Accepted Manuscript

Title: The possible role of Brazilian promontory in Little IceAge

Author: Youjia Zou Xiangying Xi

PII: S0377-0265(14)00020-7DOI: http://dx.doi.org/doi:10.1016/j.dynatmoce.2014.04.001Reference: DYNAT 931

To appear in: Dynamics of Atmospheres and Oceans

Received date: 27-10-2012Revised date: 29-3-2014Accepted date: 3-4-2014

Please cite this article as: Zou, Y., Xi, X.,The possible role of Brazilianpromontory in Little Ice Age, Dynamics of Atmospheres and Oceans (2014),http://dx.doi.org/10.1016/j.dynatmoce.2014.04.001

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Highlights

·We simulate the ITCZ movements in the Atlantic.

·We also model the ITCZ movements in the Atlantic after removing Brazilian promontory.

·The ITCZ shift affects the variations of the Gulf Stream.

·The reduction of the Gulf Stream results in a cold period in the North hemisphere.

·The Brazilian promontory may play a role in global climate change

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The possible role of Brazilian promontory in

Little Ice Age Youjia Zou1*, Xiangying Xi2 1 Department of Meteorology and Oceanography, Shanghai Maritime University. 2Management Faculty, Wuhan University of Technology. *Correspondence to: yjzou@shmtu.edu.cn

Mailing Address:

Haigang Ave 1550, Pudong District, Shanghai, Merchant Marine College, Shanghai Maritime University. P.R.China Postal code: 201306

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Abstract:

The Gulf Stream, one of the strongest currents in the world, transports approximately

31 Sv of water [Kelly et al., 1990; Baringer and Larsen, 2001; Leaman et al., 1995]

and 1.3×1015 W [Larsen, 1992] of heat into the Atlantic Ocean, and warms the vast

European continent. Thus any change of the Gulf Stream could lead to the climate

change in the European continent, and even worldwide [Harry et al., 2005]. Past

studies have revealed a diminished Gulf Stream and oceanic heat transport that was

possibly associated with a southward migration of Intertropical Convergence Zone

(ITCZ) and may have contributed to Little Ice Age (AD ~1200 to 1850) in the North

Atlantic [Lund et al., 2006]. However, the causations of the Gulf Stream weakening

due to the southward migration of the ITCZ remain uncertain. Here we use satellite

observation data and employ a model (oceanic general circulation model - OGCM)

to demonstrate that the Brazilian promontory in the east coast of South America may

have played a crucial role in allocating the equatorial currents, while the mean

position of the equatorial currents migrates between northern and southern

hemisphere in the Atlantic Ocean. Northward migrations of the equatorial currents in

the Atlantic Ocean have little influence on the Gulf Stream. Nevertheless, southward

migrations, especially abrupt large southward migrations of the equatorial currents,

can lead to the increase of the Brazil Current and the significant decrease of the

North Brazil Current, in turn the weakening of the Gulf Stream. The results from the

model simulations suggest the mean position of the equatorial currents in the

Atlantic Ocean shifted at least 180 to 260 km southwards of its present-day position

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during the Little Ice Age based on the calculations of simple linear equations and the

OGCM simulations.

Key Words

Gulf Stream; Weakening, ITCZ Migration; Equatorial Current Shift; Brazilian

Promontory; Little Ice Age

1. Introduction

The Gulf Stream is one of the world's most intensely studied current systems

[Meinen et al., 2009]. This extensive western boundary current plays an important

role in the poleward transfer of heat and salt and serves to warm the European

subcontinent [Harry et al., 2005]. Previous studies already found that the Gulf

Stream has a marked seasonal variability, with peak-to-peak amplitude in sea surface

height of 10-15 cm, and significant fluctuations in volume transport and velocity.

The fluctuations are mostly confined to the upper 200-300 m of the water column

and are a result of seasonal heating and expansion of the surface waters [Hogg and

Johns, 1995].

However, the non-seasonal variations of the Gulf Stream, which may play a

significant role in the climate change, are likely to be overlooked. Its variability on

decadal to longer timescales remains a topic of debate [Taylor and Stephens, 1998;

Rossby and Benway, 2000; Frankignoul et al., 2001]. The possibility of abrupt

changes in the Gulf Stream heat transport in response to the abrupt changes of the

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Gulf Stream volume transport is considered to be one of the key uncertainties in

predictions of climate change for the coming centuries [Lund et al., 2006; PICC,

2013]. Thus, the mechanisms responsible for the variability in the Gulf Stream

deserve major consideration by researchers.

2. Recent study on Gulf Stream transport and ITCZ

Recent studies, however, indicate that the change of the Gulf Stream in transport is

connected with the migrations of the intertropical convergence zone (ITCZ) [Lund et

al., 2006; Haug et al., 2001; Broccoli et al., 2006]. The seasonal variations of the

ITCZ can result in the seasonal changes of the Gulf Stream [Lund et al., 2006], but are

not likely to have significant impact on the climate. The newly-developed models also

reveal that the ITCZ tends to shift northward from its mean position lying at 10°N in

summer and nearly over equator in winter [Peng and Miller, 2008; Haug et al., 2001],

in correspondence with the mean position of the equatorial currents in the Atlantic

Ocean. However, anomalous southward shifts, especially large southward shifts, are

rare [Haug et al., 2001].

Nevertheless, the southward migration, which is speculated to be southward

displacement (may be a regular behavior in terms of long timescales), was indeed

observed during the Little Ice Age (LIA) by the investigations into the titanium and

iron contents in Cariaco Basin sediments (on the northern shelf of Venezuela, a highly

sensitive recorder of past climates in the tropical ocean). The new sediment records

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from Lake Titicaca also indicate that precipitation steadily increased in that region

during the LIA [Haug et al., 2001]. Pollen records from the southern margin of

Amazonia also suggest a southward expansion of humid evergreen forest during the

late Holocene [Peng and Miller, 2008; Haug et al., 2001]. These changes are

anti-correlated with decreases in precipitation indicated in the Cariaco records.

Together, they generally confirm that these climate events were associated with

southward movements of the ITCZ [Haug et al., 2001].

The Cariaco record, when combined with other records from South America [Baker et

al., 2001; Mayle et al., 2000; Maslin and Burns, 2000], unambiguously shows that

climate changes in Central and South America over the course of the Holocene are

due, at least in part , to a general southward shift of the ITCZ [Haug et al., 2001].

The fact that a large and long period southward migration of the ITCZ in tropical

Atlantic can create climate change in the North Atlantic invites questions as to why

the significant global climate events, such as Younger Dryas events [Stansell, et al.,

2010] and the LIA, originate always from the Atlantic Ocean rather than Pacific

Ocean where the similar southward migration of the ITCZ was also observed during

the past 30000 years [Koutavas and Lynch, 2004; Peng and Miller, 2008;Sachs et al.,

2009].

3. A new investigation into ITCZ and ocean currents

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We investigate the ocean currents and the ITCZ in tropical Atlantic, and find that the

special continental shelf geometry and topography in the east coast of South America

may have played a central role in the reorganizations of the South Equatorial Current

(SEC) while the mean position of the equatorial currents moves between the northern

and southern hemisphere in the Atlantic Ocean.

The SEC in the Atlantic is a broad, westward flowing current that extends from the

surface to a nominal depth of 100 m [Sramma and England, 1999]. Its northern

boundary is usually near 4°N, while the southern boundary is usually found between

15-25°S, depending primarily on longitudinal location and the season. The annual

mean transport is about 49 Sv [Dorothee et al., 2004].

The SEC veers southward as it approaches the west coast in the Pacific Ocean.

However, most of the SEC in the Atlantic Ocean (60-70%) moves northwestward

along the northern Brazilian coast across the equator as major part of North Brazil

Current (NBC). The rest of the SEC flows southwestward as part of Brazil Current

(BC) after reaching the Brazilian continental shelf because of the split by the Brazilian

promontory near Cabo De Sao Roque at a position that ranges from about 5.5°S to

10°S as shown in Fig.1. The North Equatorial Current and components of the South

Equatorial Current flowing towards the Northern Hemisphere have contributed to a

stronger Gulf Stream compared to its counterparts in the Pacific and Indian Oceans.

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Fig. 1

The NBC plays a dual role in that it first closes the wind-driven equatorial gyre

circulation and feeds a system of eastward zonal countercurrents except in boreal

summer. Secondly, it provides a conduit for cross-equatorial transport of upper-ocean

waters as part of the Atlantic Meridional Overturning Cell [Johns et al., 1998]. Along

the coast of the South America, the NBC carries very warm and salty water northwest

across the equator from about 5°30′~ 10°S [Dorothee et al., 2004]. Some of that water

feeds the Equatorial Undercurrent, but much of it continues to flow along the coast

into the Gulf of Mexico and eventually becomes major part of the Gulf Stream

[Sramma and England, 1999].

The Brazilian promontory in the east coast of northern Brazil, bulging seaward for

about 510 to 520km, acts as a “two-way switch” as the mean position of the SEC

migrates between northern and southern hemisphere. The NBC gains strength whereas

the BC diminishes and becomes weaker as the SEC displaces northward. Conversely,

the NBC weakens while the BC strengthens as the SEC shifts southward depending

on the magnitude of its displacement (Fig.2). Good anticorrelation (r≈-0.82) between

the NBC and BC demonstrates their strong linkages in transport (Fig 6). Satellite

observations showing the bifurcation point of the SEC around the Cabo De Sao Roque

also lend good support to the correlations (Fig.4).

4. Model simulations on NBC, BC and equatorial current

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The Gulf Stream transports a maximum amount of water in the fall and a minimum in

the spring [Hogg and Johns, 1995; Kelly and Gille, 1990; Zlotnicki, 1991; Kelly,

1991], in phase with the seasonal north-south shifts of the SEC. In order to better

understand the behaviors of the Gulf Stream transport, an oceanic general circulation

model (OGCM) proposed by Kim et al. (2004) has been employed and modified after

considering the geometry of the continental shelf and bottom topography. Model

simulations suggest that abnormal northward migrations of the equatorial currents in

the Atlantic Ocean result in little influence on the Gulf Stream because the central

SEC and southern SEC are relatively weaker than the northern SEC [Hogg and Johns,

1995]. However, aberrant southward migrations, especially large southward

migrations of the equatorial currents, can lead to the increase of the BC and the

significant decrease of the NBC and a weakening of the Gulf Stream. These

migrations may result in climate change in the European continent, the northern

hemisphere, and even worldwide if the state of the equatorial currents in the southern

hemisphere becomes stable for a relatively long period. (Fig.3).

Fig.2

Fig.3

In essence, the equatorial currents are driven by trade winds that are consistent with

the general position of the ITCZ, and intrinsically linked to each other through the

Hadley Cells [Billups et al., 1999; Chiang et al., 2002; Poore et al., 2004; Broccoli et

al., 2006]. The location of the ITCZ shifts to follow closely the warmest waters of the

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equatorial current [Biasutti et al., 2003]. The movements of the ITCZ always imply

the same shifts of the equatorial currents at the same time in the Atlantic Ocean [Peng

and Miller, 2008; Poore et al., 2004; Chiang et al., 2002; Vink et al., 2001]. Satellite

observations also show that the equatorial currents in the Atlantic Ocean change in

response to the displacements of the trade winds (Fig.4).

However, the mechanisms of the large and long period migrations of the ITCZ are

still debated [Billups et al., 1999; Haug et al., 2001; Chiang et al., 2002; Poore et al.,

2004; Broccoli et al., 2006]. Poore et al. (2004) and Sachs et al. (2009) suggested that

the average position of the ITCZ was linked to solar variability; and Chiang et al.

(2002) and Haug et al. (2001) speculated that it was potentially driven by

Pacific-based climate variability. Other researchers suggested the variability in the

Atlantic Meridional Overturning Circulation (AMOC) could lead to changes in the

equatorial Atlantic SST gradient and shifts of the ITCZ [Schmidt and Lynch, 2011;

Jackson and Michael, 2013].

Further investigation into the causes of the ITCZ movement is beyond the scope of

this paper. However, the model simulations have shown that the seasonal variations of

the SEC can lead to 10-13 Sv fluctuations of the NBC but are not likely to make any

significant influence on the climate because of the short periods the fluctuations

persist. However, the measurements taken at about 4°S in the upper 300 m showed

that the NBC has a significant annual cycle in this area, ranging from a maximum

transport of about 36 Sv in July-August to a minimum of 13 Sv in April-May, with an

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annual mean transport of about 26 Sv [Johns et al., 1998]. These values are consistent

with the seasonal changes of the Gulf Stream transport.

Fig.4

The model simulations also suggest that the equatorial currents in the Atlantic Ocean

shifted at least 180 ~ 260 km southwards of its present-day position (equivalently a

reduction of 3~5 Sv in the NBC transport) and persisted for about 500 ~ 600 years

during the LIA based on the calculations of simple linear equations. This is in

agreement with the diminishment of 10 percent in the Gulf Stream transport inferred

from the previous work of Lund et al., [2006] (Fig.3). The NBC transport decreased

by approximately 15~18% whereas the BC increased about 13~16% below 500m

water depth during the cool period relative to today (Fig.5 & Fig.6). The fact that the

significant climate change events always originate from the Atlantic rather than the

Pacific Ocean, where the similar anomalous southward migrations of the equatorial

currents (ITCZ) were also observed during the past 30 ky [Vink et al., 2001;Koutavas

et al., 2004; Sachs et al., 2009], has led to a speculation that the LIA is most likely

triggered, at least in part, by the Brazilian promontory whwn the SEC shifts

anomalously southward in the Atlantic.

Fig.5

Fig.6

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Although the LIA is best known as a time of cooler temperatures and alpine glacier

advances in the Northern Hemisphere, it is also characterized by anomalously dry

conditions in Central and South America and high surface salinity in the Florida

Current [Lund et al., 2006; Poore et al., 2004; Haug et al., 2001]. Previous studies

have revealed that the AMOC could account for the coincidence between cold periods

in the high-latitude North Atlantic and anomalously drier conditions over northern

South America during the LIA [Kuhlbrodt et al., 2007; Timmermann et al., 2007;

Menviel et al., 2008;Menary et al., 2011;Jackson and Michael, 2013]. This

investigation suggests that the Brazilian promontory may also play a role in the

reduction of the NBC and the increase of the BC through the reorganizations of the

SEC.

Interestingly, if the Brazilian promontory in the northeast coast is removed and

replaced by a north-south straight coastline extending well to the north and south, the

model simulations show no significant fluctuation of the Gulf Stream even if large

north-south migration of the equatorial currents occurs in the Atlantic Ocean. The

model experiments clearly suggest that the continental shelf geometry has significant

influence on the ocean currents and thereby the global climate, which is consistent

with the previous conclusions made by Xie and Kaori (2000).

We hypothesize that the anomalous southward migrations of the equatorial currents in

the Atlantic might play a role during the Younger Dryas events, because the model

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simulations suggest that a further southward shift of the SEC in the Atlantic (about

300-400 km, which is equivalent to a reduction of 8~10 Sv in the NBC ) over a longer

period (over 1000 years) would impact the thermohaline properties of the Atlantic,

and flatten the meridional temperature gradient in the North Atlantic. More

significantly, the southward displacement of the SEC can change its stratification and

its potential for deep convection by diminishing the volume transport northward and

weakening the AMOC. This would also alter the thermohaline circulations and

general circulations of atmosphere to a new stable state over a period of several

hundred years, and eventually affect the climate change in whole northern hemisphere,

and even some parts of the southern hemisphere. Indeed, there is evidence that a

significant southward shift in the position of the ITCZ occurred during the Younger

Dryas cold interval resulting in an increase in tropical North Atlantic sea surface

salinity and a reduction in Florida Current transport and AMOC [Lynch et al., 2011;

Schmidt and Lynch, 2011].

5. Discussion and conclusion

This investigation presents a plausible mechanism of the climate change in the North

Atlantic. The role of the Brazilian promontory as a potential climate modulator or

trigger is potentially significant, and may contribute to the differences between the

Pacific and Atlantic Ocean in many aspects, particularly in climate change, that have

been long overlooked. Understanding the linkages between the Gulf Stream and the

displacement of the equatorial currents and Brazilian promontory in the Atlantic

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Ocean can help us make more accurate predictions of the climate change in the next

centuries. Although the direct continuous measurement of the migrations of the

equatorial currents in the Atlantic are difficult, we can capture the long-term

variation by monitoring the changes of volume transport in the NBC and the BC

with either satellite observation or deployment of ocean moorings because these two

branches of the SEC are relatively narrow and close to the east coast of northern

South America.

Many of the uncertainties in our current understanding of the movements of the ITCZ

and the tropical Atlantic equatorial current system, and the linkages between the

equatorial current shift and climate change, need further careful investigation in the

future. For example, the mechanisms responsible for the migrations of the ITCZ and

their relationship with the equatorial currents are still debated [Lund et al., 2006; Peng

and Miller, 2008; Chiang et al., 2002 ; Haug et al., 2001; Vink et al., 2001; Nobre and

Shukla, 1996]; the causes of the faster southward but relative slower northward

displacement of the ITCZ are unknown [Peng and Miller, 2008; Haug et al., 2001];

the contributors to the period of the ITCZ in one stable state (e.g. firmly staying in

southern hemisphere) are ambiguous; and the feedback mechanisms of the

resumptions of the equatorial currents in the Atlantic are poorly understood.

When investigating the influence of the ocean currents to the climate change in the

Atlantic Ocean, it is not possible to isolate the Atlantic equatorial current system from

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an integrated Atlantic current system without considering the continental shelf

geometry and the bottom topography. It is time to take a more holistic approach and

in particular consider the combined effects of the continental shelf geometry and the

shift of the ITCZ. The Brazilian promontory may have a more important role than

presently recognized.

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Captions:

Fig. 1 Schematic plot for surface currents. Schematic plot showing the major

surface currents of the tropical Atlantic Ocean in boreal summer under normal

conditions when the North Equatorial Countercurrent flows eastward to join the

Guinea Current in the Gulf of Guinea. In other seasons, the North Equatorial

Countercurrent disappears and the surface flow moves westward in every longitude in

the western tropical Atlantic. The point of bifurcation, ranging from 5.5°S to 10°S,

separates the westward South Equatorial Current into North Brazil Current and Brazil

Current. The dotted lines indicate the boundaries between the North Equatorial

Current and North Equatorial Countercurrent and South Equatorial Current. The red

arrows denote the warm currents and the blue arrows for the cold currents.

Fig.2 Model simulations for surface currents in normal condition. Model

simulations show that most of the South Equatorial Current flow northwest under

normal conditions and become major part of the North Brazil Current across the

equator. The rest move southwest and become the Brazil Current. The divergence

occurs around Cabo de Sao Roque ranging from 5.5°S to 10°S. The dotted lines stand

for the boundaries between the North Equatorial Current and the Equatorial

Countercurrent and the South Equatorial Current. The north-south band of the

Equatorial Countercurrent is relatively smaller in boreal summer.

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Fig.3 Model simulations for surface currents in abnormal condition. Model

simulations reveal that during the anomalous southward displacement of the South

Equatorial Current (and its associated ITCZ), the westward current was bifurcated by

the Brazilian promontory that ranges from 5.5°S to 10°S. The North Brazil Current

decreased whereas the Brazil Current increased. The transport reduction in North

Brazil Current is approximately equal to the transport increase in Brazil Current. The

north-south band of the Equatorial Countercurrent is also broader than usual in boreal

summer. The dotted lines stand for the boundaries between the North Equatorial

Current and the Equatorial Countercurrent and the South Equatorial Current.

Fig.4 Monthly mean ocean surface current. The upper map is the monthly mean

ocean surface current in January and the bottom is the monthly mean in July. Satellite

observation shows a seasonal small scale north-south shift of the equatorial currents in

equatorial Atlantic from January to July in 2011. Created from

http://www.oscar.noaa.gov/datadisplay/oscar_latlon.php.

Fig.5 Transport simulations for the NBC and Brazil Current. The top panel shows

the NBC anomaly vs water depth, defined as transport at a given time minus average

transport over the period 0–1000 yr BP. Negative transport anomalies occur during

the Little Ice Age. The bottom panel shows the Brazil Current anomaly vs water depth.

Positive transport anomalies take place during the cool period.

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Fig.6 Transport simulations for the NBC and Brazil Current. The top panel shows

the estimated annual mean transport from 0–1000 yr BP for NBC in 10-yr moving

windows, where the lowest transport appeared during the Little Ice Age. The bottom

panel shows the estimated annual mean transport for Brazil Current in 10-yr moving

windows, where the highest transport occurred during the Little Ice Age. The vertical

bars represent the 95% confidence limit for the transport calculation.

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Fig. 1 Schematic plot for surface currents. Schematic plot showing under normal

conditions the major surface currents of the tropical Atlantic Ocean in boreal summer

when the North Equatorial Countercurrent flows eastward to join the Guinea Current

in the Gulf of Guinea. In other seasons the North Equatorial Countercurrent

disappears and the surface flow moves westward in every longitude in the western

tropical Atlantic. The point of bifurcation, ranging from 5.5°S to 10°S, separates the

westward South Equatorial Current into North Brazil Current and Brazil Current. The

dotted lines indicate the boundaries between the North Equatorial Current and North

Equatorial Countercurrent and South Equatorial Current. The red arrows denote the

warm currents and the blue arrows for the cold currents.

Figure

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Fig.2 Model simulations for surface currents in normal condition. Model

simulations show under the normal conditions most of the South Equatorial Current

flow northwest and become major part of the North Brazil Current across the equator

while the rest move southwest and become the Brazil Current, and they both veer at

around Cabo de Sao Roque ranging from 5.5°S to 10°S, the rest join the South

Atlantic Gyre. The dotted lines stand for the boundaries between the North Equatorial

Current and the Equatorial Countercurrent and the South Equatorial Current. The

north-south band of the Equatorial Countercurrent is relatively smaller in boreal

summer.

Figure

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Fig.3 Model simulations for surface currents in abnormal condition. Model

simulations reveal during the anomalous southward displacement of the South

Equatorial Current (ITCZ), the westward current was bifurcated by the Brazilian

promontory – ranging from 5.5°S to 10°S. The North Brazil Current decreased

whereas the Brazil Current increased. The transport deduction in North Brazil Current

is basically in line with the transport increase in Brazil Current. The north-south band

of the Equatorial Countercurrent also broadened than usual in boreal summer. The

dotted lines stand for the boundaries between the North Equatorial Current and the

Equatorial Countercurrent and the South Equatorial Current.

Figure

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Fig.4 Monthly mean ocean surface current. upper map is monthly mean ocean

surface current in January and bottom is in July. Satellite observation shows a

seasonal small scale north-south shift of the equatorial currents in equatorial Atlantic

from January to July in 2011. Created from

http://www.oscar.noaa.gov/datadisplay/oscar_latlon.php.

Figure

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Fig.5 Transport simulations for the NBC and Brazil Current. a, NBC anomaly vs

water depth, defined as transport at a given time minus average transport over the

period 0–1000 yr BP. Negative transport anomalies occur during the Little Ice Age. b,

Brazil Current anomaly vs water depth. Positive transport anomalies take place during

the cool period.

Calendar Age (yr BP)

Wa

ter D

ep

th (

m)

0 200 400 600 800 10001000

0

200

400

600

800800 -5

-4

-3

-2

-1

0

1

2

3

Calendar Age (yr BP)

Wa

ter D

ep

th (

m)

0 200 400 600 800 10001000

0

200

400

600

800800 -3

-2

-1

0

1

2

3

4

Figure

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0 200 400 600 800 10001000

21

23

25

2727

Calendar Year (yr BP)

NB

C T

ra

nsp

ort

(Sv

)

Little Ice Age

0 200 400 600 800 100010004

6

88

CalendarYear (yr BP)

BC

Tra

nsp

ort (

Sv

)

Little Ice Age

Fig.6 Transport simulations for the NBC and Brazil Current. a, Estimated annual

mean transport from 0–1000 yr BP for NBC in 10-yr moving windows, the lowest

transport appeared during the Little Ice Age. b, Estimated annual mean transport for

Brazil Current in 10-yr moving windows, the highest transport occurred during the

Little Ice Age. The vertical bars represent the 95% confidence limit for the transport

calculation.

Figure