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Chongran Zhang
,
Jing Zhang
, and
Qigang Wu

Abstract

Enhanced surface melt over the ice shelves of the Antarctic Peninsula (AP) is one of the precursors to their collapse, which can be proceeded by accelerated ground glacier flow and increased contribution to sea level rise. With the collapse of Larsen A and B and the major 2017 calving event from Larsen C, whether Larsen C is bound for a similar fate has received increasing attention. Here, the interannual variation of regional circulation over the AP region is studied using the empirical orthogonal function (EOF)/principal component (PC) analysis on the sea level pressure of ERA5. The EOF modes capture the variations of depth, location, and extent of Amundsen Sea low and Weddell Sea low in each season. Statistically significant positive correlations exist between Larsen C surface temperature and the PC time series of EOF mode 1 in winter and spring through northerly/northwesterly wind anomalies west of the AP. The PC time series of EOF mode 2 is negatively correlated with Larsen C surface temperature in autumn and summer and surface melt in summer, all due to southerly wind anomalies east of the AP. Surface energy budget analysis associated with EOF mode 2 shows that downwelling longwave radiation over Larsen C has negative statistically significant correlations with EOF mode 2 and is the major atmospheric forcing regulating the variation of Larsen C surface melt. Positively enhanced EOF mode 2 since 2004 is responsible for the recent cooling and decline of surface melt over Larsen C.

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Jing Zhang
and
John E. Walsh

Abstract

Satellite remote sensing data indicate that greenness has been increasing in the northern high latitudes, apparently in response to the warming of recent decades. To identify feedbacks of this land-cover change to the atmosphere, the authors employed the atmospheric general circulation model ARPEGE-CLIMAT, an adaptation of the Action de Recherche Petite Echelle Grande Echelle model for climate studies, to conduct a set of control and sensitivity modeling experiments. In the sensitivity experiments, they increased the greenness poleward of 60°N by 20% to mimic the manifestation of vegetation changes in the real world, and by 60% and 100% to represent potential aggressive vegetation change scenarios under global warming. In view of the direct exposure of vegetation to sunlight during the warm seasons, the authors focused their study on the results from late spring to early fall. The results revealed significant thermodynamic and hydrological impacts of the increased greenness in northern high latitudes, resulting in a warmer and wetter atmosphere. Surface and lower-tropospheric air temperature showed a marked increase, with a warming of 1°–2°C during much of the year when greenness is increased by 100%. Precipitation and evaporation also showed a notable increase of 10% during the summer. Snow cover decreased throughout the year, with a maximum reduction in the spring and early summer. The above changes are attributable to the following physical mechanisms: 1) increased net surface solar radiation due to a decreased surface albedo and enhanced snow–albedo feedback as a result of increased greenness; 2) intensified vegetative transpiration by the additional plant cover; and 3) reduced atmospheric stability leading to enhanced convective activity. The results imply that increased greenness is a potentially significant contributing factor to the amplified polar effects of global warming.

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Steve T. Stegall
and
Jing Zhang

Abstract

Wind field climatology, changes, and extremes at ~32-km resolution were analyzed for the Chukchi–Beaufort Seas and Alaska North Slope region using 3-hourly North American Regional Reanalysis (NARR) from 1979 to 2009. The monthly average wind speeds show a clear seasonal cycle with a minimum of 2–4 m s−1 in May and a maximum up to 9 m s−1 in October. The 95th percentile winds show a similar seasonality with a maximum up to 15 m s−1 in October. The 31-yr domain averaged 3-hourly wind speeds display a clear diurnal cycle over land and sea ice areas during the warm seasons. Weaker radiation during winter and larger heat capacity over open water reduce the diurnal signal in the wind field diurnal variations. There were increasing trends of areal averaged monthly mean and 95th percentile wind speeds for July through November. The strongest increase in the areal averaged 95th percentile wind speeds occurred in October from 7 m s−1 in 1979 to 10.5 m s−1 in 2009. The frequency of extreme wind events (speed above the 95th percentile winds) shows an increasing trend in all months, with the greatest increase occurring in October, showing 8% more extreme wind events in 2009 comparing to 1979. The prevailing wind direction was northeast with a frequency of 40%–60% for most of the year. The frequency for southwest and northwest winds was small (<20%) except for two anomalous areas along the Brooks Range in Alaska and the Chukotka Mountains in easternmost Russia where the frequency has increased to 35%–50% during the cold season months.

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Qigang Wu
,
Jing Zhang
,
Xiangdong Zhang
, and
Wei Tao

Abstract

The Beaufort Sea high (BSH) plays an important role in forcing Arctic sea ice and the Beaufort Gyre. This study examines the variability and long-term trends of atmospheric circulation over the Chukchi and Beaufort Seas using the ECMWF Interim Re-Analysis (ERA-Interim) for the period 1979–2012. Because of the mobility of the BSH through the year, EOF analysis is applied to the sea level pressure (SLP) field in order to investigate the principal patterns of BSH variability. In each season, the three leading EOF modes explain nearly 90% of the total variance and reflect a strengthened or weakened BSH centered over the western Arctic Ocean (EOF1), a north–south dipole-like SLP anomaly (EOF2), and a west–east dipole-like SLP anomaly (EOF3), respectively. These three EOF modes offer distinct influences on local climate in each season and have different connections with the large-scale climate variability modes in winter. In particular, the second principal component (PC2) associated with EOF2 in the autumn exhibits a tendency toward high-index polarity significant at the 5% level, and is related to strongly reduced sea ice extent.

Further, the authors have detected significant anticyclonic trends among surface wind fields associated with a strengthened BSH during summer and autumn, but significant cyclonic trends associated with a weakened BSH during early midwinter, consistent with significant trends in SLP gradients between western Arctic Ocean and the adjoining landmass. Comparison with forced trends of surface winds from various simulations from the IPCC Fifth Assessement Report (AR5) indicates that summertime changes in atmospheric circulation cannot be explained by natural external forcing or lower boundary forcings and may instead be attributable to external anthropogenic forcing.

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Xiangyu Zhang
,
Zhao Jing
, and
Peiran Yang

Abstract

Oceanic mesoscale and submesoscale eddies produce a pronounced vertical buoyancy flux, playing an important role in ocean restratification. This study used a 1-km ocean simulation to investigate the seasonality of the vertical eddy buoyancy flux (VEBF) in the Kuroshio Extension as well as its underlying dynamics. The simulated VEBF in the upper 200 m over the Kuroshio Extension has a pronounced seasonal cycle. The winter VEBF peaks in the mixed layer, whereas the summer VEBF has a much smaller magnitude but a more complicated vertical structure with a narrow peak in the shallow mixed layer and a broader and stronger peak in the seasonal thermocline. The baroclinic instability (BCI), frontogenesis, and turbulent thermal wind (TTW) balance all contribute to the VEBF seasonal cycle. In winter, large surface heat loss and intense winds destroy stratification and enhance turbulent vertical mixing in the upper ocean. These phenomena intensify VEBF by promoting its components induced by the frontogenesis and TTW balance and by triggering mixed layer instability (MLI). In summer, strong stratification associated with suppressed turbulent vertical mixing weakens the contributions of the frontogenesis and TTW balance to VEBF and shifts the dominant BCI type from the MLI to the surface Charney- and Philips-like types with greatly reduced growth rate compared with that of MLI in winter. The shallow peak of the VEBF in summer is mainly attributed to the TTW balance, whereas the BCI and frontogenesis account primarily for its deep peak.

Open access
Jing Wang
,
Shouwen Zhang
,
Hua Jiang
, and
Dongliang Yuan

Abstract

The Indian Ocean basin (IOB) mode is the dominant mode of the interannual sea surface temperature (SST) variability in the Indian Ocean, with the Indian Ocean dipole (IOD) as the second mode. An IOB event normally occurs after an El Niño or a concurrent IOD–El Niño event, the dynamics of which are traditionally believed as forced by ENSO through the Walker circulation anomalies over the tropical Indian Ocean. A strong IOB in 2020 took place after the strongest 2019 IOD on record but independent of El Niño, which challenges the traditional atmospheric bridge dynamics of the IOB event. In this study, the dynamics of the 2020 IOB event are investigated using the numerical seasonal climate prediction system of the National Marine Environmental Forecasting Center of China. It is found that the initialization of the Indian Ocean subsurface temperature during the 2019 IOD event has led to the outburst of the 2020 IOB event successfully, the dynamics of which are the propagation and the western boundary reflection of the equatorial and off-equatorial Rossby waves, inducing heat content recharge over the tropical Indian Ocean upper thermocline. In comparison, experiments of SST initialization over the tropical Indian Ocean, with the subsurface temperature in a climatological state, were unable to reproduce the onset of the 2020 IOB event, suggesting that the local air–sea interaction within the Indian Ocean basin is of secondary importance. The numerical experiments suggest that the thermocline ocean wave dynamics play an important role in forcing the IOB event. The revealed thermocline dynamics are potentially useful in climate prediction associated with IOB events.

Restricted access
Jing Duan
,
Yuanlong Li
,
Lei Zhang
, and
Fan Wang

Abstract

Interannual variabilities of sea level and upper-ocean gyre circulation of the western tropical Pacific Ocean (WTPO) have been predominantly attributed to El Niño–Southern Oscillation (ENSO). The results of the present study put forward important modulation effects by the Indian Ocean dipole (IOD) mode. The observed sea level in the WTPO shows significant instantaneous and lagged correlations (around −0.60 and 0.40, respectively) with the IOD mode index (DMI). A composite of 14 “independent” IOD events for 1958–2017 shows negative sea level anomalies (SLAs) of 4–7 cm in the WTPO during positive IOD events and positive SLAs of 6–8 cm in the following year that are opposite in sign to the El Niño effect. The IOD impacts are reproduced by large-ensemble simulations of a climate model that generate respectively 430 and 519 positive and negative independent IOD events. A positive IOD induces westerly winds over the western and central tropical Pacific and causes negative SLAs through Ekman upwelling, and it facilitates the establishment of a La Niña condition in the following year that involves enhanced Pacific trade winds and causes positive SLAs in the WTPO. Ocean model experiments confirm that the IOD affects the WTPO sea level mainly through modulating the tropical Pacific winds. Variability of the Indonesian Throughflow (ITF) induced by IOD winds has a relatively weak effect on the WTPO. The IOD’s impacts on the major upper-ocean currents are also considerable, causing anomalies of 1–4 Sv (1 Sv ≡ 106 m3 s−1) in the South Equatorial Current (SEC) and North Equatorial Countercurrent (NECC) volume transports.

Free access
Jianqi Sun
,
Jing Ming
,
Mengqi Zhang
, and
Shui Yu

Abstract

In June 2017, south China suffered from intense rainfall that broke the record spanning the previous 70 years. In this study, the large-scale circulations associated with the south China June rainfall are analyzed. The results show that the anomalous Pacific–Japan (PJ) pattern is a direct influence on south China June rainfall or East Asian early summer rainfall. In addition, the Australian high was the strongest in June 2017 during the past 70 years, which can increase the equatorward flow to northern Australia and activate convection over the Maritime Continent. Enhanced convection over the Maritime Continent can further enhance local meridional circulation along East Asia, engendering downward motion over the tropical western North Pacific and enhancing the western Pacific subtropical high (WPSH) and upward motion over south China, which increases the rainfall therein. In addition, a strong wave train pattern associated with North Atlantic air–sea interaction was observed in June 2017 at Northern Hemispheric mid- to high latitudes; it originated from the North Atlantic and propagated eastward to East Asia, resulting in an anomalous anticyclone over the Mongolian–Baikal Lake region. This anomalous anticyclone produced strong northerly winds over East Asia that encountered the southerly associated with the WPSH over south China, thereby favoring intense rainfall over the region. Case studies of June 2017 and climate research based on data during 1979–2017 and 1948–2017 indicate that the extremities of the atmospheric circulation over south Europe and Australian high and their coupling with the PJ pattern could be responsible for the record-breaking south China rainfall in June 2017.

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Xiaoqing Peng
,
Tingjun Zhang
,
Yijing Liu
, and
Jing Luo

Abstract

Freezing/thawing indices are useful for assessments of climate change, surface and subsurface hydrology, energy balance, moisture balance, carbon exchange, ecosystem diversity and productivity. Current freezing/thawing indices are inadequate to meet these requirements. We use 16 Coupled Model Intercomparison Project phase 5 (CMIP5) models available for 1850–2005, three representative concentration pathways (RCP2.6, RCP4.5, and RCP8.5) during 2006–2100, and Climatic Research Unit gridded observations for 1901–2014, to assess the performance of freezing/thawing indices derived from CMIP5 models during 1901–2005. We also analyzed past spatial patterns of freezing/thawing indices and projected these over three RCPs. Results show that CMIP5 models can reproduce the spatial pattern of freezing/thawing indices in the Northern Hemisphere but that the thawing index slightly underestimated observations and the freezing index slightly overestimated them. The thawing index agreed slightly better with observations than did the freezing index. There is significant spatial variability in the freezing/thawing indices, ranging from 0° to 10 000°C day. Over the entire Northern Hemisphere, the time series of the area-averaged thawing index derived from CMIP5 output increased significantly at about 1.14°C day yr−1 during 1850–2005, 1.51°C day yr−1 for RCP2.6, 5.32°C day yr−1 for RCP4.5, and 13.85°C day yr−1 for RCP8.5 during 2006–2100. The area-averaged freezing index decreased significantly at −1.39°C day yr−1 during 1850–2004, −1.2°C day yr−1 for RCP2.6, −4.3°C day yr−1 for RCP4.5, and −9.8°C day yr−1 for RCP8.5 during 2006–2100. The greatest decreases in the freezing index are projected to occur at high latitudes and high altitudes, where the magnitude of the decreasing rate of the freezing index is far greater than that of the increasing rate of the thawing index.

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Fuhong Liu
,
Jeremy R. Krieger
, and
Jing Zhang

Abstract

The Weather Research and Forecasting Model (WRF) and its variational data assimilation system (WRFDA) are applied to the Chukchi–Beaufort Seas and adjacent Arctic Slope region for high-resolution regional atmospheric reanalysis study. To optimize WRFDA performance over the study area, a set of sensitivity experiments are carried out to analyze the model sensitivity to model background errors (BEs) and the assimilation of various observational datasets. Observational data are assimilated every 6 h and the results are verified against unassimilated observations. In the BE sensitivity analyses, the results of assimilating in situ surface observations with a customized, domain-dependent BE are compared to those using the WRF-provided global BE. It is found that the customized BE is necessary in order to achieve positive impacts from WRFDA assimilation for the study area. When seasonal variability is incorporated into the customized BE, the impacts are minor. Sensitivity analyses examining the assimilation of different datasets via WRFDA demonstrate that 1) positive impacts are always seen through the assimilation of in situ surface and radiosonde measurements, 2) assimilating Quick Scatterometer (QuikSCAT) winds improves the simulation of the 10-m wind field over ocean and coastal areas, and 3) selectively assimilating Moderate Resolution Imaging Spectroradiometer (MODIS) retrieved profiles under clear-sky and snow-free conditions is essential to avoid degradation of assimilation performance, while assimilation of Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) retrievals has little impact, most likely due to limited data availability. Based on the sensitivity results, a 1-yr (2009) experimental reanalysis is conducted and consistent improvements are achieved, particularly in capturing mesoscale processes such as mountain barrier and sea-breeze effects.

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