Impact of African orography and the Indian summer monsoon on the low-level Somali jet

We investigated the physical mechanism for late Indian Summer Monsoon onset over Kerala (MOK). 14 early and 9 late onset years are selected based on the criteria when the onset is 5 days or more prior and after normal onset date (i.e 1 st June according to India Meteorological Department) respectively. Then, we perform composite analyses of mean May monthly and daily evolution during early and late onset years to examine the differences in monsoon circulation features prior to the MOK. We find that advection of Surface Air Temperature (SAT) from the northern to the southern China and the eastern Tibetan Plateau (TP) plays an important role to modulate the MOK processes. In the late onset years, more low-level jet (LLJ) from the Bay of Bengal (BOB) divert towards the east Asia before the onset, which is due to an extension of the low sea level pressure and high SAT over the east Asia (eastern TP, east-central China). This strengthens the low-level convergence and upper level divergence over the eastern TP and southern China. As a result, a significant amount of moisture from the BOB is transported towards the eastern TP and southern China. Thereby, a comparatively weaker LLJ and deficit low-level moisture supply over the eastern BOB maintain the key roles in modulating the MOK processes.

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Poleward shift in Indian summer monsoon low level jetstream under global warming

Climate Dynamics ◽

10.1007/s00382-014-2261-y ◽

2014 ◽

Vol 45 (1-2) ◽

pp. 337-351 ◽

Cited By ~ 49

Author(s):

S. Sandeep ◽

R. S. Ajayamohan

Keyword(s):

Global Warming ◽

Summer Monsoon ◽

Indian Summer Monsoon ◽

Low Level ◽

Poleward Shift

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Monsoon low-level jet over the gateway of Indian summer monsoon: a comparative study for two distinct monsoon years

Meteorology and Atmospheric Physics ◽

10.1007/s00703-016-0459-8 ◽

2016 ◽

Vol 128 (6) ◽

pp. 689-696 ◽

Cited By ~ 6

Author(s):

Suresh Narayanan ◽

Ajil Kottayil ◽

K. Mohanakumar

Keyword(s):

Comparative Study ◽

Summer Monsoon ◽

Indian Summer Monsoon ◽

Low Level ◽

Low Level Jet

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Influence of the Pacific–Japan Pattern on Indian Summer Monsoon Rainfall

Journal of Climate ◽

10.1175/jcli-d-17-0408.1 ◽

2018 ◽

Vol 31 (10) ◽

pp. 3943-3958 ◽

Cited By ~ 16

Author(s):

G. Srinivas ◽

Jasti S. Chowdary ◽

Yu Kosaka ◽

C. Gnanaseelan ◽

Anant Parekh ◽

...

Keyword(s):

Summer Monsoon ◽

Indian Summer Monsoon ◽

General Circulation ◽

Rossby Waves ◽

Function Analysis ◽

Circulation Model ◽

Easterly Wind ◽

Low Level ◽

The Pacific ◽

The Impact

Abstract This study discusses the impact of the Pacific–Japan (PJ) pattern on Indian summer monsoon (ISM) rainfall and its possible physical linkages through coupled and uncoupled pathways. Empirical orthogonal function analysis of 850-hPa relative vorticity over the western North Pacific (WNP) is used to extract the PJ pattern as the leading mode of circulation variability. The partial correlation analysis of the leading principal component reveals that the positive PJ pattern, which features anticyclonic and cyclonic low-level circulation anomalies over the tropical WNP and around Japan respectively, enhances the rainfall over the southern and northern parts of India. The northwestward propagating Rossby waves, in response to intensified convection over the Maritime Continent reinforced by low-level convergence in the southern flank of westward extended tropical WNP anticyclone, increase rainfall over southern peninsular India. Meanwhile, the anomalous moisture transport from the warm Bay of Bengal due to anomalous southerlies at the western edge of the low-level anticyclone extending from the tropical WNP helps to enhance the rainfall over northern India. The atmospheric general circulation model forced with climatological sea surface temperature confirms this atmospheric pathway through the westward propagating Rossby waves. Furthermore, the north Indian Ocean (NIO) warming induced by easterly wind anomalies along the southern periphery of the tropical WNP–NIO anticyclone enhances local convection, which in turn feeds back to the WNP convection anomalies. This coupled nature via interbasin feedback between the PJ pattern and NIO is confirmed using coupled model sensitivity experiments. These results are important in identifying new sources of ISM variability/predictability on the interannual time scale.

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Local Ocean-Atmosphere Interaction in Indian Summer Monsoon Multi-Decadal Variability

10.21203/rs.3.rs-788323/v2 ◽

2021 ◽

Author(s):

Dhruba Jyoti Goswami ◽

Ashok Karumuri ◽

Bhupendranath Goswami

Keyword(s):

Summer Monsoon ◽

Indian Summer Monsoon ◽

Large Scale ◽

Decadal Variability ◽

Strong Association ◽

Tropical Indian Ocean ◽

Bjerknes Feedback ◽

Low Level ◽

Increasing Trend

Abstract The significant multi-decadal mode (MDM) of the Indian summer monsoon rainfall (ISMR) during the past two millennia provides a basis for decadal predictability of the ISMR and has a strong association with the North-Atlantic variability with the Atlantic Multi-decadal Oscillation (AMO) as a potential external driver. It is also known that the annual cycles and interannual variability of ISMR and sea surface temperatures (SST) over the tropical Indian Ocean (IO) are strongly coupled. However, the role of local air-sea interactions in maintaining or modifying the ISMR MDM remains unknown. A related puzzle we identify is that the IO SST has an increasing trend during two opposite phases of the ISMR MDM, namely during an increasing phase of ISMR (1901 to 1957) as well as a decreasing phase of ISMR (1958-2007). Here, using a twentieth-century reanalysis (20CR), we examine the role of air-sea interactions in maintaining two opposite phases of the ISMR MDM and unravel that the Bjerknes feedback is at the heart of maintaining the ISMR MDM but cannot explain the increasing trend of SST in the tropical IO during the opposite phases. Large-scale low-level vorticity influence on SST and net heat flux changes through circulation and cloudiness changes associated with the two phases of the ISMR MDM together contribute to the SST trends. The decreasing trend of low-level wind convergence during the period between 1958 and 2007 is a determining factor for the decreasing trend of ISMR in the backdrop of an increasing trend of atmospheric moisture content. Consistent with the lead of the AMO with respect to ISMR by about a decade, the AMO drives the transition from one phase of ISMR MDM to another by changing its phase first and setting up low-level equatorial zonal winds conducive for the transition.

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