Weather Bee: Dry weather, record-low snow pack kept northern India warm in Oct

The month of October is almost over. While temperatures are expected to decrease in the month, this has not happened to the extent it should. Both maximum and minimum temperatures average warmer than normal for the month up to October 28.
Why is this the case? An unusually dry October is one reason, as HT had pointed out around the middle of the month. However, that may not be the only reason. Even a dry October could turn cool at night if there was snow fall in the mountains in the north. However, satellite data shows that snow pack in the northern states has hit fresh lows this season.
The temperature trends at the end of October are largely similar to what HT had reported around the middle of October. Temperatures are running warmer than normal (considered as the average temperature in the 1981-2010 period, according to the India Meteorological Department or IMD) in the northern half of India. As the accompanying maps show, both maximum and minimum temperatures are warmer than normal for almost the entire northern half of India.
However, it is useful to note some differences in the regional trends of the maximum and minimum temperatures. Parts of Bihar and West Bengal in the east and Gujarat and Rajasthan in the west (which can be counted in the northern half of India) are cooler than normal by maximum temperature but warmer than normal by minimum temperature. The divergence of these states from the rest of northern India is useful for finding the reason behind the temperature trends (more on this later).

The reason for warmer than normal temperatures in October in northern India appears to be dry weather. As the 1971-2020 average (this is the Long Period Average or LPA used by the IMD currently for tracking rain’s performance) for the first 28 days of the month shows, northern half of India receives some rain in October, although this is the rainy season largely for peninsular India. In contrast to the LPA, most of northern India has received no rain this October.

The reason for a dry October in northern India is a lack of western disturbances, which are loosely defined as storms originating in West Asia or sometimes just West of India. To be sure, parts of states like Bihar, West Bengal, Rajasthan, and Gujarat have received some rain. This is because of cyclone and cyclone-like activity in the Arabian Sea and the Bay of Bengal, a source of rain in the month for these states. Unlike these states relatively closer to the coast, the more inland states largely rely on western disturbances in October for rain. Since there haven’t been any western storms this month, most inland states have been almost completely dry.
The rain in the eastern and western ends of northern India also explains the divergence in their temperature trends with the rest of northern India. Cloudy weather does not allow much sunlight during the day and deceases the maximum temperature. However, clouds also prevent the heat accumulated during the day from being radiated back and increase minimum temperatures.
The relation between rain and temperatures described above also suggests that lack of rain is not the only factor in temperature trends in the rest of northern India. Lack of rain and clouds should have kept minimum temperatures close to normal here, which has not happened. In fact, minimum temperatures appear to have gone above normal more than maximum temperatures.
The explanation for the puzzle described above could lie in the extreme north. The reason for cooling in the northern plains in October is not just the rain that falls directly there, but also the rain and snow in the extreme north. October – past averages show – is the beginning of the period when the snow pack in the northern Himalayas starts building, another process dependent on western disturbances.
However, the lack of western disturbances combined with an already low-running snow pack from the previous snow cycle means that the current snow pack season has begun with the lowest pack since 2000, the earliest year for which this data is available. This means that winds blowing from northern India are unlikely to be as cool as they used to be.
The sum of the arguments presented here is that two of the usual factors that decrease temperatures for northern India in October are missing in action. This leaves only the southward movement of the sun in the sky. The temperatures of the past month suggest that this is not enough to cool temperatures to usual October levels in a rapidly warming world.