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1. Introduction Sunlight absorbed in the near-surface ocean in low winds causes vertical temperature gradients of T z = O (0.1–1)°C m −1 over the top few meters. The wind’s momentum is captured in this thin, stable layer termed a diurnal warm layer (DWL). This forms a diurnal jet that moves at 0.1–0.2 m s −1 . Except in low winds (<2 m s −1 ), the gradient Richardson number is often below ¼ ( Kudryavtsev and Soloviev 1990 ; Sutherland et al. 2016 ; Hughes et al. 2020a ), and therefore the
1. Introduction Sunlight absorbed in the near-surface ocean in low winds causes vertical temperature gradients of T z = O (0.1–1)°C m −1 over the top few meters. The wind’s momentum is captured in this thin, stable layer termed a diurnal warm layer (DWL). This forms a diurnal jet that moves at 0.1–0.2 m s −1 . Except in low winds (<2 m s −1 ), the gradient Richardson number is often below ¼ ( Kudryavtsev and Soloviev 1990 ; Sutherland et al. 2016 ; Hughes et al. 2020a ), and therefore the
herein as outflow jets. It should be noted that the strong outflow from 1500 to 2100 LST 29 September can be attributed to the background easterlies and not from the circulation of Typhoon Kong-rey. Associated with the first diurnal pulse there is an increase in the height of precipitating clouds as rainbands moved into range of SEA-POL ( Figs. 12b–d ). Following the first pulse and associated outflow jet, there was a lull in the radial outflow and both a decrease in height and median reflectivity of
herein as outflow jets. It should be noted that the strong outflow from 1500 to 2100 LST 29 September can be attributed to the background easterlies and not from the circulation of Typhoon Kong-rey. Associated with the first diurnal pulse there is an increase in the height of precipitating clouds as rainbands moved into range of SEA-POL ( Figs. 12b–d ). Following the first pulse and associated outflow jet, there was a lull in the radial outflow and both a decrease in height and median reflectivity of
entering the Mindanao Current. Weak eastward flow appeared at subthermocline depths below the 15°C isotherm ( Fig. 15e ) and may be a manifestation of the North Equatorial Undercurrent (NEUC) jets that, in the mean, extend across the Pacific ( Cravatte et al. 2012 ; Qiu et al. 2015 ). The upper ocean properties and velocity sections during PISTON prior to Mangkhut did show some variability from transect to transect. In glider transects along 134.3°E north of Palau repeated over a 4-yr period, Schönau
entering the Mindanao Current. Weak eastward flow appeared at subthermocline depths below the 15°C isotherm ( Fig. 15e ) and may be a manifestation of the North Equatorial Undercurrent (NEUC) jets that, in the mean, extend across the Pacific ( Cravatte et al. 2012 ; Qiu et al. 2015 ). The upper ocean properties and velocity sections during PISTON prior to Mangkhut did show some variability from transect to transect. In glider transects along 134.3°E north of Palau repeated over a 4-yr period, Schönau
-MCSs. Increased low-level wind shear has been shown to lead to an increase in convective organization, which may have aided the formation of MCSs ( Thorpe et al. 1982 ; LeMone et al. 1998 ; Lang et al. 2007 ). However, mesoscale flow features within MCSs can also act to increase low-level shear (i.e., a descending rear inflow jet). Therefore, it cannot be ruled out that the observed increase in shear may be due to processes within MCSs themselves, rather than changes background environmental conditions. An
-MCSs. Increased low-level wind shear has been shown to lead to an increase in convective organization, which may have aided the formation of MCSs ( Thorpe et al. 1982 ; LeMone et al. 1998 ; Lang et al. 2007 ). However, mesoscale flow features within MCSs can also act to increase low-level shear (i.e., a descending rear inflow jet). Therefore, it cannot be ruled out that the observed increase in shear may be due to processes within MCSs themselves, rather than changes background environmental conditions. An
and 30–60-day variations in the equatorial Indian Ocean . J. Phys. Oceanogr. , 35 , 708 – 728 , https://doi.org/10.1175/JPO2725.1 . 10.1175/JPO2725.1 Han , W. , J. P. McCreary Jr ., D. L. T. Anderson , and A. J. Mariano , 1999 : Dynamics of the eastern surface jets in the equatorial Indian Ocean . J. Phys. Oceanogr. , 29 , 2191 – 2209 , https://doi.org/10.1175/1520-0485(1999)029<2191:DOTESJ>2.0.CO;2 . 10.1175/1520-0485(1999)029<2191:DOTESJ>2.0.CO;2 Han , W. Q. , D. M
and 30–60-day variations in the equatorial Indian Ocean . J. Phys. Oceanogr. , 35 , 708 – 728 , https://doi.org/10.1175/JPO2725.1 . 10.1175/JPO2725.1 Han , W. , J. P. McCreary Jr ., D. L. T. Anderson , and A. J. Mariano , 1999 : Dynamics of the eastern surface jets in the equatorial Indian Ocean . J. Phys. Oceanogr. , 29 , 2191 – 2209 , https://doi.org/10.1175/1520-0485(1999)029<2191:DOTESJ>2.0.CO;2 . 10.1175/1520-0485(1999)029<2191:DOTESJ>2.0.CO;2 Han , W. Q. , D. M
reduces to a counterclockwise path, a result that holds across the entire dataset (1). This pathway neatly illustrates the sequence shown by Moulin et al. (2018) . Nighttime convection leaves small N and comparatively large ϵ χ early in the day. The building diurnal stratification quenches ϵ χ as N increases. At depths below z T z max , there is warming by local absorption of solar radiation, but presumably negligible shear from the diurnal jet. This state may last several hours. On the
reduces to a counterclockwise path, a result that holds across the entire dataset (1). This pathway neatly illustrates the sequence shown by Moulin et al. (2018) . Nighttime convection leaves small N and comparatively large ϵ χ early in the day. The building diurnal stratification quenches ϵ χ as N increases. At depths below z T z max , there is warming by local absorption of solar radiation, but presumably negligible shear from the diurnal jet. This state may last several hours. On the
, consequently, shear because the now-warmed layer traps the momentum input from wind. Contours and current arrows are derived from an idealized simulation (see section 3 ). Termed the diurnal jet, the near-surface velocity anomaly has been observed to be 0.1–0.3 m s −1 ( Price et al. 1986 ; Kraus 1987 ; Sutherland et al. 2016 ; Shcherbina et al. 2019 ). By isolating the near surface, DWLs (and rain layers) make the surface slippery. For both types of layers, Shcherbina et al. (2019) showed that
, consequently, shear because the now-warmed layer traps the momentum input from wind. Contours and current arrows are derived from an idealized simulation (see section 3 ). Termed the diurnal jet, the near-surface velocity anomaly has been observed to be 0.1–0.3 m s −1 ( Price et al. 1986 ; Kraus 1987 ; Sutherland et al. 2016 ; Shcherbina et al. 2019 ). By isolating the near surface, DWLs (and rain layers) make the surface slippery. For both types of layers, Shcherbina et al. (2019) showed that
altimeters by the Copernicus Marine Environment Monitoring Service (CMEMS). 2 Apart from the EICC, one other major circulation feature is the Summer or Southwest Monsoon Current (SMC). The SMC is visible in the seasonal mean during the SW monsoon as an eastward jet along 8°N between 85° and 92°E in Fig. 2d (vectors). Peak velocity in the SMC can exceed 1.5 m s −1 and northward transport has been estimated to be in the range 10–27 Sv (1 Sv ≡ 10 6 m 3 s −1 ), likely an overestimate due to the
altimeters by the Copernicus Marine Environment Monitoring Service (CMEMS). 2 Apart from the EICC, one other major circulation feature is the Summer or Southwest Monsoon Current (SMC). The SMC is visible in the seasonal mean during the SW monsoon as an eastward jet along 8°N between 85° and 92°E in Fig. 2d (vectors). Peak velocity in the SMC can exceed 1.5 m s −1 and northward transport has been estimated to be in the range 10–27 Sv (1 Sv ≡ 10 6 m 3 s −1 ), likely an overestimate due to the
. 2016 ). NASA’s Jet Propulsion Laboratory provides daily SMAP SSS data starting 1 April 2015, interpolated using the 8-day running mean. We use the most recent SMAP version 4.0, which accounts for galaxy and land contamination corrections and reduced brightness temperature biases ( Fore et al. 2016 ). Soil Moisture and Ocean Salinity (SMOS) data used in this study are the most up-to-date debiased SMOS SSS level 3 data generated by the Ocean Salinity Center of Expertise at the Centre Aval de
. 2016 ). NASA’s Jet Propulsion Laboratory provides daily SMAP SSS data starting 1 April 2015, interpolated using the 8-day running mean. We use the most recent SMAP version 4.0, which accounts for galaxy and land contamination corrections and reduced brightness temperature biases ( Fore et al. 2016 ). Soil Moisture and Ocean Salinity (SMOS) data used in this study are the most up-to-date debiased SMOS SSS level 3 data generated by the Ocean Salinity Center of Expertise at the Centre Aval de
://doi.org/10.1175/1520-0442(1998)011<0463:SAIVOA>2.0.CO;2 . 10.1175/1520-0442(1998)011<0463:SAIVOA>2.0.CO;2 Yanase , A. , K. Yasunaga , and H. Masunaga , 2017 : Relationship between the direction of diurnal rainfall migration and the ambient wind over the southern Sumatra Island . Earth Space Sci. , 4 , 117 – 127 , https://doi.org/10.1002/2016EA000181 . 10.1002/2016EA000181 Yang , S. , K. M. Lau , and K. M. Kim , 2002 : Variations of the East Asian jet stream and Asian
://doi.org/10.1175/1520-0442(1998)011<0463:SAIVOA>2.0.CO;2 . 10.1175/1520-0442(1998)011<0463:SAIVOA>2.0.CO;2 Yanase , A. , K. Yasunaga , and H. Masunaga , 2017 : Relationship between the direction of diurnal rainfall migration and the ambient wind over the southern Sumatra Island . Earth Space Sci. , 4 , 117 – 127 , https://doi.org/10.1002/2016EA000181 . 10.1002/2016EA000181 Yang , S. , K. M. Lau , and K. M. Kim , 2002 : Variations of the East Asian jet stream and Asian
