| Abstract: We present a comparison of the zonal
mean meridional circulations derived from monthly in-situ data (i.e., radiosondes
and ship reports) and from the NCEP/NCAR reanalysis product. To facilitate
the interpretation of the results, a third estimate of the mean meridional
circulation is produced by subsampling the reanalysis at the locations
where radiosonde and surface ship data are available for the in-situ calculation.
This third estimate, known as the subsampled estimate, is compared to the
complete reanalysis estimate to assess biases in conventional, in situ
estimates of the Hadley circulation associated with the sparseness of the
data sources (i.e., radiosonde network). The subsampled estimate is also
compared to the in-situ estimate to assess the biases introduced into the
reanalysis product by the numerical model, initialization process and/or
indirect data sources such as satellite retrievals. The comparisons suggest
that a number of qualitative differences between the in- situ and reanalysis
estimates are mainly associated with the sparse sampling and simplified
interpolation schemes associated with in-situ estimates. These differences
include: (1) a southern Hadley cell that consistently extends up to 200
hPa in the reanalysis, whereas the bulk of the circulation for the in-situ
and subsampled estimates tends to be confined to the lower half of the
troposphere, (2) more well-defined and consistent poleward limits of the
Hadley cells in the reanalysis compared to the in-situ and subsampled estimates,
and (3) considerably less variability in magnitude and latitudinal extent
of the Ferrel cells and southern polar cell exhibited in the reanalysis
estimate compared to the in-situ and subsampled estimates. Quantitative
comparison shows that the subsampled estimate, relative to the reanalysis
estimate, produces a stronger northern Hadley cell (\sim20%), a weaker
southern Hadley Cell (\sim20-60%), and weaker Ferrel cells in both hemispheres.
These differences stem from poorly measured oceanic regions which necessitate
significant interpolation over broad regions. Moreover, they help to pinpoint
specific shortcomings in the present and previous in-situ estimates of
the Hadley circulation. Comparions between the subsampled and in situ estimates
suggest that the subsampled estimate produces a slightly stronger Hadley
circulation in both hemispheres, with the relative differences in some
seasons as large as 20-30%. These differences suggest that the mean meridional
circulation associated with the NCEP/NCAR reanalysis is more energetic
than observations suggest. Examination of ENSO-related changes to the Hadley
circulation suggest that the in-situ and subsampled estimates significantly
overestimate the effects of ENSO on the Hadley circulation due to the reliance
on sparsely distributed data. While all three estimates capture the large-scale
region of low-level equatorial convergence near the dateline that occurs
during El Niño, the in-situ and subsampled estimates fail to effectively
reproduce the large-scale areas of equatorial mass divergence to the west
and east of this convergence area, leading to an overestimate of the effects
of ENSO on the zonal mean circulation. |