Abnormal Tides

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19 Sep to 12 Oct 2017 Water levels from Bermuda Esso Pier, St. George’s Island. Verified water level observations in green, preliminary water level observations in red. Forecast water level due to the predicted astronomical tide in blue. Water level is with respect to mean higher high water, where positive values indicate normally dry land is inundated. Alternative reference water level marks are on the right.

Over the weekend, the combination of the spring tide and a high amplitude ocean eddy resulted in localized coastal flooding around low-lying areas of Bermuda. Tides were running around 1.5 ft above expected levels which were already higher than normal thanks to a spring tide.

The role of the Astronomical Tides:

The astronomical tides are driven primarily by the gravitational effects of the Moon on the ocean. When the Moon is directly overhead, the water rises in response to the Moon’s gravitational pull. When the Moon is directly underfoot, the water rises again to balance the pull of the Moon on the opposite side of the Earth.

During “Spring” tides, the gravitational pull of the Sun on the oceans acts in the same direction as that from the Moon. This results in higher than normal tides and tidal ranges. Conversely, during “Neap” tides, the gravitational pull of the Sun is acting perpendicular to that of the Moon and lower than normal tides and tidal ranges can be expected. Looking at the blue line in the above figure, higher tides associated with the Spring tide can be seen around the 20th September and again last weekend, while lower tides associated with Neap tide can be seen around the 28th September.

Additionally, the Sun and Moon have to be aligned in space for their gravitational pull to act in the same direction. This manifests as a New Moon when the Moon is between the Sun and the Earth, and a Full Moon when the Earth is in between the Sun and the Moon. Both New and Full moon are associated with Spring tides. The Lunar cycle (including one Full and one New Moon) repeats roughly every 29 days and so you can expect a Spring tide a little more than every fortnight.

[More on Spring and Neap Tides]

Finally, the Moon follows an elliptical orbit around the Earth and so is closer or further away twice per orbit. Every ~7.5 Spring tides, the moon reaches its closest distance to Earth during a New or Full Moon. When the Moon is closer to Earth (perigee), the tides are slightly higher than normal. The opposite is true for when the Moon is furthest from Earth (apogee). Tides during last weekend’s Spring tide were higher than the 20th September’s Spring tide because the Moon was near/at perigee last weekend, and not during the 20th September.

The role of Ocean Eddies:

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Sea surface height anomalies showing a positive anomaly greater than 30 cm near Bermuda (circled in black). This was associated with an anticyclonic eddy that intensified as it tracked southwestward toward Bermuda over the last two months.

Ever present in the ocean, eddies can manifest as regions of higher (positive) or lower (negative) sea surface height anomalies. The flow around these sea surface height anomalies is often close to balanced and so they can persist for a long time as they track across the ocean surface. These anomalies are typically small, less than 30 cm.

Typical flow around a positive sea surface height anomaly is clockwise (anticyclonic), and counter-clockwise (cyclonic) for a negative sea surface height anomaly in the northern hemisphere.

Over the weekend, a positive sea surface height anomaly associated with an anticyclonic eddy was tracking near Bermuda with amplitude estimated to be more than 30 cm (1 ft) via satellite measurements. Coinciding with the spring tide and Lunar perigee, this resulted in abnormally high water levels and some coastal inundation.

See some media mention of the tides here: Royal Gazette; Bernews (1); Bernews (2)

With sea level rise associated with climate change, it is reasonable to expect this mostly nuisance level of inundating events to occur more frequently as water level anomalies don’t have to be as extreme for flooding to occur.

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Hurricane Nicole Post Storm Report Released

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NASA Terra/MODIS True Color satellite imagery of Hurricane Nicole near the time of closest point of approach 13 Oct 2016.

After each season, the National Hurricane Center prepares ‘post-storm reports’ on each tropical cyclone that formed in the North Atlantic and Eastern Pacific basins. These reports include data collected in real-time that may not have been available during operational analysis, and therefore can sometimes lead to revision of track or intensity. This was the case for 2014’s Hurricane Fay.

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Time trace of wind gusts (red ‘x’) and wind directions (green ‘triangles’) from the airport reproduced from the NHC report. We see the sharp decrease in wind gusts and sharp wind shift as the calm eye of Nicole crossed.

Today, the National Hurricane Center released its report on 2016’s Hurricane Nicole. Their analysis on observations from Bermuda suggest that widespread category one conditions occurred on the island with isolated areas seeing category two conditions. Aircraft reconnaissance measurements near the time of closest point of approach to Bermuda indicate that Nicole was still a category three hurricane with maximum sustained winds near 105 kts (120 mph). The island received impacts from the left-front quadrant of Nicole’s eyewall, missing these strongest winds (located in the right-front quadrant) and thus this was classified as a strike.

[strike, direct-hit, indirect-hitlandfall – Impact Terminology Defined]

Hurricanes are centers of extreme low pressure. Winds spiral inwards, towards centers of low pressure and in the northern hemisphere, this manifests as a counter-clockwise circulation.  Observations from Bermuda indicated that winds backed (turned counter-clockwise with time) from an easterly direction to a northerly, and then northwesterly direction suggesting that the center of circulation remained to east of the island and therefore did not make landfall, despite Bermuda entering the calm eye of the hurricane.

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Bermuda SRI Radar imagery showing steady and heavy rain rates persisting over Bermuda (red ‘+’) for hours before the drier eye and southwestern quadrant passed over the island. Reproduced from the NHC report.

Nicole’s impacts on Bermuda were also remarkable in that the hurricane made for one of the top-5 wettest meteorological days on record at the airport, and the Bermuda Weather Service was able to release a weather balloon in the eye that measured the highest precipitable water here since 1973 at 2.93″.

[What is Precipitable Water?]

Notable for an erratic early track and meteorological evolution (including two periods of rapid intensification), Hurricane Nicole will go down in the record books as the fourth early-October hurricane impact on Bermuda in three years. Once Nicole passed Bermuda, the cyclone underwent a complex transition into a powerful extratropical cyclone in the North Atlantic where it continued to produce storm force winds for several days.

Current Weather:

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Wind measurements at the Crescent, in the northern marine area via Bermuda Weather Service. Note peak in winds just before 9am 16 Feb 2017(sustained, to/gusts, middle).

Bermuda is now well into its ‘winter’ season where changeable weather associated with gales is commonplace. Yesterday’s cold front brought a brief warm and humid spell with southwesterly gales and thunderstorms that brought heavy rain to the island.

Locally severe wind gusts, mainly confined to the marine area, were also observed. The Crescent Buoy, in the northern marine area, measured a peak thunderstorm gust of 54 kts. Winds on island appear to have remained below severe levels (i.e. < 50 kts) with a peak gust of 43 kts measured at the airport prior to any thunderstorm activity.

Follow the Bermuda Weather Service for the latest official forecast, warnings, and observations for Bermuda.

Torrential January Rains

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Observations around Bermuda indicate a widespread 4-6″ of rain fell with isolated areas seeing >6″. Reports are sourced from Wunderground, WeatherLink, and Bermuda Weather Service.**Note that shaded areas on this map with fewer reports are less reliable.**

A deep long-wave upper level trough over the Eastern United States slowly edged eastwards on Thursday morning, this allowed a cold front to slowly advance towards and across Bermuda. Deep-layered flow out of the tropics allowed significant moisture transport across Bermuda. The frontal system, supported by upper level dynamics, was able to make use of that moisture in the form of an active band of heavy showers and thunderstorms that slowly progressed across the island, with a trailing region of light-moderate rains.

Rain totals for the meteorological day (0600 UTC to 0600 UTC) at the Bermuda Weather Service far exceeded several records, with 5.34″ of rain. This led to widespread flooding of low-lying and poor-drainage areas (see: Royal Gazette, Bernews). Furthermore, this rain total is roughly the amount of rain that Bermuda typically gets for the entire month of January. The 1981-2010 average January rainfall is 5.43″, or 5.30″ for 1971-2000 climate period.

Table: Records broken with yesterday’s rain, with reference to single, meteorological day records for the period 1949-present at Bermuda Weather Service.
Type of Record Previous Record
Record Wettest for the date 5 Jan 1.54″ (5 Jan 1994)
Record Wettest Jan Day 3.99″ (11 Jan 1986)
Record Wettest Winter (Dec-Jan-Feb) Day 3.99″ (11 Jan 1986)
5th Wettest Day (1.) 7.77″ (1 Jun 1996)
(2.) 6.77″ (13 Oct 2016 – Hurricane Nicole)
(3.) 6.21″ (31 Aug 1982)
(4.) 5.52″ (14 Jul 1980)
(5.) 5.34″ (5 Jan 2017)
(6.) 5.24″ (29 Oct 1967)

It is particularly impressive that this event made it into the top-5 wettest meteorological days because it occurred in winter. Rain events in winter typically produce lower totals for two main reasons, compared to summer events:

  1. these are typically associated with frontal systems that generally pass quickly
  2. there’s typically not as much atmospheric moisture available in winter

This event was associated with a frontal system, but it was progressing very slowly, and there was unusually high amount of atmospheric moisture available for rain because of the deep-layered flow from the tropics ahead of the system.

Model guidance performed well at picking up on the potential for a heavy rain event on the 5th since the end of December. This was reflected in the Bermuda Weather Service forecasts and forecaster’s discussion several days before the event.

More heavy rain is in the forecast

Yesterday’s front has progressed to the East of Bermuda, clearing its rainy weather with it. This is allowing much drier weather to settle in. However, an area of low pressure and frontal system, organizing over the lower Mississippi Valley today, will result in a similar frontal set-up to yesterday’s system over Bermuda at the end of the weekend.

Over the weekend, the low will push off the US East coast and track northeastwards around a long-wave trough over Eastern North America. As it does this, deep-layered southerly flow out of the tropics resumes ahead of a trailing cold front. This front will slowly progress eastwards toward Bermuda, passing on Sunday.

Model guidance is once again suggesting potential for a heavy rain event associated with this system. This is mentioned in the Bermuda Weather Service forecast and forecaster’s discussion. With soil freshly saturated, additional flooding in low-lying and poor-drainage areas is possible on Sunday.

A significant cool-down is then expected to start the work week. Temperatures are likely to struggle to reach 60°F on Monday as a continental polar airmass is drawn off of North America from the northwest across Bermuda.

See the Bermuda Weather Service for the latest official forecasts, warnings, and observations for Bermuda.

Tudor Hill Observatory

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Tudor Hill Marine Atmospheric Observatory (Credit: BIOS)

In a project commissioned by the National Oceanic and Atmospheric Administration’s (NOAA) Earth Systems Research Laboratory (ESRL) the Bermuda Institute of Ocean Sciences (BIOS) has been the cooperating agency responsible for measuring the concentration of key atmospheric atmospheric gas species at Tudor Hill in Bermuda. This is part of NOAA/ESRL’s Global Monitoring Division’s aim to track changes in these key gas species, particularly focusing on their sources, sinks, global trends, and distributions.

[About: Global Monitoring Division]

One of the more well known gases measured at these types of observatories, including Tudor Hill, is Carbon Dioxide. An infamous greenhouse gas emitted largely through the burning of organic matter (ie. fossil fuels), the increase in Carbon Dioxide can be seen even in the middle of the Atlantic at Tudor Hill, Bermuda.

Another interesting feature is the seasonal change in Carbon Dioxide concentration controlled mainly by the biosphere. Because there is more land in the northern hemisphere, the biosphere’s influence is disproportionally weighted to what is happening in the northern hemisphere. During northern hemisphere summer, there are more photosynthetically active plants taking in more Carbon Dioxide globally than during northern hemisphere winter.

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Measurements of concentration of Carbon Dioxide gas at Tudor Hill Bermuda since records began in 1989, including preliminary 2016 data in orange. Plotted with NOAA/ESRL’s interactive data viewer.

Attributing the trend in Carbon Dioxide to human activity is a little more complicated than just observing the trend. It comes through analysis of Carbon isotopes bonded in Carbon Dioxide. Carbon comes in two naturally occurring stable isotopes: Carbon-13 (13C) and Carbon-12 (12C). 13C has an additional neutron and therefore has slightly more mass and slightly different chemical properties.

It has been found that the biosphere (ie. plants) preferentially uptake the lighter 12C containing molecules during photosynthesis. This leaves behind less 12C in the atmosphere and so the ratio 13C/12C increases. The higher proportion of 12C in organisms is maintained even if they should become fossil fuels. When we burn organic matter (ie. fossil fuels) we release Carbon Dioxide with higher proportions of 12C into the atmosphere and the ratio 13C/12C measured in the air decreases as a result.

One way to measure the relative amount of the two stable Carbon isotopes is called “delta 13 Carbon” (δ13C). Here, the measured ratio 13C/12C is standardized by a reference ratio determined from reference research into the average properties of Carbon. That standardization is very close to one, and all the variations occur in the thousandths decimal. In practice, one is subtracted from it and then the result is multiplied by 1000.

A trend of decreasing δ13C is observed at Tudor Hill, Bermuda. This trend lends support to the idea that not only are atmospheric Carbon Dioxide concentrations increasing, but the increase might be due to human activity.

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All available delta thirteen C data for Tudor Hill, Bermuda. Records of this parameter are kept since 1991, including preliminary 2016 data points. Plotted with NOAA/ESRL’s interactive data viewer.

These trends and patterns are repeated across the world. It is clear that the upswing in Carbon Dioxide is not a local phenomenon, but a symptom of a global problem.

Many more projects, providing invaluable scientific insight rely on data like this collected at Tudor Hill in Bermuda and around the world. Bermuda has proved to offer a unique location to get continuous long time-series of the largely undisturbed samples of the low-level ambient marine atmosphere. As such, I expect research to continue or expand in Bermuda, particularly as issues such as increasing Carbon Dioxide concentrations become more pressing.

References:
Carbon Isotopes in Photosynthesis (Marion H. O’Leary, 1988)
NOAA/ESRL’s Global Monitoring Division
->Map of Observation Sites
->Tudor Hill’s Measurements via Interactive Data Viewer

Isolated Sunday Downpours

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Spatial distribution of unofficial rainfall reports around Bermuda yesterday, over 5″ of rain fell in some spots. Interpolated using data from WeatherUnderground, WeatherLink, and Bermuda Weather Service.

Late-morning, isolated showers popped up in the central parishes on Sunday dumping heavy rain from roughly 10am to 3pm with intermittent periods of thunder. The showers were largely stationary over the central parishes, at times spreading to the east. This left the western parts of the island mostly dry until isolated showers moved in from the west late in the afternoon.

While most places saw less than an inch of rain, where those showers and thunderstorms set up in the central parishes, unofficial rain totals of 2-5″ were reported. Light winds through much of the lower troposphere kept the showers that formed from moving much, and the heating of the island likely helped support the showers initiate.

A surface ridge to the south of Bermuda is maintaining light mostly westerly flow today. Meanwhile, a subtle surface trough embedded in that flow is helping to spark isolated showers and thunderstorms in the area.

Tropical Storm Fiona Approaches

Mid-August typically marks the start of the most active period for tropical cyclones in the Atlantic sometimes referred to as the Cape Verde Season. This part of hurricane season is so called because the upswing in activity in the Atlantic this time of year typically occurs from systems that form in the central and eastern Atlantic near the Cape Verde Islands. These type of systems are notorious for their long tracks that can favor stronger hurricanes once they reach the western Atlantic and threaten land. Right on cue, a series of tropical waves has begun tracking into the Atlantic from West Africa.

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Early season formation points (June 1-10) compared to formation points this time of year highlighting both the upswing in activity, and the shift into the eastern and central Atlantic, particularly near the Cape Verde Islands off the west coast of Africa. (Link)

Tropical Storm Fiona

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GOES Satellite floater un-enhanced infrared centered on Tropical Storm Fiona. (Link)

Fiona is currently a small tropical storm with maximum sustained winds near 35 kts. Its associated cluster of deep convection is supporting tropical storm force winds that only extend out to 70 nm away from the center of the storm. Fiona is currently embedded in the dry and dusty air associated with the Saharan Air Layer which is limiting the amount of deep convection the storm can sustain, while southwesterly vertical wind shear is limiting the organization of the deep convection Fiona does produce.

The Forecast

Bermuda should be monitoring Fiona carefully as the storm tracks generally northwestward, in the general direction of Bermuda. The latest forecast (as of 6pm local) from the National Hurricane Center (NHC) has Fiona continuing northwestward and weakening before degenerating into a remnant low on its final approach to Bermuda by Wednesday. With Fiona’s closest point of approach within the next 72 hours at 312 nm, Fiona is a potential threat to Bermuda (see: Local Advisory).

Fiona’s track to the northwest puts the storm over increasing sea surface temperatures; water near Bermuda is currently around 86°F (30°C) which is plenty warm to support a tropical cyclone. Aside from the increasing sea surface temperatures, Fiona’s environment is not expected to improve enough and re-strengthening is not expected.

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Map of a snapshot of sea surface temperatures yesterday – much of the Atlantic is warm enough to support a tropical cyclone using the traditional 79F (26C) threshold for formation (red contour). Warmer waters are generally more favorable for development. (Link)

However, it is important to monitor this system closely for any changes in this forecast as it approaches. In the past, marginal tropical cyclones like Fiona have struggled through dry/sheared environments, only to survive into the western Atlantic, finding it more moist and less sheared. This allowed quick reorganization and strengthening near Bermuda forcing short-notice warnings (eg. Maria (2011) and Gabrielle (2013)).

Otherwise, there are two other significant tropical waves to monitor as of 3pm local time. One such wave is southeast of Fiona, in the central tropical Atlantic. This wave is expected to track westward into the northeastern Lesser Antilles Islands/Caribbean Sea, and the NHC has given it a 60% (medium) chance for becoming a tropical depression or storm in the next 5 days. The second wave is now exiting the west coast of Senegal in Africa. The NHC is giving it a 70% (high) chance of developing in the next 5 days as it too tracks westward into the tropical central Atlantic.

Right now, showers are expected in Bermuda starting Sunday evening as a weak boundary moves into the area from the west. That boundary lingers as an area of troughiness that keeps showers and possibly thunderstorms in the region through Wednesday as Fiona, or Fiona’s remnants approach from the southeast. No adverse weather is expected from Fiona at this time.

As always, the best advice is to exercise constant vigilance with the tropics by following the latest official forecast updates from the Bermuda Weather Service and the National Hurricane Center.

Heavy Rains Summary

A well anticipated multi-day heavy rain event last weekend ended with a May’s worth of rain nearly everywhere. A band of deep, tropical moisture with total precipitable water amounts near 2 inches aligned over the island from Thursday night through Saturday afternoon. A cold front, turned stationary front remained draped nearby with associated small scale areas of low pressure developing along the front leading to isolated and brief periods of enhanced winds. The front and lows led to persistent shower and thunderstorm activity through much of the weekend resulting in some event disruption and nuisance flooding.

Officially, 3.82″ of rain was measured at the Bermuda Weather Service as a result of this rain storm. This agrees well with island-wide reports generally near 4 inches, including my PWS which measured 3.95″. In an average May, the island sees roughly 3 inches of rain.

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Distribution of rain across Bermuda showing a widespread heavy rain event. An islandwide average of 3.86″ fell. Each circle represents one observation. The shading is likely less reliable in areas of few observations. Observations from Bermuda Weather Service Facebook, Wunderground, and Davis WeatherLink.