Celeste M. Johanson

Hadley Cell Widening: Model Simulations versus Observations

Observations show that the Hadley cell has widened by about 28‐58 since 1979. This widening and the concomitant poleward displacement of the subtropical dry zones may be accompanied by large-scale drying near308Nand308S.Suchdryingposesarisktoinhabitantsoftheseregionswhoareaccustomedtoestablished rainfall patterns. Simple and comprehensive general circulation models (GCMs) indicate that the Hadley cell may widen in response to global warming, warming of the west Pacific, or polar stratospheric cooling. The combinationofthesefactorsmayberesponsiblefortherecentobservations.Butthereisnostudysofarthathas compared the observed widening to GCM simulations of twentieth-century climate integrated with historical changes in forcings. Here the Hadley cell widening is assessed in current GCMs from historical simulations of thetwentiethcenturyaswellasfutureclimateprojectionsandpreindustrialcontrolruns.Theauthorsfindthat observedwideningcannotbeexplainedbynaturalvariability.Thisobservedwideningisalsosignificantlylarger than in simulations of the twentieth and twenty-first centuries. These results illustrate the need for further investigation into the discrepancy between the observed and simulated widening of the Hadley cell.

Contribution of stratospheric cooling to satellite-inferred tropospheric temperature trends.

From 1979 to 2001, temperatures observed globally by the mid-tropospheric channel of the satellite-borne Microwave Sounding Unit (MSU channel 2), as well as the inferred temperatures in the lower troposphere, show only small warming trends of less than 0.1 K per decade (refs 1–3). Surface temperatures based on in situ observations however, exhibit a larger warming of ∼0.17 K per decade (refs 4, 5), and global climate models forced by combined anthropogenic and natural factors project an increase in tropospheric temperatures that is somewhat larger than the surface temperature increase. Here we show that trends in MSU channel 2 temperatures are weak because the instrument partly records stratospheric temperatures whose large cooling trend offsets the contributions of tropospheric warming. We quantify the stratospheric contribution to MSU channel 2 temperatures using MSU channel 4, which records only stratospheric temperatures. The resulting trend of reconstructed tropospheric temperatures from satellite data is physically consistent with the observed surface temperature trend. For the tropics, the tropospheric warming is ∼1.6 times the surface warming, as expected for a moist adiabatic lapse rate.

Simulated versus observed patterns of warming over the extratropical Northern Hemisphere continents during the cold season

A suite of the historical simulations run with the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) models forced by greenhouse gases, aerosols, stratospheric ozone depletion, and volcanic eruptions and a second suite of simulations forced by increasing CO2 concentrations alone are compared with observations for the reference interval 1965–2000. Surface air temperature trends are disaggregated by boreal cold (November-April) versus warm (May-October) seasons and by high latitude northern (N: 40°–90 °N) versus southern (S: 60 °S–40 °N) domains. A dynamical adjustment is applied to remove the component of the cold-season surface air temperature trends (over land areas poleward of 40 °N) that are attributable to changing atmospheric circulation patterns. The model simulations do not simulate the full extent of the wintertime warming over the high-latitude Northern Hemisphere continents during the later 20th century, much of which was dynamically induced. Expressed as fractions of the concurrent trend in global-mean sea surface temperature, the relative magnitude of the dynamically induced wintertime warming over domain N in the observations, the simulations with multiple forcings, and the runs forced by the buildup of greenhouse gases only is 7∶2∶1, and roughly comparable to the relative magnitude of the concurrent sea-level pressure trends. These results support the notion that the enhanced wintertime warming over high northern latitudes from 1965 to 2000 was mainly a reflection of unforced variability of the coupled climate system. Some of the simulations exhibit an enhancement of the warming along the Arctic coast, suggestive of exaggerated feedbacks.

Stratospheric Influences on MSU-Derived Tropospheric Temperature Trends: A Direct Error Analysis

Abstract Retrievals of tropospheric temperature trends from data of the Microwave Sounding Unit (MSU) are subject to biases related to the strong cooling of the stratosphere during the past few decades. The magnitude of this stratospheric contamination in various retrievals is estimated using stratospheric temperature trend profiles based on observations. It is found that from 1979 to 2001 the stratospheric contribution to the trend of MSU channel-2 brightness temperature is about −0.08 K decade−1, which is consistent with the findings of Fu et al. In the retrieval method developed by Fu et al. based on a linear combination of MSU channels 2 and 4, the stratospheric influence is largely removed, leaving a residual influence of less than ±0.01 K decade−1. This method is also found to be more accurate than the angular scanning retrieval technique of Spencer and Christy to remove the stratospheric contamination.

Atmospheric science: Stratospheric cooling and the troposphere (reply)

The success of our method for reconstructing tropospheric temperature trends is reinforced by Gillett et al., who show that our method is robust for reconstructing the tropospheric temperature trends, and that the statistical relationships between our T4, T2 and T850–300 estimates are in agreement with those independently derived from climate-model output. But Tett and Thorne use different data sets in the tropical region and suggest that our approach produces tropospheric temperature trends that are biased to warm and that it overfits the data. We argue that the differences in tropical tropospheric temperature trends between our estimate of T (Tfjws) and the T850–300 of Tett and Thorne do not invalidate our method. We also question their interpretation of the comparison between global climate model results and satellite observations.