Spring Extreme Precipitation Days in North China and Their Reliance on Atmospheric Circulation Patterns During 1979–2019

The interannual variation of springtime extreme precipitation (SEP) days in North China (NC) and their reliance on atmospheric circulation patterns are studied by using the continuous daily record of 396 rain gauges and the fifth generation of the European Centre for Medium-Range Weather Forecasts atmospheric reanalysis during 1979–2019. The SEP days are defined as the days when at least 10% of rain gauges in NC record daily precipitation no less than 10.5 mm. Results show that the number of SEP days shows large interannual variability but no significant trend in the study period. Using the objective classification method of the obliquely rotated principal analysis in T-mode, we classify the atmospheric circulation into five different patterns based on the geopotential height at 700 hPa. Three circulation patterns all have fronts and are associated with strong southerly wind, leading to 88% of SEP days in NC. The strong southerly wind may provide moisture and dynamic forcing for the frontal precipitation. The interannual variation of SEP days is related with the number of the three above-mentioned dominant circulation patterns. Further analysis shows that the West Pacific pattern could be one of the possible climate variability modes related to SEP days. This study reveals that the daily circulation pattern may be the linkage between SEP days and climate variability modes in NC.

Variation Characteristics Mathematical Calculation of O3 and Its Relationship with Meteorological Factors by Big Data Technology

Based on the data of environmental monitoring stations and meteorological stations in Qinhuangdao from May 2017 to May 2020, the variation characteristics of O3 and precursors (NO2 and CO) as well as their relationship with meteorological elements were analyzed. The results showed that the daily average concentration of O3-8 h in Qinhuangdao increased year by year. The monthly average concentration of O3-8 h was high in summer and low in winter, and the peak appeared in June. The diurnal variation of O3 concentration was unimodal structure, and the concentration increased in the afternoon, but it decreased at night. The concentration of NO2 and CO was inversely correlated with O3, and the peak value of NO2 in March could be related to frequent cold air activity and increased burning of loose coal. The meteorological elements favorable for the occurrence of ozone pollution weather in Chengde were total solar radiation irradiance greater than 1000W/m2, the daily maximum temperature greater than 33 °C, and the daily minimum relative humidity less than 40% and 65%∽80%, southerly wind or southwest wind.

Brief communication: The anomalous winter 2019 sea-ice conditions in McMurdo Sound, Antarctica

McMurdo Sound sea ice can generally be partitioned into two regimes: (1) a stable fast-ice cover, forming south of approximately 77.6∘ S around March–April and then breaking out the following January–February, and (2) a more dynamic region north of 77.6∘ S that the McMurdo Sound and Ross Sea polynyas regularly impact. In 2019, a stable fast-ice cover formed unusually late due to repeated break-out events. We analyse the 2019 sea-ice conditions and relate them to a modified storm index (MSI), a proxy for southerly wind events. We find there is a strong correlation between the timing of break-out events and several unusually large MSI events.

Impacts of CP- and EP-El Niño events on the Antarctic sea ice in austral spring

Based on observational data analyses and idealized modeling experiments, we investigated the distinctive impacts of central Pacific (CP-) El Niño and eastern Pacific (EP-) El Niño on the Antarctic sea ice concentration (SIC) in austral spring (September to November). The tropical heat sources associated with EP-El Niño and the co-occurred positive phase of Indian Ocean Dipole (IOD) excite two branches of Rossby wave trains that propagate southeastward, causing an anomalous anticyclone over the eastern Ross-Amundsen-Bellingshausen Seas. Anomalous northerly (southerly) wind west (east) of the anomalous anticyclone favor poleward (offshore) movements of sea ice, resulting in a sea ice loss (growth) in the eastern Ross-Amundsen Seas (the Bellingshausen-Weddell Seas). Meanwhile, the anomalous northerly (southerly) wind also advected warmer and wetter (colder and drier) air into the eastern Ross-Amundsen Seas (the Bellingshausen-Weddell Seas), causing surface warming (cooling) through the enhanced (reduced) surface heat fluxes and thus contributing to the sea ice melting (growth). CP-El Niño, however, forces a Rossby wave train that generates an anomalous anticyclone in the eastern Ross-Amundsen Seas, 20° west of that caused by EP-El Niño. Consequently, a positive SIC anomaly occurs in the Bellingshausen Sea. A dry version of the Princeton atmospheric general circulation model was applied to verify the roles of anomalous heating in the tropics. The result showed that EP-El Niño can remotely induce an anomalous anticyclone and associated dipole temperature pattern in the Antarctic region, whereas CP-El Niño generates a similar anticyclone pattern with its location shift westward by 20° in longitudes.

Estimating zonal Ekman transport along coastal Senegal during passage of Hurricane Fred, 30–31 August 2015

We examine the role of zonal Ekman transport along the coast of Senegal on 30 August 2015 when the tropical disturbance associated with Tropical Cyclone Fred was located to the west of Senegal, causing considerable coastal damage in the southern Senegal–Gambia domain (south of Dakar, Senegal). Ten-meter winds from three Weather Research and Forecast model simulations were used to estimate zonal Ekman transport, when the maximum values were found on 30 August. These simulations are in agreement with limited coastal observations showing increasing southerly wind speeds during 30 August but overestimated relative to the three coastal stations. The strong meridional winds translate into increased zonal Ekman transport to the coast of Senegal on 30 August and are likely responsible for some coastal flooding. Ekman transport along the coast contributes significantly to the water-level variations during swell events. The use of a coupled ocean model will improve the estimates of Ekman transport along the Guinea-Senegalese coast. The observed damage suggests that artificial and natural barriers (mangroves) should be strengthened to protect coastal communities in Senegal.

Brief communication: The anomalous winter 2019 sea ice conditions in McMurdo Sound, Antarctica

McMurdo Sound sea ice can generally be partitioned into two regimes: (1) a stable fast-ice cover, forming south of approximately 77.6° S around March/April, then breaking out the following January/February; and, (2) a more dynamic region north of 77.6° S that the McMurdo Sound and Ross Sea polynyas regularly impact. In 2019, a stable fast-ice cover formed unusually late due to repeated breakout events. We analyse the 2019 sea-ice conditions and relate them to southerly wind events using a Katabatic Wind Index (KWI). We find there is a strong correlation between breakout events and several unusually large KWI events.

Contributions of Internal Variability and External Forcing to the Recent Trends in the Southeastern Pacific and Peru–Chile Upwelling System

In a warming world context, sea surface temperature (SST) off central-south Peru, northern Chile, and farther offshore increases at a slower rate than the global average since several decades (i.e., cools, relative to the global average). This tendency is synchronous with an interdecadal Pacific oscillation (IPO) negative trend since ~1980, which has a cooling signature in the southeastern Pacific. Here, we use a large ensemble of historical coupled model simulations to investigate the relative roles of internal variability (and in particular the IPO) and external forcing in driving this relative regional cooling, and the associated mechanisms. The ensemble mean reproduces the relative cooling, in response to an externally forced southerly wind anomaly, which strengthens the upwelling off Chile in recent decades. This southerly wind anomaly results from the poleward expansion of the Southern Hemisphere Hadley cell. Attribution experiments reveal that this poleward expansion and the resulting enhanced upwelling mostly occur in response to increasing greenhouse gases and stratospheric ozone depletion since ~1980. An oceanic heat budget confirms that the wind-forced upwelling enhancement dominates the relative cooling near the coast. In contrast, a wind-forced deepening of the mixed layer drives the offshore cooling. While internal variability contributes to the spread of tendencies, the ensemble-mean relative cooling in the southeastern Pacific is consistent with observations and occurs irrespectively of the IPO phase, hence, indicating the preeminent role of external forcing.

Five-Year Climatology of Local Convections in the Dabie Mountains

Local Convection in Dabie Mountains (LCDM) occurs more frequently over the Dabie Mountains and brings severe weather to adjacent areas. In order to understand the characteristics of LCDM, their spatial distribution, the monthly and diurnal variations, and possible mechanisms are investigated. Based on radar composite reflectivity data over the 5-y period of 2014–2018 during warm seasons (April–September), a total of 195 cases of LCDM are identified. The LCDM exhibits maximum frequency on the windward slopes of the Dabie Mountains with a secondary maximum on lee slopes. It is demonstrated that LCDM peaks in July and August, while their diurnal variation exhibits a major peak in the afternoon during 12:00–16:00 local solar time (LST). Most LCDM does not leave the Dabie Mountains (NoOut-Type), accounting for 89.7% overall, and has an average 3.5 h lifespan. In contrast, the lifespans of Out-Types (i.e., LCDMs that move away from the Dabie Mountains) are longer (5.8 h on average), while most Out-Type LCDMs develop on southern slopes (‘South-Type’) and a few are also reinforced on northern slopes (‘North-Type’). The South-Type mainly produces short-duration heavy precipitation, while the ‘North-Type’ predominately generates thunderstorms high winds. It is suggested that LCDM is thermally induced, and that both the ‘South-Type’ and ‘North-Type’ are controlled by southerly wind perturbation. Lifting by upslope wind and heat sources over windward slopes has led to ‘South-Type’ development, while ascent induced by wave-like perturbations on lee slopes has led to ‘North-Type’. These mechanisms should be further investigated in future work by using field experiments and numerical simulations.

Different Enhancement of the East Asian Summer Monsoon under Global Warming and Interglacial Epochs Simulated by CMIP6 Models: Role of the Subtropical High

Southerly wind in the lower troposphere is an essential feature of East Asian summer monsoon (EASM) circulation, which is reported to be enhanced under global warming scenarios and interglacial epochs. Based on an analysis of an ensemble of CMIP6 models, this study shows that the magnitude of intensification of the EASM circulation is much smaller under global warming scenarios than during interglacial epochs. Distinct changes in the western North Pacific subtropical high (WNPSH) are responsible for the different responses of the EASM circulation. The WNPSH is substantially enhanced during interglacial epochs, which acts to strengthen the southerly wind associated with the EASM on the western flank of the WNPSH. However, the change in the WNPSH is insignificant and cannot strengthen the EASM under global warming scenarios, and the weakly enhanced EASM circulation may be a direct response to intensified heating over the Tibetan Plateau. The land–ocean thermal contrast explains the different responses of the WNPSH. During interglacial epochs, the summertime surface warming over the subtropical North Pacific is much weaker than over Eurasia due to the large thermal inertia of the ocean to increased insolation, and the WNPSH is intensified as a response to the suppressed latent heating over the subtropical North Pacific. The fast response of the WNPSH to abrupt quadrupling of CO2 without sufficient ocean warming is an analog to the interglacial epochs, but it is offset by the effect of slow oceanic warming, resulting in an insignificant change of the WNPSH under global warming scenarios.

Causes of Interdecadal Increase in the Intraseasonal Rainfall Variability over Southern China around the Early 1990s

The southern China (SC) summer rainfall exhibits prominent intraseasonal variability, which exhibits a significant increase in the early 1990s with the turning point at 1993. The SC intraseasonal rainfall events could be divided into three categories according to different propagations, including the southward-propagating (SP) events, the northwestward-propagating (NWP) events, and the northward-propagating (NP) events. This study explores the causes of the observed interdecadal increase in the intraseasonal rainfall variability over SC by comparing the SC intraseasonal rainfall events of each category between the former decadal period (P1) and the later decadal period (P2). The result indicates that such interdecadal change is due to the more frequent NP events coming from the South China Sea (SCS). Based on the moisture and vorticity budget analysis, it is revealed that the summer mean southerly wind in the middle to lower troposphere is the dominant factor of the northward propagation over the SCS, as it could induce positive meridional moisture and vorticity advection anomalies ahead of the convection. A marked interdecadal enhancement of the summer mean southerly wind over the SCS is the cause of more frequent occurrence of NP events over SC, as it provides more favorable conditions for the northward propagation. The change of the atmospheric instability over the SCS where the NP convection perturbation originates was also investigated, but no significant change was found.