Tropical Pacific Sea Surface Temperature Research Articles

Variability, teleconnection, and predictability of Korean precipitation in relation to large scale climate indices

Spatiotemporal variability, teleconnection, and predictability of the Korean precipitation related to large scale climate indices were examined based on leading patterns of observed monthly Rx5day and total precipitation through an empirical orthogonal teleconnection (EOT). Cross-correlation and lag regression analyses for the leading modes and global atmospheric circulation dataset were employed on a monthly basis. The spatial pattern of the leading EOT modes for Rx5day and total precipitation represents a northern inland mode for boreal summer and a southern coastal mode in boreal winter. The temporal evolution of the leading EOT modes exhibits increasing trends during summer season and decadal variability for winter season. The leading EOT patterns of Rx5day precipitation show more widespread coherent patterns than those of total precipitation during warm and cold seasons, while the former explains less variance in precipitation variability than the latter. The tropical ENSO forcing has a coherent teleconnection with September and November-December precipitation patterns, while the Indian Ocean dipole is identified as a driver for precipitation variability in September and November. The monsoon circulation over the western North Pacific also exhibits a significant negative correlation with winter precipitation EOTs, while tropical cyclone indices are positively correlated with the fall precipitation EOTs. The leading patterns of the September and December Rx5day precipitation time series are predictable at up to six month lead time from the tropical Pacific sea surface temperatures (SSTs), while a somewhat weak predictable response from Indian Ocean SSTs was only detected at longer lead times. In addition, predictability from the Pacific SSTs for above normal precipitation is greater than that for below normal precipitation.

Distinct Mechanisms of Decadal Subsurface Heat Content Variations in the Eastern and Western Indian Ocean Modulated by Tropical Pacific SST

Decadal variability of the subsurface ocean heat content (OHC) in the Indian Ocean is investigated using a coupled climate model experiment, in which observed eastern tropical Pacific sea surface temperature (EPSST) anomalies are specified. This study intends to understand the contributions of external forcing relative to those of internal variability associated with EPSST, as well as the mechanisms by which the Pacific impacts Indian Ocean OHC. Internally generated variations associated with EPSST dominate decadal variations in the subsurface Indian Ocean. Consistent with ocean reanalyses, the coupled model reproduces a pronounced east–west dipole structure in the southern tropical Indian Ocean and discontinuities in westward-propagating signals in the central Indian Ocean around 100°E. This implies distinct mechanisms by which the Pacific impacts the eastern and western Indian Ocean on decadal time scales. Decadal variations of OHC in the eastern Indian Ocean are attributed to 1) western Pacific surface wind anomalies, which trigger oceanic Rossby waves propagating westward through the Indonesian Seas and influence Indonesian Throughflow transport, and 2) zonal wind anomalies over the central tropical Indian Ocean, which trigger eastward-propagating Kelvin waves. Decadal variations of OHC in the western Indian Ocean are linked to conditions in the Pacific via changes in the atmospheric Walker cell, which trigger anomalous wind stress curl and Ekman pumping in the central tropical Indian Ocean. Westward-propagating oceanic Rossby waves extend the influence of this anomalous Ekman pumping to the western Indian Ocean.

Pacific Decadal Oscillation: Tropical Pacific Forcing versus Internal Variability

The Pacific decadal oscillation (PDO) is the leading mode of sea surface temperature (SST) variability over the North Pacific (north of 20°N). Its South Pacific counterpart (south of 20°S) is the South Pacific decadal oscillation (SPDO). The effects of tropical eastern Pacific (TEP) SST forcing and internal atmospheric variability are investigated for both the PDO and SPDO using a 10-member ensemble tropical Pacific pacemaker experiment. Each member is forced by the historical radiative forcing and observed SST anomalies in the TEP region. Outside the TEP region, the ocean and atmosphere are fully coupled and freely evolve. The TEP-forced PDO (54% variance) and SPDO (46% variance) are correlated in time and exhibit a symmetric structure about the equator, driven by the Pacific–North American (PNA) and Pacific–South American teleconnections, respectively. The internal PDO resembles the TEP-forced component but is related to internal Aleutian low (AL) variability associated with the Northern Hemisphere annular mode and PNA pattern. The internal variability is locally enhanced by barotropic energy conversion in the westerly jet exit region around the Aleutians. By contrast, barotropic energy conversion is weak associated with the internal SPDO, resulting in weak geographical preference of sea level pressure variability. Therefore, the internal SPDO differs from the TEP-forced component, featuring SST anomalies along ~60°S in association with the Southern Hemisphere annular mode. The limitations on isolating the internal component from observations are discussed. Specifically, internal PDO variability appears to contribute significantly to the North Pacific regime shift in the 1940s.

21st century California drought risk linked to model fidelity of the El Ni\xf1o teleconnection

Greenhouse gas induced climate change is expected to lead to negative hydrological impacts for southwestern North America, including California (CA). This includes a decrease in the amount and frequency of precipitation, reductions in Sierra snow pack, and an increase in evapotranspiration, all of which imply a decline in surface water availability, and an increase in drought and stress on water resources. However, a recent study showed the importance of tropical Pacific sea surface temperature (SST) warming and an El Niño Southern Oscillation (ENSO)-like teleconnection in driving an increase in CA precipitation through the 21st century, particularly during winter (DJF). Here, we extend this prior work and show wetter (drier) CA conditions, based on several drought metrics, are associated with an El Niño (La Niña)-like SST pattern. Models that better simulate the observed ENSO-CA precipitation teleconnection also better simulate the ENSO-CA drought relationships, and yield negligible change in the risk of 21st century CA drought, primarily due to wetting during winter. Seasonally, however, CA drought risk is projected to increase during the non-winter months, particularly in the models that poorly simulate the observed teleconnection. Thus, future projections of CA drought are dependent on model fidelity of the El Niño teleconnection. As opposed to focusing on adapting to less water, models that better simulate the teleconnection imply adaptation measures focused on smoothing seasonal differences for affected agricultural, terrestrial, and aquatic systems, as well as effectively capturing enhanced winter runoff.

Different Impacts of Northern, Tropical, and Southern Volcanic Eruptions on the Tropical Pacific SST in the Last Millennium

The impact of northern, tropical, and southern volcanic eruptions on the Pacific sea surface temperature (SST) and the different response mechanisms arising due to differences in the volcanic forcing structure are investigated using the Community Earth System Model Last Millennium Ensemble (CESM-LME). Analysis of the simulations indicates that 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, possibly reflective of the weaker magnitude of these eruptions. The Niño-3 index peaks with a lag of one and a half years after northern and tropical eruptions. Two years after all three types of volcanic eruptions, a La Niña–like SST anomaly pattern over the equatorial Pacific is observed, which seems to form an El Niño–Southern Oscillation (ENSO) cycle. The westerly wind anomaly over the western to central Pacific plays an essential role in favoring the development of an El Niño following all three types of eruptions. Thus, the key point of the question is to find the causes of the westerly wind enhancement. The 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 ocean dynamical thermostat mechanism is the fundamental cause of the anomalous westerly wind for all three types of eruptions.

What drives the decadal variation of global land monsoon precipitation over the past 50 years?

Global land monsoon precipitation (GLMP) has shown quasi‐20‐year oscillations overlying a significant downwards trend over the past 50 years (1951–2004). However, what drives this decadal variation remains an open question. Based on Coupled Model Intercomparison Project Phase 5 (CMIP5) models, in this study we explored the possible drivers for the decadal variation of GLMP and quantified their contributions. A downwards trend of −0.30 mm/day (100 years) is observed in GLMP over 1951–2004 that explains 30% of total decadal variance, and it is primarily attributed to anthropogenic aerosols (AAs) external forcing. Further analyses showed that the downwards trend in GLMP mainly results from the widespread drying over Northern Hemisphere (NH) monsoon domain. Faster growth of AAs amount in NH than Southern Hemisphere (SH) over the past 50 years results in a reduced hemispheric thermal contrast (cooler NH–warmer SH) due to more AA‐forced cooling in NH. Reduced hemispheric thermal contrast facilitates a weakened NH summer monsoon circulation and finally suppresses monsoon precipitation. Besides the downwards trend, the remaining variation of GLMP exhibited quasi‐20‐year oscillations that explain 70% of total decadal variance. The decadal oscillations are primarily attributed to internal variability, in which the central tropical Pacific sea surface temperature anomaly and Arctic Oscillation together can explain 55% of its variation.

An Improved Prediction of Indian Summer Monsoon Onset From State‐of‐the‐Art Dynamic Model Using Physics‐Guided Data‐Driven Approach

AbstractPrediction of Indian Summer Monsoon onset, which marks the beginning of the South Asian monsoon, has significant implications on monsoon characteristics and subsequent nationwide agricultural planning. Here we make the most skillful prediction of monsoon onset with a physics‐guided data‐driven model integrated with the operational monsoon prediction by Coupled Forecast System version 2. Drivers of the variability of onset, which are well simulated by Coupled Forecast System version 2, viz., tropical Pacific sea surface temperature, reversal of tropospheric temperature gradient and strengthening of westerlies over AS during the early summer are identified as potential predictors. Data‐driven method, Least Absolute Shrinkage and Selection Operator, makes the final selection of predictors for the linear regression model of onset. We obtain an improved correlation of 0.6 between simulated and observed onset dates with the proposed integrated statistical dynamical model as compared to 0.44 obtained with the operational dynamic monsoon prediction model.

Enhancement of the relationship between boreal summer precipitation over eastern China and Australia since the early 1980s

The relationship between the boreal summer (June–August, JJA) precipitation over eastern China and Australia is investigated in this study. The relationship exhibits a decadal enhancement with correlation coefficients increasing from 0.07 before the early 1980s to 0.60 thereafter. A mechanistic analysis suggests that the variability in sea surface temperature (SST) over the central tropical Pacific could play an important role in enhancing the precipitation relationship between the two regions. Over the past half century, the central Pacific SST has shown a warming trend with a significant decadal change that occurred around the early 1980s. Such changes in the central Pacific SST have resulted in stronger convective activity over the tropics, as well as eastern China and Australia since the early 1980s. In addition, since the early 1980s, the central Pacific SST stimulated anomalous twin anticyclones located on opposite sides of the equator, which changed the transportation of water vapour to eastern China and Australia. These changes in the atmospheric circulation related to central tropical Pacific SST have enhanced the impact on the precipitation variability over eastern China and Australia. Consequently, this has contributed to the covariability in the boreal summer precipitation over the two regions since the early 1980s. The result indicates that concurrent climate anomalies can provide simultaneously occurring natural disasters over Asia and Australia, which creates a challenge for international disaster prevention and mitigation.

Assessing the Relationship between MJO and Equatorial Pacific WWBs in Observations and CMIP5 Models

AbstractThis study evaluates the relationship between the Madden–Julian oscillation (MJO) and the occurrence of equatorial Pacific westerly wind bursts (WWBs). During the convective MJO phase, anomalous surface westerlies prevail in and west of the convective MJO center, providing favorable conditions for WWBs. Compared with the probability of WWBs expected under a null hypothesis that WWBs occur randomly, the convective MJO phase almost doubles the probability of a WWB occurring. Nevertheless, only 34.46% of WWBs co-occur with the convective MJO, which is much less than that reported in previous studies. We show that when the MJO and WWBs are defined using the same field with overlapping frequencies, the percentage of WWBs co-occurring with the convective MJO shows a significant increase. However, the higher percentage is simply caused by the fact that the strong WWBs during a convective MJO are more likely to be identified than those during the suppressed and neutral MJO phases. A total of 45.80% of WWBs are found to occur in the full MJO phase (both the convective and suppressed MJO phases), which is slightly higher than that expected based on randomness. Although the full MJO has statistically significant impact on the likelihood of WWBs, the influence from the full MJO on the tropical Pacific sea surface temperature anomaly is much weaker as compared to that from the WWBs. The relationships between the MJO and WWBs simulated in CMIP5 models are also assessed, and the percentage of WWBs that co-occur with the MJO simulated in models is in general less than that in observations.

Can Ningaloo Niño/Niña Develop Without El Niño–Southern Oscillation?

AbstractNingaloo Niño/Niña is the dominant climate mode in the southeastern Indian Ocean with its center of positive/negative sea surface temperature anomalies attached to Australia. Ningaloo Niño is the major cause of marine heatwaves in the region. Although oceanic variability in this region has long been considered mainly as a response to the El Niño–Southern Oscillation (ENSO), some recent studies have suggested the possible existence of local air‐sea feedback processes. Using a state‐of‐the‐art ocean‐atmosphere coupled model that realistically simulates Ningaloo Niño/Niña, whether Ningaloo Niño/Niña can occur independently of ENSO is examined. Even in an experiment in which ENSO is suppressed by strongly nudging tropical Pacific sea surface temperatures toward the model climatology, Ningaloo Niño/Niña with a similar magnitude and seasonality still develops, likely through an air‐sea interaction off Western Australia amplifying atmospheric stochastic forcing. This study is the first to show that Ningaloo Niño/Niña can develop even without ENSO.

Ocean Impacts on Australian Interannual to Decadal Precipitation Variability

In Australia, successful seasonal predictions of wet and dry conditions are achieved by utilizing the remote impact of sea surface temperature (SST) variability in tropical oceans, particularly the Pacific Ocean, on the seasonal timescale. Beyond seasonal timescales, however, it is still unclear which processes and oceans contribute to interannual-to-decadal wet/dry conditions in Australia. This research examines the interannual-to-decadal relationship between global SST anomalies (SSTAs) and Australian wet/dry variability by analyzing observational data and global climate model experiments conducted with the NCAR Community Earth System Model (CESM) and the Model for Interdisciplinary Research on Climate (MIROC). A 10-member ensemble simulation suite for 1960–2015 (CESM) and 1950–2010 (MIROC) is conducted by assimilating the observed three-dimensional ocean temperature and salinity anomalies into fully coupled global climate models. In both observational analyses and ocean assimilation experiments, the most dominant annual mean precipitation variability shows a clear relationship with SSTAs in the tropical Pacific and the Atlantic. Our partial ocean assimilation experiment, in which the ocean component of the CESM and MIROC are assimilated by the observed ocean temperature and salinity anomalies in the equatorial Pacific only, shows that the tropical Pacific SST variability is the main driver of Australian precipitation variability on the interannual-to-decadal timescales. However, our additional partial ocean assimilation experiment, in which the climate models incorporate the observed anomalies solely in the Atlantic ocean, demonstrates that the Atlantic Ocean can also affect Australian precipitation variability on the interannual-to-decadal timescale through changes in tropical Pacific SSTAs and the modulation of the global Walker circulation. Our results suggest that about a half of Australian interannual-to-decadal precipitation variability originates from the Atlantic Ocean.

A new perspective of intensified impact of El Niño‐Southern Oscillation Modoki on tropical cyclogenesis over the western North Pacific around 1990s

Previous study detected an intensified impact of the El Niño‐Southern Oscillation (ENSO) Modoki sea surface temperature (SST) anomalies on the tropical cyclone (TC) activity over the western North Pacific (WNP) after the early 1990s and attributed it to an expansion in areal coverage of the equatorial central Pacific (ECP) SST anomalies. This study identifies the contribution of SST anomalies in several other regions to this inter‐decadal change in the relationship between ENSO Modoki SST and the WNP TC genesis. Before the early 1990s, the positive ECP SST anomalies induce an anomalous lower‐level cyclone and consequently an increase in the TC genesis frequency over the southern part of the WNP, and the positive ECP and tropical Indian Ocean SST anomalies together induce an anomalous lower‐level anticyclone and accordingly a decrease in the TC genesis frequency over the northern part of the WNP. As such, the relationship between the ECP SST and the WNP TC genesis frequency is weak. After the early 1990s, the positive ECP SST anomalies with a large areal coverage induce a large anomalous lower‐level cyclone covering most of the WNP and thus an increase in the TC genesis frequency over the WNP. Meantime, the tropical northern Atlantic and western South Pacific SST anomalies during spring enhance the succeeding summer–autumn WNP atmospheric circulation response to the ECP SST anomalies through an Atlantic–Pacific teleconnection and a wind‐evaporation positive feedback, respectively. This strengthens the relationship between the ECP SST and the WNP TC genesis frequency.

Attributing the U.S. Southwest's Recent Shift Into Drier Conditions

AbstractThe U.S. Southwest experienced a strong hydroclimate trend from the 1980s to the 2010s, from cool and wet to warm and dry conditions. Attribution of this trend is challenging due to the influence of internal variability but desired by water managers eager to plan for robust signals of climate change in this water‐scarce region. Here we use an empirical method based on constructed circulation analogues to assess the contribution of atmospheric circulation variability to the recent observed hydroclimate trend. Consistent with other studies, we find the observed precipitation trend from 1983 to 2012 to be mainly due to internal atmospheric circulation variability that is driven in part by decadal‐scale tropical Pacific sea surface temperature changes. Removing this internal dynamical component brings the observed precipitation trend into closer agreement with the anthropogenically forced response in climate models, demonstrating progress toward an integrated perspective on climate change attribution.

Global climate teleconnection with rainfall erosivity in South Korea

Rainfall Erosivity Index (REI) defined as the product of rainfall kinetic energy and rainfall intensity is a well-known hydrologic indicator of the potential risk of soil erosion. Global and regional scale climatic teleconnections with REI variability over South Korea are examined. We calculate leading patterns of observed monthly REIs using the Empirical Orthogonal Teleconnection (EOT) and Function (EOF) decomposition techniques. Also we used monthly statistical analyses using cross-correlation and lag regression for the leading modes and global atmospheric circulation measurement in the Pacific and Indian Ocean. As a result, the northern inland mode is applicable during summer season and the southern coastal mode applies to fall-winter season. The temporal evolution of REI exhibits mostly increasing and depends on interdecadal oscillation patterns. The leading EOT modes explain more variance in REI than the EOF modes during warm and cold seasons. The findings from this study illustrate that the tropical ENSO forcing has the coherent association with fall and winter REI patterns, and the Indian Ocean dipole is identified as a driver for REI variability in November. The monsoon circulations over western North Pacific also exhibit significant negative correlation with the December modes. The September leading modes also show a positive correlation with the tropical cyclone activity. Leading patterns in September and November have predictability up to five month lead time from the tropical Pacific Sea Surface Temperatures (SSTs). In addition, predictability from the Pacific SSTs for above normal extreme value of REI is greater than that for below normal value in winter. In conclusion, South Korea experiences climatic teleconnection between the large scale climate indices and mid-latitude hydrologic variables.

Application of Entropy Ensemble Filter in Neural Network Forecasts of Tropical Pacific Sea Surface Temperatures.

Recently, the Entropy Ensemble Filter (EEF) method was proposed to mitigate the computational cost of the Bootstrap AGGregatING (bagging) method. This method uses the most informative training data sets in the model ensemble rather than all ensemble members created by the conventional bagging. In this study, we evaluate, for the first time, the application of the EEF method in Neural Network (NN) modeling of El Nino-southern oscillation. Specifically, we forecast the first five principal components (PCs) of sea surface temperature monthly anomaly fields over tropical Pacific, at different lead times (from 3 to 15 months, with a three-month increment) for the period 1979–2017. We apply the EEF method in a multiple-linear regression (MLR) model and two NN models, one using Bayesian regularization and one Levenberg-Marquardt algorithm for training, and evaluate their performance and computational efficiency relative to the same models with conventional bagging. All models perform equally well at the lead time of 3 and 6 months, while at higher lead times, the MLR model’s skill deteriorates faster than the nonlinear models. The neural network models with both bagging methods produce equally successful forecasts with the same computational efficiency. It remains to be shown whether this finding is sensitive to the dataset size.

Efficacy of tendency and linear inverse models to predict southern Peru's rainy season precipitation

ABSTRACTSouthern Peru receives over 60% of its annual climatological precipitation during the short period of January–March. This rainy season precipitation exhibits strong inter‐annual and decadal variability, including severe drought events that incur devastating societal impacts and cause agricultural communities and mining facilities to compete for limited water resources. Improving existing seasonal prediction models of summertime precipitation could aid in water resource planning and allocation across this water‐limited region. While various underlying mechanisms modulating inter‐annual variability have been proposed by past studies, operational forecasts continue to be largely based on rudimentary El Niño‐Southern Oscillation (ENSO)‐based indices, such as Niño3.4, justifying further exploration of predictive skill. To bridge the gap between understanding precipitation mechanisms and operational forecasts, we perform systematic studies on the predictability and prediction skill of southern Peru's rainy season precipitation by constructing statistical forecast models using best available weather station and reanalysis data sets. We construct a simple regression model, based on the principal component (PC) tendency of tropical Pacific sea surface temperatures (SST), and a more advanced linear inverse model (LIM), based on the empirical orthogonal functions of tropical Pacific SST and large‐scale atmospheric variables from reanalysis. Our results indicate that both the PC tendency and LIM models consistently outperform the ENSO‐only based regression models in predicting precipitation at both the regional scale and for individual station, with improvements for individual stations ranging from 10 to over 200%. These encouraging results are likely to foster further development of operational precipitation forecasts for southern Peru.

Causes of the active typhoon season in 2016 following a strong El Niño with a comparison to 1998

ABSTRACTThis article analysed two typhoon seasons (1998 and 2016) over the western North Pacific (WNP) with similar preceding background conditions of the tropical Pacific sea surface temperature (SST). Following the strong El Niño event in the preceding winter, the typhoon season was inactive in 1998 while active in 2016. Compared with 1998, the monsoon trough (MT) in 2016 shifted more eastwards, leading to an increase of the mid‐level water vapour, lower‐tropospheric relative vorticity and upper‐tropospheric divergence, and a decrease of the vertical wind shear. Additionally, the eastwards extension of the MT in 2016 induced more active tropical depression (TD)‐type waves than those in 1998. The analysis of the energy budget suggested that the eastwards shift of the MT during the 2016 typhoon season provided increased eddy kinetic energy for typhoon genesis, due to the joint contributions of the meridional shear of the mean zonal winds and the zonal wind convergence over the WNP. Moreover, the WNP entered a La Niña‐like SST pattern after July 2016. The relatively slower developing process of this pattern sustained the warmer SST anomalies and the anomalous cyclonic circulation, which the eastwards extension of the MT can be attributed. Hence, compared to 1998, the active typhoon season in 2016 is attributed to the eastwards extension of the MT, more active TD‐type wave activities, and slow developing process of a La Niña‐like SST pattern in 2016 typhoon season.

Dynamical and Thermodynamic Elements of Modeled Climate Change at the East African Margin of Convection

AbstractWe propose a dynamical interpretation of model projections for an end‐of‐century wetting in equatorial East Africa. In the current generation of global climate models, increased atmospheric moisture content associated with warming is not the dominant process explaining the increase in rainfall, as the regional circulation is only weakly convergent even during the rainy seasons. Instead, projected wetter future conditions are generally consistent with the El Niño‐like trend in tropical Pacific sea surface temperatures in climate models. In addition, a weakening in moisture convergence over the adjacent Congo Basin and Maritime Continent cores of convection results in the weakening of near‐surface winds, which increases moisture advection from the Congo Basin core toward the East African margin. Overall confidence in the projections is limited by the significant biases in simulation of the regional climatology and disagreement between observed and modeled tropical Pacific sea surface temperature trends to date.

Hydrological variability in the western tropical Pacific over the past 700 kyr and its linkage to Northern Hemisphere climatic change

El Niño/Southern Oscillation (ENSO) events and latitudinal shifts of the Intertropical Convergence Zone (ITCZ) cause large scale hydrologic changes in the western tropical Pacific (WTP). However, there is still a debate on the role of ENSO and ITCZ shifts on the hydroclimate in the WTP and their linkage to Northern Hemisphere climate change on glacial-interglacial timescales. For that reason, we present ice volume-corrected seawater δ18O (δ18Osw-ice, a proxy for salinity) and sea surface temperature (SST) records from IMAGES Core MD06-3047B recovered from the northern WTP to investigate the hydroclimate in the WTP during the last 700kyr. Planktic foraminifera Globigerinoides ruber based Mg/Ca-SST and δ18Osw-ice records reveal distinct glacial-interglacial variations. The δ18Osw-ice record shows that our study site is characterized by fresher conditions and thus higher precipitation during glacials than during interglacials. Our results are in good agreement with numerical model simulations, which also suggest a glacial freshening in the northern WTP. A glacial freshening of the WTP is most likely related to a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC) and/or exposure of the Maritime Continent due to sea level changes which resulted in an enhanced Pacific Walker circulation and/or southward shift of the ITCZ. In addition, SST records from the WTP and the eastern tropical Pacific (ETP) suggest an enhanced tropical Pacific zonal SST gradient and thus La Niña-like conditions, which may have contributed to fresher conditions in the WTP during glacial periods. Besides, our rainfall record is in antiphase to the Chinese speleothem δ18O records, supporting the idea of an opposing trend in precipitation changes between the WTP and East Asia on orbital timescales.

Interannual Variations and Prediction of Spring Precipitation over China

The interannual variations and the prediction of the leading two empirical orthogonal function (EOF) modes of spring (April–May) precipitation over China for the period from 1951 to 2014 are investigated using both observational data and the seasonal forecast made by six coupled climate models. The leading EOF mode of spring precipitation over China (EOF1-prec) features a monosign pattern, with the maximum loading located over southern China. The ENSO-related tropical Pacific SST anomalies in the previous winter can serve as a precursor for EOF1-prec. The second EOF mode of spring precipitation (EOF2-prec) over China is characterized by a dipole structure, with one pole near the Yangtze River and the other one with opposite sign over the Pearl River delta. A North Atlantic sea surface temperature (SST) anomaly dipole in the preceding March is found contribute to the prec-EOF2 and can serve as its predictor. A physics-based empirical (P-E) model is then formulated using the two precursors revealed by the observational analysis to forecast the variations of EOF1-prec and EOF2-prec. Compared to coupled climate models, which have little skill in forecasting the time variations of the two EOF modes, this P-E model can significantly improve the forecast skill of their time variations. A linear regression model is further established using the time series forecast by the P-E model to forecast the spring precipitation over China. Results suggest that the seasonal forecast skill of the spring precipitation over southeastern China, especially over the Yangtze River area, can be significantly improved by the regression model.