Tropical Cold Point Tropopause Research Articles

Impacts of Ozone Changes in the Tropopause Layer on Stratospheric Water Vapor

Stratospheric water vapor (SWV) changes play an important role in regulating global climate change, and its variations are controlled by tropopause temperature. This study estimates the impacts of tropopause layer ozone changes on tropopause temperature by radiative process and further influences on lower stratospheric water vapor (LSWV) using the Whole Atmosphere Community Climate Model (WACCM4). It is found that a 10% depletion in global (mid-low and polar latitudes) tropopause layer ozone causes a significant cooling of the tropical cold-point tropopause with a maximum cooling of 0.3 K, and a corresponding reduction in LSWV with a maximum value of 0.06 ppmv. The depletion of tropopause layer ozone at mid-low latitudes results in cooling of the tropical cold-point tropopause by radiative processes and a corresponding LSWV reduction. However, the effect of polar tropopause layer ozone depletion on tropical cold-point tropopause temperature and LSWV is opposite to and weaker than the effect of tropopause layer ozone depletion at mid-low latitudes. Finally, the joint effect of tropopause layer ozone depletion (at mid-low and polar latitudes) causes a negative cold-point tropopause temperature and a decreased tropical LSWV. Conversely, the impact of a 10% increase in global tropopause layer ozone on LSWV is exactly the opposite of the impact of ozone depletion. After 2000, tropopause layer ozone decreased at mid-low latitudes and increased at high latitudes. These tropopause layer ozone changes at different latitudes cause joint cooling in the tropical cold-point tropopause and a reduction in LSWV. Clarifying the impacts of tropopause layer ozone changes on LSWV clearly is important for understanding and predicting SWV changes in the context of future global ozone recovery.

Zonally resolved water vapour coupling with tropical tropopause temperature: Seasonal and interannual variability, and influence of the Walker circulation

Global stratospheric water vapour is strongly coupled to the tropical cold-point tropopause temperatures. We quantified the coupling between temperature and water vapour in the TTL on seasonal and interannual time scale using 10 years of satellite based observations, focusing on the zonal asymmetries arising from the Walker circulation. Tropical region (15° N–15° S) shows near perfect correlation values in the TTL (between ~ 15.5 km to CPT) at ascending branches of Walker circulation, indicating a strong coupling between annual cycles of cold-point and water vapour variability. Descending branches of Walker circulation are characterized by weak temperature-water vapour coupling and local lapse-rate minima at about 1–1.5 km below CPT level. The correlation pattern of deseasonalized anomalies of temperature and water vapour shows a strong zonal asymmetry, with maximum correlation values (r ~ 0.7 to 0.9) in the TTL (between 15 km and cold-point level) over Maritime Continents including western Pacific Ocean (90° E–180° E), and Atlantic Ocean (0–50° W) and minimum values (r ~ 0.2 to 0.4) over eastern Pacific and Indian Ocean. The QBO amplitude in CPT-T is more or less zonally symmetric, with peak value over the Maritime Continent and Atlantic Ocean. During ENSO event, CPT-T over the western Pacific and Maritime Continent are largely influenced with warm anomaly. Associate with this CPT-T variation, large enhancement in WMR100 is noticed over the Indian Ocean between 20° N–20° S latitude. The difference between zonal mean deseasonalized anomalies of WMR100 and SMRcpt show significant fluctuations (± 0.3 ppmv) that are roughly coincide with the Oceanic Nino Index with a 6 month lag.

Diagnosing Observed Stratospheric Water Vapor Relationships to the Cold Point Tropical Tropopause

AbstractDehydration at the tropical cold point tropopause primarily controls the entry value of water vapor to the stratosphere, with additional (uncertain) contributions from subtropical monsoonal circulations and extreme deep convection. Here we quantify the links of observed stratospheric water vapor with near‐equatorial cold point temperature (TCP), based on interannual variations of monthly zonal averages over the period 1993–2017. Water vapor observations are from combined Halogen Occultation Experiment and Aura Microwave Limb Sounder satellite measurements, and cold point temperatures are from high quality radiosondes and GPS satellite data. Interannual water vapor anomalies are highly correlated with TCP, and coherent patterns can be traced in space and time away from the tropical tropopause to quantify transport in the Brewer‐Dobson circulation, including diagnosing seasonal changes in circulation. Lagged regressions with TCP are used to reconstruct water vapor variations directly tied to the cold point, and these reconstructions account for a majority of the observed interannual water vapor variability in the lower to middle stratosphere over most of the globe. Small systematic differences from observed water vapor can identify processes not tied to zonal average TCP, and/or possible uncertainties in the satellite measurements.

Temperature Control of the Variability of Tropical Tropopause Layer Cirrus Clouds

AbstractThis study examines the temperature control of variability of tropical tropopause layer (TTL) cirrus clouds (i.e., clouds with bases higher than 14.5 km) by using 8 years (2006–2014) of observations from the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC). It is found that the temporal variability of vertical structure of TTL cirrus cloud fraction averaged between 15°N and 15°S can be well explained by the vertical temperature gradient below ~17.5 km but by the local temperature above for both seasonal and interannual time scales. It is also found that the TTL cirrus cloud fraction at a given altitude is best correlated with the temperature at a higher altitude and this vertical displacement increases with a decrease of the cirrus altitude. It is shown that the TTL cirrus cloud fractions at all altitudes are significantly correlated with tropical cold point tropopause (CPT) temperature. The plausible mechanisms that might be responsible for the observed relations between TTL cirrus fraction and temperature‐based variables are discussed, which include ice particle sediments, cooling associated with wave propagations, change of atmospheric stability, and vertical gradient of water vapor mixing ratio. We further examine the spatial covariability of TTL total cirrus cloud fraction and CPT temperature for the interannual time scale. It is found that the El Niño–Southern Oscillation and quasi‐biennial oscillation are the leading factors in controlling the spatial variability of the TTL cirrus clouds and temperatures.

An Analysis of Tropical Cold-Point Tropopause Warming in 1999

Using reanalysis datasets, the warming of the tropical tropopause in 1999 and its evolution are investigated. It is found that there is a strong rate of increase in tropical cold-point tropopause temperature (CPTT) in June 1999, with negative CPTT anomalies before June (March-April-May) and large positive anomalies after June (July-August-September). Multiple linear regression analysis shows that deep convection, the quasi-biennial oscillation (QBO), and tropical upwelling associated with the Brewer-Dobson circulation (BDC) largely explain the variations of CPTT in 1999. Before June, enhanced deep convection resulting from increased sea surface temperature (SST) over the western Pacific and enhanced tropical upwelling of the BDC lead to a higher and colder tropopause. Those two factors explain 22% and 17% of the variance in CPTT, respectively. In June, the transformation of the east phase of QBO to the west phase contributes up to more than 50% of the variance in CPTT changes. After June, reduced tropical upwelling induced by weakened wave activity results in the warmer tropical tropopause temperatures to a large extent.

Longitudinal Asymmetric Trends of Tropical Cold-Point Tropopause Temperature and Their Link to Strengthened Walker Circulation

Abstract The zonal structure of trends in the tropical tropopause layer during 1979–2014 is investigated by using reanalysis datasets and chemistry–climate model simulations. The analysis herein reveals that the tropical cold-point tropopause temperature (CPTT) trends during 1979–2014 are zonally asymmetric; that is, over the tropical central and eastern Pacific (CEP; 20°S–20°N, 160°E–100°W), the CPTT shows an increasing trend of 0.22 K decade−1, whereas over the rest of the tropical regions (non-CEP regions) the CPTT shows a decreasing trend of −0.08 K decade−1. Model simulations suggest that this zonal asymmetry in the tropical CPTT trends can be partly attributed to Walker circulation (WC) changes induced by zonally asymmetric changes of the sea surface temperatures (SSTs). The increasing (decreasing) SSTs over the western Pacific (CEP) result in a larger zonal gradient in sea level pressure over the tropical Pacific and intensified surface easterlies. The increased pressure gradient leads to enhanced convection over the Indo-Pacific warm pool and weakened convection over the CEP, facilitating a stronger WC. The downward branch of the intensified WC induces a dynamical warming over the CEP and the upward branch of the intensified WC induces a dynamical cooling over the non-CEP regions below 150 hPa. The significant warming in the upper troposphere and lower stratosphere (UTLS) caused by the WC descending and wave activity changes in the UTLS over the CEP shifts the cold-point tropopause height to a higher level, while the radiative effects of greenhouse gases, ozone, and water vapor changes in the UTLS make less important contributions to the trend of the tropical CPTT than SST changes.

TTL cooling and drying during the January 2013 stratospheric sudden warming

Extremely low water vapour concentrations (as low as 1.5 ppmv) in the tropical tropopause layer (TTL) were observed by in situ measurements during the Airborne Tropical TRopopause Experiment (ATTREX) winter 2013 deployment in February 2013. The January 2013 tropical (15°N–15°S) mean value of Microwave Limb Sounder (MLS) water vapour satellite data at 82 hPa (2.3 ppmv) was one of the lowest during the instrument record (2004–2013). The relationship between a cooling of the tropical tropopause, a sudden stratospheric warming (SSW) event and convective activity in the western Pacific is investigated using satellite data and reanalysis meteorological products to elucidate the likely origin of those extremely low water vapour concentrations. A major midwinter SSW developed on 6 January 2013. Stratospheric polar temperatures increased by ∼30 K in a matter of days and temperatures in the tropical upper troposphere and lower stratosphere (UTLS) dropped at the same time. As a result of the easterly shear phase of the Quasi‐Biennial Oscillation and the SSW, the tropical tropopause in January 2013 was anomalously cold (zonal mean of 187 K) and elevated (85 hPa). The tropical cold point tropopause (CPT) temperature and water vapour concentration at 82 hPa decreased by about 2 K and 1.5 ppmv respectively within the first 15 days of January; the water vapour change was likely a result of dehydration associated with the rapid cooling of the tropical CPT during that period.

Formation and Maintenance of the Tropical Cold-Point Tropopause in a Dry Dynamic-Core GCM

Abstract The formation of the tropical cold-point tropopause (CPT) is examined using a dry primitive equation model driven by the Held–Suarez forcing. Without moist and realistic radiative processes, the dry model successfully reproduces the zonal-mean structure of the CPT. The modeled CPT is appreciably colder (~10 K) than the prescribed equilibrium temperature, and it is maintained by upwelling in the tropical upper troposphere and lower stratosphere (UTLS). A transient simulation starting from an axisymmetric steady state without the CPT shows that the evolution and maintenance of the CPT are closely related to the zonal-mean-flow response to wave driving in the stratosphere. The transformed Eulerian-mean analysis indicates that the wave driving is mostly due to convergence of synoptic-scale waves originating from the midlatitude troposphere and propagating into the subtropical UTLS in this model simulation. The modeled CPT also shows a large sensitivity to increased baroclinicity in the equilibrium temperature. Although planetary-scale waves are not considered in this simulation, the result confirms that wave-driven upwelling in the tropical UTLS is a crucial process for the formation and maintenance of the CPT. In addition, it also implies that synoptic-scale waves may play a nonnegligible role in this mechanism, particularly in the seasons when planetary-scale wave activity in the lower stratosphere is weak.

Indo-Pacific Warm Pool Area Expansion, Modoki Activity and Tropical Cold-Point Tropopause Temperature Variations

The tropical cold-point tropopause temperature (CPTT), a potentially important indicator of global climate change, is of particular importance for understanding changes in stratospheric water vapor levels. Since the 1980s, the tropical CPTT has shown not only interannual variations, but also a decreasing trend. However, the factors controlling the variations in the tropical CPTT since the 1980s remain elusive. The present study reveals that the continuous expansion of the area of the Indo-Pacific warm pool (IPWP) since the 1980s represents an increase in the total heat energy of the IPWP available to heat the tropospheric air, which is likely to expand as a result. This process lifts the tropical cold-point tropopause height (CPTH) and leads to the observed long-term cooling trend of the tropical CPTT. In addition, our analysis shows that Modoki activity is an important factor in modulating the interannual variations of the tropical CPTT through significant effects on overshooting convection.

Tropical Cold-Point Tropopause: Climatology, Seasonal Cycle, and Intraseasonal Variability Derived from COSMIC GPS Radio Occultation Measurements

Abstract The finescale structure of the tropical cold-point tropopause (CPT) is examined using high-resolution temperature profiles derived from Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) global positioning system (GPS) radio occultation measurements for 4 yr from September 2006 to August 2010. The climatology, seasonal cycle, and intraseasonal variability are analyzed for three CPT properties: temperature (T-CPT), pressure (P-CPT), and sharpness (S-CPT). Their relationships with tropospheric and stratospheric processes are also discussed. The climatological P-CPT is largely homogeneous in the deep tropics, whereas T-CPT and S-CPT exhibit local minima and maxima, respectively, at the equator in the vicinity of deep convection regions. All three CPT properties, however, show coherent seasonal cycle in the tropics; the CPT is colder, higher (lower in pressure), and sharper during boreal winter than during boreal summer. This seasonality is consistent with the seasonal cycle of tropical upwelling, which is largely driven by stratospheric and near-tropopause processes, although the amplitude of the seasonal cycle of T-CPT and S-CPT is modulated by tropospheric circulations. On intraseasonal time scales, P-CPT and T-CPT exhibit homogeneous variability in the deep tropics, whereas S-CPT shows pronounced local variability and seasonality. The wavenumber–frequency spectra reveal that intraseasonal variability of CPT properties is primarily controlled by Kelvin waves, with a nonnegligible contribution by Madden–Julian oscillation convection. The Kelvin waves, which are excited by deep convection but often propagate along the equator freely, explain the homogeneous P-CPT and T-CPT variabilities. On the other hand, the vertically tilted dipole of temperature anomalies, which is associated with convectively coupled equatorial waves, determines the local structure and seasonality of S-CPT variability.

Cirrus cloud appearance in a volcanic aerosol layer around the tropical cold point tropopause over Biak, Indonesia, in January 2011

An aerosol layer was found 1–2 km around the tropical cold point tropopause by observations using ground‐based lidar and balloon‐borne optical particle counters (OPC) over Biak, Indonesia, in January 2011. The layer was observed throughout the survey period from 6 to 13 January. The backscattering coefficient of the layer was about 5 times larger than that of the background aerosols. The lidar‐observed depolarization ratio of the layer was very low, and the wavelength dependence of the backscattering coefficients of the layer was similar to that of stratospheric aerosols. A layer of the particles at the size of the accumulation mode was also observed by an OPC at ambient temperature, but the particles were volatile at 200°C. These properties indicate that the aerosol layer was composed of liquid phase aqueous sulfuric acid solution particles and probably originated from a volcanic eruption. It was observed by lidar that a thin cirrus cloud layer appeared within this aerosol layer. Backward trajectory analysis and satellite‐observed equivalent blackbody temperature indicate that the cirrus cloud layer probably formed in situ. The estimated upper limit of the number concentration of cloud particles was 105 m−3. The number concentration of the volatile aerosol particles (∼3 × 106 m−3) was 30 times larger than this upper limit. This upper limit, however, is comparable to the aerosol particle concentration observed by the OPC at 200°C. These results are consistent with the cirrus cloud formation with solid sulfate particles in tropical upper troposphere suggested by previous studies.

The Tropical Tropopause Layer 1960–2100

Abstract. The representation of the Tropical Tropopause Layer (TTL) in 13 different Chemistry Climate Models (CCMs) designed to represent the stratosphere is analyzed. Simulations for 1960–2005 and 1980–2100 are analyzed. Simulations for 1960–2005 are compared to reanalysis model output. CCMs are able to reproduce the basic structure of the TTL. There is a large (10 K) spread in annual mean tropical cold point tropopause temperatures. CCMs are able to reproduce historical trends in tropopause pressure obtained from reanalysis products. Simulated historical trends in cold point tropopause temperatures are not consistent across models or reanalyses. The pressure of both the tropical tropopause and the level of main convective outflow appear to have decreased (increased altitude) in historical runs as well as in reanalyses. Decreasing pressure trends in the tropical tropopause and level of main convective outflow are also seen in the future. Models consistently predict decreasing tropopause and convective outflow pressure, by several hPa/decade. Tropical cold point temperatures are projected to increase by 0.09 K/decade. Tropopause anomalies are highly correlated with tropical surface temperature anomalies and with tropopause level ozone anomalies, less so with stratospheric temperature anomalies. Simulated stratospheric water vapor at 90 hPa increases by up to 0.5–1 ppmv by 2100. The result is consistent with the simulated increase in temperature, highlighting the correlation of tropopause temperatures with stratospheric water vapor.

Ultrathin Tropical Tropopause Clouds (UTTCs): I. Cloud morphology and occurrence

Abstract. Subvisible cirrus clouds (SVCs) may contribute to dehydration close to the tropical tropopause. The higher and colder SVCs and the larger their ice crystals, the more likely they represent the last efficient point of contact of the gas phase with the ice phase and, hence, the last dehydrating step, before the air enters the stratosphere. The first simultaneous in situ and remote sensing measurements of SVCs were taken during the APE-THESEO campaign in the western Indian ocean in February/March 1999. The observed clouds, termed Ultrathin Tropical Tropopause Clouds (UTTCs), belong to the geometrically and optically thinnest large-scale clouds in the Earth's atmosphere. Individual UTTCs may exist for many hours as an only 200--300 m thick cloud layer just a few hundred meters below the tropical cold point tropopause, covering up to 105 km2. With temperatures as low as 181 K these clouds are prime representatives for defining the water mixing ratio of air entering the lower stratosphere.

Thermal variability of the tropical tropopause region derived from GPS/MET observations

Structure and variability of temperatures in the tropical upper troposphere and lower stratosphere (∼10–30 km) are studied based on Global Positioning System Meteorology (GPS/MET) observations during April 1995 to February 1997. Comparisons with several hundred colocated radiosondes demonstrates the high accuracy of GPS/MET retrievals in the tropics. Mean structure and variability of the tropical cold point tropopause is examined, and much of the subseasonal variability in cold point temperature and height appears to be related to wave‐like fluctuations (such as gravity waves or Kelvin waves). Significant correlations are found between the GPS/MET temperatures and daily, gridded outgoing longwave radiation data (a proxy for tropical deep convection), providing independent confirmation of the GPS/MET temperature fluctuations. These correlations quantify the large‐scale tropical temperature response to transient convection, showing coherent wave‐like variations over ∼12–18 km which span a hemisphere in longitude. The GPS/MET data also show clear evidence of the stratospheric quasi‐biennial oscillation (QBO) in temperatures over altitudes ∼16–40 km.

Intraseasonal Variations of Tropical Cold-Point Tropopause Temperatures

Intraseasonal variability of the tropical cold-point tropopause (CPT) and its relationship with tropical convection are investigated. Negative CPT temperature anomalies lead negative outgoing longwave radiation anomalies associated with enhanced convection by about 2–10 days throughout almost the entire lifetime of the Madden–Julian oscillation. It is proposed that the negative CPT temperature anomalies, together with positive tropospheric mean temperature anomalies found by other authors, are likely indications of Kelvin waves excited by convection centered to the west of the temperature anomalies.

Distribution and influence of convection in the tropical tropopause region

A global analysis of convective cloud in the tropical tropopause region (12–17 km) is presented. The analysis is based on high‐resolution global imagery of cloud brightness temperatures from satellites and from contemporaneous reanalysis temperatures. The coverage by deep convection decreases nearly exponentially with increasing altitude in the tropopause region. Convection is found at temperatures colder than the tropical cold point tropopause over ∼0.5% (±0.25%) of the tropics. Convection rarely penetrates more than 1.5 km above the tropopause. Large‐scale relationships between cold tropopause temperatures and deep convection indicate that where the tropopause is coldest convection penetrates most frequently. Small‐scale relationships show that the coldest diurnal tropopause temperatures occur after the diurnal peak in deep convection at tropopause levels over land. The coverage by deep convection is used to estimate the mass exchange or turnover time due to convection in the tropopause region. This turnover time is of the order of weeks at 12 km but increases to longer than a year at 18 km, with significant uncertainties in the tropopause region.

Simulations of the Interannual Variability of Stratospheric Water Vapor

Abstract Observations and model results indicate that the quasi-biennial oscillation (QBO) modulation of stratospheric water vapor results from two causes. Dynamical redistribution of water vapor from the QBO-induced mean meridional circulation dominates the observed variability in the middle and upper stratosphere. In the lower stratosphere, the QBO water vapor variability is dominated by a “tape recorder” that results from the dehydration signal accompanying the QBO variation of the tropical cold point tropopause. It is suggested that another low frequency tape recorder exists due to ENSO modulations of the tropical tropopause, but insufficiently long observations of stratospheric water vapor exist to identify this in the observations.

Tropical Cold Point Tropopause Characteristics Derived from ECMWF Reanalyses and Soundings

Abstract Tropical cold point tropopause (CPT) characteristics have been calculated using ECMWF reanalyses and high resolution radiosonde soundings obtained in TOGA COARE as well as operational sounding data. It is found that ECMWF reanalyses are suitable for investigating the morphology of the tropical CPT and the variabilities of the tropopause over the entire Tropics, despite an almost constant one-side warm bias. The daily locations of the tropical coldest CPT are clustered over the western Pacific warm pool region in January and spread out longitudinally as the year progresses. During the Indian summer monsoon, the locations of the coldest CPT are deflected northward. The influences of the quasi-biennial oscillation (QBO) and the El Nino–Southern Oscillation (ENSO) on the tropical CPT have been separated using bivariate regression. The stratospheric zonal wind shear at 50 mb leads the variation in the tropical CPT temperatures by about 6 months. The QBO signature in the tropical CPT is mainly zonally ...

Cooling trend of the tropical cold point tropopause temperatures and its implications

Operational sounding data (1973–1998) were used to determine cold point tropopause (CPT) characteristics. A cooling trend (−0.57±0.06 K/Decade during 1973–1998) in tropical CPT temperatures has been found, which is opposite to what has been hypothesized to explain the trend in stratospheric water vapor. Given this trend, the annual averages of the CPT saturation mixing ratios (SMRs) inferred from the analysis of the 1994–1997 data in the work of Dessler [1998] are substantially smaller than those from data before the mid‐1990s. This implies that while Newell and Gould‐Stewart's “stratospheric fountain” might not have been necessary to explain stratospheric water vapor during the mid‐1990s, it was necessary in most years before that. Changes in tropical convection occurrence frequency and/or strength are suggested to explain the cooling trend in the CPT temperatures. It is suggested that the observed positive trend in stratospheric water vapor is probably due to changes in the residual circulation.