Influence of modes of climate variability on the
Influence of modes of climate variability on the Northern Hemisphere midlatitude winter precipitation Asiya B 1 S* and Prasanta 1 Sanyal Department of Earth Sciences, IISER Kolkata, Mohanpur-741246, India *E-mail: abs 15 ms 027@iiserkol. ac. in Introduction • Western disturbances are atmospheric disturbances that originate in the North Atlantic-Mediterranean region, and travel towards India, and cause precipitation throughout the path. • The Western Disturbances are embedded in the mid-latitude Subtropical Westerly Jet (SWJ). • Nature of the path of these SWJ depends on the existing temperature contrast, pressure gradient and Coriolis force. • Modes of climate variability (MCV) are regular patterns which vary over a large range of space and time scales, & their fluctuations can impact global and regional temperature as well as pressure variations. Aim • To clearly understand the mechanism of the winter precipitation in the mid-latitudinal region between North Atlantic and Himalayas. • To study the role of Air Temperature (AT) and the Mean Sea Level Pressure (MSLP) in causing precipitation over this region. • To couple this extensive precipitation(PRE) event with the Western Disturbances (WDs) Indian precipitation. • To study Influence of modes of climate variability on this winter precipitation. Data & Methodology • CRU TS 4. 01 has been used for precipitation. • NCEP reanalysis for Specific humidity (Shum), MSLP, AT, zonal and meridional wind speed have been used. • Had. ISST has been used for SST • Atlantic Multi-decadal Oscillation (AMO) index is taken from https: //www. esrl. noaa. gov/psd/gcos_wgsp/Timeseries/AMO/ • Pacific Decadal Oscillation (PDO) index is taken from http: //research. jisao. washington. edu/pdo/PDO. latest. txt. • Oceanic Nino Index (ONI) index is computed using SST anomaly at NINO 3. 4 region. • North Atlantic Oscillation Index (NAO) index is computed by finding MSLP difference between Icelandic Low and the Azores High. • Moisture flux = Temporal variation –A comparison Western Disturbances precipitation a) Figure 1. (a) CRU Precipitation amount (mm/year) with moisture flux(kg/m/s) averaged from 1901 to 2016 (b) Percentage precipitation contributed by DJFM precipitation a) b) West Mediterranean region Indian Region b) a) c) b) c) d) Figure 2. (a) Upper tropospheric temperature profile in 0 C (b) corresponding MSLP amount in h. Pa (c) corresponding upper tropospheric wind pattern for four arbitrary months(DJFM, 1948 -2016). Each parameter exhibits a different pattern during every month. a) c) d) f) • e) b) Figure 3. (a) Correlation of wind and AT at 350 h. Pa (b) Correlation of wind and AT at 550 h. Pa (C) Correlation of PRE and AT at different pressure levels (meridionally averaged) (d) Correlation of wind and MSLP at 350 h. Pa (e) Correlation of wind and MSLP at 550 h. Pa (f) Correlation of MSLP and PRE. All the contours represent 95% significant correlation. Spatial correlations a) c) b) d) Figure 5. Time series of PRE at Indian region and West Mediterranean region with different climate variability modes such as (a) NAO (b) AMO (c) ONI (d) PDO in a 16 months running average. Figure 4. Correlation of PRE with (a) NAO (b) ONI (c) AMO and (d) PDO. All the contours represent 95% significant correlation. Summary Conclusion In the study region throughout the period, there is a variation in MSLP and AT. It deviates the path of SWJ every month, causing a variation in the precipitation amount received at each grid. • These variations either deflect the SWJ away from water bodies causing a lowering of PRE in some regions or attracts the moisture carrying winds, causing an increase in PRE. • As the interplay of several MCV drives the regional MSLP and AT variations, they have a different correlation with PRE at various places. References • Bjerknes, Jacob. "A possible response of the atmospheric Hadley circulation to equatorial anomalies of ocean temperature. " Tellus 18. 4 (1966): 820 -829. • Dimri, A. P. (2013), Relationship between ENSO phases with Northwest India winter precipitation. Int. J. Climatol. , 33: 1917 -1923. • Dimri, A. P. , et al. "Western disturbances: a review. " Reviews of Geophysics 53. 2 (2015): 225 -246. • Pisharoty, P. R. , and B. N. Desai. "Western disturbances and Indian weather. " Indian J. Meteorol. Geophys 7 (1956): 333 -338. Acknowledgement I want to express my gratitude to Dr. Prasanta Sanyal, Mr. Sarthak Mohanty and all Silika lab members for their keen guidance. I would like to thank NOAA, NCEP, CRU, & Hadley centre for providing me data for which my research has been possible.
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