ETM's implication on Monsoon
INTRODUCTION
Monsoon rainfall is considered to be the lifeline of India. It receives about 80% of its annual rainfall during the southwest monsoon season (June to September). Apart from its inter-annual variations, Indian Summer Monsoon Rainfall (ISMR) also exhibits prominent variations at decadal to centennial time scales. Currently weather and climate predictions are made at different time scales. However, none of them are designed to provide a lookup of the climate a few years to a decade in advance. Prior knowledge on such variations, especially at decadal to multi-decadal time scales (e.g., 10 to 20 years ahead), would have a large bearing on long-term planning for food security, water resources, and economy of the country. Past and current studies suggests that source of decadal predictability mainly comes from the ocean and its interaction with the atmosphere on the decadal time scales. i.e. the low frequency natural climate variability (initial conditions).
We identify one potential driver of the multi-decadal climate variability (DCV). In our study, we propose that the multi-decadal variability of monsoon is strongly tied with the multi-decadal periodicity in the seasonal (boreal winter-to-summer) El Nino Southern Oscillation (ENSO) transition frequencies. Particulalry, the decades populated by frequent seasonal warmings experience suppressed monsoon rainfall.
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.