Into the brine / PHOTO: Eleveight
In many parts of the northern hemisphere, spring has been a strange old time, staying much cooler than normal with lots of north and northeast winds. Apparently it has a lot to do with the jet streams and the arctic oscillation. iKitesurf.com meteorologist Shea Gibson tries to break this huge topic down into a manageable chunk here!
JET STREAM RELATIONS TO SURFACE WINDS AND WEATHER
WORDS: Shea Gibson Note: as usual there's a glossary of terms at the end of this feature (which we certainly need ourselves! - Ed) Most of us have heard of the term 'jet stream' and automatically associate it with strong winds way up in the atmosphere. There is much more to these wind and weather highways though, born from Earth's radiational heat and they ultimately reflect their presence by affecting surface winds. Notice There are actually two jet streams. We have both a polar jet stream and a subtropical stream in both hemispheres. The polar jet resides between ~50-60° latitude in both hemispheres, which represents polar air mass transport (cooler air). The polar jet tends to drive farther southwards in the cooler months and can drag the arctic air down with it at times. The subtropical jet stream typically resides around 30° latitude in both hemispheres and represents warm / moist air transport. The subtropical jet usually advances northwards in the warmer months and shows warmer air, increased moisture and flooding given the right ingredients. Conversely, it can also lead to generation of heat waves (more below) if too far north. There are times in the fall, winter and spring that the two jet streams meet closely together to create some very powerful winds aloft and instability at the surface. These are known as 'jet streaks' and can produce winds over 200mph between 30-52kft. When this happens, commercial airlines love to ride these superhighways across the Atlantic and Pacific as they gain quite a lot of forward speed. At other times there can be weaknesses where we do not see much action, hence weaker overall patterns with more stable conditions.
The two things the polar jet and the subtropical jet have in common are: 1) They both keep the transport of high and low pressure systems moving from west-to-east across the globe. This is due to the Earth's rotation, which induces the Coriolis force due to gravitational pull. This shows the counter-clockwise rotation of low pressure and clockwise rotation of high pressure that we are all familiar with in the northern hemisphere. 2) They can be influenced by teleconnection. This is where surges of energy 'upwind' across the globe can be felt 'downwind' in other countries or continents. For example, a surge of energy by a super typhoon in the western Pacific entering into (and being ripped apart by) the Pacific North Jet can re-energize over the US. This same batch of energy can then show the explosive winter Nor'easters (AKA bomb cyclone) off the northeast US coast. These can eventually transition across the Atlantic and bomb out again over portions of central/northern Europe, where storms are typically named due to their destructive force. So you see, these jet streams can work together to transport pockets of energy around the globe, much like skipping a rock.
MORE DETAIL OF HOW JET STREAMS AFFECT OUR SURFACE WINDS
As discussed in the last paragraph, these jet streams help transport high and low pressure systems. In the cooler months, low pressures can be stronger where the polar jet dips southwards and at times create strong upper lows (ULs) with one or multiple trailing strong cold fronts swinging across the surface below them. This forces winds to the surface in a southerly (convergent) flow and towards the centre in counter-clockwise fashion. Following on, the high pressure brings upper air down behind them with strong northwest or westerly surface flows (called divergence). Should the subtropical jet coincidentally ride upwards to meet the southern quadrant of the polar jet, we can see an even more powerful cut-off upper low develop to generate more powerful surface winds. In the warmer months, the polar jet recedes and the subtropical jet climbs up and over the higher mid-latitudes of the north Atlantic to allow for a strengthening of the Bermuda-Azores High, which expands pole-ward as a result. This behaviour can force ridging into France, up into Northern Germany and as far as southeastern UK. When we have multiple centres of high pressure in a region, this 'blocking pattern' domes out overhead and works drier, hotter air down over the areas underneath with weaker overall winds and the unwanted heat waves which are no good for kiters. There may also be instances where portions of Europe also benefit from the subtropical jet as it falls a bit south to bring a favourable pattern for moderate to strong SW/W/WNW sea breezes to accompany the local synoptics. Oh and don't forget: the overall jet stream across the Atlantic can be heavily influenced by El Nino and La Nina, but that is another article for another time!
WHY SUCH A STRANGE SPRING THIS YEAR?
Let's first put some focus on the polar jet, Polar Vortex and polar circulation. These all strengthen in the winter, weaken in the summer and have a stake in the weather that affects the UK/Northern EU region along with other equal latitudes around the world. They are all part of the same large-scale system and are tracked by a singular pattern index called the 'Arctic Oscillation' (AO). If the AO is negative (-AO), then we tend to have displaced cold air with wintery precip and chilly northerly winds over central and northern Europe. When the we have +AO, we tend to see warmer air ride up into central/northern areas with added storm activity and generally southerly winds. The point to all of this is we have seen some UP and DOWN swings in the AO, going from positive to negative and visa-versa. We saw the AO spike up into the positive mid-Feb 2021, drop down the negative early March 2021 and swing back up to near normal with slightly negative as of now in early May 2021. This is a likely reason we have seen such irregular patterns this spring. Stay safe and stay in the wind! Shea Gibson
RECAP OF TERMS:
JET STREAM: Upper air currents of at least 50 knots. Two types are the polar jet and the subtropical jet.
JET STREAK: When the polar and subtropical jets collide or combine to channel a very powerful pressure gradient (very strong winds).
CORIOLIS FORCE: An effect where Earth's rotation deflects moving objects to the right (east) in the northern hemi and left (west) in the southern hemi.
BOMB CYCLONE: The period of time when an upper low undergoes bombogenesis, or its peak intensity period. TELECONNECTION: Climate anomalies being related to each other at large distances, typically thousands of kilometres.
UPPER LOW: An area of Low pressure that forms at the mid level or higher altitudes of the atmosphere by warm air occluding from the cold air. BLOCKING PATTERN: A set-up where high or low pressure sets up for not allowing other systems through. Can last for short or long periods of time. iKITESURF INSIGHT:
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