• Trend of the Tropical Tropopause
• Low-frequency Variabilities of the Tropical CPT
• Summary
• References

The role of the tropical tropopause in determining stratospheric water vapor concentration was first noted by Brewer in 1949. Newell and Gould-Stewart (1981) indicated that stratospheric water vapor concentrations depend on the minimum tropical tropopause temperatures rather than their zonal mean. This is the ``stratospheric fountain'' hypothesis, which is consistent with the picture of air entering the stratosphere through the region where the tropical tropopause is coldest (e.g. the western Pacific). Dessler (1998) reexamined the ``stratospheric fountain'' hypothesis by comparing estimates of volume mixing ratio of water vapor entering the stratosphere, derived from midlatitude stratospheric observations, to annually and zonally averaged estimates of the saturation volume mixing ratio (SMR) at the tropical tropopause. Based on his calculations for the period 1994-1997, he concluded that the ``stratospheric fountain'' hypothesis is not necessary, implying that the tropospheric air can go into the stratosphere throughout the tropical tropopause at any longitude rather than through preferred regions at preferred times.

O'Sullivan and Dunkerton (1997) and Randel et al. (1998), have documented the QBO induced changes in several stratospheric constituents including water vapor. A positive trend has been found in stratospheric water vapor (Oltmans and Hofmann, 1995; Nedoluha et al., 1998). It was proposed that a warming trend of about 1 K/decade in tropical tropopause temperatures may explain the positive trend in stratospheric water vapor.

In this study we investigated the low-frequency variabilities of the tropical cold point tropopause (CPT) that are related to the stratospheric QBO and tropospheric ENSO, examined the trend in the tropical CPT temperatures, and indicated some implications for stratospheric water vapor.

Trend of the Tropical Tropopause

Cooling Trend of the Tropical CPT and Mechanism

The operational rawinsonde data between 10^oS - 10^oN from 1973 to 1998 were used to examine the trend of tropical CPT properties. We used a method similar to that used by Dessler (1998) except two differences. First, we noticed that missing data do occur near the tropopause height so that the minimum temperature may not always be properly reported in the sounding profile. In order to capture the coldest point, we discarded the few sounding profiles in which there are less than 3 records between 150 and 70 mb, then we used cubic spline fitting on the other soundings. Secondly, We try to reduce the bias due to the non-uniform locations of the sounding stations. Simply averaging the minimum SMRs from all sounding profiles in the tropics gives a cold temperature bias and underestimates the zonal mean minimum SMR since more soundings are collected over the the western Pacific, where the coldest CPTs are usually found. To minimize this bias, we divided the tropics arbitrarily into seven regions according to longitude and performed calculations similar to Dessler (1998) to get monthly mean SMR for each region. We averaged the monthly means of these regions using their areas as weights to get the monthly average for the entire tropics.

As a result of the above two improvements, we obtained annual average minimum SMRs of 3.47 ppmv for 1996, a little smaller than Dessler's (1998) 3.8 ppmv. It falls into the error range claimed by Dessler (1998), but we believe that it is more accurate. This number even more strongly supports Dessler's (1998) assertion that the ``stratospheric fountain'' is not necessary to explain the observed stratospheric dryness. However, the tropical CPT SMRs decrease at the rate of -0.460 (+/- 0.036) ppmv/decade. Corresponding temperature trend is about -0.570 (+/- 0.061) K/Decade during 1973-1998. We examined trend for each station in the tropics considering information from station history log. We did not found statistically significant positive temperature trend in the data of any station.

We proposed that the cooling trend is probably caused by changes in tropical convection activity. To prove this, We have computed the trends in tropical SST and tropical Outgoing Longwave Radiation (OLR). Trends of the tropical SST, OLR and CPT temperatures are consistent with each other. A warming trend in SST was found almost everwhere in the tropics. The warming SST tends to destabilize the static stability of the troposphere, and convection will occur more frequently and/or be more intense. Convective clouds will reach higher altitude and/or cover larger area. As an indication of this, the OLR shows a decreasing trend almost everywhere in the tropics. The OLR trend corresponds closely to the trends observed directly from the in situ rainfall observation (Waliser and Zhou, 1997). Stronger convection and more precipitation produce larger diabatic heating in the tropics, which forces a higher tropopause so that the pressures and temperatures of the tropical tropopause become lower and colder.

Implications of the Cooling Trend

One direct implication of the cooling trend is that the ``stratospheric fountain'' is still needed to explain the stratospheric dryness, at least before 1990. Dessler (1998) happened to use the data in coldest years in the period 1973-1998. The annual tropical-mean CPT temperature was 189.44 K in 1996 and 189.73 K in 1997, 1.32 K and 0.83 K below the criterion 190.76 K used by Newell and Gould-Stewart (1981) at the monthly mean and local level, respectively. An implicit assumption of Dessler (1998) was that the year-to-year variability of the tropopause-region minimum SMR was small so that his SMR calculation from 1994-1997 could be compared to those inferred entry values to examine the necessity for the ``stratospheric fountain'' hypothesis. In fact, the annual mean minimum SMRs in the late 1980s and early 1990s are about 4.4 ppmv, which is about 1.0 ppmv larger than 1996's average. The annual mean SMRs in the 1970s were even larger (4.5-5.2 ppmv). The entry value [H2O]e is increasing at a rate of 0.041 (+/- 0.007) ppmv/yr (Michelsen et al., 2000), while the SMR at the tropical CPT is decreasing at a rate of 0.046 (+/- 0.004) ppmv/yr. SMR and [H2O]_e are getting closer during the mid-1990s. This indicates that a ``stratospheric fountain'' might not be necessary during this period. However, a ``stratospheric fountain'' was necessary before that period. For instance, [H2O]_e estimated from 1985 ATMOS observations was about 3.28 ppmv while SMR at the tropical CPT in 1982 was about 4.92 ppmv which is about 1.64 ppmv larger than [H2O]e. Here, a transport time of about 3 years for air parcels to move from the tropical CPT to midlatitude middle stratosphere (Rosenlof, 1995) was considered. The annual mean SMR averaged over the entire tropics is too large to match the entry value inferred from middle latitude stratospheric observations.

There is no enough observational evidence to show how tropospheric air parcels enter the stratosphere across the tropical CPT. A few studies suggested that the western Pacific might be a ``stratospheric drain'' (Sherwood, 1999; Gettelman et al., 2000). However, the cooling trend of the tropical CPT suggests that ``stratospheric fountain'' region is the main location where air parcels get dehydrated wherever they enter the stratosphere.

Another implication of the cooling trend is that, the observed increasing trend in stratospheric water vapor is not due to changes in the tropopause temperatures. The trend contributed by methane oxidation is too small to explain the positive trend in stratospheric water vapor (Nedoluha et al., 1998). Thus the trend in water vapor is probably a consequence of dynamical processes. Primitive model experiments indicated that changes in the strength and/or the shape of tropical upwelling might be causes for the positive trend in stratospheric water vapor.

Low-frequency Variabilities of the Tropical CPT

The ECMWF ranalyses (1979-1993) were used to study low-frequency variabilities of the tropical CPT. Stratospheric zonal wind shear at 50 mbar over Singapore, and Sea Surface Temperature Anomalies (SSTA) in the Niño3.4 region are used as indices for the stratospheric QBO and tropospheric ENSO, respectively. Correlation calculations indicated that westerly wind shear at 50 mbar leads positive temperature anomaly at the tropical CPT by about 6 months and SSTA in the Niño3.4 region is simultaneously correlated with CPT temperatures. We used bivariate regression to filter QBO and ENSO signatures in the tropical CPT, then performed composite analysis using regressed time series.

QBO Signatures in the Tropical CPT

Composite analysis of the CPT temperatures for the westerly and easterly QBO wind shears shows zonally symmetric features of the QBO in CPT properties. During the westerly shear period, the composited tropical CPT is warmer by 0.2 to 0.3 K, and during easterly shear conditions it is colder by 0.2 to 0.4 K.

It takes about 3-4 months for the westerly shear at 50mb to reach 100mb and takes about 7 months for the easterly shear to propagate from 50mb to 100mb (Naujoket 1986), which gives an average of about 5-6 months. This time lag is very consistent with our analyses. The amplitude of QBO temperature perturbation can be estimated using (Andrews et al., 1987)

Given a wind change over a scale height of 10 m/s and a latitudinal scale of 1000 km, the above equation gives an estimate of about 0.79 K for temperature change over the latitudinal scale. Amplitude of the CPT temperature anomalies associated with the QBO is about 0.3-0.5 K, according to the time series of QBO signatures in the tropical CPT. Consistency in time lag and amplitude suggests that the QBO signature in the tropical CPT temperatures is probably due to the stratospheric QBO temperature anomalies that accompany the downward-propagating QBO meridional circulation.

ENSO Signatures in the Tropical CPT

The temperature anomalies associated with ENSO are larger than those associated with the QBO. They can be as low as -0.6 K during El Niño and as high as 1.0 K during La Niña over the eastern Pacific. The temperature anomalies over the western Pacific are about 0.4 K (-0.4 K) during El Niño (La Niña) events. Composites of the CPT  temperature anomalies for ENSO shows three distinct features. First, there is a East-West (E-W) dipole over the tropical Pacific. The second feature is that there are three North-South (N-S) dumbbells in the tropics. The strongest dumbbell is over the central to eastern Pacific. One dumbbell pattern is clear over the Atlantic, and similar features are also seen over the eastern Indian Ocean and the western Pacific, though less clearly. The third feature is that there is a maximum (in absolute value) over the equatorial western Pacific. These three features can be explained by changes of tropical convection activity associated with ENSO.

The annual cycle of the zonal mean tropical tropopause is driven by extratropical stratospheric wave forcing (Yulaeva et al., 1994), but the zonal asymmetrics in the tropical tropopause can be attributed to the direct response of the atmosphere to a large-scale region of tropospheric diabatic heating (Highwood and Hoskins, 1998). The mechanism proposed by Highwood and Hoskins (1998) can be extended to explain the simultaneous anti-correlation between the SSTA and the CPT temperatures over the eastern Pacific. During El Niño events, the active convection center shifts to the central to eastern Pacific. As a result, there is more precipitation in the central to eastern Pacific and so more diabatic heating there. Thus during El Niño events the tropopause is colder over the eastern Pacific. During La Niña events, the situation is reversed.

Yulaeva and Wallace (1994) investigated the QBO and ENSO signatures in global temperature and precipitation fields derived from the Microwave Sounding Unit (MSU). The ENSO associated temperature anomalies in the troposphere (MSU-2) and the lower stratosphere (MSU-4) are very consistent with the ENSO signature in the tropical CPT, in respect of the shape and positions of the E-W dipole and N-S dumbbells and the maximum anomaly over the western Pacific. Zhou and Sun (1994) extended the Gill (1980) model including a cooling over the western domain and a heating over the eastern domain, which is consistent with the distribution of diabatic heating anomalies during El Niño (Yulaeva and Wallace, 1994), to study the tropical troposphereic nonlinear steady response to two heating sources of contrasting nature. The steady  geopotential height response at the top of model domain to these two heat sources of contrasting nature shows that there is positive dumbbell over the eastern half domain and a negative dumbbell over the western half domain. These two dumbbells form a E-W (positive-negative) dipole. Additionally, there is a negative maximum geopotential height anomaly to the east of the western cooling source and this maximum anomaly is a result of wave-wave interaction.

Implication for Stratospheric Water Vapor

We expected low-frequency variabilities in the tropical CPT temperatures to be recorded in tropical stratospheric water vapor in a similar manner as the annual cycle of the tropical CPT temperatures is recorded (Mote et al. 1996). Simulations using the SUNY-SPb two-dimensional chemistry-transport model indicated that the interannual anomalies associated with the QBO circulation can explain a good deal of the observed interannual anomalies of stratospheric methane. However, it can interpret only part of the observed interannual variation of stratospheric water vapor (Randel et al. 1998). Interannual variation of stratospheric water vapor is affected by the low-frequency variability in tropical CPT temperatures, in addition to the QBO circulation. Water vapor anomalies due to the QBO circulation show large values in the middle stratosphere, consistent with the vertical structure of observations (Randel et al., 1998). However, they are too small to explain the observed interannual anomalies of water vapor in the tropical lower stratosphere. The influences of low-frequency thermal effects in CPT temperatures associated with the stratospheric QBO and the tropospheric ENSO produce low frequency ``tape recorder" signals.

Summary

Air entering the stratosphere across the tropical Cold Point Tropopause (CPT) is dehydrated by cold temperature at the CPT. Thus the tropical CPT plays an important role in determining the observed low concentration of stratospheric water vapor. The climatology of the tropical CPT has been investigated using sounding observations and ECMWF reanalyses. The CPT exhibits interannual variability associated with both the equatorial stratospheric QBO and tropospheric ENSO. The influence of the QBO on the tropical CPT is mainly zonally symmetric. However, the ENSO signature in the tropical CPT has East-West dipole and North-South dumbbell features. Both the QBO and ENSO signatures propagate upward as ``low-frequency tape recorders''. A cooling trend in the tropical CPT temperatures has been found, which in the absence of other changes would be expected to lead to a negative trend in stratospheric water vapor. Thus, the observed positive trend in stratospheric water vapor is probably caused by dynamical changes. The cooling trend also suggests that a ``stratospheric fountain'' was necessary in most years during the period 1973-1998 or the western Pacific is the main location to dehydrate air parcels wherever they enter the stratosphere across the tropical CPT.

This is a part of Xue-Long Zhou's Ph.D dissertation (The tropical cold point tropopause and stratospheric water vapor, Ph.D. dissertation, State University of New York at Stony Brook, 121pp., 2000). Three manuscripts can be obtained by contacting Xue-Long Zhou at ``xlzhou@atmos.washington.edu''

References

Zhou, X. L., M. A. Geller, and M. H. Zhang, The cooling trend of the tropical cold point tropopause temperatures and its implications, J. Geophys. Res., in press, 2000.

Zhou, X. L., M. A. Geller, and M. Zhang, Tropical Cold Point Tropopause Characteristics Derived from ECMWF Reanalyses and Soundings, J. of Clim. , in press, 2000.

Geller, M. A., X. L. Zhou, and M. H. Zhang, Simulations for interannual variability of stratospheric water vapor, J. Atmos. Sci.,  in revision, 2000.