 and T4 (T24) were combined. The trend patterns for T23 and T24 are in close agreement, suggesting internal consistencies for the trend patterns of the three channels.
Abstract
Moisture fields from the NCEP–DOE reanalysis-2 (R-2) and Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder A are validated using the Special Sensor Microwave Imager (SSM/I) retrievals over the Southern Ocean. It is shown that the spatial distributions of the annual mean statistics of the total precipitable water are similar among SSM/I, R-2, and TOVS Pathfinder A for both the eddy and mean components. However, transient statistics show that the R-2 total precipitable water agrees with SSM/I with a correlation of 0.77 over the Southern Ocean while the TOVS Pathfinder A moisture is almost uncorrelated with the SSM/I data.
Total moisture transport convergence for 1988 over the Antarctic continent is further examined using the R-2 wind and moisture data as well as the moisture retrievals from TOVS Pathfinder A. To gain a better understanding of transient and mean processes on moisture transport, the total moisture transport was decomposed into mean and eddy components. The results suggest that a mass conservation correction is necessary for the mean component, but can safely be ignored for the eddy component. With the mass conservation correction, the mean moisture transport is about the same for both the R-2 estimate alone and the estimate based on the mixed TOVS Pathfinder A moisture–R-2 wind. The computed eddy and total moisture transport convergence over Antarctica for the R-2 data agrees within 10%–15% with previous surface-data-based estimates as well as estimates from other model analyses. However, the eddy component of the mixed TOVS moisture with R-2 wind is about 60%–70% lower than the R-2 result. These differences occur because the eddy moisture amplitude of the TOVS Pathfinder A is nearly 40% lower than the R-2 data and also because the TOVS moisture has a much lower correlation with the R-2 winds. These results reflect the difficulties with the TOVS sensor in quantifying synoptic moisture transients resulting from conditional sampling problems.
Abstract
Moisture fields from the NCEP–DOE reanalysis-2 (R-2) and Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder A are validated using the Special Sensor Microwave Imager (SSM/I) retrievals over the Southern Ocean. It is shown that the spatial distributions of the annual mean statistics of the total precipitable water are similar among SSM/I, R-2, and TOVS Pathfinder A for both the eddy and mean components. However, transient statistics show that the R-2 total precipitable water agrees with SSM/I with a correlation of 0.77 over the Southern Ocean while the TOVS Pathfinder A moisture is almost uncorrelated with the SSM/I data.
Total moisture transport convergence for 1988 over the Antarctic continent is further examined using the R-2 wind and moisture data as well as the moisture retrievals from TOVS Pathfinder A. To gain a better understanding of transient and mean processes on moisture transport, the total moisture transport was decomposed into mean and eddy components. The results suggest that a mass conservation correction is necessary for the mean component, but can safely be ignored for the eddy component. With the mass conservation correction, the mean moisture transport is about the same for both the R-2 estimate alone and the estimate based on the mixed TOVS Pathfinder A moisture–R-2 wind. The computed eddy and total moisture transport convergence over Antarctica for the R-2 data agrees within 10%–15% with previous surface-data-based estimates as well as estimates from other model analyses. However, the eddy component of the mixed TOVS moisture with R-2 wind is about 60%–70% lower than the R-2 result. These differences occur because the eddy moisture amplitude of the TOVS Pathfinder A is nearly 40% lower than the R-2 data and also because the TOVS moisture has a much lower correlation with the R-2 winds. These results reflect the difficulties with the TOVS sensor in quantifying synoptic moisture transients resulting from conditional sampling problems.
