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Tropical precipitation is climatologically most intense at the heart of the intertropical convergence zone (ITCZ), but this is not always true in instantaneous snapshots. Precipitation is amplified along the ITCZ edge rather than at its center from time to time. In this study, satellite observations of column water vapor, precipitation, and radiation as well as the thermodynamic field from reanalysis data are analyzed to investigate the behavior of ITCZ convection in light of the local atmospheric energy imbalance. The analysis is focused on the eastern Pacific ITCZ, defined as the areas where column water vapor exceeds 50 mm over a specified width (typically 400–600 km) in the domain of 20°S–20°N, 180°–90°W. The events with a precipitation maximum at the southern and northern edges of the ITCZ are each averaged into composite statistics and are contrasted against the reference case with peak precipitation at the ITCZ center. The key findings are as follows. When precipitation peaks at the ITCZ center, suppressed radiative cooling forms a prominent positive peak in the diabatic forcing to the atmosphere, counteracted by an export of moist static energy (MSE) owing to a deep vertical advection and a large horizontal export of MSE. When convection develops at the ITCZ edges, to the contrary, a positive peak of the diabatic forcing is only barely present. An import of MSE owing to a shallow ascent on the ITCZ edges presumably allows an edge intensification to occur despite the weak diabatic forcing.

High-resolution observations have demonstrated the presence of strong time-mean near-surface wind convergences (NSWC) anchored across oceanic frontal zones, such as the western boundary currents. Initial analyses appeared to show a close association between this time-mean NSWC and time-mean properties of the underlying sea-surface temperature (SST), such as the gradients and second derivatives (e.g., Laplacian of SST), acting through pressure adjustment and vertical mixing mechanisms. However, a series of recent papers have revealed the instantaneous NSWC to be dominated by atmospheric fronts, and suggested the importance of air-sea processes occurring instead on shorter, synoptic timescales. In this paper, using the ERA-5 reanalysis dataset in the Gulf Stream region, we aim to reconcile these viewpoints by investigating the spatial and temporal dependence of NSWC and its relationship to SST. It is revealed that while atmospheric frontal processes govern the day-to-day variability of NSWC, the relatively weak but persistent pressure adjustment and vertical mixing mechanisms provide lower frequency modulations in conditions both with and without atmospheric fronts. In addition to their temporal characteristics, each mechanism is shown through spectral analysis to dominate on specific spatial scales. In light of recent work that has tied remote atmospheric responses to NSWC anomalies in western boundary current regions, these results emphasize the importance of oceanic frontal zones for atmospheric variability on all spatiotemporal scales.

High-resolution satellite observations and numerical experiments have revealed local enhancement of time-mean surface wind convergence along the axes of warm western boundary currents and divergence slightly poleward. A recent study has suggested that frequent occurrence of persistent atmospheric fronts and sea level pressure (SLP) troughs along a sea surface temperature (SST) front are responsible for shaping the frontal-scale wind convergence and divergence contrast as seen in the wintertime climatology near the Kuroshio Extension (KE). These events tend to induce surface wind convergence with moderate magnitude. Through atmospheric reanalysis with high-resolution SST, the present study reveals that, as in the vicinity of the KE, surface wind convergence with moderate magnitude and divergence with moderate-to-extreme magnitude are found to play a primary role in shaping the climatological-mean wind convergence–divergence contrasts across the SST fronts near the Gulf Stream (GS) and Agulhas Return Current (ARC) in winter. In contrast, strong-to-extreme convergence events associated with synoptic-scale atmospheric disturbances are found to yield horizontally uniform time-mean wind convergence. Furthermore, cluster analysis and case studies suggest that persistent atmospheric fronts and SLP troughs are responsible for inducing moderate wind convergence also near the GS and ARC. Thus, these features are consistent with their counterpart near the KE, but the impacts of the ARC tend to be substantially weaker, probably due to its cooler SST among other potential factors.

High-resolution satellite observations and numerical simulations have revealed that climatological-mean surface wind convergence and precipitation are enhanced locally around the midlatitude warm western boundary currents (WBCs) with divergence slightly to their poleward side. While steep sea surface temperature (SST) fronts along the WBCs have been believed to play an important role in shaping those frontal-scale atmospheric structures, the mechanisms and processes involved are still under debate. The present study explores specific daily scale atmospheric processes that are essential for shaping the frontal-scale atmospheric structure around the Kuroshio Extension (KE) in winter, taking advantage of a new product of global atmospheric reanalysis. Cluster analysis and case studies reveal that a zonally extending narrow band of surface wind convergence frequently emerges along the KE, which is typically observed under the surface northerlies after the passage of a developed synoptic-scale cyclone. Unlike its counterpart around the cyclone center and associated cold front, the surface convergence tends to be in moderate strength and more persistent, contributing dominantly to the distinct time-mean convergence/divergence contrast across the SST front. Accompanying ascent and convective precipitation, the band of convergence is a manifestation of a weak stationary atmospheric front anchored along the SST front or generation of a weak meso-α-scale cyclone. By reinforcing the ascent and convergence, latent heating through convective processes induced by surface convergence plays an important role in shaping the frontal-scale atmospheric structure around the KE.

The Kuroshio Extension (KE) fluctuates between its different dynamic regimes on (quasi) decadal time scales. In its stable (unstable) regime, the KE jet is strengthened (weakened) and less (more) meandering. The present study investigates wintertime mesoscale atmospheric structures modulated under the changing KE regimes, as revealed in high-resolution satellite data and data from a particular atmospheric reanalysis (ERA-Interim). In the unstable KE regime, a positive anomaly in sea surface temperature (SST) to the north of the climatological KE jet accompanies positive anomalies in upward heat fluxes from the ocean, surface wind convergence, and cloudiness. As revealed in the atmospheric reanalysis, these positive anomalies coincide with local lowering of sea level pressure, weaker vertical wind shear, warming and thickening of the marine atmospheric boundary layer (MABL), anomalous ascent, and convective precipitation. In the stable KE regime, by contrast, the corresponding imprints of sharp SST gradients across the KE and Oyashio fronts on the wintertime MABL are separated more distinctly, and so are the surface baroclinic zones along those two SST fronts. In the ERA-Interim data, such mesoscale imprints of the KE variability as above are not well represented in a period during which the resolution of SST data prescribed is relatively low. The present study thus elucidates the importance of high-resolution SST data prescribed for atmospheric reanalysis in representing modulations of the MABL structure and air–sea fluxes by the variability of oceanic fronts and/or jets, including the modulations occurring with the changing KE regimes through the hydrostatic pressure adjustment and vertical mixing mechanisms.

Mesoscale structures of the wintertime marine atmospheric boundary layer (MABL) as climatological imprints of oceanic fronts within the Kuroshio–Oyashio Extension (KOE) region east of Japan are investigated by taking advantage of high horizontal resolution of the ERA-Interim global atmospheric reanalysis data, for which the resolution of sea surface temperature (SST) data has been improved. These imprints, including locally enhanced sensible and latent heat fluxes and local maxima in cloudiness and precipitation in association with locally strengthened surface-wind convergence in the vicinities of SST fronts along the warm Kuroshio Extension and cool Oyashio to its north, are also identified in high-resolution satellite data. In addition to these mesoscale MABL features, meridionally confined near-surface baroclinic zones and zonally oriented sea level pressure (SLP) minima associated with the dual SST fronts are represented in ERA-Interim only in the period of high-resolution SST, but those imprints of the Oyashio front are missing in the low-resolution SST period. In the presence of the prevailing monsoonal northerlies, latitudinal displacements of the SLP trough, baroclinic zone, and the peak meridional gradient of the turbulent heat fluxes from each of the corresponding SST fronts are also found to be sensitive to the frontal width that depends on the SST resolution. The analysis herein suggests that the converging surface northerlies into the SLP minima can contribute positively to the formation of a surface baroclinic zone along the Kuroshio Extension, while a stronger baroclinic zone along the Oyashio front is maintained primarily through the pronounced cross-frontal contrast in sensible heat release from the ocean.

The current study is aimed at exploring the potential roles of the seasonally altering background surface wind in the seasonality of the intraseasonal oscillations (ISOs) with a focus on the sea surface temperature (SST) variability. A composite analysis of the ocean mixed layer heat budget in term of ISO phases with various satellite data is performed for boreal winter and summer. The scalar wind is found to be a dominant factor that accounts for the ocean surface heat budget, implying that the background surface wind as well as its anomaly is important for the SST variability. An easterly anomaly to the east of convection diminishes scalar wind, and thus latent heat flux, when superposed onto a background westerly wind, implying that the presence of basic westerly wind is important for the development of a warm SST anomaly ahead of the ISO convection. On the other hand, an easterly anomaly in combination with basic easterly wind magnifies scalar wind and latent heat flux and cancels out the shortwave heat flux anomaly. The seasonal migration of the background westerly wind, which is confined to a southern equatorial belt in boreal winter but spread across the northern Indian Ocean in boreal summer, may offer a mechanism that partly accounts for the seasonal characteristics of ISO propagation. The northward propagation of the SST variability associated with the boreal summer ISO is found to also involve a similar mechanism with the meridional wind modulation of scalar wind.

The known characteristics of the relationship between sea surface temperature (SST) and column water vapor (CWV) are reevaluated with recent satellite observations over tropical and subtropical oceans. Satellite data acquired by the Aqua Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E), Atmospheric Infrared Sounder (AIRS)/Advanced Microwave Sounder Unit (AMSU) suite, the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR), and the Quick Scatterometer (QuikSCAT) SeaWinds are analyzed together for 7 years from October 2002 to September 2009. CWV is decomposed into surface humidity, presumably coupled closely to SST, and the water vapor scale height as an index of vertical moisture gradient between the boundary layer and the free troposphere. Surface relative humidity is climatologically homogeneous across tropical and subtropical oceans, while the dependence of CWV on SST varies from one region to another. SST mainly accounts for the variation of CWV with the water vapor scale height, which is virtually invariant over subtropical oceans. On the other hand, over tropical oceans, the variability of CWV is explained not only by SST but also by a systematic change of the water vapor scale height. The regional contrast between tropical and subtropical oceans is discussed in the context of the regional moisture budget including vertical moisture transport through convection.

In this study the observed relationship of precipitation with column relative humidity (CRH), a metric of tropospheric humidity, is examined in order to address a known discrepancy inherent to past studies. A composite analysis of satellite data is carried out to explore the short-term (i.e., from hourly to daily) atmospheric variability for comparison with the climatology, hypothesizing that a primary cause for the discrepancy arises from a difference in the time scale of interest. The analysis is broken down into four classes on the basis of the degree of convective organization, ranging from unorganized shallow cumuli to highly organized convective systems. The CRH–precipitation relationship is found to be extremely nonlinear for the short-term variability, while the nonlinearity weakens to some degree when different convective systems in diverse humidity environments are averaged together into climatology. The weak exponential rise in the climatological CRH–precipitation curve occurs because highly organized convective systems become more frequent and intense and thus receive increasing weight in the climatological mean as the environment moistens.

In this study, cloud radiative forcing (CRF) associated with convective activity over tropical oceans is analyzed for monthly mean data from twentieth-century simulations of 18 climate models participating in phase 3 of the Coupled Model Intercomparison Project (CMIP3) in comparison with observational and reanalysis data. The analysis is focused on the warm oceanic regions with sea surface temperatures (SSTs) above 27°C to exclude the regions with cold SSTs typically covered by low stratus clouds. CRF is evaluated for different regimes sorted by pressure-coordinated vertical motion at 500 hPa (ω500) as an index of large-scale circulation. The warm oceanic regions cover the regime of vertical motion ranging from strong ascent to weak descent. The most notable feature found in this study is a systematic underestimation by most models of the ratio of longwave cloud radiative forcing (LWCRF) to shortwave cloud radiative forcing (SWCRF) over the weak vertical motion regime defined as −10 < ω500 < 20 hPa day−1. The underestimation of the ratio corresponds to the underestimation of LWCRF and the overestimation of SWCRF. Clouds in models seem to be lower in the amount of high clouds but more reflective than those in the observations in this regime. In the weak vertical motion regime, the lower free troposphere is dry. In the large-scale environment condition, the reproducibility of LWCRF is high in models adopting the scheme where the relative humidity–based suppression for deep convection occurrence is implemented. Models adopting the Zhang and McFarlane scheme show good performance without such a suppression mechanism.

The equatorial asymmetry of the east Pacific intertropical convergence zone (ITCZ) is explored on the basis of an ocean surface heat budget analysis carried out with a variety of satellite data products. The annual mean climatology of absorbed shortwave flux exhibits a pronounced meridional asymmetry due to a reduction of insolation by high clouds in the north ITCZ. Ocean mixed layer advection has the largest, if not exclusive, effect of counteracting this shortwave-exerted asymmetry. Other heat fluxes, in particular latent heat flux, predominate over the advective heat flux in magnitude but are secondary with respect to equatorial asymmetry. The asymmetry in advective heat flux stems from a warm pool off the Central American coast and, to a lesser extent, the North Equatorial Counter Current, neither of which exist in the Southern Hemisphere. The irregular continental geography presumably comes into play by generating a warm pool north of the equator and bringing cold waters to the south in the far eastern Pacific. In addition to the annual climatology, the north–south contrast in the seasonal cycle of surface heat flux is instrumental in sustaining the north ITCZ throughout the year. The northeast Pacific is exposed to a seasonal cycle that is considerably weaker than that in the southeast Pacific, arising from multiple causes including the finite eccentricity of the earth’s orbit and meridional gradient in mixed layer absorptivity. Simple experiments generating synthetic sea surface temperature (SST) illustrate that the muted seasonal cycle of heat flux forcing moderates the SST seasonal variability in the northeast Pacific and thus allows the north ITCZ to persist year round. Existing theories on the ITCZ asymmetry are briefly examined in light of the present findings.

The east Pacific double intertropical convergence zone (ITCZ) in austral fall is investigated with particular focus on the growing processes of its Southern Hemisphere branch. Satellite measurements from the Tropical Rainfall Measuring Mission (TRMM) and Quick Scatterometer (QuikSCAT) are analyzed to derive 8-yr climatology from 2000 to 2007. The earliest sign of the south ITCZ emerges in sea surface temperature (SST) by January, followed by the gradual development of surface convergence and water vapor. The shallow cumulus population starts growing to form the south ITCZ in February, a month earlier than vigorous deep convection is organized into the south ITCZ. The key factors that give rise to the initial SST enhancement or the southeast Pacific warm band are diagnosed by simple experiments. The experiments are designed to calculate SST, making use of an ocean mixed layer “model” forced by surface heat fluxes, all of which are derived from satellite observations. It is found that the shortwave flux absorbed into the ocean mixed layer is the primary driver of the southeast Pacific warm band. The warm band does not develop in boreal fall because the shortwave flux is seasonally so small that it is overwhelmed by other negative fluxes, including the latent heat and longwave fluxes. Clouds offset the net radiative flux by 10–15 W m−2, which is large enough for the warm band to develop in boreal fall if it were not for clouds reflecting shortwave radiation. Interannual variability of the double ITCZ is also discussed in brief.

Regional and temporal variability in the vertical and horizontal characteristics of tropical precipitating clouds are investigated using the Precipitation Radar (PR) and the Visible and Infrared Scanner (VIRS) on board the Tropical Rainfall Measuring Mission (TRMM) satellite. The present study focuses on the three oceanic regions (west, central, and east Pacific) together with two continental regions for comparison and the two separate time periods (February 1998 and February 2000) under different phases of the El Niño–Southern Oscillation (ENSO) in order to examine regional and ENSO-related variations. The height spectrums of storms are investigated in terms of radar echo-top height and infrared brightness temperature. The variability in the spectrum clearly correlates with the large-scale circulation and its ENSO-related change. On the basis of the height spectrum, storm systems are classified into the four categories of shallow, cumulus congestus, deep stratiform, and deep convective. The deep stratiform and deep convective categories, both of which have very cold cloud tops, are differentiated by radar echo-top heights so that deep convective systems are accompanied with an appreciable amount of large frozen particles aloft. While shallow events are dominant in the probability of occurrence over relatively cold oceans, deep convective systems take their place for warmer sea surface temperatures (SSTs). The turnover occurs at the SST threshold of 28°–29°C for all the oceanic regions and years investigated except the west Pacific in 2000, for which deep convective systems prevail over the entire range of SST. Rain correlation-scale length (RCSL) and cloud correlation-scale length (CCSL) are introduced as statistical indicators of the horizontal scale of storms. While the RCSL is 8–18 km for shallow- and cumulus congestus–type clouds without significant regional and temporal variations, the RCSL and CCSL associated with deep stratiform and deep convective systems consistently exceed 100 km and exhibit a systematic variability. The RCSL and CCSL in the central and east Pacific, particularly, increase significantly in the El Niño year.