Karen H. Rosenlof

Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming

Dropping a Notch Between 2000 and 2001, the concentration of water vapor in the stratosphere dropped by about 10%. Water vapor is an important greenhouse gas, so did the decrease affect climate and slow global warming? Solomon et al. (p. 1219, published online 28 January) used a combination of data and models to show that lower stratospheric water vapor probably has contributed to the flattening of global average temperatures since 2000, by acting to slow the rate of warming by about 25%. Furthermore, the amount of water vapor in the stratosphere probably increased between 1980 and 2000, a period of more rapid warming, suggesting how important the concentration of stratospheric water vapor might be to climate. Decreases in stratospheric water vapor after the year 2000 slowed the rate of increase in global surface temperature. Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here we show that this acted to slow the rate of increase in global surface temperature over 2000–2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% as compared to estimates neglecting this change. These findings show that stratospheric water vapor is an important driver of decadal global surface climate change.

Seasonal cycle of the residual mean meridional circulation in the stratosphere

The transformed Eulerian-mean (TEM) residual circulation is used to study the zonally averaged transport of mass in the stratosphere. The residual circulation is estimated from heating rates computed with a radiative transfer model using data from the Upper Atmosphere Research Satellite (UARS) as inputs. An annual cycle exists in the resulting circulation in the lower stratosphere, with a larger net upward mass flux across a pressure surface in the tropics during northern hemisphere winter than during northern hemisphere summer. The annual cycle in upward tropical mass flux follows the annual cycle in downward mass flux across a pressure surface in the northern hemisphere extratropics. It is argued that the annual cycle in zonal momentum forcing in the northern hemisphere stratosphere is controlling mass flux across a pressure surface in the lower stratosphere both in the tropics and in the northern hemisphere extratropics. 40 refs., 17 figs., 5 tabs.

Seasonal Variation of Mass Transport Across the Tropopause

The annual cycle of the net mass transport across the extratropical tropopause is examined. Contributions from both the global-scale meridional circulation and the mass variation of the lowermost stratosphere are included. For the northern hemisphere the mass of the lowermost stratosphere has a distinct annual cycle, whereas for the southern hemisphere, the corresponding variation is weak. The net mass transport across the tropopause in the northern hemisphere has a maximum in late spring and a distinct minimum in autumn. This variation and its magnitude compare well with older estimates based on representative 90Sr mixing ratios. For the southern hemisphere the seasonal cycle of the net mass transport is weaker and follows roughly the annual variation of the net mass flux across a nearby isentropic surface.

Stratospheric water vapor increases over the past half‐century

Ten data sets covering the period 1954–2000 are analyzed to show a 1%/yr increase in stratospheric water vapor. The trend has persisted for at least 45 years, hence is unlikely the result of a single event, but rather indicative of long-term climate change. A long-term change in the transport of water vapor into the stratosphere is the most probable cause.

Hemispheric asymmetries in water vapor and inferences about transport in the lower stratosphere

Both satellite water vapor measurements and in situ aircraft measurements indicate that the southern hemisphere lower stratosphere is drier than that of the northern hemisphere in an annual average sense. This is the result of a combination of factors. At latitudes poleward of ∼50°S, dehydration in the Antarctic polar vortex lowers water vapor mixing ratios relative to those in the north during late winter and spring. Equatorward of ∼50°S, water vapor in the lower stratosphere is largely controlled by the tropical seasonal cycle in water vapor coupled with the seasonal cycle in extratropical descent. During the tropical moist period (June, July, and August), air ascending in the Indian monsoon region influences the northern hemisphere more than the southern hemisphere, resulting in a moister northern hemisphere lower stratosphere. This tropical influence is confined to levels beneath 60 mbar at low latitudes, and beneath 90 mbar at high latitudes. During the tropical dry period (December, January, and February), dry air spreads initially into both hemispheres. However, the stronger northern hemisphere wintertime descent that exists relative to that of southern hemisphere summer transports the dry air out of the northern hemisphere lower stratosphere more quickly than in the south. This same hemispheric asymmetry in winter descent (greater descent rates during northern hemisphere winter than during southern hemisphere winter) brings down a greater quantity of “older” higher water vapor content air in the north, which also acts to moisten the northern hemisphere lower stratosphere relative to the southern hemisphere. These factors all act together to produce a drier southern hemisphere lower stratosphere as compared to that in the north. The overall picture that comes from this study in regards to transport characteristics is that the stratosphere can be divided into three regions. These are (1) the “overworld” where mass transport is controlled by nonlocal dynamical processes, (2) the “tropically controlled transition region” made up of relatively young air that has passed through (and been dehydrated by) the cold tropical tropopause, and (3) the stratospheric part of the “middleworld” or “lowermost stratosphere”, where troposphere-stratosphere exchange can occur adiabatically. Satellite water vapor measurements indicate that the base of the “overworld” is near 60 mbar in the tropics, or near the 450 K isentropic surface.

Interannual changes of stratospheric water vapor and correlations with tropical tropopause temperatures

Interannual variations of stratospheric water vapor over 1992‐2003 are studied using Halogen Occultation Experiment (HALOE) satellite measurements. Interannual anomalies in water vapor with an approximate 2-yr periodicity are evident near the tropical tropopause, and these propagate vertically and latitudinally with the mean stratospheric transport circulation (in a manner analogous to the seasonal ‘‘tape recorder’’). Unusually low water vapor anomalies are observed in the lower stratosphere for 2001‐03. These interannual anomalies are also observed in Arctic lower-stratospheric water vapor measurements by the Polar Ozone and Aerosol Measurement (POAM) satellite instrument during 1998‐2003. Comparisons of the HALOE data with balloon measurements of lower-stratospheric water vapor at Boulder, Colorado (408N), show partial agreement for seasonal and interannual changes during 1992‐2002, but decadal increases observed in the balloon measurements for this period are not observed in HALOE data. Interannual changes in HALOE water vapor are well correlated with anomalies in tropical tropopause temperatures. The approximate 2-yr periodicity is attributable to tropopause

Estimates of the stratospheric residual circulation using the downward control principle

The transformed Eulerian-mean momentum and continuity equations are used to calculate the residual mean meridional circulations for the lower stratosphere and troposphere. Momentum and temperature fluxes required for the computation are estimated from U.K. Meteorological Office (UKMO) analyzed geopotential heights. National Center for Atmospheric Research (NCAR) CCM2 model output is used to assess the errors associated with the calculation. The model comparisons showed that the method works reasonably well for solstice seasons, but is inadequate for equinox seasons. In addition, it is found that some parameterization of gravity wave drag needs to be included with the planetary wave forcing to accurately estimate the residual mean circulation using this method.

A Multidiagnostic Intercomparison of Tropical-Width Time Series Using Reanalyses and Satellite Observations

Polewardmigrationofthe latitudinaledgeofthe tropicsof0.258‐3.08decade 21 hasbeenreportedin several recent studies basedon satellite andradiosondedata and reanalysis output coveringthe past ;30 yr. The goal ofthis paperistoidentifythe extenttowhichthis largerangeoftrendscanbeexplainedbytheuse ofdifferent datasources,timeperiods,andedgedefinitions,aswellashowthewideningvariesasafunctionofhemisphere and season. Toward this end, a suite of tropical edge latitude diagnostics based on tropopause height, winds, precipitation‐evaporation, and outgoing longwave radiation (OLR) are analyzed using several reanalyses and satellite datasets. These diagnostics include both previously used definitionsand new definitions designed for more robust detection. The wide range of widening trends is shown to be primarily due to the use of different datasets and edge definitions and only secondarily due to varying start‐end dates. This study also shows that the large trends (.;18 decade 21 ) previously reported in tropopause and OLR diagnostics are due to theuse of subjectivedefinitions basedon absolutethresholds.Statistically significant Hadleycell expansion based on the mean meridional streamfunction of 1.08‐1.58 decade 21 is found in three of four reanalyses that cover the full time period (1979‐2009), whereas other diagnostics yield trends of 20.58‐0.88 decade 21 that are mostly insignificant. There are indications of hemispheric and seasonal differences in the trends, but the differences are not statistically significant.

Stratospheric water vapor feedback

Significance We show observational evidence for a stratospheric water vapor feedback—a warmer climate increases stratospheric water vapor, and because stratospheric water vapor is itself a greenhouse gas, this leads to further warming. An estimate of its magnitude from a climate model yields a value of +0.3 W/(m2⋅K), suggesting that this feedback plays an important role in our climate system. 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/(m2⋅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.

Mass fluxes of O3, CH4, N2O and CF2Cl2 in the lower stratosphere calculated from observational data

Trace gas mixing ratio data from the Upper Atmosphere Research Satellite (UARS) are combined with a radiatively derived transformed Eulerian mean (TEM) stream function to estimate the net ozone (O 3 ) mass flux from the stratosphere to the troposphere and to investigate the net fluxes of methane (CH 4 ), nitrous oxide (N 2 O), and CFC-12 (CF 2 Cl 2 ) into the lower stratosphere. The mass fluxes of ozone and long-lived tracers such as nitrous oxide, methane, and CFC-12 in the lower stratosphere can be estimated from data obtained by the cryogenic limb array etalon spectrometer (CLAES), the microwave limb sounder (MLS) for ozone, and the Halogen Occultation Experiment (HALOE) for ozone and methane. The flux of ozone from the stratosphere to the troposphere is estimated to be 510 Tg of ozone per year with a range of 450 to 590 Tg O 3 /yr. Net fluxes into the stratosphere are estimated as 27 Tg of methane per year, 8 Tg/yr of N in nitrous oxide, and 0.08 Tg/yr of CF 2 Cl 2 . There is good agreement with previous estimates within the range of uncertainty of these calculations.