Abstract

This study investigates the mechanism of the large intermodel uncertainty in the change of ENSO’s amplitude under global warming based on 31 CMIP5 models. We find that the uncertainty in ENSO’s amplitude is significantly correlated to that of the change in the response of atmospheric circulation to SST anomalies (SSTAs) in the eastern equatorial Pacific Niño-3 region. This effect of the atmospheric response to SSTAs mainly influences the uncertainty in ENSO’s amplitude during El Niño (EN) phases, but not during La Niña (LN) phases, showing pronounced nonlinearity. The effect of the relative SST warming and the present-day response of atmospheric circulation to SSTAs are the two major contributors to the intermodel spread of the change in the atmospheric response to SSTAs, of which the latter is more important. On the one hand, models with a stronger (weaker) mean-state SST warming in the eastern equatorial Pacific, relative to the tropical-mean warming, favor a larger (smaller) increase in the change in the response of atmospheric circulation to SSTAs in the eastern equatorial Pacific during EN. On the other hand, models with a weaker (stronger) present-day response of atmospheric circulation to SSTAs during EN tend to exhibit a larger (smaller) increase in the change under global warming. The result implies that an improved simulation of the present-day response of atmospheric circulation to SSTAs could be effective in lowering the uncertainty in ENSO’s amplitude change under global warming.

Abstract

Year-to-year variations in summer precipitation have great socioeconomic impacts on China. Historical rainfall variability over China is investigated using a newly released high-resolution dataset. The results reveal summer-mean rainfall anomalies associated with ENSO that are anchored by mountains in central China east of the Tibetan Plateau. These orographically anchored hot spots of ENSO influence are poorly represented in coarse-resolution datasets so far in use. In post–El Niño summers, an anomalous anticyclone forms over the tropical northwest Pacific, and the anomalous southwesterlies on the northwest flank cause rainfall to increase in mountainous central China through orographic lift. At upper levels, the winds induce additional adiabatic updraft by increasing the eastward advection of warm air from Tibet. In post–El Niño summers, large-scale moisture convergence induces rainfall anomalies elsewhere over flat eastern China, which move northward from June to August and amount to little in the seasonal mean.

Abstract

This study disentangles the changes in Indian Ocean (IO) dipole (IOD)-related SST and rainfall variability under global warming projected by the RCP8.5 runs in 29 CMIP5 models. The IOD rainfall changes consist of the thermodynamic component due to the surface moisture increase and the dynamic component due to the changes in IOD-related circulation. The IOD circulation changes are dominated by the IOD SST changes, which were further clarified using the amplitude and structural decomposition. The amplitudes of IOD SST and circulation are both decreased at rates of around 7.2% and 13.7% °C⁻¹, respectively. The structural changes in IOD SST and circulation show a pattern with increases from the eastern to the western coast of the equatorial IO, similar to the pattern of so-called extreme IOD events in previous studies. Disentangling previous mechanisms and projections, we conclude that the increased atmospheric stability suppresses the amplitudes in IOD SST and circulation, whereas the positive IOD (pIOD)-like mean-state SST changes, leading to greater warming in the west than the east, mainly alter the structure of IOD SST and circulation. Both the amplitude and structural changes in the IOD SST and circulation are robust among the CMIP5 models, but their distinct patterns and out-of-step changes lead to an uncertain projection of IOD changes defined by the dipole mode index or EOF analysis in previous studies. Furthermore, the structural changes, dominated by the pIOD-like mean-state SST changes, are significantly correlated with the historical IOD amplitude among the models. Considering the commonly overestimated IOD amplitude as an emergent constraint, the structural changes in IOD SST and circulation should not be as robust as the original multimodel projection.

Abstract

In this study, we find that the negative relationship between El Niño–Southern Oscillation (ENSO) and summer Northeast Asia (NEA; 30°–50°N, 110°–140°E) circulation, especially the geopotential height anomalies in the upper troposphere (H200), is weakened from the early 1970s, remains stable since the middle 1980s, and is strengthened dramatically after 1999/2000. The recent transitions of the ENSO–NEA H200 relationship are closely connected with the variation of circumglobal teleconnection (CGT)/Silk Road pattern (SRP), which is further modified by the interdecadal shift of the ENSO–Indian summer monsoon rainfall (ISMR) relationship and ENSO evolution. During 1980–99 when the continuing ENSOs dominate, the weakened ENSO–ISMR relationship leads to an inactive ENSO-related CGT/SRP wave train in the upper-level troposphere and then a weakened connection between ENSO and H200 over the NEA. On the other hand, when the emerging ENSOs dominate after 1999/2000, the restored ENSO–ISMR relationship reinforces the ENSO-related CGT/SRP wave train and then enhances ENSO–NEA H200 relationship. This mechanism is well simulated in the Atmospheric Model Intercomparison Project models (AMIP) and Pacific Ocean–Global Atmosphere (POGA) experiments.

Abstract

An intercomparison of the reanalysis datasets of NCEP and ECMWF is performed with respect to the response of the axial angular momentum M to the torques. While both sets satisfy the budget equations of M reasonably well (except for the time mean), this is not the case with respect to the budget equations for the difference of both sets, where the analysis data explain only a small fraction of the difference of the angular momenta in terms of the difference of the torques. It is hypothesized that the larger fraction of the difference is a manifestation of analysis error. The autocorrelation functions of the differences of the mountain and friction torques between both sets exhibit a long memory, which reflects errors in the low-frequency components of the datasets. Probability distributions of M are considered as well. It is shown that the mean torque for a given M has to be positive (negative) for negative (positive) deviations of M. It is found that the NCEP torques, as analyzed, satisfy this basic requirement. The distribution of the difference of the angular momenta cannot be explained on the basis of the corresponding difference of the torques.

Abstract

Ocean striations are composed of alternating quasi-zonal band-like flows; this kind of organized structure of currents can be found in all the world’s oceans and seas. Previous studies have mainly been focused on the mechanisms of their generation and propagation. This study uses the spatial high-pass filtering to obtain the three-dimensional structure of ocean striations in the North Pacific in both the z coordinate and σ coordinate based on 10-yr averaged Simple Ocean Data Assimilation version 3 (SODA3) data. First, we identify an ideal-fluid potential density domain where the striations are undisturbed by the surface forcing and boundary effects. Second, using the isopycnal layer analysis, we show that on isopycnal surfaces the orientations of striations nearly follow the potential vorticity (PV) contours, while in the meridional–vertical plane the central positions of striations are generally aligned with the latitude of zero gradient of the relative PV. Our analysis provides a simple dynamical interpretation and better understanding for the role of ocean striations.

Abstract

In this study, the Weather Research and Forecasting (WRF) Model and its embedded land surface and urban canopy model are used to simulate effects of urbanization on the local climate of the Las Vegas, Nevada, metropolitan area. High-resolution simulations are performed with a 3-km horizontal resolution over the city. With identical lateral boundary conditions, three land use/land cover (LULC) maps for 2006, 1992, and hypothetical 1900 are used in multiple simulations. The differences in the simulated climate among those cases are used to quantify the urban effect. The study found that urbanization in Las Vegas produces a classic urban heat island (UHI) at night but a minor cooling trend during the day. An analysis of the surface energy balance helps illustrate the major roles of the decreases in surface albedo of solar radiation and increases in the effective emissivity of longwave radiation in shaping the local climate change in Las Vegas. In addition, the emerging urban structures are found to have a mechanical effect of slowing down the climatological wind field over the urban area as a result of an increased effective surface roughness. The slowing down of the diurnal circulation leads to a secondary modification of temperature, which exhibits a complicated diurnal dependence. This suggests the need for more investigations into the coupling of thermodynamic and mechanical effects of urbanization on local climate.

Abstract

The authors investigate the change of atmospheric angular momentum (AAM) in long, transient, coupled atmosphere–ocean model simulations with increasing atmospheric greenhouse gas concentration and sulfate aerosol loading. A significant increase of global AAM, on the order of 4 × 10²⁵ kg m² s⁻¹ for 3 × CO₂–1 × CO₂, was simulated by the Canadian Centre for Climate Modelling and Analysis (CCCma) coupled model. The increase was mainly contributed by the relative component of total AAM in the form of an acceleration of zonal mean zonal wind in the tropical–subtropical upper troposphere. Thus, under strong global warming, a superrotational state emerged in the tropical upper troposphere. The trend in zonal mean zonal wind in the meridional plane was characterized by 1) a tropical–subtropical pattern with two maxima near 30° in the upper troposphere, and 2) a tripole pattern in the Southern Hemisphere extending through the entire troposphere and having a positive maximum at 60°S. The implication of the projected increase of global AAM for future changes of the length of day is discussed.

The CCCma coupled global warming simulation, like many previous studies, shows a significant increase of tropical SST and includes a zonally asymmetric component that resembles El Niño SST anomalies. In the CCCma transient simulations, even though the tropical SST and global AAM both increased nonlinearly with time, the ratio of their time increments ΔAAM/ΔSST remained approximately constant at about 0.9 × 10²⁵ kg m² s⁻¹ (°C)⁻¹. This number is close to its counterpart for the observed global AAM response to El Niño. It is suggested that this ratio may be useful as an index for intercomparisons of different coupled model simulations.

Abstract

The global atmospheric angular momentum (AAM) is known to increase with tropical eastern Pacific sea surface temperature (SST) anomalies during El Niño events. Using a reanalysis dataset, the ratio of the monthly AAM anomaly to El Niño SST anomaly (based on the Niño-3.4 index) is found to be approximately 1 angular momentum unit (=10²⁵ kg m² s⁻¹) per degree Celsius for most post-1975 El Niños. This ratio is much smaller, however, during the 1965/66 and 1972/73 El Niños, raising the possibilities that either the early reanalysis data are in error due to sparse observations, or the atmospheric response to the two early El Niños was unusual. The possibility of a severe data problem in the reanalysis is ruled out by cross-validating the AAM time series with independent measurements of length of day. The latitudinal structures of the zonal wind anomalies in 1965/66 and 1972/73 are examined for both the reanalysis and a set of general circulation model (GCM) simulations. Multiple GCM runs with specified SST produce a more positive ensemble-mean AAM anomaly in 1965 than its counterpart in the reanalysis. The GCM-simulated ensemble-mean zonal wind anomaly resembles the canonical El Niño response with accelerations of subtropical zonal jets in both hemispheres, a pattern that is almost absent in the reanalysis. On the other hand, a large spread exists among the individual ensemble members in the 1965/66 GCM simulations. Although the majority of the individual ensemble members shows the canonical El Niño response, two outliers (out of 12 runs) exhibit very small zonal wind responses in the Northern Hemisphere similar to the reanalysis. Thus, the observed AAM anomaly during 1965/66 is interpreted as an outlier with atmospheric noise being strong enough to overwhelm the canonical El Niño response. The low AAM in the 1972/73 event is related in the reanalysis to a significantly negative zonal wind response on the equator. This signal is robustly reproduced, although with a slightly smaller amplitude, in the ensemble mean and all individual ensemble members in the GCM simulations. The small ensemble standard deviation and large ensemble-mean response on the equator indicate that the negative response is due to the lower-boundary forcing related to the SST anomaly. The fact that the AAM anomaly in 1972/73 is not well correlated with the Niño-3.4 index, then, indicates that SST anomalies outside the conventional El Niño region may be responsible for the low AAM. The uncharacteristically low values of global AAM in 1965/66 and 1972/73 contribute to a low mean for the decade before 1975, which, combined with high AAM in the post-1980 era, produces a significant upward trend in AAM in the second half of the twentieth century. If the weak AAM anomalies during the two pre-1975 El Niños are due to random noise or incidental non-El Niño influences, taking them at face value would result in an overestimate of about 15%–20% in the multidecadal trend of AAM due to boundary forcing alone. Notably, a multidecadal trend in AAM is also simulated in the ensemble mean of the multiple GCM runs, but its magnitude is smaller than the observed counterpart and more consistent with the multidecadal trend of the Niño-3.4 index. The implications of these findings for climate change detection are discussed.

Abstract

The subsurface ocean response to anthropogenic climate forcing remains poorly characterized. From the Coupled Model Intercomparison Project (CMIP), a robust response of the lower thermocline is identified, where the warming is considerably weaker in the subtropics than in the tropics and high latitudes. The lower thermocline change is inversely proportional to the thermocline depth in the present climatology. Ocean general circulation model (OGCM) experiments show that sea surface warming is the dominant forcing for the subtropical gyre change in contrast to natural variability for which wind dominates, and the ocean response is insensitive to the spatial pattern of surface warming. An analysis based on a ventilated thermocline model shows that the pattern of the lower thermocline change can be interpreted in terms of the dynamic response to the strengthened stratification and downward heat mixing. Consequently, the subtropical gyres become intensified at the surface but weakened in the lower thermcline, consistent with results from CMIP experiments.