What Would Happen to Superstorm Sandy Under the Influence Of

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Geophysical Research Letters

RESEARCH LETTER What would happen to Superstorm Sandy under 10.1002/2015GL067050 the inﬂuence of a substantially warmer

Key Points: Atlantic Ocean? • Dramatic differences in storm responses under a substantially William K. M. Lau1,2,J.J.Shi3, W. K. Tao4, and K. M. Kim4 warmer Atlantic SST • Duration of storm in warm pool 1Earth System Science Interdisciplinary Center (ESSIC), University of Maryland, College Park, Maryland, USA, 2Department of determines SS-like storm intensity 3 and evolution Atmospheric Sciences, Texas A&M University, College Station, Texas, USA, Mesoscale Atmospheric Processes Laboratory, 4 • SS-like storms under warmer SST have NASA/GSFC, Greenbelt, Maryland, USA, Climate and Radiation Laboratory, NASA/GSFC, Greenbelt, Maryland, USA substantially stronger destructive power

Abstract Based on ensemble numerical simulations, we find that possible responses of Sandy-like Supporting Information: fl • Figures S1 and S3 and Movie S2 superstorms under the in uence of a substantially warmer Atlantic Ocean bifurcate into two groups. In the caption first group, storms are similar to present-day Sandy from genesis to extratropical transition, except they are • Figure S1 much stronger, with peak Power Destructive Index (PDI) increased by 50–80%, heavy rain by 30–50%, and • Video S2.1a • Video S2.1b maximum storm size (MSS) approximately doubled. In the second group, storms amplify substantially over • Video S2.2a the interior of the Atlantic warm pool, with peak PDI increased by 100–160%, heavy rain by 70–180%, and • Video S2.2b MSS more than tripled compared to present-day Superstorm Sandy. These storms when exiting the warm • Video S2.3a • Video S2.3b pool, recurve northeastward out to sea, subsequently interact with the developing midlatitude storm by • Figure S3 mutual counterclockwise rotation around each other and eventually amplify into a severe Northeastern coastal storm, making landfall over the extreme northeastern regions from Maine to Nova Scotia. Correspondence to: W. K. M. Lau, [email protected] 1. Introduction Hurricane Sandy, often referred to as Superstorm Sandy (SS), was the most destructive hurricane of the 2012 Citation: Lau, W. K. M., J. J. Shi, W. K. Tao, and hurricane season, wreaking havocs along the eastern Atlantic seaboard from Florida to Maine. It is now well K. M. Kim (2016), What would happen to recognized that SS arose from the alignment of favorable meteorological conditions in the tropics and extra- fl Superstorm Sandy under the in uence tropics, leading to its unusual track and interaction with a developing extratropical storm [Blake et al., 2013; of a substantially warmer Atlantic Ocean?, Geophys. Res. Lett., 43, Shen et al., 2013; Halverson and Rabenhorst, 2013; McNally et al., 2014]. Arguments have been advanced that doi:10.1002/2015GL067050. the alignment was a mere chance occurrence of nature, i.e., a perfect storm [Hall and Sobel, 2013; Sobel, 2014]. Some recent studies have also suggested that anthropogenic global warming could have predisposed the Received 17 NOV 2015 large-scale atmosphere-ocean to facilitate such a meteorological alignment [Greene et al., 2013; Francis and Accepted 26 NOV 2015 Vavrus, 2012]. However, Barnes et al. [2013] suggested that the northwest turn of SS-like storm is less likely based on CMIP5 projected changes of the large-scale circulation in a future warmer climate. While the debate on SS and global warming is likely to continue, there is, however, no disagreement that one of the key factors in affecting hurricane formation and intensity is sea surface temperature (SST) [Knutson et al., 2010; Emanuel, 1988] and that overall SST is rising due to global warming [IPCC, 2013]. In this study, setting aside the question of where or not SS is more likely due to anthropogenic global warming, we address a different question: What would happen to SS, if the same initial atmospheric conditions were to repeat themselves, but under the influence of a substantially warmer Atlantic Ocean? It is important to point out that, while SST provides the key thermo- dynamic forcing for tropical cyclones, dynamical forcing such as large-scale vertical wind shear and vertical motion can strongly affect the development and evolution of tropical cyclones [Wang and Lee, 2008; Knutson et al., 2010]. However, the magnitude and patterns of these dynamical forcing in a future climate are difficult, if not impossible, to estimate because of their much large variability compared to SST [Trenberth et al., 2015]. Hence, the objective of this work is limited to understanding the possible responses of SS-like storms to the thermodynamical forcing of a warmer Atlantic Ocean but under present-day atmospheric conditions. For this purpose, we use the NASA Unified-physics Weather Research Forecast (NU-WRF) regional model. The dynamical core and physical packages of the NUWRF model are the same as the Advanced Weather and Research Forecast (AWRF) model, except with improved microphysics package developed at the Goddard Space Flight Center. The AWRF and NUWRF have been used extensively for

©2015. American Geophysical Union. tropical cyclone simulation and prediction studies, and a bias by the WRF models in over-development of All Rights Reserved. storms have been noted [Davis et al., 2008; Fierro et al., 2009]. The model bias has been reduced by improved

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microphysics in the NU-WRF model [Tao et al., 2011]. For this work, the experiments were conducted using a version of the NU-WRF with a 9 km horizontal resolution and 61 vertical layers. A large domain [120 W–40 W, 5S–50 N] was chosen to allow for possible strong interactions between SS and the extratropical large-scale circulation. Two sets of 10 day, 5 member ensemble numerical experiments were carried out respectively with prescribed present October mean SST (PSST) and future SST (FSST), under the same atmospheric initial and lateral boundary conditions as for present-day SS. The 5 member integration was initialized starting 00 Z, 06 Z, 12 Z, 18 Z 22 October, and 00 Z 23 October 2012 with all integrations terminating at 00 Z 1 November. FSST was obtained by superimposing on PSST, the projected multimodel mean (MMM) SST anomalies from

33 CMIP-5 coupled models, based on a doubling of CO2 experiment. The MMM minimizes internal variability of individual climate model and provides the best model estimate of the forced response of the Earth’s climate to a prescribed emission scenario [Lau et al., 2013] (see supporting information Figure S1 for details of model attributes and experimental set up).

2. Results

As expected, when the double CO2 anomaly SST pattern (Figure S1b) is superimposed on PSST (Figure S1a), the entire Atlantic Ocean is substantially warmer. In FSST (Figure S1c), the warmest water is found in the Caribbean Sea and the Gulf of Mexico, accompanied by a warm tongue emanating from the tropics toward the extratropics away from the Atlantic coast. The total area of the warm pool (SST > 29°C) in FSST is more than doubled compared to PSST. Additionally, the water along and off the Atlantic coast from Florida to New England and the northeast coast of Canada is also much warmer by 1–3° C. This pattern of SST warming has been attributed to enhanced heat content in the upper oceans in the tropics and an increase in ocean heat transport in the Gulf Stream under global warming [Xie et al., 2010; Long et al., 2014].

2.1. Storm Track and Minimum Sea Level Pressure Under PSST, the storm track and minimum sea level pressure (MSLP) variations of SS for each of the five simulations resemble observations throughout the 10 day simulations, despite some notable model biases (Figure 1). However, the model biases are much smaller than the differences between PSST and FSST over a large fraction of the storm’s life cycle, hence ensuring the differences are robust. To facilitate discussion, the evolution of SS is divided into three phases. Phase I (00 Z 23 October–00 Z 26 October) features the initial development of SS into a Category-3 storm around 006 Z 25 October over the Caribbean warm pool (PSST > 28°C), near Cuba, with MSLP falling briefly below 960 hPa, and then rises (Figure S1b). As the storm enters Phase II (00 Z 26 October–00 Z 29 October), the storm continues to weaken until 00 Z 27 October, after which the storm steadily intensifies as it moves northward up the Atlantic seaboard. Phase III (00 Z 29 October–006 Z 31 October) commences with SS making an abrupt northwest turn toward the Jersey coast at 00–06 Z 29 October. Thereafter, SS continues to intensify, with maximum 10 m wind at approximately 40 ms 1, and MSLP falling below 940 hPa at about 18 Z 29 October. Under FSST, the initial northward migration of the storms is similar to that of PSST in Phase I (Figure 1a). During Phase II, the tracks bifurcate into two groups. Two tracks show a sharp northwest turn (hereafter referred to as FSST-NW or NW storms), while three tracks (hereafter referred to as the FSST-NE or NE storms) show a northeast recurvature, and with a delayed northwest turn in Phase III, when the storm is already far out over the North Atlantic. During Phase I, the NW storm development is similar to PSST (Figure 1b) but stronger with maximum winds at about 48 ms 1 and MSLP below 950 hPa. For NE storm, the initial development is even stronger, with MSLP dropping to 930–910 hPa, and maximum winds up to 55 ms 1 but reaching its peak speed later by ~12 h relative to PSST. During Phase II, for both NE and NW storms, the MSLP falls near or below 900 hPa, typical of a Category 5 storm around 06 Z 27 October–00 Z 28 October. In Phase III, the two NW storms briefly intensify, as they turn northwestward, and rapidly dissipate similar to those under PSST. In contrast, the three NE storms follow very different paths of development and interactions with the large-scale environment, as will be discussed next.

2.2. Evolution and Extratropical Transitions Under PSST Under PSST, the genesis of the hurricane just south of Cuba during Phase I is evident (Figure 2a). Contemporaneously, a developing midlatitude wave pattern is found near 35–50°N over the U.S. continent. During Phase II, the midlatitude system moved eastward, developing into a strong ridge over the Atlantic

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Figure 1. NHC best track observations and model simulations of Superstorm Sandy for (a) storm tracks and (b) storm center surface pressure (hPa), under different SST forcing. Labeling color conventions are solid black for observation, blue for PSST, solid red for FSST-NW, and open red for FSST-NE. All integrations end at 00 Z 1 November. Number of days refers to the length of the integrations, initiated 6 h apart.

coast, and deep troughs west and east of the ridge. The ridging off the New England coast is very pronounced at both the middle and upper troposphere (see Video S2.1), providing a strong northward steering ﬂow, for the propagation of SS up the Atlantic coast, and blocking of the northeastward movement of the storm. Within the next 24 h, the northward moving SS grows to a very large size, and subsequently merges with, and rotates counterclockwise around the advancing edge of the deepening continental trough. In Phase III and by 00 Z 29 October (Figure 2c), SS is completely secluded by the midlatitude trough, while continues intensifying and moving northwest toward the New Jersey coast [Galarneau et al., 2013]. The landfall of SS over northern New Jersey at 6–12 Z, 29 October is accompanied by an explosive growth in the size of the storm and a deepening of MSLP to ~940 hPa (see also Figure S1b).

2.3. Evolution and Extratropical Transition Under FSST Under FSST, the same initial atmospheric conditions lead to a substantially intensified storm in Phase I compared to PSST, for all five cases (two NW and three NE storms). The two NW storms are initialized at a later time, i.e., at 00 Z, and 06 Z 23 October, compared to the three NE storms which are initialized at 06 Z, 12 Z, and 18 Z 22 October, respectively. Storms within the same group behave similarly. Because of the northward propagation of the storms, the NE storms are initiated farther equatorward near the interior of the Atlantic warm pool. As will be shown in the following discussions, they also propagate slower during their development stage over the warm pool. These mean that the NE storms have longer exposure and more likely to experience stronger impacts from the underlying warmer SST. Since the life cycle of the NW storms are similar to PSST, their synoptic evolution will not be discussed here, but can be seen in Figures 2e–2h, and Video S2.2. For NE storms, the intensification over the warmer tropical water is very pronounced but with a northward propagation speed that is significantly slower than PSST and FSST-NW (see Figures 2i and 2j). By 00 Z 27 October, the storm is already a well-developed hurricane with

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Figure 2. Synoptic evolution of 500 hPa height (m) and winds (ms 1) at 2 days intervals starting at 00 Z 25 October, illustrating storm development and extratropical transition for selected case of (a–d) PSST, (e–h) FSST-NW, and (i–l) FSST-NE.

MSLP below 900 hPa when it is located off the Florida coast. Within 24 h, instead of moving northward as in PSST, the NE storm swings northeastward toward the open ocean and begins to weaken brieﬂy(seeVideoS2.3). This situation is consistent with the oft-observed recurvature of intense Atlantic hurricanes in the subtropics immediately after reaching maximum intensity [Knaff, 2009]. The eastward recurvature continues for the next 48 h. By 00 Z 29 October (Figure 2k), with the center of the storm located more than 800 km east of its PSST counterpart. In contrast to PSST (Figure 2c) and FSST-NW (Figure 2g) which are merging with the extratropical storm at this time, the NE storm continues to move eastward to sea (Figure 2k). By Phase III, a trough system with two distinct low pressure centers is formed, one over the North Atlantic identiﬁable as the remnants of the NE storm and the other over the coast of New England (Figure 2l).

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Tracking the hourly movement of the storm for individual cases (see Video S2.3), it can be seen that during Phase III; the NE storm and the developing extratropicial storm never merge but rather rotate counterclockwise with respect to each other, resembling the classic Fujiwhara effect for binary tropical cyclones [Fujiwhara, 1923, 1931; Dong and Neumann, 1983]. The Fujiwhara interaction of the two storms is illustrated in the synoptic sequence during the last 2 days of the integration (Figures 3a–3e). The hourly time series of the intensity of the two storms, defined by the minimum of the 500 hPa geopotential height, show clearly the inverse relationship and mutual amplitude oscillation with an approximate 2 day periodicity (Figure 3f). By 00 Z 1 November, the original extratropical storm has vanished, but the NE storm, i.e., what remains of its previous tropical counterpart, has deepened into a well-developed Nor’easter [Davis and Dolan, 1993; Davis et al., 1993] making landfall over the northeastern regions of Maine, U.S., and Nova Scotia, Canada. The rejuvenation of the NE storm occurs just before landfall and is likely stems from the warmer coastal water over the northeastern Atlantic seaboard in FSST (see Figure S1b). 2.4. Rainfall Structure In this section, we focus on the structural changes under FSST in rainfall directly associated with the storms, while they are tropical cyclones within the Atlantic warm pool. Detailed comparison of total accumulation and distribution of rainfall associated with the full life cycle of SS under PSST with TRMM rainfall observations can be found in Figure S3. At 12 Z 25 October, the PSST storm is found over southern Cuba, and rainfall pattern displays an asymmetric spiral structure around the center of the storm, with heavy rainfall concentrated on the northeastern sector of the storm (Figure 4a). In the next 12 h (Figure 4b), the storm propagates northward across Cuba, growing in size, and strengthening with heavy rain spiral band on the north and northeast of the storm. For FSST-NW, the evolution of the heavy rain band is similar to PSST, except that the northward propagation is slightly slower. The rain bands are more extensive and organized with the spiral rain structure more symmetric with respect to the storm center (Figures 4c and 4d). The location of the maximum rainfall in the northeastern sector of the PSST and the NW storms is consistent with a northward moving strong tropical storms under conditions of weak wind shear [Bender, 1997; Chen et al., 2006]. By comparison, the NE storm propagates slowest. At 12 Z 25 October (Figure 4e), the NE storm is still far south of Cuba. Heavy rainfall is found around and close to the eyewall, with a nearly symmetric spiral band tightly wrapped around the storm center. Here strong asymmetry in rainfall structure has developed with a local maximum in the northwestern sector of the inner core of the storm. In the next 12 h, the NE storm makes landfall over Cuba, while it continues to intensify, as evident in the dense and well-defined spiral rain band wrapping closely around the storm center. The maximum rainfall in the northwestern sector of the storm becomes more pronounced. The northwest displacement of the maximum rainfall in the inner core is consistent with the development of strong vertical wind shear along the same general direction as the motion vector for a northward propa- gating Northern Hemisphere intense tropical storm [Chen et al., 2006]. 2.5. Destructive Power In this section, we assess the destructive potential of SS and SS-like storms based on maximum winds, maximum storm size (MSS), and rainfall. For winds, we have computed the time dependent Power Dissipation fi ðÞ¼∫τ 3 τ Index (PDI), de ned as PDI t 0Vmaxd , were Vmax is the maximum sustained winds at 10 m above sea level, and τ is the time interval taken to be 1 hour [Emanuel, 2005]. For MSS, we have estimated the total storm areas in which the 10 m sustained winds exceed tropical cyclone strength (>17 ms 1). For rainfall, we have computed the hourly accumulated rainfall within a radius of 300 km around the center of the storm. Under PSST, enhanced PDI accompanied by heavy rain occurred on 25–26 October, (Figures 5a and 5c) during Phase I, when SS was located over the Caribbean. However, because of the far off-coast location of the storm, Florida and the mid-Atlantic coastal regions were not seriously affected either by winds or rainfall associated with SS under PSST. During Phase I and II, SS continued to grow in size. The PDI peaked at 012 Z 28 October, and SS grew to its maximum size (~1500 × 103 km2), while merging with the midlatitude storm. By the time of landfall at 012 Z 29 October and shortly afterward, storm surges associated with the high PDI devastated the northern Jersey shores and New York City. For NW storms, the destructive potential over the Caribbean during Phase-I is greatly enhanced, with peak PDI increased by nearly 80%, and maximum rainfall accumulation ~30% more intense than PSST. At this stage, the MSS is only slightly larger than PSST but begins to increase substantially toward the latter part of Phase I (Figure 5b). As the storm propagates northward up the Atlantic coast, the PDI heavy rainfall and

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Figure 3. (a–e) Spatial evolution pattern of 500 hPa height (m) and winds (ms 1) for a representative FSST-NE storm undergoing extratropical transition, from 00 Z 30 October through 00 Z 1 November, illustrating the Fujiwhara interaction between a Sandy-like superstorm and an extratropical storm. Only geopotential height <5450 m, and winds > 33 ms 1 are shown. (f) Amplitude modulation between the westerly trough and SS-like storm, illustrated by the hourly time series of the minimum height at the center of the two storms respectively are shown.

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Figure 4. Instantaneous rainfall distribution and sea level pressure around a typical storm during Phase I at 12 Z 25 October and 00 Z 26 October for (a, b) PSST, (c,d) FSST-NW, and (e, f) FSST-NE. Units in mm hr 1.

MSS are substantially increased relative to PSST. The larger size of the storm and the longer duration of heavy rain portend a much greater risk of damage by storm surge and flooding along the mid-Atlantic coast during Phase II in FSST. When the NW storm makes its northwest turn at 006 Z 28 October, the maximum PDI is nearly 50% higher and rainfall 40–50% heavier, and MSS nearly doubled (~3200 × 103 km2) compared to PSST. For NE storms, the damages and destruction inflicted by strong winds and heavy rainfall on the Caribbean, Southern Florida would be catastrophic, as indicated by the 100–160% increase in PDI, and 70–180% heavier rainfall, and near doubling of the storm size compared to PSST during Phase I and II. However, regions in the northern mid-Atlantic coast may be spared from the direct impact of the storm in Phase-II due to its recurvature out to the open ocean. Damage due to storm surges may still be very severe due to the strong PDI, and the exceptional MSS (6000 × 103 km2), more than three times that of PSST (Figure 5b). As evident in the PDI time variation (Figure 5a), the NE storm shows a reinvigoration in Phase III, around 00 Z 30 October–00 Z 31 October, while it is still over the open ocean, prior to making landfall over Maine and Nova Scotia between 12 Z 31 October and 00 Z 1 November (see Figures 4d and 4e). In contrast, both the PSST and FSST-NW storms are already over land and mostly dissipated during Phase III. The reinvigoration of the NE storm stems from its delayed interaction of the extratropical storm through the Fujiwhara effect (as discussed in reference to Figure 3). Since the storm reintensifies just before landfall, it could possibly have gained additional energy from the projected much warmer coastal water of the northeastern seaboard. The impacts of the transformed NE storm on the far northeastern U.S. and Canada from storm surge would be devastating.

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Figure 5. Time series of (a) hourly Power Dissipation Index, (b) maximum storm size (103 km2), and (c) mean hourly rain accumulation (mm) within 300 km radius of storm. Color scheme is grey for PSST, blue for FSST-NW, and red for FSST-NE.

It is important to stress that the foregoing discussions refer only to the destructive potential of the storms. The actual destructive power will depend on the economic infrastructure and population density of the storm affected areas [Peilke and Landsea, 1998]. Damages from ﬂoods and landslide from heavy rain will be deter- mined by additional factors such as topography, soil conditions, and vegetation cover [Philpott et al., 2008]. High tides and sea level rise from global warming could also increase the severity of storm surges, resulting in more destruction [Sobel, 2014].

3. Conclusions Based on numerical experiments with the NU-WRF regional climate model, we find that under the influence of a substantially warmer Atlantic Ocean, atmospheric conditions giving rise to present-day SS may lead to dramatically different storm responses. Depending on the time of exposure of the storm to the Atlantic warm pool, the response bifurcates into two groups. In the first group, a tropical cyclone develops, propagates, and interacts with a developing extratropical storm similar to present-day SS, except that the storm moves faster, makes landfall farther north, and packs much higher wind destructive power and heavy rain compared to

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present-day SS, with potential disastrous damages for the Caribbean and the Atlantic seaboard. In the second group, the storm is initiated near the interior of the Atlantic warm pool and propagates slower compared to the first group. As a result the storm undergoes more rapid intensification due to longer time exposure to the higher SST during its early stage of development. Here the storm intensifies rapidly to a powerful hurricane with potentially catastrophic damages over the Caribbean. After reaching maximum intensity at the poleward edge of the warm pool, the storm recurves northeastward out to sea, grows to a maximum storm size more than 3 times that of PSST, and interacts with the extratropical large-scale circulation in a manner dramatically different from the first group. The interaction of the NE storm with the developing midlatitude cyclone exhibits the Fujiwhara effect, i.e., amplitude modulation with counterclockwise rotation about each other. Eventually, the extratropical remnant of the tropical storm morphs into a typical Northeaster, reinvigorate as it passes over warmer coastal water of the northeastern seaboard, severely impacting the Maine and Nova Scotia regions. Finally, while our results bring to light the possibility that a bifurcated responses of SS-like storms under the influence of a warmer Atlantic Ocean, we note that the results may be model dependent. Additionally, we do

not expect the large-scale circulation will stay the same in a future warmer climate due to CO2 warming. Possible dynamical effects due to future changes in large-scale vertical wind shear, tropospheric humidity, and coupled SST effects have not been considered in this work. Any one or combinations of these effects could affect the present results. Including these dynamical effects in a multimodel framework and understanding their impacts in conjunction with the SST thermodynamical effects will be a challenge and should be the subject of further work.

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