Abstract

The total direct transient eddy forcing of the vorticity balance in the upper troposphere during northern winter is examined using 11 years of 2 to 8-day bandpassed global analyses. Most examinations of the importance of high-frequency eddy vorticity fluxes to the maintenance of either the climatological standing waves or low-frequency anomalous flows have focused on only the horizontal blow or the rotational component of the horizontal transient flow. The latter simplification has been shown to be questionable for planetary scales. The vorticity flux by the transient divergent flow produces a forcing of the mean streamfunction that is of comparable magnitude with the equivalent rotational term. However, the streamfunction forcing by the sum of the transient vertical advection and twisting terms largely balances the fearing by the vorticity flux convergence associated with the divergent flow. The result is that the convergence of the eddy vorticity flux by the total transient flow is not a good approximation to the total forcing of the long-term mean streamfunction by the high-frequency eddies. These results are quantified for the mean northern winter season November–March 1980/81–1990/91.

The respective roles of each transient eddy term in the vorticity equation in maintaining two large-scale, low-frequency anomalous flows are also examined. One case involves a pronounced circulation anomaly that persisted for more than a decade over the North Pacific, and the second case relates to the maintenance of extremes of the North Atlantic Oscillation. In both cases, transient vorticity fluxes systematically reinforce and help to maintain the upper-tropospheric streamfunction anomalies. Just as for the climatological standing waves, a consideration of the total transient eddy forcing of the mean anomalous streamfunction yields a different interpretation than if only the convergence of the vorticity flux is considered.

A comprehensive comparison is made among four sea surface temperature (SST) datasets: the optimum interpolation (OI) and the empirical orthogonal function reconstructed SST analyses from the National Centers for Environmental Prediction (NCEP), the Global Sea-Ice and SST dataset (GISST, version 2.3b) from the United Kingdom Meteorological Office, and the optimal smoothing SST analysis from the Lamont-Doherty Earth Observatory (LDEO). Significant differences exist between the GISST and NCEP 1961–90 SST climatologies, especially in the marginal sea-ice zones and in regions of important small-scale features, such as the Gulf Stream, which are better resolved by the NCEP product. Significant differences also exist in the SST anomalies that relate strongly to the number of in situ observations available. In recent years, correlations between monthly anomalies are less than 0.75 south of about 10°N and are lower still over the southern oceans and parts of the tropical Pacific where root-mean-square differences exceed 0.6°C.

While adequate for many purposes, the SST datasets all contain problems of one sort or another. Noise is evident in the GISST data and realistic temporal persistence of SST anomalies after 1981 is lacking. Trends in recent years are quite different between the GISST and NCEP analyses, and this can be partially traced to differences in the processing of in situ data and an increasing cold bias in the NCEP 01 data arising from incompletely corrected satellite data. Significant discrepancies also exist in centennial trends from the LDEO and GISST datasets, and these likely reflect the separate treatment of the very low frequency signal in the GISST analysis and questionable assumptions about the stationarity of statistics in the LDEO method.

Ensembles of integrations with an atmospheric general circulation model (AGCM) are used with three of the SST datasets as lower boundary conditions to show that the differences among them imply physically important differences in the atmospheric circulation. Over the Tropics, where masking by internal atmospheric variability is small, SST differences affect moist convection and systematically produce strong responses in the local divergent circulation. A case study shows that analyzed SST differences in the tropical Pacific can be as large as for a moderate El Niño. Such large discrepancies induce local rainfall anomalies up to 8 mm day⁻¹ and, in addition to the tropical circulation anomalies, are associated with global teleconnections that influence temperatures and precipitation around the world. Results also show the limitations to using AGCMs when forced by specified SSTs.

The likely sources of the problems evident in the different SST products are identified and discussed. Several of the problems are being addressed by current efforts to reprocess the SST data, which is strongly recommended, but remaining problems demand further attention and attempts to resolve them should continue. The choice among SST analyses used for AGCM simulations, for the atmospheric reanalysis projects, for identifying climate signals, and for monitoring climate is important, as known flaws in the global analyses can compromise the results.

Abstract

A chronic difficulty in obtaining reliable climate records from satellites has been changes in instruments, platforms, equator-crossing times, and algorithms. The microwave sounding unit (MSU) tropospheric temperature record has overcome some of these problems, but evidence is presented that it too contains unreliable trends over a 17-yr period (1979–95) because of transitions involving different satellites and complications arising from nonatmospheric signals associated with the surface. The two primary MSU measures of tropospheric temperature contain different error characteristics and trends. The MSU channel 2 record exhibits a slight warming trend since 1979. Its broad vertical weighting function means that the temperature signal originates from throughout the troposphere and part of the lower stratosphere; intersatellite comparisons reveal low noise levels. Off-nadir channel 2 data are combined to provide an adjusted weighting function (called MSU 2R) without the stratospheric signal, but at a cost of an increased influence of surface emissions. Land surface microwave emissions, which account for about 20% of the total signal, depend on ground temperature and soil moisture and are subject to large variations associated with the diurnal cycle. The result is that MSU 2R noise levels are a factor of 3 larger than for MSU 2 and are sufficient to corrupt trends when several satellite records are merged.

After allowing for physical differences between the satellite and surface records, large differences remain in temperature trends over the Tropics where there is a strong and deterministic coupling with the surface. The authors use linear regression with observed sea surface temperatures (SSTs) and an atmospheric general circulation model to relate the tropical MSU and surface datasets. These and alternative analyses of the MSU data, radiosonde data, and comparisons between the MSU 2R and channel 2 records, with estimates of their noise, are used to show that the downward trend in tropical MSU 2R temperatures is very likely spurious. Tropical radiosonde records are of limited use in resolving the discrepancies because of artificial trends arising from changes in instruments or sensors;however, comparisons with Australian radiosondes show a spurious downward jump in MSU 2R in mid-1991, which is not evident in MSU 2. Evaluation of reanalyzed tropical temperatures from the National Centers for Environmental Prediction and the European Centre for Medium-Range Weather Forecasts shows that they contain very different and false trends, as the analyses are only as good as the input database.

Statistical analysis of the MSU 2R record objectively identifies two stepwise downward discontinuities that coincide with satellite transitions. The first is in mid-1981, prior to which only one satellite was in operation for much of the time so the diurnal cycle was not well sampled. Tropical SST anomalies over these years were small, in agreement with the Southern Oscillation index, yet the MSU 2R values were anomalously warm by ∼0.25°C. The second transition from NOAA-10 to NOAA-12 in mid-1991 did not involve an overlap except with NOAA-11, which suffered from a large drift in its equator-crossing times. MSU 2R anomalies have remained anomalously cold since mid-1991 by ∼0.1°C. Adding the two stepwise discontinuities to the tropical MSU 2R record allows it to be completely reconciled with the SST record within expected noise levels. The statistical results also make physical sense as the tropical satellite anomalies are magnified relative to SST anomalies by a factor of ∼1.3, which is the amplification expected following the saturated adiabatic lapse rate to the level of the peak weighting function of MSU 2R.

Abstract

A modified set of FGGE Level III-b gridpoint analyses, originally produced by ECMWF, is used to diagnose the eddy energy budgets of four equal-area regions within the tropical Southern Hemisphere (0–30°S) during the SOP-1 period of 10–27 January 1979. Each region is approximately centered on a wave axis of maximum warm, rising air. Three of the four wave axes are tied to the continents of Africa, Australia, and South America, while the fourth coincides extremely well with the South Pacific Convergence Zone (SPCZ). Daily variations of the energy conversions are examined. In addition, time-averaged results of energy contents, conversions and boundary transports are compiled for a 15–day period, 10–24 January, when the, SPCZ was most active.

Results show that the eddy kinetic energy (KE) exceeds the eddy available potential energy (AE) in all four regions, with that in the SPCZ being the largest. Of the conversion and boundary flux terms, only the conversion of AE to KE is significant. Again, the region containing the largest value is the SPCZ. The main flow of energy in each region appears to consist of a generation of AE by diabatic heating, a conversion of AE to KE by thermally-direct eddy circulations, and a dissipation of KE.

The relationships among the four subareas are investigated, primarily through evaluations of the boundary fluxes of KE. Results indicate that the only significant transport between regions is a flow of KE from the SPCZ region into the South American region. Thus, it appears likely that some of the KE from the SPCZ is helping to maintain the KE of the South American region and, in particular, the South Atlantic Convergence Zone (SACZ). These results seem to be in good agreement with the modeling results produced by the NASA Goddard Laboratory for Atmospheres (GLA) General Circulation Model.

Abstract

The atmospheric state simulated by the National Center for Atmospheric Research (NCAR) Community Climate Model, version 3 (CCM3), is compared to that simulated by the NCAR Climate System Model, version 1 (CSM1). CCM3 is an atmospheric general circulation model that uses specified sea surface temperatures (SSTs) for a lower boundary condition. Observed monthly mean SSTs for 1979–93 were used in the present study. CSM1 is a coupled general circulation model in which the SSTs are determined as part of the simulation and CCM3 serves as the atmospheric component. It is found that the differences between CCM3 and CSM1 are quite small in most measures of the atmospheric circulation, consistent with the accurate and drift-free simulation of the SSTs in the coupled model. There are substantial temperature differences near the surface in the Arctic and over the ocean around Antarctica, resulting from different sea-ice distributions. The tropical precipitation also has significant differences, although neither simulation is clearly better and the errors in the two simulations tend to have opposite signs with respect to observations. In response to the change in latent heat release the tropical divergent circulation changes significantly. Middle- and high-latitude circulation changes are modest, occurring mostly in winter in association with the sea-ice changes.

Abstract

The extent to which divergent circulations, induced by tropical heating, help to maintain westerly maxima in the Southern Hemisphere subtropics during the SOP-1 of FGGE is explored using Level Bib analyses from the Goddard Laboratory for Atmospheres (GLA). The contribution of the divergent wind component to the total ageostrophic flow in the subtropics is examined, as are the roles of other forcing terms in the localized Eliassen-Palm flux zonal momentum equation. In addition, the interaction of divergent and rotational flows in the subtropics is analyzed using the complete kinetic energy budget, partitioned into rotational and divergent components.

Throughout the summertime subtropics, it is generally found that the dominant term in the zonal momentum budget is the Coriolis force applied to the diabatically driven meridional circulation. The 1argest positive tendencies due to this term are found in the entrance regions of the subtropical westerly maxima, and divergent circulations account for nearly all of the total ageostrophic flow. In the SPCZ region, however, it is found that transient eddies play an important role by partially offsetting the strong Coriolis acceleration in the entrance region of the local jet and they help accelerate the westerly flow in the exit region through both barotropic and baroclinic processes. Energetically, the dominant term in the rotational kinetic energy budget throughout the subtropical belt is the conversion of divergent to rotational kinetic energy. Furthermore, nearly all of the generated divergent kinetic energy is converted. The evidence from all of these approaches supports the view that tropical beating in transient events drives or enhances local meridional overturning in the atmosphere, which, in turn, strengthens the summer subtropical westerly jet stream.

Abstract

Monthly mean brightness temperature anomalies derived from channel 2 of the microwave sounding units (M5Us) on board NOAA satellites over the past decade are examined and compared with both weighted and pressure-level ECMWF monthly mean temperatures for the 96 months of 1982–89. Very good agreement between the MSU and channel 2 weighted ECMWF anomalies is found over most of the globe with correlation coefficients over 0.9, but the agreement falls off over the tropics, the South Atlantic, and high latitudes of the Southern Hemisphere. The ECMWF analyses agree best with the MSU data in regions of good radiosonde coverage, while lower correlations are found in regions where the analyses depend more heavily on satellite data. Systematic errors introduced into the analyses by the retrieval techniques applied to the radiance data largely explain this apparent contradiction. Additionally, changes to the analysis-forecast system at ECMWF over the decade appear as apparent changes in climate, and these discontinuities most strongly affect the tropics and are evident in regions of fewer observations.

To the extent that the weighted ECMWF data data with the MSU brightness temperatures the vertical dependence of the MSU data can be examined. Correlations of the MSU data with ECMWF temperature anomalies at individual pressure levels are highest at 300 mb over the globe, a level apparently less affected by the frequent changes and improvements at ECMWF. Over regions of good data coverage, such as the Northern Hemisphere landmasses and Australia. the MSU anomalies correlate very highly with all levels of the troposphere up to 200 mb. Thus, the MSUs appear to be an extremely useful tool for measuring global tropospheric temperature fluctuations on a monthly and longer time scale.

Problems in the ECMWF temperature record since 1982 am examined in detail for the tropics. In September 1982 the introduction of diabatic nonlinear normal-mode initialization resulted in significant temperature increases in the tropical middle troposphere, especially at 500 mb. In May 1985 tropical temperatures at 700 mb (950 inb) increased (decreased) after the implementation of the T106 spectral model with major accompanying changes to physical parameterizations. Tropical temperatures near the tropopause decreased substantially after the May 1986 enhancement of the vertical revolution of the model from 16 to 19 levels with 3 new stratosphere levels. Problems at 1000 mb are present throughout the 96-month study period and are directly related to the manner in which analyzed temperatures are obtained at ECMWF. Comparisons with temperatures obtained from radiosonde stations in the tropics show that the ECMWF analyses have clearly improved with time, especially after May 1985 after which the 1000–200-mb temperatures show much greater coherence. These results show the importance of realizing the inherent problems with operationally based gridded datasets, and they strongly support the need for reanalysis of all data using a state-of-the-art four-dimensional data assimilation system.

Abstract

FGGE Level III-b analyses, produced by the Goddard Laboratory for Atmospheres, NASA, are used to investigate the relationship between tropical heating and subtropical westerly maxima in the Southern Hemisphere during SOP-1 (5 January-4 March 1979). The mean state of two 15-day periods is examined, as well as day-to-day variations for the entire 59-day period. In Period 1 (6–20 January), the central South Pacific was extremely active convectively, while in Period 2 (3–17 February), convective activity over the western Indian Ocean was enhanced. Episodes of strong outflow in the tropics, as measured by the upper tropospheric velocity potential, were found to be well correlated with the strengthening and propagation of westerly wind maxima in the subtropics. The average location of the westerly maximum over the South Pacific and Indian oceans oceans about 16° latitude south, and slightly east, of its corresponding heat source. For a cyclone case study which is presented, however, this distance was considerably less. The response time between the upper level tropical outflow and subtropical westerly enhancement appears to be less than 12 hours; however, an exact temporal scale was difficult to identify.

Abstract

A comparison of near-global monthly mean surface temperature anomalies to those of global Microwave Sounding Unit (MSU) 2R temperatures for 1979–95 reveals differences in global annual mean trends that are shown to be largely attributable to important physical differences in the quantities that are measured. Maps of standard deviations of the monthly mean anomalies, which can be viewed as mostly measuring the size of the climate signal, reveal pronounced differences regionally in each dataset. At the surface, the variability of temperatures is relatively small over the oceans but large over land, whereas in the MSU record the signal is much more zonally symmetric. The largest differences are found over the North Pacific and North Atlantic Oceans where the monthly standard deviations of the MSU temperatures are larger by more than a factor of 2. Locally over land, the variance of the surface record is larger than that of the MSU. In addition to differential responses to forcings from the El Niño-Southern Oscillation phenomenon and volcanic eruptions, these characteristics are indicative of differences of the response to physical processes arising from the relative importance of advection versus surface interactions and the different heat capacities of land and ocean. The result is that the regions contributing to hemispheric or global mean anomalies differ substantially between the two temperature datasets. This helps to account for the observed differences in decadal trends where the surface record shows a warming trend since 1979 of 0.18°C per decade, relative to the MSU record. While a common perception from this result is that the MSU and surface measurements of global temperature change are inconsistent, the issue should not be about which record is better, but rather that both give a different perspective on the same events.

Abstract

Climatological properties for selected aspects of the thermodynamic structure and hydrologic cycle are presented from a 15-yr numerical simulation conducted with the National Center for Atmospheric Research Community Climate Model, version 3 (CCM3), using an observed sea surface temperature climatology. In most regards, the simulated thermal structure and hydrologic cycle represent a marked improvement when compared with earlier versions of the CCM. Three major modifications to parameterized physics are primarily responsible for the more notable improvements in the simulation: modifications to the diagnosis of cloud optical properties, modifications to the diagnosis of boundary layer processes, and the incorporation of a penetrative formulation for deep cumulus convection. The various roles of these physical parameterization changes will be discussed in the context of the simulation strengths and weaknesses.