mid-tropospheric circulation and surface melt on the greenland ice sheet. part ii: synoptic...

INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 18, 131±145 (1998)

MID-TROPOSPHERIC CIRCULATION AND SURFACE MELT ON THEGREENLAND ICE SHEET. PART II: SYNOPTIC CLIMATOLOGY

THOMAS L. MOTE*

Climatology Research Laboratory, Department of Geography, University of Georgia, Athens, GA 30602-2502, USAemail: [email protected]

Received 2 December 1996Revised 25 June 1997

Accepted 26 June 1997

ABSTRACT

Daily values of the spatial extent of melting on the Greenland ice sheetÐmeasured from satellite passive microwavesensorsÐare compared with a synoptic climatology of 700 hPa geopotential heights from May 1979 to June 1989. Ninecommon synoptic patterns were extracted by performing cluster analysis on component scores of a principal componentsanalysis of daily 700 hPa heights. Discrete composite analysis is used to produce maps of the geopotential height and heightdepartures for days classi®ed as each synoptic type. The mean melt extent for eight topographically de®ned regions of the icesheet are compared with the nine different synoptic patterns. Synoptic patterns that would produce onshore ¯ow are associatedwith a greater spatial extent of melting in the north and east regions of the ice sheet. The south-west regions of the ice sheethave greater melt extent during south-westerly onshore ¯ow, whereas north-westerly onshore ¯ow serves to reduce the meltextent. The strength and location of the North American trough and the Baf®n Bay low are highly related to the downstream¯ow over south Greenland and consequently to the extent of surface melting on the Greenland ice sheet. A westward displacedNorth American trough is associated with increased melting whereas an eastward displaced trough is associated with reducedmelt. # 1998 Royal Meteorological Society.

KEY WORDS: cluster analysis; synoptic climatology; 700 hPa geopotential heights; Greenland; icesheet; surface ice melt.

INTRODUCTION

Part I of this paper (Mote, 1998) examined low-frequency, mid-tropospheric teleconnections found in the summer

700 hPa geopotential height ®elds and their relationship to variations in surface melt on the Greenland ice sheet.

Although the teleconnections help explain both the interannual variation and a trend observed in the areal melt

extent, it is often dif®cult to interpret the mechanisms that may associate an anomalously high or low melt extent

with a particular atmospheric teleconnection. Part II of this paper uses a synoptic climatological approach to

produce idealized synoptic types that are more easily interpreted as to their relationship to variations in surface

melt extent.

BACKGROUND

The topography of the Greenland ice sheet plays a large role in the steering of cyclones that approach it from

North America. The cyclones are either shunted eastward into the `graveyard' of the Icelandic low, or they are

pushed through Davis Strait into Baf®n Bay. Although closed lows generally do not cross the ice sheet, the upper

level vorticity maxima may cross and subsequent cyclogenesis may occur on the east side of the ice sheet. This

CCC 0899-8418/98/020131-15 $17.50

# 1998 Royal Meteorological Society

*Correpondence to: T. L. Mote, Climatology Research Laboratory, Department of Geography, University of Georgia, Athens, GA 30602-2502, USA.

redevelopment is favoured both by the baroclinic zone resulting from the cold air drainage from the ice sheet and

lee-side cyclogenesis (Puntins, 1970). Cold air outbreaks southward are frequent on the east coast, whereas the

approach of a cyclone from the south-west or west often pulls warm air on to the west and south-east coasts

(Barry and Kiladis, 1982).

Cyclones are often steered from North America north-eastward toward Greenland by the persistent Baf®n Bay

trough in the mid-troposphere. The phase and amplitude of this trough are important in determining whether

cyclones approaching from the south-west proceed along Greenland's east or west coast. An eastward displaced

Baf®n Bay trough forces surface cyclones around the southern tip of Greenland and toward Iceland or parallel to

the east coast of Greenland, whereas a westward displaced trough steers cyclones along the west coast of

Greenland.

Barry and Kiladis (1982) charted the frequency of cyclones that approached Greenland along the west coast

through Baf®n Bay and along the east coast through the Denmark Strait for 1950±1965. The cyclones were

charted from six source regions: locally developed, Canadian Arctic north of 60�N, the North American continent

south of 60�N, the east coast of North America, and the North Atlantic. The spring (March±May) and summer

(June±August) had substantially lower cyclone frequencies in Denmark Strait than in the other two seasons. In

Baf®n Bay, the spring had the lowest frequency of cyclones. Cyclones affecting the east coast of Greenland

during the spring and summer tend to originate from North America, with both continental and east coast origins.

These are cyclones that move north-eastward and typically are directed into the region of the Icelandic low. The

west coast of Greenland is affected primarily by cyclones of Canadian Arctic and continental North American

origins during summer. The Canadian Arctic cyclones, in particular, are usually associated with the trough over

the eastern Canadian Arctic and become persistent Baf®n lows. As cyclones progress along either coast of

Greenland, they often move into the Arctic during the summer. Serezze and Barry (1988) found that Baf®n Bay

and the Greenland Sea are frequent paths for cyclones that eventually collect in the Canadian Arctic basin.

These synoptic-scale features of the climate can determine the local climatic conditions across the ice sheet,

including the summit. For example, the prevailing circulation can produce favourable conditions that encourage

the development of katabatic winds. Cyclones along the coast produce a pressure gradient force that augments the

drainage of cold air from the summit and intensi®es katabatic winds. Also, surface fronts accompanying coastal

cyclones may completely transect the ice sheet at the saddle region (Putnins, 1970).

CASE STUDIES DURING EXTREME MELT

After examining 700 hPa height patterns for a number of 3- to 5-day periods with extremely high melt extents, a

similar pattern was noted among many of the events. This can best be shown by providing a case study of the

atmospheric conditions resulting in higher or lower than average melt extent. The daily melt extents for the

period of 26 May to 3 June 1989 demonstrate the advance of melt between 26 and 30 May with its subsequent

retreat from 1 to 3 June (Mote et al., 1993). On 30 May 1989, the day with the highest melt extent for May during

1979±1991, the entire south dome of the ice sheet was experiencing snowpack melt.

The hemispheric 700 hPa height patterns demonstrate the evolution of the synoptic conditions that apparently

produced this event. On 26 May, four long waves appear in the hemispheric circulation: one trough axis at 10�E,

a second at 100�E, a third at 180�, and the ®nal one at 80�W (Figure 1(a)). Across the North American continent,

two short waves are evident in the ¯ow, one in western Canada between the North American and Paci®c troughs,

and a second that was propagating through the exit region of the North American trough.

The resulting ¯ow over Greenland on 26 May was generally zonal, in¯uenced by the short-wave pattern still

remaining over eastern Canada. At that point, the short-wave ridge was located over northern Hudson Bay.

Combined with the short-wave trough in advance, and superimposed over the long wave ¯ow, the result was

westerly ¯ow over the west coast of Greenland. On 28 May, the short wave that had been located in western

Canada was propagating though the base of the North American trough, with the short-wave ridge axis located

through St James Bay (Figure 1(b)). The eastern short wave was directly affecting Greenland by this time. The

short-wave ridge was superimposed over the North Atlantic long-wave ridge to produce a southerly geostrophic

wind component over southern Greenland. By 30 May, the ridge of the second short-wave was passing through

the North Atlantic ridge (Figure 1(c)). The height gradient between the North American trough and North

132 T. L. MOTE

# 1998 Royal Meteorological Society Int. J. Climatol. 18: 131±145 (1998)

Atlantic ridge was strongest on this day, with a difference of 300 gpm between Hudson Bay and the southern tip

of Greenland. As a result, the strongest warm air advection should have occurred on this day, which was also the

Figure 1. 700 hPa height patterns (gpm) for (a) 26 May, (b) 28 May, (c) 30 May, (d) 1 June, and (e) 3 June, 1989

GREENLAND ICE SHEET AND SYNOPTIC CLIMATOLOGY 133


day with the peak melt extent. By 1 June, the second short wave had passed through Greenland, and the ¯ow

across the western North Atlantic had become much weaker and more zonal (Figure 1 (d)). On 3 June, the North

Atlantic ridge was particularly weak as the trough of another short wave from North America was propagating

through the long-wave ridge axis (Figure 1(e)). This event, although extreme, was similar to most of the periods

with a large melt extent, particularly early or late in the season when additional sensible heat advection can

supplement the meagre net radiation available for surface melt. Additionally, the baroclinic waves that provide

the additional sensible heat may produce additional cloud cover that reduces outgoing longwave radiation.

Identifying a typical atmospheric circulation pattern or patterns for days with low melt extents during mid-

summer is more dif®cult than identifying patterns associated with extremely high melt extents. Periods of low

melt seem to evolve more slowly and last longer. Whereas the appearance of high-melt events seems to be

a result of a combination of appropriate hemispheric-scale and synoptic-scale conditions, the atmospheric

conditions associated with days of relatively low melt extent in mid-summer apparently are governed by the

phase and amplitude of hemispheric-scale atmospheric wave patterns (i.e. Rossby waves).

One example of a typical low melt extent period in mid-summer is the period of 20 July to 3 August 1986.

During this period, the North American trough was dislocated north-east from its mean position with lower than

normal heights. In the high melt extent period just described, the North American trough included a closed low

centred over Hudson Bay and another closed low in the Arctic Basin centred over the Kara and Laptev Seas

(Figure 1). In the low melt extent case, the lowest heights are over the eastern Canadian Archipelago and Baf®n

Bay rather than over Hudson Bay (Figure 2). As a result, instead of south-westerly advection from the western

Atlantic, the ¯ow over Greenland was generally westerly from northern Canada, likely reducing the amount of

sensible heat advection to the western margin of the ice sheet, where most melt occurs.

An upper-level low situated over Baf®n Bay should produce cloud cover over the western margin of the ice

sheet that reduces incoming shortwave radiation, providing less energy for melting. However, the cloud cover

should also decrease outgoing longwave radiation, providing more energy for melting. Based on the results from

Braithwaite and Olesen's (1990a) energy balance modelling over the western margin of the ice sheet, it may not

be possible to generalize about the effect of cloud cover on ablation due to the opposing in¯uence clouds have on

the shortwave and longwave radiative ¯uxes. However, Braithwaite and Olesen (1990b) found that variations in

ablation rates along the western margin of the ice sheet are largely explained by changes in the sensible heat ¯ux.

Therefore, it is likely that the changes in sensible heat advection that result from the observed circulation patterns

were the primary cause of the variations in mean melt extent.

Figure 2. 700 hPa heights (gpm) for 22 July 1986

134 T. L. MOTE


SYNOPTIC TYPING

Barry and Perry (1973) suggested that synoptic climatological analysis consists of two steps: (i) identifying and

categorizing synoptic weather types, and (ii) relating the synoptic types to local weather conditions. Both manual

and automated techniques have been used in synoptic climatological applications. Until advances in technology

made the automated classi®cation approaches viable, manual classi®cation of synoptic maps was common. These

manual classi®cations are usually based on (i) air mass characteristics, (ii) atmospheric ¯ow patterns, or (iii) parts

of the cyclone model (i.e. warm versus cold sector conditions). A few of these manual classi®cations have

included investigations of the effect of atmospheric variations on glacier melt or mass balance. For example, Alt

(1978, 1987) used a manual classi®cation of 500 hPa geopotential heights and sea-level pressure ®elds to

investigate variations in mass balance of two ice-caps in the Canadian Arctic.

Automated classi®cations are more readily reproduced, have clearly stated assumptions, and can prove much

easier to construct for large data sets. The two common automated approaches are correlation analysis (including

Kirchoffer approaches) and clustering methods. In most cases a principle components analysis (PCA) is usually

used ®rst to create orthogonal data, an orthogonal variable space is assumed in the Euclidean model for either

correlation or cluster analysis.

The ®rst type of automated classi®cation attempted was a correlation approach, ®rst discussed by Lund (1963),

and the approach has changed little since ®rst used by Lund (Yarnal, 1993). A related approach was suggested

by Kirchoffer (as cited in Yarnal, 1984). The Kirchoffer method attempts to determine the similarity of

meteorological ®elds from different maps by determining the sums-of-squares difference between pairs of maps.

However, Willmott (1987) showed that the Kirchoffer score is a simple mathematical transformation of the

correlation coef®cient. The correlation approach has been used in a number of instances to understand the

atmospheric in¯uence on the mass balance or melt rates of glaciers and ice caps. Yarnal (1984) used Kirchoffer

scores to classify 500 hPa heights ®elds over western North America and related the synoptic types to extremes in

mass balance of the Sentinel Glacier in British Columbia. Bradley and England (1979) related temperatures

during the ablation season in the Arctic to synoptic types identi®ed with a correlation-based approach. Despite the

widespread use of correlation and Kirchoffer approaches in synoptic climatology during the 1970s and early

1980s, the approach has been criticized, particularly when classifying univariate maps (Willmott et al., 1985).

Willmott (1978) suggested that covariance measures or clustering approaches are more appropriate for bivariate

map similarity exercises.

More recently, cluster analysis has been used to create synoptic types. The objective of cluster analysis is to

create relatively homogeneous groups of cases based on the climatic data available for each case. Clustering

algorithms attempt to create groups of cases in which the within-group variance is minimized while the between-

group variance is maximized (Balling, 1984). In almost all applications, the cluster analysis is preceded by a P-

mode or S-mode PCA for data decomposition, resulting in a new set of uncorrelated variables that are used in the

cluster analysis (Yarnal, 1993). The result of the cluster analysis is the classi®cation of individual cases into one

of several groups. In order to interpret each resulting synoptic type, the individual data points can be averaged

across all cases classi®ed into that synoptic type, producing an average map for that type (Crane and Barry,

1988).

Multivariate synoptic typing using cluster analysis has been advocated most vigorously by Kalkstein (e.g.

Kalkstein and Corrigan, 1986; Kalkstein et al., 1987, 1990). As noted by Yarnal (1993), few examples of cluster

analysis of univariate data ®elds, which he refers to as `map-pattern classi®cation', appear in the synoptic

climatology literature. Key and Crane (1986) did use cluster analysis for a univariate ®eld, but only in

comparison with other methodologies.

Synoptic typing method

In order to categorize circulation patterns associated with anomalous high or low melt extent, a synoptic

climatology was produced by clustering the unrotated component scores of nine principal components extracted

from daily 700 hPa height data (see Part I for discussion of the PCA analysis). The rotation performed on the

principal components in Part I of this paper was done to aid interpretation of the component loading patterns.



Rotation of the PCs is not necessary simply to create an orthogonal data set, which is necessary before conducting

the cluster analysis. Therefore, the component scores from the unrotated PCA were used in the cluster analysis.

Following the suggestion of Kalkstein et al. (1987), an average linkage algorithm was used to cluster the

unrotated component scores. The average linkage algorithm allows a case to join a cluster based on the average

Euclidean distance between the case of interest and all of the cases already belonging to the cluster (SPSS, 1988).

When Kalkstein et al. (1987) compared three common clustering algorithms for synoptic climatological analysis,

they found that the average linkage algorithm yielded the most realistic groupings and properly handled extreme

events.

The number of groups extracted from the average linkage cluster analysis was determined by examining a

scree plot of the proximity values. The proximity values at each step in the aggregation indicate the distance

between the two groups merged in that step. The magnitude of the jump in the proximity value indicates the

robustness of the solution. A large increase in the proximity value indicates that the previous solution was stable,

whereas a small increase indicates that the previous solution was unstable.

Each event is classi®ed into a group (i.e. synoptic type) by the cluster analysis. The location of the cluster

centroid in variable space can be determined by compositing the 700 hPa height data for all of the members of

that cluster. The result is a set of 700 hPa height maps that best represent the patterns observed in the original

time series of daily 700 hPa height data.

Description and frequency of synoptic types

Nine clusters, or synoptic types, were extracted with the clustering algorithm based on the proximity scree plot.

Mean departure maps for the clusters were produced by averaging the 700 hPa height data and their departures

from monthly means for each day belonging to a particular synoptic type.

Three of the nine types (Types 1, 2 and 3) accounted for approximately 71 per cent of the events during all four

summer months, and four of the remaining types (Types 6, 7, 8 and 9) accounted for only 15 per cent (Table I).

Type 1 events were the most prevalent, accounting for 30 per cent of the cases during the summer. The Type 1

events were most prevalent in May, when the Type 2 events were only the fourth most prevalent (Table I). Type 5

events were more frequent in May than any other month, and the few Type 8 events occurred only in May. Type 2

and Type 3 events become more frequent in June, and July is dominated by Types 1, 2 and 3. August is also

dominated by Types 1 and 2, but Type 4 has its highest frequency in August (Table I).

The two synoptic types with the greatest frequency are similar to the mean 700 hPa geopotential height pattern.

Type 1 (Figure 3; 29�6 per cent of cases) has a positive anomaly centre of slightly greater than 30 gpm situated

just south of Greenland. Type 2 (Figure 4; 25�3 per cent of cases) has no distinctive features in the North Atlantic.

Type 2 does show a node of positive anomalies in north-central Asia, but does not extend far into the Arctic

Basin. The actual height patterns for both types show a trough axis centred through eastern Baf®n Island in the

Canadian Archipelago. Both Type 1 and Type 2 events are not true synoptic types, but instead represent

conditions near climatology.

Table I. Frequency of days (in per cent) classi®ed as each synoptic type for eachmonth from May to August and for May±August combined

Synoptic type May June July August May±August

1 31 32 30 25 302 12 22 30 41 253 13 17 19 16 164 10 8 5 15 105 17 0 4 0 66 3 4 12 0 57 0 12 0 3 48 10 0 0 0 39 4 6 0 0 3

136 T. L. MOTE


Types 3, 5 and 7 each have a different combination of anomaly patterns in the central Arctic and the western

Atlantic. Type 3 (Figure 5; 15�8 per cent of cases) has positive anomalies in both locations as well as over central

North America, but the North American trough axis appears near its mean position. Type 5 (Figure 6; 5�7 per cent

of cases) has positive anomalies in the central Arctic and eastern Canada as well as a node of negative anomalies

centred near Iceland. Type 5 also has a node of positive anomalies stretched from St James Bay to Labrador,

which results in a closed low over Baf®n Bay in the composite 700 hPa heights. Type 7 (Figure 7; 3�9 per cent of

cases) has a node of negative anomalies over the central Arctic, with positive anomalies across Hudson Bay,

resulting in a more intense gradient in the 700 hPa heights across the Canadian Arctic, but weak, divergent ¯ow

across southern Greenland and a weakened North American trough. Type 7 gives a wave 4 pattern and may be

more of a planetary wave than a synoptic type.

Figure 3. Synoptic Type 1 700 hPa height departures (a) and 700 hPa heights (b). Dashed lines indicate negative values




The Type 4 pattern is distinctly different from the three just discussed. Type 4 (Figure 8; 9�5 per cent of cases)

has a long band of positive anomalies over Greenland, but with the greatest anomalies off the east coast of

Greenland, west of Iceland. The result is a westward displacement of the North American trough axis from its

climatological mean position, and general southerly geostrophic ¯ow across Greenland.

Type 6 (Figure 9; 4�5 per cent of cases) and Type 9 (Figure 10; 2�8 per cent of cases) have a similar structure in

the North Atlantic. Both have strong centres of positive anomalies over Baf®n Bay and western Greenland and a

node of negative anomalies between 40�N and 50�N, near the 30�W meridian. This pattern is displaced further to

the south-east in Type 9 than in Type 6. This should result in anomalous onshore ¯ow to east Greenland, but

weak ¯ow to the remainder of the island.The structure of Types 6 and 9 differs over the remainder of the Northern

Hemisphere. Type 9 has a strong negative anomaly centre over Finland, whereas Type 6 has a positive node



138 T. L. MOTE


centred over the Urals, but stretched across Western Europe. Type 6 has positive anomalies in the north-western

Paci®c and negative anomalies in the north-eastern Paci®c; Type 9 has the opposite.

The Type 8 (Figure 11; 2�9 per cent of cases) pattern has a large band of positive anomalies, greatest in the

central Arctic, stretching from Labrador across the pole to Japan. Three nodes of negative departures are situated

over the 180� meridian, off the west coast of North America and across Hudson Bay. The resulting composite

700 hPa height pattern shows an intense cut-off low just north of Hudson Bay. These cases may be associated

with a weaker circumpolar vortex or a vortex displaced toward North America.





VARIATIONS IN MELT AS RELATED TO SYNOPTIC TYPES

The purpose of the synoptic climatology is primarily to help understand the regional response of melt on the

Greenland ice sheet to atmospheric circulation. The daily melt extents were averaged for the days categorized

into each synoptic type and were compared with the mean melt extents of all other synoptic types. A one-way

analysis of variance was used to compare the means and determine the statistical signi®cance of the differences.

This should illuminate those circulation characteristics, as classi®ed into synoptic types, that are responsible for

regional variations in the surface melt.



140 T. L. MOTE


High-melt conditions

Synoptic Types 4 and 8 are associated with the highest mean melt extents for the ice sheet as a whole, and the

two types have similar regional effects. Type 4 (Figure 12) is associated with high melt extent in the south-west

and west (Regions 1±3) and the south-east (Region 8), but average melt in the north (Regions 4 and 5) and below

average melt on the east coast (Regions 6 and 7) (See part I for a map of regions). The node of positive anomalies

centred off the east coast of Greenland (Figure 8) should produce an anomalous south-westerly geostrophic wind

component over southern and western Greenland, as shown in the 700 hPa composite map. The resulting warm-

air advection should produce greater mean melt extents. However, the east coast of Greenland should experience

subsidence, which would favour cold air drainage from the summit of the ice sheet, instead of producing the

onshore ¯ow. Like Type 4, Type 9 (Figure 13) also generally produces greater melt in the south-west and west

(Regions 1±3) than the east (Regions 6 and 7), but the most anomalous values are further north (Region 3) rather

than in the south-west (Regions 1 and 2). South-westerly advection across the western margin of the ice sheet is

also favoured with this con®guration, but is due primarily to the intensi®cation and westward displacement of the

North American trough (Figure 11).


Figure 12. Melt extent during synoptic Type 4 days for each region and for the entire ice sheet. The solid squares indicate mean values, and theerror bars indicate the 95 per cent con®dence limit



Types 6 and 9 are also associated with higher than normal melt for the ice sheet as a whole, but the regional

in¯uence of these two types is much different than with Types 4 and 8. Type 6 (Figure 14) shows a tendency

toward higher melt in the north, north-east and east (Regions 4±7) than Types 4 and 8 (Figures 8 and 11). The

positive height departures over north-east Greenland and negative height departures near the south-east coast

should produce an anomalous onshore ¯ow in the east, and particularly the north-east for Type 6 events (Figure

9). This produces the highest melt extent of any synoptic type in the north-east region (Region 6). Type 9 (Figure

15) produces more melt in the north-west region (Region 5) than any other synoptic type, and produced extensive

melt in the south-east region (Region 8). The node of positive height departures in the north-west, centred over

Baf®n Bay, should result in an onshore ¯ow to the north-west (Figure 10). Meanwhile, the gradient between the

positive node south of Baf®n Bay and the negative height departures south of Iceland should also increase the

onshore ¯ow to the south-east. This type also appears to be a result of a stronger circumpolar vortex and possible

a southward displacement of the primary cyclone track over the North Atlantic to the south of Greenland.

The most striking feature for Type 7 (Figure 16) is the lower than normal melt extent for the east (Region 6)

compared with other regions of the ice sheet. The 700 hPa height anomalies for Type 7 (Figure 7) are almost

opposite of those for Type 6 in the Arctic and eastern Atlantic (Figure 8). The positive height anomalies over the

eastern Atlantic and negative height anomalies over the Arctic should result in anomalous offshore geostrophic

¯ow for the east (Region 6), reducing melt there.



142 T. L. MOTE


Low-melt conditions

The Type 5 (Figure 17) pattern produces the lowest mean melt extents for the ice sheet as a whole. Type 5 days

produce particularly low melt extents on the east (Region 7) and west (Region 3) margins of the ice sheet, but not

as low in the north (Regions 4 and 5). The negative height departures centred over Iceland, in conjunction with

the positive departures over Labrador, produce anomalous north-westerly geostrophic ¯ow over the western

margin of the ice sheet (Figure 6). This pattern may be a result of a circumpolar vortex displaced toward the

North Atlantic, placing most of Greenland under a trough at 700 hPa (Figure 6) and reducing melt. The positive

height departures over the central Arctic, however, should produce some onshore ¯ow to the north. This is likely

why Region 5 does not show the highly negative departures in mean melt extent seen in most other regions.

Synoptic Type 3 (Figure 18) days produce a slightly below normal melt extent, primarily in the north-west

(Region 4) due to the positive height departures in the central Arctic (Figure 5). This pattern favours off-shore

¯ow and cold air drainage from the summit region.

Synoptic Types 1 and 2 (charts not shown) have slightly negative mean melt extents, but all regions appear to

behave the same. This tendency for slightly negative values is likely due to the slight skewness in the

standardized melt extents toward negative values. The height patterns for Types 1 and 2 are near climatology

over the North Atlantic, which should produce melt extents near normal as well (Figures 3 and 4).





CONCLUSIONS

Initial comparisons of 700 hPa atmospheric heights associated with anomalously high or low melt extent on the

Greenland ice sheet indicate that both synoptic-scale and hemispheric-scale circulation phenomena play a role in

the surface melt variations. The synoptic-scale events are particularly important in the evolution of short-lived

`melt waves' that advance across the ice sheet, as is evidenced by several case studies, including one given in this

paper.

The synoptic climatology of 700 hPa heights shows a strong relationship to surface melt extent. For the entire

ice sheet, Types 4 and 8 have statistically signi®cant (at the 95 per cent con®dence interval) greater melt extent

that the other types. Type 8 may demonstrate the upstream development prior to ice melt. Type 5 has statistically

signi®cant lower melt extent. The synoptic types are particularly helpful in understanding the physical

mechanism for differences in melt extents for various regions of the ice sheet, particularly the contrast west and

east of the ice sheet's crest. The east and north coasts typically need onshore ¯ow to produce high melt extents.

Low pressure situated off the coasts results in subsidence that drains cold air from the summit and reduces melt

along the margin. The south-west regions of the ice sheet usually experience onshore ¯ow. In these regions,

south-westerly onshore ¯ow is necessary for a large melt extent, whereas north-westerly onshore ¯ow actually

serves to reduce the melt extent. The location and strength of the North American trough is also associated with

variations in melt on the Greenland ice sheet. A westward displaced trough results in more southerly geostrophic



144 T. L. MOTE


¯ow across Greenland and is associated with greater melt, whereas an eastward displaced trough is associated

with reduced melt.

The synoptic climatology demonstrates that the mid-tropospheric circulation plays a large role in surface melt

on the Greenland ice sheet. This is particularly important to note in regard to mass balance modelling efforts that

assume a uniform change in temperature for all locations. Climate change, accompanied by a change in the

general circulation of the Northern Hemisphere, may well produce a different response in the mass balance of the

ice sheet on either side of the crest.

ACKNOWLEDGEMENTS

This work was partially funded by NASA grant NAGW-1266 awarded to Mark Anderson, Clint Rowe and Karl

Kuivinen at the University of Nebraska-Lincoln, and NASA grants NGT-30127 and NAGW-4573 awarded to the

author. I gratefully acknowledge the assistance of Mike Palecki and Dan Leathers for their assistance on this

research and two anonymous reviewers for their comments.

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