Melt event captured by satellite and surface observations

Passive microwave satellite observations (Fig. 1a) indicate that surface melt occurred during one or more days over a broad sector of West Antarctica (termed Ross sector hereafter) in January 2016, with up to 15 melt days over parts of the eastern Ross Ice Shelf and Siple Coast. We assess the significance of this event in the context of the entire satellite record (1978–2016) using two common melt indicators1: the melt extent (area of all grid cells with at least one day of melting) and the melt index (MI) (melt area weighted by duration of the melting), both calculated for the Ross sector (red outline in Fig. 1a inset). Bearing in mind that the results are sensitive to the choice of indicator and melt algorithm2, we estimate that January 2016 was one of the three largest melt events in the Ross sector since 1978 (second behind 1991–92 for MI, and a virtual tie for first with January 2005 for melt extent).

Figure 1: The January 2016 melt event captured by satellite and surface observations. (a) Map of West Antarctica showing the number of melt days in January 2016 estimated from passive microwave satellite observations overlaid on a MODIS mosaic image62. The white crosses denote the locations of the automatic weather stations (AWSs) shown in c. The inset map outlines the boundaries of the background MODIS image (black line) and Ross sector (red line). (b) Time series of December and January melt index (bars) and November–February melt extent (blue crosses) calculated for the Ross Sector (see inset map in a) and estimated from satellite-based daily melt data. The year refers to January (for example, the 1992 melt indices are December 1991 and January 1992). No data are shown for 1988 owing to insufficient observations. (c) Time series of 10-minute near-surface temperatures from six West Antarctic AWSs whose locations are shown in a. AWS name abbreviation and elevation above sea level are given in parentheses. Orange shading highlights temperatures above −2 °C (surface melting can occur despite below-freezing near-surface temperature because of radiative heating). Full size image

The satellite observations were corroborated on the ground by a number of automatic weather stations (AWSs) that recorded near-surface temperatures near or above 0 °C for several consecutive days during 10–21 January (Fig. 1c). The onset of the melt event on 10 January was accompanied by an abrupt temperature increase at WAIS Divide and Byrd, in central West Antarctica. The temperature time series from these two sites highlight roughly two phases: Phase 1 (10–14 January), during which the temperatures were at their warmest; and Phase 2 (15–21 January), during which the temperatures gradually decreased towards their pre-event levels. The transition from Phase 1 to Phase 2 is characterized by a shift of the melt pattern towards the Transantarctic Mountains apparent in the AWS temperature time series and in the sequence of daily melt maps (Supplementary Fig. 1).

The January 2016 melt event also coincided with the AWARE field campaign11, during which comprehensive upper-air, cloud and surface radiation observations were carried out at the WAIS Divide Field Camp (star symbol in Fig. 2a). This site was just downwind and upslope (1,801 m above sea level) of the main melting region and was thus exposed to some of the same weather conditions, as evidenced by the large-scale atmospheric circulation pattern during the melt event (see Fig. 2a and results section ‘Regional atmospheric circulation’). The AWARE campaign was also notable in and of itself for providing the first routine upper-air observations from West Antarctica since 1967, when the radiosonde program ended at Byrd Station.

Figure 2: Cloud liquid water simulated by ERA-Interim and detected by CALIPSO lidar on 12 January 2016. (a) Map of cloud liquid water path (CLWP) simulated by ERA-Interim for 06:00 UTC on 12 January 2016. The black star denotes the location of WAIS Divide. The two pink lines represent the CALIPSO satellite trajectories corresponding to the vertical profiles shown in b,c. The points labelled A, B, C and D along these trajectories are identical to those shown at the bottom of the vertical profiles. The black triangle denotes where rain was witnessed by a field party on 12 January (Dr Huw Horgan, Victoria University of Wellington, personal communication). (b,c) Vertical profiles of ice/water phase retrievals from CALIPSO Lidar Level 2 data products over West Antarctica for two time windows on 12 January 2016 (4:15–4:29 UTC in b and 7:33–7:46 UTC in c). The thin black line represents the surface topography. Full size image

Cloud and radiative processes

During the short Antarctic summer, strong onshore winds may by themselves raise the ice sheet’s surface temperature (T s ) up to the melting point (through exchange of sensible heat), especially at low elevations. However, T s is ultimately controlled by the full surface energy budget (SEB), being the net of radiative (short- and longwave) and turbulent (sensible and latent) heat fluxes. Clouds exert an important influence on the SEB by modulating the radiative fluxes12,13,14, primarily by enhancing downwelling longwave radiation and attenuating incoming solar radiation. In particular, low-level liquid-bearing clouds can have a determinant role in either causing or prolonging melting conditions over ice sheets15,16.

Model estimates from the ERA-Interim Reanalysis and satellite-based cloud phase retrievals from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission for 12 January 2016 (Fig. 2 and Supplementary Fig. 2 and Supplementary Fig. 3) indicate that liquid-bearing clouds were widespread over West Antarctica during the early stage of the melt event. Note that part of the differences between ERA-Interim (Fig. 2a) and CALIPSO (Fig. 2b,c), such as over portions of the Ross Ice Shelf, can be ascribed to complete attenuation of the CALIPSO lidar signal through thick upper-level ice cloud layers. The presence of warm (that is, liquid-bearing) low-level clouds over the Ross Ice Shelf is also apparent in Moderate Resolution Imaging Spectroradiometer (MODIS) observations from 11 January 2016 (Supplementary Fig. 4). The close match between the pattern of ERA-Interim cloud liquid water path (CLWP) and the contours of the melt area on the Ross Ice Shelf highlights the potentially important role of this type of cloud in maintaining melt-prone conditions (compare Supplementary Fig. 1, Supplementary Fig. 2, and Supplementary Fig. 3 for 11–12 January 2016). In addition, the tongue of CLWP stretching from the eastern Ross Ice Shelf to the region of WAIS Divide in Fig. 2a further indicates that AWARE observations may provide insight into the cloud microphysical properties at lower elevation.

The radiosonde profiles (Fig. 3a,b) from AWARE at WAIS Divide captured the large and vertically deep temperature and moisture perturbations associated with the marine air intrusion on 10–13 January. Micropulse lidar measurements (Fig. 3d,e) yielded periods of high attenuated backscatter (>10 dB) and low depolarization ratios (<10%) below 1 km, indicating high cloud liquid water content and low ice water content17 (mixed-phase clouds). The observed CLWP (Fig. 3c) was frequently within 10–40 g m−2, that is, the range where the cloud radiative enhancement effect previously observed over Greenland15 occurs. In this range, the clouds are thick enough to enhance the downwelling longwave radiation (Fig. 4a) but thin enough to also allow shortwave radiation to reach the surface (Fig. 4b). The CLWP was within this range 30–40% of the time during 10–13 January, suggesting that this enhancement mechanism contributed to the melt event. This is further supported by the frequent and widespread occurrence of clouds with CLWP within 10–40 g m−2 simulated by ERA-Interim during the same period (Supplementary Fig. 3 and Supplementary Fig. 5). However, we also notice a significant frequency of CLWP > 40 g m−2 (Fig. 3c), under which shortwave flux is attenuated and longwave flux is similar to blackbody radiation at the cloud effective temperature. These optically thicker clouds represent a contrast to the Greenland cloud radiative enhancement effect in that they signify a more prominent role of thermal blanketing as a consequence of the warm air advection (Fig. 3a,b). The total SEB (Fig. 4e) shows a marked increase in the net energy input into the snowpack (up to 40 W m−2), mainly attributable to enhanced downwelling longwave radiation (Fig. 4a,c). This additional energy input is also apparent in the satellite brightness temperatures (Fig. 4e).

Figure 3: Cloud, moisture and temperature observations at WAIS Divide during January 2016. (a,b) Vertical profiles of temperature perturbation and water vapour mixing ratio based on 6-hourly radiosoundings. The temperature perturbation is calculated as the difference between the measured temperature and the mean temperature during the period spanning 4 December 2015 to 18 January 2016 (duration of the field campaign). (c) Precipitable water vapour (PWV) and cloud liquid water path (CLWP) estimated from microwave radiometer measurements. (d,e) Vertical profiles of attenuated backscatter and depolarization ratio based on micropulse lidar measurements from 12 January 2016. For all vertical profiles, the height is with respect to the ground level (1,801 m above sea level). Full size image

Figure 4: Surface energy budget at WAIS Divide in January 2016. (a) Downward (LW↓) and upward (LW↑) longwave radiation fluxes. (b) Downward (LW↓) and upward (LW↑) shortwave radiation fluxes. (c) Net radiation flux (LW↓ – LW↑ + SW↓ – SW↑). (d) Net turbulent flux, calculated as the sum of sensible (SHF) and latent (LHF) fluxes, where positive is energy transfer away from the surface to the atmosphere. (e) Total net energy flux into the ground (snowpack) calculated as net radiation minus net turbulent fluxes. The green crosses represent the SSMIS 19 GHz horizontally polarized brightness temperatures measured from space at the location of WAIS Divide. Full size image

Regional atmospheric circulation

We trace the immediate causes of the melt event to the presence of an amplified high-pressure ridge (blocking high) over the 90–120°W sector of the Southern Ocean (Fig. 5a–c). By creating a prominent dent in the circumpolar westerly flow, this ridge generated a strong north-south advection of warm marine air towards West Antarctica. The ridge was strongest during 10–13 January (Phase 1) but persisted through 20 January (Fig. 5c), maintaining warm conditions favourable to surface melt in the Ross sector (Phase 2). Positive sea surface temperature (SST) anomalies of >2 °C near 50°S, 120°W (Fig. 5b) may have also provided additional heat to the air travelling south (note that the positive geopotential height anomalies near 60°S, 90°W favour anticlockwise motion). Data from ERA-Interim suggest that rain fell over parts of the Ross Ice Shelf at the beginning of the event (Supplementary Fig. 6), which may have preconditioned the snow surface for prolonged melting18. Although the reanalysis data should be treated with caution, drizzle was observed at WAIS Divide on 11 January (Supplementary Fig. 7) and rain was witnessed by one field party on the Kamb Ice Stream (black triangle in Fig. 2a) on 12 January (Dr Huw Horgan, Victoria University of Wellington, personal communication).

Figure 5: Atmospheric circulation associated with the January 2016 melt event. (a) Map showing the air temperature (shaded) and wind vectors at 500 hPa at 00 UTC on 12 January 2016 based on ERA-Interim. The thick black line outlines the 5,300 meter geopotential height contour separating cold polar air masses from warmer mid-latitudes air masses. (b) Sea surface temperature anomalies (shaded) overlaid with ERA-Interim 500 hPa geopotential height (Z500) anomalies (contour lines) in January 2016. Solid red, dashed blue and solid black contour lines represent (respectively) positive, negative and zero Z500 anomalies. Anomalies are calculated with respect to the 1971–2000 period. (c) Hovmöller diagram showing daily meridionally averaged Z500 anomalies within latitudes 50–80°S during January 2016. The anomalies are with respect to the 1979–2016 January monthly mean. Full size image

Large-scale atmospheric context

On a broader scale, the melt event occurred during one of the strongest El Niño events on record (Figs 5b and 6b). Positive (anticyclonic) geopotential height anomalies in the South Pacific, such as those observed in January 2016 (Fig. 5b), are a typical signature of El Niño teleconnections, as seen both in observations6,19 and climate model simulations20,21. This type of atmospheric pattern promotes warm air advection to the Ross sector6, which explains why surface melt in this area tends to be associated with El Niño-like conditions22. For instance, the prominent melt events of December 1982 and December 1991–January 1992 (see Fig. 1b) both occurred in conjunction with El Niño conditions, characterized by a negative Equatorial Southern Oscillation Index (SOI) in Fig. 6b. Conceptually, the fact that a circulation pattern resembling the El Niño teleconnection was present during the melt event and that this pattern is favourable to warm conditions over the Ross Ice Shelf points towards a causal link between the 2015–2016 El Niño and the January 2016 melt event. However, in practice (based on observations available since 1979), the relationship between the two phenomena remains complex (see Fig. 7 and related discussion below).

Figure 6: Time series of November-January averaged climate indices. (a) Southern Annular Mode Index. (b) Equatorial Southern Oscillation Index. The series span 1957–2016. The year corresponds to the month of January. The last data point is for November 2015-January 2016. Full size image

Figure 7: 1980–2016 standardized SAM Index, Equatorial SOI, West Antarctic melt index and melt extent anomalies. Years are rank-ordered from lowest to highest melt extent. For each year (x axis), the plot displays (in the following order) the melt index (light blue), the melt extent (dark blue), the SAM Index (orange), and the equatorial SOI (red). The plot is adapted from ref. 22. Full size image

The SAM, which characterizes the strength of the westerly winds around Antarctica, is an important modulator of the tropical influence in the South Pacific sector of the Southern Ocean7. During November 2015–January 2016, the SAM Index remained predominantly positive (Fig. 6a), indicating stronger-than-normal westerly winds. This is reflected in the negative geopotential height anomalies over Antarctica in Fig. 5b. The conjunction of a strong El Niño and a positive SAM phase was unusual since, in austral summer, the latter is most often associated with La Niña-like conditions8,23 (conversely, a negative SAM phase most often occurs in conjunction with El Niño-like conditions). Furthermore, a positive SAM phase hampers meridional heat exchange between middle and high latitudes7, and is thus generally not conducive to surface melt in West Antarctica22. Thus, if anything, the positive SAM phase that prevailed before and during the melt event should have favoured colder-than-normal, not warmer-than-normal, conditions in West Antarctica.

Contribution of El Niño and the SAM

Properly understanding the mechanisms responsible for the January 2016 melt event requires investigating the potential roles of El Niño and the SAM. This in turn can provide insight into the recurrence of such event in the future (see Discussion). Figure 7 provides a means to visualize the three-way relationships between West Antarctic summer melt, the SAM, and the ENSO phenomenon (represented by the Equatorial SOI) since 1979. Note that, in this figure, the two melt indices are calculated for December–January, and the two climate indices are November–January averages. Figure 7 shows that, in general, less melt tends to occur during La Niña-like conditions (SOI > 0) and a positive SAM phase, whereas more melt tends to occur during El Niño-like conditions (SOI < 0) and a negative SAM phase. This qualitative assessment is confirmed by tallying positive and negative melt anomalies depending on the sign of the two climate indices (Supplementary Table 1). It is worth noting that the directions of the relationships are similar to those previously found between Antarctic-wide melt and the SAM Index and SOI22.

These relationships are by no means simple. For example, not all major El Niño events are accompanied by widespread surface melt in West Antarctica (for example, 1997–98); not all prominent West Antarctic melt events coincide with strong El Niño events (for example, 2005); and the magnitude of West Antarctic melt does not scale with the intensity of El Niño events. Accordingly, it is not possible to establish with certainty whether the 2015–2016 El Niño caused (in a deterministic sense) the January 2016 melt event, a problem inherent to weather and climate phenomena24. It is not uncommon for the polar jet around Antarctica to exhibit large meanders, giving rise to warm marine air intrusions3, even in the absence of an El Niño event. Following a probabilistic approach, we seek rather to assess the likelihood of the January 2016 melt event occurring given the concurrent strong El Niño and positive SAM conditions. The statistically small number of El Niño events (especially of strong events such as 1982–83, 1997–98 and 2015–16) observed since 1979 does not permit robust statistical analysis. However, climate model simulations can alleviate this issue by generating larger samples of events.

Modelling experiment set-up

We used the Community Atmosphere Model (CAM) version 4 to conduct a set of four 15-year idealized simulations (see Methods for details). Three simulations were forced with annually repeating SSTs characteristic of past strong El Niño events, representing a total of 45 El Niño events. The fourth simulation was forced with annually repeating climatological SSTs to serve as control run. A model SAM index was calculated for each simulation based on principal component analysis of Southern Hemisphere monthly 500 hPa geopotential height anomalies. Here again, we considered the average SAM Index for November–January.

Estimating surface melt occurrence in the CAM model can be problematic as the process is affected by model grid resolution, model temperature biases and model deficiencies in the placement of key atmospheric features25,26,27,28. To circumvent these issues, we used anomalies in the model near-surface air temperature as an indicator of melt-prone conditions. We calculated these anomalies for each simulation by subtracting the long-term monthly means of the control simulation, and considered the mean anomalies for December–January spatially averaged over a broad Ross sector of West Antarctica (75°–90°S; 180°–90°W). We labelled these anomalies warm events or cold events depending on their sign.

Results from model simulations

Based on the model SAM Index and series of warm and cold events, we generated a contingency table (Table 1) tallying the number of events per type (warm or cold) and phase of SAM (positive, negative or neutral) across all simulated major El Niño events. Out of 45 El Niños, warm events occur 32 times (71.1%) versus 13 times (28.9%) for cold events. This result is consistent with the known positive impact of the El Niño teleconnection pattern in the South Pacific on West Antarctic temperatures already mentioned6. Out of the 32 warm events, 15 (46.9%) occur during a negative SAM phase. Out of the 13 cold events, 8 (61.5%) occur during a positive SAM phase. The chi-square statistic is significant at P<0.01, meaning that the type of event is significantly dependent on the combined states of El Niño and SAM. Such dependence confirms findings from previous literature7,29,30.

Table 1 Contingency table tallying the count of December–January warm and cold events as a function of the phase of the November–January Southern Annular Mode in idealized CAM4 model simulations. Full size table

Expanding on the previous analysis, we also find that 9 out of the top 10 warmest events occur during a negative or neutral SAM phase, while 9 out of the top 10 coldest events occur during a positive or neutral SAM phase. This demonstrates that the Ross sector is much more likely to experience conditions favourable to surface melting during El Niño events when a negative or neutral SAM is present. Given that the SAM was in a predominantly positive phase before and during the January 2016 melt event, our model results suggest that the state of the SAM likely mitigated the magnitude (areal extent, duration) of surface melt in West Antarctica during the austral summer of 2015–2016. In other words, the 2015–2016 melt season would likely have been more prominent had the SAM been in a negative or neutral phase, more commonly associated with El Niño events.