Frank T. Keimig

20th Century Climate Change in the Tropical Andes: Observations and Model Results

Linear trend analysis of observational data combined with model diagnostics from an atmospheric general circulation model are employed to search for potential mechanisms related to the observed glacier retreat in the tropical Andes between 1950 and 1998. Observational evidence indicates that changes in precipitation amount or cloud cover over the last decades are minor in most regions and are therefore rather unlikely to have caused the observed retreat. The only exception is in southern Peru and western Bolivia where there is a general tendency toward slightly drier conditions. Near-surface temperature on the other hand has increased significantly throughout most of the tropical Andes. The temperature increase varies markedly between the eastern and western Andean slopes with a much larger temperature increase to the west. Simulations with the ECHAM-4 model, forced with observed global sea surface temperatures (SST) realistically reproduce the observed warming trend as well as the spatial trend pattern. Model results further suggest that a significant fraction of the observed warming can be traced to a concurrent rise in SST in the equatorial Pacific and that the markedly different trends in cloud cover to the east and west of the Andes contributed to the weaker warming east of the Andes in the model. The observed increase in relative humidity, derived from CRU 05 data, is also apparent in the model simulations, but on a regional scale the results between model and observations vary significantly. It is argued that changes in temperature and humidity are the primary cause for the observed glacier retreat during the 2nd half of the 20th century in the tropical Andes.

Interannual climate variability in the Central Andes and its relation to tropical Pacific and Atlantic forcing

The main spatiotemporal modes of interannual temperature and austral summer (DJF) precipitation variability in the Central Andes are identified based on a two-way principal compo- nent analysis (PCA) of 30-year (1961-1990) monthly station data and related to contemporaneous tropical Pacific and Atlantic sea surface temperature anomalies (SSTAs). In addition, various me- teorological fields, based on National Centers for Environmental Prediction / National Center for Atmospheric Research CNCEP/NCAR) reanalysis, NOAA-Outgoing Longwave Radiation (OLR) and station data, are analyzed during periods of strong positive and negative SSTA and the respec- tive composites tested for local significance using a Students t-test approach. Temperature vari- ability in the Central Andes is primarily related to E1Nifio - Southern Oscillation (ENSO) and closely follows SSTA in the central equatorial Pacific with a lag of 1-2 months. In the southern Altiplano, temperatures have significantly increased since the late 1970s. DJF precipitation is also primarily related to ENSO, featuring below (above) average precipitation during E1 Nifio (La Nifia). Precipitation over the dry western part of the Altiplano shows the closest relationship with ENSO, due to ENSO-induced atmospheric circulation anomalies. Precipitation variability over the western Altiplano features a decadal-scale oscillation, related to a similar climatic shift in the tropical Pacific domain in the late 1970s. Over the northern Altiplano the precipitation signal is re- versed in the austral summer following the peak phase of ENSO, presumably due to the temporal evolution of tropical Pacific SSTA, rapidly switching from one state to the other. No evidence for a tropical Atlantic influence on DJF precipitation was found. SSTAs in the tropical NE Atlantic, however, presumably are influenced by heating and convection over the Altiplano through an up- per air monsoon return flow, altering the strength of the NE trades that emanate from the Sahara High.

Climate Variability in the Andes of Ecuador and Its Relation to Tropical Pacific and Atlantic Sea Surface Temperature Anomalies

The main spatiotemporal modes of seasonal precipitation and temperature variability in the Andes of Ecuador (18N-48S) and their relation to tropical Pacific and Atlantic sea surface temperature anomalies (SSTAs) between 1963-92 are identified based on rotated principal component analysis and cross-correlation techniques. Outgoing longwave radiation composites are analyzed during periods of strong oceanic forcing to confirm the proposed physical mechanisms. Despite the close proximity to the Pacific, precipitation variability in the Andes of Ecuador is not related to SSTA in the tropical Pacific domain alone. The El Nino-Southern Oscillation influence is most dominant in the northwestern part of the Andes during December-February (DJF) and in the eastern Cordillera during June-August (JJA) and in both cases associated with below- (above-) average precipitation during El Nino (La Nina) years. During most of the year precipitation variability over the eastern Andes is related to a dipolelike correlation structure in the tropical Atlantic, featuring positive correlations with SSTA to the south of the ITCZ and negative correlations to the north. The proposed mechanism involves positive SSTA in the tropical South Atlantic and contemporaneous negative SSTA in the tropical North Atlantic, resulting in increased rainfall over the eastern Cordillera. The only region with slightly increased precipitation during El Nino events is confined to a narrow area along the western Andean slope between 18 and 38S in close proximity to the Pacific. However, this relationship is weak and only apparent in DJF. Temperature variability in the Andes can largely be explained by SSTA in the tropical Pacific domain. The temperature response closely follows SSTA in the Nino-3 and Nino-3.4 regions with approximately one-month lag. The northernmost part of the Andes (north of 0.58N) is the only region where temperatures are significantly correlated with tropical North Atlantic SSTA.

Interannual Variability of Summertime Convective Cloudiness and Precipitation in the Central Andes Derived from ISCCP-B3 Data

The interannual variability of austral summer [December‐January‐February‐March (DJFM)] convective activity and precipitation in the central Andes (158‐308S) is investigated between 1983 and 1999 based on in situ rain gauge measurements, International Satellite Cloud Climatology Project (ISCCP) reduced radiance satellite data (the B3 dataset), and National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) reanalysis data. Twice-daily ISCCP-B3 calibrated infrared data, corrected for limb-darkening effects and representing equivalent blackbody temperatures Tb emitted by clouds are used to derive seasonal composites of fractional cold cloud coverage F*. Comparison of in situ rain gauge measurements with F* show a good correlation when a temperature threshold Tb 5 240 K is used to derive F*. A rotated empirical orthogonal function (REOF) applied to the seasonal estimates of F* yielded three spatially separated modes of convective activity in the south, northwest, and northeast of the central Andes. Results indicate that precipitation variability in the central Andes shows less spatial coherence than previously thought, with many years showing an antiphasing of wet/dry conditions between the northern and southern part of the study area. Regression analyses confirm the crucial role of both intensity and location of upper-air circulation anomalies with easterly wind anomalies favoring wet conditions, and westerly winds producing dry conditions. Two different forcing mechanisms are identified as main causes of upper-air zonal wind anomalies in the northern and southern part of the central Andes, respectively. Easterly wind anomalies during wet summers in the northern part are in geostrophic balance with reduced meridional baroclinicity due to low-latitude (midlatitude) cooling (warming), consistent with earlier studies. Farther to the south, easterly wind anomalies during wet summers are the result of an upper-air anticyclonic anomaly centered over southeastern South America, leading to a relaxation of the upper-air westerly winds and episodic easterly transport of humid air toward the subtropical Andes. This pattern is similar to one of the leading modes of intraseasonal variability, related to extratropical Rossby wave dispersion and modulation of the position of the Bolivian high. Correlation analysis of F* with near-surface specific humidity reveals that humidity variations in the lowlands to the east are not relevant on interannual time scales for the more humid northern part of the Altiplano. In the southern Altiplano, however, there is a significant correlation between convective activity and precipitation at high elevation and the low-level humidity content to the southeast of the Andes.

Projected temperature changes along the American cordillera and the planned GCOS network

[1] Analysis of 7 GCM simulations with 2x CO2 levels shows large and statistically significant free air temperature changes (compared to controls) along the axis of the American Cordillera (from Alaska to southern Chile). At all latitudes, the modeled change in temperature increases with elevation. Temperature increases are especially large in boreal summer months from � 35–50N, and year-round in the high mountains of Peru, Bolivia and northern Chile. If these models are correct, mountain ranges that extend high into the lower troposphere are likely to experience significant warming, with implications for glacier mass balance and water resources, montane ecosystems and high elevation agricultural activities. There are few high elevation meteorological stations to validate the model projections, or to monitor future changes. The planned GCOS (Global Climate Observing System) surface network is not adequate to address the critical issues raised by these model simulations; additional high elevation observing stations are needed. INDEX TERMS: 1610 Global Change: Atmosphere (0315, 0325); 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 9350 Information Related to Geographic Region: North America; 9360 Information Related to Geographic Region: South America. Citation: Bradley, R. S., F. T. Keimig, and H. F. Diaz (2004), Projected temperature changes along the American cordillera and the planned GCOS network, Geophys. Res. Lett., 31, L16210, doi:10.1029/2004GL020229.

Atmospheric circulation anomalies associated with 1996/1997 summer precipitation events on Sajama Ice Cap, Bolivia

The analysis of atmospheric circulation anomalies related to snowfall events on Sajama volcano (Bolivian Andes) provides important information for the calibration of an ice core, recently recovered from the summit. Seventeen precipitation episodes were recorded on Sajama volcano during the 1996/1997 summer season (November 1996 to March 1997) by snow depth sensors and additional measurements of an automatic weather station located on the summit. The analysis of atmospheric circulation patterns during these events is based on zonal and meridional wind, air temperature, relative humidity, geopotential height and horizontal divergence at three pressure levels (400, 500, and 700 hPa levels), atmospheric thickness (700 hPa-400 hPa), and precipitable water (vertically integrated), all extracted from the National Centers for Environmental Prediction (NCEP) data set. Highly convective situations prevailed through most of December and January, with strong vertical motion over the Bolivian Altiplano. In February and March, increased moisture advection from the east occurred in midtropospheric levels. These results are confirmed by isobaric 5-day back trajectories and transit time analysis at the 400 hPa level. The extremely southern position of the upper air high-pressure system (“Bolivian High”) in February and March is the main reason for the unusually high precipitation amounts on the Altiplano in 1996/1997. Highly variable patterns of atmospheric circulation can lead to snowfall on Sajama during the summer months.

Climatology of surface‐based inversions in the North American Arctic

The annual cycle of surface-based inversions at nine Arctic weather stations is examined, based on a 20-year set of daily 1200 UT significant level radiosonde data. All stations are at or near the coast. Inversions in winter months are primarily the result of strongly negative net radiation at the surface, whereas in summer, inversions more commonly result from near-surface cooling of warm air masses. Inversion frequency is at a maximum in winter (generally >70% of days) when inversions range from {approximately}400 to {approximately}850 m in thickness. Inversion thickness and strength (temperature change across the inversion) are strongly related to surface temperature. Inversions may involve temperature changes of >30{degrees}C in 6{degrees}C 100 m{sup {minus}1} during periods of extreme warm air advection aloft. Midwinter inversions commonly persist for 2-4 days, but may remain undisturbed for several weeks, affecting lower tropospheric chemistry. 9 refs., 11 figs., 5 tabs.

Recent changes in the North American Arctic boundary layer in winter

Analysis of significant level radiosonde data from a network of Arctic stations reveals a systematic reduction in midwinter surface-based inversion depths over the past few decades, accompanied by a rise in surface temperature. Similar trends are observed over a wide sector, from 62°W to 162°W and from 70°N to 83°N. Possible causes for these changes include increases in warm air advection, cloud cover, ice crystals, aerosols, and greenhouse gases, but the specific reasons are difficult to identify, due to strong interactions between many potentially important factors. Nevertheless, the changes are significant for studies of Arctic haze, since the midwinter stable boundary layer has been decreasing in depth over time.

Changes in Extreme Climate Indices for the Northeastern United States, 1870–2005

The northeastern United States is one of the most variable climates in the world, and how climate extremes arechangingiscriticaltopopulations,industries,andtheenvironmentinthisregion.Along-term(1870‐2005) temperature and precipitation dataset was compiled for the northeastern United States to assess how the climate has changed. Adjustments were madeto daily temperaturesto account for changes in mean,variance, and skewness resulting from inhomogeneities, but precipitation data were not adjusted. Trends in 17 temperatureand10precipitation indicesat 40stationswereevaluatedoverthreetimeperiods—1893‐2005,1893‐ 1950, and 1951‐2005—and over 1870‐2005 for a subset of longer-term stations. Temperature indices indicate strong warming with increases in the frequency of warm events (e.g., warm nights and warm summer days) and decreases in the frequency of cold events (e.g., ice days, frost days, and the cold spell duration indicator). The strongest warming is exhibited in the decrease in frost days and the increase in growing season length. Although maximum temperatures indices showed strong warming trends over the period 1893‐1950, subsequenttrends show little changeand cooling. Few significant trends were present in the precipitation indices; however, they displayed a tendency toward wetter conditions. A stepwise multiple linear regression analysis indicated that some of the variability in the 27 indices from 1951 to 2002 was explained by the North Atlantic Oscillation, Pacific decadal oscillation, and Pacific‐North American pattern. However, teleconnection patterns showed little influence on the 27 indices over a 103-yr period.

Temporal Changes in the Observed Relationship between Cloud Cover and Surface Air Temperature

The relationship between cloud cover and near-surface air temperature and its decadal changes are examined using the hourly synoptic data for the past four to six decades from five regions of the Northern Hemisphere: Canada, the United States, the former Soviet Union, China, and tropical islands of the western Pacific. The authors define the normalized cloud cover‐surface air temperature relationship, NOCET ordT/dCL, as a temperature anomaly with a unit (one-tenth) deviation of total cloud cover from its average value. Then mean monthly NOCET time series (night- and daytime, separately) are area-averaged and parameterized as functions of surface air humidity and snow cover. The day- and nighttime NOCET variations are strongly anticorrelated with changes in surface humidity. Furthermore, the daytime NOCET changes are positively correlated to changes in snow cover extent. The regionally averaged nighttime NOCET varies from 20.05 K tenth21 in the wet Tropics to 1.0 K tenth21 at midlatitudes in winter. The daytime regional NOCET ranges from 20.4 K tenth21 in the Tropics to 0.7 K tenth21 at midlatitudes in winter. The authors found a general strengthening of a daytime surface cooling during the post‐World War II period associated with cloud cover over the United States and China, but a minor reduction of this cooling in higher latitudes. Furthermore, since the 1970s, a prominent increase in atmospheric humidity has significantly weakened the effectiveness of the surface warming (best seen at nighttime) associated with cloud cover. The authors apportion the spatiotemporal field of interactions between total cloud cover and surface air temperature into a bivariate relationship (described by two equations, one for daytime and one for nighttime) with surface air humidity and snow cover and two constant factors. These factors are invariant in space and time domains. It is speculated that they may represent empirical estimates of the overall cloud cover effect on the surface air temperature.