PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2014GL059239
Key Points:• Recent expansion of Antarctic sea ice
agrees with decreasing Southern
Ocean SST
• Southern Ocean surface climate trends
reverse sign before and after 1980
• Southern Ocean surface climate shows
pronounced multidecadal variability

Supporting Information:• Readme• Text S1• Figure S1• Table S1
Correspondence to:T. Fan,tingting@ucar.edu

Citation:
Fan, T., C. Deser, and D. P. Schneider(2014), Recent Antarctic sea ice trends inthe context of Southern Ocean surface
climate variations since 1950, Geophys.
Res. Lett., 41, 2419–2426, doi:10.1002/
2014GL059239.
Received 9 JAN 2014
Accepted 20 MAR 2014
Accepted article online 21 MAR 2014
Published online 4 APR 2014

Recent Antarctic sea ice trends in the context
of Southern Ocean surface climate
variations since 1950
Tingting Fan1, Clara Deser1, and David P. Schneider1
1

NCAR, Boulder, Colorado, USA

Abstract This study compares the distribution of surface climate trends over the Southern Ocean in australsummer between 1979–2011 and 1950–1978, using a wide variety of data sets including uninterpolated
gridded marine archives, land station data, reanalysis, and satellite products. Apart from the Antarctic Peninsula
and adjacent regions, sea surface temperatures and surface air temperatures decreased during 1979–2011,
consistent with the expansion of Antarctic sea ice. In contrast, the Southern Ocean and coastal Antarctica
warmed during 1950–1978. Sea level pressure (SLP) and zonal wind trends provide additional evidence for asign reversal between the two periods, with cooling (warming) accompanied by stronger (weaker) westerlies
and lower (higher) SLP at polar latitudes in the early (late) period. Such physically consistent trends across a
range of independently measured parameters provide robust evidence for multidecadal climate variability over
the Southern Ocean and place the recent Antarctic sea ice trends into a broader context.

1. Introduction
Antarctic sea ice extent (SIE) has shown a modest and statistically signiﬁcant increase since 1979, the period
when continuous satellite measurements are available [Parkinson and Cavalieri, 2012]. This modest expansion,
evident in all months of the year, is the result of the near cancelation of ice loss in the Bellingshausen andAmundsen Seas, and ice gain in the Ross and Weddell Seas and along the coast of East Antarctica [Cavalieri and
Parkinson, 2008; Comiso and Nishio, 2008]. The increase in Antarctic SIE is in stark contrast to the dramatic
decrease in Arctic SIE over the same time period [Cavalieri and Parkinson, 2012]. Like the Arctic, Antarctic SIE has
shown the largest percentage change in late summer and early fall since the satellite era [Turner et al., 2013].
While the decline in Arctic sea ice has been attributed, at least in part, to rising anthropogenic greenhouse gas(GHG) concentrations [Holland et al., 2006; Screen and Simmonds, 2010], the cause of the overall increase in
Antarctic SIE remains a matter of debate. Several mechanisms have been proposed, with contributions fromboth anthropogenic and natural (internal to the climate system) factors. Aiken and England [2008] and Bintanja
et al. [2013] proposed that the increase in melt water ﬂux from the Antarctic continent [Rignot et al., 2013]
promotes sea ice expansion by stabilizing the upper portion of the water column. On the other hand, Holland
and Kwok [2012] linked observed ice drift and concentration trends during April–September to changes in zonal
and meridional near-surface winds. The role of winds was further investigated in the ice-ocean modeling studyof Zhang [2013] who found that the observed westerly wind intensiﬁcation increased sea ice volume byenhancing ridge-ice production, making the ice more resilient to melting. In an earlier study, Zhang [2007]
found that warmer temperatures, while decreasing ice growth, can lead to an overall sea ice increase if ice melt
is reduced by increased upper ocean stratiﬁcation and suppressed oceanic heat transport. However, this effectis smaller than the wind effect [Zhang, 2013].
The role of strengthened westerly winds in response to stratospheric ozone depletion and increased GHG
was investigated in a coupled model context by Sigmond and Fyfe [2010] and Bitz and Polvani [2012]. Unlike
the one-way-forced simulations of Zhang [2013], these coupled modeling studies found a reduction in
Antarctic SIE. However, as pointed out by Marshall et al. (The ocean’s role in polar climate change: Asymmetric
Arctic and Antarctic responses to greenhouse gas and ozone forcing, submitted to Philosophical Transactions
A of the Royal Society, 2013), the transient and equilibrium responses of the Southern Ocean to an abrupt
increase in surface westerlies are of opposite sign, with upper ocean mixed layer processes dominating thenear-term response and slow dynamical adjustment of the meridional overturning circulation responsible forthe equilibrium response. Regardless of the sign of the response, all of these studies reported a close

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Geophysical Research Letters10.1002/2014GL059239

relationship between SIE and sea surface temperature (SST) whereby sea ice gain is associated with lower
SSTs and vice versa.
The purpose of this observational study is to place the recent Antarctic sea ice trends into a broader environmental
and temporal context. We ﬁrst examine the relationships among Antarctic sea ice concentration, Southern Ocean
SST, surface air temperature (SAT), sea level pressure (SLP), and surface zonal wind (U) trends over the period
1979–2011 using uninterpolated gridded surface marine data sets, land station archives, atmospheric reanalysis,
and satellite products. We then extend our analysis of Southern Ocean climate trends back to earlier decades(1950–1978) to illustrate the low-frequency behavior of surface climate trends over the Southern Ocean. The
data sets and methods are given in section 2. Results are presented in section 3. A discussion and summary areprovided in section 4.

2. Data and Methods
The following monthly datasets are used:
1. Sea ice concentration (SIC) based on satellite passive microwave retrievals using the “bootstrap v2” algorithm[Comiso and Nishio, 2008] on a 25 × 25 km grid for the period 1979–2011, obtained from the National Snowand Ice Data Center.
2. SST from Hadley Center SST version 3 (HadSST3) [Kennedy et al., 2011a, 2011b] and ERA-Interim (ERA-I)
Reanalysis [Dee et al., 2011]. The HadSST3 data set, on a 5° × 5° latitude-longitude grid, is based on version
2.5 of the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) but employs different quality
control and bias adjustment procedures; the data are neither interpolated nor variance adjusted (e.g., nospatial or temporal smoothing or interpolation, and missing grid boxes are not ﬁlled in). ERA-I SST are basedon a succession of different products, including the NCEP two-dimensional variational data assimilation until
June 2001, the Optimum Interpolation Sea Surface Temperature version 2 product from July to December
2001, the NCEP real-time global daily SST analysis from January 2002 to January 2009, and the operationalsea surface temperature and sea ice analysis starting February 2009.
3. SLP and near-surface zonal wind (U) data from ERA-I and from ICOADS Release 2.5 [Woodruff et al., 2011] on a
2° × 2° latitude-longitude grid. Like HadSST3, the ICOADS is based on ship and buoy measurements and contains no spatial or temporal smoothing or inﬁlling. We regridded the 2° × 2° ICOADS data to the 5° × 5°
HadSST3 grid using simple bilinear interpolation.
4. Land station SLP and SAT from the Reference Antarctic Data for Environment Research (READER; http://www.
antarctica.ac.uk/met/READER/surface/stationpt.html) and International Surface Pressure Databank (ISPD;
provided by Doctor Xuangang Yin from the National Climatic Data Center) archives.
Changes in observing practice and instrumentation have caused a spurious increase in scalar wind speed since1950 [Ward and Hoskins, 1996; Norris, 1999; Wu and Xie, 2003; Norris, 2005]. Following the recommendations of
Norris [2005; see also Deser and Phillips, 2006], we account for this spurious increase by removing a 4.7% per
decade linear trend from the ICOADS wind components. Zonal wind trend patterns from the wave- andanemometer-based sea surface wind (WASWIND) data set [Tokinaga and Xie, 2011] show good agreement withthose based on ICOADS (not shown).
We compute seasonal anomalies from the monthly anomalies, requiring a minimum of 1 month per 3 monthseason. We calculate linear trends from the seasonal anomalies using the method of least squares and assess
their statistical signiﬁcance using a Student’s t test and taking into account serial autocorrelation based on themethod of Zwiers and von Storch [1995]. For the observational archives that contain substantial amounts ofmissing data (e.g., HadSST3 and ICOADS), at least 50% of the years during 1950–2011 were required to havedata at a given grid box and season to compute a trend; otherwise, the trends were set to missing.

3. Results
3.1. Seasonal Trends During 1979–2011
Seasonal and annual trend maps of SIC, SST, and near-surface wind components over the Southern Ocean
during 1979–2011 are shown in Figure 1. The SST and wind data are from ERA-I. Cooling is evident over most of
the Southern Ocean in all seasons and the annual mean, with magnitudes approximately 0.2–0.4°C per decadeor 0.7–1.3°C over the 33 year period. Two regions of warming stand out within the domain that is otherwise

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Geophysical Research Letters10.1002/2014GL059239

Figure 1. Annual and seasonal trends during 1979–2011 of sea ice concentration (green and magenta stippling for ice gain and ice loss, respectively), SST (color
1
1
shading, °C/yr), and near-surface winds (vectors, m s yr ). See text for data sources.

cooling: the Antarctic Peninsula and adjacent Weddell, Bellingshausen and Amundsen Seas, and the sector
80°–130°E particularly in austral spring (September, October, and November (SON)). Sea ice concentration(SIC) trends are generally consistent with the SST trends: that is, regions of increasing SIC are nearly always
found in an environment of decreasing SST, and vice versa. (For visual clarity, we only discriminate the signof the SIC trends in Figure 1: their magnitudes are shown in Figure 3.) The correspondence between thesigns of the SST and SIC trends is a feature of every season and the annual mean.
Wind vector trends show less consistency from season to season than either SST or SIC trends (Figure 1). For
example, austral summer (December, January, and February (DJF)) exhibits predominantly westerly wind trends
over most of the Southern Ocean, whereas the other seasons show more regional circulation trend patterns that
include pronounced meridional wind changes (for example, the cyclonic circulation trend over the eastern
Paciﬁc sector in MAM and the anticyclonic circulation trend over the Atlantic sector in SON). The relationshipbetween the wind trends and the SIC and SST trends is complex and varies from season to season. However,regions of enhanced poleward ﬂow (e.g., warm air advection) generally correspond to areas of SST increase andsea ice loss, and vice versa. A particularly clear example is evident in SON, which shows enhanced poleward ﬂow
in the western Atlantic sector where there has been a loss of sea ice and warming of the surface ocean, andenhanced equatorward winds in the region of sea ice gain and SST decrease in the East Antarctic sector. The
importance of meridional wind anomalies in causing SST anomalies over the Southern Ocean via turbulent
heat exchange is consistent with Sallee et al. [2010]. Enhanced westerly winds are generally associated withincreased sea ice and decreased SST, particularly in DJF, MAM, and the annual mean. As discussed in Hall
and Visbeck [2002] among others, stronger westerlies increase Ekman transport of cold surface waters

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Geophysical Research Letters10.1002/2014GL059239

Figure 2. Time series of HadSST3 data coverage over the Southern Ocean (50°S–70°S) during 1900–2011 stratiﬁed by season, expressed as a percentage of grid boxes with at least one observation per season. Only austral summer (DJF; blue
curve) contains >50% continuous data coverage since 1950 (shading).

northward; the resulting divergent Ekman ﬂow near the Antarctic continent advects sea ice farther northleading to an increase in sea ice cover. The annual mean trend distributions in all three ﬁelds resemble
those in DJF but with reduced magnitude.
3.2. Extending the Record Back to 1950: Data Coverage Over the Southern Ocean
Although the sea ice record cannot be reliably reconstructed before the satellite era, the ship-based HadSST3
and ICOADS data offer the possibility of extension to earlier decades. Figure 2 shows the HadSST3 data coverageover the Southern Ocean (50°–70°S), expressed as a percentage of grid boxes with at least 1 month of data perseason, during the period 1900–2011. Data coverage is scarce in all seasons before about World War II. After
1950, more than 50% of grid boxes contain measurements in austral summer (DJF). The 50% data coverage
threshold is reached around 1980 in austral fall (MAM), and not until approximately 2005 in austral winter (JJA)
and spring (SON). Similar data sampling is found for the ICOADS variables (SLP, SST, and wind; not shown). Basedon these results, and to avoid confusion by sampling different months in different years, we have chosen to limit
our subsequent analysis to the season that provides the greatest data coverage (DJF) back to 1950. However,similar results are obtained if we consider all months of the year and if we vary our data coverage threshold to
between 30% and 70% (not shown). We emphasize that although our sampling criterion is lenient, we rely on
physical consistency among independent climate data sets to assess the reality and robustness of the results.
3.3. Surface Climate Trends: 1979–2011 Versus 1950–1978
Figure 3 compares the distribution of DJF surface climate trends over the Southern Ocean between the periods
1979–2011 and 1950–1978, based on a variety of uninterpolated gridded data sets (SST from HadSST3 and SLP
and U from ICOADS), in addition to land station records (SAT from READER; most of SLP data are taken fromREADER, except three stations along the midlatitude are from ISPD). The SIC trends for the later period are also
shown. Note that the color bar for the U trends is reversed to facilitate visual comparison with the SLP and SST
trends. Focusing ﬁrst on the period 1979–2011 (Figures 3d–3g), the widespread cooling of SSTs in the Southern
Ocean, except in the region adjacent to the Antarctic Peninsula and nearby seas, is conﬁrmed by ubiquitous
cooling in the SAT station data along the coast of East Antarctica (ﬁlled circles) and agrees well with theincreasing SIC. Likewise, the SST warming in the sector 180°–30°W adjacent to the Antarctic coast is veriﬁed byincreasing SAT at land stations and by decreasing SIC. We emphasize that the three data sets (HadSST3, land
station SAT, and SIC) are completely independent: their agreement thus provides robust evidence for the realityof the trends. North of ~50°S, SSTs and land station SAT show a warming trend, except in the far eastern Paciﬁc.
Moving on to the atmospheric variables, we ﬁnd good correspondence between the U and SLP trends over theSouthern Ocean in the recent period, with a general strengthening of the westerlies at high latitudes, in agreementwith the lowering of SLP along the Antarctic coast and raising of SLP to the north. Land station data (ﬁlled circles)
conﬁrm the marine SLP trends. The wind and SLP trends are noisier than their SST counterparts, underscoring theimportance of physical consistency to assess the robustness of the atmospheric circulation trends.

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Geophysical Research Letters10.1002/2014GL059239

Figure 3. Comparison of austral summer (DJF) surface climate trends during 1950–1978 (left column) and 1979–2011 (right column) and their zonal means (middle column). (a and e) Gridded SST trends (°C per decade) with land station SAT trends (°C per decade) superimposed as colored circles; the same color bar is used for both SST
1and SAT. (b and f) Zonal wind (U) trends (m s per decade); note that the color bar is inverted to facilitate comparison with the other climate variables. (c and g) Gridded
marine SLP trends (hPa per decade) with land station SLP trends superimposed as colored circles; the same color bar is used for both. White areas indicate insufﬁcient
data coverage for computing trends (> 60% of the years have missing data for the period indicated). (d) Sea ice concentration (SIC) trends (% per decade). The middle
columns show zonally averaged trends for 1950–1978 (red curves) and 1979–2011 (blue curves) based on the marine data sets. See text for data sources.

Trends based on the uninterpolated ship archives (HadSST3 and ICOADS) for the period 1979–2011 show moregrid-scale noise than those based on ERA-I, as expected given that they are not assimilated data products(Figure S1 in the supporting information). However, their agreement with ERA-I in terms of the large-scalepatterns is encouraging and lends conﬁdence to their utility for the purpose of this study. These conventional
data sets provide the “raw input” to any reanalysis product and that physical consistency amongst theindependently measured climate variables in these archives constitutes a strong test of the reality of the trends.
Comparing surface climate trends over the earlier period 1950–1978 with those over the later period 1979–2011
reveals a general reversal in sign (Figures 3a–3c). In particular, the early period shows warming throughout theSouthern Ocean, an aspect that is conﬁrmed by the land station SAT data. This warming trend is accompanied bya general decrease in the surface westerlies and by an increase in SLP at high latitudes and decrease at lower
latitudes, a pattern conﬁrmed by the SLP station data. In addition to the sign reversal, the early period trends
extend farther north than the late period trends. For example, the high-latitude SST warming (cooling) trend
during 1950–1978 (1979–2011) extends to ~40°S (50°S), and the high-latitude SLP increase (decrease) during
1950–1978 (1979–2011) extends to ~ 40°S (~ 60°S), with the strongest meridional gradient in the latitude band
40°–50°S (50°–70°S). This equatorward shift of the strongest gradient in the meridional SLP trends is roughly
consistent with that of the maximum amplitude of the zonal wind trends, although the data are noisy. Zonallyaveraged proﬁles conﬁrm the sign reversal and sight northward expansion of the trends in the early period
compared to the later period (middle column of Figure 3).
3.4. Southern Ocean Time Series
Complementary to the spatial information given in Figure 3, Figure 4 shows zonally averaged DJF time series ofAntarctic sea ice extent (SIE) and Southern Ocean SST, U, SLP, and SAT during 1950–2011, expressed as standardized

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Geophysical Research Letters10.1002/2014GL059239

anomalies. Low-pass-ﬁlteredversions of these records are also
shown, using a Gaussian ﬁlter (ﬁveapplications of a three-point
binomial ﬁlter) that has a halfpower point at approximately
12 years. The annual mean
(January–December) SIE is also
shown, along with a longer recordof DJF SIE based on visual satellite
imagery that begins in 1973
[Cavalieri et al., 2003]. The SST(HadSST3) and U (ICOADS) records
are averaged over the latitude
band 50°–70°S, the SLP record(ICOADS) is averaged over
55°–80°S, and the SAT record(which is limited to the period
1957–2011) is the average of all
stations south of 50°S (e.g., nine
Antarctic records plus Macquarie
Island (54.5°S, 158.9°E; the threestations clustered along theAntarctic Peninsula were ﬁrst
averaged together and then
combined with the remaining
seven SAT records so as not togive undue inﬂuence to thisregion in the Antarctic-wide
average)). Note that the SIE andU records have been inverted tofacilitate comparison with theother time series.
Figure 4. Selected zonally averaged time series, from top to bottom: Antarctic SIE,
Southern Ocean (50°–70°S) SST, Southern Ocean (50°–70°S) zonal wind (U), highlatitude SLP (55°–80°S), and Antarctic SAT. Note that the SIE and U records areinverted. The green (black) SIE curve is based on DJF (annual) means, and thedashed green curve denotes an extension of the DJF SIE record back to 1973 basedon visual satellite imagery [Cavalieri et al., 2003]; all remaining curves are DJF
averages. Thick (thin) curves are based on unsmoothed (12 year low-pass-ﬁltered)data. All records are normalized by dividing by their standard deviation.

All ﬁve independent climate
records show generally similar andphysically consistent low-frequency
behavior, with upward trends fromapproximately 1950 to 1978,
downward trends fromapproximately 1979 to 2000, andﬂat trends thereafter. All trends during both periods are signiﬁcant at the 95% conﬁdence level taking into accountserial autocorrelation, except U in the early period which is signiﬁcant at the 93% level (Table S1). These lowfrequency variations are superimposed upon large interannual ﬂuctuations. Although the reliability of the SIE record
before 1979 is questionable, the decrease in SIE during the decade of the 1970s is physically consistent with thesimultaneous warming seen in the SST record, lending some conﬁdence to its qualitative reliability.
To quantify the strength of the relationships amongst the time series shown in Figure 4, we have computed
correlation coefﬁcients between all pairs of records based on the period 1950–2011 (those with SIE are limited
to 1979–2011; Table 1). All correlations are signiﬁcant at the 95% conﬁdence level taking into account serial
autocorrelation, except for SIE with U and SAT which are signiﬁcant at the 90% level; similar results are found for
detrended data (not shown). Antarctic SIE exhibits a negative correlation with Southern Ocean SST (r = 0.56),
consistent with the trend maps discussed above. Southern Ocean SSTs in turn show positive correlations withhigh-latitude SLP (r = 0.63) and Antarctic SAT (r = 0.59), and a negative correlation with Southern Ocean zonal
wind (r = 0.49). Zonal wind and SLP are highly correlated (r = 0.66), conﬁrming the reliability of these records

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Geophysical Research Letters10.1002/2014GL059239

aTable 1. Correlation Coefﬁcients Amongst the Time Series Shown in Figure 4
SIE
SIE
SST
U
SLP
a

SST

U

SLP

SAT

0.56

0.31
0.49

0.38
0.63
0.66

0.33
0.59
0.37
0.55

Values in plain (bold) font are signiﬁcant at the 90% (95%) level.

over the past 62 years. Collectively, the overall agreement amongst the independent data sets, both in terms ofspatial pattern and temporal variability (Figures 3 and 4), provides compelling evidence for a reversal in sign ofSouthern Ocean climate trends during the late 1970s to early 1980s.

4. Summary and Discussion
We have evaluated surface climate trends over the Southern Ocean for two periods, 1979–2011 and 1950–1978,
using a variety of data sources including uninterpolated ship archives, land station data, and for the later period,
ERA-I reanalysis and satellite products. In addition, we have placed the recent trends in Antarctic sea ice
concentration into the broader environmental context of Southern Ocean surface climate trends. We focusedon austral summer (DJF) when data coverage over the Southern Ocean in the presatellite era is greatest.
During the later period, the distribution of SST trends shows notable and widespread cooling over Southern
Ocean, except for the area near the Antarctic Peninsula and adjacent West Antarctica where SST warming isobserved. The widespread SST decreases correspond to areas of sea ice expansion, while the region of SST
warming is associated with sea ice loss. Such a physically intuitive relationship suggests that SSTs can be used asa proxy for sea ice in areas near the Antarctic continent, allowing inferences of past sea ice behavior fromconventional SST measurements. For the Southern Ocean as a whole, SST has decreased by approximately 0.6°C
in DJF (0.4°C in the annual mean) while Antarctic SIC has increased by approximately 9% in DJF (12% in theannual mean) during 1979–2011. The surface cooling of the Southern Ocean since 1979 is accompanied byintensiﬁed westerly winds and a strengthened meridional SLP gradient, both in DJF and in the annual mean.
All data sets (SST, SAT, SLP, and U) show a consistent reversal in sign of the Southern Ocean surface climate trends
between 1979–2011 and 1950–1978. The independence of the various data sets provides strong evidence for thereality of the trend reversal. Early satellite records of Antarctic SIE collected from the Nimbus 5 electrically scanningmicrowave radiometer during 1973–1979, although of questionable reliability, also show a reversal in sign before
1979, suggesting they are at least qualitatively plausible. In addition to the sign reversal, the meridional extent of
the Southern Ocean surface climate trends shows a consistent northward expansion of approximately 10° of
latitude during 1950–1978 compared to 1979–2011 in all data sets. Such physically consistent trends across a
range of independently measured climate parameters provide a broader context for the recent increase in
Antarctic sea ice. We note that similar low-frequency ﬂuctuations during the past 50 years have been found insurface air temperature records [Turner et al., 2005; Jacka et al., 2004], sea level pressures [Turner et al., 2005], and in
ice core-based proxy temperature records [Schneider et al., 2006].
The mechanisms underlying the low-frequency surface climate trends over the Southern Ocean identiﬁed inthis study remain to be addressed. As mentioned in section 1, intensiﬁed surface westerly winds driven in partby stratospheric ozone depletion and GHG increase may be responsible for the surface cooling and sea ice
expansion since 1979 [Zhang, 2013]. However, coupled models participating in the Coupled Model
Intercomparison Project phase 5 (CMIP5) [Turner et al., 2013] generally do not simulate SST cooling nor sea
ice expansion over the high-latitude Southern Hemisphere [Polvani and Smith, 2013], suggesting that
decadal-scale climate variability may also be a factor. The reversal of the sign of the trends before and after
about 1980 also suggests the importance of internally generated variability. While the general sign reversalof the trends may be viewed as a response to stratospheric ozone depletion, it could also be viewed in thecontext of low-frequency variability generated by tropical dynamics [e.g., Okumura et al., 2012; Schneider
and Noone, 2012]. The additional context for observed trends provided here will help guide future work,
aimed at understanding the proximate causes for the spatial patterns and temporal evolution of surface
climate trends over the Southern Ocean since 1950.

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Geophysical Research Letters

Acknowledgments
We acknowledge helpful discussions
with Lorenzo M. Polvani, Yuko Okumura,and Ana Ordonez (who provided additional contextual analyses) during thecourse of this study. We are grateful to
Donald Cavalieri for sharing theAntarctic sea ice extent record for theperiod 1973 to 2002 and to Xungang
Yin for supplying the station data at
Puerto Montt, Christchurch, Hobart and
Ile Nouvelle-Amsterdam. We also thankthe two anonymous reviewers for theirthoughtful comments and suggestions.
The ﬁgures were produced with the
NCAR Command Language (NCL) software package. Tingting Fan is supportedby the China Scholarship Council (CSC).
David Schneider is supported by
National Science Foundation (NSF)
grants 1048899 and 1235231. NCAR issponsored by the NSF.
The Editor thanks two anonymous
reviewers for their assistance inevaluating this paper.

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