LARGE-SCALE ATMOSPHERIC & OCEANIC VARIABILITY
Introduction
This is an area which has served as a focal point for many climate researchers
over the past several decades and will continue as such for the indefinite
future. While much of my earliest work was geared towards climate prediction
(long-range forecasting) a necessary element was diagnostic analysis. In this
area I discuss only the purely diagnostic studies, although the predictive
ones certainly did explore the nature of atmospheric and oceanic variability
along with their interactions. I have divided these diagnostic studies into
two categories: (1) Extratropical and (2) Tropical & Tropical/Extratropical
Interactions.
Extratropical
The work reported in
A more sophisticated approach with broadly similar objectives (Lanzante, 1984; Lanzante and Harnack, 1984) utilized an eigenvector analysis technique which was a forerunner of singular value decomposition (SVD). This study identified large-scale SST patterns (covering both the North Pacific and North Atlantic Oceans) and their coincident atmospheric circulation patterns. In essence these are statistically coupled atmosphere-ocean patterns. Several well-known atmospheric teleconnection patterns (such as the North Atlantic oscillation and Pacific/North American pattern) appeared. In addition, two distinct warm-season "drought" modes were found with a striking similarity to patterns identified by Namias. The lag relationships showed that for the most part the dominant sequence is the atmosphere leading the ocean; this was found to be stronger in the Pacific than in the Atlantic. Again, there is not much support to the notion that the ocean leads the atmosphere.
Another study which falls into this category is Lanzante and Harnack (1983), which reports the results of my M.S thesis project. Since this also relates to the seasonal cycle it is discussed above in the appropriate section. This study examined some of the mechanisms responsible for the development of SST anomalies in the extratropical oceans during summer. It considered the effects of changes in the depth of the oceanic mixed layer as well as oceanic advective processes induced by wind-driven currents. In considering both anomalous and climatological mean SST and wind driven-currents it was found that the dominant term is the advection of the mean SST field by the anomalous wind-driven upper ocean current.
Tropical & Tropical/Extratropical Interactions
One of the earliest research projects in which I participated
(Harnack et al. 1982),
involved an examination of the relationship between some tropical
parameters (anomalies of trade winds and tropical SST's) and the extratropical
atmospheric circulation. The now well known fact that the ENSO related
signal in the tropics is transmitted to the extratropics at a lag up to
several seasons later was found. This study also examined the seasonality of
these relationships and was perhaps the first to identify the so called "spring
barrier" whereby lag relationships involving ENSO signals break down at lag
during boreal spring.
More recent work of mine (Lanzante, 1991; Lanzante, 1996) has concentrated on ENSO related SST variability in the tropics. The latter study was motivated by the inconsistencies in the literature concerning lag relationships between tropical Pacific and tropical Atlantic SST's. There were a variety of investigators who claimed either a positive, negative or no relationship between these two areas. I investigated the lag relations between SST anomalies throughout the tropics using a long historical record and identified four key areas. I found that ENSO related SST anomalies appear first in the eastern Pacific (EPAC), then appear several months later in the central Pacific (CPAC), and finally several months after this they appear in the Indian (IND) and North Atlantic (NATL) Oceans. The weakest link is the appearance in the Atlantic. I suggested that the SST anomalies in the more remote locations of the Indian and Atlantic Oceans are the result of the "atmospheric bridge" mechanism proposed by my Climate Diagnostics Group group colleagues Gabriel Lau and Mary Jo Nath.
Several years ago I completed a suite of integrations of a new coupled model involving a complex GFDL R30 atmospheric model and a simple model of the oceanic mixed layer. This coupled model was created by Mike Alexander of CIRES, located in Boulder, CO, along with his colleague Jamie Scott. The oceanic component was created by Mike and has been used quite a bit in recent years. In particular Mike has used it to study the "reemergence mechanism", whereby extratropical SST anomalies which exist in the late winter and early spring become cut off from the surface and isolated below the mixed layer during summer and early fall, only to reemerge in late fall. This and other topics will be pursued by us in Climate Diagnostics Group as well as by Mike, Jamie, and others at CDC using output from these experiments which incorporate prescribed forcing in the tropical Pacific (mimicing observations from the past ~50 years) but allow the ocean model to predict the SST's elsewhere. Mike has taken the lead in a paper which serves as a review of the "atmospheric bridge" and incorportes some results from the above coupled model runs (Alexander et al. 2002).
Plans
Although my personal involvement with the recent coupled model integrations
is on the "back-burner" I hope to return to this in the future.
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