Tuesday, October 28, 2014

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Tropical-Extratropical Interaction in Autumn

We all remember the extreme weather of October 2012; Superstorm Sandy and its destructive storm surge along the New Jersey coastline and the heavy snow accumulation further inland along the West Virginia Appalachians. Sandy's trajectory, not unlike many other late season hurricanes, particularly those that transition into full or partial extratropical storms, can be traced back to the deep southwest Caribbean.

This location, northeast of Panama and west of Columbia, is often a breading ground for late season tropical development. Storms that originate in this vicinity and move northeast toward Cuba in mid to late October, at a time when the subtropical northeasterly trade winds temporarily relax, often become exposed to the cold air intrusions from the mid-latitudes. For some local context, further west over the heart of the southern Pacific basin corollary processes can develop that dramatically impact the fate (and precipitation amounts) associated with wintertime storms in the Sierra. The interaction between the low and mid latitudes defines an elusive airmass coupling that promotes extreme weather. The current synopsis (and extended forecast) reveal similarities of such tropical-extratropical interaction that eerily resembles October 2012, with a few important exceptions.

In October 2012, warm ocean temperatures and moisture convergence in the southwest Caribbean in conjunction with a low-level inflow of momentum produced hurricane Sandy (Figure 1, left). The current forecast reveals similar ingredients and tropical development in a vicinity further west and just south of Guatemala (Figure 1, right).

Figure 1. 850 mb Geopotential heights and wind for late October 2012 (left, WRF analysis) and 2014 (right, GFS forecast). Note the cyclonic development just north of Panama (left) and just south of Guatemala (right) and the trail of westerly momentum inflow to the south and west of the cyclonic circulations in both instances.

Sandy intensified rapidly to hurricane strength as it moved northeastward before weakening just north of Cuba where it encountered cold air surging south from the mid-latitudes. The fate of the current storm remains unknown. Previous forecasts have hinted at the system redeveloping in the Caribbean and following a similar path as Sandy northeast toward Cuba.

Sea surface temperatures in the development vicinity for both events are remarkable similar as shown in Figure 2. The sea surface temperature gradient just north of the equator off the west coast of Ecuador/Columbia is believed to be responsible for the formation of a low-level momentum surge (from the west) that becomes a direct inflow into the developing cyclonic circulations in both instances. 

Figure 2. Sea surface temperatures for October 19, 2012 (left) and 2014 (right). Note the gradient off the west coast of Ecuador/Columbia just north of the equator.

In 2012, Sandy encountered a cold mid-latitude trough just north of Cuba. The cold air and the vertical wind shear that ensued led to the weakening of the hurricane environment. However, Sandy then developed mid-latitudes characteristics as it moved northward along the East coast and became a hybrid storm (a morphed system of both tropical and extratropical attributes). As it neared the mid-Atlantic coast, Sandy encountered a large block in the flow field extending southwestward from Greenland which steered the storm into the New Jersey coastline.

The existence of the block has implications to the state of the Arctic at the time and the cold air surges of mass that extended southward and preceded Sandy's arrival in the mid-Atlantic region. The similarities that exist for the current synopsis are interesting and watch worthy, although the Arctic region has not demonstrated strong cold surges southward in recent days that would precipitate a blocking pattern consistent with the 2012 scenario.

Tropical-extratropical interaction is not exclusive to hurricane activity. Cold air surges into the subtropics and tropics are known excite ongoing convection near the equator. Outflow from tropical convection and associated vapor is also known to surge northward into the mid-latitudes, often in the form of atmospheric rivers or tropical moisture plumes. The order of interaction continues to be an active area of research and is likely a nonlinear mechanism, meaning a tropical plume surge northward is a function of north and south correspondence and not exclusive to one component.

The timing of tropical plume surges, particularly over the Pacific basin, has strong implications on precipitation for the Sierra Nevada, almost exclusively during the winter months (i.e., snowfall). The correlation between tropical plumes and snow in the Sierra is strong if the interaction (and the attributes involved) align appropriately. 


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