Different impacts of Northern, Tropical and Southern volcanic eruptions on the tropical Pacific SST in the last millennium

Transcription

1 Different impacts of Northern, Tropical and Southern volcanic eruptions on the tropical Pacific SST in the last millennium Meng Zuo, Wenmin Man, Tianjun Zhou Sixth WMO International Workshop on Monsoons,Singapore 13-17, November,2017 1

2 Outline Motivation Data and Methods Response of the Pacific sea surface temperature to eruptions Mechanism of the SST formation Summary 2

3 1815 Mount Tambora 1816 year without summer 3 (Figures from google images)

4 Motivation (Timmreck 2012) Volcanic eruptions have many climatic effects on ocean dynamics, the hydroclimate, monsoons, the Arctic Oscillation, etc.

5 Relationship between volcanic eruptions and El Niño? Volcanic eruptions ENSO 5 (Figures from google images)

6 Previous study Instrumental data Reconstruction Model Simulation Equatorial Pacific tend to be in an El Niño state after tropical volcanic eruptions (Handler et al. 1984; Adams et al ; Mann et al. 2005) Different types of eruptions. (Stevesons et al. 2016; Liu et al. 2017) 6

7 The main motivation of the study is to address the following questions: Questions 1) What are the Pacific SST responses following Northern, Tropical and Southern volcanic eruptions on a monthly time scale? What are the differences between them? 2) What are the dominant mechanisms for the Pacific SST responses following these three types of eruptions? 7

8 Outline Motivation Data and Methods Response of the Pacific sea surface temperature to eruptions Mechanism of the SST formation Summary 8

9 Data and Methods CESM-LME (Last Millennium Ensemble): This model provides the largest ensemble of LM simulations from a single model currently available Volcanic-only forcing (5 members) Monthly data Methods SEA(Super epoch analysis) Monte Carlo test Criteria for volcano selection Divide volcanoes into Northern, Tropical and Southern eruptions. Reach their peaks at the same time.

10 Criteria for volcano selection Class Years Northern 1176,1213,1600,1641,1762,1835 Tropical 1001,1258,1284,1416,1809,1815,1991 Southern 1275,1341,

11 Outline Motivation Data and Methods Response of the Pacific sea surface temperature to eruptions Mechanism of the SST formation Summary 11

12 Response of the Pacific sea surface temperature to eruptions Longitude-time sections of SST and 850-hPa wind anomalies along the equatorial region following volcanic eruptions. Northern Tropical Southern There is an El Niño-like response after Northern and Tropical eruptions, and a weaker such tendency after Southern eruptions. 12

13 Evolution of the composite Niño3 index following eruptions The Niño3 index peaks (0.8 ) lagging one and a half years after Northern and Tropical eruptions; Following Southern eruptions, the maximum value of the Niño3 index is less than 0.5, but an El Niño-like SST response can still be seen. 13

14 The lead-lag correlation between the Niño3 index and zonal wind The lead-lag correlation between the Niño3 index and the 850 hpa zonal wind averaged over the western-to-central equatorial Pacific (110 o E-150 o W,5 o S-5 o N) The El Niño-like warming over the eastern Pacific results from the weakening of the trade winds. 14

15 Outline Motivation Data and Methods Response of the Pacific sea surface temperature to eruptions Mechanism of the SST formation Summary 15

16 The response of positions of the ITCZ N H The positions of the ITCZ for a no-volcano winter, the first and the second winters after three types of eruptions. T R S H Following Northern eruptions, the ITCZ shifts southward and induce a weakening of the trade winds. The southward shift of the ITCZ and the occurrence of the westerly anomaly match following Northern eruptions but do not match following Tropical and Southern eruptions. 16

17 Ocean dynamical thermostat mechanism Ocean dynamical thermostat mechanism (Clement et al. 1996) Negative radiative forcing + Negative radiative forcing over equatorial Pacific the cooling over eastern Pacific is partly balanced by vertical advection the western Pacific favors a stronger cooling decreased zonal temperature gradient westerly anomalies El Niño-like SST anomaly 17

18 The evolution of SST anomalies over western/eastern Pacific Northern Tropical Southern The cooling over the eastern Pacific is weaker than that over the western Pacific after the peak of eruptions, which is driven by the Ocean dynamical thermostat mechanism. 18

19 Response of the precipitation to eruptions Northern Tropical Southern Land-sea thermal contrast; decreased precipitation over the MC the divergent wind westerly anomaly El Niño-like response 19

20 Outline Motivation Data and Methods Response of the Pacific sea surface temperature to eruptions Mechanism of the SST formation Summary 20

21 Response of the Pacific sea surface temperature to eruptions: The Pacific features a significant El Niño-like SST anomaly 5-10 months after Northern and Tropical eruptions, and with a weaker such tendency after Southern eruptions. Two years after all three types of volcanic eruptions, a La Niña-like state over the equatorial Pacific is observed. Mechanism of the SST formation: Summary The El Niño-like warming over the eastern Pacific results from the weakening of the trade winds. A shift of the intertropical convergence zone (ITCZ) can explain the El Niño-like response to Northern eruptions, which is not applicable for Tropical or Southern eruptions. The reduction in the zonal equatorial SST gradient through the ocean dynamical thermostat mechanism, combined with the divergent wind induced by the decreased precipitation over the MC, can amplify the westerly anomalies over the equatorial Pacific, which is applicable for all three types of eruptions. 21

22 THANKS 22

23 Motivation There is a relationship between volcanic eruptions and El Niño (Handler 1984; Robock et al., 1995; Adams et al., 2003; Mann et al., 2005) + El Niño is one of the most important modes of internal climate variability and influences the global climate through atmospheric teleconnections (Alexander et al. 2002). It is of great significance to study the influence of volcanic eruptions on El Niño. Instrumental data (Handler 1984; MacCracken et al., 1984; Nicholls 1990; Self et al.,1997) Reconstruction (Adams et al., 2003) Model Simulation (Mann et al., 2005;Emile-Geay et al. 2008; McGregor et al. 2010; McGregor et al. 2011; Ohba et al. 2013; Li et al. 2013; Wahl et al. 2014; Pausata et al. 2015a; Maher et al. 2015; Stevenson et al. 2016; Lim et al. 2016; Thanh 2017; Predybaylo et al. 2017;Swingedouw et al. 2017) 23

24 Decomposition of the surface radiation flux anomalies following eruptions The change in the SST gradient is not a direct response to the radiative forcing but instead emerges through dynamic processes. 24

25 Response of the sea level pressure to eruptions Longitude-time sections of the SLP anomalies along the equatorial region following three types of eruption. The corresponding SLPs feature a lowering SLP in the eastern Pacific and rising SLP in the western Pacific. 25

26 Response of the Pacific sea surface temperature to eruptions: The Pacific features a significant El Niño-like SST anomaly 5-10 months after Northern and Tropical eruptions, and with a weaker such tendency after Southern eruptions. Two years after all three types of volcanic eruptions, a La Niña-like state over the equatorial Pacific is observed. Mechanism of the SST formation: Summary The El Niño-like warming over the eastern Pacific results from the weakening of the trade winds. A shift of the intertropical convergence zone (ITCZ) can explain the El Niño-like response to Northern eruptions, which is not applicable for Tropical or Southern eruptions. The reduction in the zonal equatorial SST gradient through the ocean dynamical thermostat mechanism, combined with the land-sea thermal contrast between the Maritime Continent (MC) and the surrounding ocean and the divergent wind induced by the decreased precipitation over the MC, can amplify the westerly anomalies over the equatorial Pacific, which is applicable for all three types of eruptions. 26