Satellite-derived ocean thermal structure for the North Atlantic hurricane season

Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Monthly Weather Review 144 (2016): 877-896, doi:10.1175/MWR-D-15-0275.1.This paper describes a new model (method) called Satellite-derived North Atlantic Profiles (SNAP) that seeks to provide a high-resolution, near-real-time ocean thermal field to aid tropical cyclone (TC) forecasting. Using about 139 000 observed temperature profiles, a spatially dependent regression model is developed for the North Atlantic Ocean during hurricane season. A new step introduced in this work is that the daily mixed layer depth is derived from the output of a one-dimensional Price–Weller–Pinkel ocean mixed layer model with time-dependent surface forcing. The accuracy of SNAP is assessed by comparison to 19 076 independent Argo profiles from the hurricane seasons of 2011 and 2013. The rms differences of the SNAP-estimated isotherm depths are found to be 10–25 m for upper thermocline isotherms (29°–19°C), 35–55 m for middle isotherms (18°–7°C), and 60–100 m for lower isotherms (6°–4°C). The primary error sources include uncertainty of sea surface height anomaly (SSHA), high-frequency fluctuations of isotherm depths, salinity effects, and the barotropic component of SSHA. These account for roughly 29%, 25%, 19%, and 10% of the estimation error, respectively. The rms differences of TC-related ocean parameters, upper-ocean heat content, and averaged temperature of the upper 100 m, are ~10 kJ cm−2 and ~0.8°C, respectively, over the North Atlantic basin. These errors are typical also of the open ocean underlying the majority of TC tracks. Errors are somewhat larger over regions of greatest mesoscale variability (i.e., the Gulf Stream and the Loop Current within the Gulf of Mexico).IFP is supported by Grants NSC 101-2628-M-002-001-MY4 and MOST 103-2111-M-002 -002 -MY3. JFP and SRJ were supported by the U.S. Office of Naval Research under the project “Impact of Typhoons on the North Pacific, ITOP.”2016-06-0

Rapid intensification of Typhoon Hato (2017) over shallow water

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Pun, I., Chan, J. C. L., Lin, I., Chan, K. T. E., Price, J. F., Ko, D. S., Lien, C., Wu, Y., & Huang, H. Rapid intensification of Typhoon Hato (2017) over shallow water. Sustainability, 11(13), (2019): 3709, doi:10.3390/su11133709.On 23 August, 2017, Typhoon Hato rapidly intensified by 10 kt within 3 h just prior to landfall in the city of Macau along the South China coast. Hato’s surface winds in excess of 50 m s−1 devastated the city, causing unprecedented damage and social impact. This study reveals that anomalously warm ocean conditions in the nearshore shallow water (depth < 30 m) likely played a key role in Hato’s fast intensification. In particular, cooling of the sea surface temperature (SST) generated by Hato at the critical landfall point was estimated to be only 0.1–0.5 °C. The results from both a simple ocean mixing scheme and full dynamical ocean model indicate that SST cooling was minimized in the shallow coastal waters due to a lack of cool water at depth. Given the nearly invariant SST in the coastal waters, we estimate a large amount of heat flux, i.e., 1.9k W m−2, during the landfall period. Experiments indicate that in the absence of shallow bathymetry, and thus, if nominal cool water had been available for vertical mixing, the SST cooling would have been enhanced from 0.1 °C to 1.4 °C, and sea to air heat flux reduced by about a quarter. Numerical simulations with an atmospheric model suggest that the intensity of Hato was very sensitive to air-sea heat flux in the coastal region, indicating the critical importance of coastal ocean hydrography.The work of I.-F.P. is supported by Taiwan’s Ministry of Science and Technology Grant MOST 107-2111-M-008-001-MY3. The work of J.C.L.C. is supported by the Research Grants Council of Hong Kong Grant E-CityU101/16. The work of I.-I.L. is supported by Taiwan’s Ministry of Science and Technology (MOST 106-2111-M-002-011-MY3, MOST 108-2111-M-002-014-MY2). The work of K.T.F.C. is jointly supported by the National Natural Science Foundation of China (41775097), and the National Natural Science Foundation of China and Macau Science and Technology Development Joint Fund (NSFC-FDCT), China and Macau (41861164027)

Ocean internal tides suppress tropical cyclones in the South China Sea

Tropical Cyclones (TCs) are devastating natural disasters. Analyzing four decades of global TC data, here we find that among all global TC-active basins, the South China Sea (SCS) stands out as particularly difficult ocean for TCs to intensify, despite favorable atmosphere and ocean conditions. Over the SCS, TC intensification rate and its probability for a rapid intensification (intensification by ≥ 15.4 m s−1 day−1) are only 1/2 and 1/3, respectively, of those for the rest of the world ocean. Originating from complex interplays between astronomic tides and the SCS topography, gigantic ocean internal tides interact with TC-generated oceanic near-inertial waves and induce a strong ocean cooling effect, suppressing the TC intensification. Inclusion of this interaction between internal tides and TC in operational weather prediction systems is expected to improve forecast of TC intensity in the SCS and in other regions where strong internal tides are present

[[alternative]]Estimation of upper-ocean thermal structure in the North West Pacific Ocean by satellite remote sensing and its application to typhoon intensity change

[[abstract]]Lack of the information on upper-ocean thermal structure is one of the identified major reasons causing unsatisfactory typhoon intensity forecast. Therefore it is critical to study the relationship between upper-ocean thermal structure typhoon intensity change. This study uses a two-layer reduced gravity ocean model (TLM_NWPO), TOPEX/Poseidon and JASON-1 sea surface height anomaly data, TRMM/TMI sea surface temperature data and climatological ocean data estimate upper-ocean thermal structure in the Northwest Pacific Ocean. The estimated profiles were validated by 2258 co-located and near co-incident in situ profiles from the Global Temperature and Salinity Profile Program (GTSPP) and the ARGO floats. It is found that the two-layer reduced gravity model is not always applicable in the entire NWPO; depends on location and month. The ‘safe zones’ where the TLM_NWPO can accurately use are defined. It is encouraging to find that most category-4 and 5 typhoons intensify in the ‘safe zones’, thus we can apply the estimated profiles to study its association with typhoon intensity change. All 33 intense and super typhoons (category-4 and 5) occur during the typhoon season (May-October) in the past 6 years (1999-2004) are studied. The sensitivity of four possible parameters (pre-typhoon SST, inner-core SST cooling, pre-typhoon Tropical Cyclone Heat Potential and inner-core Tropical Cyclone Heat Potential) are assessed. It is found that the inner-core SST cooling is the most sensitive parameter and typhoon stops intensification when the inner-core SST cooling exceeds 2.5℃. In contrast, the often emphasized pre-typhoon TCHP is found to be insensitive. It is found that TCHP is over-supplying parameter and the available TCHP is always at least an order higher than typhoons can extract, suggesting that TCHP should not be a limiting factor controlling typhoon intensification.

Upper Ocean Thermal Structure in the Western North Pacific from Satellite Altimetry

近年來有很多研究指出颱風的強度變化是跟上層海洋(0-200m)的溫度結構有密切的關系。但是要在大洋中取得上層海洋溫度結構的資訊是非常困難的，傳統上只可以透過實測的方法。而傳統實測資料的時空分佈並不足以代表整個大洋的變化，對颱風強度的研究與預報造成相當大的阻礙。因此，本研究利用先進的衛星測高技術去估計西北太平洋的上層海洋溫度結構，目的是要去彌補海洋觀測資料的不足。 首先，本研究利用測高衛星的海水面高度資料透過簡單的兩層海洋橂式去計算上層海洋的溫度結構，並利用這些資料去研究西北太平洋的超級颱風Dianmu (2004)的強度變化。發現Dianmu的強度變化對海表面溫度冷卻(SST cooling)和動態上層海洋熱容量(dynamic UOHC)最敏感。在2010年臺灣美國合作的ITOP實驗期間，我們就利用這個兩層海洋模式透過衛星的測高資料去提供每天西北太平洋上層海洋溫度結構的資訊，對整個實驗有相當大的幫助。 由於兩層模式相對簡單，而且並不適用在整個西北太平洋上。所以本研究利用>38,000個Argo實測的海洋溫度剖面去建立西北太平洋的線性回歸方法(REGWNP)，目的是要去改進衛星對上層海洋溫度結構的估計。之後，再利用>7,000個獨立的Argo剖面去驗證REGWNP的估計。驗證的結果顯示REGWNP可以準確地估計西北太平洋的上層海洋溫度結構。在颱風季節，由REGWNP估計出來的20°C等溫線深度(D20)的誤差為30m、26°C等溫線深度(D26)的誤差為20m、UOHC的誤差為20 kJ cm-2、上層100m平圴溫度(T100)的誤差為 1.5°C。另外也發現用REGWNP對比用傳統兩層模式估計出來的上層海洋溫度結構有顯著的改進；同時也非常接近用完整海洋模式計算出來的結果。 最後，研究過去西北太平洋颱風增強區域的海洋條件變化，發現從1993年到2010年之間，海洋暖渦(warm eddy)增加，冷渦(cold eddy)減少。水下溫度變化方面，在過去18年之間D20、D26和UOHC增加了9-17%，而T100增加了0.16-0.35°C。另外，正海水面異常(SSHA ≧ 10 cm)區域增加，負海水面異常(SSHA ≦ -10 cm)區域減少。這個結果顯示西北太平的海洋條件變得越來越有利颱風的增強作用。This thesis firstly demonstrates the importance of satellite-derived upper ocean thermal structure (UOTS) in typhoon research. Using a simple two-layer model, UOTS can be roughly derived from satellite altimetry and the intensity change of the Supertyphoon Dianmu (2004) is investigated. It is found that Dianmu’s intensity is very sensitive to the during typhoon SST cooling and upper ocean heat content (UOHC). During 2010, UOTS was estimated on a daily basis from satellites for the use in the large international field experiment, Impact of Typhoons on the Ocean in the Pacific (ITOP), showing the advantage of satellite-derived UOTS in typhoon-ocean research. Secondly, using >38,000 Argo temperature profiles, a linear regression method for the western North Pacific (i.e., REGWNP) is developed. Then, >7,000 in situ profiles are used to assess accuracy of REGWNP-derived UOTS. The results show that REGWNP is able to produce rather reliable UOTS. During the typhoon season, the rms difference for the depth of 20°C isotherm (D20), depth of 26°C isotherm (D26), UOHC and averaged temperature of the upper 100 m (T100) is less than 30 m, 20 m, 20 kJ cm-2, and 1.5°C, respectively. Also, it is found that REGWNP outperforms the traditional two-layer approach and is comparable to a sophisticated full ocean model for producing real-time UOTS field. Finally, based on the sea surface height anomaly (SSHA) record between 1993 and 2010, the long-term changes in ocean conditions in the western North Pacific main typhoon intensification region are investigated. It is found that the activity of warm eddies enhanced while cold eddies weakened. In terms of subsurface variability, D20, D26 and UOHC increased by 9-17%, meanwhile, T100 warmed by 0.16-0.35°C over the 18 years period. Furthermore, it is also found that the total area of positive SSHA (≧ 10 cm) features substantially increased while negative SSHA (≦ -10 cm) features deceased. These results suggest that the western North Pacific ocean conditions are becoming more favorable to typhoon intensification

Anomalous Oceanic Conditions in the Central and Eastern North Pacific Ocean during the 2014 Hurricane Season and Relationships to Three Major Hurricanes

The 2014 Northeast Pacific hurricane season was highly active, with above-average intensity and frequency events, and a rare landfalling Hawaiian hurricane. We show that the anomalous northern extent of sea surface temperatures and anomalous vertical extent of upper ocean heat content above 26 &deg;C throughout the Northeast and Central Pacific Ocean may have influenced three long-lived tropical cyclones in July and August. Using a variety of satellite-observed and -derived products, we assess genesis conditions, along-track intensity, and basin-wide anomalous upper ocean heat content during Hurricanes Genevieve, Iselle, and Julio. The anomalously northern surface position of the 26 &deg;C isotherm beyond 30&deg; N to the north and east of the Hawaiian Islands in 2014 created very high sea surface temperatures throughout much of the Central Pacific. Analysis of basin-wide mean conditions confirm higher-than-average storm activity during strong positive oceanic thermal anomalies. Positive anomalies of 15&ndash;50 kJ cm&minus;2 in the along-track upper ocean heat content for these three storms were observed during the intensification phase prior to peak intensity, advocating for greater understanding of the ocean thermal profile during tropical cyclone genesis and development