Analyses of both the observational and modeling results show that Sandy undergoes rapid deepening over a warm tropical ocean with deep tropospheric warming during the early stage, as it moves poleward. The storm weakens in the early morning 26 October, due to the tilting of upper-level warm core in the presence of intense vertical wind shear (VWS) after the storm passes across the Cuba Island. Then, from 1800 UTC 26 to 1800 UTC 28 October, Sandy experiences unique structural and intensity changes as it moves northward in parallel to the US eastern coastline. That is, widespread surface pressure falls occur over the entire storm region, pressure gradient (and radial boundary inflow) in the core region does not increase, so is the spin-up of the storm. While VWS decreases and upper-tropospheric warming at the center occurs again, it is not strong enough to induce strong central pressure falls for the intensification of the storm. Meanwhile, the areal extent of storm's cyclonic circulation increases. It transports warm-moist air to the western semicircle region of storm where a baroclinic zone develops. Convection and strong wind develop along a spiral rainband in response to frontogenesis and the gale force wind area of Sandy expands. The widespread pressure falls correspond well to an extensive area of lower-stratospheric warmer air after 26 October, causing the expansion of the cyclonic circulation of Sandy. This stratospheric warmth is associated with the tropopause undulation of eastward-moving short and long wave troughs. However, prior to landfall on 29 October, when the storm interacts with very cold-dry air in the western semi-circle region, the maximum wind near the core region increases although widespread surface pressure falls continues. This re-intensification appears to be induced by the enhancement of upper-tropospheric warming (above 500 hPa) in the core region which leads strong central pressure falls. It results in strong boundary radial inflow which helps vortex spin-up process.
In summary, the unusual intensity and structural evolution of Sandy is not only influenced by the tropospheric warming but also the warmth of lower-stratospheric air associated with tropopause undulation. Even though the lower-stratospheric air contributes significantly to central pressure falls and the expansion of storm size, it does not increase the rotational winds of Sandy.