Volume 56, Number 3 December 2012 Photo by Denice Drass

Tropical Atlantic and Tropical east pacific Areas
December 2012 ~ Mariners Weather Log
30
Table 2. Non-tropical cyclone warnings issued for the subtropical and tropical eastern North Pacific
between 1 May and 31 August 2012.
Onset Region
Peak Wind
Speed
Gale Duration Weather Forcing
0300 UTC 14 May Gulf of Tehuantepec 40 kt 15 hr Pressure Gradient
0300 UTC 22 May Gulf of Tehuantepec 40 kt 12 hr Pressure Gradient
Eastern North Pacific Ocean
Gales and significant wave events are almost exclusively
attributed to tropical systems across the tropical eastern
North Pacific during the summer months. Two significant
non-tropical warning events were documented primarily
by scatterometer data due to the lack of supporting ship
observations in the May through August 2012 time period.
Table 2 provides details on these brief gale events.
Late Season Gulf of Tehuantepec Gales
Gulf of Tehuantepec cold season wind events typically end
by the first week of April. However, in mid May 2012, there
were two occurrences of unusually strong late season winds
that briefly surged to gale force lasting 15 hours or less.
Typical cold season Gulf of Tehuantepec wind events are
initiated by strong northerly winds across the western Gulf of
Mexico. This strong flow, which advects cold air southward,
is supported by a steep pressure gradient between strong high
pressure over Mexico or southern Texas and lower pressure
over the Pacific Ocean south of Mexico. Funneling of the
wind from the Gulf of Mexico into the eastern North Pacific
Ocean is due to terrain variations over southern Mexico, and
is most pronounced across the Isthmus of Tehuantepec. These
events were atypical due to the existence of a broad area of
low pressure associated with an active early season monsoon
trough in the eastern North Pacific Ocean, which helped
initiate two tropical cyclones in May (Tropical Storm Aletta
and Hurricane Bud). As a result, moderate high pressure over
southern Texas around 1020 hPa created enough of a pressure
gradient across the region in each case to initiate gale force
winds surging into the Gulf of Tehuantepec.
The first gale event commenced around 0300 UTC May 14th.
A ridge of high pressure west of a cold front in the central
Gulf of Mexico extended southward from a 1024 hPa high
in Texas along the east coast of Mexico, while a weak trough
was located over the Bay of Campeche and a broad monsoon
trough was across the eastern North Pacific (Figure 6). In
the early morning hours of May 14th, winds increased to 20
kts at Salina Cruz.
Figure 6. National Weather Service Unified Surface
Analysis (USA) chart from 1800 UTC 13 May 2012,
centered on southern Mexico and Central America.
Moderate high pressure was building into southern
Mexico, toward the Isthmus of Tehuantepec.
An Advanced Scatterometer (ASCAT) pass at 0310 UTC
showed an unexpected increase in surface winds compared
to the previous pass, with a narrow swath of 35 kts. Based on
these data, a gale warning was immediately issued (Figure
7). An Oceansat-2 Scatterometer (OSCAT) pass nearly 3
hours later indicated peak winds of 40 kts in the Gulf of
Tehuantepec (Figure 8), while a subsequent ASCAT pass
at 1542 UTC showed winds had fallen below gale force,
but strong to near-gale winds extending more than 200 km
downstream from the Isthmus of Tehuantepec (Figure 9).
This anticyclonically curved plume of high winds eventually
became entrained into the north semicircle of the circulation
associated with Tropical Storm Aletta.
Figure 7. ASCAT wind retrieval from 14 May 2012 at 0310
UTC shows the onset of gale force winds over the Gulf of
Tehuantepec.