ALBERT

Description: Coupling of the El Niño–Southern Oscillation (ENSO) and Indian monsoon (IM) is central to seasonal summer monsoon rainfall predictions over the Indian subcontinent, although a nonstationary relationship between the two nonlinear phenomena can limit seasonal predictability. Radiative effects of volcanic aerosols injected into the stratosphere during large volcanic eruptions (LVEs) tend to alter ENSO evolution; however, their impact on ENSO-IM coupling remains unclear. Here, we investigate how LVEs influence the nonlinear behavior of the ENSO and IM dynamical systems using historical data, 25 paleoclimate reconstructions, last-millennium climate simulations, large-ensemble targeted climate sensitivity experiments, and advanced analysis techniques. Our findings show that LVEs promote a significantly enhanced phase-synchronization of the ENSO and IM oscillations, due to an increase in the angular frequency of ENSO. The results also shed innovative insights into the physical mechanism underlying the LVE-induced enhancement of ENSO-IM coupling and strengthen the prospects for improved seasonal monsoon predictions.

Description: The alternation of active and break phases in Indian summer monsoon (ISM) rainfall at intraseasonal timescales characterizes each ISM season. Both tropical and mid-latitude drivers influence this intraseasonal ISM variability. The circumglobal teleconnection observed in boreal summer drives intraseasonal variability across the mid-latitudes, and a two-way interaction between the ISM and the circumglobal teleconnection pattern has been hypothesized. We use causal discovery algorithms to test the ISM circumglobal teleconnection hypothesis in a causal framework. A robust causal link from the circumglobal teleconnection pattern and the North Atlantic region to ISM rainfall is identified, and we estimate the normalized causal effect (CE) of this link to be about 0.2 (a 1 standard deviation shift in the circumglobal teleconnection causes a 0.2 standard deviation shift in the ISM rainfall 1 week later). The ISM rainfall feeds back on the circumglobal teleconnection pattern, however weakly. Moreover, we identify a negative feedback between strong updraft located over India and the Bay of Bengal and the ISM rainfall acting at a biweekly timescale, with enhanced ISM rainfall following strong updraft by 1 week. This mechanism is possibly related to the boreal summer intraseasonal oscillation. The updraft has the strongest CE of 0.5, while the Madden–Julian oscillation variability has a CE of 0.2–0.3. Our results show that most of the ISM variability on weekly timescales comes from these tropical drivers, though the mid-latitude teleconnection also exerts a substantial influence. Identifying these local and remote drivers paves the way for improved subseasonal forecasts.

Description: This study investigates the sub-synoptic scale circulation aspects associated with the extreme rainfall event occurred over the North Indian state of Uttarakhand located in the western Himalayas (WH) during the 15–18 June 2013 period. A diagnosis based on hourly ERA5 reanalyzed circulation products archived on finer grids reveals that sustenance of heavy rains during the event period is supported by a propensity of cyclonic vorticity sources channeled toward the WH region through a narrow quasi-steady conduit in the lower troposphere from the ISM circulation. The equatorward segregating mesoscale potential vorticity (PV) structures from the quasi-stationary upper level PV anomaly (trough) during the event administered two pathways for vorticity sources. The first pathway is from the base of the trough culminating into longer horizontal conduit path from the western Arabian Sea, lending perpetual cyclonic vorticity support to the ISM environment. The second pathway is from the right flank of the trough, which promotes sustained environment of deeper mesoscale convergence zone, potentially unstable atmosphere and strong ascent over the Uttarakhand region. The convergence zone is potentially viewed as a region for strong monsoon and extratropical circulation interactions to occur on finer horizontal scales of motion, where significant vertical synchronization of positive PV advection is realized during the 16–17 June 2013 period. In addition to orographic precipitation enhancements, deeper advective synchronization noticed at sub-synoptic time periods is accredited to the nearly doubling 24-h rainfall amounts in the foothill region of Uttarakhand during the event period. The ERA5 diagnosed diabatic heating additionally indicates that precipitating systems at higher (foothill) elevations contribute to upper (lower) tropospheric heat sources.

Description: Abstract. Tropical convective activity represents a source of predictability for mid-latitude weather in the Northern Hemi-sphere. In winter, the El Niño–Southern Oscillation (ENSO) is the dominant source of predictability in the tropics and ex-tratropics, but its role in summer is much less pronounced and the exact teleconnection pathways are not well under-stood. Here, we assess how tropical convection interacts with mid-latitude summer circulation at different intra-seasonal timescales and how ENSO affects these interactions. First, we apply maximum covariance analysis (MCA) between tropical convective activity and mid-latitude geopotential height fields to identify the dominant modes of interaction. The first MCA mode connects the South Asian monsoon with the mid-latitude circumglobal teleconnection pattern. The second MCA mode connects the western North Pacific summer monsoon in the tropics with a wave-5 pattern centred over the North Pacific High in the mid-latitudes. We show that the MCA patterns are fairly insensitive to the selected intra-seasonal timescale from weekly to 4-weekly data. To study the potential causal interdependencies between these modes and with other atmospheric fields, we apply the causal discovery method PCMCI at different timescales. PCMCI extends standard correlation analysis by removing the con-founding effects of autocorrelation, indirect links and com-mon drivers. In general, there is a two-way causal interaction between the tropics and mid-latitudes, but the strength and sometimes sign of the causal link are timescale dependent. We introduce causal maps that show the regionally specific causal effect from each MCA mode. Those maps confirm the dominant patterns of interaction and in addition high-light specific mid-latitude regions that are most strongly con-nected to tropical convection. In general, the identified causal teleconnection patterns are only mildly affected by ENSO and the tropical mid-latitude linkages remain similar. Still, La Niña strengthens the South Asian monsoon generating a stronger response in the mid-latitudes, while during El Niño years the Pacific pattern is reinforced. This study paves the way for process-based validation of boreal summer telecon-nections in (sub-)seasonal forecast models and climate mod-els and therefore works towards improved sub-seasonal pre-dictions and climate projections.