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A. Raval, A. H. Oort, and V. Ramaswamy

Abstract

The authors have empirically examined the dependence of the outgoing longwave radiation (OLR) on sea surface temperature (T s), precipitable water (W), and height-mean relative humidity (RH¯). The OLR is obtained from 4 yr of data from the Earth Radiation Budget Experiment (ERBE), while T s, W, and RH¯ are obtained from objective analyses of rawinsonde and ship data. It is found that in the midlatitudes, the surface temperature explains over 80% of the variability in the clear-sky OLR (F cs) and almost half of the variability in the total OLR (F tot). It fails badly in the tropics and subtropics, however, where (T s explains only about 20% of the variability in (F cs, and is largely decoupled from F tot. The two-dimensional contour plot of the OLR binned with respect to T s and RH¯ is marked by distinct changes in the gradient that are consistent with inferences from earlier investigations. For low values of T s (<10°C), the OLR depends mainly on T s. For values of T s above 10°C, the OLR depends increasingly on RH¯. Specifically, in the tropics (T s˜25°C), the total and clear-sky OLR depend significantly on both T s and RH¯. The well-known drop in OLR in the tropics with increasing T s correlates directly to an increase in RH¯, and not to changes in T s. The authors suggest that the observed dependence of the OLR on T s and RH¯ be a minimum performance standard for climate models. This approach is illustrated by comparing the observed dependence with the results of a radiative transfer model and an R 15 general circulation model, and by discussing the strengths and limitations of using RH¯ to parameterize the OLR.

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E. O. Holopainen and A. H. Oort

Abstract

The vertically integrated atmospheric vorticity budget over the oceans offers, in principle, a possibility of determining the surface stress curl from upper wind data without the need to specify a relationship between the surface stress and surface wind. Results for the wind stress curl obtained by this vorticity method, using upper wind data for the period 1968–73, are compared with the recent stress-curl calculations by Hellerman from surface data.

The two completely independent methods give basically similar mean latitudinal distributions of the stress curl. In the midlatitudes of the Southern Hemisphere, where the transient eddies are the main mechanism of vorticity transfer. the two estimates of the basin-wide longitudinal averages of the stress curl do not deviate from each other. by ≳20%. However, in the Northern Hemisphere the agreement is less. This seemingly strange result appears to be due to the sensitivity of the vorticity method to errors in the estimates of vorticity advection by the standing waves.

It is concluded that for the time being the geographical pattern of the mean surface stress curl can, at least in the Northern Hemisphere, be estimated from surface data (using a drag formulation) more accurately than from upper wind data (using the vorticity method). Together the two methods offer a useful quality check for the upper air data.

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E. O. Holopainen and A. H. Oort

Abstract

The global distribution of the forcing of the time-mean flow due to large-scale, horizontal Reynolds stresses (uu, vv, uv) is determined from upper wind statistics for the period 1968–73. The role of this forcing in the maintenance of the vorticity and enstrophy of the time-mean flow is discussed.

The most striking effect of transient eddy stresses is the tendency to shift the subtropical maxima in the time-mean flow and the associated vorticity patterns poleward. However, significant longitudinal Variations in forcing occur, also. Calculations of the dominant terms in vorticity budgets of the North Pacific Low, the North Atlantic Low, and the Siberian High, which may he called the centers of action of winter-time circulation at sea level in the Northern Hemisphere, are presented. In all three cases, transient eddies are found to be important in maintaining the centers against the dissipative action of surface friction.

In terms of the enstrophy budget, the hemispheric and global-mean effects of transient eddies on the mean flow are small. both in December–February and June–August. In the Northern Hemisphere, where the results are most reliable, the eddies are weakly dissipative with a time scale on the order of several months.

When separating the time-mean flow into the contributions from the axisymmetric component and from the stationary disturbances, it is found that the transient eddy stresses tend to maintain the axisymmetric mean flow, but to weaken the stationary disturbances. There are significant latitudinal variations in the enstrophy forcing of the stationary disturbances. Thus eddy forcing is an important factor in maintaining the enstrophy of stationary disturbances in the extratropies, while it tends to destroy their enstrophy in the tropics.

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B. C. Carissimo, A. H. Oort, and T. H. Vonder Haar

Abstract

The poleward energy transports in the atmosphere–ocean system are estimated for the annual mean and the four seasons based on satellite measurements of the net radiation balance at the top of the atmosphere, atmospheric transports of energy at the north or south poles various types of corrections had to be made, so that the global balances are maintained. This also enabled us to estimate the uncertainties in the procedures used. The uncertainties found are similar to those reported by Hastenrath based on different satellite data sets but using the same correction method.

Finally, oceanic heat transports are computed for the annual mean and seasons. One of the crucial terms in the heat budget, the interseasonal storage of energy in the oceans, is estimated for three different layers 0–112, 0–552 and 0–550 m, enabling a further error estimate in the inferred oceanic heat transports.

The present results confirm the presence of a strong annual cycle in the transport of energy by the oceans.

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W. A. Oost, E. H. W. Worrell, J. W. Schaap, C. van Oort, and C. Kraan

Abstract

An earlier paper introduced a new type of fast-responding vector wind meter, designed for adverse conditions, such as those that exist low over the sea surface. The most critical part of this instrument, its sensor head, was difficult to construct and was suitable for wind speeds above 8 m s−1 only. In this article a new type of sensor head is introduced that is simple to construct and extends the wind speed range of the instrument downward to less than 2 m s−1.

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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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