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INTRODUCTION
Boreal summer monsoon rainfall is vital for billions worldwide, supplying the majority of annual precipitation across tropical and subtropical regions. While seasonal and interannual variations in monsoons are well documented, global boreal summer monsoon systems also exhibit significant variability on decadal to centennial timescales. Current climate prediction systems, however, are not equipped to forecast conditions several years to a decade in advance - timescales that are crucial for long-term planning in agriculture, water management, and economic stability. Emerging research points to low-frequency ocean–atmosphere interactions as key sources of decadal predictability.
In our study, we identify a potential driver of multidecadal climate variability: a Southern Hemisphere climate mode, the ETM. We propose that during decades dominated by its positive phase, global monsoon rainfall across the Northern Hemisphere is suppressed due to a weakened cross-equatorial atmospheric mass flux, which reduces the strength of the Intertropical Convergence Zone (ITCZ).
What is decadal climate predictions/variability?
Decadal predictions lies at the junction of short-range forecasts and multi-decadal to long-term forecasts.
Both the initial and the boundary conditions becomes important for decadal time scale predictions.
The inter-play between the low-frequency climate variability in the extra-tropics and tropics can provide the source for the DCV.
ETM's implication on the seasonal transition of ENSO
The seasonal transition of ENSO is defined by: ΔT= N34JJA-N34DJF
N34JJA and N34DJF are Nino3.4 SST anomalies for June-July-August and Dec-Jan-Feb, respectively.
The positive phase of ETM leads to a larger increase in the Nino3.4 SST anomalies from boreal winter to the following summer (ΔT), irrespective of the winter ENSO states.
Schematic of ETM's impact on Global Northern Hemisphere monsoons
Association of ETM with the Global Boreal Summer Monsoon
During decades with frequent positive phases of ETM (frequent seasonal warming), cross-equatorial atmospheric mass flux is significantly reduced.
This leads to a weakened Hadley circulation and reduced strength of the Intertropical Convergence Zone (ITCZ).
ETM highlights the pivotal and previously underappreciated role of Southern Hemisphere climate variability in regulating interhemispheric temperature gradients, cross-equatorial circulation, and low-frequency monsoon variability.
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