Oct 17, 2003 MISR Alex Hall, Sebastien Conil, Mimi Hughes, Greg Masi UCLA Atmospheric and Oceanic Sciences The Origin and Structure of Southern California Climate Variations

Cartoon showing the remote local impacts of the El Niño phenomenon during wintertime. The modern view of climate is that we have to understand large-scale modes, and then understand their impact on the local scale.

The Pacific--North American pattern Note the spatial scales, on the order of the scale of mid-latitude weather systems (i.e. thousands of kilometers). The El Niño phenomenon excites this pattern, so that atmospheric circulation and climate anomalies in this region are associated with El Niño.

Large-scale view of climate variability and change El Niño phenomenon Pacific North American pattern North Atlantic Oscillation Global climate change (increase in temperature, and intensification of the hydrologic cycle)

SIMULATING LOCAL CLIMATE

Experimental Design model: MM5 boundary conditions: reanalysis data from NCEP resolution: domain 1, 54 km, domain 2, 18 km, domain 3, 6 km time period: 1995 to present. One can think of this as a reconstruction of weather conditions over this time period consistent with three constraints: (1) our best guess of the large- scale conditions, (2) the physics of the MM5 model, and (3) the prescribed topography, consistent with model resolution.

PRECIPITATION

The wet season begins in October, and lasts until May…

A scatter plot of precipitation vs. elevation shows two distinct clusters with different slopes and intercepts.

It turns out that if we divide up the region into a coastal and an interior domain, this corresponds to the two clusters in the precip vs elevation plot. COASTAL INTERIOR

A scatter plot of precipitation vs. elevation shows two distinct clusters with different slopes and intercepts.

COASTAL INTERIOR If we regress precipitation against elevation, we can calculate slopes and intercepts for the two clusters. The slope measures the elevation enhancement effect. The intercept measures the precipitation that would occur if the domain were flat and located at sea level.

Slope and intercept of regression of precipitation vs elevation by calendar month coastal interior coastal interior The elevation enhancement exhibits approximately the same seasonal behavior in both the interior and coastal domains. The difference between the two domains arises mainly from the theoretical difference in precipitation that would occur the domain were flat at zero elevation. This quantity becomes very large in the coastal domain, particularly in the Dec-Mar, but is almost always zero in the interior, even during the wettest month (February).

The climatological precipitation field can be in understood in terms of the height of the coastal range: Even when synoptic scale weather systems are most active in Southern California, the coastal ranges are high enough that they wring out enough moisture to eliminate precipitation at zero elevation in the interior. However, during all seasons, the interior air is moist enough that elevation enhancement of precipitation is nearly as effective as in the coastal range. If the coastal range were higher this would not be the case.

MODES OF VARIABILITY

A view of the Santa Anas from space, taken by the Multi- angle Imaging SpectroRadiometer (MISR) on February 9, 2002.

The winds simulated by the model during the Santa Ana event of February 9-12, Note the intense flow, reaching speeds on the order of 10 meters per second, being channeled through mountain passes. The fact that the model simulates this and other Santa Ana events with the correct timing demonstrates that the conditions for Santa Ana events are contained within the boundary condition information provided to the model.

The mean surface winds from Oct-March are characterized by alongshore flow associated with the subtropical high, and a small offshore flow signal. The surface wind speed variability is strongest over the ocean, the Tehachapi and Laguna Mountains, and the complex low- lying topography north of the Los Angeles basin.

Cluster Analysis To classify the regimes of wind variability in Southern California, we performed a probabalistic cluster analysis algorithm (Smyth et al. 1999) on the October to March daily- mean winds. The clustering technique provides an quantitative means of defining preferred modes of wind variability. We chose to focus on the wet season because of the interesting combination of phenomena (Santa Ana events and precipitation) during this period. We found that the wind regimes can be well-described in terms of three clusters, which together account for 82% of the days. Note that this implies that 18% of the days do not belong to any cluster, and are unclassified.

The “Santa Ana” regime is characterized by intense offshore flow through mountain passes. Its average duration is 1.7 days and it accounts for 13% of the total days in the analysis.

The “weak northwesterly” regime is characterized by relatively weak winds over land, and moderate alongshore flow over the ocean. Its average duration is 1.6 days and it accounts for 27% of the total days in the analysis.

The “strong northwesterly” regime is characterized by weak winds over land, and strong alongshore flow over the ocean. Its average duration is 2.4 days and it accounts for 42% of the total days in the analysis. This is therefore the dominant wind regime in Southern California.

Average number of regime occurrences per month Santa Ana weak northwesterly strong northwesterly The number of cluster 1 events peaks in December, consistent with observational studies of the Santa Ana phenomenon (e.g. Raphael 2003). Clusters 2 and 3 represent vacillations of the climatological alongshore flow.

surface air temperature anomalies (K) associated with the Santa Ana cluster Santa Ana events are associated with cold temperatures in the interior, and warm temperatures in the coastal zone, consistent with the downslope flow pattern from the high desert interior.

MODES OF VARIABILITY AND HYDROLOGY

Domain averaged precipitation threshold values (cm/day) Percent of events belonging to each cluster Blah blah Santa Ana Santa Ana events are almost never associated with any precipitation event of any magnitude anywhere in the domain.

Domain averaged precipitation threshold values (cm/day) Percent of events belonging to each cluster Blah blah strong northwesterly Precipitation events somewhere in the domain occasionally coincide with the strong northwesterly regime, but they are generally involve very little rainfall.

Domain averaged precipitation threshold values (cm/day) Percent of events belonging to each cluster Blah blah weak northwesterly Some moderate precipitation events are sometimes associated with the weak northwesterly regime.

Domain averaged precipitation threshold values (cm/day) Percent of events belonging to each cluster Blah blah unclassified The overwhelming majority of all precipitation events, and all of the strong precipitation events, are associated with the unclassified winds. The strong relationship between wet and dry conditions and the wind regimes comfirms that the wind regimes correspond to real physical modes of variability.

MODES OF VARIABILITY AND THE LARGE-SCALE

These are the primary modes of pressure variability during the October-March season when the entire Siberian- Pacific-North American sector is considered. Note the wave- like patterns, with spatial scales on the order of the scale of mid-latitude weather systems (i.e. thousands of kilometers). The second mode corresponds to the well- known Pacific--North American pattern.

Here is the chance of occurrence of the Southern California wind regimes when the magnitude of the various large-scale modes is large. No one large-scale mode has significantly stronger relationship with a local regime than any of the other large- scale modes. The large-scale modes therefore have very little predictive power of the local regimes.

Large-scale sea level pressure patterns associated with the three clusters. Cluster 1 is associated with a highly localized high over the Great American desert. Cluster 2 is associated with a dipole pattern, with a low over the western U.S. and a high over the central Pacific. The pattern associated with Cluster 3 is very similar to the climatology.

Why is this pattern so important for Southern California?

Closing thoughts We have seen in our examination of (1) The geographical distribution of precipitation in Southern California (2) Relationship of local circulation regimes to larger-scale modes that spatial and temporal variability in local climate is related in a non- trivial way to larger-scale forcing. __________________________ Of course, the information coding the emergence of local phenomena is contained in larger-scale forcing. But local climate phenomena may not be sensitive to the larger-scale processes that seem important when a large-scale perspective on climate is adopted. This is critical for climate change prediction. __________________________ Interdisciplinary research