ALBERT

Beschreibung: In the recent decades, the Ross Ice Shelf (RIS) has experienced frequent summer surface melting, which accelerates ice loss and increases the instability of ice sheets. This study links the interannual variability of surface melting events over the RIS with the northerly wind anomaly over the Ross Sea sector, which is established in association with a quasi-geostrophic barotropic Rossby wave train from subtropical Australia toward West Antarctica. The Rossby wave train is regulated jointly by El Niño and atmospheric heating over western Australia. El Niño provides the major forcing of the atmospheric circulation anomalies over the Ross Sea, and most surface melting events over the RIS happened during El Niño years. In addition, the anomalous atmospheric heating over western Australia is identified as the other significant forcing that triggers the Rossby wave train. The northerly flow is sandwiched between the low and high geopotential height anomalies located respectively over the left-hand and right-hand sides of the Ross Sea, favoring strong poleward moisture and heat transport and leading to surface melting over the RIS.

Beschreibung: This study examines the fidelity of 49 coupled general circulation models (CGCMs) from phase 6 of the Coupled Model Intercomparison Project (CMIP6) in representing the influence of the El Niño–Southern Oscillation (ENSO) on Southeast Asian summer monsoon (SEASM) during ENSO decay summer. The responses of SEASM to ENSO show a large model spread among the 49 models and some of the models even simulate opposite signs of SEASM anomalies compared to the observed. The bad-performance models (BPMs) are therefore selected to be compared with both the good-performance models (GPMs) and observations to explore the possible causes of the deficiency. Results show that in the BPMs, the warm SST anomalies over the western equatorial Pacific (WEP) do not dissipate from the El Niño decay spring to summer in comparison with those in both GPMs and observations, interfering with the effect of ENSO on the SEASM. A mixed-layer heat budget analysis indicates that the SST anomalies from the late winter to the decay spring are mainly maintained by weak shortwave radiation damping effects in the BPMs, resulted from the low sensitivity of precipitation to SST anomalies due to the excessive westward extension of the cold tongue and the lower SST in the warm pool in these models. During the following summer, the warm SST anomalies, in turn, could cause a warmer zonal advection to further maintain the SST anomalies via inducing persistent westerly wind anomalies over the WEP as an atmospheric Rossby wave response.

Beschreibung: Quantification and modeling of runoff generation and hydrological processes operating in permafrost catchments are challenging. Active layer soil freeze-thaw cycles cause multifaced runoff generation processes with seasonal alternating patterns. By integrating soil temperature-based water saturation function into the soil water storage curve with a soil temperature threshold, we found that surface soil thawing induced saturation excess runoff and subsurface interflow account for approximately 66-86% and 14-34% of total spring runoff, respectively. Soil temperature significantly affects runoff generation patterns, runoff compositions, and runoff coefficients with the enlargement of the active layer. The supra-permafrost groundwater discharge decreases exponentially with active layer frozen processes during autumn runoff recession, whereas the ratio of groundwater discharge to total runoff and the direct surface runoff coefficient simultaneously increases. Based on the runoff generation processes and mechanisms, we developed a runoff generation model with temperature-induced variable source area (TVSA) for permafrost regions by coupling the active layer parameterization scheme, active layer freeze–thaw module, glacier module, and sub-permafrost groundwater module. The TVSA model was calibrated and validated using field observations in permafrost and glacier-affected Fenghuoshan and Tuotuohe watershed on the Qinghai-Tibet Plateau (QTP). Parameters related to the thermal conditions of the active layer and runoff processes were calibrated using the observed freeze and thaw depth data and the discharge data. The model successfully reproduced the freeze-thaw processes and discharges with high accuracy. The TVSA model is powerful for the systematic identification of variable source runoff generation processes and the associated physical mechanism under temperature control in permafrost basins.

Beschreibung: A weak (strong) South Asian summer monsoon strengthens (weakens) an El Niño and weakens (strengthens) a La Niña, and this observed feature can be well captured by global models. A weak (strong) monsoon causes a cyclonic (an anticyclonic) circulation over the northwestern Pacific via stimulating cold (warm) Kelvin waves. The westerly (easterly) anomaly on the southern flank of this cyclone (anticyclone) generates eastward (westward) anomaly in the mixed layer, induces warm (cold) zonal advection, and excites oceanic downwelling (upwelling) Kelvin waves, deepening (shoaling) the thermocline in equatorial eastern Pacific and resulting in cold (warm) vertical advection. The influence of monsoon-induced Pacific wind anomaly on ENSO is mainly achieved by changing the zonal advective feedback and thermocline feedback, confirmed by model sensitive experiments in which additional monsoon heating/cooling anomaly is imposed in the ENSO developing summer. An index of spring persistence barrier, which complicates the effect of ENSO on monsoons, is defined to quantify the transitive or persistent feature of ENSO transition and investigate the relationship between ENSO and monsoon. During transitive ENSO, cold SST anomaly appears in the central Pacific in summer (changed from the warm SST in preceding winter) and enhances the atmospheric anticyclone over the western Pacific and the East Asian summer monsoon by reinforcing the Walker circulation and local Hadley circulation. However, during persistent ENSO, warm SST anomaly occurs over the central Pacific in summer and exerts a weaker impact on the monsoon because of a less robust atmospheric response over the western Pacific.

Beschreibung: While it is commonly accepted that the thermal effect of the Tibetan Plateau (TP) strengthens the Asian summer monsoon, a recent analysis based mainly on idealized model experiments revealed that the TP effect weakened the Southeast Asian summer monsoon (SEASM). Based on both observational analyses and model experiments, the current study further illustrates the physical mechanism for the TP’s thermal impact on the SEASM and the modulation by sea surface temperature (SST) in the tropical Atlantic. When diabatic heating over the southern TP is enhanced, the South Asian high (SAH) intensifies and extends eastward, leading to convergence over the southeastern flank of the anomalous upper-level anticyclone and sinking motion that cause downward advection of negative vorticity. Accompanied by this anomalous anticyclonic pattern, the western Pacific subtropical high (WPSH) extends westward and the monsoon over Southeast Asia is weakened.The above TP–SEASM relationship is enhanced when SST and convection increase over the tropical Atlantic, which cause an anomalous barotropic wave train propagating southeastward from eastern North America to East Asia, inducing an eastward-extended SAH and a westward-extended WPSH. The anomalous heating over the tropical Atlantic also modulates the Walker circulation, inducing ascending motions over the Maritime Continent and the eastern tropical Indian Ocean and a lower-level anticyclone over Southeast Asia as a Gill-type response. Thus, a warming tropical Atlantic can intensify the TP’s thermal forcing, weaken the SEASM, and then modulate the TP–SEASM relationship through both the extratropical wave train and the tropical zonal circulation.

Beschreibung: The presence of gas hydrates is well known in the marine realm southwest off Taiwan due to the widespread distribution of bottom simulating reflectors in seismic records. To learn more about gas hydrate systems and their dynamics at passive and active margins, we drilled boreholes with MARUM-MeBo200 seafloor drill rig at areas where geophysical indications for gas hydrates have been detected in the past. Several gas hydrate proxies like negative chloride anomalies in the pore water, cold spots detected by infrared thermal scans on cores, increased resistivity, and lithological parameters clearly showed the presence of hydrates in the drilled sections. However, gas hydrate was not recovered by MeBo most likely because of small crystal sizes which dissociated during recovery from the seafloor. Three holes were drilled at southern summit of Formosa Ridge down to 126 m below seafloor (mbsf) and recovered sediments from which in situ hydrate saturation values were estimated between 1 and 10% at 15–42 mbsf and even higher values of up to 38% below 100 mbsf. The latter are probably related to the sealing effect of carbonate precipitation which occur at 85–95 mbsf directly above the hydrate-enriched layer. Four holes were drilled at Four-Way Closure Ridge where a maximum drilling depth of 143.90 m was reached. Hydrate presence starts in 65 mbsf continuing down-core with a range of 1–29% gas hydrate saturation in fine-grained homogenous clay. An abrupt change to higher gas hydrate saturation values of up to 80% occurs below 109 mbsf where silty and sandy turbidite layers are often intercalated. Such high gas hydrate contents only occur in the sand layers and not in the fine-grained sediments intercalated to the sand deposits.