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DOI 10.1007/s00382-014-2227-0Clim Dyn (2015) 44:807825

State of the tropical Pacific Ocean and its enhanced impact on precipitation over East Asia during marine isotopic stage 13

M. P. Karami N. Herold A. Berger Q. Z. Yin H. Muri

Received: 28 August 2013 / Accepted: 19 June 2014 / Published online: 16 July 2014 Springer-Verlag Berlin Heidelberg 2014

associated teleconnections with the extra-tropics favored increased precipitation over the EASM. As compared to PrI, it is found that the summer (JuneJulyAugust) sea surface temperature (SST) is warmer in the eastern tropi-cal Pacific Ocean and colder to the west. In concert with previous studies, we show that colder summer SSTs in the central tropical Pacific during MIS-13 promotes an upper-level teleconnection between the tropical Pacific Ocean and EASM. It also contributes to the strengthening of the north-ern Pacific subtropical high and, therefore, the transport of more moisture into the EASM. We suggest that the reduced eastwest SST difference in the tropical Pacific in summer helps to maintain the teleconnection between the tropical Pacific and EASM. The correlation between tropical Pacific SSTs and the EASM was higher in our MIS-13 simula-tions, further supporting the enhancement of their relation-ship. It is found that the pure impact of El Nio Southern Oscillation on EASM precipitation increases by up to 30 % in MIS-13 for HadCM3 while it is minor for CCSM3. Bet-ter constraining the spatio-temporal variability of tropical Pacific SST during the interglacials may thus help explain the anomalously strong EASM during MIS-13 which has been observed from geological records.

Keywords Paleoclimate modeling MIS-13 ENSO Teleconnection East Asian summer monsoon

1 Introduction

Given the Earths current interglacial state, past intergla-cial periods provide good candidates for studying potential climate change. Quaternary interglacials exhibited changes in atmospheric and oceanic circulations and are subject to high spatio-temporal resolution data archives, making them

Abstract Multiple terrestrial records suggest that marine isotopic stage 13 (MIS-13), an interglacial period approxi-mately 0.5 million years ago, had the strongest East Asian summer monsoon (EASM) of the last one million years. This is unexpected given that, compared to other intergla-cials, MIS-13 was globally cooler with a lower CO2 con-centration. We use two coupled atmosphereocean general circulation models, the Hadley Centre Coupled Model, version 3 (HadCM3) and Community Climate System Model, version 3.0 (CCSM3), to simulate the climate of MIS-13 forced with different insolation and greenhouse gas concentrations relative to the pre-industrial (PrI) situ-ation. Both models confirm a stronger EASM during MIS-13 compared to PrI. Here we specially focus on analyzing the impact of the tropical Pacific Ocean on the EASM. Our simulations suggest that the mean climatic state in the tropical Pacific during MIS-13 was La Nia-like and that

M. P. Karami (*) N. Herold A. Berger Q. Z. Yin H. Muri Georges Lematre Centre for Earth and Climate Research (TECLIM), Earth and Life Institute (ELI), Universit Catholique de Louvain, Place Louise Pasteur 3, Box L4.03.08, 1348 Louvain-La-Neuve, Belgiume-mail: pashakarami@gmail.com

Present Address: M. P. Karami Geotop, Universit du Qubec Montral (UQAM), Montreal, Canada

N. Herold Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH 03824, USA

H. Muri Department of Geosciences, Meteorology and Oceanography Section, University of Oslo, Blindern, Postboks 1022, 0315 Oslo, Norway

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ideal candidates for investigating the sensitivity of climate to forcings.

The interglacial stage of ~0.5 million years ago, identi-fied as marine isotopic stage 13 (MIS-13), was character-ized by a relatively low CO2 concentration (Luthi et al. 2008), cool Antarctic temperatures (Jouzel et al. 2007) and high benthic 18O values (e.g., Lisiecki and Raymo 2005) related to higher global ice volume and/or colder deep-ocean temperatures. However, it featured the strongest East Asian summer monsoon (EASM) of the last one million years, an insight derived mainly from loess records from northern China (e.g. Kukla et al. 1990; Guo et al. 1998) and paleosols from southern China (Yin and Guo 2006). The strongest EASM in MIS-13 was proposed as a paradox (Yin and Guo 2008) given that CO2 and CH4 concentra-tions were relatively low during MIS-13 and its insolation is not abnormal as compared to other interglacials. Strong African and Indian monsoons during MIS-13 were also suggested based on marine sediments from the equatorial Indian Ocean (Bassinot et al. 1994) and the Mediterranean Sea (Rossignol-Strick et al. 1998).

This study investigates the role of the tropical Pacific Ocean in enhancing the EASM during MIS-13. In previous model studies of MIS-13, the impacts of insolation, green-house gas (GHG) concentrations and ice sheets were inves-tigated (Yin et al. 2008, 2009; Sundaram et al. 2012; Muri et al. 2012, 2013). The first model simulations of MIS-13 climate were performed by Yin et al. (2008, 2009) using an Earth system model of intermediate complexity, LOVE-CLIM. They found that strong summer insolation in the Northern Hemisphere (NH) is chiefly responsible for the more intense EASM of MIS-13 compared to pre-industrial (PrI). However, model comparison between MIS-13 and all the other interglacials (in particular MIS-5e) showed that insolation alone did not make the MIS-13 EASM excep-tionally strong (Yin and Berger 2012). Therefore, the role of other factors was also needed to be investigated. The presence of a Eurasian ice sheet further increased (by 5 %) the precipitation in the EASM region through a topograph-ically-induced atmospheric wave train (Yin et al. 2008, 2009). Whether there were in fact additional ice sheets in the NH during MIS-13 is still unclear from geological evidence (Guo et al. 2009). The results of Yin et al. (2008, 2009) have since been confirmed by general circulation models (Sundaram et al. 2012; Muri et al. 2012, 2013).

Based on hydrographic reconstructions from the South China Sea, Yu and Chen (2011) conjectured the importance of the tropical dynamics and the eastwest sea surface tem-perature (SST) gradient of the Equatorial Pacific in MIS-13 climate. In the modern climate, the tropical Pacific Ocean is an important component of climate variability. The El Nio-Southern Oscillation (ENSO) is the dominant source of interannual variability in the tropical Pacific Ocean and

strongly influences global precipitation (Ropelewski and Halpert 1987) and the EASM (e.g., Chang et al. 2000) through changing atmospheric circulation and teleconnec-tions (e.g., Hoskins and Karoly 1981). El Nio (La Nia), which is the positive (negative) phase of ENSO, is driven by significantly warm (cold) SST in the equatorial eastern Pacific Ocean in winter. Mean summer precipitation over China usually increases after onset of the El Nio (Shen and Lau 1995; Chang et al. 2000). Moreover, the mean climatic state of the tropical Pacific varies on interdecadal to millennial time scales, which modifies the atmospheric teleconnections accompanying ENSO events (Mller and Roeckner 2008) and influences EASM precipitation. For instance, changes in the ENSO-EASM relation on interdec-adal time scales (e.g., Wu and Wang 2002; Lee et al. 2008), was linked to the interdecadal changes in the background state of the tropical Pacific and Indian Oceans (Chang et al. 2000). At precessional frequencies, El Nio- or La Nia- like configurations are found in the mean-state of the tropi-cal Pacific depending on the time of perihelion (Clement et al. 1999).

The atmospheric-oceanic interaction in the tropical Pacific which was very likely to contribute to a stronger EASM in MIS-13 has not been explored. We use two fully coupled general circulation models, Hadley Centre Cou-pled Model, version 3 (HadCM3) and Community Cli-mate System Model, version 3.0 (CCSM3), to explore the ocean and atmosphere interactions in the tropical Pacific and their teleconnection with the EASM during MIS-13. This work goes beyond previous studies of MIS-13 (e.g., Muri et al. 2013) by showing that the changes in the mean state as well as in the interannual variability of the tropi-cal Pacific increased the ENSOEASM relationship and contributed to the higher EASM precipitation during this period. In addition to unraveling the causes for the climate of MIS-13, this study also contributes to our understand-ing of the relationship between the tropical Pacific Ocean and East Asian monsoon under different CO2 concentra-tion and insolation.

This paper is organized as follows: Sect. 2 briefly describes the models and boundary conditions. In Sect. 3, the difference in the mean climate between MIS-13 and PrI, and the subsequent difference in the monsoons are investigated. This will be followed by our proposed mecha-nism concerning the enhanced relationship between the tropical Pacific and EASM in MIS-13. In Sect. 4, we will analyze the interannual variability around the mean state of each model experiment and will quantify the contribution of ENSO events to the enhancement of the EASM. Lastly, Sect. 5 draws conclusions concerning the role of tropical Pacific Ocean in enhancement of the EASM and the often disregarded relationship between mean climate and interan-nual variability.

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2 Model descriptions

Full descriptions of each models configuration is presented in Muri et al. (2012) for HadCM3 and Herold et al. (2012) for CCSM3. Here, we summarize the main information of the models and the boundary conditions. The HadCM3 is a fully coupled atmosphereocean general circulation model (AOGCM). Its atmospheric component has 19 verti-cal levels and horizontal resolution of 3.75 2.5 (lon-gitude latitude). The oceanic component has 20 vertical layers with 1.25 1.25 horizontal resolution. Documen-tation of the model are explained in e.g. Pope et al. (2000) and Gordon et al. (2000). HadCM3 produces ENSO with a period in 34 year band (Collins et al. 2001), and has a realistic representation of the monsoons (Turner and Slingo 2009). The CCSM3 (Collins et al. 2006) is an AOGCM of a similar generation to HadCM3, both of which were a part of the Fourth Assessment Report by the IPCC. The atmospheric component of the CCSM3 represents 26 vertical levels with a horizontal T31 spectral resolution (~3.75 3.75). The ocean component represents 25 ver-tical levels with a nominal ~31 horizontal resolution. The version of the CCSM3 used in this study simulates ENSO variability with as much skill as higher resolution versions of the model (Yeager et al. 2006), though exhibits too high a frequency compared to observations (Deser et al. 2006).

Two simulations were performed for each model; a PrI and a MIS-13 simulation. Only the climate response to insolation and GHGs is analyzed while ice sheets are kept at their present-day states. The largest difference between the PrI and MIS-13 simulations are their astronomical con-figurations (Table 1). MIS-13 has a larger eccentricity and its NH summer occurred at perihelion, which leads to a higher summer insolation in the NH compared to our PrI simulations [the summer solstice daily insolation at 65N is 50 W m2 (10 %) larger during MIS-13 than PrI]. This dif-ference in insolation forcing induces significant changes in the climate system as shown in previous studies (Yin et al. 2008; Muri et al. 2013) as well as in this study.

For HadCM3, the PrI control run is for the year 1850, and to simulate the MIS-13 climate the astronomical parameters and GHG concentrations at 506 ka BP are used (Table 1; Muri et al. 2013). Both HadCM3 experiments

have the same insolation and GHG as Yin et al. (2008) where the justification of the forcing was explained. The HadCM3 experiments were run for 800 years. The PrI CCSM3 simulation is that of Herold et al. (2012) and the MIS-13 CCSM3 simulation was set up in an identical fashion to the interglacial simulations in that study. The PrI simulation of CCSM3 was run for 1,300 years (thus a total length of 1,300 years) and the MIS-13 one was ini-tiated from the PrI simulation at year 500 (of 1,300) and run for 1,000 years. The CCSM3 MIS-13 experiment has the same astronomical parameters as the HadCM3 one, but has slightly different GHG concentrations (Table 1). This is because the HadCM3 MIS-13 experiment has followed the strategy of Yin et al. (2008) where the average of GHG concentrations over MIS-13 was used, whereas the CCSM3 MIS-13 experiment has followed the strategy of Yin and Berger (2012) where interglacial climates were simulated under peak forcings. This slight difference in GHG concen-trations between the two models is equivalent to ~0.13 W/m2 radiative forcing (Myhre et al. 1998) and thus would not likely change our conclusions.

Given that our focus is on the potential role of the tropi-cal Pacific in enhancing EASM, a comprehensive inter-model comparison is not done. However, recent studies provide comprehensive overviews of various models skill in reproducing interglacial climate (e.g., Lunt et al. 2013). The HadCM3 and CCSM3 underestimate high latitude warmth and thus also misrepresent the equator to pole tem-perature gradient based on Lunt et al. (2013). HadCM3 however does a better job than CCSM3 in reproducing interglacial temperatures.

3 Climatology of MIS13

3.1 Annual/summer atmosphere and ocean MIS-13 climatologies

In both HadCM3 and CCSM3, the annually-averaged global SST of MIS-13 is generally found to be lower than PrI. This cooling is mainly due to its lower CO2 concentration (Yin and Berger 2012). The strongest cooling particularly occurs in the tropical Pacific and the North Atlantic Ocean for HadCM3, and in the western

Table 1 Astronomical parameters (Berger 1978) and greenhouse gas concentrations used for simulating the PrI and MIS-13 climate in HadCM3 and in CCSM3

Experiment Obliquity () Eccentricity Longitude of perihelion () CH4 (ppb) N2O (ppb) CO2 (ppm)

PrI 23.446 0.016724 102.04 760 270 280HadCM3 MIS-13 23.377 0.034046 274.05 510 280 240CCSM3 MIS-13 23.377 0.034046 274.05 508 258 247

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North Pacific and North Atlantic Ocean for CCSM3 (Fig. 1). In the tropical Pacific, both models show cool-ing, though this is stronger in HadCM3. The eastern tropical Pacific Ocean (250280E) shows a smaller decrease in temperature relative to the central and west-ern tropical Pacific. One possible explanation for this is the westward shift of the Pacific cold tongue (where the SST is minimum along the tropical Pacific) which is further discussed in Sect. 3.2. Also, the presence of strong ocean dynamics (e.g., upwelling) in the eastern tropical Pacific Ocean has been known to cause different responses to climate change relative to the western tropi-cal Pacific (Clement et al. 1996).

Figure 2 shows the modeled precipitation for NH sum-mer (JuneJulyAugust; JJA) as our particular interest is the summer monsoon. The PrI values of the two models (Fig. 2a, b) are qualitatively in agreement although there are differences in the tropics and East Asia. For the differ-ence in precipitation between MIS-13 and PrI, we focus on eastern China where most EASM proxy data were collected, i.e., approximately 2040N and 100120E (e.g., Yin and Guo 2008; their Fig. 2). The MIS-13 JJA precipitation increases in both models in MIS-13 except for central-eastern China in the CCSM3. The increase in EASM precipitation is larger in the HadCM3 and cov-ers eastern China in agreement with proxy data. Overall, HadCM3 seems to perform better than CCSM3 in captur-ing increased precipitation throughout the EASM region when compared to the data of Yin and Guo (2008). For the modern simulations of EASM, it was also found that the HadCM3 reproduced the EASM well (Lei et al. 2013) while CCSM3 had deficient rainfall over EASM (Meehl et al. 2006).

The position and strength of the western Pacific subtrop-ical high is known to play a dominant role in the variability and distribution of EASM precipitation (Zhou et al. 2008 and references therein). In our MIS-13 simulations, JJA sea level pressure (SLP) values for both models exhibit west-ward extension and strengthening of the western Pacific subtropical high and deepening of Asian low (Fig. 3). Such a change is in favor of increasing the monsoon intensity by bringing more moisture from the Indian and Pacific Oceans to East Asia (Fig. 3). The total moisture transported to East Asia in our PrI simulations mainly originates from the Indian Ocean and South China Sea (Fig. 3a, c). In CCSM3 MIS-13, it can be seen that the moisture coming from the northern Pacific Ocean into EASM is increased compared to CCSM3 PrI (Fig. 3b). In the case of HadCM3 MIS-13, the additional moisture comes not only from the northern Pacific Ocean, but also from the Indian Ocean and South China Sea. More moisture from the Pacific Ocean dur-ing MIS-13 suggests an increase in its contribution to the EASM during MIS-13.

3.2 La Nia-type mean climate of the tropical Pacific

All the fields in this section are presented as annual mean values averaged over 5S5N.

The cooling of the tropical Pacific SSTs (as shown in Sect. 3.1) is not uniform along the equator and is largest in the eastern-central equatorial Pacific Ocean around 240E (Fig. 4a). The Pacific cold tongue has shifted westward in our MIS-13 simulations by ~10 compared to PrI ones. The difference in SST (annual mean) between the cold tongue and the western tropical Pacific shows an increase in MIS-13 for both models. However, the difference in SST between the easternmost and western tropical Pacific Ocean is decreased in MIS-13. This is important to con-sider when interpreting proxy SST records from the tropi-cal Pacific (cf., Mohtadi et al. 2006).

The magnitude of the zonal wind stress during MIS-13 has decreased in the eastern tropical Pacific (230280E) compared to PrI, while it has increased to the west (150230E; Fig. 4b).

The mean thermocline depth, where the largest vertical oceanic temperature gradient occurs, is shown in Fig. 4c. The thermocline along the equator is typically approximated by the depth of the 20 C isotherm (e.g., Merkel et al. 2010), which was found to be a good approximation for our MIS-13 simu-lations and thus adopted here. Compared to the PrI, MIS-13 in CCSM3 shows an overall shoaling of the thermocline with a reduced eastwest slope. In HadCM3, the MIS-13 thermo-cline is shallower in the east, deeper in the west and, therefore, has steeper tilt. The enhanced vertical velocity in the eastern tropical Pacific (not shown) also indicates a stronger upwelling consistent with the shoaling of the thermocline in both mod-els. When the eastwest tilt of the thermocline is compared between MIS-13 and PrI, its relation with the corresponding zonal wind stress is non-linear. Between 230E and 280E, the shallower thermocline does not follow the weakened zonal wind stress. This is likely to be related to the stronger meridi-onal wind (Fig. 12 in Appendix) in the eastern equatorial Pacific (mainly south of the equator) which causes stronger upwelling through Ekman pumping and, therefore, a shal-lower thermocline. The equatorial Pacific thermocline com-puted in both PrI and MIS-13, is deeper in HadCM3 than in CCSM3 although HadCM3 exhibits stronger zonal and merid-ional winds. The difference in the thermocline of the two mod-els might be an ocean-related process (e.g., Timmermann et al. 2005) and/or due to the difference in the vertical resolution of the ocean component of the models.

The sea-level pressure differences between our MIS-13 and PrI simulations (in both models) show a positive and negative anomaly in the eastern and western tropical Pacific, respectively (Fig. 13 in Appendix). Regarding the Walker circulation, the atmospheric vertical velocity at the equator versus longitude shows the upwelling and

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downwelling branches (Fig. 4d; note that negative velocity is upward). HadCM3 MIS-13 shows westward shift (west-ward extension) of the convection center in the western tropical Pacific and smaller vertical velocity in the west-ern Pacific (150170E). CCSM3 MIS-13, however, shows increased convection in the western Pacific (150180E) and no westward extension of the Walker circulation.

The shoaling of the thermocline in the eastern tropical Pacific, the presence of the cold SST anomaly in the cen-tral tropical Pacific, the westward shift of the cold tongue and Walker circulation, and stronger zonal wind stress in the central equatorial Pacific all indicate that MIS-13 was subject to a La Nia-like mean climatic state, relative to PrI conditions.

3.3 Strengthened relationship between tropical Pacific SST and East Asian monsoon

Higher summer insolation and the subsequent increase of landocean thermal contrast (i.e., between East Asia and Western Pacific Ocean) in MIS-13 was found as an impor-tant factor for the enhancement of EASM precipitation (Yin et al. 2008). We find that MIS-13 landocean thermal con-trast reaches its maximum in June, and starts to decrease in July and August (not shown). This is while, EASM precipi-tation of MIS-13, continues to increase in July and August, and has its maximum in July for CCSM3 and in August for

HadCM3 (Fig. 5d). This suggests that other factors, next to the landocean thermal contrast, could have also played a role in MIS-13 to explain the EASM precipitation evolu-tion in June, July and August. The changes in the tropical Pacific SST, is seen a good candidate.

The SST difference between MIS-13 and PrI shows a warming anomaly in the eastern equatorial Pacific and a cooling one in the central equatorial Pacific during summer (JJA) which coincides with the increase in EASM precipi-tation (Fig. 5a, b). These SST anomalies affect the atmos-pheric circulation by modifying the tropical convection. Summer-time cooling in the central tropical Pacific was suggested to have a critical role in maintaining the western Pacific subtropical high and modifying the EASM precipi-tation (Fan et al. 2013). The SST difference between MIS-13 and PrI has larger seasonal variability in the eastern equatorial Pacific (260280E) than in the western equato-rial Pacific (140160E; Fig. 5a, b) which also affects the tropical convection. Such behavior is normally linked to the higher asymmetry in the distribution of SSTs on the north-ern and the southern sides of the eastern equatorial Pacific (Li and Philander 1996; Thuburn and Sutton 2000).

Changes in the eastwest SST gradient in the tropical Pacific has been suggested to have a larger influence on the tropical convection than a uniform zonal cooling/warm-ing (Yin and Battisti 2001; Chiang 2009). The eastwest SST difference in MIS-13 (Fig. 5c) is smaller than in PrI

Fig. 1 Annually-averaged sea surface temperature (SST; C). Left column is CCSM3 and right one is HadCM3. a and b are PrI val-ues, c and d are the difference between MIS-13 and PrI. The colored-

shaded areas in c and d are significant at the 95 % confidence level based on the Students t test and the white-shaded areas are non-sig-nificant values

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from May to October in CCSM3 (the maximum decrease is reaching 1 C), and from June to December in HadCM3 (the maximum decrease is reaching 3 C). At around the same period, the EASM precipitation (averaged over 2040N and 100120E) is found to be higher in MIS-13, especially in HadCM3 (Fig. 5d). Accordingly, less winter precipitation coincides with the enhanced eastwest SST difference in MIS-13. Hence, the eastwest tropical Pacific SST gradient and EASM precipitation are found to be anti-correlated (around 0.8 with 95 % significance) in MIS-13 which is not the case in PrI. Thus the teleconnection between the tropical Pacific and EASM was present dur-ing MIS-13 though not in PrI. We propose that the reduced eastwest SST difference and the cooling in the central tropical Pacific Ocean in MIS-13 are contributing factors in providing more rainfall over EASM through maintaining the teleconnection between the tropical Pacific and EASM as will be discussed in Sect. 3.4.

3.4 Teleconnection between tropical Pacific and East Asian monsoon

Changes in the tropical Pacific convection affect the atmos-pheric circulation in the extra-tropical regions through atmospheric teleconnections (e.g., Seager et al. 2010) which can vary with the seasonal cycle (Alexander et al. 2002). Atmospheric teleconnections during modern ENSO (El Nio and La Nia) events are normally attributed to the Ross by wave train generated due to changes in tropical Hadley circulation (Trenberth et al. 1998) or tropical dia-batic heating (De Weaver and Nigam 2004). These wave

(a)

(c)

(e) (f)

(d)

(b)

Fig. 2 Summer (June-JulyAugust) precipitation (mm/day). Left column is CCSM3 and right one is HadCM3. a and b are PrI values, c and d are the difference between MIS-13 and PrI, e and f are also the difference between MIS-13 and PrI but zoomed into the East-Asian region

Fig. 3 Summer (June-JulyAugust) sea-level pressure superim-posed on the vertically-integrated moisture transport. Left column is CCSM3 and right one is HadCM3. a and b are PrI values, c and d are the difference between MIS-13 and PrI. Please note the different color legends for the isobars in c and d

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trains which are excited towards higher latitudes modify the westerlies and are an important component of the tel-econnection between the tropics and sub-tropics.

To assess the change in teleconnection between the tropi-cal Pacific and EASM during MIS-13, we analyze the zonal eddy wind field anomalies (differences between MIS-13 and PrI) at 250 mb. The eddy wind represents the stationary waves (Mller and Roeckner 2008), and its anomaly shows for instance changes in the wave amplitude and ENSO-related wave trains (e.g., De Weaver and Nigam 2004). We will mainly focus on the summer (JJA) teleconnection as both of our models show excess precipitation during these months.

The positive summer SST anomaly in the eastern tropi-cal Pacific and the negative anomaly in the western tropi-cal Pacific (Fig. 5a, b) slows down the Walker circulation by causing anomalous ascending in the east and anomalous descending in the west, respectively (Fig. 5 e, f; note that the negative velocity is upward). It should be noted that the maxi-mum weakening of the Walker circulation in MIS-13 relative to PrI occurs mainly around the summer season consistent with the reduced eastwest SST difference. The subsequent change in the vertical velocity and convection is transferred to the middle and upper troposphere through a Gill-type effect (Gill 1980). In the Gill response, two anticyclones (cyclones) form to the west of a warm (cold) anomaly in the middle-upper troposphere, straddling the equator. Although our mod-els are more complex than Gills simple model, some of these theoretically-predicted features can be seen by comparing the

tropical Pacific wind anomaly (Fig. 6) with the correspond-ing SST anomalies (Fig. 5). We highlight the cyclones (C) straddling the equator by drawing schematic circles. The Gill-like cyclones which are positioned on both sides of the equa-tor and extend to 20 north and south (Fig. 6) correspond to the cold SST anomaly in the eastern-central tropical Pacific (Fig. 5). The anomalous easterlies on the northern flank of the cyclone C1 interact with the westerly jet and its meander which in return reinforces the teleconnection between the tropical Pacific and other regions of the globe. This interac-tion promotes the upper-level anomalous cyclone C2 in the western north Pacific that connects the tropical Pacific to East Asia. We therefore suggest that the tropical Pacific in MIS-13 had a larger share or at least played a modulating role in providing moisture over East Asia through a summer telecon-nection between the EASM and the tropical Pacific. Due to the westward propagation of the SST anomalies from June to August (cf., Thuburn and Sutton 2000), the corresponding cyclones (C1) also move westward (Figs. 5, 6). This might cause an additional interaction between C1 and the westerly jet, and hence a stronger teleconnection.

Similar characteristics and patterns were also found for the wind field anomaly in the observations of the modern La Nia (Yuan and Yan 2013). Moreover, the SST and wind anomaly shown in Figs. 5 and 6, have similar characteris-tics as the La Nia Modoki (Ashok et al. 2007) which has been suggested to promote more precipitation over the monsoon front (Fan et al. 2013). The cooling anomaly in

Fig. 4 Annual mean average over 5S5N in the tropical Pacific for a sea surface temper-ature (C), b zonal wind stress (N/m2), c depth of thermocline (m), d vertical wind velocity at 700 mb (Pa/s) note that the negative velocity is upward. CCSM3 experiments are in Red and the ones of HadCM3 are in blue

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the central tropical Pacific and the development of the asso-ciated upper-level anomalous cyclone in the western north Pacific as discussed above also resembles the Pacific-East Asian teleconnection of Wang et al. (2000). Although this teleconnection was suggested to operate between the East Asian winter monsoon and ENSO in the modern climate, it could also operate in summer as found in our results.

4 MIS13 ENSO characteristics

The interannual variability of the tropical Pacific SST and EASM precipitation around their climatological means (Sect. 3) will be discussed in this section.

4.1 Was ENSO persistent in MIS-13?

As ENSO is the main mode of interannual variability in the modern tropical Pacific, it is of interest to know

whether ENSO-related events were present in MIS-13 and if so, to what extent they influenced EASM. To find the dominant modes of interannual variability in the tropical Pacific SST, the Empirical Orthogonal Function (EOF) was calculated from our 1,200 months of SST data (taken from the last 100 years of our simulations) in the region between 15S15N and 145E280E, which cover NINO regions 14. EOFs are mathematical tool and known to show the spatial mode (pattern) of vari-ability and correlations. Before calculating the EOFs the monthly mean climatology was subtracted from the origi-nal monthly time-series to produce a time-series of 1,200 monthly anomalies. Our focus is on the first two EOFs that together, as will be shown, explain more than 50 % of the total variance.

In the EOF plots, the regions with the same sign vary in phase with each other, and the larger values correspond to the higher amplitudes of the variability. The first EOF (EOF1) as the dominant mode shows the canonical ENSO

Fig. 5 Annual cycle of the SST difference between MIS-13 and PrI in the tropical Pacific versus longitude a CCSM3 b HadCM3. c annual cycle of SST difference between western (147E) and east-ern (276E) tropical Pacific, d annual cycle of the EASM precipita-

tion (averaged over 2040N and 100120E). Annual cycle of the vertical velocity at 700 mb in the tropical Pacific versus longitude e CCSM3 f HadCM3 (note that the negative velocity is upward). The panels related to the tropical Pacific are first averaged over 5S5N

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pattern in both MIS-13 and PrI (Fig. 7ad). The principal components corresponding to the EOF1 vary at time scales of 35 years as would be expected for an ENSO mode (not shown). The regions of maximum SST variability in EOF1 plots are more to the west compared to the observations, particularly in the CCSM3 runs (cf., Deser et al. 2006). The EOF1 of MIS-13 and PrI in CCSM3 are almost spa-tially identical. HadCM3 MIS-13 has its maximum EOF1 variability in the central equatorial Pacific (220240E). The percentage of the first EOFs relative to the total vari-ance for the MIS-13 experiments (41 % in CCSM3 and 39 % in HadCM3) are smaller than those computed for PrI (51 % in CCSM3 and 53 % in HadCM3). This indi-cates a smaller contribution of the first mode in the total variability of tropical Pacific SSTs in MIS-13. The second EOF (EOF2) exhibits a dipole pattern on either side of the equatorial Pacific (Fig. 7eh) and has a larger percent-age in MIS-13 (11 % in CCSM3 and 13 % in HadCM3) than in PrI (9 % in CCSM3 and 5 % in HadCM3). As with EOF1, the CCSM3 MIS-13 and PrI simulations have simi-lar EOF2 patterns. This is not the case in HadCM3 where MIS-13 and PrI have different EOF2 patterns, particularly in the northern and southern parts of the region considered.

The EOF2 varies on the time scales of 23 years, slightly shorter than EOF1.

To investigate the frequency of variability in the tropical Pacific SST, the spectrum of SST in the NINO3.4 region were computed. For this, the monthly mean cycles of SSTs were first resolved and a 12-month moving average filter was applied following Douglass (2011). The spectrum (Fig. 8a, b) shows a dominant variability at the interannual time scale which is associated with ENSO. The CCSM3 PrI run shows a broad spectral peak (above the 99 % con-fidence level) in the 1.54 year band, with the highest peak occurring at 4 years. Peaks in the CCSM3 MIS-13 are lower in magnitude and are limited to a narrower band between 1.5 and 2.5 years. Similar results are found for the power spectrum of NINO3.4 SSTs in HadCM3 where the significant peaks (above the 99 % confidence level) in MIS-13 have smaller amplitudes and are concentrated in a nar-rower band (25.5 year band) than in PrI (27 year band).

The EOF of the tropical Pacific SST and spectral analy-sis of NINO3.4 SST show, therefore, that ENSO-type inter-annual variability remained present in MIS-13 and was occurring more frequently than in PrI. The probability den-sity function (not shown) for NINO3.4 SST also confirms

Fig. 6 Eddy wind field anomaly (difference between MIS-13 and PrI) at 250 mb calculated after removing the zonal mean is shown for June, July and August from top to down. Left column is CCSM3 and

right is HadCM3. We show the cyclones by schematic circles. Vectors smaller than 0.5 m/s are not shown

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these findings and shows that MIS-13 could have been characterized by more La Nia events than El Nio ones.

To determine the amplitude of ENSO, the standard deviation of SST in the NINO3.4 region (averaged over 5S5N; 190E240E) is used (e.g., Collins et al. 2001). In the HadCM3, the ENSO amplitude in MIS-13 (0.82 C) is found 15 % smaller than that in PrI (0.96 C). Similarly for CCSM3, we also find that the ENSO ampli-tude in MIS-13 (0.92 C) is 19 % smaller than that in PrI (1.14 C). By applying the F-test, the difference in the amplitude of ENSO between MIS-13 and PrI (in

both CCSM3 and HadCM3) is found statistically sig-nificant (to 95 % significance). The ENSO amplitudes in HadCM3 experiments are at least 11 % smaller than those in CCSM3 with significance of 95 %. This can be due to the fact that the tropical Pacific thermocline in HadCM3 is deeper than in CCSM3.

4.2 Enhanced relation between ENSO and the EASM

The spectral analysis of the modeled EASM precipitation (averaged over 2040N and 100120E) is shown in Fig. 8

Fig. 7 The first two empirical orthogonal functions (addressed as EOF1 and EOF2) of SST in the Pacific Ocean as a function of latitude and longitude. Right column is for MIS-13 simulations and the left column is for PrI

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(panels c and d). It demonstrates that interannual variability of the EASM in both MIS-13 and PrI has a dominant period between 2 and 4 years, and thus appears largely synchronous with ENSO. The EOF1 of precipitation calculated over the region between 20 and 50N and 90E140E also varies at the same time scale as ENSO (i.e., 23 years) in both mod-els suggesting a possible impact of ENSO on the interan-nual variability of EASM. EOF1 explains 12 % of the total variance in both the PrI and MIS-13 simulations of CCSM3. In HadCM3, however, MIS-13 has a larger variance for the EOF1 of precipitation (22 %) than PrI (10 %) which indi-cates a larger contribution of the ENSO in the total variability of EASM precipitation in MIS-13. The pattern of EOF1 in both PrI runs (Fig. 14 Appendix) have opposite sign over southeast (2535N and 100120E) and northeast China (3545N and 100120E). This suggests that ENSO has opposite impacts on those regions as is also found for the modern climate that the ENSOEASM relation may differ between the northern and the southern parts of EASM (e.g., Wu and Wang 2002; Lee et al. 2008). The CCSM3 MIS-13 simulation has a similar pattern as the PrI one, but with a shift to the north. The EOF1 of HadCM3 MIS-13 shows negative values over large parts of China which indicates that the impact of ENSO would be distributed more uniformly.

To check if the EASM variability is correlated to the tropical Pacific SST and whether this relation differed

during MIS-13, the Pearsons coefficient of correlation between 1,200 months of modelled NINO3.4 SST and the modelled precipitation was computed (Fig. 9). Both models show an increase in correlation towards more positive val-ues in the EASM region in MIS-13 compared to PrI. A pos-itive correlation implies that higher (lower) precipitation is associated with warmer (colder) NINO3.4 SSTs relative to their respective means. The increase in correlation is con-sistent with the enhanced teleconnection between the tropi-cal Pacific and East Asia shown in Sects. 3.3 and 3.4. It can also be seen that the HadCM3 MIS-13 simulation has a larger correlation coefficient in the EASM region compared to the CCSM3 MIS-13 simulation. The positively increased correlation between NINO3.4 SST and precipitation in our MIS-13 simulations can be the result of the ENSO (as the dominant mode of the tropical Pacific SST variability) and/or the seasonal change in insolation, that force the tropical temperature and the EASM precipitation in a similar fash-ion. We will calculate the contribution of ENSO in Sect. 4.3. The correlation coefficients between the NINO3.4 SST anomalies (difference between MIS-13 and PrI) and pre-cipitation anomalies were also calculated to find the pos-sible relation between the changes in SST and precipitation of MIS-13 relative to PrI. The positive correlation shows that NINO3.4 SST and precipitation anomalies have a tendency to be greater or less than their respective means

(a) (b)

(c) (d)

Fig. 8 Fast Fourier Transform spectrum after resolving the monthly mean cycles by a 12-month moving average filter for: a NINO3.4 SST in CCSM3 b NINO3.4 SST in HadCM3 c EASM precipita-tion in CCSM3 d EASM precipitation in HadCM3. Continuous

lines are for MIS-13, dashed lines are for PrI, black lines are 99 % confidence lines. NINO3.4 is the region averaged over 5S5N and 190E240E. EASM precipitation was first averaged over 2040N and 100120E

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simultaneously. CCSM3 has both positive and negative correlation coefficients over EASM while HadCM3 has mainly positive values (Fig. 15 Appendix).

The same correlation analysis as above was computed between precipitation and the North Atlantic Oscillation (NAO) index (not shown), but did not show a significant change over the EASM region. However, the mean-cli-mate of MIS-13 has positive NAO-like features (Muri et al. 2013) which strengthens the relationship between the tropical Pacific Ocean and EASM (Wu et al. 2012) consist-ent with our results. The correlation of the Indian Ocean Dipole Index with EASM also did not show a difference between MIS-13 and PrI (Muri et al. 2013). Thus ENSO took on great importance in controlling EASM variability during MIS-13 than during PrI.

4.3 Isolating the impact of ENSO

Observations show that ENSO events (both El Nio and La Nia phases) in the modern climate strongly influence global precipitation (Ropelewski and Halpert 1987). In our model runs, we also find that the precipitation changes dur-ing the modeled ENSO events (e.g., Figure 8). Therefore, it is of interest to compute the pure contribution of ENSO events to the total change in precipitation of MIS-13. In other words, we are interested to calculate how much of

the increased EASM precipitation in MIS-13 is related to changes in ENSO. First, NINO3 SST anomalies were cal-culated with respect to the mean climatology. ENSO events were defined to be those where the December NINO3 SST anomaly exceeds one standard deviation (El Nio) or falls below minus one standard deviation (La Nia) of NINO3 SST (e.g., Merkel et al. 2010). Those model years in which the NINO3 SST anomaly fell between minus one and plus one standard deviation were defined as non-ENSO or nor-mal years.

We define the pure impact of ENSO on precipitation as the corresponding change in precipitation compared to the background precipitation (mean precipitation of non-ENSO years). To compute this, the background precipitation was subtracted from the precipitation of the ENSO years and the average of the resulting values for both MIS-13 and PrI were calculated. Then, the precipitation difference between MIS-13 and PrI were separated into their difference in the impact of ENSO and in background precipitation (Fig. 10). As can be seen, the difference in the pure impact of ENSO on precipitation between MIS-13 and PrI does not change in CCSM3 but increases in HadCM3 (i.e., more rainfall) for most of the regions throughout the globe. It can be shown that the pure ENSO-driven precipitation over east China reaches up to 30 % of the total precipitation differ-ence between MIS-13 and PrI. This is consistent with the

(a) (b)

(c) (d)

Fig. 9 Pearson correlation map between NINO3.4 SST and precipi-tation. The color contour/bar shows the value of correlation coef-ficients. Only the correlation coefficients with a significance level

larger than 95 % are shown. Top panels are for CCSM3 runs and lower panels are for HadCM3

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increased EOF1 variance of EASM precipitation in MIS-13 compared to PrI for HadCM3 (Sect. 4.2). The differ-ence in the pure impact of ENSO on precipitation between

HadCM3 and CCSM3 illustrates one of the mechanisms contributing towards HadCM3s better representation of MIS-13 EASM.

Fig. 10 The difference in precipitation between MIS-13 and PrI (Fig. 2) separated into two parts: ENSO-driven and non-ENSO driven. Left panels are the difference in precipitation between MIS-13

and PrI after taking into account only the pure impact of ENSO, right panels are the difference in precipitation between MIS-13 and PrI for their non-ENSO years

(a) (b)

(d)(c)

Fig. 11 Summer (JJA) precipitation change driven by ENSO teleconnection shown as the difference between El Nio and La Nia precipitation composites for: a CCSM3 MIS-13 b CCSM3 PrI c HadCM3 MIS-13 d HadCM3 PrI

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4.4 ENSO teleconnection

To indentify the difference in ENSO teleconnections between MIS-13 and PrI, a composite analysis of summer (JJA) precipitation was performed. The average effect of El Nio and La Nia on precipitation can be determined by these composite maps. By averaging precipitation for the El Nio years, La Nia years and the non-ENSO years, their corresponding so-called composite anomalies were obtained. The difference between the El Nio and La Nia precipitation composites shows changes in their corre-sponding pattern of precipitation which is associated with the difference in their teleconnections (Fig. 11). The El Nio and La Nia composite difference in precipitation in PrI has a similar pattern to that of the modern-day obser-vation (e.g., Figure 8 in Deser et al. 2006) over southern and central east-China but with smaller magnitudes. In both models a double ITCZ-like structure and the rainfall distri-bution over the equatorial Pacific has disappeared in MIS-13, likely due to the cooling of the central tropical Pacific. Over east China, larger precipitation differences between El Nio and La Nia composites can be seen in MIS-13 suggesting increased ENSO teleconnection during MIS-13.

5 Summary and conclusions

With the aim of better understanding the strong EASM dur-ing the relatively cool MIS-13 interglacial, we investigated the role of the tropical Pacific Ocean. Two coupled general circulation models, HadCM3 and CCSM3, were used to study the climate of MIS-13 with different insolation and GHGs than the present day. Results from both models con-firm the increased EASM precipitation during MIS-13 com-pared to PrI. Overall, we leave more confidence to the results from HadCM3 given its better performance than CCSM3 in capturing EASM in MIS-13 and at present-day (Lei et al. 2013). It was also shown that the western Pacific subtropi-cal high was strengthened and extended westward in MIS-13 providing more moisture to the EASM. The additional mois-ture in MIS-13 came from the northern Pacific Ocean for CCSM3, and from the Indian Ocean and South China Sea as well as the northern Pacific Ocean for HadCM3.

We suggested that MIS-13 had a La Nia-like mean climate in the tropical Pacific and the associated telecon-nection with the extra-tropics acted to increase precipita-tion over EASM. In MIS-13, the increasing trend of the EASM precipitation during summer was not found for the landocean thermal contrast. Summer-time cooling in the central tropical Pacific was suggested to promote more rainfall over the EASM through maintaining the summer western Pacific subtropical high (cf., Fan et al. 2013). In MIS-13, the eastwest SST gradient in the tropical Pacific

was reduced during NH summer (reaching 3 C for HadCM3) and was anticorrelated (around 0.8 with 95 % significance) with EASM precipitation. The reduced eastwest SST gradient promoted an upper-level anomalous cyclone in the western north Pacific which was the main system connecting the tropical Pacific to the EASM. Thus, the changes in the tropical Pacific SST contributed to the intense EASM of MIS-13 next to the larger land-sea ther-mal contrast driven by the higher summer insolation (e.g., Yin et al. 2008).

Based on our modelling analysis we conclude that ENSO variability was present in MIS-13 with a smaller amplitude but higher frequency than in PrI. On the other hand, the precipitation rate in the EASM-region showed larger correlation with ENSO in MIS-13. This means that although ENSO variability had smaller amplitude in MIS-13 compared to PrI, it had a larger influence on the EASM. This we relate to the enhanced teleconnection between the tropical Pacific and East Asia during MIS-13. Moreover, the pure impact of ENSO on increasing EASM precipita-tion during MIS-13 was investigated. It was shown that in HadCM3, the ENSO-related precipitation was stronger in MIS-13 than in PrI, and accounted for up to 30 % of the total precipitation difference between MIS-13 and PrI. This was not the case in CCSM3 in which the ENSO-related precipitation did not significantly differ between MIS-13 and PrI. This could also be one reason explaining better representation of EASM in HadCM3 compared to CCSM3.

Our results suggest that the state of the tropical Pacific during the past interglacials could be quite different from today, which in turn could have changed the relationship between the tropical Pacific and the EASM. Moreover, it could also affect the ENSO properties and teleconnections as was shown in our results. Future research constraining the state of the tropical Pacific could serve the dual purpose of resolving the oceanic response of this vitally important region to interglacial forcing as well as constraining the enigmatic EASM during MIS-13.

Acknowledgments This work and M. P. Karami were supported by the European Research Council Advanced Grant EMIS (No 227348 of the Programme Ideas). Q. Z. Yin is supported by the Bel-gian National Fund for Scientific Research (F.R.S.-FNRS). H. Muri is supported by the Research Council of Norway (Grant agreement 229760). We are grateful to the reviewers for their constructive com-ments and suggestions. We thank Dr. Fred Kucharski, Dr. Carlos Almeida, Gauillame Lenoir and Dr. Tobias Bayr for helpful discus-sions. Access to computer facilities was facilitated through sponsor-ship from S. A. Electrabel, Belgium. We are also grateful to CISM staff at Universit catholique de Louvain for their technical support.

Appendix

See Figs. 12, 13, 14, 15.

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Fig. 13 Annual of average of sea-level pressure. First row is PrI, second row is MIS-13 and the third row is the difference between MIS-13 and PrI. Left and right columns are related to CCSM3 HadCM3 experiments, respectively

Fig. 12 Annual mean average over 5S5N in the tropical Pacific for the meridional wind stress (Nm2)

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State ofthe tropical Pacific Ocean andits enhanced impact onprecipitation overEast Asia duringmarine isotopic stage 13Abstract 1 Introduction2 Model descriptions3 Climatology ofMIS-133.1 Annualsummer atmosphere andocean MIS-13 climatologies3.2 La Nia-type mean climate ofthe tropical Pacific3.3 Strengthened relationship betweentropical Pacific SST andEast Asian monsoon3.4 Teleconnection betweentropical Pacific andEast Asian monsoon

4 MIS-13 ENSO characteristics4.1 Was ENSO persistent inMIS-13?4.2 Enhanced relation betweenENSO andthe EASM4.3 Isolating the impact ofENSO4.4 ENSO teleconnection

5 Summary andconclusionsAcknowledgments References