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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 mass flux.
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.
Linear regression of normalized PC2 of SLP EOF2 against JJAS minus DJF mean meridional circulation (MMC). Unit is Kg/s. (b) Same as (a) but the global JJAS minus DJF rainfall (Units: mm/day). (c) Reconstructed ISMR (P’) using a simple multiple linear regression model where P’ = m1·F W +m2·F N +m 0 , where F W and F N represent the frequency of seasonal warming and seasonal neutral transition in a sliding window of 21 years, respectively. P is the original multidecadal ISMR (21 years running mean). Gray curve represents AMO index. Inset image shows the linear regression (mm/day) of normalized PC2 against rainfall anomalies at each grid point over India.
Association of ETM with the Global and the Indian Monsoon:
During decades with frequent positive phases of ETM (frequent seasonal warming), global Hadley cell is weaker and global Monsoon is suppressed.
The low-frequency variability of the Indian Summer Monsoon Rainfall (ISMR) is strongly related to the seasonal warming frequencies.
During decades with frequent seasonal warming, monsoon rainfall in central India and the western coast of India experience strong and significant negative departure.
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