Recent studies indicate that the influence of midlatitude SST fronts extends through the marine atmospheric boundary layer (MABL) into the free atmosphere, with implications for climate variability. To better understand the mechanisms of this ocean-to-atmosphere influence, SST-induced MABL convergence is explored here with the Weather Research and Forecasting mesoscale model in an idealized, dry, twodimensional configuration, for winds crossing from cold to warm SST and from warm to cold SST.For strong cross-front winds, O(10 m s 21 ), changes in the turbulent mixing and MABL depth across the SST front lead to MABL depth-integrated convergence in the cold-to-warm case and depth-integrated divergence in the warm-to-cold case. The turbulent stress divergence term changes over a shorter length scale than the pressure gradient and Coriolis terms, such that the MABL response directly above the SST front is governed by nonrotating, internal boundary layer-like physics, which are consistent with the vertical mixing mechanism. An important consequence is that the increment in the cross-front surface stress diagnoses the vertical motion at the top of the MABL. These physics are at variance with some previously proposed SST frontal MABL models in which pressure adjustments determine the MABL convergence.The SST-induced MABL convergence results in vertical motion that excites a stationary internal gravity wave in the free atmosphere, analogous to a mountain wave. For a 15 m s 21 cross-front wind, the gravity wave forced by an SST increase of 38C over 200 km is comparable to that forced by an 80-m change in topography.

Downdrafts of air cooled by evaporating raindrops are an essential component of mesoscale convective systems (MCSs). Here we use surface wind observations from the Advanced SCATterometer (ASCAT) to identify MCS downdrafts over the western equatorial Pacific Ocean as regions of horizontal wind divergence exceeding 10 −4 s −1 . More than 1300 downdrafts are identified over the observation period (2009)(2010)(2011)(2012)(2013)(2014). The downdraft signal in the surface winds is validated with satellite measurements of brightness temperature and rainfall rate, and surface buoy measurements of air temperature; composite analysis with these measurements indicates that ASCAT detects downdrafts that lag the peak convection by 8-12 h. While ASCAT resolves mesoscale downdrafts in regions of light rain, a composite against buoy air temperature indicates that ASCAT fails to resolve the stronger convective-scale downdrafts associated with heavy rainfall at squall fronts. Nevertheless, the global observations by the satellite scatterometer open a new avenue for studying MCSs.

Satellite observations of infrared brightness temperature and rainfall have shown offshore propagation of diurnal rainfall signals in some coastal areas of the tropics, suggesting that diurnal rainfall is coupled to land‐sea breeze circulations. Here we utilize satellite observations of surface winds and rainfall to show the offshore copropagation of land breeze and diurnal rainfall signals for 300–400 km from the east coast of India into the Bay of Bengal. The wind observations are from the 2003 Quick Scatterometer (QuikSCAT)‐SeaWinds “tandem mission” and from 17 years of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI); the rainfall observations are from the TRMM 3B42 product and from TMI. The surface wind convergence maximum leads the rainfall maximum by 1–2 h in the western part of the bay, implying that the land breeze forces the diurnal cycle of rainfall. The phase speed of the offshore propagation is approximately 18 m s−1, consistent with a deep hydrostatic gravity wave forced by diurnal heating over India. Comparisons with a cloud system‐resolving atmospheric model and the ERA‐Interim reanalysis indicate that the models realistically simulate the surface land breeze but greatly underestimate the amplitude of the rainfall diurnal cycle. The satellite observations presented in this study therefore provide a benchmark for model representation of this important atmosphere‐ocean‐land surface interaction.

Satellite observations and modeling studies show that midlatitude SST fronts influence the marine atmospheric boundary layer (MABL) and atmospheric circulation. Here we use the Weather Research and Forecasting (WRF) mesoscale model to explore the atmospheric response to a midlatitude SST front in an idealized, dry, two-dimensional configuration, with a background wind V oriented in the along-front direction.The SST front excites an along-front wind anomaly in the free atmosphere, with peak intensity just above the MABL. This response is nearly quasigeostrophic, in contrast to the inertia-gravity wave response seen for crossfront background winds. The free atmosphere response increases with the background wind V , in contrast to previously proposed SST frontal MABL models.The MABL winds are nearly in Ekman balance. However, a cross-front wind develops in the MABL due to friction and rotation, such that the MABL cross-front Rossby number ε ≈ 0.2. The MABL vorticity balance and scaling arguments indicate that advection plays an important role in the MABL dynamics. Surface wind convergence shows poor agreement with MABL depthintegrated convergence, indicating that the MABL mixed-layer assumption may not be appropriate for SST frontal zones with moderate to strong surface winds.Satellite observations show a robust influence of SST fronts on surface winds (Xie 2004; Chelton 34 et al. 2004;O'Neill et al. 2005;Small et al. 2008; Chelton and Xie 2010;O'Neill 2012; O'Neill 35 et al. 2012), such that surface wind speed and surface stress magnitude increase over warm SST, 36 and decrease over cold SST (Fig. 1). Likewise, the wind stress divergence response is strong when 37 surface winds are oriented in the cross-front direction (perpendicular to SST contours), and the 38 wind stress curl response is strong when surface winds are oriented in the along-front direction. 39 These wind stress patterns are ubiquitous (Chelton et al. 2004) but here we focus on the midlatitude 40 situation. 41 The marine atmospheric boundary layer (MABL) dynamics are distinct for the two cases de-42 picted in Fig. (1) (Schneider and Qiu 2015). For the case of "strong" cross-front winds [defined as 43 having O(1) cross-front Rossby number ε = U/ f L, where U is the maximum cross-front compo-44 nent of the MABL wind, f is the Coriolis parameter, and L is the length scale of the SST front], 45 the MABL momentum budget is dominated by changes in the advection and turbulent stress diver-46 gence terms (Samelson et al. 2006; Spall 2007b; Small et al. 2008; Kilpatrick et al. 2014, hereafter 47 KSQ). The air-sea heat flux is vigorous due to the high wind speed and large air-sea temperature 48 difference, but the MABL temperature adjusts to SST over a longer length scale than the turbulent 49 stress divergence term. For winds blowing from cold to warm SST, the MABL response resem-50 bles the "vertical mixing mechanism" (Wallace et al. 1989; Hayes et al. 1989), whereby enhanced 51 downward mixing of momentum increases the surface stress and redu...

The generation of variance by anomalous advection of a passive tracer in the thermocline is investigated using the example of density-compensated temperature and salinity anomalies, or spiciness. A coupled Markov model is developed in which wind stress curl forces the large-scale baroclinic ocean pressure that in turn controls the anomalous geostrophic advection of spiciness. The ''double integration'' of white noise atmospheric forcing by this Markov model results in a frequency (v) spectrum of large-scale spiciness proportional to v 24 , so that spiciness variability is concentrated at low frequencies.An eddy-permitting regional model hindcast of the northeast Pacific confirms that time series of large-scale spiciness variability are exceptionally smooth, with frequency spectra } v 24 for frequencies greater than 0.2 cpy. At shorter spatial scales (wavelengths less than ;500 km), the spiciness frequency spectrum is whitened by mesoscale eddies, but this eddy-forced variability can be filtered out by spatially averaging. Large-scale and long-term measurements are needed to observe the variance of spiciness or any other passive tracer subject to anomalous advection in the thermocline.

Satellite observations from the Moderate Resolution Imaging Spectroradiometer andSea-viewing Wide Field-of-view Sensor reveal a "tongue" of elevated near-surface chlorophyll that extends into the Southern California Bight from Point Conception. A local chlorophyll maximum at the western edge of the bight, near the Santa Rosa Ridge, indicates that the chlorophyll is not solely due to advection from Point Conception but is enhanced by local upwelling. Chlorophyll in the bight peaks in May and June, in phase with the seasonal cycle of wind stress curl. The spatial structure and seasonal variability suggest that the local chlorophyll maximum is due to a combination of bathymetric influence from the Santa Rosa Ridge and orographic influence from the coastline bend at Point Conception, which causes sharp wind stress curl in the bight. High-resolution glider observations show thermocline doming in May-June, in support of the local upwelling effect. Despite the evidence for local wind stress curl-forced upwelling in the bight, we cannot rule out alternative mechanisms for the local chlorophyll maximum, such as iron supply from the ridge. Covariability between chlorophyll, surface wind stress, and sea surface temperature (SST) indicates that nonseasonal chlorophyll variability in the bight is closely related to SST, but the spatial patterns of SST influence vary by time scale: Subannual chlorophyll variability is linked to local wind-forced upwelling, while interannual chlorophyll variability is linked to large-scale SST variations over the northeast Pacific. This suggests a greater role for nonlocal processes in the bight's low-frequency chlorophyll variability.Plain Language Summary Satellite observations of ocean color reveal a "tongue" of enhanced chlorophyll in the Southern California Bight, indicating that this is a region of high biological productivity. The satellite observations and local observations from automated gliders suggest that the enhanced chlorophyll in the bight is due to local upwelling, rather than chlorophyll being simply carried downstream from Point Conception. The seasonal cycle of chlorophyll is in phase with the local wind stress forcing, which peaks in May and June. In contrast to the seasonal variability, the substantial year-to-year chlorophyll variations are controlled by large-scale climate variability over the northeast Pacific, that is, El Niño and marine heat wave events.

Satellite scatterometer observations of surface winds over the global oceans are critical for climate research and applications like weather forecasting. However, rain‐related errors remain an important limitation, largely precluding satellite study of winds in rainy areas. Here we utilize a novel technique to compute divergence and curl from satellite observations of surface winds and surface wind stress in rainy areas. This technique circumvents rain‐related errors by computing line integrals around rainy patches, using valid wind vector observations that border the rainy patches. The area‐averaged divergence and wind stress curl inside each rainy patch are recovered via the divergence and curl theorems. We process the 10 year Quick Scatterometer (QuikSCAT) data set and show that the line‐integral method brings the QuikSCAT winds into better agreement with an atmospheric reanalysis, largely removing both the “divergence bias” and “anticyclonic curl bias” in rainy areas noted in previous studies. The corrected QuikSCAT wind stress curl reduces the North Pacific midlatitude Sverdrup transport by 20–30%. We test several methods of computing divergence and curl on winds from an atmospheric model simulation and show that the line‐integral method has the smallest errors. We anticipate that scatterometer winds processed with the line‐integral method will improve ocean model simulations and help illuminate the coupling between atmospheric convection and circulation.

Satellite scatterometers provide the only regular observations of surface wind vectors over vast swaths of the world oceans, including coastal regions, which are of great scientific and societal interest but still present challenges for remote sensing. Here we demonstrate systematic scatterometer wind errors near Hawaii's Big Island: Two counter‐rotating lee vortices, which are clear in the International Comprehensive Ocean‐Atmosphere Data Set ship‐based wind climatology and in aircraft observations, are absent in the Jet Propulsion Laboratory and Remote Sensing Systems scatterometer wind climatologies. We demonstrate similar errors in the representation of transient Catalina Eddy events in the Southern California Bight. These errors likely arise from the nonuniqueness of scatterometer wind observations, that is, an “ambiguity removal” is required during processing to select from multiple wind solutions to the geophysical model function. We discuss strategies to improve the ambiguity selection near coastal mountains, where small‐scale wind reversals are common.