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The possible role of Brazilian promontory in Little Ice Age
Accepted Manuscript Title: The possible role of Brazilian promontory in Little Ice Age Author: Youjia Zou Xiangying Xi PII: DOI: Reference:
S0377-0265(14)00020-7 http://dx.doi.org/doi:10.1016/j.dynatmoce.2014.04.001 DYNAT 931
To appear in:
Dynamics
Received date: Revised date: Accepted date:
27-10-2012 29-3-2014 3-4-2014
of
Atmospheres
and
Oceans
Please cite this article as: Zou, Y., Xi, X.,The possible role of Brazilian promontory in Little Ice Age, Dynamics of Atmospheres and Oceans (2014), http://dx.doi.org/10.1016/j.dynatmoce.2014.04.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights ·We simulate the ITCZ movements in the Atlantic.
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·We also model the ITCZ movements in the Atlantic after removing Brazilian promontory.
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·The ITCZ shift affects the variations of the Gulf Stream.
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·The reduction of the Gulf Stream results in a cold period in the North hemisphere.
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·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
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Department of Meteorology and Oceanography, Shanghai Maritime University. Management Faculty, Wuhan University of Technology.
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Correspondence to: [email protected]
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Youjia Zou1*, Xiangying Xi2
Mailing Address:
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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]
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and 1.3×1015 W [Larsen, 1992] of heat into the Atlantic Ocean, and warms the vast
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European continent. Thus any change of the Gulf Stream could lead to the climate
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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
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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
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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
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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
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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
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Gulf Stream; Weakening, ITCZ Migration; Equatorial Current Shift; Brazilian
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Promontory; Little Ice Age
1. Introduction
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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
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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
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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.
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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
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2. Recent study on Gulf Stream transport and ITCZ
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deserve major consideration by researchers.
Recent studies, however, indicate that the change of the Gulf Stream in transport is
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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
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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
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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],
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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
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late Holocene [Peng and Miller, 2008; Haug et al., 2001]. These changes are
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anti-correlated with decreases in precipitation indicated in the Cariaco records.
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Together, they generally confirm that these climate events were associated with
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southward movements of the ITCZ [Haug et al., 2001].
The Cariaco record, when combined with other records from South America [Baker et
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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
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due, at least in part , to a general southward shift of the ITCZ [Haug et al., 2001].
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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
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(SEC) while the mean position of the equatorial currents moves between the northern
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and southern hemisphere in the Atlantic Ocean.
The SEC in the Atlantic is a broad, westward flowing current that extends from the
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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
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15-25°S, depending primarily on longitudinal location and the season. The annual
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mean transport is about 49 Sv [Dorothee et al., 2004].
The SEC veers southward as it approaches the west coast in the Pacific Ocean.
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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
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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
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waters as part of the Atlantic Meridional Overturning Cell [Johns et al., 1998]. Along
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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
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feeds the Equatorial Undercurrent, but much of it continues to flow along the coast
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into the Gulf of Mexico and eventually becomes major part of the Gulf Stream
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[Sramma and England, 1999].
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The Brazilian promontory in the east coast of northern Brazil, bulging seaward for
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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
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understand the behaviors of the Gulf Stream transport, an oceanic general circulation
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model (OGCM) proposed by Kim et al. (2004) has been employed and modified after
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considering the geometry of the continental shelf and bottom topography. Model simulations suggest that abnormal northward migrations of the equatorial currents in
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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,
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1995]. However, aberrant southward migrations, especially large southward migrations of the equatorial currents, can lead to the increase of the BC and the
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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
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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
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response to the displacements of the trade winds (Fig.4).
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observations also show that the equatorial currents in the Atlantic Ocean change in
However, the mechanisms of the large and long period migrations of the ITCZ are
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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
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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
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Pacific-based climate variability. Other researchers suggested the variability in the Atlantic Meridional Overturning Circulation (AMOC) could lead to changes in the
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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.
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Fig.4
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The model simulations also suggest that the equatorial currents in the Atlantic Ocean
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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
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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
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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
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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
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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
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Current [Lund et al., 2006; Poore et al., 2004; Haug et al., 2001]. Previous studies
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have revealed that the AMOC could account for the coincidence between cold periods
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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;
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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
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reduction of the NBC and the increase of the BC through the reorganizations of the
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SEC.
Interestingly, if the Brazilian promontory in the northeast coast is removed and
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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,
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and flatten the meridional temperature gradient in the North Atlantic. More
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significantly, the southward displacement of the SEC can change its stratification and
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its potential for deep convection by diminishing the volume transport northward and weakening the AMOC. This would also alter the thermohaline circulations and
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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,
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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
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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;
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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
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variation by monitoring the changes of volume transport in the NBC and the BC
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with either satellite observation or deployment of ocean moorings because these two
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branches of the SEC are relatively narrow and close to the east coast of northern
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South America.
Many of the uncertainties in our current understanding of the movements of the ITCZ
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and the tropical Atlantic equatorial current system, and the linkages between the equatorial current shift and climate change, need further careful investigation in the
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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
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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
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shift of the ITCZ. The Brazilian promontory may have a more important role than
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presently recognized.
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Captions:
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Fig. 1 Schematic plot for surface currents. Schematic plot showing the major
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surface currents of the tropical Atlantic Ocean in boreal summer under normal
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conditions when the North Equatorial Countercurrent flows eastward to join the Guinea Current in the Gulf of Guinea. In other seasons, the North Equatorial
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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,
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separates the westward South Equatorial Current into North Brazil Current and Brazil Current. The dotted lines indicate the boundaries between the North Equatorial
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Current and North Equatorial Countercurrent and South Equatorial Current. The red
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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
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Equatorial Current (and its associated ITCZ), the westward current was bifurcated by
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the Brazilian promontory that ranges from 5.5°S to 10°S. The North Brazil Current
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decreased whereas the Brazil Current increased. The transport reduction in North Brazil Current is approximately equal to the transport increase in Brazil Current. The
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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
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Current and the Equatorial Countercurrent and the South Equatorial Current.
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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
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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.
Page 22 of 29
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
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windows, where the lowest transport appeared during the Little Ice Age. The bottom
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panel shows the estimated annual mean transport for Brazil Current in 10-yr moving
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windows, where the highest transport occurred during the Little Ice Age. The vertical
Ac ce p
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an
bars represent the 95% confidence limit for the transport calculation.
Page 23 of 29
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Figure
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Fig. 1 Schematic plot for surface currents. Schematic plot showing under normal
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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
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in the Gulf of Guinea. In other seasons the North Equatorial Countercurrent
ed
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
ce pt
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
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warm currents and the blue arrows for the cold currents.
Page 24 of 29
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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
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flow northwest and become major part of the North Brazil Current across the equator
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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
ed
Atlantic Gyre. The dotted lines stand for the boundaries between the North Equatorial Current and the Equatorial Countercurrent and the South Equatorial Current. The
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summer.
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north-south band of the Equatorial Countercurrent is relatively smaller in boreal
Page 25 of 29
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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
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Equatorial Current (ITCZ), the westward current was bifurcated by the Brazilian
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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
ed
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
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dotted lines stand for the boundaries between the North Equatorial Current and the
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Equatorial Countercurrent and the South Equatorial Current.
Page 26 of 29
ce pt
ed
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an
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Figure
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
Ac
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.
Page 27 of 29
Figure
0 3
2
1
0
400 -1
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Water Depth (m)
200
-2
600
-3
800 0
200
400
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-4
600
800
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Calendar Age (yr BP)
-5
1000
0
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400
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600
800 0
3
2
1
ed
Water Depth (m)
200
4
200
400 600 Calendar Age (yr BP)
0
-1
-2
800
1000
-3
Ac
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.
Page 28 of 29
Figure
Little Ice Age
25
23
21 200
400 600 Calendar Year (yr BP)
800
1000
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0
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NBC Transport (Sv)
27
an M
6
Little Ice Age
200
400 600 CalendarYear (yr BP)
800
1000
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4 0
ed
BC Transport (Sv)
8
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
Ac
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.
Page 29 of 29
S0377-0265(14)00020-7 http://dx.doi.org/doi:10.1016/j.dynatmoce.2014.04.001 DYNAT 931
To appear in:
Dynamics
Received date: Revised date: Accepted date:
27-10-2012 29-3-2014 3-4-2014
of
Atmospheres
and
Oceans
Please cite this article as: Zou, Y., Xi, X.,The possible role of Brazilian promontory in Little Ice Age, Dynamics of Atmospheres and Oceans (2014), http://dx.doi.org/10.1016/j.dynatmoce.2014.04.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights ·We simulate the ITCZ movements in the Atlantic.
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·We also model the ITCZ movements in the Atlantic after removing Brazilian promontory.
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·The ITCZ shift affects the variations of the Gulf Stream.
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·The reduction of the Gulf Stream results in a cold period in the North hemisphere.
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·The Brazilian promontory may play a role in global climate change
Page 1 of 29
The possible role of Brazilian promontory in Little Ice Age
*
Department of Meteorology and Oceanography, Shanghai Maritime University. Management Faculty, Wuhan University of Technology.
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2
Correspondence to: [email protected]
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1
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Youjia Zou1*, Xiangying Xi2
Mailing Address:
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Haigang Ave 1550, Pudong District, Shanghai, Merchant Marine College, Shanghai Maritime University. P.R.China Postal code: 201306
Page 2 of 29
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]
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and 1.3×1015 W [Larsen, 1992] of heat into the Atlantic Ocean, and warms the vast
cr
European continent. Thus any change of the Gulf Stream could lead to the climate
us
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
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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
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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
te
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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
Ac ce p
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
Page 3 of 29
during the Little Ice Age based on the calculations of simple linear equations and the OGCM simulations. Key Words
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Gulf Stream; Weakening, ITCZ Migration; Equatorial Current Shift; Brazilian
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Promontory; Little Ice Age
1. Introduction
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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
M
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
te
d
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.
Ac ce p
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
Page 4 of 29
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
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2. Recent study on Gulf Stream transport and ITCZ
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deserve major consideration by researchers.
Recent studies, however, indicate that the change of the Gulf Stream in transport is
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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
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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
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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],
Ac ce p
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
Page 5 of 29
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
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late Holocene [Peng and Miller, 2008; Haug et al., 2001]. These changes are
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anti-correlated with decreases in precipitation indicated in the Cariaco records.
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Together, they generally confirm that these climate events were associated with
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southward movements of the ITCZ [Haug et al., 2001].
The Cariaco record, when combined with other records from South America [Baker et
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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
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due, at least in part , to a general southward shift of the ITCZ [Haug et al., 2001].
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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
Page 6 of 29
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
ip t
(SEC) while the mean position of the equatorial currents moves between the northern
us
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and southern hemisphere in the Atlantic Ocean.
The SEC in the Atlantic is a broad, westward flowing current that extends from the
an
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
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15-25°S, depending primarily on longitudinal location and the season. The annual
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d
mean transport is about 49 Sv [Dorothee et al., 2004].
The SEC veers southward as it approaches the west coast in the Pacific Ocean.
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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.
Page 7 of 29
Fig. 1
The NBC plays a dual role in that it first closes the wind-driven equatorial gyre
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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
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waters as part of the Atlantic Meridional Overturning Cell [Johns et al., 1998]. Along
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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
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feeds the Equatorial Undercurrent, but much of it continues to flow along the coast
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into the Gulf of Mexico and eventually becomes major part of the Gulf Stream
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[Sramma and England, 1999].
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The Brazilian promontory in the east coast of northern Brazil, bulging seaward for
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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
Page 8 of 29
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
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understand the behaviors of the Gulf Stream transport, an oceanic general circulation
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model (OGCM) proposed by Kim et al. (2004) has been employed and modified after
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considering the geometry of the continental shelf and bottom topography. Model simulations suggest that abnormal northward migrations of the equatorial currents in
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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,
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1995]. However, aberrant southward migrations, especially large southward migrations of the equatorial currents, can lead to the increase of the BC and the
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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
Ac ce p
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
Page 9 of 29
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
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response to the displacements of the trade winds (Fig.4).
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observations also show that the equatorial currents in the Atlantic Ocean change in
However, the mechanisms of the large and long period migrations of the ITCZ are
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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
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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
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Pacific-based climate variability. Other researchers suggested the variability in the Atlantic Meridional Overturning Circulation (AMOC) could lead to changes in the
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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
Page 10 of 29
annual mean transport of about 26 Sv [Johns et al., 1998]. These values are consistent with the seasonal changes of the Gulf Stream transport.
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Fig.4
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The model simulations also suggest that the equatorial currents in the Atlantic Ocean
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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
an
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
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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
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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
Ac ce p
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
Page 11 of 29
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
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Current [Lund et al., 2006; Poore et al., 2004; Haug et al., 2001]. Previous studies
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have revealed that the AMOC could account for the coincidence between cold periods
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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;
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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
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reduction of the NBC and the increase of the BC through the reorganizations of the
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SEC.
Interestingly, if the Brazilian promontory in the northeast coast is removed and
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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
Page 12 of 29
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,
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and flatten the meridional temperature gradient in the North Atlantic. More
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significantly, the southward displacement of the SEC can change its stratification and
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its potential for deep convection by diminishing the volume transport northward and weakening the AMOC. This would also alter the thermohaline circulations and
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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,
M
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
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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;
Ac ce p
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
Page 13 of 29
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
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variation by monitoring the changes of volume transport in the NBC and the BC
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with either satellite observation or deployment of ocean moorings because these two
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branches of the SEC are relatively narrow and close to the east coast of northern
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South America.
Many of the uncertainties in our current understanding of the movements of the ITCZ
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and the tropical Atlantic equatorial current system, and the linkages between the equatorial current shift and climate change, need further careful investigation in the
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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
Ac ce p
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
Page 14 of 29
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
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shift of the ITCZ. The Brazilian promontory may have a more important role than
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presently recognized.
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Baringer, M. O., J. C.Larsen, 2001. Sixteen years of Florida Current transport at 27°N.
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Captions:
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Fig. 1 Schematic plot for surface currents. Schematic plot showing the major
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surface currents of the tropical Atlantic Ocean in boreal summer under normal
us
conditions when the North Equatorial Countercurrent flows eastward to join the Guinea Current in the Gulf of Guinea. In other seasons, the North Equatorial
an
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,
M
separates the westward South Equatorial Current into North Brazil Current and Brazil Current. The dotted lines indicate the boundaries between the North Equatorial
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d
Current and North Equatorial Countercurrent and South Equatorial Current. The red
Ac ce p
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.
Page 21 of 29
Fig.3 Model simulations for surface currents in abnormal condition. Model simulations reveal that during the anomalous southward displacement of the South
ip t
Equatorial Current (and its associated ITCZ), the westward current was bifurcated by
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the Brazilian promontory that ranges from 5.5°S to 10°S. The North Brazil Current
us
decreased whereas the Brazil Current increased. The transport reduction in North Brazil Current is approximately equal to the transport increase in Brazil Current. The
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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
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Current and the Equatorial Countercurrent and the South Equatorial Current.
te
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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
Ac ce p
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
ip t
windows, where the lowest transport appeared during the Little Ice Age. The bottom
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panel shows the estimated annual mean transport for Brazil Current in 10-yr moving
us
windows, where the highest transport occurred during the Little Ice Age. The vertical
Ac ce p
te
d
M
an
bars represent the 95% confidence limit for the transport calculation.
Page 23 of 29
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Figure
us
Fig. 1 Schematic plot for surface currents. Schematic plot showing under normal
an
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
M
in the Gulf of Guinea. In other seasons the North Equatorial Countercurrent
ed
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
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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
Ac
warm currents and the blue arrows for the cold currents.
Page 24 of 29
cr
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Figure
us
Fig.2 Model simulations for surface currents in normal condition. Model simulations show under the normal conditions most of the South Equatorial Current
an
flow northwest and become major part of the North Brazil Current across the equator
M
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
ed
Atlantic Gyre. The dotted lines stand for the boundaries between the North Equatorial Current and the Equatorial Countercurrent and the South Equatorial Current. The
Ac
summer.
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north-south band of the Equatorial Countercurrent is relatively smaller in boreal
Page 25 of 29
cr
ip t
Figure
us
Fig.3 Model simulations for surface currents in abnormal condition. Model simulations reveal during the anomalous southward displacement of the South
an
Equatorial Current (ITCZ), the westward current was bifurcated by the Brazilian
M
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
ed
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
ce pt
dotted lines stand for the boundaries between the North Equatorial Current and the
Ac
Equatorial Countercurrent and the South Equatorial Current.
Page 26 of 29
ce pt
ed
M
an
us
cr
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Figure
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
Ac
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.
Page 27 of 29
Figure
0 3
2
1
0
400 -1
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Water Depth (m)
200
-2
600
-3
800 0
200
400
cr
-4
600
800
an
us
Calendar Age (yr BP)
-5
1000
0
M
400
ce pt
600
800 0
3
2
1
ed
Water Depth (m)
200
4
200
400 600 Calendar Age (yr BP)
0
-1
-2
800
1000
-3
Ac
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.
Page 28 of 29
Figure
Little Ice Age
25
23
21 200
400 600 Calendar Year (yr BP)
800
1000
us
cr
0
ip t
NBC Transport (Sv)
27
an M
6
Little Ice Age
200
400 600 CalendarYear (yr BP)
800
1000
ce pt
4 0
ed
BC Transport (Sv)
8
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
Ac
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.
Page 29 of 29