, during the main IOZM season at both sides of the Indian Ocean Walker cell. The link between Sacred/

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SOND) every 2e7 years, causing pronounced rainfall anomalies in East Africa, western Indonesia, Australia, and India (e.g., Ashok et al., 2001; Black et al., 2003). Under normal conditions, a strong gradient in sea surface temperature (SST) between the warmer Eastern Equatorial Indian Ocean (EEIO) and the cooler Western Equatorial Indian Ocean (WEIO) causes weak westerly winds along the equator, contributing to the normal Indian Ocean Walker Cir- culation, with rising air and deep atmospheric convection over Indonesia and descending air and dry conditions in East Africa.

During IOZM positive (IOZMþ) events, a breakdown of the normal SST gradient and equatorial windﬁeld leads to anomalously dry conditions in western Indonesia while enhancing the Octo- bereDecember (OND) rainy season in East Africa (Fig. 1). The event ceases when the normal boreal summer monsoon circulation de- velops, cooling the WEIO and relaxing anomalous easterly winds (Webster et al., 1999; Schott and McCreary, 2001).

In addition to its direct effects on East African and Indonesian precipitation, the IOZM has the ability to effect large changes in regional hydrology through its connection with the Indian monsoon and the El Ni~no-Southern Oscillation (ENSO) (Ashok et al., 2001; D'Arrigo and Smerdon, 2008; Ummenhofer et al., 2011). Both an IOZMþ event and an El Ni~no event necessitate anomalous cooling in the Indo-Paciﬁc warm pool and weakened vertical ascent

and convection over Indonesia, and in some but not all years the two co-occur (Saji et al., 1999; Saji and Yamagata, 2003a), possibly triggered by ENSO-induced zonal shifts in Walker cell anomalies (Fischer et al., 2005). IOZMþ events can also be triggered by meridional Hadley cell perturbations in the absence of ENSO dynamics (Fischer et al., 2005), and numerous other studies have established that the IOZM's interannual ﬂuctuations are distinct from ENSO and should be considered a separate Indian Ocean phenomenon (e.g.,Saji and Yamagata, 2003b; Behera et al., 2006;

Schott et al., 2009).

While the IOZM operates independently from ENSO, teleconnections to the Paciﬁc Ocean can play an important role in triggering and enhancing IOZM events via local air-sea interactions, ocean dynamics, and atmospheric teleconnections (Li et al., 2003;

Behera et al., 2006; Gnanaseelan and Vaid, 2010). Interannual to decadal variability in Paciﬁc SSTs can induce stronger/more frequent IOZMþevents by preconditioning the EEIO with a shal- lower thermocline (Annamalai et al., 2005; Schott et al., 2009). This preconditioning is associated with changes in the Indonesian Throughﬂow as well as an atmospheric teleconnection to equatorial winds over the Indian Ocean (Annamalai et al., 2005). Air-sea interactions in the warm pool are also integral to the generation and termination of IOZM events (Cai et al., 2013and refs therein).

SST variability in the EEIO induces a positive feedback with wind speed and evaporative heat loss, further enhancing cold anomalies.

However, cool SSTs also suppress clouds and convection, warming the EEIO via increased shortwave radiation (Cai et al., 2013).

In addition to pronounced interannual variability, the IOZM also exhibits low-frequency variability. The power spectrum of observed IOZM SSTs exhibits a spectral peak at ~10 years (90%

signiﬁcance level), suggesting that periodicity at decadal and potentially longer timescales is intrinsic to the IOZM system (Ashok et al., 2004a, 2003). Twentieth century SST observations reveal multi-decadal modulations of the IOZM, with periods of time characterized by more frequent/intense IOZM negative (IOZM-) events (~1880e1920), more frequent/intense IOZM positive

Fig. 1.Schematic illustration of the anomalous components of wind, precipitation, and sea surface temperature observed during an IOZM positive event. Adapted fromSaji et al.

(1999), Saji et al. (2006), andSchott et al. (2009). Red boxes outline the regions used to calculate the Zonal Wind Index, Ueq, and the Dipole Mode Index, discussed in text.

Table 1

List of abbreviations.

IOZM Indian Ocean Zonal Mode

IOZMþ() IOZM positive (negative)

SST Sea Surface Temperature

EEIO Eastern Equatorial Indian Ocean

WEIO Western Equatorial Indian Ocean

OND OctobereDecember

SOND SeptembereDecember

ENSO El Ni~no-Southern Oscillation

DMI Dipole Mode Index

ZWI Zonal Wind Index

LS Link Strength

B. Konecky et al. / Quaternary Science Reviews 103 (2014) 1e18 2

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SOND) every 2e7 years, causing pronounced rainfall anomalies in East Africa, western Indonesia, Australia, and India (e.g., Ashok et al., 2001; Black et al., 2003). Under normal conditions, a strong gradient in sea surface temperature (SST) between the warmer Eastern Equatorial Indian Ocean (EEIO) and the cooler Western Equatorial Indian Ocean (WEIO) causes weak westerly winds along the equator, contributing to the normal Indian Ocean Walker Cir- culation, with rising air and deep atmospheric convection over Indonesia and descending air and dry conditions in East Africa.

and convection over Indonesia, and in some but not all years the two co-occur (Saji et al., 1999; Saji and Yamagata, 2003a), possibly triggered by ENSO-induced zonal shifts in Walker cell anomalies (Fischer et al., 2005). IOZMþ events can also be triggered by meridional Hadley cell perturbations in the absence of ENSO dynamics (Fischer et al., 2005), and numerous other studies have established that the IOZM's interannual ﬂuctuations are distinct from ENSO and should be considered a separate Indian Ocean phenomenon (e.g.,Saji and Yamagata, 2003b; Behera et al., 2006;

Table 1

LS Link Strength

SOND) every 2e7 years, causing pronounced rainfall anomalies in East Africa, western Indonesia, Australia, and India (e.g.,Ashok et al., 2001; Black et al., 2003). Under normal conditions, a strong gradient in sea surface temperature (SST) between the warmer Eastern Equatorial Indian Ocean (EEIO) and the cooler Western Equatorial Indian Ocean (WEIO) causes weak westerly winds along the equator, contributing to the normal Indian Ocean Walker Cir- culation, with rising air and deep atmospheric convection over Indonesia and descending air and dry conditions in East Africa.

and convection over Indonesia, and in some but not all years the two co-occur (Saji et al., 1999; Saji and Yamagata, 2003a), possibly triggered by ENSO-induced zonal shifts in Walker cell anomalies (Fischer et al., 2005). IOZMþ events can also be triggered by meridional Hadley cell perturbations in the absence of ENSO dynamics (Fischer et al., 2005), and numerous other studies have established that the IOZM's interannualﬂuctuations are distinct from ENSO and should be considered a separate Indian Ocean phenomenon (e.g.,Saji and Yamagata, 2003b; Behera et al., 2006;

Table 1

LS Link Strength

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and convection over Indonesia, and in some but not all years the two co-occur (Saji et al., 1999; Saji and Yamagata, 2003a), possibly triggered by ENSO-induced zonal shifts in Walker cell anomalies (Fischer et al., 2005). IOZMþ events can also be triggered by meridional Hadley cell perturbations in the absence of ENSO dynamics (Fischer et al., 2005), and numerous other studies have established that the IOZM's interannualﬂuctuations are distinct from ENSO and should be considered a separate Indian Ocean phenomenon (e.g.,Saji and Yamagata, 2003b; Behera et al., 2006;

Table 1

LS Link Strength

In addition to its direct effects on East African and Indonesian precipitation, the IOZM has the ability to effect large changes in regional hydrology through its connection with the Indian monsoon and the El Ni~no-Southern Oscillation (ENSO) (Ashok et al., 2001; D'Arrigo and Smerdon, 2008; Ummenhofer et al., 2011). Both an IOZMþevent and an El Ni~no event necessitate anomalous cooling in the Indo-Paciﬁc warm pool and weakened vertical ascent

and convection over Indonesia, and in some but not all years the two co-occur (Saji et al., 1999; Saji and Yamagata, 2003a), possibly triggered by ENSO-induced zonal shifts in Walker cell anomalies (Fischer et al., 2005). IOZMþevents can also be triggered by meridional Hadley cell perturbations in the absence of ENSO dynamics (Fischer et al., 2005), and numerous other studies have established that the IOZM's interannualﬂuctuations are distinct from ENSO and should be considered a separate Indian Ocean phenomenon (e.g.,Saji and Yamagata, 2003b; Behera et al., 2006;

Table 1 List of abbreviations.

LS Link Strength

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(IOZMþ) events (~1960e2000), and periods of lower overall ac- tivity (~1920e1950) (Kripalani and Kumar, 2004; Ihara et al., 2008).

Multi-decadal variations in the equatorial Indian Ocean windﬁeld, SST, and thermocline depth are also apparent in observations and in modeling simulations (Saji and Yamagata, 2003b; Ashok et al., 2004a; Annamalai et al., 2005; Tozuka et al., 2007).

Low-frequency behavior makes the IOZM potentially important to and detectible in low-resolution paleoclimate proxy reconstructions. Indeed, paleoclimate studies have invoked“IOZM- like” dynamics to explain low-frequency patterns in regional paleoclimate from decadal up to orbital timescales (e.g., Stager et al., 2005; Grifﬁths et al., 2010; Gupta et al., 2010). These

“IOZM-like” dynamics are not directly analogous to those of an individual IOZM event, whose onset and cessation are directly tied to a given year's seasonal cycle. However, a low-frequency,“IOZM- like”mode could arise from periods of more frequent/intense IOZM events (e.g.,Tozuka et al., 2007), intrinsic low-frequency variability in Indian Ocean surface ocean characteristics (e.g.,Annamalai et al., 2005), and/or feedbacks with the Indian monsoon circulation and the Indonesian Throughﬂow (Ashok et al., 2001; Annamalai et al., 2003, 2005).

Understanding low-frequency variations in the IOZM is critical for decadal-to century-scale climate prediction in the circum- Indian Ocean region. This is particularly true under 21^stcentury global climate change scenarios, as a recent increase in IOZM intensity has been linked to increased greenhouse gas concentrations and warming (Cai et al., 2009) and increasingly strong positive feedbacks with the Asian monsoon circulation (Abram et al., 2008).

However, future variability of the IOZM is uncertain due to uncertainties in ENSO and Asian monsoon teleconnections (Abram et al., 2008; Cai et al., 2009), which are strongly affected by inter- decadal variability in the Paciﬁc Ocean (Zhang et al., 1997; Ashok et al., 2004a). In some model simulations, changes in the mean state of the tropical Indian and Paciﬁc Oceans are associated with increased frequency of IOZMþevents, particularly events occurring in consecutive years (Saji et al., 2006; Cai et al., 2009). However, IOZM event frequency may not increase in the 21^stcentury, despite a mean state of the Indian Ocean that is projected to become more

“IOZMþ-like”in its mean wind and SST structure (Cai et al., 2013).

Paleoclimate records can help to illuminate the interactions between IOZM intensity, low-frequency variability, and changes in mean state. On paleoclimate timescales, corals, lake level records, tree rings, and output from paleoclimate model simulations have all been interpreted to reﬂect IOZM variability on interannual to multidecadal time-scales (e.g.,Stager et al., 2005; Zhao et al., 2005;

Abram et al., 2008, 2007; D'Arrigo et al., 2008). However, recent paleoclimate proxy reconstructions and model simulations suggest that continental rainfall in the circum-Indian Ocean region has a nonstationary relationship with Indian Ocean SST variability on decadal and longer timescales (Zinke et al., 2009; Coats et al., 2013).

The relationship between Indian Ocean regional rainfall and ENSO, as well as IOZM teleconnections, may also be nonstationary on these timescales (Ashok et al., 2001, 2004b; Timm et al., 2005).

Unstable behavior in the IOZM itself, and/or nonstationarity in the IOZM's relationship with rainfall, must be addressed in order to improve our understanding of the role of the IOZM in paleoclimate as well as its role in regulating regional precipitation variability in the future.

1.2. Tracking past and present IOZM variations with precipitation isotopes

Oxygen and hydrogen isotopes of precipitation (d¹⁸^O^precip^and d^Dprecip) are an increasingly important proxy for modern and past climate processes in the tropics (e.g.,Vuille et al., 2012; Conroy

et al., 2013; Moerman et al., 2013). In the Indo-Paciﬁc, changes in the Walker circulation associated with the IOZM alter rainfall as well as the O and H isotopic composition of rainfall in East Africa and Indonesia. These anomalies, observed in precipitation and precipitation isotopic data from stations distributed throughout East Africa and Indonesia, were reproduced by an isotope-enabled GCM simulation from 1950 to 1994 (Vuille et al., 2005a). The IOZM produces isotopic anomalies by altering the Walker circulation over the Indian Ocean, changing the rainout and distillation processes that accompany anomalous vertical ascent (descent), upper-level divergence (convergence), and increased (decreased) convection over East Africa (Indonesia) during the SOND season. Because both sides of the Walker cell are affected in an opposite manner, a distinctive east/west spatial pattern characterizes the precipitation isotopic response to the IOZM, with signiﬁcant changes of opposite direction occurring in East Africa and Indonesia. IOZMþevents are associated with negative (¹⁸O-, D-depleted) anomalies in East Af- rica and positive (¹⁸O-, D-enriched) anomalies in western Indonesia.

An increasing number of high-resolution proxy records of d¹⁸^O^precip ^andd^D^precipspanning the past millennium (1000 C.E.- present) is becoming available from sediments, corals, and spe- leothems in the circum-Indian Ocean region. These records have suggested considerable variations in hydrology over the past millennium (Zinke et al., 2004; Newton et al., 2006; Fleitmann et al., 2007; Partin et al., 2007; Sinha et al., 2007; Abram et al., 2008;

Grifﬁths et al., 2010; Tierney et al., 2010, 2011; Yan et al., 2011;

Konecky et al., 2013, 2014). Many but not all of these records are characterized by pronounced multi-decadal variability (Zinke et al., 2004; Fleitmann et al., 2007; Konecky et al., 2013), and some show distinctive centennial-scale features or even millennium-long trends (Tierney et al., 2011; Yan et al., 2011; Konecky et al., 2013, 2014). Changes in the Indian or Pacific Ocean Walker circulation have been proposed to explain O- and H-isotopic variations on all timescales, as well as interactions with the Indian, East Asian, and Australasian monsoon circulation. The Northern Hemisphere Me- dieval Climate Anomaly (MCA; ~1000e1200 C.E.) and Little Ice Age (LIA; 1550e1800 C.E.) have received particular attention, with po- tential ramifications for solar forcing and Northern Hemisphere climate changes to affect regional atmospheric circulation. The timing of these multi-century events varies from site to site, and age model uncertainties often preclude a direct comparison of these variations to each other and to Northern Hemisphere temperature records (Masson-Delmotte et al., 2013). Nonetheless, records from both sides of the Indian Ocean suggest that the Indian Ocean Walker circulation may have responded strongly to external forcings on multi-decadal and longer timescales, leading to both precipitation and isotopic anomalies over the continents (Griffiths et al., 2010; Tierney et al., 2013). This highlights the importance of systematically testing IOZM relationships in isotopic proxy records, especially records that directly trackd¹⁸Ôprecip/d^Dprecip.

In this study, we examine low-frequency behavior of the IOZM/

d¹⁸^O^precip^{(and IOZM/}d^D^precip) relationship over two important time periods: the historical period (1870e2003), and the past millennium (~1000 C.E. to present). The historical period, for which a critical mass of SST observations is available, provides the longest and best-constrained timeframe to observe IOZM behavior, including several multi-decadal cycles. The past millennium provides an important test case to observe multi-decadal to centennial variability in IOZM behavior and to assess its regional impacts.

These proxy records comprise the only network of d¹⁸^O^precip^/ d^D^precip-based proxy reconstructions from the core IOZM-affected regions of East Africa and Indonesia that are of high enough resolution to assess low-frequency timescales of variability over the past millennium. Together, these two timeframes provide

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Oxygen and hydrogen isotopes of precipitation (d¹⁸^Oprecipand d^D^precip) are an increasingly important proxy for modern and past climate processes in the tropics (e.g.,Vuille et al., 2012; Conroy

An increasing number of high-resolution proxy records of d¹⁸^O^precip ^andd^D^precipspanning the past millennium (1000 C.E.- present) is becoming available from sediments, corals, and spe- leothems in the circum-Indian Ocean region. These records have suggested considerable variations in hydrology over the past millennium (Zinke et al., 2004; Newton et al., 2006; Fleitmann et al., 2007; Partin et al., 2007; Sinha et al., 2007; Abram et al., 2008;

Konecky et al., 2013, 2014). Many but not all of these records are characterized by pronounced multi-decadal variability (Zinke et al., 2004; Fleitmann et al., 2007; Konecky et al., 2013), and some show distinctive centennial-scale features or even millennium-long trends (Tierney et al., 2011; Yan et al., 2011; Konecky et al., 2013, 2014). Changes in the Indian or Pacific Ocean Walker circulation have been proposed to explain O- and H-isotopic variations on all timescales, as well as interactions with the Indian, East Asian, and Australasian monsoon circulation. The Northern Hemisphere Me- dieval Climate Anomaly (MCA; ~1000e1200 C.E.) and Little Ice Age (LIA; 1550e1800 C.E.) have received particular attention, with po- tential ramifications for solar forcing and Northern Hemisphere climate changes to affect regional atmospheric circulation. The timing of these multi-century events varies from site to site, and age model uncertainties often preclude a direct comparison of these variations to each other and to Northern Hemisphere temperature records (Masson-Delmotte et al., 2013). Nonetheless, records from both sides of the Indian Ocean suggest that the Indian Ocean Walker circulation may have responded strongly to external forcings on multi-decadal and longer timescales, leading to both precipitation and isotopic anomalies over the continents (Griffiths et al., 2010; Tierney et al., 2013). This highlights the importance of systematically testing IOZM relationships in isotopic proxy records, especially records that directly trackd¹⁸Ô^precip^/d^D^precip^.

These proxy records comprise the only network of d¹⁸^O^precip^/ d^Dprecip-based proxy reconstructions from the core IOZM-affected regions of East Africa and Indonesia that are of high enough resolution to assess low-frequency timescales of variability over the past millennium. Together, these two timeframes provide

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Oxygen and hydrogen isotopes of precipitation (d¹⁸^O^precip^and d^D^precip) are an increasingly important proxy for modern and past climate processes in the tropics (e.g.,Vuille et al., 2012; Conroy

An increasing number of high-resolution proxy records of d¹⁸^O^precip^and d^D^precipspanning the past millennium (1000 C.E.- present) is becoming available from sediments, corals, and spe- leothems in the circum-Indian Ocean region. These records have suggested considerable variations in hydrology over the past millennium (Zinke et al., 2004; Newton et al., 2006; Fleitmann et al., 2007; Partin et al., 2007; Sinha et al., 2007; Abram et al., 2008;

These proxy records comprise the only network of d¹⁸^Oprecip/ d^D^precip-based proxy reconstructions from the core IOZM-affected regions of East Africa and Indonesia that are of high enough resolution to assess low-frequency timescales of variability over the past millennium. Together, these two timeframes provide (IOZMþ) events (~1960e2000), and periods of lower overall ac-

tivity (~1920e1950) (Kripalani and Kumar, 2004; Ihara et al., 2008).

“IOZM-like”dynamics are not directly analogous to those of an individual IOZM event, whose onset and cessation are directly tied to a given year's seasonal cycle. However, a low-frequency,“IOZM- like”mode could arise from periods of more frequent/intense IOZM events (e.g.,Tozuka et al., 2007), intrinsic low-frequency variability in Indian Ocean surface ocean characteristics (e.g.,Annamalai et al., 2005), and/or feedbacks with the Indian monsoon circulation and the Indonesian Throughﬂow (Ashok et al., 2001; Annamalai et al., 2003, 2005).

An increasing number of high-resolution proxy records of d¹⁸^O^precip^andd^D^precipspanning the past millennium (1000 C.E.- present) is becoming available from sediments, corals, and spe- leothems in the circum-Indian Ocean region. These records have suggested considerable variations in hydrology over the past millennium (Zinke et al., 2004; Newton et al., 2006; Fleitmann et al., 2007; Partin et al., 2007; Sinha et al., 2007; Abram et al., 2008;

d¹⁸^Oprecip(and IOZM/d^Dprecip) relationship over two important time periods: the historical period (1870e2003), and the past millennium (~1000 C.E. to present). The historical period, for which a critical mass of SST observations is available, provides the longest and best-constrained timeframe to observe IOZM behavior, including several multi-decadal cycles. The past millennium provides an important test case to observe multi-decadal to centennial variability in IOZM behavior and to assess its regional impacts.

These proxy records comprise the only network of d¹⁸^O^precip^/ d^D^precip-based proxy reconstructions from the core IOZM-affected regions of East Africa and Indonesia that are of high enough resolution to assess low-frequency timescales of variability over the past millennium. Together, these two timeframes provide

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“IOZM-like”dynamics are not directly analogous to those of an individual IOZM event, whose onset and cessation are directly tied to a given year's seasonal cycle. However, a low-frequency,“IOZM- like”mode could arise from periods of more frequent/intense IOZM events (e.g.,Tozuka et al., 2007), intrinsic low-frequency variability in Indian Ocean surface ocean characteristics (e.g.,Annamalai et al., 2005), and/or feedbacks with the Indian monsoon circulation and the Indonesian Throughﬂow (Ashok et al., 2001; Annamalai et al., 2003, 2005).

An increasing number of high-resolution proxy records of d¹⁸^O^precip^andd^D^precipspanning the past millennium (1000 C.E.- present) is becoming available from sediments, corals, and spe- leothems in the circum-Indian Ocean region. These records have suggested considerable variations in hydrology over the past millennium (Zinke et al., 2004; Newton et al., 2006; Fleitmann et al., 2007; Partin et al., 2007; Sinha et al., 2007; Abram et al., 2008;

d¹⁸^Oprecip(and IOZM/d^Dprecip) relationship over two important time periods: the historical period (1870e2003), and the past millennium (~1000 C.E. to present). The historical period, for which a critical mass of SST observations is available, provides the longest and best-constrained timeframe to observe IOZM behavior, including several multi-decadal cycles. The past millennium provides an important test case to observe multi-decadal to centennial variability in IOZM behavior and to assess its regional impacts.

These proxy records comprise the only network of d¹⁸^Oprecip/ d^D^precip-based proxy reconstructions from the core IOZM-affected regions of East Africa and Indonesia that are of high enough resolution to assess low-frequency timescales of variability over the past millennium. Together, these two timeframes provide