[1] Observations of temperature, winds, and atmospheric trace gases suggest that the transition from troposphere to stratosphere occurs in a layer, rather than at a sharp ''tropopause.'' In the tropics, this layer is often called the ''tropical tropopause layer'' (TTL). We present an overview of observations in the TTL and discuss the radiative, dynamical, and chemical processes that lead to its timevarying, three-dimensional structure. We present a synthesis definition with a bottom at 150 hPa, 355 K, 14 km (pressure, potential temperature, and altitude) and a top at 70 hPa, 425 K, 18.5 km. Laterally, the TTL is bounded by the position of the subtropical jets. We highlight recent progress in understanding of the TTL but emphasize that a number of processes, notably deep, possibly overshooting convection, remain not well understood. The TTL acts in many ways as a ''gate'' to the stratosphere, and understanding all relevant processes is of great importance for reliable predictions of future stratospheric ozone and climate.

We show here that stratospheric water vapor variations play an important role in the evolution of our climate. This comes from analysis of observations showing that stratospheric water vapor increases with tropospheric temperature, implying the existence of a stratospheric water vapor feedback. We estimate the strength of this feedback in a chemistry-climate model to be +0.3 W/(m 2 ·K), which would be a significant contributor to the overall climate sensitivity. One-third of this feedback comes from increases in water vapor entering the stratosphere through the tropical tropopause layer, with the rest coming from increases in water vapor entering through the extratropical tropopause.climate change | lowermost stratosphere | overworld D oubling carbon dioxide in our atmosphere by itself leads to a global average warming of ∼1.2°C. However, this direct warming from carbon dioxide drives other changes, known as feedbacks, that increase the eventual warming to 2.0-4.5°C. Thus, much of the warming predicted for the next century comes not from direct warming by carbon dioxide but from feedbacks.The strongest climate feedback is the tropospheric water vapor feedback (1, 2). The troposphere is the bottom 10-15 km of the atmosphere, and there are physical reasons to expect it to become moister as the surface warms (3)-and, indeed, both observations (4-6) and climate models (7, 8) verify this. Because water vapor is itself a greenhouse gas, tropospheric moistening more than doubles the direct warming from carbon dioxide.Stratospheric water vapor is also a greenhouse gas (9) whose interannual variations may have had important climatic consequences (10). This opens the possibility of a stratospheric water vapor feedback (11, 12) whereby a warming climate increases stratospheric water vapor, leading to additional warming. In this paper, we investigate this possibility. AnalysisMicrowave Limb Sounder Observations of the Overworld. Stratospheric water vapor can best be understood by subdividing the stratosphere into two regions: the overworld, that part of the stratosphere above the altitude of the tropical tropopause (∼16 km), and the lowermost stratosphere, that part of the extratropical stratosphere below that altitude (13) (see also figure 1 of ref. 14). Air enters the overworld exclusively through the tropical tropopause layer (TTL), where cold temperatures regulate the humidity of the air (14, 15) (we hereafter refer to the water content of air entering the overworld as H 2 O ov-entry ). Variations in H 2 O ov-entry can therefore be traced to variations in TTL temperatures. Fig. 1 shows monthly average tropical 82-hPa (∼18-km altitude) water-vapor volume-mixing-ratio anomalies observed by the Aura Microwave Limb Sounder (MLS) (16) (all tropical averages in this paper are over 30°N-30°S; anomalies are the remainder after the average annual cycle has been subtracted). These data are a good approximation of H 2 O ov-entry because this air has just entered the overworld and production of water from methane oxidation is negli...

Abstract. We present a hypothesis on the dehydration and transfer of air from the tropical troposphere into the stratosphere. The hypothesis is based on the existence of a thick "tropopause layer," in which vertical and horizontal mixing are both significant. Air is rapidly alehydrated upon entering this layer in vigorous convective overshoots, then slowly ascends through the layer before fully entering the stratosphere. Dehydration and genuine entry into the stratosphere are separate processes that happen on much different time scales.

A model of convective and advective transport across the tropical tropopause is described. In this model overshooting convective turrets inject dehydrated tropospheric air into a tropical "tropopause layer" (TTL) bounded approximately by the 50 and 150 hPa surfaces, a layer similar to the "entrainment zone" at the top of the planetary boundary layer. The overshooting process occurs only in limited regions. In the TTL, mixtures of overshooting and ambient air undergo buoyancy-driven settling, then slowly loft through the TTL and eventually enter the main stratosphere throughout the tropics. We find that for reasonable parameter settings the combined action of convection, isentropic mixing, and advection by the large-scale circulation in the model can produce realistic water vapor and ozone profiles while balancing the energy budget. Some of the observed peculiarities that can be simulated are: i) the widespread absence of vapor saturation at the tropopause despite tropical mean upward motion; ii) an ozone minimum below the mean tropopause, and iii) the typical location of stratiform cloud tops below the mean tropopause. In contrast to inferences from typical "cold trap" models, the relative humidity of air crossing the tropopause is found to be sensitive to ice microphysics.

Between 2003 and 2008, the global‐average surface temperature of the Earth varied by 0.6°C. We analyze here the response of tropospheric water vapor to these variations. Height‐resolved measurements of specific humidity (q) and relative humidity (RH) are obtained from NASA's satellite‐borne Atmospheric Infrared Sounder (AIRS). Over most of the troposphere, q increased with increasing global‐average surface temperature, although some regions showed the opposite response. RH increased in some regions and decreased in others, with the global average remaining nearly constant at most altitudes. The water‐vapor feedback implied by these observations is strongly positive, with an average magnitude of λq = 2.04 W/m2/K, similar to that simulated by climate models. The magnitude is similar to that obtained if the atmosphere maintained constant RH everywhere.

Satellite and in situ water vapor and ozone observations near the base of the overworld (θ ≈ 380‐K potential temperature) are examined in summertime northern midlatitudes, with a focus on how their horizontal variations are influenced by deep convection. We show that summertime convection has a significant effect on the water vapor budget here, but only a small effect on the ozone budget. Using a simple model, we estimate that convection increases model extratropical water vapor at 380 K by 40% but decreases model extratropical ozone by only a few percent, relative to what would occur without convection. In situ data show that this convective injection occurs up to at least ∼390 K. This raises the possibility that the convectively moistened air might travel isentropically to the tropics and ascend into the stratospheric overworld without passing through the cold point. We argue that trends in convective moistening should be examined as possible contributors to observed trends in lower stratospheric water vapor, at least during summer months.

We examine variations in water vapor in air entering the stratosphere through the tropical tropopause layer (TTL) over the past three decades in satellite data and in a trajectory model. Most of the variance can be explained by three processes that affect the TTL: the quasi-biennial oscillation, the strength of the Brewer-Dobson circulation, and the temperature of the tropical troposphere. When these factors act in phase, significant variations in water entering the stratosphere are possible. We also find that volcanic eruptions, which inject aerosol into the TTL, affect the amount of water entering the stratosphere. While there is clear decadal variability in the data and models, we find little evidence for a long-term trend in water entering the stratosphere through the TTL over the past 3 decades.

A satellite‐borne precipitation radar is used to study the penetration of convection bearing large particles to altitudes around the tropical tropopause, a region now known as the tropical tropopause layer (TTL). This overshooting convection has been identified as potentially important in the dehydration of air entering the stratosphere. The global distribution of the radar reflectivity tops in the TTL follows the interseasonal and interannual patterns of the surface precipitation rates. The amount of overshooting convection is ∼5% of total deep convection and ∼1.5% of the total convective rain. In agreement with previous studies, the radar observations show that continental convection typically extends to higher altitudes than oceanic convection.