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Thermally driven wind circulation near ocean fronts
Phys. Chem. Earth, VoI, 23, No. 5-6, pp. 605 607, 1998 1998 Elsevier Science Ltd. All fights reserved oo79-1946/98/$,see front matter
Pergamon PII: S0079- ! 946(98)00082-2
Thermally Driven Wind Circulation near Ocean Fronts Yuri S. GesheHn Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, N.S., Canada B2Y 4A2
Received 25 April 1997; accepted 19 August 1997
Abstract. A relationship between the sea surface temperature gradient and wind speed is investigated. The climatological analysis is confined to the frontal zones of the Newfoundland Basin and Kuroshio areas. The findings are demonstrated to differ for both regions and different data sets. The inferred component of thermally driven wind is compared to the Hs~ 1984, estimate. In the Newfoundland Basin, the thermal wind circulation is simulated better than in the Kuroshio area. The reason for this is addressed. © 1998 Elsevier Science Ltd. All fights reserved
1 Introduction The areas of ocean fronts (such as the Gulf Stream, Kuroshio) are unique in the zones of sharp sea surface temperature (SST) gradients. The structure of the marine atmospheric boundary layer differs when the wind is parallel to and perpendicular to the oceanic front. The latter is expected to induce atmospheric secondary flow similar to a sea breeze circulation. There is no universally agreed term for that sort of circulation if the horizontal temperature gradient is caused by phenomena other than the coast line. Hsu. 1984, called it sea-breeze-like winds and developed a simple model for the winds driven by sharp SST gradients. The mmlogy between actual sea breeze and the phenomenon being considered is not close because the former one is associated with the diurnal variability of the air temperature over the land, whereas the latter is not associated with such variability. Therefore, we suggest the use of more appropriate term: thermally driven wind (TDW) and employ this term in the following discussion.
Correspondence to: Yuri S. Geshelin
Some evidence for the modulation of surface momentum fluxes by SST variations was provided in the JASIN experiment (Guymer et al., 1983), suggesting the feasibility of TDW. The JASIN data sets were also used by Businger and Shaw (1984), and the conclusion was made that mesoscale changes in SST are capable of inducing secondary flow. However, the JASIN SST gradients were too small to allow a quantitative description of the phenomenon. The more convincing experimental evidence of TDW was furnished in the FASINEX experiment (Friehe and Williams, 1988; Weller, 1991). It was shown that a rise in cross-front profile of surface wind speed was associated with the sharpest SST gradients. Kolinko and Geshelirt 1995, described TDW in the Newfoundland Basin on a climatological scale. The purpose of this study is to investigate the climatology of the phenomenon at two major hydrological fronts - the North Atlantic Current and the Kumshio - and to compare the results with the Hsu, 1984, theory.
2 Data setsand analysis
This study employs two data sets: Comprehensive Ocean Almosphere Data Set (COADS) and the data collected in the SECTIONS program in the Newfoundland Basin on board Soviet research vessels. The latter data set is referred to as SECTIONS a ~ , Both data sets span the period 19811991 and provide spatial resolution sufficient for such a study. The data sets contain a variety of meteorological and oceanogrephic characteristics; sea surface temperature, wind speed amplitude end wind direction were used in this study. Whereas SECTIONS d~_~ are available for the Newfoundland Basin only, subsets from COADS were derived both for the Newfoundland Basin and the Kuroshio zone. The analysis is confined to the winter period only: the period of the most intensive air-sea interaction. Maps of 605
606
Y.S. Geshelin: Thermally Driven Wind Circulation near Ocean Fronts
the two areas are shown in Fig.l where the dashed lines separate cold water masses (to the north of the line) from warm (to the south of the line). 50W
60W
40W :0 :50N
50N
Conversely, when the air is warmed from below, it is convectively destabilized. For each data category the wind speeds were then binned by SST. To assess confidence intervals, the standard deviation for each bin was computed. The number of data used for each case is given in Table 1. Table 1. Numberof data used for each category.
•L
The Newfoundland The Newfoundland, The Kuroshio, Basin, SECTIONS Basin,COADS COADS
•°
o °
"°
North winds South winds
i
1,968 626
3,099 1,074
21,224 1i,339
40N
40N
3 Results a |
60W
50W 130E
I
40W
50N
150E 50N
40N
40N
120E
140E
3°NI. i"
30N
• • Bf, * ~
I 20N
b
I 120E
130E
140E
'
20N 150E
Fig.l. The study areas: a - the NewfoundlandBasin,b - the zone of the Kuroshio.Dashed lines are schematicallocationsof the strongestSST gradientsin winter.
The method of analysis follows Kolinko and Geshelin, 1995. For each of the two study areas, two data categories were selected: (1) winds blowing from the cold underlying surface onto the warm one, and (2) from warm to cold. Such a choice was made because the structure of the marine boundary layer undergoes significant changes as one regime gives way to the other. When the air mass travels over a cold surface, it becomes more stable.
The results are given in Fig.2 where the terms north curve and south curve are employed. The north curve is based on all cases when the winds are blowing from the cold underlying surface to the warm one. In both cases the winds are blowing along the SST coordinate which is roughly perpendicular to the hydrological front. This coordinate allows us to observe the effect of adjustment of the air mass to the underlying surface and to detect TDW. In Fig.2, one can see the apparent rise of the north curve in the SST range 13-15°C. It is this SST range that falls on the sharpest SST gradients in the Newfoundland Basin (Clarke et al., 1980). This suggests that the sharpness of the gradient has an impact on the wind speed profile. According to the Hsu, 1984, theory, as the air travels from the cold sea surface to the warm one, it is sped up by the effect of the sharp SST gradient below. Using observational data collected on a single day, Hsu showed that the thermally caused increase in wind speed was 6 m.s"~. The contribution of the climatological TDW-component to the net wind speed can be inferred from Fig.2 as the magnitude of the rise in the north curve - the maximum value exceeds the minimum one by 3 m.s" (Fig.2a). Because the exact agreement between climatologies and a case study is not expected, the values seem to be in a reasonable agreement. To apply Hsu's model on a climatological scale, a separate study needs to be done. In the case of south curves (the air travels onto the cold surface), no significant response to the hydrological front is observed (Fig.2). This is because the vertical circulation similar to actual sea breeze circulation is achieved only in the case where the winds are blowing from the cold side of the front (Hsu, 1984). Under these conditions, the air counterflow can develop at higher elevations. Once the winds are blowing from the warm side, there is no air counterflow in the upper layer. In the case of the
Y. S. Geshelin: Thermally Driven Wind Circulation near Ocean Fronts -) IUL m's"l 15
13 12 11 10
. . . . 8 10 12 14 16 18
9 6
SST (C) 15
•
,
•
,
!f
14
7
12
5
11
/ t
10b/
Ix
'
6
'
8
•
•
•
,
.
.
14
1213,',,A l,
.
.
'
2
.
.
.
We have provided evidence of the thermally driven wind in the vicinities of two major ocean fronts - the Gulf Stream and the Kuroshio and investigated the phenomenon on a climatological scale. The contributions of the thermally driven wind to the net wind climatology in the Newfoundland Basin based on SECTIONS and COADS
•
data sets are 3.0 and 1.5-2.0 m.s"j respectively. The first estimate is closer to what can be inferred from a simple theory (Hsu, 1984), applied to a case study. In the Kuroshio
•
6
"
8
•
•
•
10 12 14 16 18
8
.
.
.
.
.
.
7
Acknowledgements. The author wishes to thank Drs F. Dobson and C. Mason for valuable comments.
6
References t
.
.
area, the T D W - c o m p o n e n t is smaller (1.0-1.5 m,s "j) but still distinguishable.
.
' ,,-,
t ! "
,
SST(C)
.>/
, , 'y?
!0 9
.
•
3
10 12 14 16 18
.
4 Conclusions
SST (C)
•
SST(C) 15
associated component amounts to 1.0 -1.5 m.s ") (Fig.2c). We contend that this is smaller than in the Newfoundland Basin because the Kuroshio SST gradients are weaker than those in the G u l f Stream area.
%
4
I~,
9
from the rise in the north curve in the range 20-24°C: the
. . . . . 8 10 12 14 16 18
2" 6
8
,
most of COADS winds were observed by trained amateurs and lack quality control. The TDW effect in the Kuroshio zone can be inferred
O,, m.s"l . . . . .
8
607
%
3 2
.
10 12 14 16 18 20 22 24 SST (C)
•
•
•
"
°
•
•
10 12 14 16 18 20 22 24 SST (C) north curve south curve
.-)
Fig.2. Wind speed amplitude IUI (a,b,c) and standard deviation o. (d,e,0 versus SST: a,d - The Newfoundland Basin, SECTIONS; b,e - The Newfoundland Basin, COADS; c,f- the Kuroshio, COADS.
SECTIONS data set the effect of the "thermal breeze" is better seen than in COADS. The COADS-based estimate of the TDW-component is 1.5-2.0 m.s "~ (Fig.2b). The difference between the two estimates is not significant and is presumably due to the fact that the SECTIONS data were collected by professional meteorologists whereas
Businger, J.A. and Shaw, WJ., The response of the marine boundary layer to mesoscale variations in the sea-surface temperature, Din. Oceans Atmos., 8, 26%281, 1984. Clarke R.A., Hill H.W., Reinger R~F., and Warren B.A. Current system south and east of the Grand Banks of Newfoundland, ,/.Physical Oceonogr., 10, NO.I, 25-65, 1980. Friehe, A.A. and Williams, E.R., Aircraft measurements in FASINEX, in Seventh Conference on Ocean-Atmosphere Interaction, 38-40, American Meteorological Society,Boston, Mass., 1988. Guymer, T.H., Businger, J.A., Katsaros,K.B., Shaw, WJ., Taylor, P.K., Large, W.G., and Payne, R.E., Transfer processesat the air-sea interface, Phil. Trmu. R. Soc. Local., A308, 253-273, 1983. Hsu, S.A., Sea-bReze-like winds across the north wali of tic Gulf Stream: an analytical model, J. ofGeoph. Res., g9, NO.C2, 2025-2028, 1984. Kolinko, A.V. and Geshelin, Y.S., Breeze and driR effects in the Gulf Stream asen, Meteorologia i gideologia (in Ru.~ian), NO.I, 110-113, 1995. Wcllar, R.A., Owrview of the Frontal Air-Sen Int~action Experiment (FASINEX): a study of air-sea interaction in a region of strong oceanic gradients, J ofGeoph. Res., 96, NO,C5, 8501-8516, 1991.
Pergamon PII: S0079- ! 946(98)00082-2
Thermally Driven Wind Circulation near Ocean Fronts Yuri S. GesheHn Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, N.S., Canada B2Y 4A2
Received 25 April 1997; accepted 19 August 1997
Abstract. A relationship between the sea surface temperature gradient and wind speed is investigated. The climatological analysis is confined to the frontal zones of the Newfoundland Basin and Kuroshio areas. The findings are demonstrated to differ for both regions and different data sets. The inferred component of thermally driven wind is compared to the Hs~ 1984, estimate. In the Newfoundland Basin, the thermal wind circulation is simulated better than in the Kuroshio area. The reason for this is addressed. © 1998 Elsevier Science Ltd. All fights reserved
1 Introduction The areas of ocean fronts (such as the Gulf Stream, Kuroshio) are unique in the zones of sharp sea surface temperature (SST) gradients. The structure of the marine atmospheric boundary layer differs when the wind is parallel to and perpendicular to the oceanic front. The latter is expected to induce atmospheric secondary flow similar to a sea breeze circulation. There is no universally agreed term for that sort of circulation if the horizontal temperature gradient is caused by phenomena other than the coast line. Hsu. 1984, called it sea-breeze-like winds and developed a simple model for the winds driven by sharp SST gradients. The mmlogy between actual sea breeze and the phenomenon being considered is not close because the former one is associated with the diurnal variability of the air temperature over the land, whereas the latter is not associated with such variability. Therefore, we suggest the use of more appropriate term: thermally driven wind (TDW) and employ this term in the following discussion.
Correspondence to: Yuri S. Geshelin
Some evidence for the modulation of surface momentum fluxes by SST variations was provided in the JASIN experiment (Guymer et al., 1983), suggesting the feasibility of TDW. The JASIN data sets were also used by Businger and Shaw (1984), and the conclusion was made that mesoscale changes in SST are capable of inducing secondary flow. However, the JASIN SST gradients were too small to allow a quantitative description of the phenomenon. The more convincing experimental evidence of TDW was furnished in the FASINEX experiment (Friehe and Williams, 1988; Weller, 1991). It was shown that a rise in cross-front profile of surface wind speed was associated with the sharpest SST gradients. Kolinko and Geshelirt 1995, described TDW in the Newfoundland Basin on a climatological scale. The purpose of this study is to investigate the climatology of the phenomenon at two major hydrological fronts - the North Atlantic Current and the Kumshio - and to compare the results with the Hsu, 1984, theory.
2 Data setsand analysis
This study employs two data sets: Comprehensive Ocean Almosphere Data Set (COADS) and the data collected in the SECTIONS program in the Newfoundland Basin on board Soviet research vessels. The latter data set is referred to as SECTIONS a ~ , Both data sets span the period 19811991 and provide spatial resolution sufficient for such a study. The data sets contain a variety of meteorological and oceanogrephic characteristics; sea surface temperature, wind speed amplitude end wind direction were used in this study. Whereas SECTIONS d~_~ are available for the Newfoundland Basin only, subsets from COADS were derived both for the Newfoundland Basin and the Kuroshio zone. The analysis is confined to the winter period only: the period of the most intensive air-sea interaction. Maps of 605
606
Y.S. Geshelin: Thermally Driven Wind Circulation near Ocean Fronts
the two areas are shown in Fig.l where the dashed lines separate cold water masses (to the north of the line) from warm (to the south of the line). 50W
60W
40W :0 :50N
50N
Conversely, when the air is warmed from below, it is convectively destabilized. For each data category the wind speeds were then binned by SST. To assess confidence intervals, the standard deviation for each bin was computed. The number of data used for each case is given in Table 1. Table 1. Numberof data used for each category.
•L
The Newfoundland The Newfoundland, The Kuroshio, Basin, SECTIONS Basin,COADS COADS
•°
o °
"°
North winds South winds
i
1,968 626
3,099 1,074
21,224 1i,339
40N
40N
3 Results a |
60W
50W 130E
I
40W
50N
150E 50N
40N
40N
120E
140E
3°NI. i"
30N
• • Bf, * ~
I 20N
b
I 120E
130E
140E
'
20N 150E
Fig.l. The study areas: a - the NewfoundlandBasin,b - the zone of the Kuroshio.Dashed lines are schematicallocationsof the strongestSST gradientsin winter.
The method of analysis follows Kolinko and Geshelin, 1995. For each of the two study areas, two data categories were selected: (1) winds blowing from the cold underlying surface onto the warm one, and (2) from warm to cold. Such a choice was made because the structure of the marine boundary layer undergoes significant changes as one regime gives way to the other. When the air mass travels over a cold surface, it becomes more stable.
The results are given in Fig.2 where the terms north curve and south curve are employed. The north curve is based on all cases when the winds are blowing from the cold underlying surface to the warm one. In both cases the winds are blowing along the SST coordinate which is roughly perpendicular to the hydrological front. This coordinate allows us to observe the effect of adjustment of the air mass to the underlying surface and to detect TDW. In Fig.2, one can see the apparent rise of the north curve in the SST range 13-15°C. It is this SST range that falls on the sharpest SST gradients in the Newfoundland Basin (Clarke et al., 1980). This suggests that the sharpness of the gradient has an impact on the wind speed profile. According to the Hsu, 1984, theory, as the air travels from the cold sea surface to the warm one, it is sped up by the effect of the sharp SST gradient below. Using observational data collected on a single day, Hsu showed that the thermally caused increase in wind speed was 6 m.s"~. The contribution of the climatological TDW-component to the net wind speed can be inferred from Fig.2 as the magnitude of the rise in the north curve - the maximum value exceeds the minimum one by 3 m.s" (Fig.2a). Because the exact agreement between climatologies and a case study is not expected, the values seem to be in a reasonable agreement. To apply Hsu's model on a climatological scale, a separate study needs to be done. In the case of south curves (the air travels onto the cold surface), no significant response to the hydrological front is observed (Fig.2). This is because the vertical circulation similar to actual sea breeze circulation is achieved only in the case where the winds are blowing from the cold side of the front (Hsu, 1984). Under these conditions, the air counterflow can develop at higher elevations. Once the winds are blowing from the warm side, there is no air counterflow in the upper layer. In the case of the
Y. S. Geshelin: Thermally Driven Wind Circulation near Ocean Fronts -) IUL m's"l 15
13 12 11 10
. . . . 8 10 12 14 16 18
9 6
SST (C) 15
•
,
•
,
!f
14
7
12
5
11
/ t
10b/
Ix
'
6
'
8
•
•
•
,
.
.
14
1213,',,A l,
.
.
'
2
.
.
.
We have provided evidence of the thermally driven wind in the vicinities of two major ocean fronts - the Gulf Stream and the Kuroshio and investigated the phenomenon on a climatological scale. The contributions of the thermally driven wind to the net wind climatology in the Newfoundland Basin based on SECTIONS and COADS
•
data sets are 3.0 and 1.5-2.0 m.s"j respectively. The first estimate is closer to what can be inferred from a simple theory (Hsu, 1984), applied to a case study. In the Kuroshio
•
6
"
8
•
•
•
10 12 14 16 18
8
.
.
.
.
.
.
7
Acknowledgements. The author wishes to thank Drs F. Dobson and C. Mason for valuable comments.
6
References t
.
.
area, the T D W - c o m p o n e n t is smaller (1.0-1.5 m,s "j) but still distinguishable.
.
' ,,-,
t ! "
,
SST(C)
.>/
, , 'y?
!0 9
.
•
3
10 12 14 16 18
.
4 Conclusions
SST (C)
•
SST(C) 15
associated component amounts to 1.0 -1.5 m.s ") (Fig.2c). We contend that this is smaller than in the Newfoundland Basin because the Kuroshio SST gradients are weaker than those in the G u l f Stream area.
%
4
I~,
9
from the rise in the north curve in the range 20-24°C: the
. . . . . 8 10 12 14 16 18
2" 6
8
,
most of COADS winds were observed by trained amateurs and lack quality control. The TDW effect in the Kuroshio zone can be inferred
O,, m.s"l . . . . .
8
607
%
3 2
.
10 12 14 16 18 20 22 24 SST (C)
•
•
•
"
°
•
•
10 12 14 16 18 20 22 24 SST (C) north curve south curve
.-)
Fig.2. Wind speed amplitude IUI (a,b,c) and standard deviation o. (d,e,0 versus SST: a,d - The Newfoundland Basin, SECTIONS; b,e - The Newfoundland Basin, COADS; c,f- the Kuroshio, COADS.
SECTIONS data set the effect of the "thermal breeze" is better seen than in COADS. The COADS-based estimate of the TDW-component is 1.5-2.0 m.s "~ (Fig.2b). The difference between the two estimates is not significant and is presumably due to the fact that the SECTIONS data were collected by professional meteorologists whereas
Businger, J.A. and Shaw, WJ., The response of the marine boundary layer to mesoscale variations in the sea-surface temperature, Din. Oceans Atmos., 8, 26%281, 1984. Clarke R.A., Hill H.W., Reinger R~F., and Warren B.A. Current system south and east of the Grand Banks of Newfoundland, ,/.Physical Oceonogr., 10, NO.I, 25-65, 1980. Friehe, A.A. and Williams, E.R., Aircraft measurements in FASINEX, in Seventh Conference on Ocean-Atmosphere Interaction, 38-40, American Meteorological Society,Boston, Mass., 1988. Guymer, T.H., Businger, J.A., Katsaros,K.B., Shaw, WJ., Taylor, P.K., Large, W.G., and Payne, R.E., Transfer processesat the air-sea interface, Phil. Trmu. R. Soc. Local., A308, 253-273, 1983. Hsu, S.A., Sea-bReze-like winds across the north wali of tic Gulf Stream: an analytical model, J. ofGeoph. Res., g9, NO.C2, 2025-2028, 1984. Kolinko, A.V. and Geshelin, Y.S., Breeze and driR effects in the Gulf Stream asen, Meteorologia i gideologia (in Ru.~ian), NO.I, 110-113, 1995. Wcllar, R.A., Owrview of the Frontal Air-Sen Int~action Experiment (FASINEX): a study of air-sea interaction in a region of strong oceanic gradients, J ofGeoph. Res., 96, NO,C5, 8501-8516, 1991.