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On the Cooling and Freshening of Antarctic

Intermediate Water in the South Pacific Ocean

Between 1970 and 2018

By Alexis Racioppi

Senior Honors Thesis

Environment, Ecology, & Energy Program University of North Carolina at Chapel Hill

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Abstract

Global warming is a driver of sometimes subtle shifts in climate patterns. One such area where subtle changes occur is deep ocean water formation, where alterations in processes regulating water density may impact surface conditions that determine water mass density (e.g., precipitation/evaporation, warming/cooling). The Antarctic Intermediate Water mass (AAIW) forms as an amalgamation of multiple water masses at the Antarctic Polar Front and spreads northward into all ocean basins. This feature of the AAIW makes it especially interesting for identifying how climate change impacts deep water formation, as the presence of the AAIW in all of the southern ocean basins allows scientists to use it as an early warning sign of climate change. Modern climatic processes, such as increased precipitation and rising surface ocean temperatures, influence the conservative temperature and salinity characteristics of the waters contributing to the volume of the AAIW, thus influence the characteristics of the AAIW itself. The AAIW should retain the signatures of temperature and salinity change at the time of its formation as it sinks and travels throughout the South Pacific Ocean during thermohaline circulation. In March and April of 2018, the expectation of preserved temperature and salinity change was tested aboard the Sea Education Association vessel SSV Robert C. Seamans. This senior honors thesis research sought to calculate rates of temperature and salinity change in the South Pacific AAIW from 1970 to 2018 on multiple time scales at both the upper boundary (700-800 m) and core (950-1050 m) of the water mass. CTD data collected during the six-week SEA cruise from Lyttelton, NZ, to Pape’ete, French Polynesia, were compared to historical cast data from the 2018 version of NOAA’s World Ocean Database to derive these temperature and salinity trends. For the sampled region of the South Pacific in my study, the AAIW was found to be experiencing freshening at both upper boundary and core depth zones, with increased

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Introduction

The long-term overturning cycle of the oceans means that climate changes since the

industrial revolution and from centuries ago can simultaneously impact the conditions of ocean

waters from surface to depth (Gebbie and Huybers, 2019). Surface condition shifts in the North

Atlantic and Southern Ocean, where much of the volume of the global deep sea is derived, are

particularly relevant to understanding change in subsurface water masses. This includes the

Antarctic Intermediate Water (AAIW), the creation of which is dependent on thermohaline

circulation of waters forming in the poleward oceans.

Thermohaline circulation is a global ocean current pattern that describes the constant

movement of deep seawater between ocean basins and its upwelling to return as surface waters

(Figure 1; Stommel and Arons, 1960; Ramsdorf, 2006). This process depends on the

temperature and salinity of different water masses. Together, temperature (thermo-) and salinity

(-haline) determine the density of seawater. More saline water is denser than water containing

less salt; water that has a lower temperature is denser than warmer water. These two parameters

combine to yield a range of seawater densities, thus dictating how different water masses interact

and travel. During thermohaline circulation, slow-moving currents force the horizontal transport

of water while convective processes drive the vertical transport of unique water parcels, where

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Figure 1. Global overturning (thermohaline) circulation schematic shown from a Southern Ocean perspective. Figure from: Talley, 2011.

In today’s oceans, global thermohaline circulation begins in the North Atlantic where

cold, salty (dense) water formed at the surface sinks, becoming a deep ocean water mass that

travels south along the seafloor towards Antarctica (Figure 2). This movement is slow

(centimeters per year), but eventually the North Atlantic Deep Water (NADW) encounters the

Antarctic Divergence Zone (55˚S), where surface waters separate due to Ekman transport

influences on both the Antarctic Coastal Current (pushing water towards Antarctica) and the

strong Antarctic Circumpolar Current (pushing water away from Antarctica)(Gordon, 1971).

This divergence allows the long-traveled NADW to upwell, reaching the surface and moving

either towards Antarctica, joining the Antarctic Circumpolar Water to later be subducted as

Antarctic Bottom Water (AABW), or away from Antarctica, northward via Ekman transport

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obtains the name Antarctic Surface Water, and encounters the sub-polar low around 60-50˚S

latitude. This atmospheric low pressure zone is characterized by high levels of precipitation

(Segar, 2018), which drives further freshening while slight warming occurs by heat transfer from

the air above. Eventually, the Antarctic Surface Water mass reaches the Antarctic Polar Front, a

convergence zone marking the northern boundary of the Antarctic Circumpolar Current (Gordon,

1971). Here, it meets and mixes slightly with warmer (less dense) Subantarctic Surface Waters.

Being the denser water mass, the Antarctic Surface Water subducts (sinks) beneath the

Subantarctic waters at this boundary, settling around 1000m depth as the relatively cold and

fresh, newly named, Antarctic Intermediate Water (Sloyan and Rintoul, 2001; Figure 2). The

majority of AAIW by volume forms near the tip of South America and is then distributed into

every ocean basin by the Antarctic Circumpolar Current (ACC) as it speeds unhindered around

Antarctica (Talley, 2011).

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In this story of deep ocean circulation and the creation of AAIW, salinity and temperature

play a large role determining the vertical stratification for deep ocean water masses. Salinity and

temperature are both “conservative properties” of water, meaning that they are not influenced by

biological, chemical, or physical processes once they are set (Segar, 2018). In the ocean, a water

mass (e.g. NADW, AABW, AAIW, etc.) refers to a volume of water with defined lateral and

depth boundaries, having a specific location of formation and nearly uniform salinity and

temperature values throughout. The temperature and salinity of a water mass are established at

the air-sea interface and retained over its lifetime. It is possible to differentiate between water

masses based on these conservative properties, as well as track their historical variations

(England 1991). Temporal changes in key properties are possible following shifts in prevailing

atmospheric and surface ocean conditions and/or decadal to long-term climate at the location of

water mass formation, each of which determine the temperature and salinity signatures of

newly-created waters destined for the deep sea. Thus, these deep waters carry the signature of the global

climate at the time of their formation as well as some influence of local climate change as they

transport heat across the surface of the planet (Rhamstorf, 2006). Though it is a fundamental

process in heat transport, similar to atmospheric circulation, the entire circuit of thermohaline

circulation requires about 1,000 years, enabling a long climatic historical signal to be carried

within deep ocean waters (Gebbie and Huybers, 2019).

Other factors also control global ocean density patterns, including the amounts of

precipitation, evaporation, and ice formation/melt that occur at any location on the ocean’s

surface resultant from local climate. These processes can have dramatic impacts on surface water

salinity, and thus the characteristics of locally-forming water masses (Rhein et al., 2013).

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ice formation yield increased sea surface salinity (Segar, 2018). The main process that affects

surface water temperature, thus heat content of deep waters, is the temperature of the atmosphere

immediately above; locally, air is warm or cool based on solar input at its geographic position

and radiative influences from the Earth’s surface. The upper ocean either absorbs atmospheric

heat energy or loses it via conduction and convection, the latter common in the polar regions

with strong, persistent wind regimes (Sekma et al., 2013).

In recent years, changes in ocean salinity and temperaturepatterns have been observed in

many parts of the world, including surface regions where deep water masses are known to form.

On average, global oceans to 700m depth warmed at a rate of 0.015 degrees Celsius from 1970

to 2010 with more prominent increases in the North Atlantic (Rhein et al., 2013). Additionally,

statistically significant salinity changes were detected in 43.8% of the world’s surface ocean

from 1950 to 2008 (Durak and Wijffels, 2010), with patterns of salinity increase and decrease

resembling the mean salinity field. These surface water changes are reflected in deep water

masses like the AABW, which has become 0.06 degrees Celsius warmer and 0.004  0.001 psu

fresher per decade from 1994 to 2016, potentially as a result of glacial calving in Southern Ocean

(Menezes et al., 2017). Interestingly, this freshening rate increases to 0.008  0.001 psu when

only considering data from the study’s most modern decade (2007 to 2016), suggesting a

possible acceleration of the freshening rate in recent years (Menezes et al., 2017). The NADW

likewise showed changes at its formation region, exhibiting an estimated cooling trend from

1955 to 2005 (Mauritzen et al., 2012) and a freshening rate of -0.01 psu per decade from the

1960s to the 2000s (Dickson et al., 2002).

Sediment data have documented the influence of changing surface ocean conditions on

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Atlantic after atmospheric warming (Broecker, 2000; Rahmstorf, 2006). Such dynamics could

have sufficiently weakened thermohaline circulation that a lack of heat transport to the North

Atlantic allowed Arctic glaciers to rapidly form (Rahmstorf, 2006; Clark et al., 2002). This

ocean-atmosphere coupled shift has been suggested as a mechanism for the Little Ice Age, which

followed a slight warm period (Broecker, 2000). Occurring less than 1000 years ago, the

remnants of cooling from the Little Ice Age may still be detectable in old Pacific waters (Gebbie

and Huybers, 2019). The potential for such dramatic climate shifts following ocean salinity and

temperature variations highlights the importance of monitoring ongoing changes of these

parameters in today’s water masses.

The AAIW is an especially interesting water mass to study because it migrates into in all

sectors of Southern Hemisphere oceans, from the northern edge of the Antarctic Polar Front to as

far north as 20 degrees South (England and Santoso, 2002; Figure 3). In general, AAIW waters

reside at depths of 700-1300m, with average salinity readings of 34.1-34.6 psu and average

temperatures of 2.4-7 degrees Celsius (Emery, 2001; England and Santoso, 2002; Schmidtko and

Johnson, 2012; Menezes, 2017). However, like other extensive water masses, it too has

experienced temperature and salinity changes. In the Indian Subtropical Gyre near 32˚S,

freshening (-0.13 psu total from 1930-1990) and cooling (-0.33˚C total from 1930-1990) of the

water mass were observed (Bindoff & McDougall, 2000). A cooling trend has likewise been

found for the AAIW in the southwest Pacific (-0.4˚C from 1970 to 1990; Johnson and Orsi,

1997) and salinity trends at 50˚S in the southwest Atlantic (-0.02 psu over 40 years; Curry et al.,

2003) and along 17˚S in the South Pacific (-0.064 psu from 1930-1995; Wong et al., 1999) agree

that the AAIW has experienced freshening. However, opposite trends were observed in the

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salinity (0.0041 psu) were reported at the water mass’ central depths (32.5˚S; Schnieder et al.

2005).

These conflicting trends in salinity and temperature changes highlight geographic

variations in climate-driven properties for the AAIW and underscore the need to observe and

understand water mass changes on local scales, especially where little inter-basin mixing occurs

once waters are distributed via the ACC. Though inter-basin mixing is minimal for the AAIW, it

is predicted that subtle density differences within the water mass may result in different depth

zones of the AAIW exhibiting their own distinct temperature and salinity characteristics that

experience non-cohesive temporal change.

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Though past research on the AAIW documented changing levels of salinity and

temperature, data on these trends must be recorded regularly and locally, as well as analyzed at

multiple depths within in the water mass in order to understand zonal differences. This research

aimed to quantify and contextualize rates of AAIW temperature and salinity change from

1970-2018 in its boundary and core depth zones by addressing three objectives:

1. To determine and compare decadal rates of temperature and salinity change (1970-2018) to

more recent rates on a shorter time scale (2000-2018).

2. To compare rates of temperature and salinity change with and without the addition of

isolated SEA cruise S-278’s 2018 hydrocast data.

3. To compare rates of temperature and salinity change for the AAIW’s upper boundary and

core depth zones.

Methods

In 2018, the Sea Education Association’s sailing research vessel, SSV Robert C. Seamans,

followed the S-287 cruise track (Figure 4) beginning in Lyttelton, NZ (March 31st departure) and

ending in Pape’ete, Tahiti, French Polynesia (sampling concluded April 25th in the eastern

Pacific Subtropical Gyre). This month-long research cruise was part of the SEA Semester

Program and consisted of students from around the world working together with research faculty

to learn field techniques and conduct research on the future of the oceans and climate.

Between 0930 and 1200 each day aboard ship, weather permitting, a hydrographic

carousel with CTD (Conductivity, Temperature, Depth) probe (SBE19PlusV2; SeaBird

Electronics, Bellvue, WA) was lowered into the ocean and continuously collected salinity,

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cast yielded data in averaged 5-m bins from the sea surface to depths up to 1500m. At the time of

each cast, the latitudinal and longitudinal position was recorded.

Using CTD casts along the S-278 cruise track, the AAIW was identified by locating

where the water mass’ characteristic salinity and temperature minima and maxima (34.1-34.6 psu

and 2.4-7˚C) appeared in a Temperature-Salinity diagram (Emery, 2001; England and Santoso,

2002; Schmidtko and Johnson, 2012; Menezes, 2017; Figure 5). This process also permitted

identification of the depth range of data within each profile belonging to the AAIW, enabling the

identification of the water mass’ core depth zone (950-1050m)

Figure 4. Map of S-278 CTD casts along a 2018 cruise track from Lyttelton, NZ to Pape’ete, French Polynesia. The red box outlines the area from which historical data was collected for time series analysis. *Box spans 32.5 – 33.5 ˚S and 160 – 170 ˚W.

Historical data was compiled from the World Ocean Database (2018 version) using the

Select and Search tool on NOAA’s National Centers for Environmental Information website

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repository was queried by geographic location (bounds: West -170, East -160, North -32.5, and

South -33.5) and date (1970-2018). The red box created by these coordinates (Figure 4) was

selected as the acceptable geographic range for historic data comparisons because it was the

narrowest range of latitude and longitude that contained at least one cast per time period, as well

as an S-278 cast. Depth (meters), temperature (˚C), and salinity (psu) data were extracted for all

World Ocean Database water column profiles (sourced from CTDs, expendable

bathythermographs (XBTs), and profiling floats) that met these time and location criteria.

All extracted historical data were compiled in a Microsoft Excel document with S-278

data that met the same geographic requirements (027-HC). In this file, cast data were first

separated into five decadal time period bins (1970-79, 1980-89, 1990-99, 2000-09, 2010-18) and

eight bins of shorter, more recent intervals, hereafter “fine” bins, (2000-03, 2004-05, 2006-07,

2008-09, 2010-11, 2012-13, 2014-15, and 2016-18).

For each time bin, all data were run through two sets of filters, each corresponding to a

different target depth range. Quality control filters for temperature and salinity were applied to

remove any outliers well outside the established AAIW ranges. The first set of filters focused on

the AAIW boundary zone (700-800m) and included temperatures between 5.9-7.2˚C and

salinities of 34.3-34.5 psu. All remaining data were used to calculate average temperature,

salinity, and respective standard deviation values for each time bin; rates of change were also

determined for both parameters within the 700-800m depth zone, calculated separately for the

decadal and fine time bins. Data from the geographically relevant S-278 cast (027-HC) were

filtered using the same procedures. In all cases, S-278 temperature and salinity data were

included in the 2018 year for one set of rate calculations, and then copied to their own time bin

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separate and highlight the latest observations from the massive amount of data that existed before

the SEA Semester voyage in 2018.

Second, the data were filtered for the 950m –1050m depth range, selected to represent the

“core” of the AAIW located using the T-S plot peak generated with S-278 data. Here, quality

control filters included temperatures between 4.45-5.71˚C and salinities of 34.3-34.42 psu. Filter

adjustments from the first target depth to the second were made in order to accommodate the

water mass’ characteristics at different zones. Once again, the remaining data were used to

calculate average temperature, salinity, and respective standard deviation values for each set of

time bins. As before, rates of change for both parameters were determined for the core

(950-1050m) depth zone, calculated separately for the decadal and fine time bins. Filtered S-278 data

(027-HC) for this depth zone yielded its own average temperature and salinity values which were

included in a second set of rate calculations for each set of time bins (decadal and fine).

Rates of change for temperature and salinity were compared between the two target depth

zones and within each target depth between the two timescales. In addition, rates of change

excluding S-278 data were compared to rates including S-278 data in all cases.

Results

This ocean hydrography dataset contained 645 historical and modern water column

profiles binned into two series of time intervals. In the series of decadal bins, the number of

profiles in each time bin ranged from one profile in the 1970-79 and 1980-89 bins to 12 during

1990-99, 149 from 2000-09, and a maximum of 482 between 2010-18. When broken into smaller

time bins for the years with greater available data (2000-2018), the number of profiles in each

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the 2012-13 bin. The decadal 2010-18 bin and fine 2016-18 bin each contained data for a single

modern profile measured during SEA’s S-278 cruise in 2018.

The AAIW was located using T-S plots with bounds of 2.4-7.2 ˚C and 34.3-34.5 psu. The

core depths of the water mass were identified using the temperature and salinity values at the

peak of the AAIW T-S curve (Figure 5). The values found at this peak consistently occurred

between 950-1050m depth. CTD depth profile measurements from the S-278 cruise and

historical ocean database resulted in the determination of rates of salinity and temperature

change in two depth regions of the AAIW over multiple time periods and scales.

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For the core depth zone and the upper boundary depth zone, time binned data is presented

in a series of box plots for salinity (Figure 6) and temperature (Figure 7). Salinity showed a

decreasing trend for the 700-800m boundary depth zone in both time bin configurations (Figure

6a, 6b). The fine time scale rate of salinity change for this depth zone was 1.59x the speed of the

decadal time scale (Table 1). For each plot in Figure 6, rates of change were determined once

excluding the white S-278 box and once including the white S-278 box in order to separate the

most recent data from the earlier part of the 2010-18 or 2016-2018 time periods. With the

inclusion of the S-278 CTD data, the rate of decadal salinity change for the upper boundary zone

increased to 1.18x its previous value while the fine time series rate increased to 1.22x its

previous value (Table 1). Rates of salinity change are consistently of lower magnitude in the

core depth zone than in the upper boundary zone (Table 1). The 950-1050m core depth zone also

shows a trend of decreasing salinity in both time bin configurations (Figure 6c, 6d). The fine

time scale rate of salinity change for this depth zone was 1.2x the speed of the decadal time scale

(Table 1). With the inclusion of the S-278 CTD data, the decadal rate of salinity change for the

core depth zone increased to 1.13x its previous value while the fine time series rate increased to

1.28x its previous value (Table 1). It is important to note that all S-278 data represent only one

CTD cast, while the historical data incorporates multiple casts within a time range and is an

average. Additionally, the middle 50% of data points for the core depths are generally spread

over a narrower range of salinity values than the middle 50% of data points for the boundary

depths of the AAIW. According to these calculations, both depth ranges of the AAIW examined

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Sequential changes in mean salinity at upper boundary and core depths are summarized at

the decadal scale (Table 3, 4) and at finer scale over the most recent 20 years (Tables 5,6;mean

values shown as “x” symbols in Figure 6).

Figure 6. AAIW salinity change over time was analyzed at the decadal (a, c) and fine (b, d) scales for two depth ranges. Boxes represent data within the 25-75th percentiles while whisker

extent represents the rest of the data values, excluding outliers represented as dots beyond the whiskers, within each time period. Horizontal lines within boxes indicate median values while “x” symbols indicate mean values. For each panel, the rightmost box (white) represents S-278 data (2018).

00-03 04-05 06-07 08-0 10-11 12-1 14-15 16-18 S278 04-05 06-0 08-09 10-1 12-13 14-1 16-18 S278

00-03 04-05 06-07 08-0 10-11 12-1 14-15 16-18 S278 04-05 06-0 08-09 10-1 12-13 14-1 16-18 S278

b a

70-79 80-89 90-99 00-09 10-18 S278

d c

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Table 1. Rates of Salinity Change (psu/yr)

Time scale 700-800m 950-1050m

Decade -0.0017 -0.0015

Fine -0.0027 -0.0018

Decade S-278 -0.002 -0.0017

Fine S-278 -0.0033 -0.0023

Temperature showed a decreasing trend for the 700-800m boundary depth zone in both

time bin configurations (Figure 7a, 7b). The fine time scale rate of temperature change for this

depth zone was 8.44x the speed of the decadal time scale (Table 2). As with salinity, for each

plot in Figure 7, rates of change were determined once excluding the white S-278 box and once

including the white S-278 box. With the inclusion of the S-278 CTD data, the rate of decadal

temperature change for the upper boundary zone increased to 3.33x its previous value while the

fine time series rate increased to 1.70x its previous value (Table 2). For rates of temperature

change that are in the same direction (negative/decreasing), rates are of a lower magnitude in the

core depth zone than in the upper boundary zone (Table 2). The 950-1050m core depth zone also

shows a trend of decreasing temperature on a fine time scale, but increasing temperature on a

decadal scale (Figure 7c, 7d). With the inclusion of the S-278 CTD data, the decadal rate of

temperature change for the core depth zone increased to 1.33x its previous rate of warming while

the fine time series rate decreased to 0.61x its previous rate of cooling (Table 2). Once again, it

is important to note that all S-278 data represent only one CTD cast, while the historical data

incorporates multiple casts within a time range and is an average. Additionally, the 950-1050m

core zone is shown to have consistently lower temperature values than the 700-800m upper

boundary zone (Figure 7). According to these calculations, the upper boundary zone of this

AAIW segment are cooling on both time scales, while the core zone appears to warming on a

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Sequential changes in mean temperature at upper boundary and core depths are

summarized at the decadal scale (Tables 3, 4) and at finer scale over the most recent 20 years

(Tables 5,6; mean values shown as “x” symbols in Figure 7) after extracting mean values from

the appropriate plots in Figure 7.

Figure 7. Temperature change over time was analyzed at the decadal (a, c) and fine (b, d) scales for two depth ranges.

00-03 04-05 06-07 08-0 10-11 12-13 14-15 16-18 S278 04-05 06- 08-09 10-1 12-13 14-1 16-18 S278

00-03 04-05 06-07 08-0 10-11 12-13 14-15 16-18 S278 04-05 06- 08-09 10-1 12-13 14-1 16-18 S278

b a

70-79 80-89 90-99 00-09 10-18 S278

d c

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Table 2. Rates of Temperature Change (˚C/yr)

Time scale 700-800m 950-1050m

Decade -0.0009 0.0024

Fine -0.0076 -0.0021

Decade S-278 -0.003 0.0032

Fine S-278 -0.0129 -0.0013

Table 3. Decadal temperature and salinity averages of the AAIW from 700-800m.

Time Period Avg. Temperature (˚C) Standard Deviation (Temperature) Avg. Salinity (psu) Standard Deviation (Salinity) Number of data points

1970-79 6.55 0.17 34.39 0.012 41

1980-89 6.59 0.16 34.45 0.013 101

1990-99 6.55 0.22 34.38 0.019 777

2000-09 6.54 0.22 34.37 0.017 1568

2010-18 6.53 0.20 34.35 0.013 5868

S-278 6.42 0.16 34.33 0.008 20

Table 4. Decadal temperature and salinity averages of the AAIW from 950-1050m.

Time Period Avg. Temperature (˚C) Standard Deviation (Temperature) Avg. Salinity (psu) Standard Deviation (Salinity) Number of data points

1970-79 5.00 0.21 34.36 0.003 41

1980-89 5.05 0.15 34.41 0.005 101

1990-99 5.11 0.24 34.34 0.009 777

2000-09 5.06 0.24 34.34 0.011 1568

2010-18 5.11 0.24 34.32 0.008 5868

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Table 5. Fine time scale temperature and salinity averages of the AAIW from 700-800m. Time Period Avg. Temperature (˚C) Standard Deviation (Temperature) Avg. Salinity (psu) Standard Deviation (Salinity) Number of data points

2000-03 6.60 0.18 34.38 0.013 985

2004-05 6.60 0.18 34.38 0.013 71

2006-07 6.57 0.13 34.37 0.015 13

2008-09 6.43 0.25 34.36 0.018 499

2010-11 6.40 0.16 34.35 0.011 187

2012-13 6.40 0.16 34.35 0.010 216

2014-15 6.49 0.16 34.35 0.011 895

2016-18 6.55 0.20 34.35 0.014 4550

S-278 6.42 0.16 34.33 0.008 20

Table 6. Fine time scale temperature and salinity averages of the AAIW from 950-1050m.

Time

Period TemperatureAvg.

(˚C)

Standard Deviation (Temperature)

Avg. Salinity

(psu) DeviationStandard

(Salinity)

Number of data points

2000-03 5.08 0.22 34.34 0.012 985

2004-05 5.16 0.24 34.35 0.009 71

2006-07 5.14 0.14 34.34 0.010 13

2008-09 4.99 0.26 34.34 0.009 499

2010-11 5.00 0.23 34.32 0.011 187

2012-13 5.00 0.20 34.32 0.006 216

2014-15 5.08 0.21 34.32 0.006 895

2016-18 5.13 0.25 34.32 0.008 4550

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The AAIW forms at the Antarctic Polar Front as upwelling NADW meets Antarctic

Surface Water and freshens before subducting underneath warmer, less dense surface waters

settling between those waters and denser, underlying deep water masses. This formation process

determines the ways in which AAIW temperature and salinity may be impacted by both

long-term (hundreds of years) and short-long-term (decades) climatic processes as it is created. In fact,

much of the data from this study supports the assertion that two separate mechanisms may be

acting to induce observed variations in the AAIW from 1970-2018. The first leverages the

centuries-long deep water memory of surface water cooling during the transition from the

Medieval Warm Period to the Little Ice Age (Gebbie and Huybers, 2019). The second is a more

recent, anthropogenically-induced climatic shift to a warmer lower atmosphere, thus warmer

surface oceans, inducing freshwater influxes to the surface ocean via precipitation and possibly

ice melt (Rhein et al., 2013).

Both of these processes appeared to influence the results pertaining to the first objective

of this study: to determine and compare decadal rates of temperature and salinity change

(1970-2018) to more recent rates on a shorter time scale (2000-(1970-2018). Overwhelmingly, the AAIW in

this segment of the South Pacific experienced freshening (decreased salinity) on both time scales

(Figure 6; Table 1). These findings agree with those reported by Bindoff and McDougal (2000),

Curry and colleagues (2003) and Wong and colleagues (1999) that the AAIW is freshening,

though the rates of core freshening determined by those investigations are, respectively, faster in

the Indian Subtropical Gyre, and slower in the southwest Atlantic and South Pacific at 17˚S than

the rates of change calculated here (Table 1). Freshening in multiple basins was expected for the

AAIW (Rhein et al., 2013) and geographical constraints may explain the differences between

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(Figure 3) is in a position such that its subsequent transport by the ACC delivers it first to the

Atlantic basin, then the Indian basin, and last the Pacific basin. Thus, it is possible that salinity

differences between ocean basins are influenced by the time the AAIW has spent in circulation,

interacting with the many waters that contribute to the ACC in varying geographic locations

(Talley, 2011; Figure 1), before being delivered to a specific basin. These varying geographic

inputs support the observation of differing freshening rates in different AAIW locations; near

33˚S in the southwest Pacific (this study) rates were larger in magnitude than those found at

more northerly Pacific latitudes (Wong et al., 1999) or in the southwest Atlantic (Curry et al.,

2003), whereas freshening rates at a comparable latitude in the Indian Subtropical Gyre (Bindoff

and McDougal, 2000) are on the same order of magnitude to those reported here.

Moving forward from the general trend of increasing salinity, this study found that recent

freshening rates for the AAIW are consistently greater in magnitude than decadal rates of

freshening (Table 1). This pattern has also been observed in the NADW by Menezes and

colleagues (2017); although the NADW contributes to AAIW formation, intermediate water

freshening in the South Pacific cannot be attributed to changes in NADW. Thermohaline ocean

circulation is too slow for the influence of 1994-2016 North Atlantic freshening to influence the

Southern Ocean. A more plausible explanation for the freshening trends found in this study relies

on the mechanism of anthropogenic climate change since the industrial revolution. Increases in

atmospheric water vapor drive the delivery of more precipitation to the subpolar lows (Seidel,

2002) and rising air temperatures hasten ice along portions of the Antarctic coastline, together

causing water masses that incorporate the heavily influenced Southern Ocean surface waters, like

the AAIW, to freshen. As atmospheric temperatures continue to warm, further freshwater

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vapor)-driven feedback loop, could explain the freshening increase observed in this study (Rhein et al.,

2013).

When considering temperature trends for this study’s first objective, the AAIW had a

consistent cooling pattern on both decadal and recent time scales in the upper boundary depth

zone, while the core of the water mass exhibited warming on a decadal scale and cooling on a

more recent fine time scale (Table 2). The trend of AAIW cooling is supported by the findings

of Bindoff and McDougal (2000) and Johnson and Orsi (1997); Bindoff and McDougal (1994)

link it to warming and freshening in the surface waters of Antarctic Surface Water causing a

vertical displacement of density surfaces in the water column. This anthropogenic warming of

the surface oceans (Rhein et al., 2013) could explain why cooling trends in the AAIW upper

boundary zone appear to be faster and why the core depths switch from warming to cooling in

modern years (Table 2). These trends are further supported by observations at core AAIW

depths by Schnieder et al. (2005) in the southeast Pacific. A recent study has also suggested that

sub-surface Pacific waters may still be cooling as a result of North Atlantic surface water cooling

during the transition to the Little Ice Age (Gebbie and Huybers, 2019). This mechanism of deep

ocean water masses preserving and transporting centuries-past cooling trends could thus serve as

a possible explanation for the cooling AAIW. It perhaps serves as a baseline upon which the

previously discussed modern cooling trends compound to produce faster modern rates of cooling

in the water mass than the decadal scale trends.

The fingerprints of anthropogenic climate change (i.e. increasing precipitation over the

subpolar lows and global surface warming) are further supported as a mechanism for the above

trends when considering the results of objective two: to compare rates of temperature and salinity

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the isolated S-278 data to the mix consistently increased the magnitude of calculated freshening

rates for the AAIW in this study (Table 1). As S-278 cruise represented the most recent data, its

effect as a unique data point on salinity trends supports well-documented hastening

anthropogenic climate change in the modern era, namely increased high latitude precipitation

and polar ice melt over the past decade (Rhein et al., 2013). Temperature trends continued the

pattern of increased modern cooling in the upper boundary depths when the isolated S-278 cast

data were included, again supporting that this trend is influenced by accelerating surface

warming due to anthropogenic climate change (Rhein et al., 2013). In the core zone, however,

the addition of modern S-278 data increased the magnitude of decadal warming and decreased

the magnitude of more recent freshening. Together with the trends of increased cooling in the

upper boundary zone at both time scales and only modern cooling in the core zone, it is

suggested that a warm temperature anomaly likely appears in the S-278 data. These data could

also be the reason for the decreased modern freshening that the core depths experienced in this

study (Table 2).

As only a single S-278 CTD cast at a single location was used for computing these rates, it is

possible that this anomaly could have been reduced by utilizing a larger 2018 dataset. Using

more than a single cast for the time period would have produced more meaningful and

statistically robust insights into the most recent AAIW trends. The 1970-79 and 1980-89 time

bins for decadal analysis also contained only one CTD cast. As with the 2018 time bin, trends

calculated using these two decadal bins would be more statistically robust had there been more

geographically relevant data to include in each time period.

The final objective of this study was to compare rates of temperature and salinity change

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upper boundary zone experienced faster freshening in all cases relative to the AAIW core (Table

1). This trend was fitting for a water mass for which salinity plays such an important role in

determining density (Durack and Wjiffles, 2010); freshening decreases seawater density. Thus, it

is expected that the freshest AAIW water should remain at the upper water mass boundary while

the more saline, but still freshening, waters would be at deeper core depth zones. Waters within

the AAIW that experience the most freshening will be less dense and sink less deeply than those

experiencing less freshening. Larger freshening rates would be expected to be observed in the

upper boundary of the water mass. This study was limited by the S-278 modern cast data, which

did not extend to the lower AAIW boundary layer, and it would be interesting to investigate this

salinity trends across all depths of the water mass. According to the observations reported here,

the lower boundary depth zone of the water mass would be expected to also exhibit freshening

but at a slower rate than the waters above it.

Furthermore, the upper boundary depth zone of the AAIW was observed to experience equal

or greater rates of cooling than the core depth zone, especially considering that, on a decadal

scale, the core depth zone had a warming trend (Table 2). The within-water mass temperature

discrepancy renders it hard to tease apart the mechanisms driving change. One possible

explanation could be linked to the already existing temperature differences in the two depth

zones. The AAIW’s core depth zone has consistently lower average temperatures than the upper

boundary depth zone (Figure 7), suggesting that within the AAIW formation region near the

Polar Front, these core waters are subducted earlier (i.e. farther south) than the upper boundary

waters sourced from a warmer, more northerly portion of the formation region. In this case, the

proportions of sinking water contributing to each of the examined depth zones may have shifted

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proportionally largest volume origin. The core depths may therefore show temperature changes

associated with more poleward waters (more warming) while the upper boundary depths reflect

changes on the subtropical edge of the Polar Front (more cooling) (Rhein et al., 2013). Once

again, it would be interesting to extend this analysis to the lower boundary depths of the AAIW,

where colder waters indicating an even more southward subduction could potentially be

experiencing even stronger decadal warming trends than the core depth zone.

Global Implications of Cooling and Freshening AAIW

The cooling and freshening of the AAIW have the potential to impact large scale

thermohaline circulation and its ability to transport heat on Earth and maintain the global heat

budget. While water mass cooling implies increasing density and the freshening implies

decreasing density, the density of the AAIW is believed more suspectable to the latter change of

freshening (Durack and Wjiffles, 2010). With the strong and accelerating freshening trends

found in this study and other scientific studies cited earlier, it is possible that the overall density

of the AAIW is decreasing. As the AAIW depends on its density to subduct beneath surface

waters near the Polar Front and settle at intermediate depths, a decreased density could limit its

future ability to do so; limited AAIW formation could in turn influence global thermohaline

circulation velocities and volumes. Sinking of one water mass is what allows upwelling of

another water mass during this circulation. Thus, an interruption in one section of the

thermohaline circulation, however remote it may seem, could have consequences thousands of

kilometers away. An interruption may cause related slow-downs in deep water mass movements

around the planet and possibly result in an intensification of stratification in the global ocean

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Any such slowdown of worldwide deep ocean currents would vastly disrupt the global heat

budget, which relies on the deep ocean to remove thermal energy from the atmosphere and store

it for 1,000 years before being ventilated in upwelling zones. With inhibited or halted deep ocean

circulation, the ocean cannot effectively remove thermal energy from the Earth’s surface; this

could result in even further atmospheric warming and hysteresis (Broecker 2000; Rahmstorf,

2006). This grim situation is currently unlikely (Rhein et al., 2013), but gradual trends towards

such a state are not out of range of possibility. These findings demonstrate consideration for such

eventual impacts of anthropogenic climate change are warranted. Clearly the mechanisms

driving climate change have both natural and anthropogenic sources, as shown in the long-term

and short-term trends in rates of AAIW temperature and salinity change. Historical effects of

climate transitions surrounding the Little Ice Age are found in the salinity signals of NADW.

However, climate change in more recent centuries is driven by human activities which intensify

historical trends.

Though these findings support the potential for more than one mechanism forcing temperature

(cooling) and salinity (freshening) change in the South Pacific AAIW, the influence of each

process cannot be distinguished conclusively by the results of this or other studies conducted to

date. However, reported observations do clearly indicate that rates of salinity and temperature

change in the AAIW increased in more recent years. Furthermore, this study concluded that it is

essential to analyze changes in these parameters at multiple depths within a water mass, as not all

depth zones of a water mass may experience change in the same way. In order to better

understand how such changes in the AAIW would influence thermohaline circulation and the

global heat budget, it is imperative that these trends continue to be monitored across the AAIW,

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Acknowledgements

This project was supported by Sea Education Association faculty, staff and students onboard the SSV Robert C. Seamans during SEA Semester S-278; data collection and analysis would not have been possible without their help. Thanks to the David and Vicki Craver Trust Fund to JEC for financially supporting my presentation of this material at the AGU 2020 Ocean Sciences Meeting. Thank you also to the UNC Marine Sciences Department for bestowing the Hill Fund Award to support the original thesis project that was put on hold due to COVID-19 restrictions. Many thanks to Honors Carolina and the Michael P. and Jean W. Carter Research Fund for their financial contribution to the original thesis project, and again to Honors Carolina for supporting the transition to a new thesis project amid pandemic challenges.

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