Tropical Atlantic and Tropical East Pacific Areas September through December 2014

Scott Stripling and Michael Formosa
Tropical Analysis and Forecast Branch,
National Hurricane Center, Miami, Florida
NOAA National Center for Environmental Prediction

North Atlantic Ocean to 31N and Eastward to 35W, including the Caribbean Sea and the Gulf of Mexico

Atlantic Highlights

The autumn period of September through December 2014 was moderately active in terms of gale conditions across the TAFB Area of Responsibility (AOR). The 16 non-tropical warnings issued for the Tropical North Atlantic during this period were nearly average for this season, based on the recent 8-year seasonal average of 17 warnings.

Table 1 below shows the non-tropical warning events that occurred across the Tropical Atlantic, Gulf of Mexico, and Caribbean Sea during this period. While only 2 warning events occurred in October, November and December provided active periods, with 8 and 6 events occurring, respectively. The upper atmospheric pattern strongly influenced gale frequency during that time, with a series of 4 gale warnings issued during a 4 day span in late November, and 4 gale warnings issued during the last 6 days of December.

Table 1

The upper atmospheric pattern spanning North America and the adjacent ocean basins transitioned in September and October with the development of a persistent middle to upper level trough across the eastern third of the U.S. This pattern supported a cold front entering northwest portions of the Gulf of Mexico during the late afternoon hours of 13 Oct. This front was reinforced by a secondary front and a strong northerly wind surge as high pressure moved southward from western Texas into southeastern Mexico on 14 Oct. This northerly surge produced the first gale event of the season, across portions of the southwestern Gulf of Mexico. This pattern evolution would repeat itself several times throughout the season.

A high amplitude middle to upper level flow pattern across North America and the north Atlantic further evolved during November, with the low pressure across the eastern U.S. expanding and deepening, producing a broad trough prevailing from the Hudson Bay southward across the Great Lakes region and into the southeastern U.S., (Figure 1). This upper atmospheric pattern maximized in intensity during the second half of November, bringing extremely cold arctic air into the central and eastern U.S., and a series of strong cold fronts with northerly gales across the Gulf of Mexico.

The first of these strong Gulf of Mexico cold fronts initiated northerly gales on 13 Nov, followed by another cold front producing northerly gales on 17 Nov, followed later in the month by a strong cold front sweeping southeastward across much of the region 26 through 27 Nov. Gale force winds developed behind this third front in the northerly flow across southwest portions of the Gulf of Mexico, and ahead of the front across the western Atlantic on the 26th, and then behind the front in cold northerly flow as the front moved across the northwestern Caribbean Sea on the 27th. This third front penetrated most of Central America and reached the eastern Tropical Pacific, with strong high pressure behind the front acting as the main forcing mechanism for a storm force wind event through the Gulf of Tehuantepec.

13-14 November Gulf of Mexico Gale

The strongest and longest duration gale event occurred 13-14 Nov, when northerly gales developed across the western Gulf of Mexico behind a cold front, then spread southward across southwest portions of the gulf, where gales persisted through 14 Nov, before finally ending by 00 UTC 15 Nov. This frontal system was supported by the deep upper level trough prevailing across the Great Lakes Region digging south and southwestward into the central U.S., which was common for most of November as shown in Figure 1. On 13 Nov, a 1045 HPa surface high shifted from western Canada southeastward and into the Great Plains, then built southward across Texas and was funneled between the cold front and the eastern slopes of the Sierra Madre Occidentales of northern Mexico. This drove the cold front quickly south and southeastward across the Gulf of Mexico, while bringing extremely dry and cold arctic air across much of the central U.S., (Figure 2). This initiated northerly gales across the Gulf of Mexico west of the cold front beginning 0600 UTC 13 Nov that spread southward into western portions of the Bay of Campeche from 1200 to 1800 UTC 13 Nov. Some of the first confirmation of this gale event came at 1200 UTC when the tanker British Robin (MGSH7) reported NW gales to 37 kts behind the front across the northwest gulf, while the cargo vessel Cap Jervis (A8VL7) also reported N gales to 35 kts in the western Bay of Campeche.

This narrow southward building ridge, trapped between the eastern slopes of the mountainous terrain of interior Mexico and the cold front across the Gulf of Mexico is depicted in Figure 3, and occurred several times during the period, typically leading to gale force wind events in either the western Gulf of Mexico, or through the Gulf of Tehuantepec in the eastern Pacific.

During the late morning and afternoon hours of 13 Nov, very cold and dense air drove quickly southward behind the front and across the western Gulf and into the western Bay of Campeche, with winds there increasing to 40 to 45 kts southward of 26N, as depicted by a 1642 UTC ASCAT pass, Figure 4.

By 1200 UTC 14 Nov the strong high center across the U.S. shifted eastward into the Missouri Valley and had pushed the cold front southeastward, reaching from central Florida to the central Bay of Campeche. This weakened modestly both the pressure gradient behind the front and the strong cold air advection across western portions of the gulf, and allowed winds to diminish to around 35 kt. The high pressure then shifted eastward into the Ohio Valley and weakened to 1032 HPa by 00 UTC 15 Nov, nudging the cold front into extreme south Florida, with gales ending across the southwest Gulf of Mexico. Figure 5 shows the wind and seas observed by PEMEX buoy BMO across the southwestern Gulf of Mexico behind this front, as well as for two additional gale events occurring in November. The time series of wind speed and Significant Wave Height (SHW) show winds peaking at 20 m/s (40 kts) during the 13-14 Nov event, and SWH near 6 m (19.7 ft) with maximum wave heights near 11 m (36 ft). Very few ship observation were available during this gale event and the associated marine conditions observed at BMO for this event are certainly a good reason to avoid this area when gales spread across the full length of the gulf.

Eastern North Pacific Ocean South of 30N and East of 140W

Pacific Highlights

The fall and winter months are an active time for gale and storm events in this portion of the Eastern Pacific. The majority of the events typically occur in the Gulf of Tehuantepec. Thus far in the 2014-2015 season, there were 14 Gulf of Tehuantepec gale and storm events, three (3) Gulf of California gale events, and two (2) other Eastern Pacific gale events.

Table 2
Table 3

The Gulf of Tehuantepec wind events are usually driven by mid-latitude cold frontal passages through the narrow Chivela Pass in the Isthmus of Tehuantepec between the Sierra Madre de Oaxaca Mountains on the west and the Sierra Madre de Chiapas Mountains on the east. The northerly winds from the southwest Gulf of Mexico funnel through the pass delivering stronger winds into the Gulf of Tehuantepec. The events are of various duration with the longer events associated with reinforcing secondary cold fronts in the Gulf of Mexico. The events are usually void of precipitation in the Gulf of Tehuantepec, thus scatterometer passes are not rain contaminated and wind retrievals are of the highest quality. The Gulf of Tehuantepec gale and storm events this season totaled 606 hours, a 6% decrease from last season’s 642 hours. The 2011-12 and 2012-13 seasons each had 492 hours.

The strongest event of the season thus far occurred in the period 26-30 Nov 2014. Strong northerly winds in the Gulf of Mexico behind a cold front on 26 Nov 2014 funneled through the Chivela Pass resulting in a storm event in the Gulf of Tehuantepec. (Figure 6). Note that the 1026 hPa High over the western Gulf of Mexico significantly increased the surface pressure gradient over the Isthmus of Tehuantepec. This wind event commenced at 0600 UTC 26 Nov 2014 as a gale. Winds increased to storm force 12 hours later and lasted until 29 Nov 0000 UTC when winds once again decreased to gale force. The event ended on 30 Nov 0000 UTC 2014 and lasted a total of 90 hours, the longest duration event of the season to date.

A European Advanced Scatterometer (ASCAT) pass captured the event in both the Bay of Campeche and the Gulf of Tehuantepec (Figure 7). An isolated storm force wind barb was depicted over the Gulf of Tehuantepec within a larger area of 45 kt winds. Gale force winds extended southward to 14N between 94W and 96W, while 20 kt winds extended as far south as 10N between 95W and 99W. The Statendam (PHSG) traversed both the gale and storm areas while sailing WNW on 27 Nov 2014.

A new scatterometer instrument, NASA’s International Space Station Rapid Scat-terometer (ISS-RapidScat), also captured the event in the Gulf of Tehuantepec on 28 November 2014. (Figure 8). The pass showed several 50- kt wind barbs in the Gulf of Tehuantepec

The RapidScat’s scatterometer hardware consists of 1990’s warehoused QuikScat test equipment, a new reflector antenna, and a new nadir adaptor mounting device. The instrument was delivered by the SpaceX’s cargo resupply spacecraft in 2014, and was integrated into the International Space Station’s equipment. The scatterometer requires no interaction from Space Station astronauts. The Space Station’s orbit has also proven better in sampling the earth’s tropics than the other sun-synchronous satellites. This is expected to be a highly valuable operational data source for TAFB marine forecasters.

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