Comparison of the effect of land-sea thermal contrast on interdecadal variations in winter and summer blockings
The influence of winter and summer land-sea surface thermal contrast on blocking for 1948–2013 is investigated using observations and the coupled model intercomparison project outputs. The land-sea index (LSI) is defined to measure the changes of zonal asymmetric thermal forcing under global warming. The summer LSI shows a slower increasing trend than winter during this period. For the positive of summer LSI, the EP flux convergence induced by the land-sea thermal forcing in the high latitude becomes weaker than normal, which induces positive anomaly of zonal-mean westerly and double-jet structure. Based on the quasiresonance amplification mechanism, the narrow and reduced westerly tunnel between two jet centers provides a favor environment for more frequent blocking. Composite analysis demonstrates that summer blocking shows an increasing trend of event numbers and a decreasing trend of durations. The numbers of the short-lived blocking persisting for 5–9 days significantly increases and the numbers of the long-lived blocking persisting for longer than 10 days has a weak increase than that in negative phase of summer LSI. The increasing transient wave activities induced by summer LSI is responsible for the decreasing duration of blockings. The increasing blocking due to summer LSI can further strengthen the continent warming and increase the summer LSI, which forms a positive feedback. The opposite dynamical effect of LSI on summer and winter blocking are discussed and found that the LSI-blocking negative feedback partially reduces the influence of the above positive feedback and induce the weak summer warming rate.
KeywordsLand-sea thermal contrast Blocking Asymmetric warming Double-jet
This work was jointly supported by the National Science Foundation of China (41521004 and 41705047) and the Foundation of Key Laboratory for Semi-Arid Climate Change of the Ministry of Education in Lanzhou University and the China 111 project (No. B13045), and the Foundation of Key Laboratory for Semi-Arid Climate Change of the Ministry of Education in Lanzhou University from the Fundamental Research Funds for the Central Universities (lzujbky-2017-bt04). We thank the World Climate Research Program’s Working Group on Coupled Modeling, which is responsible for archiving CMIP outputs, and the climate modeling groups (listed in Table 3 of this paper) for producing and making available their model outputs. For the CMIP the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and leads development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. S.M. is supported by the German Federal Ministry of Education and Research Grant no 01LN1304A.
- Andrews DG, Holton JR, Leovy CB (1987) Middle atmosphere dynamics. Academic press, San Francisco, pp 489Google Scholar
- Dunn-Sigouin E, Son S-w (2013) Northern Hemisphere blocking frequency and duration in the CMIP5 models. J Geophys Res 118:1179–1188Google Scholar
- Egger J (1978) Dynamics of blocking highs. J Atmos Sci 35:1788–1801. doi: 10.1175/1520-0469(1978)035<1788:DOBH>2.0.CO;2
- Kaas E, Branstator G (1993) The relationship between a zonal index and blocking activity.J Atmos Sci 50:3061–3077. doi: 10.1175/1520-0469(1993)050<3061:TRBAZI>2.0.CO;2
- Li J, Wang J (2003) A modified zonal index and its physical sense. Geophys Res LettGoogle Scholar
- Rikus L (2015) A simple climatology of westerly jet streams in global reanalysis datasets part I: mid latitude upper tropospheric jets, Clim Dynam. doi: 10.1007/s00382-015-2560-y
- Shutts GJ (1983) The propagation of eddies in diffiuent jetstreams : eddy vorticity forcing of’ blocking’ flow fields. Q J R Meteorol Soc 737–761. doi: 10.1002/qj.49710946204
- Sun L, Perlwitz J, Hoerling M (2016) What caused the recent “Warm Arctic, cold continents” trend pattern in winter temperatures?. Geophys Res Lett 5345–5352. doi: 10.1002/2016GL069024
- Wallace JM, Zhang Y, Bajuk L (1996) Interpretation of interdecadal trends in Northern Hemisphere surface air temperature. J Clim 9:249–259. doi: 10.1175/1520-0442(1996)009<0249:IOITIN>2.0.CO;2
- Zhu Z, Zhu B (1982) The nonlinear equilibrium states of ultra-long waves induced by zonal asymmetric thermal forcing and blocking situation. Sci Sinica 25:1201–1212 (Chinese).Google Scholar