SST Gradients and the East Asian Early-Summer Monsoon

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1 January 3, :58 The Global Monsoon System: Research and Forecast (3rd Edition) in x 7.5in b2503-ch01 page 3 3 Chapter 1 SST Gradients and the East Asian Early-Summer Monsoon Richard H. Johnson and Michael D. Toy Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA National Center for Atmospheric Research, Boulder, CO 80303, USA johnson@atmos.colostate.edu The East Asian summer monsoon is characterized by strong, moist southwesterly flow over a broad expanse that includes the seas adjacent to China and the Pacific Ocean south of Japan. Relatively strong sea surface temperature (SST) gradients, particularly at the time of summer monsoon onset, exist over these waters as a result of the previous winter s cold outbreaks. The flow of warm air over cooler water can lead to low-level convergence through the suppression of vertical momentum transport in the boundary layer as a result of increased stability. Given sufficient moisture and instability, deep convection can break out. Studies have shown that this process is operative off the coast of East Asia during the summer monsoon and frequently contributes to extensive bands of precipitation. Most of the precipitation enhancement occurs over the open ocean; however, an important exception is in the area of Taiwan where the SST gradient intersects the island during May and June. A recent study using data from the 2008 SoWMEX/TiMREX suggests that the SST gradient upstream of Taiwan can increase rainfall over the island by locally enhancing low-level convergence offshore. 1. Introduction Flow of air across gradients in sea surface temperature (SST) can have a significant impact on the structure of the atmospheric boundary layer. There are several ways in which the impacts can be felt. First, flow across SST gradients can lead to changes in near-surface stability, surface stress and fluxes, such that the wind profile is modified (Sweet et al. 1981; Businger and Shaw 1984). A second effect is the modification of turbulent kinetic energy, leading to changes in the vertical transfer of momentum and boundary layer depth (Hayes et al. 1989; Wallace et al. 1989). In addition to these effects, there can be hydrostatic pressure changes across SST gradients that can drive low-level convergence in the tropics (Lindzen and Nigam 1987). These processes are reviewed in Xie (2004), Small et al. (2008), and Chelton and Xie (2010). To illustrate how SST gradients can modify the vertical transfer of momentum, a schematic diagram is presented in Fig. 1 (from Chelton and Xie 2010). It shows flow of air from cool to warm SSTs across a meandering SST frontal boundary. Winds are stronger on the warm side of the front due to enhanced buoyancy fluxes and downward mixing of higher momentum air from aloft. Where the flow is perpendicular to the front, there is low-level convergence of the flow and the sea surface stress. Where the flow is parallel to the front, positive vorticity is generated as well as a positive curl of the surface stress. Changes in the surface wind stress can The Global Monsoon System: Research and Forecast (3rd Edition) Edited by C. P. Chang et al. c 2016 by World Scientific Publishing Co.

2 January 3, :58 The Global Monsoon System: Research and Forecast (3rd Edition) in x 7.5in b2503-ch01 page 4 4 R.H.JohnsonandM.D.Toy Fig. 1. Idealized depiction of surface flow across a meandering SST front (adapted from Chelton and Xie 2010). Green shaded area indicates convergence of both the surface flow and surface stress τ. Red shaded area indicates positive vorticity and positive curl of the surface stress. feed back and alter the properties of the upper ocean (Small et al. 2008). The flow configuration depicted in Fig. 1 is representative of that climatologically occurring over East Asian coastal seas and northwest Pacific Ocean during the summer monsoon. Namely, southwesterly monsoon flow passes over warm waters in the south toward cooler waters in the north. This situation for the month of June is illustrated in Fig. 2. Airflow extends from the southern South China Sea and western Pacific Ocean to the Japan/Korea region, where the SST is considerably lower. Although there is not a sharp frontal boundary like that depicted in Fig. 1, there is a flow across an SST gradient such that air mass transformation and modification of the boundary layer occur. The condition of warm-to-cool climatological airflow first begins in mid-may with the onset of the East Asian summer monsoon. The air-sea interaction processes just described will be reviewed for the region of the East Asian summer monsoon. 2. Previous Investigations of Air-Sea Interaction Impacts on Rainfall The advent of global, satellite-based measurements of surface winds, SST, and precipitation Fig climatological mean June SST from Lamont-Doherty Earth Observatory website ( iridl.ldeo.columbia.edu/) based on 1 Reynolds et al. (2002) SST data. White arrows schematically represent typical near-surface monsoonal airflow. over the past two decades has permitted the documentation of a myriad of fascinating and complex interactions between the ocean and atmosphere (Xie 2004; Small et al. 2008; Chelton and Xie 2010). Particularly prominent are situations where flow across relatively strong SST gradients modifies the boundary layer, cloudiness, and precipitation, such as the eastern Pacific cold tongue (de Szoeke and Bretherton 2004), Tropical Instability Waves in the Pacific Ocean (Chelton et al. 2001; Liu et al. 2000; Hashizume et al. 2002), the Gulf Stream (Minobe et al. 2008), and the Kuroshio and its extension (Nonaka et al. 2003; Xu et al. 2011; Miyama et al. 2012). An investigation has been carried out by Tokinaga et al. (2009) of air-sea interaction processes associated with warm air flowing over cooler water in the northwest Pacific Ocean, as depicted in Fig. 2. They used ship and satellite observations, and reanalysis data to show that strong SST gradients associated with the Kuroshio Extension impact clouds and precipitation over an extensive area. Results

3 January 3, :58 The Global Monsoon System: Research and Forecast (3rd Edition) in x 7.5in b2503-ch01 SST Gradients and the East Asian Early-Summer Monsoon page SST Gradients Over the Northern South China Sea Fig. 3. June July climatology: (a) QuikSCAT surface convergence (color in 10 5 s 1 ) and AMSR-E SST (thin contours at 1 C intervals) superimposed on negative pressure vertical velocity of JRA-25 at 700 hpa (thick contours greater than Pa s 1 at Pa s 1 intervals) and (b) vertical section of negative pressure velocity (color in 10 2 Pa s 1 ) and horizontal convergence (contours at s 1 ) along 145 E. From Tokinaga et al. (2009). of their study are shown in Fig. 3. Strong convergence and upward motion at 700 hpa can be seen at the southern edge of the strong SST gradient (Fig. 3a). As evident from Fig. 3b, the surface convergence produces deep upward motion in this convectively unstable region of the western Pacific. In another study, Tanimoto et al. (2009) showed that warm southerlies across the Kuroshio Extension can lead to fog formation over the cooler water. The onset of the East Asian summer monsoon over the South China Sea (SCS) typically occurs around mid-to-late May (Wu and Zhang 1998; Ding and Chan 2005). It is marked by a reversal of the winds from north-easterlies to southwesterlies over the northern SCS, where a strong SST gradient exists as a remnant of the winter monsoon (Chu and Chang 1997). Xie et al. (2002) showed that for the East China Sea, the cooling is enhanced due to the shallow offshore waters, which should also be a factor for the northern SCS. The mean conditions in this region for May are shown in Fig. 4. A fairly strong SST front exists over the northern SCS with convergence of the surface winds quite noticeable along the boundary. Although the SST moderates somewhat as the monsoon season progresses, an SST gradient still exists in June, as shown Fig. 5. The mean surface winds for June are generally southerly and a deceleration of the flow is evident as air crosses the strong SST gradient southwest and northeast of Taiwan. The reduction in wind speed, and hence increase in low-level convergence, associated with the passage of the air from warm to cool water Fig. 4. Ten-year ( ) May mean QuikSCAT winds and SST (1 data; Reynolds et al. 2002). Note convergence along SST front over northern SCS.

4 January 3, :58 6 The Global Monsoon System: Research and Forecast (3rd Edition) in x 7.5in b2503-ch01 page 6 R. H. Johnson and M. D. Toy Fig. 5. As in Fig. 4, but for June. is consistent with the mechanism proposed by Wallace et al. (1989). The existence of enhanced low-level convergence upstream of Taiwan during the early summer monsoon or Mei-yu period when the atmosphere is convectively unstable suggests a possible impact of strong SST gradients upwind of Taiwan on island rainfall. The Mei-yu period is a time of very heavy rainfall over Taiwan (Chen et al. 2007). The possibility of air-sea interaction affecting Taiwan rainfall has been recently investigated by Toy and Johnson (2014). This work is the subject of the next section. 4. Effects of SST Gradients on Taiwan Rainfall during TiMREX In May-June 2008, a field campaign SoWMEX (Southwest Monsoon Experiment)/ TiMREX (Terrain-influenced Monsoon Rain-fall Experiment) was conducted over Taiwan and the surrounding region to investigate the mechanisms for heavy rainfall over Taiwan (Jou et al. 2011). Instrumentation deployed in the experiment included aircraft, special soundings, ships, research radars, and surface stations. These observations and a numerical model have been used in Toy and Johnson (2014) to investigate the potential impact of the SST gradient upstream of Taiwan on island rainfall. The focus of the Toy and Johnson (2014) study is on the June period of heavy rainfall over southern Taiwan when there was moist, southerly flow impinging on the island (Davis and Lee 2012; Xu et al. 2012). In order to determine the effects of the SST gradient, the Advanced Research Weather and Forecasting Research (WRF) model (ARW; Skamarock et al. 2008) was used to carry out two simulations: (1) a control run with the observed SST field (Fig. 6a, CTRLSST, from the daily realtime global SST (RTG SST) analyses of the National Centers for Environmental Prediction (NCEP) on a 0.5 grid) and (2) a companion run with a smoothed SST field (Fig. 6b, SMTHSST). The SMTHSST field was constructed in such a way so as to reduce the gradient upstream (to the southwest) of the island thereby examining its impact on Taiwan rainfall. The SST distribution upwind of Taiwan in Fig. 6b is not unlike that observed in a number of past years during the early summer monsoon. The SST difference plot (Fig. 6c) shows that compared to SMTHSST, the CTRLSST distribution is characterized by cooler water over the northern Taiwan Strait and warmer water off the south tip of Taiwan. The WRF is used with a convectionpermitting horizontal grid spacing of 3 km with a stretched vertical grid having 60 levels from the surface to 50 hpa. The following parameterizations are used: the WRF Single-Moment 6-Class Microphysics scheme (Hong and Lim 2006), Yonsei University boundary layer (Hong et al. 2006, Dudhia shortwave radiation, and rapid radiative transfer model (Mlawer et al. 1997) for longwave radiation. The control run was initialized with the NCEP Global Operational Analysis fields on the boundaries shown in Fig. 6 starting at 0000 UTC on 12 June with boundary values updated every six hours, and run up until 18 June An additional six runs were initialized with random perturbations using the WRF Model data assimilation (WRFDA) system (Barker et al. 2012). Results

5 January 3, :58 The Global Monsoon System: Research and Forecast (3rd Edition) in x 7.5in b2503-ch01 page 7 SST Gradients and the East Asian Early-Summer Monsoon 7 Fig. 6. (a) Control SST for Jun and (b) Smoothed-SST used in simulations, and (c) SST difference between the two. Arrow in (a) depicts direction of near-surface flow. shown are the means from the 7-member ensemble runs. Simulations were carried out with and without the island of Taiwan in the model in order to separate out the effects of the SST gradient from flow blocking by the high terrain of the island. For the sake of brevity, the no-taiwan results will not be shown here. The accumulated rainfall over the five-day June period from the control simulation is shown along with the Tropical Rainfall Measuring Mission (TRMM) rainfall estimate in Fig. 7. In addition to the large rainfall totals along the south coast of China and over the north-central SCS, the model shows a rainfall maximum along the southwest coast of Taiwan (Fig. 7a). The distribution of the rainfall over the entire domain agrees quite well with TRMM 3B42 estimates (Fig. 7b); however, TRMM places the rainfall maximum offshore whereas the model places it onshore. An operational rainfall product based on composite radar data prepared by Taiwan (not shown) gives better agreement with the model results in having the maximum rainfall just inland of the coastline. The TRMM product, which uses a merger of microwave and outgoing longwave radiation data, likely places the maximum rainfall too far to the southwest as a result of northeasterly flow aloft advecting cirrus from the coastal precipitation offshore. The north end of Taiwan received considerably less precipitation than the south end during this period of the experiment. The simulated rainfall from the SMTHSST run, along with the rainfall difference (CTRLSST SMTHSST), is shown in Fig. 8. The general pattern of rainfall over the large domain in the SMTHSST (Fig. 8a) is similar to that for the CTRLSST (Fig. 7a), as might be expected. However, there are important differences between the two simulations in several areas related to the different SST distributions (Fig. 8b). With respect to Taiwan, there is an increase in rainfall up to 100 mm along the southwest coast in the control simulation, indicating that the stronger SST gradient

6 January 3, :58 The Global Monsoon System: Research and Forecast (3rd Edition) in x 7.5in b2503-ch01 page 8 R. H. Johnson and M. D. Toy Fig. 7. Accumulated rainfall over model domain from 0000 UTC 13 Jun through 0000 UTC 18 Jun from (a) WRF CTRLSST simulation and (b) TRMM 3B42 data. The SST ( C) is overlaid (contours). is enhancing rainfall there. On the other hand, there is a reduction in rainfall over the central and northern Taiwan Strait as a result of the cooler water in the control simulation. The increased rainfall in the CTRLSST simulation over and just offshore the southwest coast of Taiwan is associated with increased convergence in that location (not shown). In order to understand how the SST gradient increases precipitation in this region, it is necessary to look at the modification of the Fig. 8. (a) Accumulated rainfall (color shading) over model domain from 0000 UTC 13 Jun through 0000 UTC 18 Jun from WRF SMTHSST simulation. SST ( C) is overlaid (contours). (b) Rainfall difference (color shading) between CTRLSST and SMTHSST simulations. SST difference between CTRLSST and SMTHSST simulations is overlaid (0.25 C contour interval with negative values indicated by dashed lines). boundary layer by the changing conditions of the underlying ocean. To do so, plots of the vertical structure of the lower atmosphere are presented for the CTRLSST and SMTHSST simulations (Fig. 9). The longitude of this plot is 120 E,

7 May 19, :24 The Global Monsoon System: Research and Forecast (3rd Edition) in x 7.5in b2503-ch01 page 9 SST Gradients and the East Asian Early-Summer Monsoon 9 Fig. 9. Vertical cross sections along 120 E of mean 0000 UTC 13 Jun 0000 UTC 14 Jun (a),(b) wind speed, (c),(d) horizontal divergence (color shading) and potential temperature (K) (contours, with 0.2 K interval for θ 302 K), and difference (CTRLSST SMTHSST) in (g) wind speed and (h) horizontal divergence. (e),(f) Mean 0000 UTC 13 Jun 0000 UTC 18 Jun buoyancy flux (red curve) and SST (black curve) along 120 E. (i) Difference (CTRLSST SMTHSST) in accumulated rainfall along 120 E from 0000 UTC 13 Jun through 0000 UTC 18 Jun. Letters S and M indicate latitude of ship (99810) and Makung (46734) radiosonde stations referred to in Toy and Johnson (2014). The bold brown line indicates the latitude range of the island of Taiwan for reference.

8 January 3, :58 The Global Monsoon System: Research and Forecast (3rd Edition) in x 7.5in b2503-ch01 page R. H. Johnson and M. D. Toy just off the west coast of Taiwan (Fig. 8). The wind speed profiles (Figs. 9a,b) are broadly similar for both simulations with a low-level jet developing within the latitude band of Taiwan. This feature is a south-westerly barrier jet that forms to the west of Taiwan owing to flow blocking by the island (Li and Chen 1988; Yeh and Chen 2002; Wang et al. 2005). However, within the latitude band encompassed by the TiMREX research ship (S) and Makung (M), there is an important difference in the flow very close to the surface. Namely, there is a deceleration of the flow in CTRLSST that is not present in SMTHSST. This feature is reflected in a localized convergence maximum between S and M in CTRLSST (Fig. 9c), which is weaker in SMTHSST (Fig. 9d). These two plots also show a much stronger near-surface frontal zone in this location in CTRLSST than SMTHSST. This frontal zone is anchored by the sharp poleward decline of the surface buoyancy flux across the SST front. This effect has been shown to be operative on the synoptic scale by Hotta and Nakamura (2011). Figures 9e,f illustrate the differing SST gradients and buoyancy fluxes across the span of Taiwan. The increased SSTs north of the front in the SMTHSST simulation serve to enhance the downward mixing of momentum from the barrier jet, thereby increasing the surface buoyancy fluxes and weakening the near-surface frontal boundary near 23 N. This process, in turn, leads to weaker convergence and less rainfall than in the CTRLSST case. The model difference fields of wind speed, divergence, and rainfall (Figs. 9g,h,i) show that the increased convergence and rainfall in the CTRLSST occurs at the leading (southern) edge of the SST front. The greater low-level stability and reduced downward mixing of momentum in the CTRLSST simulation near N support the arguments of Hayes et al. (1989) and Wallace et al. (1989) for SST gradient impacts on boundary layer convergence. The above findings illustrated for June in Fig. 9 (a period prior to the breakout of deep convection) have been repeated for a longer, convectively active period (13 17 June) with similar results obtained (Toy and Johnson 2014). These simulations were designed to demonstrate that the effects of the SST gradient on the low-level flow were independent of the existence of precipitating systems. In addition, simulations were carried out with the island of Taiwan removed and the results were qualitatively unchanged (not shown), implying that the SST gradient impacts on rainfall over southern Taiwan are essentially independent of topographic effects. Furthermore, the mechanism proposed by Lindzen and Nigam (1987) involving hydrostatic effects of air residing over cooler/warmer water was also investigated by Toy and Johnson (2014) and found to additionally contribute to some extent to the greater localized convergence over southern Taiwan in CTRLSST vs. SMTHSST. 5. Summary and Conclusions Strong SST gradients exist off the coast of East Asia during the early summer monsoon as a result of the previous winter s cold air outbreaks over the ocean. Such gradients have been shown to influence low-level divergence, clouds, and precipitation, particularly near strong SST gradients like those found in the Kuroshio Extension. There is typically a strong SST gradient over the northern South China Sea around the time of the onset of the summer monsoon in midto-late May, a period referred to as the Mei-yu in Taiwan. The effect of this SST gradient on rainfall over Taiwan during the 2008 SoWMEX/ TiMREX field campaign has been investigated using the Advanced Research WRF (Toy and Johnson 2014). Through a comparison of rainfall distributions in two experiments, one with the observed SST and one with a smoothed SST distribution, it is found that for the SoWMEX/TiMREX mid-june heavy rainfall

9 January 3, :58 The Global Monsoon System: Research and Forecast (3rd Edition) in x 7.5in b2503-ch01 page 11 SST Gradients and the East Asian Early-Summer Monsoon 11 period that the stronger SST gradient increases rainfall over southern Taiwan by enhancing lowlevel convergence along the SST front. A reduction in vertical mixing of higher-momentum air aloft to the surface occurs as the south-tosouthwesterly flow crosses from warm to cooler water, consistent with the mechanism proposed by Wallace et al. (1989). Since there is yearto-year variability in the strength of the SST gradient over the northern South China Sea as a result varying intensity of previous winter monsoons and other factors, these findings have important implications regarding seasonal prediction of the intensity and location of Mei-yu precipitation over Taiwan. Acknowledgments This research has been supported by the National Science Foundation under Grant AGS We acknowledge high-performance computing support from Yellowstone (ark:/ 85065/d7wd3xhc) provided by NCAR s Computational and Information Systems Laboratory, sponsored by the National Science Foundation. We also thank Brian McNoldy for creating several of the figures, Rick Taft for assistance with preparation of the article, and three anonymous reviewers for their helpful comments on the manuscript. References Barker, D. and coauthors, 2012: The Weather Research and Forecasting Model s Community Variational/Ensemble Data Assimilation System: WRFDA. Bull. Amer. Meteor. Soc., 93, Businger, J. A. and W. J. Shaw, 1984: The response of the marine boundary layer to mesoscale variations in sea-surface temperature. Dyn. Atmos. Oceans, 8, Chelton, D. B. and S.-P. Xie, 2010: Coupled oceanatmosphere interaction at oceanic mesoscales. Oceanography, 23, Chen, C.-S., Y.-L. Chen, C.-L. Liu, P.-L. Lin, and W.-C. Chen, 2007: Statistics of heavy rainfall occurrences in Taiwan. Wea. Forecasting, 22, Chu, P. C. and C.-P. Chang, 1997: South China Sea warm pool in boreal spring. Adv. Atmos. Sci., 14, Daisuke Hotta and Hisashi Nakamura, 2011: On the significance of the sensible heat supply from the ocean in the maintenance of the mean baroclinicity along storm tracks. J. Climate, 24, Davis, C. A. and W.-C. Lee, 2012: Mesoscale analysis of heavy rainfall episodes from SoWMEX/ TiMREX. J. Atmos. Sci., 69, de Szoeke, S. P. and C. S. Bretherton, 2004: Quasi-Lagrangian large eddy simulations of cross-equatorial flow in the East Pacific atmospheric boundary layer. J. Atmos. Sci., 61, Ding, Y. and J. C. L. Chan, 2005: The East Asian Summer Monsoon: An overview. Meteor. Atmos. Phys., 89, Hashizume, H., S.-P. Xie, M. Fujiwara, M. Shiotani, T. Watanabe, Y. Tanimoto, W. T. Liu, and K. Takeuchi. 2002: Direct observations of atmospheric boundary layer response to slow SST variations on the Pacific equatorial front. J. Climate, 15, Hayes, S. P., M. J. McPhaden, and J. M. Wallace, 1989: The influence of sea-surface temperature on surface wind in the eastern equatorial Pacific: Weekly to monthly variability. J. Climate, 2, Hong, S.-Y., Y. Noh, and J. Dudhia, 2006: A new vertical diffusion package with an explicit treatment of entrainment processes. Mon. Wea. Rev., 134, Hong, S.-Y. and J.-O. J. Lim, 2006: The WRF single-moment 6-class microphysics scheme (WSM6). J. Korean Meteor. Soc., 42, Jou, B. J.-D., W.-C. Lee, and R. H. Johnson, 2011: An overview of SoWMEX/TiMREX. The Global Monsoon System: Research and Forecasts, 2nd Edition, C.-P.Changet al., Eds. World Scientific, pp Li, J. and Y.-L. Chen, 1998: Barrier jets during TAMEX. Mon. Wea. Rev., 126, Lindzen, R. S. and S. Nigam, 1987: On the role of sea surface temperature gradients in forcing low-level winds and convergence in the tropics. J. Atmos. Sci., 44, Liu, W. T., X. Xie, P. S. Polito, S.-P. Xie, and H. Hashizume, 2000: Atmospheric manifestation of tropical instability waves observed by

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