Analysis of 17 years (1980-1996) of tropical Pacific thermocline variations indicates that short period (about 18 months) cycles in the early 1990s exhibit a relatively small meridional scale. The long period (4-5 year) cycles in the 1980s, associated with major basic-wide warming/cooling events, have much wider meridional scales. The dominant mode of interannual variation of thermocline depth can be approximately described by a basin-scale east-west seesaw oscillation, with an eastward propagation of thermocline depth anomalies occurring in the equatorial waveguide during the transition phases. The long-period ENSO cycles associated with the major basic-wide warming/cooling events involve a substantial change of heat storage in the western North Pacific (WNP) (5-15°N, 130-170°E). The tendency of thermocline displacement and local wind stress curl in the WNP exhibit a coherent, broad spectral peak on a 8-20 month time scale. The deepening (rising) of the thermocline occurs in phase with the local anticyclonic (cyclonic) wind stress forcing, suggesting the essential roles of the in situ wind forcing in thermocline adjustment. The surface wind variation in the western Pacific is found to play a critical role in the phase transition of ENSO cycles. During the mature phases of major warm episodes, there is a rapid establishment of an anomalous anticyclonic wind stress curl over the WNP. The anticyclonic wind stress deepens the WNP thermocline and starts a recharge of heat content in the warm pool. Meanwhile, the easterly anomalies to the south of the anticyclonic center elevate the thermocline in the equatorial western Pacific, and trigger an eastward migration of the rising thermocline along the equator, leading to cooling in the east. The transition from cooling to warming during the mature phases of cold episodes involves a similar process, but with opposite anomalies.
To investigate experimentally 3D chaotic Lagrangian motion in steady baroclinic waves, long-term particle tracking was conducted on the wavenumber-3, 4 and 5 flows in a differentially-heated rotating annulus fluid. With the use of fluorescent solution microcapsules as tracer particles and an automated tracking system, the trajectories were observed in each wavenumber for more than 10 hours. Tracer particles in all the observed trajectories preferred to track the same cyclic route of the flow regions for about half of the total time and were chaotically trapped in the vortices where they resided for an indefinite time. The preference of this cyclic route has been expected from the numerical investigation of the Lagrangian motion made by Sugata and Yoden (1994) and supports their Lagrangian view of the heat transport. Furthermore, a few interesting results were obtained for the wavenumber dependence of the Lagrangian motion: a transition rate, which is the number of the region transitions per time, grows slightly as the wavenumber increases; on the Lagrangian view, the total inward heat flux of wavenumber-5 is considered distinctively larger than those of wavenumber-3 and 4.
Ocean and atmosphere variability is investigated over the whole Atlantic basin in a 51-year record of sea surface temperature (SST). In the tropics, SSTs variations are separated into two time scales: decadal (8-16 years) and interannual (<5 years). A strong cross-equatorial gradient mode dominates decadal SST anomalies with centers of action having opposite polarities at 15°N and 15°S, while interannual variations are characterized by SST anomalies of the same polarity throughout the tropics. Sea level pressure (SLP) regressions onto the cross-equatorial gradient index reveal extratropical teleconnections associated with the North Atlantic Oscillation (NAO), and with the South Atlantic. At the same time, a Pan-Atlantic spatial pattern is also found in SST regressions. Lagged regressions of SLP on the cross-equatorial SST gradient index shows the extratropical decadal oscillation that is out of phase between hemispheres. These results suggest that an extratropical forcing could excite the tropical Atlantic variability.
A singular value decomposition (SVD) analysis between the Northern Hemisphere winter-mean 500-hPa geopotential height (Z500) and near-global surface temperature (land-surface air temperature + sea surface temperature) fields was conducted for the recent three decades. The leading mode, referred to as Polar/Eurasia (PEA), is considered as a hemispheric-wide atmospheric internal variability rather than a local teleconnection pattern. Winter mean Z500 of the leading mode is closely related to Eurasian surface air temperature from autumn to spring throughout the period. Both interannual and interdecadal time scales are dominant during the three decades in the time scales. No linear relationship between the tropical SST anomalies is found in this mode. The second and third modes can be identified as the Pacific/North American (PNA) and Western Pacific (WP)-type teleconnection patterns, respectively. Although they are also considered as internal modes of atmospheric variabilities, their interannual variabilities are also influenced by the tropical Pacific SST anomalies. These results are compared with the more popular SVDanalysis between geopotential height and SST, for which the leading mode exhibits the so-called “ENSO (El Niño/Southern Oscillation) related” winter circulation pattern. It is shown that this ENSO-related mode can be reconstructed from the linear combination of the second (PNA) and third (WP) modes of the present SVD mentioned above. These results suggest that inter-El Niño differences of atmospheric circulation in midlatitudes can be interpreted as the different strengths of the PNA and WP patterns on one hand, and on the other hand by the different magnitude of the PEA, which is linearly independent of ENSO. The characteristics of the vertical structure of the atmospheric variability were further examined by conducting a SVD analysis of 100-hPa temperature and Z500. The PEA and PNA patterns in the Z500 field extracted as the first and second SVD modes, respectively, exhibit contrasting features in association with lower stratospheric temperature. The PNA pattern is confined to the troposphere, while PEA extends further into the stratosphere. This suggests they are produced by different mechanisms.
The moisture budget over the Atlantic Ocean and South America is evaluated for two contrasting 5-year periods, 1985-1989 (wet) and 1990-1994 (dry), of rainfall over Northeast Brazil (NEB) using NCEP/NCAR reanalysis data. The linear correlation coefficients between the 5-year averages of rainfall over NEB and sea surface temperature (SST) anomalies over the Atlantic Ocean show a dipole configuration over the Tropical Atlantic. It is found that evaporation is higher over the Tropical North Atlantic, and lower over the Tropical South Atlantic during the 5-year period of higher rainfall over NEB. Higher evaporation causes lower SST and vice versa. This seems to be the main mechanism which generates dipole configuration in the Atlantic SST. During the rainy season (February through May) in NEB moisture is transported towards NEB and converges, providing the necessary moisture source for the rainfall. This low level convergence is associated with the rising branch of a local meridional circulation cell, which in turn is generated by the SST dipole. This series of interlinked mechanisms seems to fit in a physical picture responsible for the rainfall decadal variation over NEB.
Features of the moisture transport and the moisture balance within the Asian summer monsoon region in 1991 are studied utilizing the 24-hour prediction data by a global weather prediction model in relation to changes of precipitation and the major circulation systems (CSs) defined in Part I of the present paper. The change during May, June and July is characterized by a shift of the intense rainfall areas, with the strong moisture sink from the equatorial zone over the Indian Ocean and Indonesia, to the subtropical area of the Indian subcontinent and East Asia. The southwesterly moisture transport in 30-80°E is mainly due to CS-3 (the clockwise circulation over the Indian Ocean), whereas the southwesterly moisture transport in 80-110°E and 110-140°E is mainly due to CS-4 (clockwise circulation around Indonesia) and CS-5 (circulation around the Pacific subtropical anticyclone), respectively. The confluence between the adjacent CSs yields strong southerly moisture transport into the monsoon rainfall areas and large moisture-flux convergence. It is an important fact that the seasonal variation of the moisture flux crossing the boundary of a large domain, which is bounded by 10°S, 40°N, 35°E and 140°E, and the variation of cross equatorial moisture transport from the southern hemisphere, are significantly smaller than the variations of the moisture transport within the domain. The moisture balance calculation also indicates that the essential feature of the summer monsoon is the formation of a pair of moisture source region, and the adjacent moisture sink region, within the large domain. The moisture source regions form under the subsidence and convective stable stratification. The moisture sink regions appear under the ascent motion and convective unstable stratification. Several parameters such as “moisture influx ratio” and “rainfall production ratio” are defined to discern the characteristics of the moisture balance. The temporal and spatial variations of these parameters depict well the features of moisture balance within the Asia summer monsoon region.
Heavy snowfall occurs every year in orographic areas of Japan facing the Japan Sea. The riming growth process contributing to the formation of snowfall was studied by radar and microphysical analyses. The observational results can be summarized as follows. Radar analyses showed that snow clouds which formed over the Japan Sea moved from the sea toward the land, and changed from the developing stage to the mature stage and then to the dissipating stage as they moved inland. When the clouds reached an orographic area, they developed again due to topographic updraft. The microphysical measurements showed that rime mass increased with an increase in snowfall intensity, and with an increase in wind speed when the mass flux of ice crystals was subtracted. Besides, it was supposed from the figure of wind speed vs. riming proportion relationship that rime mass increased with an increase in wind speed even if the advection effect of cloud droplets is subtracted. These indicate in the case of a strong wind that falling ice crystals can wash out many cloud droplets in clouds where cloud droplets increased, resulting in a snowfall of high riming proportion and high intensity. Thus, the riming growth process contributes significantly to the formation of orographic snowfall. The observational results also showed that the deposition and aggregation growth processes contributed to the formation of orographic snowfall. It was shown that the liquid water content in snow clouds depends on not only the production rates of cloud droplets due to topography-induced updraft, but also the mass flux of ice crystals in clouds. The observational results showed that two cases in which there are only a small mass flux of ice crystals, as compared with the amounts of cloud droplets, have a high potential for artificial seeding experiments.
In order to estimate sea ice albedo around the marginal sea ice zone of the southwestern Okhotsk Sea, we conducted the measurement of albedo aboard the ice breaker Soya in early February of 1996 and 1997. Using upward and downward looking pyranometers mounted at the bow of the ship, we obtained albedo data. We also measured ice concentration and thickness quantitatively by a video analysis. The observations show a good correlation between albedo and ice concentration. From a linear regression, sea ice albedo (ice concentration=100%) is estimated to be 0.64±0.03 at the 95% confidence level. The developed snow grains on sea ice due to sea water and/or solar radiation may be responsible for this somewhat lower value, compared with that over the snow-covered land fast ice in the polar region. Deviations of the observed values from this regression have a statistically significant correlation with solar zenith cosine at the 99% level, and with ice thickness at the 95% level. The linear regression formula which predicts albedo is also derived as the variables of ice concentration and solar zenith cosine. Although the regression coefficients are both statistically significant, the coefficient of ice concentration is much more significant in this formula than that of solar zenith cosine. The deviation of the observed albedo from this regression seems to be mainly caused by ice surface conditions rather than by ice thickness or cloud amount. All these results suggest that snow cover on sea ice plays an important role in determining the surface albedo. We also did albedo observations of dark nilas with snow-free surface, they were estimated as 0.10 and 0.12 for ice thickness of 1 to 1.5cm and 2 to 3cm, respectively.
The orographic modification of a precipitating convective cloud, which formed in association with typhoon 9304 and travelled over the sea, was studied by mainly analyzing the dual-Doppler radar system observational data. From July 24 1900 LST, to July 25 0000 LST, many conective clouds landed at the southeastern coast of the Kii Peninsula. Although they reached the coast in various stages of development, most of them showed similar features. Their radar-echoes were intensified 10 to 20km off the coast before landing, and the radar-echoes were intensified again or broadened over land after their radar-echo intensities were reduced near the coast. Before the first intensification of convective radar-echoes over the sea, the echoes were intensified in their upper rear-parts 30 to 40km off the coast, and intensified in their front-parts near the coast. By averaging horizontal winds derived from the dual-Doppler radar system observational data for about two hours, it was shown that the ambient wind speed decreased near the coast, and the horizontal convergence zone larger than 10-4s-1 existed along the coast line about 10km off the coast. On the basis of these observational results the travelling convective clouds orographic modification, and the efficient formation of precipitation in them were discussed, taking into account the results of numerical experiment about the Kii Peninsula orographic effect on ambient winds. It can be inferred that the large rainfall amount recorded in association with the typhoon around the Kii Peninsula coast was caused as a result of the integration of travelling convective clouds orographic modification.
Aerosol samples were collected at Syowa, Antarctica (69°00'S, 39°35'E), from February 13, 1993 to January 29, 1994. All samples were analyzed by ion-chromatography to examine the mass concentrations of sodium (Na+), sulfate (SO42-), and methanesulfonate (MSA or CH3SO3-). The concentrations of MSA ranged from a mean value of 1.4ng m-3 during the winter (from June to August 1993) to 32ng m-3 during the summer (from February to March 1993, and from December 1993 to January 1994). The mean concentration of non-sea-salt sulfate (nss-SO42-) was 34ng m-3 during the winter, whereas during the summer it was 147ng m-3. The mean molar ratio of MSA to nss-SO42- was 0.25 during the summer, but values during the winter decreased to 0.04. This resulted from the difference in amplitudes between the seasonal variations of MSA and nss-SO42- concentrations. A similar variation was also found at other sampling sites over a broad area of the southern oceans. Since the interaction between the atmospheric boundary layer and the free troposphere seems to regionally vary, it is difficult to explain the seasonal variation of the MSA/nss-SO42- molar ratio by only the transport of air parcels from the free atmosphere. The seasonal variation of MSA/nss-SO42- molar ratio appears to be mainly the result of the seasonal variation of the dimethylsulfide (DMS of CH3SCH3) oxidation process.