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Wmo/Tcp/Www Ninth International Workshop On WMO/TCP/WWW NINTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES (IWTC-9) Topic (3.2): Intensity change: External influences Rapporteur: Scott A. Braun NASA Goddard Space Flight Center [email protected] Phone: +1 301 614 6316 Working Group: Heather Archambault (NOAA GFDL) I-I Lin (Pacific Science Association) Yoshiaki Miyamoto (Keio University) Michael Riemer (Johannes Gutenberg-Universität) Rosimar Rios-Berrios (NCAR) Elizabeth Ritchie-Tyo (UNSW-Canberra) Balachandran Sethurathinam (Regional Weather Forecasting Centre and Area Cyclone Warning Centre), L. Nick Shay (University of Miami) Brian Tang (University at Albany – State University of New York) Abstract: This report focuses on recent (2014-2018) advances regarding external influences on tropical cyclone (TC) intensity change. Special attention is given to the influences of sea-surface temperature (SST) and ocean heat content, vertical wind shear, trough interactions, dry and/or dusty environmental air, and situations in which multiple factors may act in concert to impact TC intensity change. Studies on ocean interactions highlight the important roles of warm- and cold-core eddies, as well as freshwater plumes and coastal barrier waters, in the modification of surface fluxes and their impacts on intensity. Studies of the impact of vertical wind shear highlight the role that shear plays in modulating the structure and intensity of inner-core convection and its feedback to intensity. Vertical wind shear also significantly impacts the predictability of TC intensity, likely because of its interaction with the convection and TC vortex. While dry environmental air is often an inhibiting influence, the response of a TC can be varied, with dry air in some situations actually favoring intensification and in other cases preventing secondary eyewall formation and the associated impacts on intensity. The effects on storm intensity of aerosols, especially in the Saharan Air Layer, remains a source of significant uncertainty. Complex interactions between the various environmental influences on intensity can affect the timing of rapid intensification (RI), with vertical wind shear often decreasing the likelihood of RI, but SSTs, dry air, convection, and the TC vortex contributing to significant uncertainty in the timing of RI. Rapid weakening typically occurs when a TC crosses over a sharp SST gradient, experiences increasing wind shear, and drier environmental air. Finally, multiple modes 3.2.1 of variability, including the Pacific Decadal Oscillation, El Niño Southern Oscillation, and the Madden-Julian Oscillation are found to influence TC intensification. 3.2.1 Introduction Much like tropical cyclogenesis, TC intensity is greatly impacted by the storm environment, aspects of the ocean or atmosphere that exist independent of the storm, typically on meso- to synoptic spatial scales. Critical processes related to the environment include ocean interactions [related to sea-surface temperature (SST) and ocean heat content (OHC)], vertical wind shear, humidity, and potentially aerosols-cloud- precipitation interactions. These basic processes are often tied to specific oceanic features (e.g., warm/cold-core eddies, seasonal or interannual SST anomalies) and weather systems (upper-level troughs/lows, or modes of large-scale tropical variability, Saharan air layer outbreaks). This section focuses on these environmental, or external, influences on TC intensity change and is organized as follows. The first several sections (3.2.2-3.2.5) highlight individual topics such as the impacts of ocean processes, vertical wind shear (VWS), trough interactions, dry air, and aerosol interactions. Since most storms are not impacted just by a single process, but by a combination of these processes, Section 3.2.6 summarizes findings related to multi-factor influences such as joint ocean-VWS or VWS-dry air impacts and the influences of large-scale modes of variability such as El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and Madden-Julian Oscillation (MJO). 3.2.2 Ocean influences The findings of this group of papers with respect to oceanic impacts on TCs focused on SSTs, warm- and cold-core eddies (WCE, CCE), climate variability such as El Niño, and climate change. The impact of mesoscale ocean eddies on tropical cyclone intensities was assessed by Ma et al. (2017) using a combination of observations and coupled models. The SST data showed that CCEs tend to promote cooling given that the thermocline lies closer to the surface whereas TC interaction Figure 1. Best track of Isaac (black: Tropical with WCEs points to small SST storm; green: Cat 1 Hurricane) relative to decreases as the depth of the the complex ocean circulation and ocean thermocline is much deeper. Moisture heat content in the Gulf of Mexico from disequilibrium induced by the WCE is Meyers et al. (2014) for 25 August 2012 track dependent and vanishes after (WCE: Warm core eddy; CCE Cold core departure of the TC due to the eddy; LC: Loop Current). (From Fig. 1 of dependence of surface fluxes on wind Jaimes et al. 2016) 3.2.2 stress. Jaimes et al. (2016) found that North Atlantic Hurricane Isaac (2012) strengthened as it moved over a WCE, with sea-surface warming (positive feedback mechanism) of 0.5 K measured over a 12-h interval and with enhanced enthalpy fluxes during this intensification stage (Fig. 1). Enthalpy fluxes were enhanced during Isaac’s intensification stage due to an increase in moisture disequilibrium between the ocean and atmosphere over the WCE. This enhanced buoyant forcing from the ocean is an important intensification mechanism in TCs over WCEs. As shown in Fig. 2, Walker et al. (2014) found that slow translation over a CCE caused the rapid collapse Figure 2. SST on 15 Sept 2005 and hurricane Kenneth of Northeast Pacific track (14-30 Sept) where intensity are shown with Hurricane Kenneth (2005) colored circles. The 26°C isotherms reveal limits to as sea surface cooling Kenneth’s development and triangles depict CCE caused enthalpy fluxes to locations from 21 Sept to 1 Oct. The white box depicts become negative, cutting the area shown in panels for SSTs (panels a,b,c) and off fuel to Kenneth (e.g., sea-surface height anomaly (SSHA) (panels e,f,g) on 16, the time available for 21 and 27 Sept respectively relative to 6-hr posutions mixing/upwelling is longer and intensity of Kenneth. Panels d,h depict the along- for slower speeds). This track time evolution of SST and SSHA where the stars study challenged the are the position on 16 and 21 Sept. respectively. Notice notion that Kenneth’s the extreme cooling on 21 Sept in panel b. (From Fig. 1 weakening was due only of Walker et al. 2014) to VWS (Pasch 2006). Several papers (Newinger and Toumi 2015; Androulidakis et al. 2016; Yang and Wang 2017) examined TC interaction with fresh water river plumes and barrier layers that include the effects of both salinity and temperature. In general, fresh water can increase stability and may reduce TC-induced cooling; hence, they may be more 3.2.3 favorable for TC intensification. However, Newinger and Toumi (2015) found that an additional factor, ocean color, also needs to be considered. In the river plume region, the turbid-colored plume can block sunlight from the deeper ocean, thus cooling the upper subsurface ocean (typically –0.3 K in the upper 100-m temperature). They found that this negative impact from ocean color can offset the positive impact from fresh water on TC intensification. TC interaction with coastal waters is of high interest because most existing studies focus on interactions over the open ocean. Seroka et al. (2016) reported that due to coastal baroclinic processes, there can be significant ahead-of-the-eye cooling of >6 K in the Mid-Atlantic Bight, which explained Hurricane Irene’s (2011) rapid decay prior to landfall. They emphasized the need for improved representation of coastal processes in current TC coupled forecasting models since rapid and significant intensity change just prior to landfall can have large implications for disaster mitigation. As noted by Foltz et al. (2018), SST and TC intensification relationships vary considerably from basin to basin as suggested in previous studies. For example, SST explained less than 4% of the variance in TC intensification rates in the Atlantic, 12% in the Western North Pacific, and 23% in the Eastern Pacific. While along-track SST variability is lower in the Atlantic due to smaller horizontal gradients compared to the Pacific, along-track gradients in OHC are quite large in the Atlantic basin, which can lead to strong air-sea interactions as recently observed in Hurricane Isaac (see Fig. 1). The more important rationale in the Western Pacific is that SST tends to vary out of phase with VWS and TC outflow temperatures (higher SST with lower VWS and lower outflow temperatures, all leading to greater intensification). Presumably, this strengthens the relationship between SST and TC intensification more in the Western Pacific than in the Eastern Pacific or Atlantic. Foltz and Balaguru (2016) studied the El Niño conditions of 2014–2015 and the rapid intensification (RI) of Hurricane Patricia in the Eastern Pacific. Warm SSTs with a deeper than normal thermocline contributed to Patricia's RI, leading to an increase in its maximum potential intensity by 14 m s-1. In this oceanic regime where Patricia Figure 3. Depth of the 26oC isotherm (m: upper intensified, SST was 1.5 K higher panel) and OHC (kJ cm-2: lower panel) from and and sea-surface height was 10– average of El Niño years (02,04, 06, 09: black 14 cm higher compared to curve) and a one standard deviation average conditions during the last (gray) relative to the 2015 El Niño at 104.5W, extreme El Niño in 1997, 17.5N along Patricia’s track. Notice OHC is double emphasizing the extraordinary that of previous El Niño years.
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