A new Atmospheric wave is now active over the tropics, spreading its influence on the weather patterns across North America and Europe. It will even play a role in tropical storm development in the first half of the month.
Global weather is a very complex system, with thousands of different factors influencing its development on different scales. But regardless of time and place, the weather is globally connected into one extensive system.
We will look at one of these global weather factors, now active as an atmospheric wave in the tropics. You will learn what it is and how it plays its role in the weather development during the Summer when the otherwise strong Polar Vortex dynamics are generally absent.
SUMMER IS HERE
Meteorological Summer covers the 3-monthly period of June, July, and August. These are the warmest three months over the Nothern Hemisphere and represent the summer months also in the statistics.
Below we have the global temperature anomaly analysis for June 2022. We can see that the month was overall warmer over Europe and most of the United States and northern Canada. Colder than average temperatures were present over the western Arctic Ocean and Greenland.
In the mid-latitudes, we see less precipitation than usual in the eastern United States. The exception is southern Florida. More precipitation was over the northwestern United States and parts of central Europe.
The pressure pattern behind June has featured high-pressure zones over much of the United States and Europe. In addition, we can see the deep low-pressure zone over Greenland and a high-pressure area expanding from Aleutians to northern Canada.
One of the main factors behind the pattern this Summer is the La Nina. We can see it as an area of cold ocean anomalies in the tropical Pacific Ocean. We will release a special update on the La Nina and its expected influence on the upcoming cold season 2022/2023.
But while the La Nina is a low-frequency background influence, we have many shorter-period atmospheric waves also emerging from the tropics.
A lot of the global variability is driven by invisible tropical “waves” in the atmosphere. The largest and most dominant source of short-term variability from the tropical regions is the Madden-Julian Oscillation wave, known simply as MJO.
MJO is an eastward-moving disturbance of thunderstorms, clouds, rain, winds, and pressure anomalies. It moves across the Earth on the equator in about 30 to 60 days.
It also has the power to influence the weather patterns further north over the North Hemisphere, as there is a strong connection between the tropics and the global weather.
The MJO consists of two parts: one is the enhanced rainfall (wet) phase, and the other is the suppressed rainfall (dry) phase. The graphic below from NOAA Climate shows two main components of this wave: increased storms and rainfall (lower pressure) and reduced storms and drier weather (higher pressure) on the other side. Image by NOAA Climate.
We can see here that on the top, the air is diverging (moving away) over the wet phase and converging (moving together) over the dry phase. This horizontal movement of air is called the Velocity Potential in the tropics.
How do we track these MJO waves? First, we track the MJO by looking at these larger-scale air movements and the areas where the air rises and subsides.
The graphic below shows exactly that. The horizontal movement of air in the upper levels (~12-14km), where cold colors indicate lower pressure and wet weather, and warm colors show drier weather with fewer clouds and precipitation.
You can see that the movement of the wave is arranged into phases. Each can have a different influence on the weather patterns, so we need to keep track of how it is moving around the globe. This way, we can easily define where the wave is and what influence it brings along.
Monitoring of the MJO is critical during the Atlantic Hurricane Season. That is because the wet/enhanced phase of the MJO wave can provide favorable conditions for tropical systems to form and intensify over the Atlantic Ocean.
Below we have a great visualization of the MJO phases as it moves across the globe. The animation shows cloud patterns in different phases during the wave activity. For example, blue means more clouds and rainfall, and brown areas are drier with fewer clouds.
We have also produced a video animation showing the global atmospheric moisture as precipitable water to put everything into perspective. We can see how the tropical regions connect with the mid-latitudes, creating “atmospheric rivers” and connecting to the global weather system.
AN MJO WAVE EMERGES
Looking at the latest analysis, we can see the two phases (wet/dry) of the MJO wave. The convective/wet phase is currently over Indonesia and Australia. The dry/suppressed phase is currently over the Americas and the Atlantic Ocean. Looking at the diagrams above, this corresponds to the phase 4 pattern.
But looking a week ahead, we can see an evolution of the wave. The wet/enhanced phase is strong over the western Pacific Ocean. You can see the dry/suppressed phase is forecast over the eastern Pacific and the Atlantic Ocean.
MJO WEATHER FORECAST
We now know what the MJO wave is and how it moves across the globe. Below is an ECMWF ensemble forecast of the MJO phases on a diagram. It is quite easy to read, as we can see in which phases it is moving and on which day.
We began the month in phase 4, moving into 5 for the first half of July. The amplitude is forecast to reduce into the second half of the month, meaning a reduced influence of the MJO wave on the weather patterns.
We will now look at the weather pattern forecast for the next two weeks, looking for the MJO signal from the Pacific Ocean to the Atlantic.
GROWING WEATHER IMPACT
Below we have the 1-week forecast from GFS, filtered for MJO. We can see the convective phase of the MJO progressing into the western Pacific Ocean. The dry phase is forecast to move over the Atlantic Ocean, keeping the tropical activity low, and progress into phase 5 of the MJO.
Below is the Phase 5 pressure anomaly composite for the Summer season. It suggests a high-pressure ridge in the eastern United States, northern Canada, and western Europe. We also have a low-pressure system over the western United States and western Canada. NAO is in the negative mode.
These composites use historical data for each phase in different seasons. It aims to uncover a signal that each MJO phase produces at a certain time of the year. Think of it as guidance on what to expect influence-wise from these waves.
Looking at the pressure anomaly forecast for early July, we can see the high-pressure zones over western Europe and the United States. The low-pressure area is over eastern Canada, the northeastern United States, and north and central Europe. Overall we can see the common elements of the MJO phase 5 influence.
Looking at the temperature anomalies, we can first see a cold anomaly in the Arctic, thanks to the strong low-pressure system. But that is unrelated directly to the MJO. Next, we can see a cooler airmass spreading from Scandinavia into central Europe.
Looking at North America, we can see a strong contrast over Canada between the cooler east and warmer west. Over the United States, temperatures remain mostly warmer than normal, except for the northeastern United States and parts of the northern United States.
Precipitation-wise over North America, we can see more rainfall over the Midwest and the eastern United States. Over the western United States, we have normal precipitation and drier conditions over the southern and southeastern parts of the country.
Looking at the development in the Tropics, we can see no activity in the near future. This is because the suppressed phase of the MJO is moving over the Atlantic Ocean. That limits any significant tropical storm development, at least in the first half of the month.
6-10 DAY OFFICIAL WEATHER OUTLOOK
NOAA also regularly releases its weekly forecasts for the United States. Below is the temperature forecast in the 6-10 day range, which covers early July. This forecast calls for warmer than normal weather over much of the central and southern United States.
We also have the official precipitation outlook below, which calls for more precipitation over the northern and eastern United States. Drier weather conditions are forecast over the western and southern United States.
MID-MONTH WEATHER PATTERN
Looking at the mid-mont, the MJO will weaken with the convective phase over the eastern Pacific. The suppressed phase slowly progresses out of the Atlantic Ocean and over Africa. This is not yet a supportive pattern for tropical storm formation.
The ECMWF pressure forecast for this period shows the low-pressure area over the eastern United States and a ridge in the west. A similar east low/west high pattern is over Europe. A strong low-pressure anomaly remains over the Arctic regions.
The 850mb temperature anomaly for mid-month shows warmth returning over central Europe as the high-pressure zone expands. Warm anomalies continue over Canada, while a cold anomaly drops down into the central and eastern United States.
Below we have a surface temperature anomaly forecast over North America. The forecast calls for colder than normal air spreading into the Midwest and reaching down towards the southeastern United States. Hotter conditions prevail over the western United States.
The cooler air is also drier, as seen by the drier anomalies over the Midwest and the Ohio valley. More precipitation is expected over the southern United States.
NOAA 8-14 DAY WEATHER OUTLOOK
This process is captured on the official NOAA temperature outlook in the 8-14 day timeframe. NOAA is forecasting cooler than normal conditions over parts of the eastern United States. Conversely, warmer weather prevails over the western and southern United States.
NOAA precipitation outlook also calls for wetter conditions over the southern United States. However northern half of the country is forecast to receive less precipitation than normal due to the cooler air also being direr.
EXTENDED RANGE WEATHER PATTERNS
Looking further into the second half of July, we will use the ECMWF extended ensemble forecast.
The pressure pattern forecast for week 3 shows a high-pressure area remaining over Canada. That hints at possible lower pressure over the eastern United States. In addition, a low-pressure area is over Scandinavia, while a ridge expands over the rest of Europe.
The temperature forecast for Europe shows strong warm anomalies under high pressure. This hints at a likely significant heatwave over central Europe, with high temperatures and little precipitation. In the meantime, we see colder anomalies over far northern Europe under low pressure.
The temperature forecast for North America shows cooler than normal temperatures remaining mostly in the eastern United States. However, warmer anomalies are expected to remain in the western and southern United States and Canada under high pressure.
Precipitation-wise, more rainfall is expected in southern and far eastern the United States. Drier conditions prevail over the Midwest and the southwestern United States.
Looking at the tropical development in this range, we have a low probability of tropical storm development. Some indication is shown along the east coast of the United States, but that is more likely a product of the false interpretation of low-pressure systems.
LATE MONTH EXTENDED TRENDS
The pressure pattern forecast for late July shows continued high pressure over Canada and a likely low-pressure response in the eastern United States. This would promote a more northerly flow into the eastern United States and warmth in the west. In addition, a high-pressure zone remains over Europe.
Looking at the temperature forecast for Europe in this period, the warm anomalies over the central parts persist. Therefore, this is a likely continuation of the heatwave.
Looking at the temperature forecast for North America, we see the warm anomalies expanding and covering much of the western and central United States. However, a neutral to cool anomaly remains over the southeastern part of the country, linked to a likely low-pressure area.
The precipitation forecast shows more precipitation trending in the eastern part of the country. Drier conditions prevail in the western United States.
Also, looking at the tropical forecast, we see little in the way of any significant development. But at this range, resolving individual storms in an ensemble system is complicated. The MJO suppressed phase will move out of the Atlantic in this range, so late July or early August can be a favorable period for new tropical activity.
We will move from extended to long-range, looking at the latest seasonal trends for the rest of the warm season.
WARM SEASON 2022 OUTLOOK
The model of choice is the ECMWF, which is one of the most reliable models for long-range forecasting. Of course, in reality, a lot can change in each individual year/season. But generally, the ECMWF model is at the top regarding ” reliability “.
But no long-range/seasonal forecasting system can be called reliable. We are only forecasting trends and how the weather patterns are evolving on a large scale and over longer time periods.
The forecast period we will focus on is July-August-September (JAS 2022). This period covers the rest of the meteorological Summer and the end of the warm season.
In the pressure pattern forecast from ECMWF below, we can see a persistent high-pressure system extending into the northwestern United States and northern Canada. A low-pressure area is forecast to settle over northwestern Europe.
A secondary high-pressure area is found over the northeastern United States. It will have a regional effect on the weather development in the eastern United States and eastern Canada.
The global temperature distribution follows this pattern. Over North America, we see warm peak anomalies over the central and northwestern United States and central Canada. That is the warm air mass under the high-pressure anomaly.
Over Europe, we see a neutral area over northwestern regions under the low-pressure zone. Warmer than normal temperatures continue over the southern half of Europe under the high-pressure zone.
The global precipitation forecast shows drier conditions over most of the central and northern United States. But parts of the southwestern and eastern United States and eastern Canada have a higher chance of wetter conditions.
Over Europe, we see drier than normal conditions over much of the southern half of Europe. Conversely, wetter conditions prevail over northern Europe under the influence of the low-pressure forecast zone.
The NOAA official Summer temperature outlook shows that most of the United States are warmer than normal. The main warm anomalies are focused on the western half of the United States. Another warm zone is in the northeast, under the secondary high-pressure zone.
The official Summer precipitation forecast is quite similar to the model forecast. We have an equal-to-higher probability for more precipitation in the eastern United States and over parts of the southwest. But most of the northwestern and central United States is forecast to have a drier rest of the warm season.
The problem with low precipitation is typically the persistence of drought conditions in the southern and western United States. Below we have the latest drought analysis from NOAA, which shows the current drought conditions across the United States.
Most of the western half of the United States is already under some drought conditions. The driest conditions prevail in the southern United States. Over the southwest, the monsoon will alleviate some of the drought conditions, while the drought conditions in the southern United States prevail.
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