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JoséP. Peixoto
and
Abraham H. Oort

Abstract

The present paper deals with the analysis of the time-average relative humidity fields in the atmosphere. Twice-daily estimates of relative humidity are used.

After some theoretical considerations on the relations between relative humidity, other moisture parameters, and temperature, a critical analysis of the various sources of data is made considering their possible limitations. Various methods of computing relative humidity are formulated and discussed.

The global distribution of relative humidity at various levels shows that it is not zonally uniform with centers of various intensities at all latitudes. The global maps show maxima in the equatorial zone and minima in the dry subtropical belts around 30°N and 30°S. The land–sea contrast and variations related to the orographic relief are also apparent. The general pattern of relative humidity is similar at all levels but its magnitude decreases with altitude. The seasonal analyses show a similar pattern as the annual analyses but are slightly shifted toward the summer pole.

The saturation deficit is also evaluated. Cross sections of the saturation deficit show that the maxima are found in the middle to lower troposphere at subtropical latitudes, being most intense in the Northern Hemisphere during the summer season.

The temporal variability of the relative humidity due to transient eddies exhibits a bimodal structure with maxima in the midlatitudes of each hemisphere around 700 mb. The stationary eddy distributions are less pronounced than the transient ones and do not change substantially from one season to another.

To validate our results, several comparisons are made using independent sources of humidity data as well as cloud distributions at various levels. Thus, COADS data are used to obtain independent analyses of the surface relative humidity over the oceans, and satellite observations by SAGE are used at the 300-mb level. The rawinsonde-SAGE differences are on the order of 10% in the Tropics and 20% in the high latitudes, due in part to a clear-sky (dry) bias in the SAGE data. Our results are further compared with those obtained from operational analyses by the ECMWF. The differences do not exceed 5% in the Tropics but tend to be larger in the tropical upper troposphere and at all levels in the extratropies of the Southern Hemisphere, where the radiosonde network is quite sparse. In view of the obvious connections between the moisture distribution in the atmosphere and cloudiness, a cloud climatology is used to cheek its consistency with the present results. The latitudinal and interseasonal variations of cloudiness and relative humidity are similar, with maxima in the equatorial belt and at high latitudes and minima in the subtropics that shift poleward during summer and equatorward during winter.

Finally, some comments are made on the radiosonde-observing systems in the light of recent satellite studies of humidity. Mainly at the upper levels systematic localized differences are found between electrical hygristor and organic sensors, but the differences almost disappear in the middle and lower troposphere.

In spite of the shortcomings, limitations, and errors of the radiosonde network, the present analyses describe for the first time the large-scale, three-dimensional characteristics of the relative humidity in the global atmosphere.

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Richard D. Rosen
,
David A. Salstein
, and
JoséP. Peixoto

Abstract

Maps of the streamfunction field for the vertically integrated flux of water vapor over the Northern Hemisphere are presented for each of six annual periods. This approach reveals the existence of a broad-scale cellular pattern in the flow that is not so readily apparent from the more conventional analyses of its zonal and meridional components. In addition, changes in the moisture flow from one year to the next are clearly pictured, a particular good example of which is provided by the behavior of the streamfunction over the Pacific during the period considered.

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Richard D. Rosen
,
David A. Salstein
, and
JoséP. Peixoto

Abstract

Annual mean holds of the zonal and meridional components of the vertically integrated water vapor flux in the atmosphere have been computed objectively from six years of Northern Hemispheric upper air data. Emphasis is placed on the longitudinal structure of these fields in order to identify regional contributions to the annual variations of the zonally averaged moisture fluxes. These temporal changes, particularly those associated with the meridional transport of latent heat. appear large enough to impact significantly on the maintenance of the atmospheric energy balance. While the distribution and quality of some upper air data make it difficult to estimate the degree to which the analyses precisely duplicate atmospheric behavior everywhere, this study provides documentation of a wide range of interannual variability. In this regard, the behavior of one of the years chosen for study contrasts sharply with that of the rest of the sample, as was similarly noted for other quantities evaluated by Rosen et at. (1976).

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Richard D. Rosen
,
David A. Salstein
,
JoséP. Peixoto
,
Abraham H. Oort
, and
Ngar-Cheung Lau

Abstract

A number of Northern Hemisphere circulation fields and statistics are derived for the months of January and June 1979 from level III-b analyses produced by GFDL using a 4-dimensional data assimilation scheme which incorporates measurements from a wide variety of sources. In particular, hemispheric maps and zonal cross sections of the wind, specific humidity, and the eddy fluxes of momentum, heat and moisture are examined. Certain quantities related to the atmosphere's energy cycle are also considered. These fields and statistics are compared with those derived from analyses that rely solely on the conventional rawinsonde station data taken during the same months. In the case of the monthly mean zonal and meridional winds, we also present results based on the level III-b analyses of the ECMWF.

The station-based analyses yield zonal mean statistics and hemispheric integrals that are generally comparable to those from the level III-b analyses. For example, the intensity of the Northern Hemisphere Hadley cell in January produced by the station analyses lies between those of the III-b analyses, which differ by as much as 35%. On regional scales, however, there are some large differences in the circulation fields between the station-based and level III-b analyses over areas of sparse station coverage. For example, the station-based analysis of the 200 mb field of transient eddy momentum flux in January does not include a significant region of northward flux over the northeast Pacific that is contained in the GFDL analysis. It is not yet clear, though, to what extent model biases may be affecting the GFDL analysis in this or in other station-sparse areas. In the case of the subtropical Pacific jet in January, the station-based analysis appears to underestimate its extent, but there are also considerable differences between the two level III-b analyses in this region. In addition, the GFDL analyses often appear to be noisy. Improvements in the level III-b analyses need to be made before full confidence can be placed in results based on modern data assimilation techniques.

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David S. Gutzler
,
Richard D. Rosen
,
David A. Salstein
, and
JoséP. Peixoto

Abstract

Interannual fluctuations of observed winter seasonal mean 850 mb temperatures over the Northern Hemisphere during 1958–73 are documented and compared with midtropospheric height variations. Interannual temperature variance maxima are found over the Eurasian and North American continents, in striking contrast to the height field which exhibits variance maxima over the midlatitude oceans. Patterns of interannual variability are defined objectively using eigenvector analysis. The first two spatial eigenvectors of temperature variability describe hemisphere-scale patterns. The gravest eigenvector contains elements of the Eurasian and Pacific–North American (PNA) height patterns defined in earlier studies. One-point correlation maps confirm the strong positive correlation between temperature fluctuations over Siberia and western Canada found in the first eigenvector but indicate that other elements of intercontinentality are not so strong as the eigenvectors suggest.

To isolate more regionalized patterns of variability, therefore, the leading temperature eigenvectors are subjected to varimax rotation. The leading rotated pattern contains North American centers coincident with the PNA height pattern and an additional Caribbean center, and its temporal fluctuations are highly correlated with a PNA index derived from 500 mb height anomalies. Over the Asian continent, two temperature patterns are found that incorporate north–south anomaly dipoles not clearly depicted in height patterns. One of these patterns also describes the tendency for positive correlation between temperatures over the Gulf of Alaska and Siberia. Another pattern contains two centers over Europe and a broad center across western and central Asia. A fifth rotated pattern describes temperature fluctuations associated with the North Atlantic Oscillation in the sea level pressure field.

Time series associated with patterns containing centers over Siberia and northwest North America are also correlated with interannual fluctuations of hemisphere-averaged temperature. In particular, temporal fluctuations of the PNA height pattern or the leading rotated temperature pattern are significantly correlated with hemisphere-mean seasonal temperature anomalies at 850 mb and at the surface.

The roles of anomalous temperature advection and transient eddy heat flux divergence in the heat balance of seasonal temperature anomalies are examined using correlation statistics. Over much of the hemisphere, seasonal anomalies of temperature advection appear to maintain seasonal temperature anomalies, while eddy heat flux divergence anomalies tend to dissipate them. However, advection anomalies are poorly correlated with temperature anomalies over the Asian continent west of about 90°E, so another term in the heat balance must play a dominant role. We speculate that radiative forcing due to snow cover anomalies may be important in this region.

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