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1. The Recent Decline and Recovery of Indian Summer Monsoon Rainfall: Relative Roles of External Forcing and Internal Variability

   https://doi.org/10.1175/JCLI-D-19-0833.1  

   Huang, Xin; Zhou, Tianjun; Turner, Andrew; Dai, Aiguo; Chen, Xiaolong; Clark, Robin; Jiang, Jie; Man, Wenmin; Murphy, James; Rostron, John; et al (June 2020, Journal of Climate)

   null (Ed.)

   Abstract The Indian summer monsoon (ISM) rainfall affects a large population in South Asia. Observations show a decline in ISM rainfall from 1950 to 1999 and a recovery from 1999 to 2013. While the decline has been attributed to global warming, aerosol effects, deforestation, and a negative-to-positive phase transition of the interdecadal Pacific oscillation (IPO), the cause for the recovery remains largely unclear. Through analyses of a 57-member perturbed-parameter ensemble of model simulations, this study shows that the externally forced rainfall trend is relatively weak and is overwhelmed by large internal variability during both 1950–99 and 1999–2013. The IPO is identified as the internal mode that helps modulate the recent decline and recovery of the ISM rainfall. The IPO induces ISM rainfall changes through moisture convergence anomalies associated with an anomalous Walker circulation and meridional tropospheric temperature gradients and the resultant anomalous convection and zonal moisture advection. The negative-to-positive IPO phase transition from 1950 to 1999 reduces what would have been an externally forced weak upward rainfall trend of 0.01 to −0.15 mm day −1 decade −1 during that period, while the rainfall trend from 1999 to 2013 increases from the forced value of 0.42 to 0.68 mm day −1 decade −1 associated with a positive-to-negative IPO phase transition. Such a significant modulation of the historical ISM rainfall trends by the IPO is confirmed by another 100-member ensemble of simulations using perturbed initial conditions. Our findings highlight that the interplay between the effects of external forcing and the IPO needs be considered for climate adaptation and mitigation strategies in South Asia. 

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2. A Multidecadal-Scale Tropically Driven Global Teleconnection over the Past Millennium and Its Recent Strengthening

   https://doi.org/10.1175/JCLI-D-20-0216.1  

   Feng, Xiaofang; Ding, Qinghua; Wu, Liguang; Jones, Charles; Baxter, Ian; Tardif, Robert; Stevenson, Samantha; Emile-Geay, Julien; Mitchell, Jonathan; Carvalho, Leila M.; et al (April 2021, Journal of Climate)

   null (Ed.)

   Abstract In the past 40 years, the global annual mean surface temperature has experienced a nonuniform warming, differing from the spatially uniform warming simulated by the forced responses of large multimodel ensembles to anthropogenic forcing. Rather, it exhibits significant asymmetry between the Arctic and Antarctic, with intermittent and spatially varying warming trends along the Northern Hemisphere (NH) midlatitudes and a slight cooling in the tropical eastern Pacific. In particular, this “wavy” pattern of temperature changes over the NH midlatitudes features strong cooling over Eurasia in boreal winter. Here, we show that these nonuniform features of surface temperature changes are likely tied together by tropical eastern Pacific sea surface temperatures (SSTs), via a global atmospheric teleconnection. Using six reanalyses, we find that this teleconnection can be consistently obtained as a leading circulation mode in the past century. This tropically driven teleconnection is associated with a Pacific SST pattern resembling the interdecadal Pacific oscillation (IPO), and hereafter referred to as the IPO-related bipolar teleconnection (IPO-BT). Further, two paleo-reanalysis reconstruction datasets show that the IPO-BT is a robust recurrent mode over the past 400 and 2000 years. The IPO-BT mode may thus serve as an important internal mode that regulates high-latitude climate variability on multidecadal time scales, favoring a warming (cooling) episode in the Arctic accompanied by cooling (warming) over Eurasia and the Southern Ocean (SO). Thus, the spatial nonuniformity of recent surface temperature trends may be partially explained by the enhanced appearance of the IPO-BT mode by a transition of the IPO toward a cooling phase in the eastern Pacific in the past decades. 

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3. Tropical Volcanic Eruptions and Low Frequency Indo‐Pacific Variability Drive Extreme Indian Ocean Dipole Events

   https://doi.org/10.1029/2023GL103991  

   Tiger, Benjamin_H; Ummenhofer, Caroline_C (October 2023, Geophysical Research Letters)

   Abstract Volcanic eruptions can have significant climate impacts and serve as useful natural experiments for better understanding the effects of abrupt, externally forced climate change. Here, we investigate the Indian Ocean Dipole's (IOD) response to the largest tropical volcanic eruptions of the last millennium. Post‐eruption composites show a strong negative IOD developing in the eruption year, and a positive IOD the following year. The IOD and El Niño‐Southern Oscillation (ENSO) show a long‐term damped oscillatory response that can take up to 8 years to return to pre‐eruptive baselines. Moreover, the Interdecadal Pacific Oscillation (IPO) phase at the time of eruption controls the IOD response to intense eruptions, with negative (positive) IPO phasing favoring more negative (positive) IOD values via modulation of the background state of the eastern Indian Ocean thermocline depth. These results have important implications for climate risk in low‐likelihood, high‐impact scenarios, particularly in vulnerable communities unprepared for IOD and ENSO extremes. 

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4. Interannual to Decadal Variability of Tropical Indian Ocean Sea Surface Temperature: Pacific Influence versus Local Internal Variability

   https://doi.org/10.1175/JCLI-D-20-0807.1  

   Zhang, Lei; Wang, Gang; Newman, Matthew; Han, Weiqing (December 2020, Journal of Climate)

   Abstract The Indian Ocean has received increasing attention for its large impacts on regional and global climate. However, sea surface temperature (SST) variability arising from Indian Ocean internal processes has not been well understood particularly on decadal and longer timescales, and the external influence from the Tropical Pacific has not been quantified. This paper analyzes the interannual-to-decadal SST variability in the Tropical Indian Ocean in observations and explores the external influence from the Pacific versus internal processes within the Indian Ocean using a Linear Inverse Model (LIM). Coupling between Indian Ocean and tropical Pacific SST anomalies (SSTAs) is assessed both within the LIM dynamical operator and the unpredictable stochastic noise that forces the system. Results show that the observed Indian Ocean Basin (IOB)-wide SSTA pattern is largely a response to the Pacific ENSO forcing, although it in turn has a damping effect on ENSO especially on annual and decadal timescales. On the other hand, the Indian Ocean Dipole (IOD) is an Indian Ocean internal mode that can actively affect ENSO; ENSO also has a returning effect on the IOD, which is rather weak on decadal timescale. The third mode is partly associated with the Subtropical Indian Ocean Dipole (SIOD), and it is primarily generated by Indian Ocean internal processes, although a small component of it is coupled with ENSO. Overall, the amplitude of Indian Ocean internally generated SST variability is comparable to that forced by ENSO, and the Indian Ocean tends to actively influence the tropical Pacific. These results suggest that the Indian-Pacific Ocean interaction is a two-way process. 

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5. Role of Tropical Variability in Driving Decadal Shifts in the Southern Hemisphere Summertime Eddy-Driven Jet

   https://doi.org/10.1175/JCLI-D-19-0604.1  

   Yang, Dongxia; Arblaster, Julie M.; Meehl, Gerald A.; England, Matthew H.; Lim, Eun-Pa; Bates, Susan; Rosenbloom, Nan (July 2020, Journal of Climate)

   null (Ed.)

   Abstract The Southern Hemisphere summertime eddy-driven jet and storm tracks have shifted poleward over the recent few decades. In previous studies, explanations have mainly stressed the influence of external forcing in driving this trend. Here we examine the role of internal tropical SST variability in controlling the austral summer jet’s poleward migration, with a focus on interdecadal time scales. The role of external forcing and internal variability are isolated by using a hierarchy of Community Earth System Model version 1 (CESM1) simulations, including the pre-industrial control, large ensemble, and pacemaker runs. Model simulations suggest that in the early twenty-first century, both external forcing and internal tropical Pacific SST variability are important in driving a positive southern annular mode (SAM) phase and a poleward migration of the eddy-driven jet. Tropical Pacific SST variability, associated with the negative phase of the interdecadal Pacific oscillation (IPO), acts to shift the jet poleward over the southern Indian and southwestern Pacific Oceans and intensify the jet in the southeastern Pacific basin, while external forcing drives a significant poleward jet shift in the South Atlantic basin. In response to both external forcing and decadal Pacific SST variability, the transient eddy momentum flux convergence belt in the middle latitudes experiences a poleward migration due to the enhanced meridional temperature gradient, leading to a zonally symmetric southward migration of the eddy-driven jet. This mechanism distinguishes the influence of the IPO on the midlatitude circulation from the dynamical impact of ENSO, with the latter mainly promoting the subtropical wave-breaking critical latitude poleward and pushing the midlatitude jet to higher latitudes. 

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