Shangfeng Chen

An analysis on the physical process of the influence of AO on ENSO

The influence of the spring AO on ENSO has been demonstrated in several recent studies. This analysis further explores the physical process of the influence of AO on ENSO using the NCEP/NCAR reanalysis data over the period 1958–2010. We focus on the formation of the westerly wind burst in the tropical western Pacific, and examine the evolution and formation of the atmospheric circulation, atmospheric heating, and SST anomalies in association with the spring AO variability. The spring AO variability is found to be independent from the East Asian winter monsoon activity. The spring AO associated circulation anomalies are supported by the interaction between synoptic-scale eddies and the mean-flow and its associated vorticity transportation. Surface wind changes may affect surface heat fluxes and the oceanic heat transport, resulting in the SST change. The AO associated warming in the equatorial SSTs results primarily from the ocean heat transport in the face of net surface heat flux damping. The tropical SST warming is accompanied by anomalous atmospheric heating in the subtropical north and south Pacific, which sustains the anomalous westerly wind in the equatorial western Pacific through a Gill-like atmospheric response from spring to summer. The anomalous westerly excites an eastward propagating and downwelling equatorial Kelvin wave, leading to SST warming in the tropical central-eastern Pacific in summer-fall. The tropical SST, atmospheric heating, and atmospheric circulation anomalies sustain and develop through the Bjerknes feedback mechanism, which eventually result in an El Niño-like warming in the tropical eastern Pacific in winter.

Recent trends in winter temperature extremes in eastern China and their relationship with the Arctic Oscillation and ENSO

Interannual variations in the number of winter extreme warm and cold days over eastern China (EC) and their relationship with the Arctic Oscillation (AO) and El Niño-Southern Oscillation (ENSO) were investigated using an updated temperature dataset comprising 542 Chinese stations during the period 1961–2011. Results showed that the number of winter extreme warm (cold) days across EC experienced a significant increase (decrease) around the mid-1980s, which could be attributed to interdecadal variation of the East Asian Winter Monsoon (EAWM).Probability distribution functions (PDFs) of winter temperature extremes in different phases of the AO and ENSO were estimated based on Generalized Extreme Value Distribution theory. Correlation analysis and the PDF technique consistently demonstrated that interannual variation of winter extreme cold days in the northern part of EC (NEC) is closely linked to the AO, while it is most strongly related to the ENSO in the southern part (SEC). However, the number of winter extreme warm days across EC has little correlation with both AO and ENSO. Furthermore, results indicated that, whether before or after the mid-1980s shift, a significant connection existed between winter extreme cold days in NEC and the AO. However, a significant connection between winter extreme cold days in SEC and the ENSO was only found after the mid-1980s shift. These results highlight the different roles of the AO and ENSO in influencing winter temperature extremes in different parts of EC and in different periods, thus providing important clues for improving short-term climate prediction for winter temperature extremes.

Regional changes in the annual mean Hadley circulation in recent decades

Previous studies suggest that the zonally averaged Hadley circulation (ZAHC) has experienced a robust poleward expansion, and its trend in intensity displays inconsistency among different data sets. This study examines changes in regional HC intensity and poleward edge using six reanalyses, outgoing longwave radiation, and precipitation data sets. HCs in six regions, including Africa (AFHC), the Indian Ocean (IOHC), the western Pacific (WPHC), the eastern Pacific (EPHC), South America (SAHC), and the Atlantic (ATHC), are investigated. Intensity trends in the Northern Hemisphere (NH) WPHC and ATHC and the Southern Hemisphere (SH) EPHC and ATHC are in agreement with each other in the six reanalyses. Furthermore, regional HCs in these domains appear to be intensifying, although not all of the reanalyses show statistically significant trends. For the poleward edge, its trend in the NH AFHC, IOHC, EPHC, SAHC, and ATHC is significantly larger than zero, and the northern HC poleward edge exhibits uniform poleward migrations in these five regions. In the SH, only the trend in the SAHC poleward edge is significantly different from zero. Furthermore, the trend in the SH SAHC poleward edge is significantly larger than those in the SH AFHC, IOHC, and ATHC. The results indicate that the poleward migration of the southern ZAHC poleward edge during recent decades that has been identified by previous studies may be attributed mainly to the poleward migration of the southern SAHC poleward edge. Further analyses suggest that changes in regional HC poleward edges could have a significant impact on regional precipitation anomalies.

The Changing Relationship between Interannual Variations of the North Atlantic Oscillation and Northern Tropical Atlantic SST

AbstractThe relationship between interannual variations of boreal winter North Atlantic Oscillation (NAO) and northern tropical Atlantic (NTA) sea surface temperature (SST) experienced obvious interdecadal changes during 1870–2012. Similar interdecadal changes are observed in the amplitude of NTA SST anomalies. The mean NTA SST change may be a plausible reason for several changes in the NAO–NTA SST connection. Under a higher mean NTA SST, NTA SST anomalies induce larger wind anomalies over the North Atlantic that produce a tripole SST anomaly pattern and amplify NTA SST anomalies. Comparison of the evolution of anomalies between 1970–86 and 1996–2012 unravels changing roles of El Nino–Southern Oscillation (ENSO) and extratropical atmospheric disturbances in the formation of NTA SST anomalies. During 1970–86, ENSO events play a key role in initiating NTA SST anomalies in the preceding spring through atmospheric circulation changes. With the decay of ENSO, SST anomalies in the midlatitude North Atlantic wea...

An interdecadal change in the influence of the spring Arctic Oscillation on the subsequent ENSO around the early 1970s

Previous studies suggested that the springtime Arctic Oscillation (AO) influences the El Niño-Southern Oscillation (ENSO) outbreak in the following winter. Using the HadISST, HadSLP2r, ERSSTv3b and NCEP-NCAR reanalysis data for the period 1948–2012, this analysis further reveals that the AO–ENSO relationship experienced a pronounced interdecadal shift. The spring AO influence on the subsequent ENSO is weak before 1970; while the influence becomes strong and statistically significant in the 1970s and 1980s. We then compare the spring AO associated circulation, SST and precipitation anomalies between the PRE (1949–1968) and POST (1970–1989) epochs to explore this interdecadal change of the AO–ENSO relationship. The spring AO-related anomalies of atmospheric circulation over the North Pacific mid-latitudes, cyclonic circulation over the subtropical western-central Pacific, and westerly winds in the tropical western-central Pacific are found to be stronger in the POST epoch than in the PRE epoch. The intensity of spring Pacific synoptic-scale eddy activity is seen to experience a significant interdecadal change around the early-1970s from a weak regime to a strong regime. Thus the strength of synoptic-scale eddy feedback to the low frequency flow becomes stronger after 1970. In the POST epoch, the strong synoptic-scale eddy feedback provides a favorable condition for the formulation of the spring AO-related cyclonic circulation and westerly wind anomalies over the western North Pacific. The tropical SST, precipitation and atmospheric circulation anomalies sustain and develop from spring to winter through the positive Bjerknes feedback, leading to an El Niño-like warming in the tropical central-eastern Pacific in the following winter.

Dominant Modes of Interannual Variability in Eurasian Surface Air Temperature during Boreal Spring

AbstractThis study investigates interannual variations of surface air temperature (SAT) over mid- and high latitudes of Eurasia during boreal spring and their association with snow, atmospheric circulation, and sea surface temperature (SST) changes. The leading mode of spring SAT variations is featured by same-sign anomalies over most regions. The second mode features a tripole anomaly pattern with anomalies over the central part opposite to those over the eastern and western parts of Eurasia. A diagnosis of surface heat flux anomalies suggests that snow change contributes partly to SAT anomalies in several regions mainly by modulating surface shortwave radiation but cannot explain SAT changes in other regions. Atmospheric circulation anomalies play an important role in spring SAT variability via wind-induced heat advection and cloud-induced surface radiation changes. Positive SAT anomalies are associated with anomalous westerly winds from the North Atlantic Ocean or with anomalous anticyclone and souther...

Intensified impact of northern tropical Atlantic SST on tropical cyclogenesis frequency over the western North Pacific after the late 1980s

Previous studies suggest that spring SST anomalies over the northern tropical Atlantic (NTA) affect the tropical cyclone (TC) activity over the western North Pacific (WNP) in the following summer and fall. The present study reveals that the connection between spring NTA SST and following summer–fall WNP TC genesis frequency is not stationary. The influence of spring NTA SST on following summer–fall WNP TC genesis frequency is weak and insignificant before, but strong and significant after, the late 1980s. Before the late 1980s, the NTA SST anomaly-induced SST anomalies in the tropical central Pacific are weak, and the response of atmospheric circulation over the WNP is not strong. As a result, the connection between spring NTA SST and following summer–fall WNP TC genesis frequency is insignificant in the former period. In contrast, after the late 1980s, NTA SST anomalies induce pronounced tropical central Pacific SST anomalies through an Atlantic–Pacific teleconnection. Tropical central Pacific SST anomalies further induce favorable conditions for WNP TC genesis, including vertical motion, mid-level relative humidity, and vertical zonal wind shear. Hence, the connection between NTA SST and WNP TC genesis frequency is significant in the recent period. Further analysis shows that the interdecadal change in the connection between spring NTA SST and following summer–fall WNP TC genesis frequency may be related to the climatological SST change over the NTA region.

Regional change in snow water equivalent–surface air temperature relationship over Eurasia during boreal spring

Present study investigates local relationship between surface air temperature and snow water equivalent (SWE) change over mid- and high-latitudes of Eurasia during boreal spring. Positive correlation is generally observed around the periphery of snow covered region, indicative of an effect of snow on surface temperature change. In contrast, negative correlation is usually found over large snow amount area, implying a response of snow change to wind-induced surface temperature anomalies. With the seasonal retreat of snow covered region, region of positive correlation between SWE and surface air temperature shifts northeastward from March to May. A diagnosis of surface heat flux anomalies in April suggests that the snow impact on surface air temperature is dominant in east Europe and west Siberia through modulating surface shortwave radiation. In contrast, atmospheric effect on SWE is important in Siberia and Russia Far East through wind-induced surface sensible heat flux change. Further analysis reveals that atmospheric circulation anomalies in association with snowmelt over east Siberia may be partly attributed to sea surface temperature anomalies in the North Atlantic and the atmospheric circulation anomaly pattern associated with snowmelt over Russia Far East has a close association with the Arctic Oscillation.

On the weakened relationship between spring Arctic Oscillation and following summer tropical cyclone frequency over the western North Pacific: A comparison between 1968–1986 and 1989–2007

This study documents a weakening of the relationship between the spring Arctic Oscillation (AO) and the following summer tropical cyclone (TC) formation frequency over the eastern part (150°–180°E) of the western North Pacific (WNP). The relationship is strong and statistically significant during 1968–1986, but becomes weak during 1989–2007. The spring AOrelated SST, atmospheric dynamic, and thermodynamic conditions are compared between the two epochs to understand the possible reasons for the change in the relationship. Results indicate that the spring AO leads to an El Ni˜no-like SST anomaly, lower-level anomalous cyclonic circulation, upper-level anomalous anticyclonic circulation, enhanced ascending motion, and a positive midlevel relative humidity anomaly in the tropical western–central Pacific during 1968–1986, whereas the AOrelated anomalies in the above quantities are weak during 1989–2007. Hence, the large-scale dynamic and thermodynamic anomalies are more favorable for TC formation over the eastern WNP during 1968–1986 than during 1989–2007.

Modulation of the connection between boreal winter ENSO and the South Asian high in the following summer by the stratospheric quasi-biennial oscillation

This study investigated the modulation effect of boreal winter quasi-biennial oscillation (QBO) on the connection between winter El Nino–Southern Oscillation (ENSO) events and the variability of the following summers South Asian high (SAH) by using European Centre for Medium-Range Weather Forecasts Interim Reanalysis data for the period of 1979–2013. The results suggest that the boreal summer SAH is more significantly influenced by preceding ENSO events in the easterly phase of the QBO than in the westerly phase. The change in the ENSO-SAH relationship in the different QBO phases may be attributable to the change in the ENSO-induced sea surface temperature (SST) anomalies over the tropical Indian Ocean (TIO). Specifically, ENSOs influence on the following summers TIO SST is more significant when the QBO is in its easterly phase than westerly phase. Further analysis showed that the change in the connection between ENSO and the TIO SST in the different QBO phases may be attributable to change in the anomalous tropical Indian Ocean cell (IOC) associated with ENSO. In particular, the anomalous IOC induced by anomalous Walker circulation over the tropical Pacific is stronger and located further west in QBO easterly than westerly phase. This leads to stronger descending motion and larger SST anomalies over the TIO in the QBO easterly phase. These larger SST anomalies over the TIO in the QBO easterly phase could exert a more significant influence on the tropospheric temperature through moist adjustment, which subsequently results in stronger SAH variability.