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Minghao Yang
,
Chongyin Li
,
Xiong Chen
,
Yanke Tan
,
Xin Li
,
Chao Zhang
, and
Guiwan Chen

Abstract

The reproducibility of climatology and the midwinter suppression of the cold-season North Pacific storm track (NPST) in historical runs of 18 CMIP6 models is evaluated against the NCEP reanalysis data. The results show that the position of the climatological peak area of 850-hPa meridional eddy heat flux (υT850) is well captured by these models. The spatial patterns of climatological υT850 are basically consistent with the NCEP reanalysis. Generally, NorESM2-LM and CESM2-WACCM present a relatively strong capability to reproduce the climatological amplitude of υT850 with lower RMSE than the other models. Compared with CMIP5 models, the intermodel spread of υT850 climatology among the CMIP6 models is smaller, and their multimodel ensemble is closer to the NCEP reanalysis. The geographical distribution in more than half of the selected models is farther south and east. For the subseasonal variability of υT850, nearly half of the models exhibit a double-peak structure. In contrast, the apparent midwinter suppression in the NPST represented by the 250-hPa filtered meridional wind variance (υυ250) is reproduced by all the selected models. In addition, the present study investigates the possible reasons for simulation biases regarding climatological NPST amplitude. It is found that a higher model horizontal resolution significantly intensifies the climatological υυ250. There is a significant in-phase relationship between climatological υυ250 and the intensity of the East Asian winter monsoon (EAWM). However, the climatological υT850 is not sensitive to the model grid spacing. Additionally, the climatological low-tropospheric atmospheric baroclinicity is uncorrelated with climatological υυ250. The stronger climatological baroclinic energy conversion is associated with the stronger climatological υT850.

Open access
Minghao Yang
,
Chongyin Li
,
Xin Li
,
Yanke Tan
,
Xiong Chen
, and
Chao Zhang

Abstract

Based on the daily NCEP reanalysis, the present study investigates the interdecadal change in the relationship between the winter North Pacific storm track (WNPST) and the East Asian winter monsoon (EAWM), and evaluates the WNPST–EAWM relationship in 17 CMIP6 models. The results show that the out-of-phase WNPST–EAWM relationship underwent an interdecadal change in the mid-1980s. The WNPST–EAWM relationship became less significant during period 2 (P2; 1990–2015). The atmospheric circulation anomaly related to the EAWM during period 1 (P1; 1955–80) is more robust than that during P2. The interdecadal weakening WNPST–EAWM relationship may be attributed to the interdecadal damping WNPST–EAWM interaction. The EAWM-related anomalous baroclinic energy conversion and moisture effects, including meridional and vertical eddy moisture fluxes, contribute to the significant attenuation of the WNPST during P1. The transient eddy-induced dynamic forcing and thermal forcing anomalies, as well as the barotropic process represented by the local Eliassen–Palm flux divergence associated with WNPST, can also significantly manipulate the upper-tropospheric jet during P1. However, the atmospheric circulation and interaction between the WNPST and EAWM during P2 are not as significant as those during P1. The effect of ENSO on the WNPST is significantly different before and after the mid-1980s. After the mid-1980s, the WNPST shows the characteristic of moving equatorward during El Niño events. It seems that ENSO takes over the WNPST from the EAWM after the mid-1980s. In addition, except for BCC-ESM1, CanESM5, and SAM0-UNICON, most of the CMIP6 models cannot reproduce the significant out-of-phase WNPST–EAWM relationship.

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Zhijin Li
,
Yi Chao
,
James C. McWilliams
, and
Kayo Ide

Abstract

A three-dimensional variational data assimilation (3DVAR) scheme has been developed within the framework of the Regional Ocean Modeling System (ROMS). This ROMS3DVAR enables the capability of predicting meso- to small-scale variations with temporal scales from hours to days in coastal oceans. To cope with particular difficulties that result from complex coastlines and bottom topography, unbalanced flows, and sparse observations, ROMS3DVAR includes novel strategies. These strategies include the implementation of three-dimensional anisotropic and inhomogeneous error correlations based on a Kronecker product, application of particular weak dynamic constraints, and implementation of efficient and reliable algorithms for minimizing the cost function. The formulation of ROMS3DVAR is presented here, and its implementation off the West Coast is currently under way.

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Yi Chao
,
Zhijin Li
,
John D. Farrara
, and
Peter Hung

Abstract

A two-dimensional variational data assimilation (2DVAR) method for blending sea surface temperature (SST) data from multiple observing platforms is presented. This method produces continuous fields and has the capability of blending multiple satellite and in situ observations. In addition, it allows specification of inhomogeneous and anisotropic background correlations, which are common features of coastal ocean flows. High-resolution (6 km in space and 6 h in time) blended SST fields for August 2003 are produced for a region off the California coast to demonstrate and evaluate the methodology. A comparison of these fields with independent observations showed root-mean-square errors of less than 1°C, comparable to the errors in conventional SST observations. The blended SST fields also clearly reveal the finescale spatial and temporal structures associated with coastal upwelling, demonstrating their utility in the analysis of finescale flows. With the high temporal resolution, the blended SST fields are also used to describe the diurnal cycle. Potential applications of this SST blending methodology in other coastal regions are discussed.

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Xianjin Li
,
Yi Chao
,
James C. McWilliams
, and
Lee-Lueng Fu

Abstract

The upper Pacific Ocean Current and temperature have been simulated by a three-dimensional ocean general circulation model (OGCM) with two different vertical-mixing schemes. One corresponds to the modified Richardson number–dependent scheme of Pacanowski and Philander (PP); the other is adapted from the newly developed K-Profile Parameterization (KPP) scheme. The performance of both schemes in a Pacific OGCM is evaluated under the same model configuration and boundary conditions. Model and data comparisons are made for the mean state, annual cycle, and interannual-to-interdecadal variability. In the Tropics, both the PP and KPP schemes produce reasonably realistic tropical thermal and current structures; however, KPP is better than PP in several important aspects. For example, the KPP scheme simulates a more realistic thermocline and significantly reduces the cold surface temperature bias in the eastern equatorial Pacific. The depth of the maximum Equatorial Undercurrent simulated by the KPP scheme is much closer to the observation. In the extratropics the KPP scheme is significantly better than the PP scheme in simulating the thermal and current structures, including the annual mean, annual cycle, and interannual-to-interdecadal variability.

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Xiangzhou Song
,
Chunlin Ning
,
Yongliang Duan
,
Huiwu Wang
,
Chao Li
,
Yang Yang
,
Jianjun Liu
, and
Weidong Yu

Abstract

Six-month buoy-based heat flux observations from the poorly sampled tropical southeastern Indian Ocean are examined to document the extremes during three tropical cyclones (TCs) from December 2018 to May 2019. The most striking feature at the mooring site (16.9°S, 115.2°E) during the TCs is the extensively suppressed diurnal cycle of the net surface flux (Qnet), with a mean daytime (nighttime) reduction of 470 (131) W m−2, a peak decrease at approximately noon of 695 W m−2 and an extreme drop during TC Riley of 800 W m−2. The mean surface cooling in the daytime is primarily contributed by the 370 W m−2 decrease in shortwave radiation associated with the increased cloudiness. The air–sea turbulent heat fluxes increase by approximately 151 W m−2 in response to the enhanced wind speed under near-neutral boundary conditions. The daily mean rainfall-induced cooling is 8 W m−2, with a maximum magnitude of 90 W m−2. The mean values, seasonal variation, and synoptic variability of the characteristic heat fluxes are used to assess the new reanalysis data from ERA5 and MERRA2 and the analyzed OAFlux. The overall performance of the high-frequency net heat flux estimates at the synoptic scale is satisfactory, but the four flux components exhibit different quality levels. A serious error is that ERA5 and MERRA2 poorly represent TCs, and they show significant daily mean Qnet biases with opposite directions, −59 W m−2 (largely due to the overestimated latent heat with a bias of −76 W m−2) and 50 W m−2 (largely due to the overestimated shortwave radiation with a bias of 41 W m−2), respectively.

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Chao Li
,
Ying Sun
,
Francis Zwiers
,
Dongqian Wang
,
Xuebin Zhang
,
Gang Chen
, and
Hui Wu

Abstract

On the basis of a newly developed observational dataset and a suite of climate model simulations, we evaluate changes in summer mean wet bulb globe temperature (WBGT) in China from 1961 through 2080. We show that summer mean WBGT has increased almost everywhere across China since 1961 as a result of human-induced climate change. Consequently, hot summers as measured by summer mean WBGT are becoming more frequent and more conducive to heat stress. Hot summers like the hottest on record during 1961–2015 in western or eastern China are now expected occur once every 3–4 years. These hot WBGT summers have become more than 140 times as likely in eastern China in the present decade (2010s) as in the 1961–90 baseline period and more than 1000 times as likely in western China. The substantially larger influence in western China is associated with its stronger warming signal, which is likely due to the high Bowen ratio of sensible to latent heat fluxes of dry soils and increases in absorbed solar radiation from the decline in mountain snow cover extent. Observation-constrained projections of future summer mean WBGT under the RCP8.5 emissions scenario indicate that, by the 2040s, almost every summer in China will be at least as hot as the hottest summer in the historical record, and by the 2060s it will be common (on average, every other year) for summers to be as much as 3.0°C hotter than the historical record, pointing to potentially large increases in the likelihood of human heat stress and to a massive adaption challenge.

Open access
Siyan Dong
,
Ying Sun
,
Chao Li
,
Xuebin Zhang
,
Seung-Ki Min
, and
Yeon-Hee Kim

Abstract

While the IPCC Fifth Assessment Working Group I report assessed observed changes in extreme precipitation on the basis of both absolute and percentile-based extreme indices, human influence on extreme precipitation has rarely been evaluated on the basis of percentile-based extreme indices. Here we conduct a formal detection and attribution analysis on changes in four percentile-based precipitation extreme indices. The indices include annual precipitation totals from days with precipitation exceeding the 99th and 95th percentiles of wet-day precipitation in 1961–90 (R99p and R95p) and their contributions to annual total precipitation (R99pTOT and R95pTOT). We compare these indices from a set of newly compiled observations during 1951–2014 with simulations from models participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6). We show that most land areas with observations experienced increases in these extreme indices with global warming during the historical period 1951–2014. The new CMIP6 models are able to reproduce these overall increases, although with considerable over- or underestimations in some regions. An optimal fingerprinting analysis reveals detectable anthropogenic signals in the observations of these indices averaged over the globe and over most continents. Furthermore, signals of greenhouse gases can be separately detected, taking other forcing into account, over the globe and over Asia in these indices except for R95p. In contrast, signals of anthropogenic aerosols and natural forcings cannot be detected in any of these indices at either global or continental scales.

Open access
Zhiyu Li
,
Wenjun Zhang
,
Malte F. Stuecker
,
Haiming Xu
,
Fei-Fei Jin
, and
Chao Liu

Abstract

The present work investigates different responses of Arctic surface air temperature (SAT) to two ENSO types based on reanalysis datasets and model experiments. We find that eastern Pacific (EP) ENSO events are accompanied by statistically significant SAT responses over the Barents–Kara Seas in February, while central Pacific (CP) events coincide with statistically significant SAT responses over northeastern Canada and Greenland. These impacts are largely of opposite sign for ENSO warm and cold phases. During EP El Niño in February, the enhanced tropospheric polar vortex over Eurasia and associated local low-level northeasterly anomalies over the Barents–Kara Seas lead to anomalously cold SAT in this region. Simultaneously, the enhanced tropospheric polar vortex leads to enhanced sinking air motion and consequently reduced cloud cover. This in turn reduces downward infrared radiation (IR), which further reduces SAT in the Barents–Kara Seas region. Such a robust response cannot be detected during other winter months for EP ENSO events. During CP El Niño, the February SATs over northeastern Canada and Greenland are anomalously warm and coincide with a weakened tropospheric polar vortex and related local low-level southwesterly anomalies originating from the Atlantic Ocean. The anomalous warmth can be enhanced by the local positive feedback. Similar SAT signals as in February during CP ENSO events can also be seen in January, but they are less statistically robust. We demonstrate that these contrasting Arctic February SAT responses are consistent with responses to the two ENSO types with a series of atmospheric general circulation model experiments. These results have implications for the seasonal predictability of regional Arctic SAT anomalies.

Full access
George Maier
,
Andrew Grundstein
,
Woncheol Jang
,
Chao Li
,
Luke P. Naeher
, and
Marshall Shepherd

Abstract

Extreme heat is the leading weather-related killer in the United States. Vulnerability to extreme heat has previously been identified and mapped in urban areas to improve heat morbidity and mortality prevention efforts. However, only limited work has examined vulnerability outside of urban locations. This study seeks to broaden the geographic context of earlier work and compute heat vulnerability across the state of Georgia, which offers diverse landscapes and populations with varying sociodemographic characteristics. Here, a modified heat vulnerability index (HVI) developed by Reid et al. is used to characterize vulnerability by county. About half of counties with the greatest heat vulnerability index scores contain the larger cities in the state (i.e., Athens, Atlanta, Augusta, Columbus, Macon, and Savannah), while the other half of high-vulnerability counties are located in more rural counties clustered in southwestern and east-central Georgia. The source of vulnerability varied between the more urban and rural high-vulnerability counties, with poverty and population of nonwhite residents driving vulnerability in the more urban counties and social isolation/population of elderly/poor health the dominant factor in the more rural counties. Additionally, the effectiveness of the HVI in identifying vulnerable populations was investigated by examining the effect of modification of the vulnerability index score with mortality during extreme heat. Except for the least vulnerable categories, the relative risk of mortality increases with increasing vulnerability. For the highest-vulnerability counties, oppressively hot days lead to a 7.7% increase in mortality.

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