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Ruichen Zhu
,
Haiyuan Yang
,
Zhaohui Chen
,
Zhiyou Jing
,
Zhiwei Zhang
,
Bingrong Sun
, and
Lixin Wu

Abstract

A variety of submesoscale coherent vortices (SCVs) in the Kuroshio Extension region have been reported by recent observational studies, and the preliminary understanding of their properties, spatial distribution, and possible origins has progressively improved. However, due to relatively sparse in situ observations, the generation mechanisms of these SCVs and associated dynamic processes remain unclear. In this study, we use high-resolution model simulations to fill the gaps of the in situ observations in terms of the three-dimensional structures and life cycles of SCVs. Vortex detection and tracking algorithms are adopted and the characteristics of warm-core and cold-core SCVs are revealed. These vortices have finite Rossby numbers (0.25–0.4), and their horizontal structures can be well described by the Taylor vortex model in terms of the gradient wind balance. The vertical velocity field is characterized by a distinct dipole pattern with upwelling and downwelling cells at the vortex edge. It is very likely that both types of SCVs are generated along the eastern Japan coast through flow–topography interactions, and the Izu–Ogasawara Ridge and Hokkaido slope are found to be two important generation sites where topography friction produces extremely low potential vorticity. After leaving the boundary, SCVs can propagate over long distances and trap a water volume of ∼1011 m3.

Open access
Haiyuan Yang
,
Lixin Wu
,
Ping Chang
,
Bo Qiu
,
Zhao Jing
,
Qiuying Zhang
, and
Zhaohui Chen

Abstract

Using eddy-resolving Community Earth System Model (CESM) simulations, this study investigates mesoscale energetics and air–sea interaction at two different time-scale windows in the Kuroshio Extension (KE) region. Based on an energy budget analysis, it is found that both baroclinic and barotropic pathways contribute to eddy energy generation within the low-frequency window (longer than 3 weeks) in this region, while both air–sea heat fluxes and wind stresses act as prominent eddy killers that remove energy from the ocean. In contrast, within the high-frequency window oceanic variability is mainly fed by baroclinic instability and regulated by turbulent thermal wind (TTW) processes, while the positive wind work is derived primarily from ageostrophic flow, i.e., Ekman drift, and along with air–sea heat fluxes has little influence on geostrophic mesoscale eddies.

Full access
Shengpeng Wang
,
Zhao Jing
,
Qiuying Zhang
,
Ping Chang
,
Zhaohui Chen
,
Hailong Liu
, and
Lixin Wu

Abstract

In this study, the global eddy kinetic energy (EKE) budget in horizontal wavenumber space is analyzed based on 1/10° ocean general circulation model simulations. In both the tropical and midlatitude regions, the barotropic energy conversion from background flow to eddies is positive throughout the wavenumber space and generally peaks at the scale (L e ) where EKE reaches its maximum. The baroclinic energy conversion is more pronounced at midlatitudes. It exhibits a dipolar structure with positive and negative values at scales smaller and larger than L e , respectively. Surface wind power on geostrophic flow results in a significant EKE loss around L e but deposits energy at larger scales. The interior viscous dissipation and bottom drag inferred from the pressure flux convergence act as EKE sink terms. The latter is most efficient at L e while the former is more dominant at smaller scales. There is an evident mismatch between EKE generation and dissipation in the spectral space especially at the midlatitudes. This is reconciled by a dominant forward energy cascade on the equator and a dominant inverse energy cascade at the midlatitudes.

Full access
Xueying Zhang
,
Xu Dong
,
Jing Zeng
,
Shugui Hou
,
Paul C. J. P. Smeets
,
Carleen H. Reijmer
, and
Yetang Wang

Abstract

MODIS (Moderate Resolution Imaging Spectroradiometer) land surface temperature measurements in combination with in situ air temperature records from 119 meteorological stations are used to reconstruct a monthly near-surface air temperature product over the Antarctic Ice Sheet (AIS) by means of a neural network model. The product is generated on a regular grid of 0.05° × 0.05°, spanning from 2001 to 2018. Comparison with independent in situ air temperature measurements shows low uncertainty, with a mean bias of 0.09°C, a mean absolute error of 2.23°C, and a correlation coefficient of 97%. Furthermore, the performance of the reconstruction is better than ERA5 (the fifth-generation ECMWF reanalysis model) against in situ measurements. For the 2001–18 period, the MODIS-based near-surface air temperature product yields annual warming in the East Antarctica, but cooling in the Antarctic Peninsula and West Antarctica. However, they are not statistically significant. This product can also be used to investigate the impact of the Southern Hemisphere annual mode (SAM) on year-to-year variability of air temperature. The enhanced positive phase of SAM in recent decades in austral summer has a cooling effect on East and West Antarctica. In addition, the dataset has the potential application for climate model validation and data assimilation due to the independence of the input of a numerical weather prediction model.

Full access
Jing Duan
,
Yuanlong Li
,
Fan Wang
,
Aixue Hu
,
Weiqing Han
,
Lei Zhang
,
Pengfei Lin
,
Nan Rosenbloom
, and
Gerald A. Meehl

Abstract

The subtropical oceans between 35° and 20°S in the Southern Hemisphere (SH) have exhibited prevailingly rapid sea level rise (SLR) rates since the mid-twentieth century, amplifying damages of coastal hazards and exerting increasing threats to South America, Africa, and Australia. Yet, mechanisms of the observed SLR have not been firmly established, and its representation in climate models has not been examined. By analyzing observational sea level estimates, ocean reanalysis products, and ocean model hindcasts, we show that the steric SLR of the SH subtropical oceans between 35° and 20°S is faster than the global mean rate by 18.2% ± 9.9% during 1958–2014. However, present climate models—the fundamental bases for future climate projections—generally fail to reproduce this feature. Further analysis suggests that the rapid SLR in the SH subtropical oceans is primarily attributable to the persistent upward trend of the southern annular mode (SAM). Physically, this trend in SAM leads to the strengthening of the SH subtropical highs, with the strongest signatures observed in the southern Indian Ocean. These changes in atmospheric circulation promote regional SLR in the SH subtropics by driving upper-ocean convergence. Climate models show systematic biases in the simulated structure and trend magnitude of SAM and significantly underestimate the enhancement of subtropical highs. These biases lead to the inability of models to correctly simulate the observed subtropical SLR. This work highlights the paramount necessity of reducing model biases to provide reliable regional sea level projections.

Full access
Jing-Jia Luo
,
Ruochao Zhang
,
Swadhin K. Behera
,
Yukio Masumoto
,
Fei-Fei Jin
,
Roger Lukas
, and
Toshio Yamagata

Abstract

Climate variability in the tropical Indo-Pacific sector has undergone dramatic changes under global ocean warming. Extreme Indian Ocean dipole (IOD) events occurred repeatedly in recent decades with an unprecedented series of three consecutive episodes during 2006–08, causing vast climate and socioeconomic effects worldwide and weakening the historic El Niño–Indian monsoon relationship. Major attention has been paid to the El Niño influence on the Indian Ocean, but how the IOD influences El Niño and its predictability remained an important issue to be understood. On the basis of various forecast experiments activating and suppressing air–sea coupling in the individual tropical ocean basins using a state-of-the-art coupled ocean–atmosphere model with demonstrated predictive capability, the present study shows that the extreme IOD plays a key role in driving the 1994 pseudo–El Niño, in contrast with traditional El Niño theory. The pseudo–El Niño is more frequently observed in recent decades, coincident with a weakened atmospheric Walker circulation in response to anthropogenic forcing. The study’s results suggest that extreme IOD may significantly enhance El Niño and its onset forecast, which has being a long-standing challenge, and El Niño in turn enhances IOD and its long-range predictability. The intrinsic El Niño–IOD interaction found here provides hope for enhanced prediction skill of both of these climate modes, and it sheds new light on the tropical climate variations and their changes under the influence of global warming.

Full access
Peter A. Bieniek
,
Uma S. Bhatt
,
John E. Walsh
,
T. Scott Rupp
,
Jing Zhang
,
Jeremy R. Krieger
, and
Rick Lader

Abstract

The European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) has been downscaled using a regional model covering Alaska at 20-km spatial and hourly temporal resolution for 1979–2013. Stakeholders can utilize these enhanced-resolution data to investigate climate- and weather-related phenomena in Alaska. Temperature and precipitation are analyzed and compared among ERA-Interim, WRF Model downscaling, and in situ observations. Relative to ERA-Interim, the downscaling is shown to improve the spatial representation of temperature and precipitation around Alaska’s complex terrain. Improvements include increased winter and decreased summer higher-elevation downscaled seasonal average temperatures. Precipitation is also enhanced over higher elevations in all seasons relative to the reanalysis. These spatial distributions of temperature and precipitation are consistent with the few available gridded observational datasets that account for topography. The downscaled precipitation generally exceeds observationally derived estimates in all seasons over mainland Alaska, and it is less than observations in the southeast. Temperature biases tended to be more mixed, and the downscaling reduces absolute bias at higher elevations, especially in winter. Careful selection of data for local site analysis from the downscaling can help to reduce these biases, especially those due to inconsistencies in elevation. Improved meteorological station coverage at higher elevations will be necessary to better evaluate gridded downscaled products in Alaska because biases vary and may even change sign with elevation.

Full access
Qiong Wu
,
Hong-Qing Wang
,
Yi-Zhou Zhuang
,
Yin-Jing Lin
,
Yan Zhang
, and
Sai-Sai Ding

Abstract

Three infrared (IR) indicators were included in this study: the 10.8-μm brightness temperature (BT10.8), the BT difference between 12.0 and 10.8 μm (BTD12.0–10.8), and the BT difference between 6.7 and 10.8 μm (BTD6.7–10.8). Correlations among these IR indicators were investigated using MTSAT-1R images for summer 2007 over East Asia. Temporal, spatial, and numerical frequency distributions were used to represent the correlations. The results showed that large BTD12.0–10.8 values can be observed in the growth of cumulus congestus and associated with the boundary of different terrain where convection was more likely to generate and develop. The results also showed that numerical correlation between any two IR indicators could be expressed by two-dimensional histograms (HT2D). Because of differences in the tropopause heights and in the temperature and water vapor fields, the shapes of the HT2Ds varied with latitude and the type of underlying surface. After carefully analyzing the correlations among the IR indicators, a conceptual model of the convection life cycle was constructed according to these HT2Ds. A new cloud convection index (CCI) was defined with the combination of BTD12.0–10.8 and BTD6.7–10.8 on the basis of the conceptual model. The preliminary test results demonstrated that CCI could effectively identify convective clouds. CCI value and its time trend could reflect the growth or decline of convective clouds.

Full access
Jiawenjing Lan
,
Jun Yang
,
Yongyun Hu
,
Xiang Li
,
Jiaqi Guo
,
Qifan Lin
,
Jing Han
,
Jian Zhang
,
Shuang Wang
, and
Ji Nie

Abstract

For modern Earth, the annual-mean equatorial winds in the upper troposphere are flowing from east to west (i.e., easterly winds). This is mainly due to the deceleration effect of the seasonal cross-equatorial Hadley cells, against the relatively weaker acceleration effect of coupled Rossby and Kelvin waves excited from tropical convection and latent heat release. In this work, we examine the evolution of equatorial winds during the past 250 million years using one global Earth system model, the Community Earth System Model version 1.2.2 (CESM1.2.2). Three climatic factors different from the modern Earth—solar constant, atmospheric CO2 concentration, and land–sea configuration—are considered in the simulations. We find that the upper-tropospheric equatorial winds change sign to westerly flows (called equatorial superrotation) in certain eras, such as 250–230 and 150–50 Ma. The strength of the superrotation is below 4 m s−1, comparable to the magnitude of the present-day easterly winds. In general, this phenomenon occurs in a warmer climate within which the tropical atmospheric circulation shifts upward in altitude, stationary and/or transient eddies are relatively stronger, and/or the Hadley cells are relatively weaker, which in turn are due to the changes of the three factors, especially CO2 concentration and land–sea configuration.

Free access
Shaofeng Hua
,
Baojun Chen
,
Yubao Liu
,
Gang Chen
,
Yang Yang
,
Xiaobo Dong
,
Zhen Zhao
,
Yang Gao
,
Xu Zhou
,
Rong Zhang
, and
Jing Duan

Abstract

Airborne microphysical measurements of a frontal precipitation event in North China were used to evaluate five microphysics schemes for predicting the bulk properties of ice particles. They are the Morrison and Thompson schemes, which use predetermined categories, the 1-ice- and 2-ice-category configurations of the Predicted Particle Properties (P3) scheme and the Ice-Spheroids Habit Model with Aspect-Ratio Evolution (ISHMAEL) scheme, which model the evolution of particle properties, and the spectral bin fast version (SBM_fast) microphysics scheme within the Weather Research and Forecasting (WRF) Model. WRF simulations with these schemes successfully reproduced the observed temperature and the liquid and total water content profiles at corresponding times and locations, allowing for a credible comparison of the predictions of particle properties with the aircraft measurements. The simulated results with the 1-ice-category P3 scheme are in good agreement with the observations for all the particle properties we examined. The 2-ice-category P3 scheme overestimates the spectrum width and underestimates the number concentration, which can be alleviated by reducing the ice collection efficiency. The simulation with the SBM_fast scheme deviates from the observed ice particle size distributions since the mass–diameter relationship of snow-sized particles adopted in this scheme may not be applicable to this stratiform cloud case.

Free access