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Yu Ping Guan and Rui Xin Huang

Abstract

The classical two-box model of Stommel is extended in two directions: replacing the buoyancy constraint with an energy constraint and including the wind-driven gyre. Stommel postulated a buoyancy constraint for the thermohaline circulation, and his basic idea has evolved into the dominating theory of thermohaline circulation; however, recently, it is argued that the thermohaline circulation is maintained by mechanical energy from wind stress and tides. The major difference between these two types of models is the bifurcation structure: the Stommel-like model has two thermal modes (one stable and another one unstable) and one stable haline mode, whereas the energy-constraint model has one stable thermal mode and two saline modes (one stable and another one unstable). Adding the wind-driven gyre changes the threshold value of thermohaline bifurcation greatly; thus, the inclusion of the wind-driven gyre is a vital step in completely modeling the physical processes related to thermohaline circulation.

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Chien-Ben Chou and Huei-Ping Huang

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The use of the Advanced Very High Resolution Radiometer (AVHRR) data in a one-dimensional variational scheme is examined to retrieve cloud parameters and atmospheric profiles. The variational scheme used TIROS Operational Vertical Sounder radiance data for retrieval. The AVHRR data were used in the partly cloudy and cloudy cases to provide initial guesses for cloud parameters in the iterative scheme, to detect the presence of cirrus clouds, and to determine the sea surface temperature used in retrieval. Sensitivity tests showed that the error in the initial guesses of cloud parameters has substantial impact on the accuracy of the retrieved fields; this sensitivity increases with increased cloudiness. Cloud parameters deduced from AVHRR data are nearly optimal, in terms of maximizing the efficiency of convergence, as the initial guesses for the retrieval scheme. In the absence of cirrus cloud, a retrieval procedure incorporating AVHRR initial guesses produced temperature and humidity profiles for partly cloudy cases that are about as accurate as those for clear cases. In both cases the maximum improvement made in the retrieval procedure over background error was about 0.2 K in the temperature profile, and 0.05 (in logarithm of mixing ratio) in the humidity profile. For partly cloudy cases, best retrieval results were obtained for a low cloud top, or a middle cloud top but with small cloud fraction. Cirrus cloud remains a problem, as its presence generally degrades the quality of retrieval.

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Girish Nigamanth Raghunathan and Huei-Ping Huang

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This study performs an updated analysis of Northern Hemisphere retrograde disturbances that were first identified by classical observational studies as one of the dominating coherent structures in the higher latitudes on the submonthly time scale. Analyzing 8–30-day bandpass-filtered data based on reanalysis, a set of criteria on the phase and amplitude of zonal wave-1 Fourier coefficients of geopotential height anomalies at 250 mb (1 mb = 1 hPa) and 60°N are used to identify strong retrograde-wave events in the spirit of Madden and Speth. The new catalog of retrograde-wave events from 1979 to 2017 is used to extract basic statistics and structures of retrograde waves across all major events. The results broadly agree with those reported in the classical observational studies, reaffirming the robustness of the phenomenon. The new catalog can be used to aid further studies on the mechanisms and predictability of retrograde waves. As an example, an analysis of isentropic potential vorticity over the Pacific sector for selected retrograde-wave events reveals the common occurrence of an extrusion of low-PV air into the higher latitudes, followed by a westward shift of the low-PV patch and vortex shedding. Future directions of research surrounding the retrograde-wave phenomenon are discussed.

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Kaiming Hu, Shang-Ping Xie, and Gang Huang

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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.

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Gang Huang, Kaiming Hu, and Shang-Ping Xie

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The correlation of northwest (NW) Pacific climate anomalies during summer with El Niño–Southern Oscillation (ENSO) in the preceding winter strengthens in the mid-1970s and remains high. This study investigates the hypothesis that the tropical Indian Ocean (TIO) response to ENSO is key to this interdecadal change, using a 21-member ensemble simulation with the Community Atmosphere Model, version 3 (CAM3) forced by the observed history of sea surface temperature (SST) for 1950–2000. In the model hindcast, the TIO influence on the summer NW Pacific strengthens in the mid-1970s, and the strengthened TIO teleconnection coincides with an intensification of summer SST variability over the TIO. This result is corroborated by the fact the model’s skills in simulating NW Pacific climate anomalies during summer increase after the 1970s shift.

During late spring to early summer, El Niño–induced TIO warming decays rapidly for the epoch prior to the 1970s shift but grows and persists through summer for the epoch occurring after it. This difference in the evolution of the TIO warming determines the strength of the TIO teleconnection to the NW Pacific in the subsequent summer. An antisymmetric wind pattern develops in spring across the equator over the TIO, and the associated northeasterly anomalies aid the summer warming over the north Indian Ocean by opposing the prevailing southwest monsoon. In the model, this antisymmetric spring wind pattern is well developed after but absent before the 1970s shift.

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Yanyan Huang, Huijun Wang, and Ping Zhao

Abstract

The summer (June–August) Asian–Pacific Oscillation (APO) measures the interannual variability of large-scale atmospheric circulation over the Asian–North Pacific Ocean sector. In this study, the authors assess the predictability of the summer APO index interannual variability and the associated atmospheric circulation anomalies using the 1959–2001 hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF), Centre National de Recherches Météorologiques (CNRM), and the Met Office (UKMO) general circulation models from the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) project. The results show that these models predict the summer APO index interannual variability well and have higher skill for the North Pacific than for the Asian upper-tropospheric temperature. Meanwhile, the observed APO-related atmospheric circulation anomalies in the South Asian high, the tropical easterly wind jet over the Asian monsoon region in the upper troposphere, the subtropical anticyclone over the North Pacific, and the summer southwest monsoon over Asia in the lower troposphere are reasonably well predicted in their spatial patterns and intensities. Compared with the observations, however, these models display low skill in predicting the long-term varying trends of the upper-tropospheric temperature over the Asian–North Pacific sector or the APO index during 1959–2001.

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Ping Huang, Xiao-Tong Zheng, and Jun Ying

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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.

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Wenping Jiang, Gang Huang, Ping Huang, and Kaiming Hu

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The northwest Pacific anticyclone (NWPAC) anomalies during post–El Niño summers are a key predictor of the summer climate in East Asia and the northwestern Pacific (NWP). Understanding how this will change under global warming is crucial to project the changes in the variability of the northwest Pacific summer monsoon. Outputs from 18 selected coupled models from phase 5 of the Coupled Model Intercomparison Project show that the anomalous NWPAC response to El Niño will likely be weakened under global warming, which is attributed to the decreased zonal contrast between the tropical Indian Ocean (TIO) warming and the NWP cooling during post–El Niño summers. Under global warming, the NWPAC anomalies during the El Niño mature winter are weakened because of decreased atmospheric circulation in response to El Niño–Southern Oscillation (ENSO), which leads to the weakening of local air–sea interaction and then decreases the cold NWP SST anomalies. Furthermore, the decreased surface heat flux anomalies, the weakened anticyclone anomalies over the southeastern Indian Ocean, and the slackened anomalous easterlies over the north Indian Ocean weaken the warm TIO SST anomalies. However, the strengthened tropospheric temperature anomalies could enhance the anomalous TIO warming. Although the changes in TIO SST anomalies are indistinctive, the weakening of the SST anomaly gradient between the TIO and the NWP is robust to weaken the NWPAC anomalies during post–El Niño summers. Moreover, the positive feedback between the TIO–NWP SST anomalies and the NWPAC anomalies will enhance the weakening of NWPAC under global warming.

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Wenping Jiang, Ping Huang, Gang Huang, and Jun Ying

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An excessive westward extension of the simulated ENSO-related sea surface temperature (ENSO SST) variability in the CMIP5 and CMIP6 models is the most apparent ENSO SST pattern bias and dominates the intermodel spread in ENSO SST variability among the models. The ENSO SST bias lowers the models’ skill in ENSO-related simulations and induces large intermodel uncertainty in ENSO-related projections. The present study investigates the origins of the excessive westward extension of ENSO SST in 25 CMIP5 and 25 CMIP6 models. Based on the intermodel spread of ENSO SST variability simulated in the 50 models, we reveal that this ENSO SST bias among the models largely depends on the simulated cold tongue strength in the equatorial western Pacific (EWP). Models simulating a stronger cold tongue tend to simulate a larger mean zonal SST gradient in the EWP and then a larger zonal advection feedback in the EWP, favoring a more westward extension of the ENSO SST pattern. In addition, with the overall improvement in the EWP cold tongue from CMIP5 to CMIP6, the excessive westward extension bias of ENSO SST in CMIP6 models is also reduced relative to those in CMIP5 models. The results suggest that the bias and intermodel disagreement in the mean-state SST have been improved, which improves ENSO simulation.

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Joseph Egger, Klaus-Peter Hoinka, Klaus Weickmann, and Huei-Ping Huang

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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.

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