Subseasonal prediction, climate variability & change, and stratosphere-troposphere coupling
The most commonly-used diagnostic of the strength of the stratospheric polar vortex is the zonal-mean zonal wind at 10 hPa (~30 km) and 60°N (U10-60), which is westerly during winter. It is an easy diagnostic to compute and understand, which probably helped drive its uptake. Reversals of U10-60 to easterlies indicate either a major sudden …
Continue reading "The importance of the polar vortex at 100 hPa"
Only a few years ago, ECMWF's then-twice-weekly 51-member extended-range forecasts were not publicly available -- something that is almost hard to comprehend nowadays, as we have daily, 101-member ensemble forecasts available for free on the ECMWF website. The ensemble size is spectacular, and increases forecast reliability. But I'm more interested in what we gain from …
Continue reading "What’s that coming over the hill, is it a weak vortex?"
The title might seem rather obvious - or a surprising statement coming from me! But what I seek to achieve in this short blog post is provide a simple reminder that historically cold weather has indeed happened without a sudden stratospheric warming (or significant stratospheric polar vortex disruption). The role of the stratosphere can sometimes …
Continue reading "There’s more to the UK winter than the stratosphere"
The stratospheric polar vortex is currently at record-strong levels, based on the metrics of 10 hPa 60°N zonal-mean zonal-wind and 60-90°N average temperature. This is likely to be due to a combination of the timing and duration of the major Sudden Stratospheric Warming (SSW) in January: the duration of easterlies (most of January) shielded the …
24 January 2009. This was the 'central date' (defined as the day on which the 10 hPa 60°N zonal-mean zonal wind reverses from westerly to easterly) of a remarkable, record-breaking major sudden stratospheric warming event. There are several reasons why this event is worth a revisit. The January 2009 SSW set a large number of …
Non-downward propagating SSWs? Major stratospheric sudden warming events (SSWs) attract widespread attention because they are now known to have significant impacts on the tropospheric circulation (e.g. Baldwin and Dunkerton 2001, hereafter BD01). Anomalies in the stratospheric circulation (often expressed as the Northern Annual Mode (NAM) index, or polar cap geopotential height anomalies) propagate downwards through …
I study the stratosphere, the layer of atmosphere above the troposphere that extends from about 10-50 km above the surface. Some people might joke (I hope) that "nobody cares about the stratosphere", primarily because, unlike the troposphere, it contains no 'weather' in the traditional sense. Being very dry (...literally), it can't be seen on visible …