A spectral aqua-planet atmospheric general circulation model (AGCM) is forced with a series of zonally constant sea surface temperature (SST) distributions which are symmetric about the equator. For every oceanic forcing, the AGCM is run twice; a first time keeping all spectral modes and a second time with only the zonally symmetric ones. Parameterizations and boundary conditions remain the same in all cases thus allowing a consistent comparison of 3-D and 2-D flows. The comparative study shows that the structure of the tropical mean state of the full model is basically captured by the zonally symmetric model and that eddy fields merely modify this structure. This shows that the structure of the tropical mean state is mainly determined by the shape of the effective SST forcing. We confirm previous studies where the shape and strength of the Hadley circulation is comparable in the 3-D and 2-D experiments for cases with a well pronounced single ITCZ. So the underestimation of the Hadley circulation often found in idealized zonally symmetric models is not only due to the neglect of large-scale eddy fields. A new result is that both the full as well as the zonally symmetric model show the phenomenon of ITCZ splitting if the SST distribution gets flat enough at the equator. So ITCZ splitting can be explained by purely zonally symmetric mechanisms and is not necessarily induced by eddy fields. Eddies seem to stabilize single ITCZ circulation regimes. For SST distributions where ITCZ splitting occurs, multiple states exist in the zonally symmetric model. The atmosphere switches, for the same SST, between a single and double ITCZ state introducing a long-term variability in the tropics without the influence of mid-latitudes and without atmosphere-ocean coupling.
To gain insight into properties of in-cloud vertical motion and precipitation production in the tropics, three-dimensional outputs from an aquaplanet experiment using a 3.5-km mesh global cloud-system resolving model (GCRM) were analyzed. Probability distributions of precipitation and latent heating in the 10°N–10°S domain are evaluated in comparison with Tropical Rainfall Measurement Mission (TRMM) observations. Despite biases of generally higher precipitation top height (PTH) and deficiencies near the melting level, the model reproduced the general morphology of the precipitation and total latent heating profiles. Relationship between PTH and cloud top height (CTH) in the simulated clouds reproduced clear contrast between deep and shallow convection in active and suppressed environments, respectively. The simulated in-cloud vertical velocities were on the order of O(0.1 m s-1) in anvil clouds and O(1 m s-1) in updraft cores, as in the range of those in previous observations. Focusing on relatively strong upward motion, the maximum in-cloud vertical motion (w_max) was defined in each column. Probabilities of w_max had double peaks (z = 1–4 and 7–12 km) with minima in the middle troposphere. Vigorous upward motions most frequently occurred in the upper troposphere as the active portion of well-organized convective systems. They were often surrounded by updrafts with w_max height in the lower to middle troposphere, forming a group of updraft regions (w_max > 1 m s-1) with horizontal scale of O(10 km). In the regions of compensating subsidence, updrafts tended to be capped below the middle troposphere and small in horizontal size. In both regions the updrafts were accompanied by cold pools of their characteristic horizontal scale. Finally, time evolutions of in-cloud updrafts were analyzed to explore the roles of in-cloud updrafts at different altitude. It was found that the updrafts with w_max height in the middle troposphere produced the heaviest surface precipitation, preceded by moisture transport in the lower to middle troposphere. This suggests that middle tropospheric updrafts most efficiently produced surface precipitation through tight linkage between dynamics and cloud processes, although their occurrence was rare.
The terrestrial carbon dioxide (CO2) budget interacts with the Earth's climate system on diurnal to centennial and longer time scales, making it critical for climatic prediction and stabilization. Atmospheric observations and global syntheses of CO2 data indicate that the terrestrial biosphere is one the major sources of interannual variability, but the underlying mechanisms operating on different time-scales and the potential impacts of this on future projections remain unclear. Here it is shown that the El Niño and Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) regime affect temporal variability in the terrestrial carbon budget with different time scales. The terrestrial carbon budget, estimated using a process-based model (VISIT) for the period 1910–2005, was correlated with the indices of PDO, AMO, and ENSO with various smoothing periods and lag lengths. ENSO showed the highest short-term correlation, corresponding to interannual terrestrial variability, whereas PDO and AMO had higher correlations at the decadal time scale. Such correlations with the meteorological regimes occurred heterogeneously over the land surface. This study suggests that long-term monitoring is needed to elucidate the temporal variability, and that decadal predictability of climate and terrestrial models should be improved further.
The variability and predictability of the East Asian (EA) winter climate has been studied, based on observed datasets and multi-model ensemble (MME) hindcast experiments archived at the APEC Climate Center (APCC). The focus is on the leading modes of wintertime variability over the eastern to northeastern part of Asia, which are identified based on multivariate EOF analysis of the monthly 850 hPa wind and temperature. The leading EA climate mode is characterized by continental-scale temperature anomalies covering a broad region from the northwestern flank of the Siberian high to northeast Asia. The second mode is associated with fluctuations of temperature and monsoon northerlies over the EA locations of Korea, Japan and eastern coastal China. Moreover, the first mode is found to be influenced by the Scandinavian (SCA) pattern, while the second mode is closely associated with the Eurasia (EU) pattern. In general, the dominant circulation patterns of the EA wintertime variability from each MME member model compare well with their observational counterparts. However, the temporal variations of these modes are difficult to reproduce in the model simulations. The variation of the leading mode is found to be better predicted by most models, which leads to better predictions of the winter climate over continental northeast Asia, compared to the second mode. The MME performance is further assessed in the context of circulation changes during ENSO. It is found that most models have difficulty in capturing both the timing and strength of the observed second EA climate mode variations. Analysis based on observations shows that there is Rossby wave activity from Eurasia in early boreal winter during ENSO years. The Eurasian wave train, however, is either too weak or absent in the model simulations. Overall, these results highlight the difficulties in forecasting EA winter climate in the current framework of seasonal climate prediction.
The mid-Pliocene warm period (∼3 million years ago; 3 Ma) is one of the plausible scenarios which provide insight into the climate system in a globally warmer world as projected by climate models for the future. The reconstructed sea surface temperature (SST) by the Pliocene Research, Interpretation and Synoptic Mapping phase 3 (PRISM3) reveals that salient warming occurs in the higher latitudes together with weakening of surface cooling in the equatorial and coastal upwelling regions. The sensitivity of an atmospheric general circulation model (AGCM) is studied by prescribing the surface condition based on the PRISM3 paleoenvironmental reconstructions. The simulated Walker circulation generally slows down, inducing convergent anomaly over the African continent and divergent anomaly over the Asian monsoon region at the lower troposphere; and vice versa at the upper troposphere in response to the reduced east-west gradient of the tropical SST in that period. The ascending branch of the Hadley cell expands poleward, implying a weakening of the meridional circulation in response to the warmer SST in the higher latitudes. To identify the physical reason for the modulation of the wet-dry climatological pattern in low latitudes, additional sensitivity experiments were conducted by changing the configurations of ice-sheet cover, vegetation and zonal patterns of the SST. The results indicate that the reduction of meridional and zonal gradient of tropical SST is the key factor for the expansion of proxy-suggested wetter climate over Africa.
This study examines the stability criterion of a saturated air parcel in an unsaturated environment using the parcel method for parcels of the same virtual temperature as their environment. By considering the lapse rate of virtual temperature in a moist adiabatic process, we get the results that the lapse rate of temperature for the criterion is larger than the usual moist adiabatic temperature lapse rate by about 20% of the difference between dry and moist adiabatic lapse rates. In general, the dominant circulation patterns of the EA wintertime variability from each MME member model compare well with their observational counterparts. However, the temporal variations of these modes are difficult to reproduce in the model simulations. The variation of the leading mode is found to be better predicted by most models, which leads to better predictions of the winter climate over continental northeast Asia, compared to the second mode. The MME performance is further assessed in the context of circulation changes during ENSO. It is found that most models have difficulty in capturing both the timing and strength of the observed second EA climate mode variations. Analysis based on observations shows that there is Rossby wave activity from Eurasia in early boreal winter during ENSO years. The Eurasian wave train, however, is either too weak or absent in the model simulations. Overall, these results highlight the difficulties in forecasting EA winter climate in the current framework of seasonal climate prediction.
Baiu is a Japanese name for rain and the rainy season that occurs in boreal early summer in the western North Pacific (WNP). This work proposes a methodology to determine the Baiu onset and closing dates using only an atmospheric parameter, the equivalent potential temperature (θe) in the lower troposphere, to reveal the interannual variability of Baiu season. First, the location of the Baiu front is determined by the maximum of the negative meridional gradient of θe, which identifies the zonal boundary of air masses. Then, the Baiu onset and closing dates are fixed on the basis of the northward movement of the Baiu front and development of the southern air mass in the WNP. The mean onset date was estimated to be June 2 near Japan, which is comparable to the date published by the Japan Meteorological Agency (JMA) for South Kyushu, although the correlation was not significant between the two interannual variations. The meridional shift of the monsoon southwesterlies was concurrent with early or late onset dates. The mean closing date was July 21, about 1 week earlier than the JMA date for North Tohoku, but the correlation was significant between the two interannual variations. The closing date varied with an anomalous meridional dipole in atmospheric circulation, largely extending from the eastern Eurasian continent to the central North Pacific. The correlation was insignificant between the interannual variations of the onset and closing dates, although both had dominant 2–4-year periodicity. There was also no significant correlation with typical large-scale interannual variations (i.e., the El Niño/Southern Oscillation, the tropospheric biennial oscillation of the Asian monsoon, and the North Atlantic Oscillation). Hence, these three interannual variations do not appear to be direct causes of the modulation of the onset and closing dates of the Baiu season near Japan.
The effects of future tropical Pacific sea surface temperature (SST) changes on regional precipitation projections are statistically studied for December–January–February (DJF) and June–July–August (JJA) in the Coupled Model Intercomparison Project phase 3 (CMIP3) experiments. The present climate precipitation responses to Niño3 SST variability appear as an uncertainty with regards to future regional precipitation changes among the CMIP3 model projections. Compared with the CMIP3 models projecting La Niña-like Pacific SST changes, the models projecting El Niño-like Pacific SST changes tend to simulate more precipitation in future DJF over the tropical central Pacific, southeastern North America and the tropical western Indian Ocean, and less over the tropical northwestern Pacific, the tropical South Pacific and tropical South America. For JJA, the models projecting El Niño-like Pacific SST changes tend to simulate greater future precipitation in the tropical central Pacific and less over the Maritime Continent and around Central America. Interestingly, the present climate features of the delayed JJA precipitation response to previous DJF Niño3 SST variability also appear as differences in future JJA precipitation changes between the models projecting future El Niño-like and La Niña-like Pacific SST changes in DJF. Compared to the later models, the former models have a tendency to show more precipitation south of Japan and south of the equator in the central to eastern Pacific, and less in the subtropical northwestern Pacific. CMIP3 model analysis indicates that the projected El Niño-like SST changes are related to the present precipitation climatology of the models in the near-equatorial eastern Pacific for each DJF and JJA season, suggesting the importance of realistically simulating present precipitation climatology in the tropical Pacific for future projections.
This study investigated the properties of heavy precipitation and its associated vertical motion in an aquaplanet experiment with a 3.5-km mesh global cloud-system resolving model (GCRM). The statistics of precipitation and vertical velocity were examined in terms of the precipitation top height (PTH) and the maximum in-cloud vertical velocity in each column (w_max) for the grid points with the top 1% and 1–10% of the surface precipitation rate (pr_sfc) in the 10°N–10°S domain. To support the findings, realistic simulation cases were also analyzed. In the columns with the top 1% (1–10%) of pr_sfc, peak frequencies of w_max height were found at z = 4–6 (1–4) km with the PTH several kilometers above that. Thermodynamic conditions were more humid and warmer in these columns than in the columns with average precipitation. These results were common to all simulation cases. Composite time evolution of convection with heavy surface precipitation was also examined for the aquaplanet experiment. The results suggest that the vigorous upward motion in the middle (lower) troposphere for columns with the top 1% (1–10%) of pr_sfc enabled efficient moisture transport from the boundary layer to the middle troposphere.
This work contrasts the climatic impacts of so-called warm-pool (WP) and cold-tongue (CT) El Niño on the atmospheric circulation over the western North Pacific (WNP). It is found that the anomalous atmospheric circulation over the WNP is nearly opposite in response to these two types of El Niño events in developing autumn. A weak anomalous anticyclone appears over the WNP during CT El Niño, whereas a weak anomalous cyclone emerges in the same region during WP El Niño. These nearly opposite autumn responses of atmospheric circulation have a significant impact on East Asian climate, and southern China autumn rainfall in particular, although this contrast tends to diminish as El Niño events enter their mature phase.
Using two kinds of reanalysis data, this study explores latitudinal (southward) extension of cooling and upwelling signals in the stratosphere associated with major stratospheric sudden warmings (SSWs) in Northern winter. A composite analysis of SSWs reveals that the cooling and upwelling signals do extend to Southern mid-latitudes exceeding about 30°S. A further examination on event-to-event variability shows that the SSW-associated cooling in the equatorial stratosphere tends to be stronger when strong wave driving extends more equatorward to subtropical latitudes. Such changes to the wave driving are notable for several cases in horizontal patterns of planetary wave breaking.
To understand the cloud condensation nuclei (CCN) ability of Asian mineral dust particles, we investigated the activation spectrum (fraction) and the hygroscopicity parameter κ of the certified reference materials for Asian mineral dust (CAD) particles. The CAD particles were dispersed in dry conditions using a rotating brush generator and then measured with a cloud condensation nuclei counter and a condensation particle counter. The concentration ratios of CCN to condensation nuclei (CN) of the CAD particles were 0.7 at supersaturation (SS) = 0.2% and 0.8 at SS = 0.4%. This suggests that Asian mineral dust particles, by themselves, may effectively act as CCN. The mean hygroscopic parameter κ of the CAD particles estimated from the relationship between the measured critical supersaturation and dry diameter was 0.014. Using this κ-value, we simulated the change in the size distribution from CCN to droplets during adiabatic ascent by a parcel model. The simulation suggested that the CAD (low hygroscopicity) particles do not grow large enough to promote collision-coalescence in the early stage of cloud formation. The hygroscopic parameter κ of internally mixed particles of CAD and sea salt was calculated by using the κ-Köhler theory, and their activation and subsequent condensation growth processes during adiabatic ascent were also simulated with the parcel model. The simulation results suggest that the hygroscopicity of Asian mineral dust particles was considerably enhanced, and droplets activated on them grew much larger than those activated on pure mineral dust particles.