Indo-Pacific warm pool expansion modulates MJO lifecycle

The Madden–Julian Oscillation (MJO) is the most dominant mode of intraseasonal
variability in the tropics, characterized by an eastward propagating zonal circulation pattern
and rain bands. MJO is very crucial phenomenon due to its interactions with other
timescales of ocean-atmosphere like El Niño Southern Oscillation, tropical cyclones,
monsoons, and the extreme rainfall events all across the globe. MJO events travel almost
half of the globe along the tropical oceans, majorly over the Indo-Pacific Warm Pool
(IPWP) region. This IPWP region has been warming during the twentieth and early twenty-
first centuries in response to increased anthropogenic emissions of greenhouse gases and
is projected to warm further. However, the impact of the warming of the IPWP region on
the MJO life cycle is largely unknown. Here we show that rapid warming over the IPWP
region during 1981–2018 has significantly changed the MJO life cycle, with its residence
time decreasing over the Indian Ocean by 3–4 days, and increasing over the Indo-Pacific
Maritime Continent by 5–6 days. We find that these changes in the MJO life cycle are
associated with a twofold expansion of the Indo-Pacific warm pool. The warm pool has
been expanding on average by 2.3 × 105 km2 per year during 1900–2018 and at an
accelerated average rate of 4 × 105 km2 per year during 1981–2018. The accelerated
warm pool expansion has increased moisture in the lower and middle troposphere over
IPWP and thereby increased the gradient of lower-middle tropospheric moisture between
the Indian Ocean and western Pacific. This zonal gradient of moisture between the Indian Ocean
and west Pacific and the increased subsidence over the Indian ocean due to increased
convective duration of MJO over maritime continent are likely the reasons behind the
changing lifecycle of MJO.