Warm Episode over Indian Sub-continent March 2010 By Richard Grumm National Weather Service State College Contributions by Lance Bosart SUNY Albany

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1 Warm Episode over Indian Sub-continent March 2010 By Richard Grumm National Weather Service State College Contributions by Lance Bosart SUNY Albany 1. INTRODUCTION A massive subtropical ridge formed over western Asia in mid-march 2010 (Fig. 1) and moved over the Indian sub-continent. The high temperature at New Delhi (VIDP), India, approached or exceeding 38C (100 F) for a period of about 10 days. New Delhi is normally a warm location with daily highs around and in excess of 45C or greater observed frequently from mid-april through early June. Features of note with this early season warm episode included: An anomalously 500 hpa ridge with height anomalies in excess of 3 standard deviations above normal A strong subtropical jet over northwest India and Pakistan over the ridge Above normal 850 hpa temperatures with 2 to 3SD above normal anomalies over northwestern. These data are of no value over Tibet. A close 22C contour over much of western India over a 5 day period. The surge of warm air into the region may be related to the positive phase of ENSO when the tropics are typically quite warm. This may produce a stronger than normal sub-tropical jet (STJ) which may be related to surges of the larger subtropical anticyclone poleward. This may produce some of these anomalous surges of warm air and a strong STJ into higher latitudes. This note shows some of the key anomalies associated with the Indian warm episode of March METHODS The Japanese reanalysis data was used to get the daily and the time-averaged pattern over the Indian sub-continent. Anomalies were computed using these data and the NCEP Global re-analysis climatological data. All anomalies here are shown as standardized anomalies. The time-averaged fields were computed using all 6-hourly data from UTC daily. For brevity times are stated as day and hour such that 15/1200 UTC implies 15 March 2010 at 1200 UTC. 3. RESULTS Figure 1 shows the 5-day time-averaged fields for the period of 15/1200 UTC. The key features include the large 500 hpa ridge with large height anomalies over Afghanistan and the Himalayan plateau. The latter location should be used with caution. The key feature in the mean is the 500 hpa ridge and 500 hpa height anomalies (Fig. 1a) and the 20 to 22C 850 hpa temperature and anomalies beneath the 500 hpa ridge (Fig. 1b). During this period the pressure and PW anomalies were not significant with only a hint of above normal PW air along the West Coast of India (Fig. 1d). The evolution of the ridge with daily data is shown in Figures 2-6. At 15/1200 UTC the center of the ridge was the Persian Gulf and the warmest air at 850 hpa was well to the west with some 1SD above normal air at 850 hpa over northwestern India (Fig. 2b). The ridge slowly drifted to the east on 16/1200, 17/1200 and 18/1200 UTC (Figs. 3-5). Above normal PW air surged along the West Coast and into western India on the 18 th (Fig. 5d).

2 Figure 1. The time-averaged pattern and standardized anomalies over India for the period of 1200 UTC 15 to 1200 UTC 20 March Data shown include a) 500 hpa heights (m) and height anomalies, b) 850 hpa temperatures (C ) and anomalies, c) mean sea-level pressure (hpa) and anomalies and d) precipitable water (mm) an anomalies. Over India and south of the Himalayas, the 500 hpa ridge peaked in intensity on the 20 th and 21 st (Figs. 6-7). By 22/1200 UTC and after the 500 hpa heights returned toward more normal values and the 850 hpa temperatures trended downward suggesting this surge was ending. Figure 9 shows the mean pattern from 20/1200 to 26/1200 UTC. These data show the height anomalies focused over northern India with no 5880 m contour and above normal 850 hpa temperatures. Unlike the earlier part of the episode, the PW values were above normal during the latter period. The 200 hpa winds and STJ are shown in Figure 10. These are daily images showing the jet going over the ridge from 15/1200 UTC through 23/1200 UTC. On the northern edge of the ridge, a strong westerly STJ was present with 3 to 5SD above normal westerly within the core of the STJ. In the weak height gradient about the ridge, the winds were lighter than normal from the Arabian Peninsula eastward to India and into southwestern Asia. The weaker than normal winds persisted over India and moved to the south with time as the ridge built over the region. 4. CONCLUSIONS A large subtropical ridge built over southwestern Asia in mid-march This ridge produced warm weather over the region with an extended period of above normally warm temperatures over India. Some locations in India saw daily highs

3 never 40F for an extended period of time. In a country known for warm weather, this surge of warm air in mid-march was about a month ahead of the normal surge of persistent warm weather over the region. The subtropical ridge was a key player in this early season warm episode. A strong subtropical ridge with a strong STJ on its northern edge were associated with this early season warm up.

4 Figure 2. The pattern over India at 1200 UTC 15 March 2010 showing the field and standardized anomalies over India for the period of 1200 UTC 15 to 1200 UTC 20 March Data shown include a) 500 hpa heights (m) and height anomalies, b) 850 hpa temperatures (C ) and anomalies, c) mean sea-level pressure (hpa) and anomalies and d) precipitable water (mm) an anomalies.

5 Figure 3. As in Figure 2 except valid at 1200 UTC 16 March 2010.

6 Figure 4. As in Figure 2 except valid at 1200 UTC 17 March 2010.

7 Figure 5. As in Figure 2 except valid at 1200 UTC 18 March 2010.

8 Figure 6. As in Figure 2 except valid at 1200 UTC 20 March 2010.

9 Figure 7. Figure 2 except valid at 1200 UTC 21 March 2010.

10 Figure 8. As in Figure 2 except valid at 1200 UTC 22 March 2010.

11 Figure 9. As in Figure 1 except for the period of 1200 UTC 20 to 1200 UTC 26 March 2010.

12 Figure 10. GFS 200 hpa winds and wind anomalies at 1200 UTC from a-i 15 to 23 March 2010.