Diurnal precipitation and high cloud frequency variability over the Gulf Stream and over the Kuroshio
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Recent studies show mid-latitude western boundary currents (WBCs) substantially influence the atmosphere aloft, and an important feature is enhanced rain band over the WBCs in climatological mean field. However, how such long-term, climate phenomena are related to shorter, weather timescale phenomena are generally remained to be explored. In this paper, diurnal precipitation and cloud variations are investigated global mid-latitude oceans with emphasis on air–sea interactions over WBCs using satellite-derived precipitation and outgoing longwave radiation (OLR) datasets. Strong 24-h period components of precipitations are found over the Gulf Stream in summer and over the Kuroshio in the East China Sea in early summer (Baiu–Meiyu season), respectively. Similar diurnal precipitations are not observed in WBCs in the Southern Hemisphere year around. The diurnal precipitation cycles over the Gulf Stream and the Kuroshio exhibit peak phases in the early to late morning for the Gulf Stream and late morning to early afternoon for the Kuroshio, with southeastward phase propagations. High cloud frequency derived from OLR data exhibit consistent diurnal cycles. A substantial difference of diurnal cycles between the Gulf Stream and the Kuroshio regions are associated with the large-scale Baiu–Meiyu rain and cloud bands for the latter region. Diurnal precipitation and high cloud variability is found in the vicinity of the Kuroshio itself, embedded in the Baiu–Meiyu rain and cloud bands distributing in a wider area without a strong diurnal component. The spatial and seasonal distributions of the diurnal variability over these WBCs strongly suggests that the diurnal precipitation and cloud cycles are essential aspects of deep heating mode of atmospheric response recently reported for these WBCs. These results indicate that these WBCs in the Northern Hemisphere play an important role in modulating short-term precipitation variations, and on the other hand diurnal variability can be a substantial agent for the mid-latitude air–sea interaction.
KeywordsClimate process Air–sea interaction Western boundary currents Mid-latitude Scale-interaction Observational data analysis
How mid-latitude oceans fronts, associated with western boundary currents (WBCs) and meso-scale eddies, influence the overlying atmosphere is a rapidly developing research field. In particular, air–sea interactions over four mid-latitude WBCs, i.e., the Gulf Stream, the Kuroshio and its extension, the Agulhas Return Current, and the Brazil-Malvinas currents, have attracted much attention (see reviews by Chelton et al. 2004; Xie 2004; Small et al. 2008; Chelton and Xie 2010). Among these four regions, prominent tropospheric responses to the WBCs above the Marine Atmospheric Boundary Layer (MABL) were reported for the Gulf Stream (Minobe et al. 2008, 2010; Czaja and Blunt 2011), the Kuroshio Extension (Tokinaga et al. 2009; Czaja and Blunt 2011) and the Kuroshio in the East China Sea (Xu et al. 2011; Sasaki et al. 2012), characterized by enhanced rain accompanied by ascent. Also, cloud distribution and tropospheric temperatures are modified over the Agulhas Return Current region (Liu et al. 2007; Shimada and Minobe 2011). These studies, however, mainly investigated seasonal or annual mean climatologies. The atmospheric responses found in the time mean fields must involve time varying phenomena on shorter timescales. This is especially true for precipitation, because its occurrence is highly intermittent and is closely related to shorter timescale phenomena such as synoptic scale disturbances and diurnal variability.
Most of the previous studies that have investigated the relation between shorter timescale phenomena and air–sea interaction over the mid-latitude WBCs have focused on synoptic scale variations. Synoptic scale disturbances strongly modify air–sea heat fluxes in WBC regions as found in observations (e.g., Alexander and Scott 1997; Zolina and Gulev 2003; Shaman et al. 2010; Rudeva and Gulev 2011) and also in modeling studies (e.g., Nonaka et al. 2009; Taguchi et al. 2009; Brayshaw et al. 2011). The sensible and latent heat fluxes are important in maintaining the surface baroclinicity (e.g., Hoskins and Valdes 1990; Nakamura et al. 2004, 2008), which is crucial for developments of extratropical cycles (Hoskins et al. 1985) and thus important in determining the nature of the storm tracks (e.g., Inatsu et al. 2003; Inatsu and Hoskins 2004; Brayshaw et al. 2008, 2011; Ogawa et al. 2012). In particular, surface storm tracks and high wind distributions are strongly affected by WBCs (Sampe and Xie 2007; Joyce et al. 2009; Booth et al. 2010), and atmospheric latent heating associated with precipitation plays an important role in cyclone development over WBC regions (e.g., Kuo et al. 1991; Kuwano-Yoshida et al. 2010; Booth et al. 2012). Furthermore, an observational study by Nakamura et al. (2012) showed that wintertime extratropical cyclone tracks are modified by the Kuroshio’s large meander south of Japan, which was confirmed in the numerical experiment by Hayasaki et al. (2013).
In contrast to the large body of work on synoptic scale variability, the linkage between air–sea interaction over the WBCs and diurnal variability is much less well studied. Most studies of diurnal precipitation cycles focused their attention on diurnal variations over land (e.g., Wallace 1975; Oki and Musiake 1994) or in the tropics (e.g., Janowiak et al. 1994; Yang and Slingo 2001; Nesbitt and Zipser 2003; Kikuchi and Wang 2008), probably reflecting data availability in early studies and also the overall tendency for strong diurnal variations over land and/or the tropics. However, one can see that a relatively strong diurnal cycle of precipitation occurs over the Gulf Stream in a global analysis (Fig. 2 in Kikuchi and Wang 2008) or in a regional analysis focused on North America (Fig. 6 in Dirmeyer et al. 2012) in summer, though the possible dependency of this diurnal precipitation to the Gulf Stream was not mentioned in these papers. Diurnal cloud variations over the Gulf Stream region were also reported a decade ago by Alliss and Raman (1995), though their analysis was limited to the western portion of the Gulf Stream (west of 71°W) using 6-hourly data, which is coarsely sampled compared with hourly or 3-hourly sampled data used in recent studies (e.g., Dai et al. 2007; Kikuchi and Wang 2008).
Given the fact that the oceanic diurnal precipitations are generally stronger in the tropics than in the mid-latitudes, it is worthwhile to note that tropical-like atmospheric responses to mid-latitude WBCs have been identified in recent studies. Minobe et al. (2010) found a stark difference in the atmospheric response over the Gulf Stream between the summer and winter seasons. In summer, maximum upward winds are found in the middle of the troposphere, just above the enhanced precipitation over the Gulf Stream, accompanied by horizontal convergence in the lower troposphere. Atmospheric heating has its maximum also in the mid-troposphere, where convective latent heating dominates. The authors referred to this type of atmospheric response as the deep heating mode. In winter, upward winds have their maximum in the lower troposphere (850–700 hPa), and the corresponding horizontal convergence is mostly limited to the MABL, though summer time convergence is not confined in this layer. The corresponding atmospheric heating consists of sensible heating (including vertical diffusion of heat in the MABL) and convective latent heating just above the top of the MABL, resulting in total atmospheric heating that exhibits a maximum in the lower troposphere. The authors referred to this type of atmospheric response as the shallow heating mode, and showed that a bimodal variation of the atmospheric heating profile exists between deep and shallow distributions.
A deep heating mode as energetic as that over the Gulf Stream is also found in the atmospheric response over the Kuroshio in the East China Sea in June, i.e., middle of early summer rain season in this region (Sasaki et al. 2012). This long-lasting precipitation phenomenon prevailing from southeast China to Japan is called Baiu in Japan and Meiyu in China. The Baiu–Meiyu rainband is caused by basin–scale interactions between the atmosphere, land and the ocean. Moisture transport by low-level jets along the western flank of the subtropical high is a key factor in generating the Baiu–Meiyu rainband (e.g., Ninomiya and Akiyama 1992), whereas horizontal temperature advection from the Tibetan Plateau also plays an important role in determining its location (Sampe and Xie 2010). Sasaki et al. (2012) found that a localized atmospheric response to the Kuroshio, with the deep heating mode, is embedded in the large-scale Baiu–Meiyu rainband. A weak deep heating mode is also observed over the Kuroshio Extension in summer (Minobe et al. 2010; Tokinaga et al. 2009), and over the East China Sea in spring (Xu et al. 2011). The presence of tropical-like atmospheric responses to the Gulf Stream may be related to the aforementioned diurnal precipitation over the Gulf Stream. If so, is similar diurnal precipitation variability also found over the Kuroshio in the East China Sea? Can prominent diurnal precipitations be found in WBCs in the Southern Hemisphere? Minobe et al. (2010) suggested that for an energetic deep heating mode to exist, sea-surface temperature (SST) warmer than 24–26 °C may be necessary, and further suggested that the absence of the deep heating mode (i.e. as strong as over the Gulf Stream) for the WBCs in the Southern Hemisphere due to the relatively cold SSTs in those regions.
The purpose of the present paper is, therefore, to investigate diurnal variability of precipitations over mid-latitude WBCs. For this purpose, we analyze a satellite-derived precipitation dataset globally in order to investigate the overall connection of diurnal variations to the WBCs and also regionally to determine the detailed spatio-temporal structures. The rest of the present paper is organized as follows. In Sect. 2, we describe the datasets and analysis method. In Sect. 3, diurnal precipitations are analyzed globally. This global analysis indeed reveals strong diurnal precipitation variability in summer over the Gulf Stream and over the Kuroshio in the East China Sea. Thus, in the following two sections (Sects. 4, 5), we conduct closer analyses for these two regions. Discussion and conclusions are presented in Sect. 6.
2 Data and method
We analyzed a satellite-derived precipitation dataset and an outgoing longwave radiation (OLR) dataset. The precipitation dataset is GSMaP MVK version 5 (Ushio et al. 2009). The rain rates in this dataset are estimated using a Kalman filter from multiple micro-wave satellites combined with an infra-red (IR) dataset provided by National Oceanic and Atmospheric Administration–Climate Prediction Center (NOAA–CPC), and are provided on an hourly, 0.1° × 0.1° grid from March 2000 to December 2010. The IR data are used for moving vectors of estimated precipitation and also for the estimation of the precipitation itself from brightness temperatures. It should be noted that most of the previous studies of diurnal precipitation used the TRMM 3B42 dataset, which is on a three-hourly 0.25° × 0.25° grid (Huffman et al. 2007), whereas the GSMaP MVK has higher spatial and temporal resolutions than that.
We also analyzed a global 3-hourly, 1° × 1° OLR dataset of NASA/GEWEX Surface Radiation Budget (SRB) project (Stackhouse et al. 2004). This dataset is available from NASA Langley Research Center’s Atmospheric Science Data Center for a period from July 1982 to December 2007, and analyzed in this study from 2000 to 2007. The data was briefly analyzed by Minobe et al. (2008), who reported frequent daytime high cloud occurrence along the Gulf Stream based on occurrence frequency of OLR levels lower than 160 W/m2, which roughly corresponds to a cloud-top height of about 300 hPa. Following their approach, we also examine occurrence rate of OLR smaller than 160 W/m2 as a measure of high cloud frequency. In order to know whether the major features found in the OLR-based high cloud frequency are consistent with independent cloud data, we also examine cloud amount for high-level clouds of D2 dataset of International Satellite Cloud Climatology Project (ISCCP) for a period from 2000 to 2009 (Rossow and Schiffer 1999). Spatial resolution D2 dataset is 2.5° × 2.5°, coarser than that of OLR, with the 3-hourly sampling interval. High-level clouds in ISCCP D2 dataset are defined clouds that have cloud top pressures are smaller than 440 hPa, and thus include lower clouds than those used for OLR-based high-cloud frequency.
Diurnal climatologies are calculated for each month and also for each season, and from these the diurnal amplitude and phase of the 24-h Fourier component are estimated. The phase is expressed by the local solar time (LST) for the maximum of this Fourier component, where LST is calculated from coordinated universal time (UTC) with longitudinal distance between each grid point and 0°. Also, we examine relative amplitude, given by the amplitude divided by the mean value. If diurnal variability consists of pure sinusoidal wave of 24-h component, relative amplitude is in a range between 0 and 100 %. In this case, a relative amplitude of 100 % means that the daily minimum is 0 % and the maximum is 200 % of the mean value. A relative amplitude can be larger than 100 %, when precipitation occur in a narrower time range than that expected from a sinusoidal wave.
In order to show geographical relations between atmospheric and oceanic structures, we also use climatological satellite-derived SST and surface geostrophic current velocity data. SST data used for calculating climatological SSTs are optimal interpolation SST ver. 2 on a daily, 0.25° × 0.25° grid (Reynolds et al. 2007). Weekly surface geostrophic current velocities, estimated from satellite altimeter, are provided by AVISO on a 1/3° × 1/3° grid (http://www.aviso.oceanobs.com/en/data/product-information/duacs/ssaltoduacs-products.html).
3 Global view
The relative amplitudes (see Sect. 2) of the diurnal precipitation over the Gulf Stream are about 40–70 %, and over the Kuroshio are 30–60 % (Fig. 2b). These values are much higher than general values, found to be 10–30 % over much of the ocean (Dai et al. 2007). The relative amplitudes over the Gulf Stream and the Kuroshio indicate a possibility that the diurnal cycle of precipitation in these regions provides an important contribution to the mean rain rate.
Figure 2c shows phases at which the 24-h precipitation component reaches its maximum in LST. A prominent feature is the contrasting phase over land and ocean. It is known that the diurnal cycle tends to have its maximum over the ocean in local morning, while over land it peaks in the local afternoon to evening (e.g., Dai et al. 2007; Kikuchi and Wang 2008). The phases over the Northern Hemisphere WBCs follow these general tendencies.
4 Gulf Stream analysis
The phase of diurnal precipitation in July over the Gulf Stream (Fig. 6b) indicates that the diurnal precipitation peaks in the morning (about 5–11 LST). As mentioned previously, this early morning maximum is a common feature of diurnal precipitation cycles over the ocean (Yang and Slingo 2001; Dai et al. 2007; Kikuchi and Wang 2008). The phase generally increases seaward from the Gulf Stream, suggesting an southeastward phase propagation, with a phase speed in the range of 10–30 m/s. Seaward phase propagations of tropical diurnal precipitation cycles were reported in previous studies with speeds of 10–20 m/s, possibly due to gravity waves (Yang and Slingo 2001; Kikuchi and Wang 2008). For the aforementioned two amplitude maxima, the phase of the off-shore maximum slightly leads that of the near-shore maximum, indicating that precipitation occurs slightly earlier over the Gulf Stream after the separation than those before the separation with seaward propagation in respective regions. The phases over land indicate that the diurnal precipitation tend to peak in the late afternoon and early evening (about 15–19 LST), with phases in the Florida peninsula leading those along the coast north of 32°N.
As noted above, the diurnal precipitation cycles over the Gulf Stream and over land have roughly opposite phases. In light of this, it is natural to consider whether the diurnal precipitation cycle over the ocean is tightly related to that over land. For example, is diurnal precipitation variability over the Gulf Stream strongly influenced by the diurnal precipitation over North America? Or are these two diurnal variations are essentially independent? In order to gain some insight into these questions, we conduct a lag-correlation analysis using raw precipitation data. Here, lag-correlations at each grid point are calculated with respect to the area-averaged precipitation time series. Two reference time series are chosen; one is taken for the aforementioned near-shore diurnal amplitude maximum over the Gulf Stream (31–33°N, 78–76°W), and the other is taken from a region of strong diurnal precipitation cycle over land, along the same latitudinal band (31–33°N, 83–81°W). The observed significance of the correlations is estimated based on lag-correlations of large-lags, where data can be assumed to be uncorrelated. More specifically, we calculate lag correlations with large lags from 7 to 14 days every 6 h. In order to examine the significance at a specific grid point these large-lag correlations ±5° in latitudes and longitudes are collected. The collected correlations are then sorted in order their absolute correlations. If the observed absolute correlation is higher than top 5 % of the collected large-lag absolute correlations, the observed correlation is considered to be significant at the 5 % significance level. This approach is better than the conventional theoretical estimation of significance, because raw hourly rain rate does not follow a Gaussian distribution, which is assumed for the theoretical estimation.
The strongest diurnal cycle is found in June over the Kuroshio in the East China Sea (Fig. 10c). In this month, mean rain rate is also the strongest, consistent with Sasaki et al. (2012) (Fig. 10d). The maximal diurnal amplitude is as high as 0.36 mm/h to the west of Ryukyu Islands, which is about 30 % larger than the maximum amplitude during July in the North Atlantic (0.28 mm/h), and this is the largest diurnal precipitation amplitude over the mid-latitude global oceans in all calendar months. In June, the secondary amplitude maximum is seen to the south of Kyushu, and two patches of relatively high amplitude are found in the North Pacific to the south of Japan. These spatial features are not seen in the average precipitation, which is more smoothly distributed from the East China Sea to the North Pacific. The strong diurnal precipitations in the East China Sea are limited to June and not found in either May or July (Fig. 10a, c, e). This is in contrast to the diurnal cycle over the Gulf Stream, which exhibits similar spatial structures during all summer months. The rapid month to month transition of diurnal precipitations is closely related to the mean precipitation change, which is governed by the northward migration of the Baiu–Meiyu rainband (e.g., Wang and LinHo 2002). In the rest of this section, we closely examine diurnal cycles over the Kuroshio in June.
Figure 12b shows the phase distribution of the 24-h components of precipitation over the Kuroshio region. Peak time for large diurnal amplitudes (say 0.2 mm/h or more) is 10–13 LST, few hours later than that over the Gulf Stream. Also, around the large diurnal amplitudes, phases indicate southeastward increase with the speeds of 10–20 m/s. The propagation direction is roughly the same as that in the Gulf Stream region, but the propagation speed over the Kuroshio is somewhat slower.
Phases of the 24-h component of high cloud occurrence rate exhibit peaks in the early afternoon (12–16 LST) over regions of strong amplitudes, straddling the East China Sea and the western North Pacific (Fig. 13d). In this region, phases of high cloud occurrence are delayed by 2–3 h compared with those of precipitation, and this delay is slightly longer than is found over the Gulf Stream. An southeastward phase propagation is found for the large amplitude region, consistent with precipitation. The phase and amplitude distributions of high-cloud frequency derived from OLR data are generally consistent with those of high-cloud amount of ISCCP D2 dataset (not shown).
6 Discussion and conclusions
Diurnal variations over the mid-latitude oceans are analyzed using an hourly, satellite-derived precipitation dataset (GSMaP MVK ver. 5) and 3-hourly OLR data from the SRB project. Previous studies have also described diurnal precipitation cycles, including the mid-latitude oceans (e.g., Dai et al. 2007), although the link between diurnal precipitation cycles and WBCs is analyzed comprehensively in this study. It is found that over mid-latitude oceans, poleward of 25°N/S, strong diurnal cycles occur over the Northern Hemisphere WBCs, i.e., the Gulf Stream and the Kuroshio, in the boreal summer season (Fig. 2). No prominent diurnal precipitation is found over the WBCs in the Southern Hemisphere the whole year round (Figs. 2, 3).
There are some common features but also differences in the diurnal cycles between the regions over the Gulf Stream and the Kuroshio. In both regions, maximum amplitudes of diurnal precipitation occur roughly over the current axes with a pattern contracted westward compared with the rainbands, more specifically, just before and after the separation at the Cape Hatteras for the Gulf Stream (Fig. 4) and west of Ryukyu Islands in the East China Sea for the Kuroshio (Fig. 10). The maximum amplitudes are slightly larger over the Kuroshio than over the Gulf Stream. The diurnal phases commonly exhibit southeastward propagation, with peak times for the large amplitudes being from early to late morning (5–11 LST) for the Gulf Stream (Fig. 6b) and from late morning to noon (10–13 LST) over the Kuroshio in the East China Sea (Fig. 12b). Occurrence frequencies of high clouds exhibit diurnal cycles consistent with those of the precipitation. The relative amplitudes of high cloud occurrence are larger than those of precipitation, suggesting that the diurnal precipitation is mainly due to deep convection, consistent with studies of tropical diurnal precipitation over the oceans (e.g., Janowiak et al. 1994; Yang and Slingo 2001). Near the Gulf Stream, diurnal precipitation cycles over North America are roughly 180° out-of-phase of those over the ocean (Fig. 6b). The lag-correlation analysis shows that a direct link between oceanic and terrestrial diurnal cycles is not strong (Fig. 7), if it exists at all, and thus they probably originate different physical mechanisms.
The diurnal cycles are likely to be closely associated with the deep heating mode of the atmospheric response to the WBCs. The spatial and seasonal distributions of the strong deep heating mode found in previous studies (Minobe et al. 2010; Sasaki et al. 2012) and those of diurnal precipitation appear to be similar to one another. This is not surprising, because as discussed in Sect. 1, it is known that diurnal precipitation variability is strong in the tropics, and the deep heating mode is a tropical-like atmospheric response to ocean fronts over SSTs warmer than 24–26 °C (Minobe et al. 2010). It is noteworthy that diurnal precipitation variability over the Gulf Stream and the Kuroshio has higher relative amplitudes than those over most of the tropics. Furthermore, contributions of diurnal cycles to average rain rate are larger over the WBCs than in the ITCZ and SPCZ, and thus diurnal variability can be an important agent for producing in mean precipitations in the WBC regions. Consequently, it is strongly suggested that the diurnal precipitation and cloud cycles are intrinsic and important aspects of the deep heating mode.
A natural question is what mechanism is responsible for the diurnal cycles over the Gulf Stream and the Kuroshio? This is an open question not only for the diurnal cycles over these WBCs but also for diurnal cycles over the tropical oceans, which have been much more widely studied (e.g., Dai et al. 2007; Kikuchi and Wang 2008). Yang and Smith (2006) summarized twelve proposed mechanisms for diurnal precipitation. Among these mechanisms, a morning maximum may be produced by two mechanisms. One mechanism is static radiation convection, in which radiative cooling from the top of clouds during the night time destabilizes the upper cloud, and thus increases convection and precipitation (e.g., Kraus 1963; Randall et al. 1991). Another mechanism is static radiation convection with enhanced moistening (Tao et al. 1996; Sui et al. 1997). In this mechanism, enhanced night time radiative cooling increases humidity, and thus amplifies condensation and precipitation. It is interesting to examine whether one of these, or other, mechanisms play a role over the WBCs in future study. The southeastward phase propagations observed over the Gulf Stream and East China Sea are somewhat reminiscent of previously reported seaward phase propagations of diurnal precipitation cycles in the tropics (Yang and Slingo 2001; Kikuchi and Wang 2008). It is suggested that the tropical seaward phase propagation is due to gravitation waves, which can generated by topographic effects (Mapes et al. 2003) or land convection (Zuidema 2003). Another possible mechanism for the phase propagation is advection by background winds, as has been suggested for diurnal cloud developments over the Gulf Stream region (Alliss and Raman 1995).
In order to identify the mechanism, atmospheric numerical models should be useful. Indeed, Fig. 6 of Dirmeyer et al. (2012) shows that enhanced diurnal precipitation over the Gulf Stream are somewhat reproduced by several numerical models. However, in that paper, which focuses on diurnal precipitations over North America, phases over the North Atlantic are masked out due to small amplitudes for most of models, and thus it is not clear whether these models can reproduce the observed phase distribution. A model whose phase distribution over the western North Atlantic was shown in that paper is the Nonhydrostatic ICosahedral Atmospheric Model (NICAM), which exhibits reasonable phase difference between the off-shore region and near-shore region along the Gulf Stream, but the seaward phase increase is not seen (Fig. 5 in their paper). Nevertheless, a reasonable step for identifying the mechanism for the diurnal cycle in real world may be to examine these and other model outputs to assess the degree to which they can reproduce the diurnal cycles and to then identify the responsible mechanisms.
It should be worth noting that successfully capturing the diurnal cycle in numerical atmospheric models can be essential in producing mean realistic distributions of precipitation and thus atmospheric heating. Moisture used for the precipitation over the Gulf Stream and the Kuroshio is supplied by the local evaporation and moisture transport from the south along the west flank of the subtropical high (Minobe et al. 2010; Sasaki et al. 2012). The diurnal cycle can thus play an essential role in turning the moisture into precipitation in the relevant locations. If this hypothesis is correct, numerical model tunings, which primarily focus on the diurnal cycle, can improve not only the diurnal cycle itself but also mean precipitation distributions. This hypothesis can be examined by changing parameterizations or radiation forcings in a model. According to this hypothesis, the diurnal cycle can be important not only in the context of weather, but also from a climate perspective. Consequently, the diurnal cycles over the Gulf Stream and the Kuroshio, which may be an intrinsic aspect of the deep heating mode, link the ocean and the atmosphere, and also possibly weather and climate.
Animation of diurnal climatology of rain rate over the Gulf Stream region in July (and also over the Kuroshio region in June examined in Sect. 5) is found at author’s home page, http://www.sci.hokudai.ac.jp/~minobe/animation/diurnal_precip_WBCs/.
Many thanks for Dr. C. O’Reilly for comments on the English and also on the science, and for two anonymous reviewers for their helpful and constructive comments. This work was supported by the Japan Society for the Promotion of Science (Grant-in-Aid for Scientific Research 22106008 and 22244057).
- Dirmeyer PA, Cash BA, Kinter JL, Jung T, Marx L, Satoh M, Stan C, Tomita H, Towers P, Wedi N, Achuthavarier D, Adams JM, Altshuler EL, Huang BH, Jin EK, Manganello J (2012) Simulating the diurnal cycle of rainfall in global climate models: resolution versus parameterization. Clim Dyn 39(1–2):399–418. doi: 10.1007/s00382-011-1127-9 CrossRefGoogle Scholar
- Huffman GJ, Adler RF, Bolvin DT, Gu GJ, Nelkin EJ, Bowman KP, Hong Y, Stocker EF, Wolff DB (2007) The TRMM multisatellite precipitation analysis (TMPA): quasi-global, multiyear, combined-sensor precipitation estimates at fine scales. J Hydrometeorol 8(1):38–55. doi: 10.1175/jhm560.1 CrossRefGoogle Scholar
- Kubota M, Tomita H (2007) The present state of the J-OFURO air–sea interaction data product. Flux News, WCRPGoogle Scholar
- Mapes BE, Warner TT, Xu M (2003) Diurnal patterns of rainfall in northwestern South America. Part III: diurnal gravity waves and nocturnal convection offshore. Mon Wea Rev 131 (5):830–844. doi: 10.1175/1520-0493(2003)131<0830:dporin>2.0.co;2
- Nakamura H, Sampe T, Tanimoto Y, Simpo A (2004) Observed associations among storm tracks, jet streams and midlatitude oceanic fronts. In: Wang C, Xie SP, Carton JA (eds) The Earth’s climate: the ocean–atmosphere interaction, Geophys. Monogr., vol 147. Amer. Geophys. Union, pp 329–346Google Scholar
- Nakamura H, Sampe T, Goto A, Ohfuchi W, Xie S-P (2008) On the importance of midlatitude oceanic frontal zones for the mean state and dominant variability in the tropospheric circulation. Geophys Res Lett 35 (15). doi: 10.1029/2008gl034010
- Ninomiya K, Akiyama T (1992) Multiscale features of Baiu, the summer monsoon over Japan and the East-Asia. J Meteor Soc Jpn 70(1B):467–495Google Scholar
- Nonaka M, Nakamura H, Taguchi B, Komori N, Kuwano-Yoshida A, Takaya K (2009) Air–sea heat exchanges characteristic of a prominent midlatitude oceanic front in the South Indian Ocean as simulated in a high-resolution coupled GCM. J Clim 22(24):6515–6535. doi: 10.1175/2009jcli2960.1 CrossRefGoogle Scholar
- Ogawa F, Nakamura H, Nishii K, Miyasaka T, Kuwano-Yoshida A (2012) Dependence of the climatological axial latitudes of the tropospheric westerlies and storm tracks on the latitude of an extratropical oceanic front. Geophys Res Lett 39. doi: 10.1029/2011gl049922
- Saha S, Moorthi S, Pan HL, Wu XR, Wang JD, Nadiga S, Tripp P, Kistler R, Woollen J, Behringer D, Liu HX, Stokes D, Grumbine R, Gayno G, Wang J, Hou YT, Chuang HY, Juang HMH, Sela J, Iredell M, Treadon R, Kleist D, Van Delst P, Keyser D, Derber J, Ek M, Meng J, Wei HL, Yang RQ, Lord S, Van den Dool H, Kumar A, Wang WQ, Long C, Chelliah M, Xue Y, Huang BY, Schemm JK, Ebisuzaki W, Lin R, Xie PP, Chen MY, Zhou ST, Higgins W, Zou CZ, Liu QH, Chen Y, Han Y, Cucurull L, Reynolds RW, Rutledge G, Goldberg M (2010) The NCEP climate forecast system reanalySIS. Bull Am Meteor Soc 91(8):1015–1057. doi: 10.1175/2010bams3001.1 CrossRefGoogle Scholar
- Shimada T, Minobe S (2011) Global analysis of the pressure adjustment mechanism over sea surface temperature fronts using AIRS/Aqua data. Geophys Res Lett 38. doi: 10.1029/2010gl046625
- Stackhouse PW, Gupta SK, Cox SJ, Colleen MJ, Zhang T, Chiacchio M (2004) Twelve-year surface radiation budget data set. GEWEX News 14(4):10–12Google Scholar
- Taguchi B, Nakamura H, Nonaka M, Xie S-P (2009) Influences of the Kuroshio/Oyashio Extensions on Air–Sea Heat Exchanges and Storm-Track activity as revealed in regional atmospheric model simulations for the 2003/04 cold season. J Clim 22(24):6536–6560. doi: 10.1175/2009jcli2910.1 CrossRefGoogle Scholar
- Ushio T, Sasashige K, Kubota T, Shige S, Okamoto K, Aonashi K, Inoue T, Takahashi N, Iguchi T, Kachi M, Oki R, Morimoto T, Kawasaki ZI (2009) A Kalman filter approach to the global satellite mapping of precipitation (GSMaP) from combined passive microwave and infrared radiometric data. J Meteor Soc Jpn 87:137–151. doi: 10.2151/jmsj.87A.137 CrossRefGoogle Scholar
- Yang S, Smith EA (2006) Mechanisms for diurnal variability of global tropical rainfall observed from TRMM. J Clim 19(20):5190–5226. doi: 10.1175/JCLI3883.1
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