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Patrick F. Cummins, Lawrence A. Mysak, and Kevin Hamilton

Abstract

The Rossby wave field generated by the annual cycle of the observed wind stress curl over the North Pacific Ocean (15°N–53°, 100°W–175°E) has been obtained through numerical integration of the linearized, reduced-gravity vorticity equation in spherical coordinates. The dominant source region of Rossby waves is adjacent to the eastern boundary between 20°–44°N. More specifically this source is shown to be made up essentially of two distinct parts: a southern region off California-Baja California, which was first identified by White and Saur, and a northern region corresponding to a generation area first proposed by Mysak. In addition, a second, midocean generation region has been identified over the central North Pacific from 35° to 45°N, 150° to 160°W.

The behavior of the model is strongly affected by wave refraction due to the variation of phase velocity with latitude as described in Schopf et al. As waves emanate from the eastern boundary they are refracted such that the wavenumber vector, initially aligned zonally, becomes reoriented to the northwest. Associated with this is a turning of the group velocity vector and of wave rays towards the southwest.

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Kevin Hamilton, Albert Hertzog, François Vial, and Georgiy Stenchikov

Abstract

The longitudinal dependence of interannual variations of tropical stratospheric wind is examined in a detailed general circulation model simulation and in the limited observations available. A version of the SKYHI model is run with an imposed zonally symmetric zonal momentum source that forces the zonal-mean zonal wind evolution in the tropical stratosphere to be close to an estimate of the observed zonal wind based on radiosonde observations at Singapore during the period 1978–99. This amounts to a kind of simple assimilation model in which only the zonal-mean wind field in the tropical stratosphere is assimilated, and other quantities are allowed to vary freely. A total of five experiments were run, one covering the full 1978–99 period and four for 1989–99.

The results at and above about 30 hPa are fairly simple to characterize. When the zonal-mean wind near the equator at a particular level is easterly, the monthly mean wind has only very small zonal contrasts. When mean westerlies are present near the equator, significant zonal asymmetries occur at low latitudes, most notably easterly anomalies over South America and westerly anomalies in the eastern Pacific region. These anomalies generally display a continuous meridional phase propagation with the extratropical quasi-stationary eddy field in the winter hemisphere. The net result is a significantly weaker peak-to-peak amplitude of the quasi-biennial oscillation (QBO) in zonal wind over the South American sector than over the rest of the equatorial band. The zonal contrast in QBO amplitude near 10 hPa exceeds 10%.

In the lower stratosphere the zonal asymmetries in the prevailing wind are fairly small. Asymmetries seem to reflect the upward extension of the tropospheric Walker circulation, and are less strongly modulated by the quasi-biennial oscillation in zonal-mean circulation.

The model results were checked against limited station observations at Nairobi (1.3°S, 36.7°E), Singapore (1.4°N, 103.9°E), Rochambeau (4.8°N, 52.4°W), and Bogota (4.7°N, 74.1°W). Overall reasonable agreement was found between the monthly mean zonal winds in the model simulation and these station data. The low-latitude wind field in monthly mean NCEP gridded analyses was also examined. These analyses have some obviously unrealistic features in the tropical stratosphere, but some of the behavior seen in the SKYHI model simulations can be identified as well in the NCEP analyses.

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Chunxi Zhang, Yuqing Wang, Axel Lauer, and Kevin Hamilton

Abstract

The Weather Research and Forecasting (WRF) model V3.3 has been configured for the Hawaiian Islands as a regional climate model for the region (HRCM). This paper documents the model configuration and presents a preliminary evaluation based on a continuous 1-yr simulation forced by observed boundary conditions with 3-km horizontal grid spacing in the inner nested domain. The simulated vertical structure of the temperature and humidity are compared with twice-daily radiosonde observations at two stations. Generally the trade wind inversion (TWI) height and occurrence days are well represented. The simulation over the islands is compared with observations from nine surface climatological stations and a dense network of precipitation stations. The model simulation has generally small biases in the simulated surface temperature, relative humidity, and wind speed. The model realistically simulated the magnitude and geographical distribution of the mean rainfall over the Hawaiian Islands. In addition, the model simulation reproduced reasonably well the individual heavy rainfall events as seen from the time series of pentad mean rainfall averaged over island scales. Also the model reproduced the geographical variation of the mean diurnal rainfall cycle even though the observed diurnal cycle displays quite different features over different islands. Comparison with results obtained using the land surface dataset from the official release of the WRF model confirmed that the newly implemented land surface dataset generally improved the simulation of surface variables. These results demonstrate that the WRF can be a useful tool for dynamical downscaling of regional climate over the Hawaiian Islands.

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Kevin Hamilton, R. John Wilson, and Richard S. Hemler

Abstract

The large-scale circulation in the Geophysical Fluid Dynamics Laboratory “SKYHI” troposphere–stratosphere–mesosphere finite-difference general circulation model is examined as a function of vertical and horizontal resolution. The experiments examined include one with horizontal grid spacing of ∼35 km and another with ∼100 km horizontal grid spacing but very high vertical resolution (160 levels between the ground and about 85 km). The simulation of the middle-atmospheric zonal-mean winds and temperatures in the extratropics is found to be very sensitive to horizontal resolution. For example, in the early Southern Hemisphere winter the South Pole near 1 mb in the model is colder than observed, but the bias is reduced with improved horizontal resolution (from ∼70°C in a version with ∼300 km grid spacing to less than 10°C in the ∼35 km version). The extratropical simulation is found to be only slightly affected by enhancements of the vertical resolution. By contrast, the tropical middle-atmospheric simulation is extremely dependent on the vertical resolution employed. With level spacing in the lower stratosphere ∼1.5 km, the lower stratospheric zonal-mean zonal winds in the equatorial region are nearly constant in time. When the vertical resolution is doubled, the simulated stratospheric zonal winds exhibit a strong equatorially centered oscillation with downward propagation of the wind reversals and with formation of strong vertical shear layers. This appears to be a spontaneous internally generated oscillation and closely resembles the observed QBO in many respects, although the simulated oscillation has a period less than half that of the real QBO.

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Axel Lauer, Ralf Bennartz, Kevin Hamilton, and Yuqing Wang

Abstract

An important parameter often adjusted to achieve agreement between simulated and observed radiative fluxes in climate models is the rain formation efficiency. This adjustment has been justified as accounting for the effects of subgrid-scale variability in cloud properties, but this tuning approach is rather arbitrary. This study examines results from a regional climate model with precipitation formation schemes that have been conventionally tuned, and it compares them with simulations employing a recently developed scheme that uses satellite observations to explicitly account for the subgrid-scale variability of clouds (“integral constraint method”). Simulations with the International Pacific Research Center’s Regional Atmospheric Model (iRAM) show that the integral constraint method is capable of simulating cloud fields over the eastern Pacific that are in good agreement with observations, without requiring model tuning. A series of global warming simulations for late twenty-first-century conditions is performed to investigate the impact of the treatment of the precipitation formation efficiency on modeled cloud–climate feedbacks. The results with the integral constraint method show that the simulated cloud feedbacks have similar patterns at all the model resolutions considered (grid spacings of 50, 100, and 200 km), but there are some quantitative differences (with smaller feedbacks at finer resolution). The cloud responses to global warming in simulations with a conventionally tuned autoconversion scheme and with the integral constraint method were found to be quite consistent, although differences in individual regions of ~10%–30% are evident. No conclusions can be drawn from this study on the validity of model tuning for thick clouds and mixed phase or ice clouds, however.

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Chunxi Zhang, Yuqing Wang, Kevin Hamilton, and Axel Lauer

Abstract

A 20-yr simulation with a fine-resolution regional atmospheric model for projected late twenty-first-century conditions in Hawaii is presented. The pseudo-global-warming method is employed, and the boundary conditions are based on a multimodel mean of projections made with global coupled models run with a moderate greenhouse gas emissions scenario. Results show that surface air temperature over land increases ~2°–4°C with the greatest warming at the highest topographic heights. A modest tendency for the warming to be larger on the leeward sides of the major islands is also apparent. Climatological rainfall is projected to change up to ~25% at many locations. The currently wet windward sides of the major islands will have more clouds and receive more rainfall, while the currently dry leeward sides will generally have even less clouds and rainfall. The average trade wind inversion–base height and the mean marine boundary layer cloud heights are projected to exhibit only small changes. However, the frequency of days with clearly defined trade wind inversions is predicted to increase substantially (~83% to ~91%). The extreme rainfall events are projected to increase significantly. An analysis of the model’s moisture budget in the lower troposphere shows that the increased mean rainfall on the windward sides of the islands is largely attributable to increased boundary layer moisture in the warmer climate. Rainfall changes attributable to mean low-level circulation changes are modest in comparison.

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Kevin Hamilton, R. John Wilson, J. D. Mahlman, and L. J. Umscheid

Abstract

The long-term mean climatology obtained from integrations conducted with different resolutions of the GFDL “SKYHI” finite-difference general circulation model is examined. A number of improvements that have been made recently in the model are also described. The versions considered have 3° × 3.6°, 2° × 2.4°, and 1° × 1.2° latitude–longitude resolution, and in each case the model is run with 40 levels from the ground to 0.0096 mb. The integrations all employ a fixed climatological cycle of sea surface temperature. Over 25 years of integration with the 3° model and shorter integrations with the higher-resolution versions are analyzed. Attention is focused on the December–February and June–August periods.

The model does a reasonable job of representing the atmospheric flow in the troposphere and lower stratosphere. The simulated tropospheric climatology has an interesting sensitivity to horizontal resolution. In common with several spectral GCMs that have been examined earlier, the surface zonal-mean westerlies in the SKYHI extratropics become stronger with increasing horizontal resolution. However, this “zonalization” of the flow with resolution is not as prominent in the upper troposphere of SKYHI as it is in some spectral models. It is noteworthy that—without parameterized gravity wave drag—the SKYHI model at all three resolutions can simulate a realistic separation of the subtropical and polar night jet streams and a fairly realistic strength of the lower-stratospheric winter polar vortex.

The geographical distribution of the annual-mean and seasonal precipitation are reasonably well simulated. When compared against observations in an objective manner, the SKYHI global precipitation simulation is found to be as good or better than that obtained by other state-of-the-art general circulation models. However, some significant shortcomings remain, most notably in the summer extratropical land areas and in the tropical summer monsoon regions. The time-mean precipitation simulation is remarkably insensitive to the horizontal model resolution employed. The other tropospheric feature examined in detail is the tropopause temperature. The whole troposphere suffers from a cold bias of the order of a few degrees Celcius, but in the 3° SKYHI model this grows to about 6°C at 100 mb. Interestingly, the upper-tropospheric bias is reduced with increasing horizontal resolution, despite that the cloud parameters in the radiation code are specified identically in each version.

The simulated polar vortex in the Northern Hemisphere winter in the upper stratosphere is unrealistically confined to high latitudes, although the maximum zonal-mean zonal wind is close to observed values. Near the stratopause the June–August mean temperatures at the South Pole are colder than observations by ∼65°C, 50°C, and 30°C in the 3°, 2°, and 1° simulations, respectively. The corresponding zonal-mean zonal wind patterns display an unrealistically strong polar vortex. The extratropical stratosphere stationary wave field in the Northern Hemisphere winter is examined in some detail using the multiyear averages available from the 3° SKYHI integration. Comparison with comparable long-term mean observations suggests that the model captures the amplitude and phase of the stationary waves rather well.

The SKYHI model simulates the reversed equator-pole temperature gradient near the summer mesopause. The simulated summer polar mesopause temperatures decrease with increasing, horizontal resolution, although even at 1° resolution the predicted temperatures are still warmer than observed. The increasing resolution is accompanied by increased westerly driving of the mean flow in the summer mesosphere by dissipating gravity waves. The present results suggest that the SKYHI model does explicitly resolve a significant component of the gravity waves required to produce the observed summer mesopause structure.

The semiannual oscillation near the tropical stratopause is reasonably well simulated in the 3° version. The main deficiency is in the westerly phase, which is not as strong as observed. There is also a second peak in the amplitude of the semiannual wind oscillation at the top model level (0.0096 mb) corresponding to the observed mesopause semiannual oscillation. This simulated mesopause oscillation is weaker (by a factor of ∼3) than that observed. The simulation in the tropical stratopause and mesosphere changes quite significantly with increasing resolution, however. In the tropical lower stratosphere of the 3° model the zonal-mean zonal wind displays a very weak (∼3 m s−1 peak to peak) interannual variation, which-while rather irregular-does display a roughly biennial period and the downward phase propagation that is characteristic of the observed quasi-biennial oscillation.

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Yoshio Kawatani, Kevin Hamilton, Kaoru Sato, Timothy J. Dunkerton, Shingo Watanabe, and Kazuyoshi Kikuchi

Abstract

Observational studies have shown that, on average, the quasi-biennial oscillation (QBO) exhibits a faster phase progression and shorter period during El Niño than during La Niña. Here, the possible mechanism of QBO modulation associated with ENSO is investigated using the MIROC-AGCM with T106 (~1.125°) horizontal resolution. The MIROC-AGCM simulates QBO-like oscillations without any nonorographic gravity wave parameterizations. A 100-yr integration was conducted during which annually repeating sea surface temperatures based on the composite observed El Niño conditions were imposed. A similar 100-yr La Niña integration was also conducted. The MIROC-AGCM simulates realistic differences between El Niño and La Niña, notably shorter QBO periods, a weaker Walker circulation, and more equatorial precipitation during El Niño than during La Niña. Near the equator, vertical wave fluxes of zonal momentum in the uppermost troposphere are larger and the stratospheric QBO forcing due to interaction of the mean flow with resolved gravity waves (particularly for zonal wavenumber ≥43) is much larger during El Niño. The tropical upwelling associated with the Brewer–Dobson circulation is also stronger in the El Niño simulation. The effects of the enhanced tropical upwelling during El Niño are evidently overcome by enhanced wave driving, resulting in the shorter QBO period. The integrations were repeated with another model version (MIROC-ECM with T42 horizontal resolution) that employs a parameterization of nonorographic gravity waves in order to simulate a QBO. In the MIROC-ECM the average QBO periods are nearly identical in the El Niño and La Niña simulations.

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Peter Hamilton, Jimmy C. Larsen, Kevin D. Leaman, Thomas N. Lee, and Evans Waddell

Abstract

Transports were calculated for four sections of the Florida Current from Key West to Jupiter, Florida, using a moored current-meter array and voltages from cross-channel telephone cables at the western and northern ends of the Straits of Florida. In addition, moored arrays were used to estimate transport through the Northwest Providence, Santaren, and Old Bahama Channels that connect the Florida Current to the southwestern part of the North Atlantic Ocean. Transport measurements were obtained for an 11-month period from December 1990 to November 1991. Mean transports of ∼25 Sv (1 Sv ≡ 106 m3 s−1) for the flow across the western ends of the straits, which agree quite well with recent estimates of 23.8 ± 1 Sv entering the Gulf of Mexico through the Yucatan Channel, were obtained from both the Key West to Havana cable and the moored array. This estimate is about 5 Sv less than the generally accepted transport through the northern end of the straits at 27°N. This difference was partially accounted for by inflows through the side channels with more transport from the Old Bahama than the Northwest Providence Channel. The variability in the southern part of the straits was larger than at 27°N and included large diversions of the Florida Current south of the Cay Sal Bank and into the Santaren Channel that were caused by large meanders of the flow. The variability of transport in the side channels contributed to the variability of the Florida Current and reduces the correlations of the transports at the ends of the straits. Therefore, the well-measured transport at 27°N is not an accurate indicator of the transport of the Loop Current out of the Gulf of Mexico.

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Axel Lauer, Kevin Hamilton, Yuqing Wang, Vaughan T. J. Phillips, and Ralf Bennartz

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Cloud simulations and cloud–climate feedbacks in the tropical and subtropical eastern Pacific region in 16 state-of-the-art coupled global climate models (GCMs) and in the International Pacific Research Center (IPRC) Regional Atmospheric Model (iRAM) are examined. The authors find that the simulation of the present-day mean cloud climatology for this region in the GCMs is very poor and that the cloud–climate feedbacks vary widely among the GCMs. By contrast, iRAM simulates mean clouds and interannual cloud variations that are quite similar to those observed in this region. The model also simulates well the observed relationship between lower-tropospheric stability (LTS) and low-level cloud amount.

To investigate cloud–climate feedbacks in iRAM, several global warming scenarios were run with boundary conditions appropriate for late twenty-first-century conditions. All the global warming cases simulated with iRAM show a distinct reduction in low-level cloud amount, particularly in the stratocumulus regime, resulting in positive local feedback parameters in these regions in the range of 4–7 W m−2 K−1. Domain-averaged (30°S–30°N, 150°–60°W) feedback parameters from iRAM range between +1.8 and +1.9 W m−2 K−1. At most locations both the LTS and cloud amount are altered in the global warming cases, but the changes in these variables do not follow the empirical relationship found in the present-day experiments.

The cloud–climate feedback averaged over the same east Pacific region was also calculated from the Special Report on Emissions Scenarios (SRES) A1B simulations for each of the 16 GCMs with results that varied from −1.0 to +1.3 W m−2 K−1, all less than the values obtained in the comparable iRAM simulations. The iRAM results by themselves cannot be connected definitively to global climate feedbacks; however, among the global GCMs the cloud feedback in the full tropical–subtropical zone is correlated strongly with the east Pacific cloud feedback, and the cloud feedback largely determines the global climate sensitivity. The present iRAM results for cloud feedbacks in the east Pacific provide some support for the high end of current estimates of global climate sensitivity.

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