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Hui Liu and Jun Li

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Hyperspectral infrared (IR) sounders, such as the Atmospheric Infrared Sounder (AIRS) and the Infrared Atmospheric Sounding Interferometer (IASI), provide unprecedented global atmospheric temperature and moisture soundings with high vertical resolution and accuracy. In this paper, the authors investigate whether advanced IR soundings of water vapor and temperature observations can improve the analysis of a tropical cyclone vortex and the forecast of rapid intensification of a tropical cyclone. Both the IR water vapor and temperature soundings significantly improve the typhoon vortex in the analysis and the forecast of the rapid intensification of Typhoon Sinlaku (2008). The typhoon track forecast is also substantially improved when the full spatial resolution AIRS soundings are assimilated. This study demonstrates the potential important application of high spatial and hyperspectral IR soundings in forecasting tropical cyclones.

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Hui Li and Ryan L. Sriver

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Tropical cyclone (TC)-induced ocean vertical mixing can alter the upper-ocean temperature structure, influencing ocean heat content variability and meridional ocean heat transport. TC–ocean interactions can influence tropical variability on seasonal to interannual time scales. Here the impacts of TCs on the global ocean and the associated feedbacks are investigated using a hierarchy of high-resolution global ocean model simulations featuring the Community Earth System Model (CESM). The aim is to understand the potential impact of the model’s self-generated transient TC events on the modeled global ocean. Two ocean-only simulations are performed using the atmosphere boundary conditions from a fully coupled preindustrial CESM simulation configured with 0.25° atmosphere resolution and the nominal 1° ocean resolution (with ~0.25° meridional resolution in the tropics). The high-resolution coupled model is capable of directly simulating TC events with wind structure and climatology generally consistent with observations. TC effects at the ocean–atmosphere boundary are filtered out in one of the ocean simulations (OCN_FILT) while fully retained in the other (OCN_TC) in order to isolate the effect of the TCs on regional and global ocean variability across multiple time scales (from intraseasonal to interdecadal). Results show that the model-simulated TCs can 1) alter surface and subsurface ocean temperature patterns and variability; 2) affect ocean energetics, including increasing ocean mixed layer depth and strengthening subtropical gyre and meridional overturning circulations; and 3) influence ocean meridional heat transport and ocean heat content from seasonal to interannual time scales. Results help provide insights into the model behavior and the physical nature of the effect of TCs within the Earth system.

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Hui Li, Alexey Fedorov, and Wei Liu

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This study compares the impacts of Arctic sea ice decline on the Atlantic Meridional Overturning Circulation (AMOC) in two configurations of the Community Earth System Model (CESM) with different horizontal resolution. In a suite of model experiments we impose radiative imbalance at the ice surface, replicating a loss of sea ice cover comparable to the observed during 1979-2014, and find dramatic differences in the AMOC response between the two models. In the lower-resolution configuration, the AMOC weakens by about one third over the first 100 years, approaching a new quasi-equilibrium. By contrast, in the higher-resolution configuration, the AMOC weakens by ~10% during the first 20-30 years followed by a full recovery driven by invigorated deep water formation in the Labrador Sea and adjacent regions. We investigate these differences using a diagnostic AMOC stability indicator, which reflects the AMOC freshwater transport in and out of the basin and hence the strength of the basin-scale salt-advection feedback. This indicator suggests that the AMOC in the lower-resolution model is less stable and more sensitive to surface perturbations, as confirmed by hosing experiments mimicking Arctic freshening due to sea ice decline. Differences between the models’ mean states, including the Atlantic mean surface freshwater fluxes, control the differences in AMOC stability. Our results demonstrate that the AMOC stability indicator is indeed useful for evaluating AMOC sensitivity to perturbations. Finally, we emphasize that, despite the differences in the long-term adjustment, both models simulate a multi-decadal AMOC weakening caused by Arctic sea ice decline, relevant to climate change.

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Wenhong Li, Laifang Li, Rong Fu, Yi Deng, and Hui Wang

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This study investigates the changes of the North Atlantic subtropical high (NASH) and its impact on summer precipitation over the southeastern (SE) United States using the 850-hPa geopotential height field in the National Centers for Environmental Prediction (NCEP) reanalysis, the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), long-term rainfall data, and Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) model simulations during the past six decades (1948–2007). The results show that the NASH in the last 30 yr has become more intense, and its western ridge has displaced westward with an enhanced meridional movement compared to the previous 30 yr. When the NASH moved closer to the continental United States in the three most recent decades, the effect of the NASH on the interannual variation of SE U.S. precipitation is enhanced through the ridge’s north–south movement. The study’s attribution analysis suggested that the changes of the NASH are mainly due to anthropogenic warming. In the twenty-first century with an increase of the atmospheric CO2 concentration, the center of the NASH would be intensified and the western ridge of the NASH would shift farther westward. These changes would increase the likelihood of both strong anomalously wet and dry summers over the SE United States in the future, as suggested by the IPCC AR4 models.

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Li Tao, Tim Li, Yuan-Hui Ke, and Jiu-Wei Zhao

Abstract

A Pacific–Japan (PJ) pattern index is defined based on the singular value decomposition (SVD) analysis of summertime 500-hPa height in East Asia and precipitation in the tropical western North Pacific (WNP). The time series of this PJ index shows clearly the interannual and interdecadal variations since 1948. Idealized atmospheric general circulation model (AGCM) experiments were carried out to understand the remote and local SST forcing in causing the interannual variations of the PJ pattern and interdecadal variations of the PJ-like pattern. It is found that the PJ interannual variation is closely related to El Niño–Southern Oscillation (ENSO). A basinwide warming occurs in the tropical Indian Ocean (TIO) during El Niño mature winter. The TIO warming persists from the El Niño peak winter to the succeeding summer. Meanwhile, a cold SST anomaly (SSTA) appears in the eastern WNP and persists from the El Niño mature winter to the succeeding summer. Idealized AGCM experiments that separate the TIO and WNP SSTA forcing effects show that both the remote eastern TIO forcing and local WNP SSTA forcing are important in affecting atmospheric heating anomaly in the WNP monsoon region, which further impacts the PJ interannual teleconnection pattern over East Asia. In contrast to the interannual variation, the interdecadal change of the PJ-like pattern is primarily affected by the interdecadal change of SST in the TIO rather than by the local SSTA in the WNP.

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Fei Liu, Tim Li, Hui Wang, Li Deng, and Yuanwen Zhang

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The authors investigate the effects of El Niño and La Niña on the intraseasonal oscillation (ISO) in the boreal summer (May–October) over the western North Pacific (WNP). It is found that during El Niño summers, the ISO is dominated by a higher-frequency oscillation with a period of around 20–40 days, whereas during La Niña summers the ISO is dominated by a lower-frequency period of around 40–70 days. The former is characterized by northwestward-propagating convection anomalies in the WNP, and the latter is characterized by northward- and eastward-propagating convective signals over the tropical Indian Ocean/Maritime Continent. The possible mechanisms through which El Niño–Southern Oscillation (ENSO)-induced background mean state changes influence the ISO behavior are examined through idealized numerical experiments. It is found that enhanced (weakened) mean moisture and easterly (westerly) vertical wind shear in the WNP during El Niño (La Niña) are the main causes of the strengthened (weakened) 20–40-day northwestward-propagating ISO mode, whereas the 40–70-day ISO initiated from the Indian Ocean can only affect the WNP during La Niña years because the dry (moist) background moisture near the Maritime Continent during El Niño (La Niña) suppresses (enhances) the ISO over the Maritime Continent, and the ISO propagates less over the Maritime Continent during El Niño years than in La Niña years.

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Wenhong Li, Laifang Li, Rong Fu, Yi Deng, and Hui Wang

Abstract

Recently Diem questioned the western ridge movement of the North Atlantic subtropical high (NASH) reported in a 2011 paper of Li et al. This reply shows more analysis that further strengthens the conclusions originally put forth by Li et al. Diem’’s analysis of the trend in the western ridge of the NASH was based on the data over a 30-yr period (1978–2007), whereas the main conclusions in Li et al. were drawn according to the data over a 60-yr period (1948–2007). Over the last 60 years, the NASH has shown a significant trend of westward movement, the meridional movement of the western ridge of the NASH has enhanced in the recent three decades, and the potential impact of global warming cannot be ruled out in an attempt to explain these changes of the NASH.

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Qingqing Li, Yihong Duan, Hui Yu, and Gang Fu

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In this study, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to simulate Typhoon Rananim (2004) at high resolution (2-km grid size). The simulation agrees well with a variety of observations, especially for intensification, maintenance, landfall, and inner-core structures, including the echo-free eye, the asymmetry in eyewall convection, and the slope of the eyewall during landfall. The asymmetric feature of surface winds is also captured reasonably well by the model, as well as changes in surface winds and pressure near the storm center.

The shear-induced vortex tilt and storm-relative asymmetric winds are examined to investigate how vertical shear affects the asymmetric convection in the inner-core region. The inner-core vertical shear is found to be nonunidirectional, and to induce a nonunidirectional vortex tilt. The distribution of asymmetric convection is, however, inconsistent with the typical downshear-left pattern for a deep-layer shear. Qualitative agreement is found between the divergence pattern and the storm-relative flow, with convergence (divergence) generally associated with asymmetric inflow (outflow) in the eyewall. The collocation of the inflow-induced lower-level convergence in the boundary layer and the lower troposphere and the midlevel divergence causes shallow updrafts in the western and southern parts of the eyewall, while the deep and strong upward motion in the southeastern portion of the eyewall is due to the collocation of the net convergence associated with the strong asymmetric flow in the midtroposphere and the inflow near 400 hPa and its associated divergence in the outflow layer above 400 hPa.

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Hui Yu, Peiyan Chen, Qingqing Li, and Bi Tang

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Forecasts of tropical cyclone (TC) intensity from six operational models (three global models and three regional models) during 2010 and 2011 are assessed to study the current capability of model guidance in the western North Pacific. The evaluation is performed on both V max and P min from several aspects, including the relative error, skill assessment, category score, the hitting rate of trend, and so on. It is encouraging to see that the models have had some skills in the prediction of TC intensity, including that two of them are better than a statistical baseline in V max at several lead times and three of them show some skill in intensity change. With dissipated cases included, all the models have skills in category and trend forecasting at lead times longer than 24 h or so. The model forecast errors are found to be significantly correlated with initial error and the observed initial intensity. A statistical calibration scheme for model forecasting is proposed based on such an attribute, which is more effective for P min than V max. The statistically calibrated model forecasts are important in setting up a skillful multimodel consensus, for either the mean or the statistically weighted mean. The V max forecasts converted from the calibrated P min consensus based on a statistical wind–pressure relationship show significant skill over the baseline and a skillful scheme is also proposed to deal with the delay of the model forecasts in operation.

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Hui Wang, Rong Fu, Arun Kumar, and Wenhong Li

Abstract

The variability of summer precipitation in the southeastern United States is examined in this study using 60-yr (1948–2007) rainfall data. The Southeast summer rainfalls exhibited higher interannual variability with more intense summer droughts and anomalous wetness in the recent 30 years (1978–2007) than in the prior 30 years (1948–77). Such intensification of summer rainfall variability was consistent with a decrease of light (0.1–1 mm day−1) and medium (1–10 mm day−1) rainfall events during extremely dry summers and an increase of heavy (>10 mm day−1) rainfall events in extremely wet summers. Changes in rainfall variability were also accompanied by a southward shift of the region of maximum zonal wind variability at the jet stream level in the latter period. The covariability between the Southeast summer precipitation and sea surface temperatures (SSTs) is also analyzed using the singular value decomposition (SVD) method. It is shown that the increase of Southeast summer precipitation variability is primarily associated with a higher SST variability across the equatorial Atlantic and also SST warming in the Atlantic.

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