Entlang eines 1000 km langen Streifens der US-Ostküste zwischen Massachusetts und North Carolina stieg in den letzten Jahrzehnten der Meeresspiegel schneller an als im globalen Durchschnitt. Was steckt hinter dieser anomal schnellen Entwicklung? Das Forscherduo Kenigson und Han nahm das Phänomen jetzt näher unter die Lupe und veröffentlichte das Ergebnis im Dezember 2014 im Fachblatt Journal of Geophysical Research.
In ihrer Studie fanden sie starke Hinweise darauf, dass sich hinter der Anomalie vor allem ein Effekt der sogenannten Atantischen Multidekadenoszillation (AMO) verbirgt, die den Meeressspiegel mal schneller und mal langsamer ansteigen lässt. Die Effekte sind regional gestaffelt, was die derzeitige Beschränkung auf die „U.S. mid-Atlantic coast“ erklärt. Im Folgenden die Kurzfassung der Arbeit:
Detecting and understanding the accelerated sea level rise along the east coast of the United States during recent decades
A “hot spot” of accelerated sea level rise has recently been detected between Cape Hatteras and Cape Cod. The acceleration in the long-term trend, however, is difficult to isolate from transient acceleration due to variability, particularly the ∼60 year cycle associated with the Atlantic Multidecadal Oscillation (AMO). The Empirical Mode Decomposition (EMD) and Ensemble EMD (EEMD) methods have been used to isolate oscillations and provide robust acceleration estimates for the trend. Yet the reliability of these methods in detecting accelerated sea level rise, particularly given the limited lengths of tide gauge records, has not been fully tested. Here, the EMD and EEMD methods are applied to both tide gauge observations and synthetic sea level time series constructed as a sum of oscillations extracted from tide gauge records and trends with prescribed acceleration rates. The successively truncated synthetic and observed data are analyzed with (E)EMD, and estimates of the acceleration error based on the record length are produced. Generally, EEMD provides more stable acceleration estimates than EMD, and the error decreases as the record length increases, although not monotonically. Records exceeding two multidecadal oscillation periods in length provide superior estimates over shorter records. In addition, the AMO may have contributed significantly to the rapid acceleration detected in the hot spot during recent decades. These findings have important implications for improved detection of regional sea level acceleration in a warming climate.
Bereits zuvor hatten mehrere Studien zeigen können, dass es sich bei der Anomalie um ein rein natürliches Phänomen handelt (siehe unseren Blogbeitrag „Beschleunigter Meeresspiegelanstieg an Teilen der US-Ostküste liegt noch vollständig im Bereich der natürlichen Variabilität“).
Es wird immer klarer, dass die natürliche Variabilität des Meeresspiegels zunächst vollkommen verstanden werden musss, bevor man sich an Prognosen heranwagen sollte. Im September 2014 erschien im Journal of Geophysical Research eine diesbezügliche Studie von Thompson & Mitchum, in der die Autoren an der nordamerikanischen Ostküste nach Schwankungen des Meeresspiegels suchten. Dabei ging es den Wissenschaftlern um Veränderungen im Abstand von Jahr zu Jahr, jedoch auch um Trends im Maßstab von einigen Jahrzehnten. Interessanterweise konnten Thompson & Mitchum feststellen, dass offenbar ein einheitliches Schwankungsmuster existiert, das vom kanadischen Nova Scotia bis in die Karibik reicht. Es muss also Antriebskräfte geben, die im gesamten Gebiet wirken. Im Folgende der Abstract der Studie:
Coherent sea level variability on the North Atlantic western boundary
Interannual to decadal sea level variability on the North Atlantic western boundary is surprisingly coherent over substantial distances stretching from the Caribbean to Nova Scotia. The physical mechanisms responsible for this basin-scale, low-frequency coherence are explored in a diagnosis of simulated ocean fields from GECCO, which reproduces the observations to good approximation. Coastal sea level variability on the western boundary is known to be influenced by meridional divergence in the boundary current resulting in a geostrophic tilting of the sea surface. This mechanism is found to be of leading order along some stretches of the boundary, but it does not account for the coherence spanning the western North Atlantic. Instead, the coherence along the entire boundary is accounted for by vertical divergence resulting in the uniform rise and fall of the sea surface west of the 295°E meridian. The vertical divergence is found to be due to net vertically integrated zonal transport across this meridian resulting from meridional variation in the Sverdrup transport over the basin interior.
Einige Monate später, im Dezember 2014, legte ein Team um Philip Woodworth im Journal of Geophysical Research nach. Die Wissenschaftler fanden, dass die von Jahr zu Jahr auftretenden Schwankungen im Meeresspiegelanstieg an der Ostküste der USA wohl vor allem von Veränderungen in den küstennahen Winden herrühren. Hier die Kurzfassung:
Mean sea-level variability along the northeast American Atlantic coast and the roles of the wind and the overturning circulation
The variability in mean sea level (MSL) during 1950–2009 along the northeast American Atlantic coast north of Cape Hatteras has been studied, using data from tide gauges and satellite altimetry and information from the Liverpool/Hadley Centre (LHC) ocean model, thereby providing new insights into the spatial and temporal scales of the variability. Although a relationship between sea level and the overturning circulation can be identified (an increase of approximately 1.5 cm in MSL for a decrease of 1 Sv in overturning transport), it is the effect of the nearshore wind forcing on the shelf that is found to dominate the interannual sea-level variability. In particular, winds are found to be capable of producing low-frequency changes in MSL (“accelerations”) in a narrow coastal band, comparable to those observed by the tide gauges. Evidence is presented supporting the idea of a “’common mode” of spatially coherent low-frequency MSL variability, both to the north and south of Cape Hatteras and throughout the northwest Atlantic, which is associated with large spatial-scale density changes from year to year.