Search

The western Pacific subtropical high (WPSH) substantially affects the climate in the Pacific and East Asia. Previous studies have revealed that the springtime Indo‐Pacific warm pool (IPWP) sea surface temperature zonal gradient (SSTG) could be used as a predictor of the subsequent summertime WPSH's intensity. Here, we find that the interannual variability of the springtime IPWP SSTG has greatly decreased after the late 1990s, accompanied by the weakened relationship between the springtime IPWP SSTG and the following summertime WPSH, which may reduce the efficiency of the springtime IPWP SSTG as a key predictor for the summertime WPSH in recent decades. This observed recent weakening IPWP SSTG–WPSH relationship could be largely contributed by the decadal shift of the El Niño–Southern Oscillation (ENSO) and the WPSH around the late 1990s. The ENSO regime shift from the eastern Pacific (EP) type to the central Pacific (CP) type could alter the spatial pattern of the springtime IPWP sea surface temperature (SST) dipole and further weaken the local air–sea interaction between the underlying IPWP SST and the WPSH. From another perspective of the WPSH decadal shift, the WPSH‐related first leading mode before (after) the late 1990s, characterized by a large‐scale uniform (dipole) pattern with an oscillating period of ~4–5 year (~2–3 year), tended to promote a stronger (weaker) linkage with the springtime IPWP SSTG. In addition, the recent enhancement of the tropical Atlantic SST influences is considered to possibly promote the decadal shifts of the ENSO and the WPSH‐related leading mode. After the springtime tropical Atlantic SST was added as a predictor, the predicting skills of the empirical equation for the summertime WPSH could be substantially improved. The results herein have important implications for the further improvement of the seasonal WPSH prediction, which is of great practical significance in the prevention and mitigation of climate disasters. [ABSTRACT FROM AUTHOR]

Initialization and the spin‐up effect are critical to model performance in the four‐dimensional variational data assimilation (4DVar) system. The incremental analysis update (IAU) technique is able to combat these problems. This study introduces the IAU technique into the 4DVar framework (IAU‐4DVar) of the operational China Meteorological Administration Global Forecast System (CMA‐GFS) and evaluates the performance of the IAU‐4DVar. In this IAU‐4DVar framework, the initial increments optimally estimated are split up into equally weighted forcing terms and integrated into the tangent model and its adjoint model. Then, the optimal estimation of the analysis increment is gradually introduced into the model integration during the assimilation window. A set of two‐month assimilation and forecast experiments were established and executed in this study, and the preliminary results show that the IAU technique can reduce the spin‐up effect of the initial analysis increment from the operational 4DVar system with digital filter initialization (DFI‐4DVar) by retaining the balance in dynamics and thermodynamics. In the optimal minimization process, the IAU‐4DVar converges faster than the DFI‐4DVar with a reduction of the computational cost of up to ca 40%. In addition, the introduction of IAU‐4DVar into the CMA‐GFS model could have a positive impact on the analysis and forecast skills relative to the operational DFI‐4DVar. [ABSTRACT FROM AUTHOR]

It is well‐known that the increasing zonal‐mean trend of sea‐ice concentration (SIC) surrounding Antarctica is up to 2015 so this paper first summarized this trend with opposing regional trends in each season along the latitude band 60°–70°S during 1979–2015. The surface wind trends can partly explain the observed regional SIC trends. By analysing the surface wind, as well as the tropospheric geopotential height (GPH) and temperature trends, the study revealed that the increasing zonal‐mean SIC trend and opposing regional SIC trends surrounding Antarctica are vertically associated with GPH and temperature trends in the troposphere. This relationship is also found in extreme years for seven SIC maximum and four SIC minimum autumns from 1979 to 2015, particularly reconfirmed in the two SIC minimum autumns of 2017 and 2018, indicating that a positive regional SIC anomaly is vertically associated with an anomalously cold air mass centred around 850 hPa in the troposphere which is located beneath a negative GPH anomaly at the upper troposphere, and vice versa. [ABSTRACT FROM AUTHOR]

ABSTRACT In the summer of 2010, western Russia experienced extreme heat, which was noted for its exceptional spatial spread, long duration, high intensity and impacts. We use an anomaly-based approach to decompose atmospheric variables into daily climatic components and anomalies from two reanalysis datasets. We show that a surface heat wave event results from a downward extension of an anomalously warm air column below a centre of positive geopotential height anomalies in the upper troposphere. Therefore, we use this approach to analyse all summer regional heat wave events with spatial scales larger than 2000 km and durations greater than 5 days over western Eurasia from 1980 to 2014. Our results demonstrate that the rapid increase in regional heat wave events over western Eurasia since 2010 is a direct response to the increasing frequency of large-scale, quasi-stationary positive centres of maximum height anomalies in the upper troposphere. [ABSTRACT FROM AUTHOR]