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Skip to content. Previous post. Next post. Strong sensible heating is observed below hPa, while latent heating, mainly due to convection, occurs above hPa and gradually decreases in magnitude above hPa Fig. The combined latent and sensible heating in winter yields a major peak from the surface to hPa and a secondary peak in the midtroposphere. The diabatic heating over the Florida Current parallelogram in Fig. The winter shallow maximum is due to sensible heating not shown , while the summer deep maximum is caused by convective heating, which displays a pronounced peak in the middle and upper troposphere Fig.
The summertime middle-tropospheric maximum of latent heating in this region is consistent with the aforementioned strong precipitation Fig.
The shallow diabatic heating maximum found over the Gulf Stream proper in winter and the deep maximum over the Florida Current in summer are related to a bimodal behavior of the maximal heating heights Fig. The shallow total diabatic heating and its components Fig. Consequently, the bimodal behavior of atmospheric heating is closely related to different atmospheric responses to the Gulf Stream. The shallow heating mode is mainly for the wintertime atmospheric response over the Gulf Stream proper, while the deep heating mode is for the summertime atmospheric response over the Florida Current.
We have examined seasonal variations of the atmospheric response to the Gulf Stream based on high-resolution satellite data and ECMWF operational analysis and forecasts. The response may be classified into two modes with distinct spatiotemporal variations. The winter mode is best seen over the Gulf Stream proper, characterized by strong surface convergence highly correlated with the SLP Laplacian Figs.
Precipitation is enhanced over surface convergence, accompanied by a high cloud fraction of midlevel clouds Figs. Local evaporation supplies much of the moisture for precipitation over the central and eastern Gulf Stream proper Fig.
Wind convergence is confined in the MABL, and the shallow convergence is closely associated with upward motion in the troposphere Fig. Over the Gulf Stream proper, sensible and latent heating reaches an annual maximum in the winter season Fig.
The total heating is large from the surface to hPa. For the convenience of discussion, this atmospheric response mode is referred to as the shallow heating mode.
The shallow heating mode may occur in other major ocean fronts. The summer mode is most pronounced over the Florida Current and the western Gulf Stream proper, characterized by strong precipitation Fig.
Suggestive of deep convection, upward motions penetrate into the upper troposphere but do not have prominent signatures of enhanced surface wind convergence Figs. Over the Florida Current, precipitation exceeds local evaporation, and the additional moisture is supplied by advection from the south as part of the basin-scale anticyclonic circulation over the North Atlantic Fig. Latent heating by convection dominates in summer with a maximum at — hPa Fig. This atmospheric response mode may be referred to as the deep heating mode.
Indeed, the height of maximal diabatic heating exhibits a prominent bimodal structure with one peak in the MABL and the other peak in the middle troposphere, corresponding to the shallow and deep heating modes, respectively Fig. Boundary layer adjustments lead to locally enhanced evaporation and surface wind convergence on the warm flank of the Gulf Stream.
In a companion paper using an atmospheric general circulation model, Kuwano-Yoshida et al. In one experiment that removes the Gulf Stream SST front, the local enhancement of convective precipitation disappears along the Gulf Stream, with much less effect on large-scale condensation.
In their model, the atmosphere features a deep layer of conditional instability that enables deep convection in summer over the Gulf Stream, consistent with our observational results. Our results suggest two necessary conditions for the deep heating mode.
First, SST needs to exceed a certain threshold. A visual inspection of rain rate, high-level cloud, and vertical winds in the upper troposphere Figs. The second necessary condition for the deep heating mode is moisture transport from low-latitudes by the large-scale atmospheric circulation. For the precipitation maximum in the summertime rainband over the Florida Current, a substantial portion of the moisture is transported from the south on the west flank of the North Atlantic subtropical high Fig.
The numerical study of Miyasaka and Nakamura showed that the subtropical high is primarily caused by a land—sea thermal contrast between maritime radiative cooling over the North Atlantic and continental sensible heating over North Africa.
Consequently, the moisture transport from low latitudes is an external condition to ocean-to-atmosphere influences of the Gulf Stream, and depends on the atmospheric circulation determined by larger-scale processes. These necessary conditions suggest that the deep heating mode may occur in the western part of other ocean basins in the summer season. The preference for the summer season is due to the high SST requirement. Western boundary currents transport heat poleward, helping to maintain high SSTs in the western basin.
In addition, southerlies prevail in summer on the west flank of the subtropical anticyclone over the cool ocean, transporting moist air from low latitudes as in the North Atlantic.
The western North Pacific meets both conditions for the deep heating mode. Very recently, Tokinaga et al. They also documented prominent surface wind convergence associated with SLP adjustments in the winter season December—February. These results indicate that over the Kuroshio Extension, the shallow and deep heating modes are mainly at work in the winter and summer seasons, respectively. The baiu rainband is anchored by the westerly wind jet in the midtroposphere, and SST gradients are hypothesized to help its extension east of Japan Sampe and Xie Our additional analysis indicates that upward motions over the Kuroshio Extension are accompanied by shallow and deep horizontal convergence in the winter and summer seasons, respectively Figs.
In winter, sensible heating in the MABL and latent heating just above dominate, as in the shallow heating mode over the Gulf Stream proper Figs. The summertime latent heating over the Kuroshio Extension, however, is much weaker than that over the Florida Current, with comparable sensible heating in the boundary layer. Thus, over the summer Kuroshio Extension, the deep heating mode is at work but with larger contributions from the shallow heating mode than over the Florida Current.
Another interesting difference in the atmospheric response between the Gulf Stream and the Kuroshio Extension is that the slanted structure of the wintertime upward wind over the Kuroshio Extension is much more prominent than that over the Gulf Stream. Such a tilt is clearly visible in the horizontal wind convergence over the Kuroshio Extension Fig. Ascent is nearly collocated with the diabatic heating for both shallow and deep heating modes over the Gulf Stream, opposite to the argument of Hoskins and Karoly who suggested that diabatic heating is balanced by horizontal advection in the midlatitudes.
In our analysis, vertical temperature advection dominates over horizontal advection for the deep heating mode Fig. For the shallow heating mode, large diabatic heating from the surface to hPa is not balanced by temperature advection calculated from monthly-mean data, suggestive of an active role of weather disturbances Fig. It is beyond our scope to investigate reasons for the discrepancy between the Hoskins and Karoly scale analysis and our observational result, but we note that their assumption of long waves is not satisfied for the atmospheric response to the Gulf Stream front and that the scale analysis does not consider the influence of synoptic disturbances.
Over the Kuroshio Extension during summer, Sampe and Xie note a similar balance between convective heating and adiabatic cooling in ascending motion.
Further studies are necessary to understand how the deep and shallow heating modes are controlled by environmental conditions of the atmosphere and ocean. In particular, interactions between mean conditions studied in the present paper and shorter-term variations including synoptic variability e.
Also, since the Gulf Stream is part of the upper limb of the Atlantic meridional overturning circulation, which can give rise to a source of decadal predictability Keenlyside et al. We thank M. Inatsu, M. Watanabe, K.
Yamazaki, J. Small, L. Ghil for discussions. Constructive comments from anonymous reviewers were quite helpful toward improving the presentation. Citation: Journal of Climate 23, 13; Sign in Sign up. Advanced Search Help. Journal of Climate. Sections Abstract 1.
Introduction 2. Data a. Operational analysis and reanalysis data b. Satellite data c. Gridded in situ observations 3. Oceanic condition 4. Surface wind convergence and precipitation 5. Cloud and lightning 6. Vertical winds 7. Atmospheric heating 8.
Conclusions and discussion. Export References. Reverdin , H. Giordani , and G. Hoskins , and M. Schlax , M. Freilich , and R. Hautala , and K. Ghil , and E.
Joyce , and S. Kwon , and L. Latif , J. Jungclaus , L. Kornblueh , and E. Xie , N. Iwasaka , and T. Shapiro , and E. Robinson , I. Hall , S. Peng , and R. Minobe , and S-P. Zheng , W. Pichel , C-Z. Zou , P. Xie , P. Polito , S-P. Xie , and H. Xie , and P. Kuwano-Yoshida , N. Komori , S-P. Xie , and R. Sampe , A.
Goto , W. Ohfuchi , and S-P. Chelton , and S. Chelton , S. Esbensen , and F. Cornillon , and D. Robinson , and M. Grell , and Y. Skyllingstad , D. Esbensen , L.
Cornillon , and T. Fett , J. Kerling , and P. Nakamura , T. Kagimoto , and S. Rogers , W. Wang , and B. Tanimoto , and S-P. Tanimoto , S-P. Xie , T. Sampe , H. Tomita , and H. Graham , and C. Mitchell , and C. Enfield , S-k Lee , and C. Lakhtakia , and J. Jin , and J. Hafner , Y. Tanimoto , W. Liu , H. Tokinaga , and H. View in gallery top Rain rate, middle evaporation, and bottom their difference in left winter and right summer of ECMWF data.
View in gallery a Winter and b summer lightning flash density shade. View in gallery left Monthly mean upward velocity and right horizontal convergence for the region shown in Fig. View in gallery a , c Seasonal development of diabatic heating rate and b wintertime and d summertime vertical profiles of total diabatic heating rate line with open circle , convective heating thick solid line , large-scale condensation heating thick dashed line , and sensible heating thin solid line over the top Gulf Stream proper and bottom Florida Current the respective areas shown in Fig.
View in gallery Vertical profiles of vertical thick solid line and horizontal thick dashed line mean temperature advection and total temperature advection thin line top in winter over the Gulf Stream proper and bottom in summer over the Florida Current: areas as in Fig. View raw image top Rain rate, middle evaporation, and bottom their difference in left winter and right summer of ECMWF data. View raw image a Winter and b summer lightning flash density shade. View raw image left Monthly mean upward velocity and right horizontal convergence for the region shown in Fig.
View raw image a , c Seasonal development of diabatic heating rate and b wintertime and d summertime vertical profiles of total diabatic heating rate line with open circle , convective heating thick solid line , large-scale condensation heating thick dashed line , and sensible heating thin solid line over the top Gulf Stream proper and bottom Florida Current the respective areas shown in Fig.
View raw image Vertical profiles of vertical thick solid line and horizontal thick dashed line mean temperature advection and total temperature advection thin line top in winter over the Gulf Stream proper and bottom in summer over the Florida Current: areas as in Fig. Chart I. Tracks of Centers of Anticyclones, December, Author: P. Previous Article Next Article. Editorial Type: Article. Article History. Download PDF. Full access. Introduction How the midlatitude ocean influences the overlying atmosphere on climate time scales is a long-standing question.
Operational analysis and reanalysis data We use the monthly fields on a 0. Satellite data We analyze a suite of satellite observations of sea surface wind, cloud fraction, lightning flash frequency as well as sea surface height.
Gridded in situ observations Following Tokinaga et al. Oceanic condition This section describes briefly surface oceanic conditions and their seasonal changes. Surface wind convergence and precipitation Figure 2 shows the seasonal mean convergence of equivalent neutral wind velocity at 10 m, estimated from QuikSCAT observations.
Vertical winds The vertical wind also exhibits seasonally dependent structures. Atmospheric heating Results in the previous sections show that the wintertime and summertime responses to the Gulf Stream system are different in a number of aspects. Conclusions and discussion We have examined seasonal variations of the atmospheric response to the Gulf Stream based on high-resolution satellite data and ECMWF operational analysis and forecasts.
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