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(zpwen@fudan)

(zpwen@fudan.edu.cn). OUTLINE. 1. Motivation. 2. Data and model. 3. Interdecadal Change in the Tropical Precipitation Anomaly during the Boreal Spring. Outline. 4. Effect of different precipitation mode on atmospheric teleconnection. 5. Summary.

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(zpwen@fudan)

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  1. (zpwen@fudan.edu.cn)

  2. OUTLINE 1.Motivation 2. Data and model 3. Interdecadal Change in the Tropical Precipitation Anomaly during the Boreal Spring Outline 4. Effectof different precipitation mode on atmospheric teleconnection 5. Summary

  3. Interdecadal Change in the Tropical Precipitation Anomaly during the Boreal Spring

  4. 1. Motivation • // The importance of the variability of tropical spring precipitation • The variability of tropical precipitation has received much attention due to its pronounced fluctuation on different time–scales and its tremendous impacts on the atmospheric circulation both in the tropics and in the extratropics(Hoskins and Karoly 52 1981; McBride et al. 2003; Wu et al. 2012; Guo et al. 2015, 2017; Chu et al. 2017). • An accumulation of evidence suggested that the precipitationduring the boreal spring [March–May (MAM)] is also characterized by energetic variations on both the interannualand interdecadal time–scales, which are distinguish with those in other seasons (e.g. Cai and Cowan 2009; Taschetto and England 2009; Feng and Li 2011; Feng et al. 2011; Guo et al. 2016). • On the interannual time scale,MAM was considered to be the timewhen the impacts of central Pacific (CP) and eastern Pacific (EP) El Niño are the most different from each other (Cai and Cowan 2009; Taschetto and England 2009).

  5. 1. Motivation // Inter-decadal change around 1990s A frequent occurrence of CP El Niño (Yu et al. 2012; Xiang et al. 2013; Yeh et al. 2015). Source: Yeh et al, 2015

  6. 1. Motivation // Inter-decadal change around 1990s Rainfall associated with two types of ENSO events Rainfall variability from year to year Guoet al, 2015 Wenget al, 2009

  7. 1. Motivation // Inter-decadal change around 1990s Recent researches were suggestive of the importance of the springseason in the CP ENSO events (e.g. Cai and Cowan 2009; Taschetto and England 2009; Feng and Li 2011) Caiand Cowan 2009

  8. 1. Motivation JJA SON DJF EOF (MAM_rainfall) 1980-2013

  9. 1. Motivation Focus here: Q1: Why does the leading mode of tropical rainfall variation in spring experience a pronounced inter-decadal change around the late 1990s? Q2: Why was the inter-decadal change of the leading mode of precipitation anomalies occurred only in boreal spring?

  10. 2.Data and model • // Datasets • Monthly precipitation data from Global Precipitation Climatology Project (GPCP) • Monthly quantities derived from ERA-Interim • Monthly SST derived from HadISST NCEP Global Ocean Data Assimilation System (GODAS) Simple Ocean Data Assimilation (SODA) reanalysis version 2.0.2 Monthly wind stress from Tropflux reanalysis • 1980-2016 March-April-May (MAM) mean • // Model LBM is a simple dry model, based on a linearized atmospheric general circulation model(AGCM) developed at CCSR dry linear baroclinic model (LBM, Watanabe and Kimoto 2000)

  11. Leadingmode of spring rainfall EOF (MAM_rainfall) 1980-2013

  12. Leading mode of spring rainfall R=0.12 R=0.80 EOF (MAM_rainfall) 1999-2013 EOF (MAM_rainfall) 1980-1998

  13. Leading mode of spring rainfall Table 1 Variances of the first two modes of tropical Pacific rainfall in spring during different periods.

  14. Leading mode of spring rainfall Source: Xiang et al, 2013 Regression onto PC1 SST & U925

  15. Leading mode of spring rainfall • // The main mode of the spring precipitation shifts around the late 1990s Regression map onto PC1 SVD for [1980-1998/99-2013] Guo et al. JC, 2016

  16. Leading mode of spring rainfall Q1: Why the leading mode of tropical rainfall variation in spring experiences a pronounced inter-decadal change around the late 1990s? A1: Diagnose the variability of oceanic feedback processes over time based on the area-averaged form of linear equation for SSTAs, which proposed by Jin et al. (2006).

  17. Possible reasons of its mode shifts Zonal advection (ZA) feedback Thermocline (TH) feedback Ekman pumping (EK) feedback anomalous vertical advection of mean vertical SST gradient: anomalous zonal advection of mean zonal SST gradient: mean vertical advection of anomalous vertical temperature gradient: Source: Jin et al. 2006 The main dynamic coupling term that we discuss here could be described by the following formation: Source: Xiang et al. 2012

  18. Running-t Test: 90% ZA/Sum:90% Bar: Summation

  19. Possible reasons of its mode shifts ~ 0.3°C Gradient: east-west

  20. Possible reasons of its mode shifts Correlation coefficient of u-current and the zonal feedback over CP: 0.99 Running-t Test: 95%(CP)

  21. Possible reasons of its mode shifts Q: Why is the zonal current anomaly against the wind in the CP? Source: Su et al. 2010 Running-t Test: 95%(CP)

  22. Possible reasons of its mode shifts Shading: anomaly Contour: climatology

  23. Possible reasons of its mode shifts Schematic U: current

  24. Possible reasons of its mode shifts • Q2: • The inter-decadal change of the leading mode of precipitation anomalies was observed only in boreal spring. Why? • A2: • Life cycle of ENSO-like SSTA distribution might change. • Variance of variables in spring is most unstable. EF(post90): insignificant

  25. Causes of unique shift in spring Shading：SSTA Contour：SSTt interval: 0.15ºC/month

  26. Causes of unique shift in spring Shading：SSTA Contour：SSTt interval: 0.15ºC/month

  27. Causes of unique shift in spring

  28. Causes of unique shift in spring • Q2: • The inter-decadal change of the leading mode of precipitation anomalies was observed only in boreal spring. Why? • A2: • Life cycle of ENSO-like SSTA distribution might change. • Variance of variables in spring is most unstable. Shading：feedback

  29. Causes of unique shift in spring Red line: anomaly year Black line: average

  30. Interdecadal change in the spring precipitation anomaly pattern over tropical Pacific. Summary The zonal advection feedback processes over the CP

  31. Causes of unique shift in spring. Summary Red line: anomaly year Black line: average

  32. Effectof different precipitation mode on atmospheric teleconnection

  33. 1. Motivation • // The diabatic heating variability associated with tropical anomalous precipitation has significant influence on the circulations both in the tropics and in the extratropics.(Hoskins and Karoly 1981; McBride et al. 2003; Wu et al. 2012; Guo et al. 2015, 2017; Chu et al. 2017) The atmopsheric response of barotropic stream function (contour) in a simple atmospheric model imposed into different forcing (shading) (Zheng et al. 2014) Focus here on: • The impact of such interdecadal change in the precipitation modeon the extra-tropical atmospheric circulation?

  34. 2015/16 El Niño event • // Does the main mode shift still exist after 2015/16? It needs to revisit the robustness of the interdecadal change in precipitation anomaly after extending the studying period to 2016. Nino3.4 index https://www.climate.gov/news-features/blogs/enso/september-2016-enso-update-cooling-our-heels • Whether the interdecadal change in the precipitation mode in GWW2016 is still robust after taking into account the 2015/16 El Niño event remains unknown.

  35. 2015/16 El Niño event • // Does the main mode shift still exist after 2015/16? EOF for [1980-2016] MAM Precip. for 1983/98/2016 Shading: Precip（99%） Contour: SST

  36. 2015/16 El Niño event • // Does the main mode shift still exist after 2015/16? EOF for [1980-1998/99-2016] EOF for [1980-2016] Post-99 Pre-98

  37. Effect of different precipitation mode • // Observational evidence

  38. Effect of different precipitation mode • // Observational evidence Regression map onto Pre98 PC1 Regression map onto Post99 PC1 Shading: GPH（95%） Contour: Heat Vector: UV

  39. Effect of different precipitation mode • // Possible mechanism of the distinct teleconnection associated with EP and CP type precipitation patterns Following Plumb (1985), the horizontal component of the Plumb flux in the form of geostrophic wind can be written as: The barotropicvorticity equation Eliassen–Palm flux Rossbywave source [RWS] (Sardeshmukh and Hoskins 1988; Shimizu and de Cavalcanti 2011) → to examine the generation of Rossby waves → the propagation of Rossby waves Advection of absolute vorticity by divergence flow Vortex stretching term

  40. Effect of different precipitation mode • // Possible mechanism of the distinct teleconnection associated with EP and CP type precipitation pattern the weak generation and propagation of Rossby waves Pre-1998 Shading: RWS（95%） Contour: Heat Vector: EP flux Post-1999 significant negative RWS anomalies, accompanied by a striking northward/ northeastward propagation of Plumb flux

  41. Effect of different precipitation mode • // Possible mechanism of the distinct teleconnection associated with EP and CP type precipitation pattern Advection of absolute vorticity by divergence flow Vortex stretching term Shading: Vortex stretching term（95%） Adverse effect Reinforced effect Shading: Vorticity advection term

  42. Effect of different precipitation mode • // Possible mechanism of the distinct teleconnection associated with EP and CP type precipitation pattern (in the form of RWS anomaly) Anomaly induced by heat forcing Basic state The S3 term offset S1 possibly due to the heat–induced northerly wind over the studying area and the divergence to its north. Shading: RWS term（95%） Contour: mean vorticity Vector: anomalous divergence wind

  43. Effect of different precipitation mode • // Possible mechanism of the distinct teleconnection associated with EP and CP type precipitation pattern Anomaly induced by heat forcing Basic state The CP–type precipitation pattern is associated with a well–organized convergence flow over the WNP and divergence flow over the CP. Shading: RWS term（95%） Contour: mean vorticity Vector: anomalous divergence wind

  44. Numerical simulation // LBM experiments . (Watanabe and Kimoto 2000). Vertical profile of the specific heating (K day–1) at sigma levels

  45. Numerical simulation // LBM experiments The joint effect of WP and EP for EXP.1 heating anomalies Shading: GPH 300hPa Contour: heat forcing Vector: UV wind The conjunct impacts of WNP and CP forcing on the extra–tropical circulation

  46. Numerical simulation • // Role of basic state change before and after 1998 whether such an interdecadal change in basic flow before and after 1998 is responsible for the teleconnection excitation and propagation？ Shading: Vorticity（95%） Vector: divergence wind

  47. Numerical simulation • // Role of basic state change before and after 1998 The spatial distribution of the negative/positive precipitation anomalies seems to hold the key to the significant teleconnection appearance. Shading: GPH 300hPa Contour: heat forcing Vector: UV wind

  48. Summary • // CP–type precipitation mode is closely related to a significant teleconnection pattern extending from the tropics to North Atlantic Ocean, while the teleconnection associated with the EP–type precipitation mode is unclear. The CP–type precipitation pattern, which could induce a well–organized vorticity anomaly center and significant divergence southerly, possibly results in the effective generation of Rossby wave. • // Experiments decided to inspect the sensibility to the basic state suggested that the change in the spring basic flow before and after 1998 plays a minor role in inducing extratropicalteleconnectioncompared with the tropical precipitation forcing.

  49. Guo Y., Z. Wen*, and R. Wu, 2016: Interdecadal Change in the Tropical Precipitation Anomaly around the late 1990s during the Boreal Spring. Journal of Climate, 29, 5979-5997. DOI: 10.1175/JCLI-D-15-0462.1. Guo Y., Z. Wen*, R. Chen, X. Li and X. Yang, 2019: Impact of Boreal Spring Precipitation Pattern Change in late 1990s over Tropical Pacific on the atmospheric teleconnection. Climate Dynamics. 52: 401–416. DOI: 10.1007/s00382-018-4149-8

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