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Dehai Luo
,
Yao Yao
, and
Aiguo Dai

Abstract

In Part I of this study, it is revealed that decadal variations of European blocking, in its intensity, duration, and position, during 1978–2011 are modulated by decadal changes in the frequency of North Atlantic Oscillation (NAO) events associated with background Atlantic conditions. In Part II, reanalysis data are analyzed to first show that a T-bone-type structure of the climatological-mean blocking frequency in the Euro-Atlantic sector roughly results from a combination of the blocking frequency distributions along the southeast–northwest (SE–NW) direction associated with negative-phase NAO (NAO) events and along the southwest–northeast (SW–NE) direction associated with positive-phase NAO (NAO+) events.

A nonlinear multiscale interaction (NMI) model is then used to examine the physical processes behind the blocking frequency distributions. This model shows that the combination of eastward- and westward-displaced blocking frequency patterns along the SW–NE and SE–NW directions associated with NAO+ and NAO events leads to a T-bone-type frequency distribution, as seen in reanalysis data. Moreover, it is found that the westward migration of intense, long-lived blocking anomalies over Europe following NAO+ events is favored (suppressed) when the Atlantic mean zonal wind is relatively weak (strong). This result is held for the strong (weak) western Atlantic storm track. This helps explain the findings in Part I. In particular, long-lived blocking events with double peaks can form over Europe because of reintensification during the NAO+ decay phase, when the mean zonal wind weakens. But the double-peak structure disappears and becomes a strong single-peak structure as the mean zonal wind strengthens.

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Aiguo Dai
and
Kevin E. Trenberth

Abstract

To evaluate the performance of version 2 of the Community Climate System Model (CCSM2) in simulating the diurnal cycle and to diagnose the deficiencies in underlying model physics, 10 years of 3-hourly data from a CCSM2 control run are analyzed for global and large-scale features of diurnal variations in surface air temperature, surface pressure, upper-air winds, cloud amount, and precipitation. The model-simulated diurnal variations are compared with available observations, most of which were derived from 3-hourly synoptic reports and some new results are reported for surface air temperatures. The CCSM2 reproduces most of the large-scale tidal variations in surface pressure and upper-air winds, although it overestimates the diurnal pressure tide by 20%–50% over low-latitude land and underestimates it over most oceans, the Rockies, and other midlatitude land areas. The CCSM2 captures the diurnal amplitude (1°–6°C) and phase [peak at 1400–1600 local solar time (LST)] of surface air temperature over land, but over ocean the amplitude is too small (≤0.2°C). The CCSM2 overestimates the mean total cloud amount by 10%–20% of the sky from ∼15°S to 15°N during both December– January–February (DJF) and June–July–August (JJA) and over northern mid- and high-latitude land areas in DJF, whereas it underestimates the cloud amount by 10%–30% in the subtropics and parts of the midlatitudes. Over the marine stratocumulus regions west to the continents, the diagnostic cloud scheme in the CCSM2 underestimates the mean stratocumulus amount by 10%–30% and does not simulate the observed large diurnal variations (∼3%–10%) in the marine stratocumulus clouds even when driven by observational data. In the CCSM2, warm-season daytime moist convection over land starts prematurely around 0800 LST, about 4 hours too early compared with observations, and plateaus from 1100 to 1800 LST, in contrast to a sharp peak around 1600–1700 LST in observations. The premature initiation of convection prevents convective available potential energy (CAPE) from accumulating in the morning and early afternoon and intense convection from occurring in the mid to late afternoon. As a result of the extended duration of daytime convection over land, the CCSM2 rains too frequently at reduced intensity despite the fairly realistic patterns of rainy days with precipitation >1 mm day−1. Furthermore, the convective versus nonconvective precipitation ratio is too high in the model as deep convection removes atmospheric moisture prematurely. The simulated diurnal cycle of precipitation is too weak over the oceans, especially for convective precipitation. These results suggest that substantial improvements are desirable in the CCSM2 in simulating cloud amount, initiation of warm-season deep convection over land, and in the diurnal cycle in sea surface temperatures.

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Adam Hugh Monahan
and
Aiguo Dai

Abstract

The spatial structure of asymmetries in sea surface temperature (SST) and surface air temperature (SAT) between average El Niño and La Niña events is considered. It is demonstrated that in historical SST and SAT reconstructions, the anomaly spatial pattern that changes sign between El Niño and La Niña events (the “linear” signal) strongly resembles that of principal component analysis (PCA) mode 1, while that which does not change sign (the “nonlinear” signal) resembles the pattern of PCA mode 2. The linear and nonlinear patterns also strongly resemble the standard deviation and skewness fields, respectively. Furthermore, temporal subsampling of long (130 yr) SST reconstructions suggests that the magnitude of the nonlinear signal and its similarity to PCA mode 2 are functions of the strength of ENSO, as measured by the standard deviation of the PCA mode-1 time series. Finally, it is found that of several coupled general circulation models (GCMs) considered, the spatial and temporal structure of the El Niño–La Niña asymmetry is captured only by the GFDL R30 model, despite large biases in its covariance structure.

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Aiguo Dai
,
Kevin E. Trenberth
, and
Taotao Qian

Abstract

A monthly dataset of Palmer Drought Severity Index (PDSI) from 1870 to 2002 is derived using historical precipitation and temperature data for global land areas on a 2.5° grid. Over Illinois, Mongolia, and parts of China and the former Soviet Union, where soil moisture data are available, the PDSI is significantly correlated (r = 0.5 to 0.7) with observed soil moisture content within the top 1-m depth during warm-season months. The strongest correlation is in late summer and autumn, and the weakest correlation is in spring, when snowmelt plays an important role. Basin-averaged annual PDSI covary closely (r = 0.6 to 0.8) with streamflow for seven of world's largest rivers and several smaller rivers examined. The results suggest that the PDSI is a good proxy of both surface moisture conditions and streamflow. An empirical orthogonal function (EOF) analysis of the PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature and an El Niño– Southern Oscillation (ENSO)-induced mode of mostly interannual variations as the two leading patterns. The global very dry areas, defined as PDSI < −3.0, have more than doubled since the 1970s, with a large jump in the early 1980s due to an ENSO-induced precipitation decrease and a subsequent expansion primarily due to surface warming, while global very wet areas (PDSI > +3.0) declined slightly during the 1980s. Together, the global land areas in either very dry or very wet conditions have increased from ∼20% to 38% since 1972, with surface warming as the primary cause after the mid-1980s. These results provide observational evidence for the increasing risk of droughts as anthropogenic global warming progresses and produces both increased temperatures and increased drying.

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Chunlüe Zhou
,
Aiguo Dai
,
Junhong Wang
, and
Deliang Chen
Open access
Matthew T. Jenkins
,
Aiguo Dai
, and
Clara Deser

Abstract

The dynamic and thermodynamic mechanisms that link retreating sea ice to increased Arctic cloud amount and cloud water content are unclear. Using the fifth generation of the ECMWF Reanalysis (ERA5), the long-term changes between years 1950–79 and 1990–2019 in Arctic clouds are estimated along with their relationship to sea ice loss. A comparison of ERA5 to CERES satellite cloud fractions reveals that ERA5 simulates the seasonal cycle, variations, and changes of cloud fraction well over water surfaces during 2001–20. This suggests that ERA5 may reliably represent the cloud response to sea ice loss because melting sea ice exposes more water surfaces in the Arctic. Increases in ERA5 Arctic cloud fraction and water content are largest during October–March from ∼950 to 700 hPa over areas with significant (≥15%) sea ice loss. Further, regions with significant sea ice loss experience higher convective available potential energy (∼2–2.75 J kg−1), planetary boundary layer height (∼120–200 m), and near-surface specific humidity (∼0.25–0.40 g kg−1) and a greater reduction of the lower-tropospheric temperature inversion (∼3°–4°C) than regions with small (<15%) sea ice loss in autumn and winter. Areas with significant sea ice loss also show strengthened upward motion between 1000 and 700 hPa, enhanced horizontal convergence (divergence) of air, and decreased (increased) relative humidity from 1000 to 950 hPa (950–700 hPa) during the cold season. Analyses of moisture divergence, evaporation minus precipitation, and meridional moisture flux fields suggest that increased local surface water fluxes, rather than atmospheric motions, provide a key source of moisture for increased Arctic clouds over newly exposed water surfaces during October–March.

Significance Statement

Sea ice loss has been shown to be a primary contributor to Arctic warming. Despite the evidence linking large sea ice retreat to Arctic warming, some studies have suggested that enhanced downwelling longwave radiation associated with increased clouds and water vapor is the primary reason for Arctic amplification. However, it is unclear how sea ice loss is linked to changes in clouds and water vapor in the Arctic. Here, we investigate the relationship between Arctic sea ice loss and changes in clouds using the ERA5 dataset. Improved knowledge of the relationship between Arctic sea ice loss and changes in clouds will help further our understanding of the role of the cloud feedback in Arctic warming.

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Yao Yao
,
Dehai Luo
,
Aiguo Dai
, and
Ian Simmonds

Abstract

Part I of this study examines the relationship among winter cold anomalies over Eurasia, Ural blocking (UB), and the background conditions associated with Arctic warming over the Barents and Kara Seas (BKS) using reanalysis data. It is found that the intensity, persistence, and occurrence region of UB-related Eurasian cold anomalies depend strongly on the strength and vertical shear (VS) of the mean westerly wind (MWW) over mid–high-latitude Eurasia related to BKS warming.

Observational analysis reveals that during 1951–2015 UB days are 64% (54%) more frequent during weak MWW (VS) winters, with 26.9 (28.4) days per winter, than during strong MWW (VS) winters. During weak MWW or VS winters, as frequently observed during 2000–15, persistent and large UB-related warming is seen over the BKS together with large and widespread midlatitude Eurasian cold anomalies resulting from increased quasi stationarity and persistence of the UB. By contrast, when the MWW or VS is strong as frequently observed during 1979–99, the cold anomaly is less intense and persistent and confined to a narrow region of Europe because of a rapid westward movement of the strong UB. For this case, the BKS warming is relatively weak and less persistent. The midlatitude cold anomalies are maintained primarily by reduced downward infrared radiation (IR), while the surface heat fluxes, IR, and advection all contribute to the BKS warming. Thus, the large BKS warming since 2000 weakens the meridional temperature gradient, MWW, and VS, which increases quasi stationarity and persistence of the UB (rather than its amplitude) and then leads to more widespread Eurasian cold events and further enhances the BKS warming.

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Dehai Luo
,
Xiaodan Chen
,
Aiguo Dai
, and
Ian Simmonds

Abstract

Winter atmospheric blocking circulations such as Ural blocking (UB) have been recognized to play an important role in recent winter Eurasian cooling. Observational analyses performed here reveal that the winter warming in the Barents–Kara Seas (BKS) related to the recent decline of sea ice concentration (SIC) has been accompanied by a large increase in the mean duration of the UB events. A new energy dispersion index (EDI) is designed to help reveal the physics behind this association and show how the BKS warming can influence the mean duration of UB events. This EDI mainly reflects the role of the meridional potential vorticity (PV) gradient in the blocking persistence and it characterizes the changes in energy dispersion and nonlinearity strength of blocking. The meridional PV gradient combines the relative vorticity gradient (related to the nonuniform meridional shear of the mean zonal wind) and the mean zonal wind strength. It is revealed that the BKS warming leads to a significant lengthening of the UB duration because of weakened energy dispersion and intensified nonlinearity of the UB through reduced meridional PV gradient. Furthermore, the duration of the UB is found to depend more strongly on the meridional PV gradient than the mean westerly wind strength, although the meridional PV gradient includes the effect of mean westerly wind strength. Thus, the meridional PV gradient is a better indicator of the change in the blocking duration related to Arctic warming than the zonal wind strength index.

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Shuangmei Ma
,
Tianjun Zhou
,
Aiguo Dai
, and
Zhenyu Han

Abstract

In this study, daily precipitation (P) records for 1960–2013 from 632 stations in China were homogenized and then applied to study the changes in the frequency of dry (P = 0) and trace (0 < P < 0.1 mm day−1) days and all precipitation events (P ≥ 0.1 mm day−1), and the frequency and accumulated amount of precipitation at different intensities. Over China as a whole, very heavy precipitation (P ≥ 50 mm day) events have increased significantly from 1960 to 2013, while light (0.1 ≤ P < 10 mm day−1) and moderate (10 ≤ P < 25 mm day−1) events have decreased significantly, accompanying the significant increases of dry days and decreases of trace days. This indicates a shift from light to intense precipitation, implying increased risks of drought and floods over China since 1960. Although the consistent increases of dry days and decreases of trace days and light and total precipitation days are seen over most of China, changes in other precipitation categories exhibit clear regional differences. Over the Yangtze River valley and southeast China, very heavy precipitation events have increased while light precipitation events have decreased. However, positive trends are seen for all precipitation categories over northwest China, while trends are generally negative over southwest, northeast, and northern China. To examine the association with global warming, the dependence of the precipitation change for each intensity category over China on global-mean temperature was analyzed using interannual to decadal variations. Results show that dry and trace days and very light and very heavy precipitation events exhibit larger changes per unit global warming than medium-intensity precipitation events.

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Dehai Luo
,
Wenqi Zhang
,
Linhao Zhong
, and
Aiguo Dai

Abstract

In this paper, an extended nonlinear multiscale interaction model of blocking events in the equivalent barotropic atmosphere is used to investigate the effect of a slowly varying zonal wind in the meridional direction on dipole blocking that is regarded as a nonlinear Rossby wave packet. It is shown that the meridional gradient of potential vorticity ( PV y = PV / y ) prior to the blocking onset, which is related to the background zonal wind and its nonuniform meridional shear, can significantly affect the lifetime, intensity, and north–south asymmetry of dipole blocking, while the blocking dipole itself is driven by preexisting incident synoptic-scale eddies. The magnitude of the background PV y determines the energy dispersion and nonlinearity of blocking. It is revealed that a small background PV y is a prerequisite for strong and long-lived eddy-driven blocking that behaves as a persistent meandering westerly jet stream, while the blocking establishment further reduces the PV y within the blocking region, resulting in a positive feedback between blocking and PV y . When the core of the background westerly jet shifts from higher to lower latitudes, the blocking shows a northwest–southeast-oriented dipole with a strong anticyclonic anomaly to the northwest and a weak cyclonic anomaly to the southeast as its northern pole moves westward more rapidly and has weaker energy dispersion and stronger nonlinearity than its southern pole because of the smaller PV y in higher latitudes. The opposite is true when the background jet shifts toward higher latitudes. The asymmetry of dipole blocking vanishes when the background jet shows a symmetric double-peak structure. Thus, a small prior PV y is a favorable precursor for the occurrence of long-lived and large-amplitude blocking.

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