To broaden the range of known circulations and to test existing theory, a variety of issues are examined concerning the dynamics of flows in thick, thin, and transitional atmospheric layers. The circulations are produced numerically using a primitive equation model subject to simple heating functions. To confine the motions to a thin upper layer, the heating is chosen to produce a flow with either an exponential (EXP) vertical structure, or one that is linear (LIN) aloft while vanishing below. Five sets of solutions are created to define the terrestrial and jovian axisymmetric states, some basic terrestrial states, and the transitional jovian states for the two structures. The axisymmetric cases examine how the surface drag, static stability, rotation rate, and layer thickness influence the flow character. The standard theory is extended to allow for a weaker drag and the solutions confirm that at lower rates the Hadley cells become wider, and the thermal fronts sharper and double. In the absence of any drag, the cells disappear and a thermal wind prevails globally. But in the absence of a background static stability, the cells become more intense and create their own stable temperature field. For normal parameter values, the Hadley cells adhere to the theoretical form as the rotation rate increases, except when their width falls below 3° of latitude. Furthermore, when the heated layer is thin and the jets are confined aloft, the cells develop vertically bimodal amplitudes, while remaining deep and exhibiting the usual widths. The basic 3-D terrestrial cases examine the role of the heating rate, static stability, surface drag, and rotation rate on the flow character. The mean jets exist within a limited latitudinal range, with their location being as much dependent on the heating amplitude as on the heating distribution. When the background static stability is absent, the standard circulation theory becomes less valid as the cells and baroclinic instability become more intense and act together to stabilize low and middle latitudes. However, when the drag is reduced, the baroclinic instability becomes much weaker and confined to lower levels because of suppression by the jet’s stronger barotropic component. Other forms of baroclinic instability can be produced by creating double-jet flows, either by increasing the rotation rate or by adding an extra source of baroclinicity in low latitudes. The transitional jovian cases examine how the multiple jets behave as the active layer is varied between thick and thin for the LIN and EXP structures. In all cases, the jet widths remain constant with latitude, but their amplitudes vary, peaking either in low or middle latitudes depending on how the baroclinicity is distributed. An extra baroclinicity in low latitudes produces a jet whose barotropic instability can drive an equatorial superrotation, regardless of layer thickness. The eddy-driven jets have a similar dynamics for all layer thicknesses but, unlike the steady LIN jets, the EXP jets also migrate equatorward and, on rare occasions, poleward.
Mechanisms that maintain decadal sea surface temperature (SST) anomalies in the midlatitudes North Pacific are investigated using observational data. Focussing on the seasonality of decadal SST anomalies, individual heat budget analyses were conducted for the ocean mixed layer in a southern region (25°N-35°N, 175°E-145°W), and a northern region (39°N-49°N, 165°E-155°W) of the North Pacific. It was revealed that the decadal SST anomaly in the southern region, which was pronounced in winter, was significantly maintained by meridional Ekman temperature advection that occurred during November-anuary. On the other hand, the decadal SST anomaly in the northern region, which was observed in almost all months, was maintained by the sum of the effects of sensible and latent heat flux, and the net long wave radiation flux at the sea surface, and the meridional Ekman temperature transport during the period of October-ecember. In the Kuroshio extension region, which overlaps the southwest portion of the northern region, the anomalous temperature transport by entrainment in the seasonal thermocline was found to be very important for the maintenance of decadal SST anomaly.
Ground-based sky radiometers were used to measure direct solar irradiance and solar aureole radiance for several years at Sapporo, Tsukuba, and Tokyo, Japan. From these measurements, we computed aerosol optical thickness at 0.5 μm, τ(0.5), and the A° ngstro¨m exponent, α, and volume size distributions within a column. The optical thickness at Sapporo increased markedly over a short period of time following Asian dust events, and a forest fire in Siberia. The columnar volume size distributions observed during the Asian dust events showed a peak radius of 2.0-3.0 mm. Backward trajectory analyses suggest that the particles producing this springtime event originated in the Loess Plateau and Gobi Desert, and reached Sapporo via southern China. The columnar size distribution during the forest fire case showed an increase in the density of particles with a peak radius ∼0.2 μm. Trajectory analysis clearly linked the atmospheric changes over Sapporo with a forest fire in Siberia. The aerosol optical thickness, τ(0.5), has a clear seasonal cycle at Sapporo, with a vernal maximum and an autumnal minimum. The Ångström exponent, α, has a clear seasonal cycle at both Tokyo and Tsukuba, where early-winter maxima and springtime minima are observed, but at Sapporo the seasonal cycle is weaker, with a summer maximum and a vernal minimum. Aerosols were classified into four types (Types I∼IV) based on τ(0.5), and α data observed at the three sites. Aerosols with a τ(0.5) smaller than the total mean of τ(0.5), but greater than or equal to the total mean of α τ(0.5) < τ(0.5), α ≥ α were classified as Type I; aerosols with τ(0.5) ≥ τ(0.5) and α ≥ α were Type II;those with τ(0.5) < τ(0.5) and α < α were Type III; and those with τ(0.5) ≥ τ(0.5), α < α were Type IV.The most common aerosol type, that is, the background aerosol, was Type I (∼40%) at all three sites.Type-IV aerosols at all three sites showed the same seasonal cycle (spring maximum), suggesting largescalephenomena such as Asian dust events may contribute to the production and transport of thisaerosol type. The behavior of Type-II aerosols differed at the three sites, indicating that local phenomenaare important in the production and transport of Type-II aerosols. The emission of manufactured aerosolsand subsequent gas-to-particle processes may contribute to Type-II aerosol formation. Type-III aerosolsat Sapporo were characterized by a seasonal cycle opposite to that of Type-IV aerosols, suggestingthat large particles have different sources and/or transport processes in spring and autumn. The atmospheric turbidity retrieved from the sky radiometer is compared, β(SR), at Sapporo with that calculated using direct solar radiation measurements, β(DSR). Both showed the same seasonal cycle, but β(SR) was slightly smaller (∼0.02) than β(DSR). Sky radiometers can retrieve the optical properties of aerosols under cloudy conditions if there are no clouds around the solar aureole. The seasonal mean of τ(0.5) (or α) under cloudy conditions was 1.5 to 1.8 (1.1 to 1.2) times larger (smaller) than under clear sky conditions. In other words, the direct effect of aerosols that exist in clear air between clouds cannot be ignored. We call this the hybrid effect of aerosols.
Convective cells in two deep snow bands were examined over the Sea of Japan with an instrumented aircraft and dual-Doppler radars. A combination of the relatively warm air stream (westerly) at lower levels secluded near the center of the low and the polar air stream (west-southwesterly) at middle and upper levels produced a deep unstable stratification to the southwestern quadrant of a well-developed low, where snow bands A1 and B1 were respectively observed at around 1600 and 1700 JST on 28 Jan. 1993. Radar data showed that both snow bands were in quasi-steady state within a dual Doppler radar observation area. However, the two snow bands showed quite different development; the maximum re-flectivities in bands A1 and B1 were 15 dBZ and 30 dBZ, respectively. Aircraft data showed that the accretional growth of snow particles was the primary precipitation formation mechanism in the convective cells embedded in both snow bands. From aircraft and radar data, it appears that vertical wind shear within the cloud layer influenced the precipitation process. In the cell embedded in snow band A1, the updraft at middle and upper levels tilted downshear due to a strong wind shear and precipitation embryos initiated in the updraft were advected downshear from the main supercooled cloud water region. Therefore, these embryos did not grow to large graupel particles. By comparison, in the cell embedded in snow band B1, the updraft tilted upshear at lower levels and stood almost erect at middle and upper levels due to weak wind shear. Therefore, precipitation embryos initiated in the updraft grew into 5 mm graupel particles as they descended through the main supercooled cloud water region. In addition, re-circulation of graupel particles, which had descended in a vicinity of the updraft core, seemed to play an important role to efficiently produce larger graupel particles.
The anomalous response of the tropical atmosphere to the recently discovered Indian Ocean Dipole has been studied in the present article, using an Atmospheric General Circulation Model (AGCM), and the NCEP/NCAR Reanalysis. Our AGCM study shows that the response of the atmosphere to the IOD is of dipole—like pattern in the circulation, and is baroclinic. An anomalous circulation in the zonalvertical plane is induced, with the subsidence over colder pole and the upward motion over the warmer pole, modulating the Walker circulation over the equatorial Indian Ocean, as observed. An anomalous circulation in the meridional-vertical plane from the eastern pole of the dipole towards India, and the Bay of Bengal is simulated, which affects the Indian summer monsoon rainfall. The model atmosphere is heated by the anomalous surplus of the latent heat, the net long-wave radiation, and sensible heat fluxes over the warmer pole of the IOD. This results in an excess of net vertically integrated moisture convergence, and excess latent heat release in the atmospheric column above the warm pole. This energy, together with the anomalous convergence of enthalpy, is converted into anomalous mechanical energy and leads to the divergence of mechanical energy flux. This anomalous divergence of the mechanical energy in the upper tropospheric part of the column causes propagation of the disturbances to the surrounding regions. The mean seasonal circulation during the boreal summer is crucial for maintenance of the anomalous energy distribution and propagation during the IOD event. Transient high frequency variability does not appear to contribute much to the energy conversions.
Using the European Centre for Medium-Range Weather Forecasts reanalysis data, we examine whether an onset mechanism of the Australian summer monsoon proposed by Kawamura et al., which incorporates possible air-sea feedback processes, can apply to the South Asian summer monsoon system as well. In their mechanism, a combination of the increase in sea surface temperatures and dry intrusion into the layer at 600-850 hPa level over the ocean off, the equatorial side of a continent, plays a crucial role in enhancing potentially convective instability prior to the onset. It is found that the onset mechanism is able to apply to the abrupt onset of the Indian summer monsoon at the beginning of June, rather than the earliest onset over the Indochina peninsula in middle May. This is consistent with the observational fact that abruptness of the monsoon onset is most evident over the coastal regions of the Indian subcontinent. The Indochina peninsula is characterized by a relatively slow onset, although its date is earliest. The asymmetry of the transition speed between the onset and retreat regimes also has similar regional features. If the mechanism operates efficiently on the Indian subcontinent and adjacent oceans, it is anticipated that the onset of the Indian summer monsoon is delayed, as compared to that over Southeast Asia because subsidence in the periphery of a sub-continental scale thermal low, resulting from intensification of land-ocean thermal contrast, inhibits convection. This may be one of the possible reasons why the two major onsets seen in the South Asian summer monsoon system are clearly distinguished from each other.
From an examination of the analyses of the NCEP/NCAR reanalysis data for the years 1979 to 2000, it is found that the persistence of tropical 200 hPa geopotential heights, representing tropospheric mean temperatures, exhibits a ‘barrier’ during the boreal autumn, especially between September and October. On the other hand, the persistence of indices associated with the El Niño/Southern Oscillation (ENSO), which is intimately related to variations of the tropical tropospheric mean temperature, have been shown to decrease during the spring. >From this feature, a tropical climatic year that starts in October is proposed. The ‘annual’ average of the tropical zonal mean 200 hPa heights for the tropical climatic year introduced above is presently de-fined as the tropical year index (TYI). Associated with the TYI, a zonally uniform tropical temperature anomaly is prominent at the 700-150 hPa levels over the period from November to the following September. On the other hand, the tropical temperatures at the 1000-850 hPa levels are not zonally uniform, but vary almost simultaneously with the underlying tropical sea surface temperatures. The temperature anomalies are found to be larger in the upper troposphere (0.4-0.6 K), than in the lower troposphere (0.2-0.3 K). The tropical tropospheric warming (or cooling) is not directly connected with the increase (or decrease) in the amount of the precipitation over the entire tropics. The mean mass of tropical water vapor, within the 700-300 hPa levels, gradually increases after the peak of ENSO events, unlike the tropospheric temperatures.
The irregularity of El Niño is investigated with a 400-year simulation of a coupled ocean-atmosphere general circulation model. The model produces irregular El Niños with peak sea surface temperature (SST) anomalies ranging from 1°C to 4°C in the equatorial central-eastern Pacific. In the equatorial Pacific, the temporal phase relationship of the upper ocean heat content (OHC) anomaly relative to SST, and wind stress anomalies can be explained by the “recharge oscillator” mechanism. A difference of the zonal mean OHC anomaly between the equator and the northern subtropics arises before the development of equatorial SST anomaly. It is found that a larger OHC anomaly is accumulated on the equator as a precursor of strong El Niño. The heat-budget analysis suggests that horizontal advection in the ocean interior is a major contributor to the build-up of the larger OHC anomaly during the recharge phase, which is associated with the zonal-mean wind-curl anomaly in the off-equatorial North Pacific. This also implies that the surface heating in the subtropics is a potential contributor through meridional heat transport. Besides the aspect of amplitude irregularity, the model El Niño shows irregularities in frequency and seasonal phase locking. Possible linkages between these irregularities are discussed.
Results of the split-window cloud retrieval method and the new Meteosat Second Generation cloud analysis method (MSG/CLA), have been compared for MODIS data over the west Atlantic Ocean. Very good agreement is obtained for the classification of optically thick ice and water clouds. Differences are found for thin cirrus, thin water clouds and at cloud edges. These differences are explained by the fact that MSG/CLA also uses spectral channels of 3.9, 6.2, and 8.7 mm in addition to the split-window, which provides information over and above the split-window observations. Some of the disagreement at cloud edges is interpreted as inter-channel miss-alignment. The analysis in this study also confirms that an optically thin water cloud can be correctly classified by the MSG/CLA method.